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UKOMADU KINGSLEY EZE PG/M.ENG/11/59404
EFFECT OF BAFFLES ON OIL WATER SEPARATOR
ENGINEERING
A PROJECT WORK SUBMITTED TO THE DEPARTMENT OF CIVIL ENGINEERING, FACULTY OF ENGINEERING, UNIVERSITY
OF NIGERIA, ENUGU CAMPUS
IJEOMAH CLARA
Digitally Signed by: Content manager’s Name
DN : CN = Webmaster’s name
O= University of Nigeria, Nsukka
OU = Innovation Centre
DECEMBER, 2012.
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EFFECT OF BAFFLES ON OIL WATER SEPARATOR
BY
UKOMADU KINGSLEY EZE
PG/M.ENG/11/59404
IN PARTIAL FULEILLMENT OF THE REQUIREMENT OF MASTER OF ENGINEERING IN CIVIL ENGINERRING
(WATER RESOURCES AND ENVIRONMENTAL ENGINEERING)
ENGR. PROF. J.C. AGUNWAMBA
(SUPERVISOR)
DEPARTMENT OF CIVIL ENGINEERING
FACULTY OF ENGINEERING
UNIVERSITY OF NIGERIA, NSUKKA
DECEMBER, 2012.
3
CERTIFICATION
UKOMADU KINGSLEY EZE, a Postgraduate Student in the Department of Civil
Engineering, with Reg. No. PG/M.Eng/11/59404, has satisfactorily completed
the requirements of the research work for the degree of masters in
Engineering in Civil Engineering. The work embodied in this thesis is the
original and has not been submitted in full for any other diploma or degree in
this or any other University.
……………………………
Ukomadu Kingsley Eze
(student)
……………………………. …………………………
Engr. Prof. J. C. Agunwamba Engr. Prof. O.O.Ugwu
(SUPERVISOR) (HEAD OF DEPARTMENT)
……………………………. …………………………….
(DEAN, FACULTY OF ENGINEERING) (EXTERNAL EXAMINER)
4
DEDICATION
This work is dedicated to the Almighty God for his boundless grace that
sustained me all through the period of this programme. In all my journeys
which were quite numerous, He preserved me.
5
ACKNOWLEDGEMENT
I wish to express my profound gratitude to my project supervisor
and lecturer, Engr. Prof. J.C. Agunwamba, who taught me, meticulously
supervised this research work. My thanks goes to my parent Mr. Cecilia
Ukomadu who made my programme a reality, my, friends, classmates and all
my well wishers.
I also wish to use this opportunity to appreciate the workers in the
laboratory who gave me successive directive.
6
ABSTRACT
Gravity oil water separators with varied no baffles and varied locations
of baffle for the removal of free oils from wastewater was developed.
Performance test was carried out to determine its removal efficiencies. The
primary component of the separators include introducing different number of
baffles on the channels and also varying locations of baffle in other channels,
using five flowrates for enhance gravity separation and coalescence oil
droplets. The removal efficiency (E) of the separators was found to be inversely
proportional to influent flowrate, effluent concentration, and directly
proportional to retention time (t). Three different oils were used at different
influent concentration; for engine oil, Coil= 15300mg/1, for groundnut oil, Coil=
1300mg/1, for hydraulics oil, Coil= 35700mg/1. For channel which different
number of baffles was introduced, the highest separator efficiency for the
three oils was achieved at 94%, 86%, 90% at the lowest flowrate (Q) of 0.3x10-4
m3/s. at the highest no of baffles, at longer retention time. There was almost a
linear progression of oil removal efficiencies with no of baffles, the value
correlation coefficient increases with decrease in the flowrate used, fig 4.5,
flowrate (Q) of 3.0x10-4 , R2 = 0.992 and at flowrate 9.0x10-4, R2 = 0.982.
Keywords: baffles, free oils, oil-water separation removal efficiencies
7
LIST OF ABBREVIATIONS
Ci Influent oil concentration (mg/1)
Co effluent oil concentration )mg/1)
d oil dropled diameter (m)
vs particle setting velocity in m/s
q flowrate (m3/5)
vr rising velocity of oil droplets
V terminal velocity (m/s)
v horizontal velocity (m/s)
R radius of the particles in metres
g Acceleration due to gravity (M/S2)
r oil density (kg/m3)
OPEC - organization of petroleum exporting countries
NOC - National oil companies
NNPC - Nigeria Nation Petroleum Company
PSC - Production Sharing Contracts
JOA - Joint Operating Agreement
8
MOU - Memorandum of Understanding
WSF - Water Soluble Fraction
CWA - Clean Water Act
OWS - Oil Water Separator
API - American Petroleum Institute
NRE - Reynold Number
9
TABLE OF CONTENTS
Certification - - - - - - - - - - i
Dedication - - - - - - - - - - - ii
Acknowledgement - - - - - - - - - iii
Abstract - - - - - - - - - - - iv
List of Abbreviations - - - - - - - - - v
Table of content - - - - - - - - - - vi
CHAPTER ONE
1.0 Introduction - - - - - - - - - 1
1.1 Significance of Study - - - - - - - - 4
1.2 Research Objectives - - - - - - - - 4
1.3 Research Problems - - - - - - - - 4
1.4 Scope of Research - - - - - - - - 4
1.5 Research limitation - - - - - - - - 5
CHAPTER TWO
2.0 Literature Review - - - - - - - - 6
10
2.1 History of oil and gas industry in Nigeria - - - - - 6
2.1.1 The TOA and how it operates - - - - - - - 8
2.1.2 Challenges of the TOA - - - - - - - - 9
2.1.3 The PSC and how it operates - - - - - - - 9
2.2 Overview of oil exploration - - - - - - - 12
2.2.1 Exploration surveying - - - - - - - - 13
2.2.2 Exploration Drilling - - - - - - - - 15
2.2.3 Development and production - - - - - - - 18
2.2.4 Decommissioning and inhabitation - - - - - - 22
2.3 Crude oil emulsification - - - - - - - 23
2.4 Physical and chemical characteristics of oil - - - - - 25
2.4.1 Surface tension - - - - - - - - - 25
2.4.2 Viscosity - - - - - - - - - -25
2.4.3 Solubility - - - - - - - - - - 26
2.5 Behavior of oil in marine environments - - - - - 26
2.5.1 Marine Environment - - - - - - - - 26
2.5.2 Weathering processes - - - - - - - - 28
2.5.3 Spreading and drifting - - - - - - - - 29
2.5.4 Dissolution - - - - - - - - - 29
2.5.5 Dispersion - - - - - - - - - - 29
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2.5.6 Photochemical oxidation - - - - - - - 30
2.5.7 Shoreline interactions - - - - - - - - 31
2.6 Impact of oil on marine environment - - - - - -
31
2.6.1 Physical contamination - - - - - - - - 31
2.6.2 Effects of oiling on sea birds - - - - - - - 33
2.7 Chemical Response Technology - - - - - - 33
2.7.1 Natural Biodegradation - - - - - - - - 33
2.7.2 Enhanced biodegradation - - - - - - - 35
2.7.3 Oil sinking agents - - - - - - - - 35
2.7.4 Sorbents - - - - - - - - - - 36
2.7.5 Dispersion - - - - - - - - - - 39
2.8 Oil water separation - - - - - - - - 40
2.8.1 Separation theory - - - - - - - - -
42
2.8.2 Stoke’s law and a sphere falling through a quiescent viscous liquid 45
2.9 What is oil water separator - - - - - - - 46
2.9.1 Types of oil water separators - - - - - - - 46
2.9.2 Gravity type separator - - - - - - - - 48
12
2.9.3 Enhanced oil water separators - - - - - - -
48
2.9.4 Water polluted by oil - - - - - - - - 57
2.10 Review of Previous Work - - - - - - - 58
2.11 Two phase horizontal flow - - - - - - - 64
2.11.1 Models for oil water flows - - - - - - - 65
2.11.2 Fundamentals of a separated flow model - - - - - 67
2.12 Baffles - - - - - - - - - - 71
1.12.1 Number of baffles - - - - - - - - 73
1.12.2 Importance of baffles - - - - - - - -
CHAPTER THREE: METHODOLOGY
3.1 Separator Development - - - - - - - - 73
3.2 Applicability of varied no of baffles - - - - - - 74
3.3 Separator and baffles design - - - - - - - 75
3.4 Experimental procedure - - - - - - - -
76
3.5 Determination of oil concentration - - - - - - 77
3.6 Determination of oil removal efficiency - - - - - 78
3.7 Tracers studies - - - - - - - - - 78
3.7.1 Tracer studies - - - - - - - - - 79
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3.8 Dispersion number - - - - - - - - 79
CHAPTER FOUR
4.0 Data collection and analysis - - - - - - - 80
4.1 Data discussion - - - - - - - - - 85
4.1.1 Effect of no of baffles on oil removal efficiency - - - - 85
4.1.2 Oil removal efficiency as a function of flowrates (Q) and initial
concentration - - - - - - - - - - 86
4.2 Dispersion number - - - - - - - - 86
CHAPTER FIVE
5.0 Conclusion and Recommendation - - - - - - 88
5.1 Conclusion - - - - - - - - - - 88
5.2 Recommendation - - - - - - - - -
88
References - - - - - - - - - - 89
14
Appendix - - - - - - - - - - - 95
15
CHAPTER ONE: INTRODUCTION
In refineries, chemical plants, electric power plants and many other
industrial facilities the separation of various oil and water mixtures can cause
problems. These problems are often the result of imperfect understanding of the
nature of the mixtures and how to take advantage of their properties to
accomplish the required separations. In addition, many states and cities require
treatment of storm water from parking lots and other facilities where cars and
trucks may be present to treat storm water to ensure the oil and fuel that may
have leaked from the vehicles does not enter the rivers, streams and lakes. This
course will give an overview of many of the industrial and also storm water
processing situations that may arise and also some of the means for solving the
problems with pros and cons of many possible designs as well as some
suggestions on determining the nature and extent of the problems and possible
solutions. For purposes of this discussion, oil means hydrocarbons except where
specifically noted otherwise.
TYPES OF SEPARATIONS:
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Four main types of separations are likely in industrial plants and numerous
more similar situations exist from time to time. The four most common are:
(i)Water from Oil where main flow is Mostly Oil
(ii)Oil from Water where main flow is Mostly Water
(iii)Emulsions
(iv); Non-Hydrocarbon oils
WATER FROM OIL WHERE MAIN FLOW IS MOSTLY OIL
Separating water from continuous flows of oil is commonly required in oil
production applications, oil refineries and chemical plants as well as some places
where it is essential that the hydrocarbons not be contaminated with water. The
possible problems with water contamination were first emphasized during the
last part of World War II when it was found that airplanes could fly high enough to
cause the water to freeze in the fuel lines.
The pilots found this unreasonably inconvenient because it caused the
engines to stop, so equipment was designed to ensure that only tiny amounts of
water were allowed to remain in the aviation fuel It was alsnd that refinery
processes operated easier and better and corrosion problems were avoided by
17
removing the water from the hydrocarbons. Numerous types of equipment have
been designed to cope with the widely varying problems of removing the water
from the oil and several of these are discussed below. The problems in removing
water from oil vary widely mostly because of the widely varying viscosity of
hydrocarbons that must be treated.
OIL FROM WATER WHERE MAIN FLOW IS MOSTLY WATER
Separating oil from a continuous stream of water is commonly done in oil
refineries, chemical plants, and other industrial facilities for resource recovery as
well as environmental reasons. It is also practiced in some oil field situations
where the flow from the wells is primarily water. The beginnings of the
application of scientific principles to these separations began in 1948 when the
American Petroleum Institute (API) commissioned a study by the University of
Wisconsin to prepare a method for designing separators to recover oil from the
main refinery wastewater streams. This design is still used, but it was not
originally designed for environmental purposes and does not generally produce
an effluent suitable for discharge to rivers, streams or lakes. This method requires
a large residence time and is therefore bulky and costly, so modified design “API.
18
Type” systems are often used. Since the 1948 study2, numerous designs
have been used to remove oil from water and several are discussed below. The
newer designs make it possible to remove oil from the water down to less than 10
mg/l.
EMULSIONS:
An emulsion is a mechanical mixture, not a solution, consisting of droplets
of one immiscible fluid dispersed in another continuous fluid. A good definition,
offered by 10, is: "An emulsion is an apparently homogenous mixture in which
one liquid is dispersed as droplets throughout a second immiscible liquid." In the
case of water and oil, two types of emulsion are common, depending on which is
the continuous phase.
1. Oil in water emulsions.
2. Water in oil emulsions.
A third type, water in oil in water, is possible but very uncommon.
Emulsions can be very difficult to separate and because of the extreme variations
in type, causes, and treatment are outside the scope of study.
19
1.1 SIGNIFICANCE OF STUDY
This study is of great importance to the oil and gas industries, especially the
downstream sector; it will contribute to the enhancement of oil-water separators,
in its efficiency of oil removal.
1.2 OBJECTIVE OF THE STUDY
- The objective of this project is to ascertain the effect varied number of
baffles on oil-water separator.
- Varying flowrate five times in each of the five channel proposed
- tracer studies experiment was performed to know the hydraulic pathway
of flow
1.3 RESEARCH PROBLEMS
Due to the fact that effect of baffles on oil-water separators is a new research
topic, availability of data from previous work was a challenge, some information
was sighted from journals, web and other works related to the topic
1.4 SCOPE OF WORK
20
The scope of this study is check the oil removal efficiencies of five channels,
introducing different number of baffles in each of them, and varying five different
flowrates in each of them. Three oils were used differently in mixture of oil/water
to carry out the experiment; samples were collected to test for oil concentration.
1.5 RESEARCH LIMITATIONS
The research limitation was that there was no consistent power supply in
laboratory when samples were taken to test for oil concentration. The
material used for construction of my separator must have reacted with the
mixture to alter the experimental results for oil concentration. The oil used
was adhering to the body of the material used thereby reducing its volume.
21
CHAPTER TWO: LITERATURE REVIEW
2.1 HISTORY OF OIL AND GAS INDUSTRY IN NIGERIA
Oil and gas operations commenced in Nigeria effectively in 1956, with the
first commercial find in that year by the then Shell D’Arcy. Before this time, that
is, from November 1938, almost the entire country was covered by a concession
granted to the company to explore for petroleum resources. This dominant role
22
of Shell in the Nigerian oil industry continued for many years, until Nigeria’s
membership of the Organization of Petroleum Exporting.
Countries (OPEC) in 1971, after which the country began to take a firmer
control of its oil and gas resources, in line with the practice of the other members
of OPEC. This period witnessed the emergence of National Oil Companies (NOCs)
across OPEC member countries, with the sole objective of monitoring the stake of
the oil‐producing countries in the exploitation of the resource. Whereas in some
OPEC member countries the NOCs took direct control of production operations, in
Nigeria, the Multinational Oil Companies (MNOCs) were allowed to continue with
such operations under Joint Operating Agreements (JOA) which clearly specified
the respective stakes of the companies and the Government of Nigeria in the
ventures.
This period also witnessed the arrival on the scene of other MNOCs such as
Gulf Oil and Texaco (now ChevronTexaco), Elf Petroleum (now Total), Mobil (now
ExxonMobil), and Agip, in addition to Shell, which was already playing a dominant
role in the industry. These other companies were also operating under JOAs with
NNPC, with varying percentages of stakes in their respective acreages. To date,
the above companies constitute the major players in Nigeria’s oil industry, with
23
Shell accounting for a just little less than 50% of Nigeria’s total daily production,
which currently stands at about 2.4 million barrels of oil per day. JOAs are also still
dominant in the oil industry in Nigeria, accounting for over 90% of total oil and
gas production in Nigeria today. The emergence of offshore oil and gas operations
and the granting of deep water acreages to the oil producing companies has
however witnessed a shift from JOA regimes to Production Sharing Contracts
(PSCs), with implications for the operation and regulation of the oil industry in
Nigeria. This shift is attributable to a number of factors ranging from the
complexity of operations in the offshore terrain, (which makes regulation under a
JOA more difficult), to dwindling resources of the country, (which makes funding
under the JOAs precarious for the government). At a time when the Nigerian
government is intent on increasing oil and gas reserves and the country’s
production capacity without the necessary funds to back it up, a funding
arrangement which achieves those objectives without having a negative impact
on the scarce resources available for investment in other sectors of the economy
is imperative. A number of oil and gas projects using the PSC model are due to
come on stream soon and the successes recorded so far in this area have
encouraged the government to consider extending PSC arrangements to other
areas of the industry which had hitherto operated under JOAs This paper
24
examines these contractual models in the Nigerian oil and gas industry, their
respective strengths and drawbacks, and the current shift in emphasis from JOAs
to PSCs, adducing reasons for this shift, and what this portends for investment in
the sector in Nigeria. The aim is to show the long term effects of this shift on the
investment climate and the overall development of the Nigerian economy, in
which oil and gas plays a central role.
2.1.1 THE JOA AND HOW IT OPERATES
Modelled after partnership agreements, the JOA operates as a form of
partnership between the joint venture partners, which spells out the participatory
interest of each of the partners and also designates one of the partners as the
operator of the venture. In Nigeria, the NNPC represents the interest of the
government in the joint ventures, whereas the respective MNOCs operate the
different ventures with varying participatory interests. The JOA governs the
relationship between the parties, including budget approval and supervision,
crude oil lifting and sale in proportion to equity, and funding by the partners. In
addition to the JOA, a Memorandum of Understanding (MOU) governs the
manner in which revenues from the venture are allocated between the partners,
including payment of taxes, royalties and industry margin. The income derived
25
from the operation is also shared in proportion to the equity interests of the
parties to the venture, with each party bearing the cost of its royalty and tax
obligations in the same proportion. Allocations are also made from the revenue to
take care of operating and technical costs.
2.1.2 CHALLENGES OF THE JOA
Some of the constraints associated with the JOA include poor funding, due
mainly to the imbalance in the financial capacity of the different joint venture
partners, especially the government which has other pressures on its resources,
leading often to reduction in operations and consequential loss in revenue. JOA is
also constrained by allegations of gold plating of operating costs by the
non‐operators of the venture, which often leads to mutual suspicion between the
parties, and the rather unfair distribution of revenues, especially in the situation
of upsides from high oil prices. Additionally, the Operator also faces peculiar
challenges in Nigeria such as the need to meet the incessant demands by oil
producing communities for development programmes in their areas–demands
which could lead to disruptions in operations from time to time. With the
expansion of the Nigerian oil and gas industry, acreages started being allocated in
the shallow and deep offshore areas, and this introduced the need for a different
26
regime, as it brought its own unique challenges in terms of funding and technical
complexity. This led to the introduction of PSCs in the new offshore and inland
basin acreages, which is gradually assuming prominence in the entire industry.
2.1.3 THE PSC AND HOW IT OPERATES
As the name implies, PSCs focus on the sharing of the output of oil and gas
operations in agreed proportions between the Oil Company, as a contractor to
the government, and the NOC, as the representative of government interests in
the venture. This form of contracts originated in Indonesia in 1966 and was
modelled along the lines of share cropping in agriculture, where the owner of the
land grants a farmer the rights to grow crops on his land and shares the proceeds
with the farmer in agreed proportions after the harvest. Under a PSC, the
contractor, usually a foreign oil company bears the entire cost and risk of
exploration activities, and only reaps the rewards after a commercial find. In the
event of a commercial discovery, the contractor recovers its costs fully from
allocation of oil, referred to as ‘Cost Oil’. Allowance is also made from
production for royalties, after which the remainder of the production, called
‘Profit Oil’, is shared in agreed proportions between the company and the
government as represented by the NOC. The Oil Company thereafter pays income
27
tax on its profits from the venture. The oil and all the installations remain the
property of the host government throughout the duration of the contract. In
Nigeria, this form of contractual arrangement is relatively new, and covers mostly
acreages in the shallow and deep offshore areas and the inland basins. The major
operators in Nigeria are still largely the holders of the PSCs but there have also
been new entrants, made up of independent foreign oil companies, which enter
into partnerships with indigenous companies to bid for oil blocks, and thereafter
operate it in line with predetermined contractual arrangements.
In addition to the specific contracts signed with the individual companies,
the main law which regulates the operation of PSCs in Nigeria is the Deep
Offshore and Inland Basin Production Sharing Contracts Act No. 9, Laws of the
Federation of Nigeria, 1999. This law sets out the general framework for the
operation of PSCs, including the applicable royalties, tax regimes, and the manner
in which costs and profits are allocated between the parties.
It provides for payment of a flat rate of 50% tax on petroleum profits by
PSC operators, and sets different royalty regimes, depending on the water depth
in which the operation is carried out, ranging from 12% for water depths of
28
200‐500m, to 0% for water depths in excess of 1,000m. PSCs in inland basins
attract a flat royalty of 10%.
In addition to royalties, taxes and its share of profit oil, the government
also earns revenue from signature bonuses paid by the oil companies upon
successful bids. Most forms of payments under PSCs operating in Nigeria are
made in oil, as the law provides for cost oil, tax oil, royalty oil and profit oil.
Investment Tax Credits and Allowances are also available to the investors at the
rate of 50% of the value of such investments. Some of the advantages associated
with PSCs include the relative flexibility in the management of the operations, and
the fact that there is no financial burden on the host government, and even after
a commercial find, the payment to the contractor is in oil, which does not attract
any direct financial cost. Leveraging on the technical know‐how and experience of
the companies in such operations, the government can focus its energies in other
areas of the economy while trusting that the oil and gas industry will develop at
an acceptable pace without the usual trappings of cash call constraints. However,
PSCs have some drawbacks such as the risky nature of the operation. For
instance, in the event of an unsuccessful operation, millions of dollars could be
completely lost ‐ unless the local laws allow for costs from one acreage to be
transferred to another, which is not always the case, and would depend on the
29
provisions of the PSC entered into by the parties. Also, the fact that the
contractor is usually allowed a relatively unfettered hand to draw up and execute
its programme could lead to allegations of gold plaiting of costs. The long term
nature of transactions in the oil industry however usually mitigates some of these
difficulties. The tendency is usually for both parties to strive to make room for
flexibility in drawing up the terms, and also make provisions for renegotiation in
the event that particular provisions are later found to be causing undue hardship.
In recent times, there has been a conscious shift in the contractual structure in
the oil and gas industry in Nigeria from JOAs.
2.2 OVERVIEW OF OIL EXPLORATION
The oil and gas industry comprises two parts: ‘upstream’—the exploration
and production sector of the industry; and downstream’—the sector which deals
with refining and processing of crude oil and gas products, their distribution and
marketing. Companies operating in the industry may be
regarded as fully integrated, (i.e. have both upstream and downstream interests),
or may concentrate on a particular sector, such as exploration and production,
commonly known as an E&P company, or just on refining and marketing (a R&M
30
company). Many large companies operate globally and are described as ‘multi-
nationals’, whilst other smaller companies concentrate on specific areas of the
world and are often referred to as ‘independents’.
Frequently, a specific country has vested its interests in oil and gas in a
national company, with its name often reflecting its national parenthood. In the
upstream sector, much reliance is placed upon service and upon contractor
companies who provide specialist technical services to the industry, ranging from
geophysical surveys, drilling and cementing, to catering and hotel services in
support of operations. This relationship between contractors and the oil
companies has fostered a close partnership, and increasingly, contractors are fully
integrated with the structure and culture of their clients. Scientific exploration for
oil, in the modern sense, began in 1912 when geologists were first involved in the
discovery of the Cushing Field in Oklahoma, USA. The fundamental understand
the activities involved. This section briefly describes the process, but those
requiring more in-depth information should refer to literature available from
industry groups and academia. Table 1 provides a summary of the principal steps
in the process and relates these to operations on the ground.
2.2.1 EXPLORATION SURVEYING
31
In the first stage of the search for hydrocarbon-bearing rock formations,
geological maps are reviewed in desk studies to identify major sedimentary
basins. Aerial photography may then be used to identify promising landscape
formations such as faults or anticlines. More detailed information is assembled
using a field geological assessment, followed by one of three main survey
methods: magnetic, gravimetric and seismic.
The Magnetic Method depends upon measuring the variations in intensity
of the magnetic field which reflects the magnetic character of the various rocks
present, while the Gravimetric Method involves the measurements of small
variations in the gravitational field at the surface of the earth. Measurements are
made, on land and at sea, using an aircraft or a survey ship respectively.
A seismic survey, as illustrated in Figure 1 on page 6, is the most common
assessment method and is often the first field activity undertaken. The Seismic
Method is used for identifying geological structures and relies on the differing
reflective properties of sound waves to various rock strata, beneath terrestrial or
oceanic surfaces. An energy source transmits a pulse of acoustic energy into the
ground which travels as a wave into the earth. At each point where different
geological strata exist, a part of the energy is transmitted down to deeper layers
32
within the earth, while the remainder is reflected back to the surface. Here it is
picked up by a series of sensitive receivers called geophones or seismometers on
land, or hydrophones submerged in water. Special cables transmit the electrical
signals received to a mobile laboratory, where they are amplified and filtered and
then digitized and recorded on magnetic tapes for interpretation.
Dynamite was once widely used as the energy source, but environmental
considerations now generally favour lower energy sources such as vibroseis on
land (composed of a generator that hydraulically transmits vibrations into the
earth) and the air gun (which releases compressed air) in offshore exploration. In
areas where preservation of vegetation cover is important, the shot hole
(dynamite) method is preferable to vibroseis.
2.2.2 EXPLORATION DRILLING
Once a promising geological structure has been identified, the only way to
confirm the presence of hydrocarbons and the thickness and internal pressure of
a reservoir is to drill exploratory boreholes. All wells that are drilled to discover
hydrocarbons are called ‘exploration’ wells, commonly known by driller dcats’.
The location of a drill site depends on the characteristics of the underlying
geological formations. It is generally possible to balance environmental protection
33
criteria with logistical needs, and the need for efficient drilling. For land-based
operations a pad is constructed at the for a single exploration well occupies
between 4000–15 000 m2. The type of pad construction depends on terrain, soil
conditions and seasonal constraints. Operations over water can be conducted
using a variety of self-contained mobile offshore drilling units (MODUs), the
choice of which depends on the depth of water, seabed conditions and prevailing
meteorological conditions,— particularly wind speed, wave height and current
speed. Mobile rigs commonly used offshore include jackups, semi-submersibles
and drillships, whilst in shallow protected waters barges may be used.
Land-based drilling rigs and support equipment are normally split into
modules to make them easier to move Drilling rigs may be moved by land, air or
water depending on access, site location and module size and weight. Once on
site, the rig and a self-contained support camp are then assembled. Typical
drilling rig modules include a derrick, drilling mud handling equipment, power
generators, cementing equipment and tanks for fuel and water (see Figure 2).
The support camp is self-contained and generally provides workforce
accommodation, canteen facilities, communications, vehicle maintenance parking
areas ,a helipad for. remote sites, fuel handling and storage areas, and provision
34
or the collection, treatment and disposal of wastes. The camp should occupy a
small area (typically 1000 m2), and be located from immediate area of the drilling
rig—upstream from the prevailing wind direction.
Once drilling commences, drilling fluid or mud is continuously circulated
down the drill pipe and back to the surface equipment. Its purpose is to balance
underground hydrostatic pressure, cool the bit and flush out rock cuttings. The
risk of an uncontrolled flow from the reservoir to the surface is greatly reduced by
using blowout preventers —a series of hydraulically actuated steel rams that can
close quickly around the drill string or casing to seal off a well. Steel casing is run
into completed sections of the borehole and cemented using provide to maintain
the integrity of the borehole and isolates underground formations. Drilling
operations are generally conducted around-the clock. Time taken to drill a bore
hole depends on the depth of the hydrocarbon bearing formation and the
geological conditions, but it is commonly of the order of one on two months.
Where a hydrocarbon formation is found, initial well tests—possibly lasting
another month—are conducted to establish flow rates and formation pressure.
These tests may generate oil, gas and formation water—each of which needs to
be disposed of. After drilling and initial testing, the rig is usually dismantled and
moved to the next site. If the exploratory drilling has discovered commercial
35
quantities of hydrocarbons, a wellhead valve assembly may be installed. If the
well does not contain commercial quantities of hydrocarbon, the site is
decommissioned to a safe and stable condition and restored to its original state
or an agreed after use. Open rock formations are sealed with cement plugs to
prevent upward migration of wellbore fluids. The casing wellhead and the top
joint of the casings are cut below the ground level and capped with a cement
plug.
When exploratory drilling is successful, more wells are drilled to determine
the size and the extent of the field. Wells drilled to quantify the hydrocarbon
reserves found are called ‘out step’ or ‘appraisal’ wells. The appraisal stage aims
to evaluate the size and nature of the reservoir, to determine the number of
confirming or appraisal wells required, and whether any further seismic work is
necessary. The technical procedures in appraisal drilling are the same as those
employed for exploration wells, and the description provided above applies
equally to appraisal operations. A number of wells may be drilled from a single
site, which increases the time during which the site is occupied. Deviated or
directional drilling at an angle from a site adjacent to the original discovery
36
borehole may be used to appraise other parts of the reservoir, in order to reduce
the land used or ‘foot print’.
2.2.3 DEVELOPMENT AND PRODUCTION
Having established the size of the oil field, the subsequent wells drilled are
called ‘development’ or ‘production’ wells. A small reservoir may be developed
using one or more of the appraisal wells. A larger reservoir will require the drilling
of additional production wells. Multiple production wells are often drilled from
one pad to reduce land requirements and the overall infrastructure cost. The
number of wells required to exploit the hydrocarbon reservoir varies with the size
of the reservoir and its geology. Large oilfields can require a hundred or more
wells to be drilled, whereas smaller fields may only require ten or so.
The drilling procedure involves similar techniques to those described for
exploration; however, with a larger number of wells being drilled, the level of
activity obviously increases in proportion. The well sites will be occupied for
longer, and support services— workforce accommodation, water supply, waste
management, and other services—will correspondingly increase. As each well is
drilled it has to be prepared for production before the drilling rig departs. The
heavy drill pipe is replaced by lighter weight tubing in the well and occasionally
37
one well may carry two or three strings of tubing, each one producing from
different layers of reservoir rock. At this stage the blow out preventer is replaced
by a control valve assembly or ‘Christmas Tree’. Most new commercial oil and gas
wells are initially free flowing: the underground pressures drive the liquid and gas
up the well bore to the surface. The rate of flow depends on a number of factors
such as the properties of the reservoir rock, the underground pressures, the
viscosity of the oil, and the oil/gas ratio. These factors, however, are not constant
during the commercial life of a well, and when the oil cannot reach the surface
unaided, some form of additional lift is required, such as a pumping mechanism
or the injection of gas or water to maintain reservoir pressures. It is now quite
common to inject gas, water, or steam into the reservoir at the start of the field’s
life in order to maintain pressures and optimize production rates and the ultimate
recovery potential of oil and gas. This in turn may require the drilling of additional
wells, called injection wells. Other methods of stimulating production can be
used, such as hydraulic fracturing of the hydrocarbon bearing formation, and acid
treatment (particularly in limestone) to increase and enlarge flow channels. /Once
the hydrocarbon reaches the surface, it is routed to the central production facility
which gathers and separates the produced fluids (oil, gas and water). The size and
38
type of the installation will depend on the nature of the reservoir, the volume and
nature of produced fluids, and the export option selected.
The production facility processes the hydrocarbon fluids and separates oil,
gas and water. The oil must usually be free of dissolved gas before export.
Similarly, the gas must be stabilized and free of liquids and unwanted
components such as hydrogen sulphide and carbon dioxide. Any water produced
is treated before disposal. A schematic representation of a typical crude oil
processing facility is shown in Figure 3. Routine operations on a producing well
would include a number of monitoring, safety and security programme,
maintenance tasks, and periodic down hole servicing using a wire line unit or a
work over rig to maintain production. The operator will be able to extract only a
portion of the oil present using primary recovery (i.e. natural pressure and simple
pumping) but a range of additional recovery methods are available as discussed
above. For example, secondary recovery uses water flood or gas injection, and
tertiary methods employing chemicals, gases or heat may also be used to increase
the efficiency of oil recovery. The infrastructure required for development drilling
in onshore operations is similar to that described above for exploration.
39
However, once drilling is completed, the individual wellhead assemblies
and well sites are considerably smaller than when the drill rig was on site.
Typically, each well requires an area of some 10 m2 surrounded by a security
fence. Often the well sites are concentrated within a central area, which includes
processing facilities, offices and workshops, and this would typically occupy an
area of several hectares, depending upon the capacity of the field. Since the
production operation is a long-term development, the temporary facilities used in
exploration are replaced by permanent facilities and are subject to detailed
planning, design and engineering and construction. The temporary workforce
associated with exploration activity is replaced by a permanent workforce, usually
accommodated in the local area and, where desirable, fully integrated with the
local community: indeed a large proportion of the workforce may be recruited
locally and receive specialized training. Similarly, the local infrastructure will need
to provide variety of requirements in addition to labor, such as materials supplies,
education, medical, etc.
In offshore production developments, permanent structures are necessary
to support the required facilities, since typical exploration units are not designed
for full scale production operations. Normally, a steel platform is installed to serve
as the gathering and processing centre and more than 40 wells may be drilled
40
directionally from this platform. Concrete platforms are sometimes used. If the
field is large enough, additional ‘satellite’ platforms may be needed, linked by sub
sea flow lines to the central facility. In shallow water areas, typically a central
processing facility is supported by a number of wellhead platforms. Recent
technological developments, aimed at optimizing operations, include remotely
operated subsea systems which remove the requirement for satellite platforms.
This technology is also being used in deep water where platforms are unsuitable,
and for marginal fields where platforms would be uneconomic. In these cases,
floating systems—ships and semi-submersibles—‘service’ the subsea wells on a
regular basis.
Recent advances in horizontal drilling have enhanced directional drilling as
a means of concentrating operations at one site and reducing the ‘footprint’ on
land of production operations (Figure 5) and the number of platforms offshore.
The technology now enables access to a reservoir up to Several kilometers from
the drill rig, while technology is developing to permit even wider range. This
further minimizes the ‘footprint’ by reducing the need for satellite wells. It also
allows for more flexibility in selecting a drill site, particularly where environmental
concerns are raised.
41
2.2.4 DECOMMISSIONING AND REHABILITATION
The decommissioning of onshore production installations at end of their
commercial life, typically 20–40 years, may involve removal of buildings and
equipment, restoration of the site to environmentally-sound conditions,
implementation of measures to encourage site re-vegetation, and continued
Monitoring of the site after closure. Planning for decommissioning is an integral
part of the overall management process and should be considered at the
beginning of the development during design, and is equally applicable to both
onshore and offshore operations. Section 6 provides more detailed discussion on
decommissioning and rehabilitation.
By their nature, most exploration wells will be unsuccessful and will be
decommissioned after the initial one-to-three months of activity. It is, therefore,
prudent to plan for this from the outset, and ensure minimal environmental
disruption. Decommissioning and rehabilitation will, subsequently, be simplified.
2.3 CRUDE OIL EMULSIFICATION
2 Crude oil is seldom produced alone because it generally is commingled with
water. The water creates several problems and usually increases the unit cost of
42
oil production. The produced water must be separated from the oil, treated, and
disposed of properly. All these steps increase costs. Furthermore, sellable crude
oil must comply with certain product specifications, including the amount of basic
sediment and water (BS&W) and salt, which means that the produced water must
be separated from the oil to meet crude specifications (Miller etal, 1993),
Produced water may be produced as “free” water (i.e., water that will settle out
fairly rapidly), and it may be produced in the form of an emulsion. A regular
oilfield emulsion is a dispersion of water droplets in oil. Emulsions can be difficult
to treat and may cause several operational problems in wet-crude handling
facilities and gas/oil separating plants. Emulsions can create high-pressure drops
in flow lines, lead to an increase in emulsifier use, and sometimes cause trips or
upsets in wet-crude handling facilities. The problem is usually at its worst during
the winter because of lower surface temperatures. These emulsions must be
treated to remove the dispersed water and associated inorganic salts to meet
crude specifications for transportation, storage, and export and to reduce
corrosion and catalyst poisoning in downstream processing facilities.
Emulsions occur in almost all phases of oil production and processing: inside
reservoirs, well bores, and wellheads; at wet-crude handling facilities and gas/oil
43
separation plants; and during transportation through pipelines, crude storage,
and petroleum processing (Kokal, etal, 2001) .
The primary focus is on the fundamentals and the application of available
technologies in resolving emulsions. The chapter looks at the characteristics,
occurrence, formation, stability, handling, and breaking of produced oilfield
emulsions. There are several good general references available for more detailed
and diversified discussions on crude oil emulsions. A comprehensive presentation
and further basic information can be found in an encyclopedia of emulsion
technology,1–3 Becher’s classic book4 on the subject, and recent books on
petroleum emulsions.
• Sec. 12.1 provides a brief introduction to the occurrence, types, and
characteristics of emulsions.
It deals with the fundamental nature of emulsions including their
definitions, how they form, and their physical properties. It also includes a
subsection on viscosities of emulsions.
• An emulsion is dispersion (droplets) of one liquid in another immiscible Liquid
(Aske etal, 2002). Types of Emulsions. Produced oilfield emulsions can be
44
classified into three broad groups: water-in-oil, oil-in-water, and multiple or
complex emulsions. Water-in-oil emulsions consist of water droplets in a
continuous oil phase, and oil-in-water emulsions consist of oil droplets in a water-
continuous phase. In the oil industry, water-in-oil emulsions are more common
(most produced oilfield emulsions are of this kind); therefore, the oil-in-water
emulsions are sometimes referred to as “reverse” emulsions.(Aveyard, etal,1996).
Multiple emulsions are more complex and consist of tiny droplets suspended in
bigger droplets that are suspended in a continuous phase.
2.4 PHYSICAL AND CHEMICAL CHARACTERISTICS OF OIL
2.4.1 SURFACE TENSION
Surface tension is the force of attraction between the surface molecules of
a liquid. This force and the viscosity determine the rate of spread over the surface
of water or land, or into the ground. Oils with low specific gravities such as light
crude’s and lighter fuel oils generally have a greater potential spreading rate.
Surface tension decreases with increasing temperature and increase the rate of
spreading of a spill (Cohen, 2005).
2.4.2 VISCOSITY
45
Viscosity is the property of a fluid (gas or liquid), by which it resists a
change in shaper or movement. The lower the viscosity, the more easily it flows.
Viscosity changes with temperature and the lower the temperature, the higher
the viscosity. The viscosity of crude oil depends on its content of light fractions
and the ambient temperature. As oil weathers, its viscosity increases due to the
progressive loss of low molecular weight, volatile fractions. (Fischel, 2005).
The viscosity of spilled oil influences the rate of spreading of the spill, the
adhesion abilities of the oil, its penetration into
the soil and beach sediments as well as the ability of the pumps used in a clean-up
operation to remove oil from the surface.
2.4.3 SOLUBILITY
Solubility is a process by which a substance (solute) will dissolve in another
substance (solvent). The solubility of oil in water is extremely low (generally less
than 5 p.p.m.). this process is very important in relation to the toxicity of
hydrocarbons to aquatic organisms because certain “slightly” soluble
hydrocarbons and various mineral salts present in oil are dissolved in the
surrounding water.
46
2.5 BEHAVIOUR OF OIL IN MARINE ENVIRONMENTS
2.5.1 Marine Environment
For anyone involved in combating pollution in marine environments, it is
essential to have some knowledge of these environments, of typical processes
taking place in the oceans and of the interactions with shores. These processes
will get more complicated when oil is spilled in large quantities over the surface
and may be more seriously disturbed by inappropriate human action than if
Nature was left to deal with the oil spills. Seawater is a complex solution of
dissolved minerals, elements and salts. As water (H20) is a compound of hydrogen
and oxygen, these two are the most abundant elements. Sodium chloride (NaCl)
forms the majority of dissolved salts, with magnesium, calcium and potassium
chlorides and carbonates forming the rest. (Alegundro, 1987). Seawater, being a
mixture of many different types of salts, has virtually the same composition
wherever and whenever readings are taken. In mid-oceans the ratio between
these various salts is remarkably constant, whereas in inshore waters this ratio
may vary considerably due to the input of water by rivers.
The density of pure water 40C is 1,000 kg M-3. If it is heated or cooled, the
density of water decreases. Saline water has a higher density and lower freezing
47
point than pure water, depending on the content of salts. Wind action produces a
mixed layer on the surface to a depth of 50 to 200m which is almost isothermal in
the vertical. A zone below this extends for another 500 to 1,000m over which the
temperature decreases with the oil spills. Seawater is a complex solution of
dissolved minerals, elements and salts. As water (H20) is a compound of hydrogen
and oxygen, these two are the most abundant elements. Sodium chloride (NaCl)
forms the majority of dissolved salts, with magnesium, calcium and potassium
chlorides and carbonates forming the rest. (Alegundro, 1987). Seawater, being a
mixture of many different types of salts, has virtually the same composition
wherever and whenever readings are taken. In mid-oceans the ratio between
these various salts is remarkably constant, whereas in inshore waters this ratio
may vary considerably due to the input of water by rivers.
The density of pure water 40C is 1,000 kg M-3. If it is heated or cooled, the
density of water decreases. Saline water has a higher density and lower freezing
point than pure water, depending on the content of salts. Wind action produces a
mixed layer on the surface to a depth of 50 to 200m which is almost isothermal in
the vertical. A zone below this extends for another 500 to 1,000m over which the
temperature decreases fluctuations about their mean values. Where the density
of the water increase strongly with depth, the strong hydrostatic equilibrium
48
introduces different eddy sizes in the horizontal and in the vertical. Fluids with
marked density stratification, with light fluids on top of dense fluids, are very
stable, but if there is relative motion between two layers, then the boundary
between the two will deform. If the relative motion is strong, then the boundary
may become unstable and turbulence ensures and mixing will occur. The process
of vertical circulation being driven by vertical density differences is called
conversion, whereas horizontal circulation of water is called advection.
Convection occurs when the air temperature drops below the temperature of the
surface water. The surface water cools and sinks and is replaced by subsurface
water, which also cools and sinks.
2.5.2 WEATHERING PROCESSES
When crude oil or petroleum products are released into marine environments
they are immediately subjected to a wide variety of weathering processes such
as:
- Spreading and drift;
- Dissolution and advection;
- Photochemical oxidation
- Water-in-oil emulsification;
49
2.5.3 SPREADING AND DRIFTING
A major process, which affects the behavior of crude oil and refined
products during the first hours after release into the sea, provided that the pour
point is lower than the ambient temperature, is the spreading. This process
increases the overall surface area of the spill, thus enhancing mass transfer via
evaporation and dissolution. In calm water this leads to a continuous decrease in
the oil slick thickness in a circular pattern, which finally reaches a minimum value
ho = 10 -2 to 10-3 cm.
2.5.4 DISSOLUTION
Dissolution may have important biological consequences, although it is of
less importance in terms of the overall mass balance of an oil spill. The extent of
dissolution depends on the point of oil release. Subsurface releases of crude oil,
which happen in offshore operations, enhance the dissolution of lower molecular
weight aromatic components.
2.5.5 DISPERSION
Dispersion of whole oil droplets is the most important process in the break-
up and disappearance of a surface slick. The sea surface turbulence has a direct
50
impact on droplet dispersion, although dispersion of oil droplets into the water
column or processes of spontaneous emulsification from calm seas can also occur.
The dispersion rates are given in table 1.7 as percentage per day for various sea
states independent of individual crude oil characteristics and were derived from
observations of real spills.
Natural dispersion is in fact the net result of three separate processes:
- The initial process of globulation, i.e. the formation of oil droplets from a
slick under the influence of breaking waves;
- The process of dispersion, i.e. the transport of oil droplets into the water
column as a net result of the kinetic energy of the oil droplets supplied by
the breaking waves and the rising forces;
- The process of coalescence of the oil droplets within the slick.
2.5.6 PHOTOCHEMICAL OXIDATION
Several mechanisms are proposed for photochemical oxidation of oil:
- Free radical oxidation in the presence of oxygen;
- Singlet oxygen initiation of hydro peroxide formation;
- Ground-state triplet oxygen combining with free radicals to form peroxides.
51
Rates of photo-oxidation are considered to be dependent on the wave length, but
hey are also affected by turbidity and SPM concentrations, particularly for higher
molecular weight aromatics. The presence of inhibition such as sulphur
compounds or beta-carotenes can restrict the formation of radicals or inhibit
singlet oxygen-mediated peroxide formation. Humic substances may reduce the
photolysis rates of UV –sensitive compounds, but humic materials can also
photosynthesis transformation of organic compounds through an intermediate
transfer of energy to molecular oxygen. Some photo-oxidized compounds have
enhanced water solubility and consequently are quickly removed from surface
slicks and diluted in underlying waters. (Buist etal, 1997)
2.5.7 SHORELINE INTERACTIONS
Fresh oil can penetrate coarse grained intertribal sediments quite rapidly.
Ultimately oil can reach the depth of down to 0.5m in gravel and coarse sand.
With recurring high tides, dissolved components may be removed from the
interstitial waters, but significant amounts of higher molecular weight (grater
than n-C12) aliphatic and aromatic components remain for several years.(Clauss
etal, 2000)
2.6 IMPACT OF OIL ON MARINE ENVIRONMENT
52
2.6.1 PHYSICAL CONTAMINATION
When oil is spilled on a calm water surface, only soluble components in the
oil affect organisms in the underlying water. Most waters however, are not calm
and waves and currents mix oil into the underlying water. The growth of marine
organisms depends basically on the quantity and quality of the primary
production of phytoplankton (algae). Apart from the toxic effects of oil, marine
micorfauna can experience indirect food effects since algal production can be
changed after an oil spill. Type phytoplankton. After a relatively large spill
(>0.11/m2), rapid mechanical removal of oil from the water surface is necessary.
Treatment of the oil with a dispersant will generally aggravate the effects
caused by high dissolved oil concentration in the water as a direct result of
dispersion. Alternative combat methods, such as mechanical methods are
needed. To prevent long-term effects, transportation of oil to the sediments must
be prevented. Depending upon the oil and the mixing energy, natural break up of
the slick occurs with droplets of oil remaining in the water column (Blanchi, 2001).
Water soluble fractions (WSF) cause immediate toxic effects on marine and
freshwater organisms. The WSF ahs also been used for bioassay testing, which is
difficult with oil that adheres to vessel walls and organisms, particularly with flow-
53
through systems. Field exposures are increasingly diluted with time. The generally
used bioassay expression (96hLC50), i.e. concentration of oil, producing
50%mortality of the test organisms, does not permit a direct comparison of
results. This concept multiples the time of exposure expressed as p.p.m. days or
p.p.m.h. A premise is made here that all the organism will respond to a toxicant 1
p.p.m. for 20 hours. There are obvious limits to this concept because some
organisms may be killed instantly when concentrations are high when
concentrations are low and the time is long, many organisms can metabolize
hydrocarbons and live without apparent adverse effects. Some bioassays show
the greatest mortality during the first 24 hours of a 4 day exposure, whereas
other bioassays show little mortality during the first day, but increased mortality
after that.
2.6.2 EFFECTS OF OILING ON SEA BIRDS
The effects of oiling on birds may be twofold:
- External effects associated with oiling of plumage;
- Internal effects associated with the pathological effects of ingested oil.
54
The external effects of oil are the most noticeable and most immediately
debilitating. Oil destroys the waterproofing and insulating properties of the
plumage. The bird will suffer from chilling and it is often unable to fly or remain
afloat in the water. The bird has difficulty in obtaining food or escaping predators.
In addition to the decreased foraging abilities of the bird, the presence of oil in
the environment usually results in loss of food. Irritation or ulceration of the eyes
and clogging of the body openings and mouth often accompany oil
contamination. The weakened bird becomes susceptible to secondary infections,
both bacterial and fungal.
2.7 CHEMICAL RESPONSE TECHNOLOGY
2.7.1 NATURAL BIODEGRADATION
Natural cleansing, allowing oil to be degraded and removed by natural
means, could for some spills be the most ecologically sound method, although it is
not an active method and it takes a sound method, although it is not an active
method and it takes a long time too be fully effective. On the open ocean, where
there is no threat to any sensitive onshore or offshore habitats or species, the
spills will evaporate weather, disperse and degrade naturally. But such a spill
should be monitored in order to intervene if the by oil results in the formation of
55
a continuous film or slick of oil, which tends to spread continuously. In open seas,
this only film is undesirable because it constitutes a barrier to the transfer to
seawater of air and light, which are indispensable to the support of marine life. In
coastal waters, oily slicks damage crustacean beds and beaches.
Active biodegradation of oil droplets suspended in the water column
requires the presence of a high concentration of Microorganisms at the oil/water
interface, but they are present in limited quantities in seawater. Thus it is
necessary to speed up the proliferation of these microorganisms in order to
stimulate biodegradation. The organisms need not only the oxygen and carbon
contained in seawater and oil respectively, but also nitrogen and phosphorus,
which in natural seawater are present in small quantities, preventing the required
rapid rate of development of these bacteria. Thus natural biodegradation of oil is
a slow process, requiring months and sometimes years to complete this
process.(Lindstedt Siwa, 1995)
On shorelines with high energy environments such as exposed rocky
intertidal habitats the natural processes result in more rapid cleansing. In some
low energy habitats such as marshes, if oiling is light, natural cleansing might be
preferred, because almost all the clean-up methods involving pedestrian or
56
vehicular traffic are potentially damaging, in some cases even more so than the
spill itself. This traffic causes disruption of natural water circulation patterns,
damages plant root system and works oil into the sediments. Burning and
cropping the vegetation often delay recovery compared with natural cleansing. So
in most cases the natural cleansing process should be considered first.(Little
etal,1998).
2.7.2 ENHANCED BIODEGRADATION
When oil is spilled onto the sea surface, natural processes such as
evaporation, dissolution and biodegradation play a large role in the fate of the oil
spill. These processes are usually safe, but they are slow with the exception of
evaporation. Such organisms as bacterial yeasts and fungi are ubiquitous and can
metabolize most of the oil components, but the process is very slow. The time
necessary in natural conditions for degradation varies from two to six months and
sometimes even longer (Cubit etal, 1997).
Hydrocarbon biodegradation is a biological oxidation and therefore most
microorganisms involved are aerobic species. Biodegradation is limited by several
factors, of which the most important are:
57
- Oxygen content of the water;
-Temperature, as the biological activity is related to temperature: oxygen content
increase, when temperature decreases; Nutrient availability – in the vicinity of an
oil slick carbon is abundantly available, but usually there is a lack of nitrogen and
phosphorus, which are essential for microbiological activity.
2.7.3 OIL SINKING AGENTS
Removing from the water surface could be achieved by sinking it to the
bottom by applying sinking agents such as sand, clay, chalk, fly ash or cement.
Applied to an oil slick they adsorb oil, the density of the resulting oil/agent mass
above 1 kg/1 and the mass sinks to the sea floor, remaining there for a long time.
To prevent long-term effects such transportation of oil to the sediment should be
prevented for the following reasons.(Doherty etal,2002)
- Bottom flora and fauna are likely to be adversely affected, because of the
smothering effect exerted by the oil/agent mass;
- The oil passes through a broad spectrum of marine life between the surface
and the bottom, possibly clogging the gills of organisms;
58
- The sunken oil/agent mass will be migrating over wider areas of sea floor
than originally affected and therefore spreading the adverse effects of the
spill;
- The oil/agent mass can foul fish bottom nets and gear and otherwise taint
commercially valuable species;
- Under certain turbulence and temperature conditions, the entrapped oil
can be released, resulting in recurrence of surface slicks and possible
shoreline contamination.
For the above reasons this method of removal of oil from the water surface
is not recommended and in some countries it is prohibited.
2.7.4 SORBENTS
Sorbents are commonly used for the final clean-up of trace amounts of oil
or to remove oil from areas increasable to skimmers. Sorbents are very effective
as “polishing” methods, i.e. in removing a thin layer from the water surface or a
small amount of oil from beaches. Ecological effects of sorbent use are minimal
except when deployed and recovered by hand in habitats sensitive to disturbance
(marshes) or when they are not properly recovered.
59
Absorbents functions by capillary action and the more porous the
substance, the greater the opportunity for uptake of oil into its capillaries. But this
capability will depend upon the specific gravity and viscosity of the spilled oil,
because lighter oil will draw further up into the capillaries. The absorptive
capacity will depend on the surface area to which the oil can adhere. A good
sorbent picks up oil strongly and picks up water weakly. The oil is taken up not
only by surface adsorption, but also by entrapment in voids such as porce and/or
capillaries. That is best on heavy, highly viscous oils and their performance on
light performance on lighter oils or more freely flowing water-insoluble organic
liquids can be poor. The uptake time of a god sorbent is fast-less than one
minute-but it must retain oil when it is lifted off the water. (Delvigne Gerard,
2006)
In order to improve the ability of the sorbent to attract oil instead of water
and to stay afloat on the water surface, sorbents are treated with oleophilic
compounds, which attract oil, and hydrophobic agents, which repel water. Some
materials which attract oil rapidly allow it to drain out easily again. There are
three groups of sorbents:
- natural organic;
60
- mineral based;
- synthetic organic
Natural sorbents absorb three to six times their weight in oil, are nontoxic
and relatively non-persistent in the environment as they are biodegradable. The
oil I trapped in natural cross linked strands, rather than by strand capillarity. Peat
moss and chicken feathers, treated to increase their oil sorption potential are
often used, but they absorb water as well as oil and sink rapidly when saturated
with water, resulting in severe clean-up problems. Recovery of large volumes of
naturally sorbents requires considerable manpower and extensive disposal of oil-
contaminated sorbents by burning or burial. Natural sorbents release low
viscosity oils, while being lifted out of the water. To minimize this problem some
natural sorbents are available in compressed sheets. Sawdusi, pine bark and
wood chips are available world-wide. Peat dried to a water content of less than
30% is an excellent sorbent, but it will sink unless pretreated Mineral-based
sorbents have slightly better recovery efficiency than natural sorbents (four to
eight times their own weight). They show several disadvantages such as:
- Distribution difficulties in windy weather because of their very low specific
weight;
61
- Potential respiratory irritations due to inhalation of their dust;
- Persistence in the environment because of no biodegradation; abrasiveness
of sorbents causing sometimes damage to recovery devices.
Synthetic organic sorbents show extremely high recovery efficiency quite
independent of oil viscosity. They usually are in the form of plastic foams or
plastic fibers, which are quite easy to spread and to recover. Some synthetic foam
can be reused after the oil has been squeezed out, but they readily absorb water.
2.7.5 DISPERSION
Chemical Dispersion
The key components of dispersants are surface-active agents (surfactants),
which are partly oil soluble and partly water soluble. They reduce the interfacial
tension between oil and water, which, when added to wave and wind energy,
breaks up a surface oil slick into small droplets, dispersing into the upper few
meters of water column. Thus the slick no longer moves with winds and surface
currents, but enters the water column, where it is rapidly diluted, quickly losing its
lower molecular weight fractions and moves with the subsurface currents. But
dispersing slicks introduces a large amount of oil into the upper water column
62
which is generally recognized as disadvantages of this method, because some
impact to water column organisms at the site of dispersion would be
expected.(Kerley 2001)
Using dispersants to mitigate oil spills has remained controversial, despite
considerable testing in both the laboratory and the field. One of the major
concerns, which has not been satisfactorily resolved, is how effective are various
dispersants on different oils over a range of environmental conditions. Laboratory
experiments can not accurately simulate the real world, while field experiments
are difficult to monitor and control. Concentration test results show that the
amount of oil entering the water column from a t rated surface slick is
dramatically higher than from an untreated slick under the same wave conditions
with concentration sup to 70 p.p.m. compared to insignificant amounts in natural
dispersion. Even when no agitation was applied to a treated slick, higher
concentrations were observed in the water under the slick than under an
untreated slick in 25cm waves. In the case of an untreated slick, the formation of
water-in-oil emulsions appeared to inhibit dispersion. When dispersant was
applied, 10 cm non breaking waves deposited a significant amount of oil in the
water column, though the exact amount could not be computed due to
undetermined losses to the rest of the basin. In the first hour about 16% of oil
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entered the water column. Increasing the wave height increased threat of entry
of oil into the water column.
The advantages of dispersion in removing oil from the water surface are
lost if the oil droplets do not stay in the water column. Once the oil droplets have
entered the water, the stability of the oil-in-water emulsion formed depends on
both the turbulence maintaining the drops in the emulsion and the size of the
drops. If the level of turbulence is low, the size of droplets is particularly
important.
2.8 OIL WATER SEPARATION
The separation of water from oil that is collected in any oil spill recovery
operation is a continuing and necessary requirement during every stage of the
effort. Its importance is reflected in the cost of transport and storage of large
volumes of oily water, the salvage value of separated oil and the added labor
costs associated with long-term recovery operations.(Knut Gaaseidnes etal,1999).
Oil/water separators are devices commonly used on Air Force installations as a
method to separate oils from a variety of wastewater discharges. They are
typically installed in industrial and maintenance areas and receive oily wastewater
64
generated during processes such as aircraft and vehicle maintenance and
washing. The effluent from oil/water separators is typically discharged to either a
sanitary sewer system or a storm sewer. Discharges of domestic and industrial
wastewater are regulated under the Clean Water Act (CWA). Properly designed,
installed, and operated, oil/water separators provide a treatment system for
handling oily wastewater that prevents the entry of unacceptable levels of
contamination to a storm sewer or sanitary sewer system. However, oil/water
separators are generally not designed to separate solids or high concentrations of
oil from water, such as might occur, for example, when a large quantity of oil or
sludge is spilled or poured into a wash bay drain. Thus, it is important for all
personnel who discharge wastewater into an oil/water separator to understand
how they function, including their limitations, in order to prevent them from
becoming sources of environmental pollution.
2.8.1 SEPARATION THEORY
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We have therefore the situation of separation of either oily droplets from
the water (deoiling), or draining of emulsified water from a chocolate mousse
type of water-in-oil emulsion. In both cases we are talking about separation, or
settling and coalescing of droplets. When looking into the secret ‘black boxes’ of
the different engineering companies that calculate and dimension separators for
oil, water and gas, it is both astonishing and a bit disappointing to see how much
they rely on Stokes’ law. In-house corrections and experimental factors might be
included in the calculations, but still basically they are using his simple equation.
This is however, a very good starting point to begin to understand the separation
processes.
Stokes’ law in its original form in MKS units can be expressed as follows:
V= h/t = (D2g(p-p1)18v - - - - - - - (2.01)
Where: v =terminal velocity of droplet (m/s), h = travel distance of droplet
(m), t = time (s), D = droplet diameter (m), g ¼ = acceleration due to gravity (9.81
m/s2), r = water density (kg/m3), r = oil density (kg/m3), y ¼ viscosity of the
continuous phase (pa·s) From an analysis of eqn (1) you can determine most of
what needs to be done to enhance the separation of the phases. The droplet
diameter is squared and therefore has considerable influence on the rate of
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separation. It is important to increase the droplet size. In practical separator
engineering this is done by coalescing devices such as oleophilic meshes, porous
media, coalescence plates, etc. Applying oscillating electrostatic fields and heat to
enhance collisions between droplets, adding surfactants to lower the oilwater
interfacial tension destabilizes the natural emulsifiers and lowers the electrostatic
repellent forces between the droplets. Heater treatment units affect the surface
chemistry of the complex oil spill emulsions and of oil droplet coalescence
processes and are very important to the separation process. The details of these
processes are far beyond the aim of this paper, but the interested reader can find
an updated review by (NN/Separation Technology, 1993.)
The droplet separation velocity is also increased if we are able to increase the
acceleration beyond that normally provided by gravity (i.e. 9.81 m/s2). This can be
done in a centrifuge or hydro cyclone which has been developed for just this
purpose. Sophisticated equations have been derived to show separation
efficiency and rate centrifuges and hydro cyclones, but for our general analysis it
is convenient to put oneself in the position of one of the droplets to be separated
and just ‘feel’ the force produced by the high speed rotation and respond by
faster and more energetic separation from the continuous phase. Such devices
greatly exceed the normal ‘g’ force used in ‘gravity’ separators. This gives rise to
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the possibility of construction of very compact separation devices which are able
to separate very small droplets. The problem with this type of separation device is
the high shear force in a two phase system at the point of entering and leaving
the separator. Very often the phases re-mix and the fluid leaves the separator as
a milky dispersion product.
The rate of separation is also increased if we can increase the density
difference of the phases. This might be done by adding buoyant gas bubbles to
the oil droplets or heavy particles to water droplets in w/emulsions (chocolate
mousse). The application of gas bubble flotation is widely used in the industry and
one method for creating gas bubbles is to apply a vacuum above a saturated
liquid. This process named vacuum flotation forms gas bubbles in the dispersed
oil droplets and can be very effective in clearing out oily water. It is believed that
part of the reported efficiency of the MSRC oil-water separator is due to this
process because of the separators placement at the suction side of the oil cargo
pump. The opposite process that of adding heavy particles to an emulsion to
adhere to the water droplet and enhance the water droplet separation rate is
more rare. This process also has the ability to perforate the rigid interfacial films
that are formed when emulsions are weathered, and form channels to drain the
water phase from the rigid oily film matrix. The principal author has separated
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heavy emulsion by adding demulsifies and sand in the size of 2–4mm with good
efficiency. When using this process care must be taken to find the right chemical
to prevent the formation of a new waste in the form of oily sand.
Finally the rate of separation of droplets can be increased by lowering the
viscosity of the continuous phase. When the continuous phase is water, little can
be done. However in the case of a w/o-emulsion, heating the oil phase will
generally lower the viscosity drastically. In the standard heater treatment units
used by the petroleum industry to break and separate heavy w/o-emulsions,
applying heat both enhances the chemical activity and increases the settling rate
by lowering the continuous phase viscosity.
Continuous heating of a large volume of recovered oil spill can be both
difficult and expensive.( Murdock etal,1995)
2.8.2 STOKES’ LAW AND A SPHERE FALLING THROUGH A QUIESCNET
VISCOUS FLUID.
If the particles are falling in the viscous fluid based on their own weight due
to gravity, then a terminal velocity (also known as settling velocity) is reached
69
when this frictional force combined with the buoyancy force exactly balances the
gravitational force and the resulting terminal (settling) velocity is given by
Vs = 2 (Ps-Pf)gR2
9 µ ---------------------------(2.1)
Where
g = gravitational acceleration in m/s2
R = Radius of the particle in m
Vs = particle settling velocity in m/s. vertically down ward if
Ps>Pf
upward if Ps<Pf from the above equation for Vs, it can be seen that the
variables are the viscosity of the continuous liquid, specific gravity difference
between the continuous liquid and the particle and the particle size. A negative
velocity is referred to as the particle or droplet rise velocity (Hydro-Flo, 2011).
2.9 WHAT IS AN Oil/WATER SEPARATOR?
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Oil/water separators (O/WS) are "in-line" devices used to remove oils and
greases (and sometimes solids) from industrial waste streams and storm water
discharges. O/WSs operate by employing various physical or chemical separation
methods, including gravity separation, filters, coagulation/flocculation, and
flotation. However, the use of any separation process depends on the properties
of the oil in the oil/water mixture (AFI, 1994).
2.9.1 TYPES OF OIL WATER SEPARATORS
According to the US Environmental Protection Agency (2005), oil/water
separators used to pre-treat wastewater are usually of two kinds – Conventional
Gravity oil/water Separators and Enhanced Gravity oil/water Separators. Their
design is based on the specific gravity difference between oil and the wastewater.
CONVENTIONAL OIL/WATER SEPARATOR
A good example of conventional oil/water separator is the API oil/water
separator designed according to the standards and specifications of API. The
separator has 3 chambers: an influent chamber, the main chamber and an
effluent chamber.
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a) American Petroleum Institute (API) separators: are long vaults with
baffles designed to remove sediment and hydrocarbon loadings from urban runoff.
Large API separators may include sophisticated mechanical equipment for
removing oil from the surface and settled solids from the bottom. Note: Many
studies conducted on the east coast refer to this multi-chambered (generally three
chambers) design with baffles as a “water quality inlet
72
(b) SPILL CONTROL (SC) SEPARATORS: are the least expensive and
Complex of the three. The device is a simple underground vault or manhole
with a “T” outlet designed to trap small spills.
2.9.2 Gravity type separators
The simplest separators of this type are nothing more than large holding
tanks which may be coupled together in series. Debris laden oily water is pumped
in and allowed to separate under the action of gravity alone. The surfaced oily
flotsam may be skimmed or pumped off and the separated water is drained or
73
pumped off from near the bottom of the tank. When such tanks are coupled in
series the oil from the top of the first tank is fed into the second tank and any
dispersed water remaining in the oil will have additional opportunity to separate
and of course the water quality removed from the bottom of the second tank will
exceed that of the first. For cases where the pumping is continuous, the pumping
rate determines the residence time and this affects the degree of separation.
However it is doubtful if it will be less than 15 p.p.m. at any pumping rate. Figure
1 is an example of coupled separation tanks.
2.9.3 ENHANCED OIL WATER SEPARATOR
PARALLEL PLATE OIL/WATER SEPARATORS
Typically, Parallel Plate Separators are similar to API separators but they
include titled parallel plate assemblies (also known as parallel packs) which
provide short vertical distance for the small oil droplets to travel before they
encounter a fixed surface where they coalesce with other droplets and continue
to rise along the plates to the water surface.
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(3) Coalescing Plate (CP) Separators: include a series of parallel inclined plates to
encourage separation of materials of different densities. The plates are typically
made of fiberglass or polypropylene and are closely spaced to improve the
hydraulic conditions in the separator and promote oil removal. Under (CP)
separators we have;
(A Inclined Plate Separators: Inclined plate separators have been used
successfully for many years. These systems are.
Usually made in large modules constructed of fiberglass corrugated plates
packaged in steel or stainless steel frames. The oil droplets entering the system
rise until they reach the plate above then migrate along the plate until they reach
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the surface. Plates in this type system are often 3/4" apart, but may be as much
as 4" apart. These separators are also known as CPI (corrugated plate interceptor)
separators. Advantages of this system include improved efficiency at removing
both solids and oil (over API type separators) and resistance to plugging with
solids. Figure 7 shows a Schematic of a typical inclined plate separator. Separators
of this general type may be configured in several orientations, with the flow
perpendicular to the plates as shown below, parallel to the plates in up flow
manner and
Parallel to the plates in down
flow manner. The design varies with the manufacturer and exact service required.
(B) Inclined Plate Separator: Flat Corrugated (Horizontal Sinusoidal) Plate
Separators Flat corrugated plate separators often use horizontal oleophilic
polypropylene plates stacked one on top of another in vertical stacks and
fastened into packs with rods or wires. The system uses a combination of laminar
flow coalescence and oleophilic attraction.
Slowing the flow of water to low velocities where laminar flow regimes
exist minimizes turbulence. Turbulence causes mixing of the oil and water and
76
reduces oil droplet sizes. Stroke’s law states that larger droplets will rise faster
and thus separate better. The oleophilic nature of the plates allows the oil
droplets to attach and encourages them to coalesce into larger ones which will
rise faster. These plates provide better separation than could be arrived at
without plates. The advantages of this system are that the plate packs are
modular and relatively small in size compared to the inclined plate modules.
Corrugated plates in this type system are spaced a nominal 0.25" to 0.5" apart.
Because the plates are corrugated, rise distances of droplets in the vertical
direction are greater than the perpendicular distance between Plates. The oil
droplets must rise approximately 0.4" for the nominal 0.25" spacing and 0.7" for
the nominal 0.5" spacing. Because spacing varies slightly due to variations in plate
molding and assembly the spacing are referred to as nominal 0.25" and 0.511
while varying somewhat from these dimensions. Figure 9 provides a detail of part
of a separator pack and includes a graphic depiction of rise distances. Because the
vertical rise distance to be covered is less than for the inclined plate systems, the
same size particle is separated in less time. Conversely, the same amount of space
time provided within the plate area causes effective separation of smaller size
particles. Disadvantages of this system are possible plugging of the plate packs by
solids and possible damage to the plates by solvents.
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(C) Multiple Angle Plate Separators: Multiple angle plate separators were
developed to take advantage of the virtues of the horizontal sinusoidal separator
plates while eliminating many of the disadvantages. The plates are corrugated in
both directions, making a sort of "egg-carton" shape. Spacers are built into the
plates for two spacing’s (nominal 0.25" and 0.5", or 8 mm and 16 mm).
Advantages of the multiple-angle system are:
The plates are designed to shed solids to the bottom of the separator,
avoiding plugging and directing the solids to a solids collection area. In inclined
plate systems, solids must slide down the entire length of the plates whereas in
the multiple angle systems the solids only have to slide a few inches before
encountering one of a multitude of solids removal holes. The solids drop directly
to the bottom of the separator. The double corrugations provide surfaces that
slope at least a forty-five (45) degree angle in all directions so that coalesced oil
can migrate upward. The holes in the plates that constitute the oil rise paths and
solids removal paths also provide convenient orifices for insertion of cleaning
wands.
The advantages of the above ground units are that they are factory
fabricated and require a minimum of field installation time. Most large units are
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designed utilizing plates installed in in-ground vaults. The primary advantages of
vault installations are that the cost per unit flow is minimized and the below-
grade installation is both convenient for gravity flow applications and does not
waste valuable plant area. A typical large underground vault system utilizing
multiple angle plates is shown below during installation at a facility in Canada.
(D) Coalescing Tube Separators: Coalescing tube separators utilizing perforated
plastic tubes for separation have been used for separation of oil and water. The
advantages of the use of this type separator are low cost and enhanced
separation due to the oleophilic nature of the packing. The disadvantage is that
the oil separation from the tubes is more or less random and therefore not
optimized. These separators are usually made with the tubes in the vertical
position but some are constructed with the tubes horizontal. Operation of the
two designs is substantially the same.
(E) Packing Type Separators: One other system that can be used for coalescence
is routing the emulsion through a bed . This type of coalescer is often used in
conjunction with gravity separation or inclined plate separation as a polishing
stage. Similar packs have been made of other materials, including stainless steel
and polypropylene. Systems of this type can be efficient, but the tightly packed
79
coalescing media can experience plugging problems. Coalescing media of this type
is often used as a second stage after a plate type first stage of separation. In this
type application, it is common to use plastic woven mesh of the type often used
as demister pads in distillation columns.
(F) Gravity Type Coalescing Separators
By creating a situation where oily water is forced to pass through some
type of oleophilic medium oil Separation of oil and water droplets will tend to
adhere and collect. They will join together by cohesive attraction thereby increase
their overall diameters and reduce the population density of smaller droplets.
Since the rate of separation of oil from water is directly proportional to the
square of droplet diameter this is the simplest and most significant action which
can be taken to separate an oil-water mix. Figure 2 displays an example of such a
separator. This separator is of a type generally employed in the petroleum
industry and is very efficient at processing oily waste which is free of large debris
and which has a fairly low viscosity (>2500 cSt). Several model sizes ranging from
15 gpm units to 300 gym (21–429 BBL/h) can be found in the World
Catalog(2002), however, only the 50 gum (72 BBL/h) unit falls within range of the
suggested weight size specifications recommended above. Some of the problems
80
presented by this type of separator when used on board a vessel of opportunity
such as a barge or ship are:
- Severe loss of separation efficiency due to rolling and pitching which causes
water to spill over the weir into the oil reservoir.
- Ineffective separation when oil or emulsion content exceeds 40–50% of the
influent evidenced by an increase of free water in the oil effluent and higher
concentration of hydrocarbon in the water effluent
- Requires prescreening system to facilitate removal of particles greater than
6mm in diameter if needed at a particular spill site.
(G) Inverted cone separators
A unique design for oil–water separation found in the World Catalogue which is
also a coalescing device but which differs in the fact that the oily water is pumped
in under pressure below the inverted cone and just above the coalescing medium
Oil which moves into the oleophilic medium of the coalescer slows down and
cohesive forces between particles of oil, being greater than the adhesive forces of
the oil and the oleophilic medium, cause oil particles to join together diminishing
the population density of the smaller oil droplets. Both the separated oil and
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water exit the unit by gravity flow. A significant difference between this device
and the others discussed so far is that separated oil collects in a deep vertical
column assuring that little if any water will exit with the separated oil even when
employed moving platform such as a ship or barge. Even though the weight and
size of the listed available units exceed the targeted future expectations for water
borne separators, the manufacture specifies that they can be used on board
vessels larger than 65 ft (20 m) in length. Both the through put capacity and
effluent quality are shown to be reasonably close to the desired values.
(H) Skimmer/separators
An oil skimmer concept which has been on the market for many years is now
listed in the chapter on oil/ water separators ‘. Quite possibly it might be called a
floating oil-water separator as it would seem to eliminate some of the obvious
problems that exist in trying to develop suitable transportable separators For
example there would be no need to be concerned with size and weight of the
separator as it is an integral part of the skimmer design. The flow capacities of the
two units listed in the World Catalog are ambiguous and have no relation to the
desired throughput capacities listed in the first paragraph of this chapter. A
search of early product literature from this company reveals that the recovery
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rate for the 33 ft (10 m) unit ‘1000 gpm (1573 BBL/h)’ is based on a vessel speed
of 3 fp (1.8 knots) and a slick thickness of 1 inch (25.4 mm). The recovery rate for
the 66 ft (20 m) unit ‘3300 gpm (4905 BBL/h)’ is based on a vessel speed of 4.7 fps
(2.8 knots) and the same slick thickness.
No ‘effluent’ measurements are provided because ‘the water phase is in
open
It would seem to the authors that some means to sample and efficiently test the
‘effluent’ directly beneath these vessels on a continual basis should be required
before these units are classified as oil separators, without which they are only
collectors of spilled oil and emulsions.
(I) Centrifugal Separators
One of the centrifugal oil-water separation units tested by the US Coast
Guard and MSRC was Vortoil, an oil spill separation system made by Conoco
Specialty Products. It employs a surge tank for its first stage
and hydro cyclones in the final two stages. The initial stage provides gravity
separation and emulsion breaker treatment. From there oily water is pumped
from the tank bottom to the first hydro cyclone. These are long conical devices
83
designed for continuous flow through operation wherein the oily mixture is
caused to rotate at extremely high angular velocities causing a radial separation
to occur with the most dense medium being pressed to the outer wall and the
least dense forming a central core where it is systematically extracted and
returned to the surge tank. The water and any oil it may retain due to the
extraction process are then fed to the second hydro cyclone for further
separation, thereby reducing the water effluent to its lowest possible
hydrocarbon level.
By the very nature of their design hydro cyclones function best when the
influent oil/water ratio is under the 25% range. At a 25% oil/water ratio, the
central core of oil in the hydro cyclone occupies half the overall radius. When the
influent oil/water ratio rises to 50% the central core of oil will move to occupy
approximately 70% of the radius. In one test of the unit where the oil content of
the influent was 76%, the water in the effluent oil went up to an unacceptable
level of 86%. When tests were run with mousse mixtures (25 932 cP) where the
influent mousse/water ratio ranged from 15 to 61%, the water effluent ran from
103 to 122 p.p.m. oil and an oil effluent 1–2% water.
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The performance of the Vortoil system was rated as successful for cases
where influent oil/water ratios were kept below 50%. Such ratios could be easily
maintained at a spill site by selection of the proper skimming or pumping systems.
2.9.4 WATER POLLUTED BY OIL
Water polluted by oil may result from domestic, industrial or municipal
discharge of untreated oily wastewater into the environment – land and surface
water bodies. Water pollution by oil may also result from oil spillage – accidental
or sabotage. Oil in wastewater discharged on land may be washed into surface
water bodies during rainfall.
Oil forms a thin layer on top of water and act as lid on the surface of the
water which inhibits sunlight penetration into the water (limiting the
photosynthesis of marine plants and phytoplankton; this as well as decreasing
fauna population, affects the food chain in the ecosystem)n and it also inhibits
oxygen dissolution in the water. Animals and plants living in the water cannot
breathe effectively, the oil coats the feathers of water birds and the fur of marine
animals causing than to become sick and if there is a great amount of oil on their
bodies, they die. Even the insects that live on the water surface are also badly
affected. Oil can kill an animal by blinding it and making it defenseless. Surface
85
water bodies polluted with oil are also unsuitable for human recreational
activities. Owing to the fact that oil floats on top of water, less sunlight penetrates
into the water.
i) Prevention: Secondary containment, Active environmental/spill
response plan, double hulling, use of oil/water Separator
ii) Cleanup & Recovery: Bioremediation, bioremediation accelerators,
Boom, skimmers, sorbets, chemical and biological agents, shovels and
other road equipment, controlled burning, use of dispersants, watch &
wait, dredging, skimming, solidifying, vacuum & centrifuge.
2.10 REVIEW OF PREVIOUS WORK
Law Poung etal.(2006) carried out a research work on a novel oil-water
separator with multiple angles parallel coalescence frustum for removal of
physically emulsified and free oils from wastewater. Performance test was carried
out to determine its removal efficiency. The primary component of the separators
includes a series of inverted and upright frustums-shaped coalescence plates to
form a multiple angle plate arrangement for enhance gravity separation and
coalescence of oil droplets. The oil removal efficiency (E), of the separator was
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found to be inversely proportional to influent flowrate ( Q,) and directly
proportional to retention time.
Svetgoff, (1989) provides a brief introduction to the occurrence, types, and
characteristics of emulsions.
It deals with the fundamental nature of emulsions including their
definitions, how they form, and their physical properties. It also includes a
subsection on viscosities of emulsions. The stability of emulsions including the
formation of films or “skins” on the droplets. Factors that affect emulsion stability
include heavy polar material in the oil (asphaltenes, waxes), very fine solids,
temperature, droplet size, pH, and brine composition, discuss how to measure
emulsion starts, visits the mechanisms of emulsification. The factors that
destabilize emulsions include temperature, shear, removal of solids, and control
of emulsifying agents’ also discusses the practical aspects of emulsification and
highlights methods of emulsion breaking including thermal, mechanical, electrical,
and chemical method.
Bill Ball, Htc Lnc (2005) worked on separation of oil-water mixture by
designing a wash tank. It was accomplished using an atmospheric separation,
typically in low- cost storage tanks designed such that liquid flows down into the
87
tank and exists under a cone or spreader and the spreader bottom is made up of
inverted v-shaped notches. Like saw-teeth the v-notches distributes the flow of oil
uniformly through the water phase washing the water out of the oil.
Kanshik (2001) carried out the effect of baffles on the performance of a
triangular built-in-storage solar heaterThe triangular design of improves the
natural convection between the absorbing plate and the water, leading to better
performance of the system. A night insulation cover is used at the top surface of
the heater to reduce the losses from the system during the off-sunshine hours.
Another way to reduce the losses from the system is by using an insulating baffle
plate in the tank. This plate divides the tank water into two portions; the water
above this plate is known as the upper column and below is known as the lower
column. The water of the two columns is in contact with each other with the help
of the incoming and outgoing vents provided in the baffle plate. The proposed
system is studied by varying the vent area, the water mass in the two columns
and under a constant flow rate for different durations. The effects of the thermal
conductivity and thickness of the baffle plate on the water temperature are also
investigated. It is found that the presence of the baffle plate greatly improves the
system performance during off-sunshine hours.
88
Word, Appl. (2001) carried a research on numerical Modeling of the Effect
of the Baffle Location on the Flow Field, Sediment Concentration and Efficiency of
the Rectangular Primary Sedimentation Tanks. In order to find the proper position
of a baffle in a rectangular primary sedimentation tank, computational
investigations were performed. Also laboratory experiments are conducted to
verify the numerical results, so the velocity profile, vertical distribution of the
suspended sediment concentration and removal efficiency of the sedimentation
tank was measured. The results of the present study indicate that a uniform flow
field in the settling zone is enhanced when the baffle position provides small
volume of circulation regions. So the maximum concentration of the suspended
sediments inside the settling zone and the highest value of removal efficiency are
achieved. , The results of measurements of suspended sediment removal
efficiency are shown. An optimum efficiency could be achieved. As the baffle
distance from D/L was increased from 0.125 to 0.40, the removal rate value of the
relative location of the reaction baffle existed, under which the highest removal
decreased from 22.53% to 15.48%. With increasing distance of the baffle location
from the inlet slot, the return of the circulation flow increased and the jet effect
intensified, at the bottom of the baffle. As the distance from D/L was 0.125 to
17.3%, therefore the smaller the baffle distance, the weaker its energy dissipation
89
effect Buser ,(2007) In this study, a range of critical air velocities and loading rates
were evaluated to determine the effect of baffle location of the pre-separator's
collection efficiency when using cotton gin waste. None of the treatments
significantly affected the over-sized cyclone or over-all collection efficiency. The
pre-separator collection efficiency was higher (81%) when the baffle placed at
one-third the overall width of the pre-separator from the inlet than when placed
at one-half (78%) or two-thirds (75%). The pre-separator collection efficiency was
79.4% at 18.3-m s-1 (3600-fpm) inlet velocity which was significantly higher than
78.2% at 20.3 m s-1 (4000 fpm) and 78.5% at 22.4 m s-1 (4400 fpm). Loading rate
did significantly affect the pre-separator efficiency, but not to the extent of inlet
velocity. The sieve analysis indicated that the pre-separator removed the majority
of material larger than 180 µm; however, the pre-separator did allow a
substantial amount of lint to pass through to the cyclone. The baffle-type pre-
separator performed well in reducing the course material loading rate entering.
According to Brentwood Industries Inc. (2011), more separation (higher flow rate
or better effluent) is accomplished in conventional Oil/Water Separator by adding
volume to the conventional Oil/Water Separator which is the only way to create
the necessary surface area for improved treatment.
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In as much as the above claim is true, Brentwood did not give any scientific
or mathematical relationship that relates size of the conventional oil/water
separator to separation efficiency or separator performance.
Brentwood(2006) went further to recommend adding PVC parallel plate
coalescing media as an inexpensive and easy way to increase treatment without
adding volume to existing steel conventional separators. The problem associated
with adding parallel plate coalescing media such as plugging by solid particles
(settle able and floatable), frequent/regular rigorous maintenance and the need
to assign a dedicated staff to a coalescing Oil/Water Separator often discourage
many companies from converting their conventional oil/water separators to
coalescing Plate Oil/Water Separators. Installing Coalescing Plate Separator in an
environment where large quantity of solid particles are likely, would make
construction of a grit chamber mandatory thereby resulting in higher cost.
For inflows from small drainage areas (fueling stations, maintenance shops
etc), a coalescing plate type separator is typically considered due to space
limitation, but if plugging of the plates is likely then a new design for the
conventional (Baffle type) oil/water Separator should be considered on an
experimental basis (King county Surface Water Management; 2005)
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Parallel Plate or Coalescing Plate Separators are usually recommended only
for light oil loadings when a higher level of oil removal is required; when the
wastewater stream contains minimal solids concentrations and when the facility
or company is committed to the additional maintenance procedures required in
keeping the coalescing pack free of debris (US Army Corps of Engineers, 2010).
Parallel plate oil/water separators function on the same principle as conventional
gravity separators but they require less space.
Conventional oil/water separator relies on a large water volume and this
correlates to a tank size that can be 5 times the size of an equally sized coalescing
separator but by using the coalescing media, the size of the tank is reduced and a
higher performance is attained than the original conventional gravity separator
(Pan America Environmental, 2011). In as much as Pan America Environmental
assertion is true, it did not give any mathematical relationship to relate various
sizes of the conventional oil/water separator to separation efficiency.
The inlet of conventional oil/water separator is often fitted with diffusion
baffles to reduce turbulent flow that might prevent effective separation of the oil
and that might re-suspend settled pollutants (US Environmental Protection
Agency; 2005). In as much as the assertion of the US Environmental Protection
92
Agency is true, it did not state any mathematical relationship between number of
baffles and degree of oil/water separation. It did not also state the number of
baffles to be considered optimum for any size of separator.
2.11 TWO PHASE HORIZONTAL FLOW
According to Vedapuri (1999), Russel et al (1959) studied oil-water flow in a
transparent horizontal pipeline using white mineral oil with a viscosity of 18 cp at
77 degF. They found three distinct flow patterns: bubble flow, stratified and
mixed. Charles et al (1961), defined four flow patterns in their equal density
oil/water flow in 2.5cm pipes: water droplets in oil, concentric water with oil
flowing in the core, oil slugs in water and oil bubble in water. Three different oils
(viscosity 6.29, 16.8 and 65 cp) were used in their studies. They found that the
resulting oil-water flow patterns were mostly independent of the oil viscosities.
Oglesby (1979), studied flow patterns of oil/water mixtures in small
diameter horizontal pipes using a mixture of SN-250 and diesel oil and mixed the
oils in order to obtain a range of viscosities for his experiments conducted in a
3.8cm internal diameter pipe. Flow regimes from segregated to homogenous
were observed with increase in mixture velocity. Oglesby also reported a slug-
flow pattern near the point of inversion. There was a drastic change in the
93
pressure drop at the inversion point and the magnitude of this chance increased
with the increasing mixture velocity and oil viscosity.
Malhotra (1995), reported three distinct flow patterns (bubble, semi-
segregated and semi-mixed) for LVT 200 – water mixture. Bubble flow is observed
for a mixture velocity in the range 0/4 – 0.6 m/s and an input oil percentage less
than 10%. Semi-segregated flow is observed for oil cut above 20% and mixture
velocity in the range 0.4 to 0.8 m/s semi-mixed flow is observed. The volume
occupied by the oil/water dispersion increases with an increase in mixture
velocity. For velocities greater than 1.4 m/s almost homogenous mixture is
obtained. Extensive work on the variation of water percentage across a pipe
cross-section for two different oils (viscosity 2 cp and 100 cp) in horizontal pipes
was also done. For the high viscosity oil, not much mixing was observed at the
interface. The viscous oil layer held up in the pipe occupying a much greater cross
section than that calculated based on the input water cut. This results in a slow-
moving oil phase and the compressed water phase at the bottom travels at a very
high velocity. The in situ water layer velocity was three times the mixture velocity
for certain water cuts. The increased water layer velocity leads to higher corrosion
rates.
94
2.11.1 MODELS FOR OIL-WATER FLOWS
Duckler (1984), classified models of defining two-phase flows into three
categories
1. Homogeneous models
2. Dimensional Analysis and Similarity
3. Separated Models
Universal Approach
Flow pattern dependent method
The homogenous model treats the two phases as a single pseudo fluid,
where the properties of the two phases are treated as those of a homogeneous
mixture. This allows for simplification into single phase flow equations.
Arirachakaran et al. (1989) developed a model to predict pressure gradient in
homogeneous flow. The oil-water mixture was considered as a single phase fluid
and friction factor correlations were developed for laminar and turbulent flows as
follows:
For laminar flow (NRe < 1500):
Fm = 64 ----------------------------------------------------------------------(2.20)
95
NRe
For turbulent flow (NRe > 1500):
1 = 1.74 – 2.0 log 2E + 18.7 ---------------------------------------(2.21)
Fm d NRe fm
Where
NRe = Homogenous two phase Reynolds number
Fm = Mixture Moody friction factor
E = Pipe wall roughness, ft
d = Pipe diameter, ft
The dimensional analysis and similarity technique makes use of
experimental data to correlate relationships among various dimensionless
variables. Dimensional analysis is used to allow for a more accurate definition of
the problem under consideration by reducing the number of separate variables in
the problem to a smaller number of dimensional groups of variables. The virtue of
dimensional analysis is that it provides correlations that are easy to apply.
96
However dimensional analysis does have limitation in application. The
dimensionless parameters developed by this technique can be used only under
similar experimental situations for which these parameters were created.
Separated models assume that the two phases are flowing side by side. The
separated flow model provides that the two phases have differing properties as
well as differing velocities. There are a number of separated flow models that can
be considered, the simplest being where flow pattern configuration in the pipe is
disregarded (i.e., Universal Approach). The most complex models use separate
equations for continuity of mass, momentum and energy for each phase and
involve simultaneous solution of the equations taking into account flow pattern
effects. Brauner and Maron (1989) analyzed the flow of two immiscible stratified
layers for a wise range of viscosity and density differentials. Utilizing an adjustable
definition for the hydraulic diameter based on the relative velocity of the two
layers, they analyzed all possible flow patterns: laminar or turbulent – turbulent
flows.
2.11.2 FUNDAMENTALS OF A SEPARATED FLOW MODEL
For two-phase stratified flow of oil and water in a circular pipe. Mass
balance yields the following equations:
97
Qo = UoAo = UsoA (2.30)
Qw = UwAw = UswA (2.31)
Qo + Qw = UoAo + UwAw = (Uso |+ Usw)A. (2.32)
A = Pipe cross-sectional area, m2
Qo = volumetric flow rate of the oil phase, m3/s
Qw = volumetric flow rate of the water phase, m3/s
Ao = cross sectional area occupied by the oil layer, m2
Aw = cross-sectional area occupied by the water layer, m2
Uo = in situ velocity of the oil layer, m2/s
Uw = in situ velocity of the water layer, m2/s
Uso = superficial velocity of the oil layer, m2/s
Uso = superficial velocity of the water layer, m2/s
If the height of the oil-water interface is h from the bottom of the pipe, then the cross sectional area occupied by the water phase is given by
Aw = D2 n –cos-1 (2h – 1) + (2h – 1) 1 – (2h – 1)2 (2.40)
4
98
Where
h = h/D, the dimensionless film height
D = diameter of the pipe, m
A momentum balance is carried out for the oil and water layers as follows:
For the water phase
Aw dp – twSw – tiISiI – PwAwg sin a = O (2.50)
dx
For the oil phase
Ao dp – toSo + tiSi – PoAo g sin a = O (2.60)
Dx
Where
t0 = shear stress at wall for oil, N/m2
tw = shear stress at wall for water, N/rn2
ti = interfacial shear stress, N/m2
99
a = pipe inclination, (ais positive for upward flow)
So = portion of pipe circumference in contact with the oil phase, m
Sw = portion of pipe circumference in contact with the water phase,
Si = width of the interface, m
dp = pressure gradient, N/m3
dx
Po = density of the oil phase, kg/m3
Pw = density of the water phase, kg/m3
The pressure drop is the same in both phases. Eliminating the pressure drop term
yields,
t0 so– twSw – ti Si +Si + (P0 – Pw)g sin a = O (2.70)
dx Aw AO Aw
according to Taitel and Duckler (1976), the shear stresses are evaluated by using a
Blasius type equation as below.
to = Fo PoU2o (2.71)
100
2
tw = Fw pwU2w
2
Where F is fanning friction factor that depends on the Reynolds number. The
friction factors are defined as
Fo = Co Do Uo -Uo (2.80)
Vo
Fw = Cw Dw Uw -Uw (2.81)
Vw
The constants C and n are given the following values: for laminar flow C –
16 and N= 1 and for turbulent flow C = 0.046 and N = 0.2. It should be noted here
that the Reynolds numbers for the two fluids are based on the equivalent
hydraulic diameters. The equivalent hydraulic diameters are defined according to
whether the upper layer is the faster one or vice-versa (Brauner, 1989). The
interface is considered as free surface when the velocities of the phases on each
side of the interface is of comparable levels. Where the velocities are different,
the interfacial surface has to be added to the wetted perimeter in the faster
101
phase. In contrast to gas-liquid flow, in liquid-liquid system, the velocities of the
two phases may be similar and alternatively one phase velocity exceeds the other.
Thus,
When Uo > Uw
Do = 4Ao (2.90)
So+Si
Dw = 4Aw (2.91)
Sw
When Uo < Uw
Do = 4Ao (2.92)
So
Dw = 4Aw (2.93)
Sw+Si
When Uo = Uw
102
Do = 4Ao (2.94)
So
Dw = 4Aw (2.95)
Sw
The interfacial stress between the two layers ti is defined as
ti = Fi p(Uw – Uo) (Uw – Uo) (2.10)
2
P = the density of the faster layer (kg/m3)
Fi = the friction factor of the faster layer
All parameters shown in equation 2.91 can be expressed as a function of interface
height. Hence solving equation 2.91 for given oil and water flow rates, one can
predict the height of the interface.
2.12 BAFFLES
103
Most common baffle types
Most common baffles are straight flat plates of metal that run along the
straight side of vertically oriented cylindrical tank or vessel. This baffle design is
often referred to as "standard baffles". There are many nuances. Some may have
the baffles extend into the bottom dish or bottom head. Some may go up the
straight side only partially (so-called partial baffles). Some baffles are flush with
the side wall, but the majorities have a space between the baffles and the tank's
wall. They are off-wall.
2.12.1 Number of baffles
Most vessels will have at least 3 baffles. 4 is most common and is often
referred to as the "fully baffled" condition. This basically means that adding any
more baffles doesn't significantly add to the power consumption of the impellers.
2.12.3 Importance of baffles
Baffles are needed to stop the swirl in a mixing tank. Almost all impellers rotate in
the clockwise or counter-clockwise direction. Without baffles, the tangential
velocities coming from any impeller(s) causes the entire fluid mass to spin. It may
look good from the surface seeing that vortex all the way down to the impeller, but
104
this is the worst kind of mixing. There is very little shear and the particles go
around and around like in a Merry-Go-Round. This is more like a centrifuge than a
mixer. The presence of baffles stops that.
CHAPTER THREE: METHODOLOGY
3.1 SEPARATOR DEVELOPMENT
In this research 5 different channels separation technique with varied no of
baffles was proposed to enable wastewater landed with free oil to flow into the
channel systems and allow the horizontal velocity of flow (Vh) to decelerate with
distance from the inlet. The reduction in horizontal velocity of flow Vh would
enhance high efficiency of oil removal. This reduction in horizontal velocity of
105
flow due to the presence of different baffles promotes optimum oil – liquid
separation.
However, the fluid elements flows more easily in reduced number of baffles
and escapes quicker from the oil droplets and flow more easily.
Furthermore, as the oil droplets suspension gets concentrated, it gets
denser, and the oil removal efficiency gets reduced. The tiny or small oil droplets
coalesce and rises by the force of gravity and from bigger droplets, eventually the
oil droplets floats to the surface thus, the separation of oil and Warf water could
be achieved.
The proposed separation technique is very much dependent on the
arrangement and orientation of the varied no of baffles and the baffles spacing,
other factors such as influent;
(1) Concentration
(2) Flow rates
(3) Viscosity and
106
(4) Specific gravity, using properties, volume of the system are undoubtedly
playing important roles in removal or separation efficiency of the oil and
Warf water. The features of the proposed separator;
(a) Rectangular separation channels with varied no of baffles concerning the
horizontal velocity.
(b) Equal interval of baffle spacing
(c) Two outlet openings, one top and one below
3.2 APPLICABILITY OF VARIED NO OF BAFFLES
1) To promote laminar flow
2) To promote optimum oil water separation efficiency
By using the principle of a maximum number of baffles, a minimum distance
for lighter oil droplets to rise and heat the baffle. The baffle assists oil droplets to
coagulate and float to the surface and be collected. The use of baffle facilities
reduces the velocity of flow and the oil to be captured, reducing the
concentration of effluent and enhancing oil removal efficiency. The design of the
baffles was such that it was 0.2 meters spacing intervals from each other, and the
channel was 1.5 meter long, so the oil has to travel a distance of 15 cm in other to
be collected.
107
Droplets are released, when they become large enough that the buoyancy
due to their size overcomes the attractive forces holding the droplets, the force
trying to tear off the droplets is the frictional force is proportional to the flow
velocity of the water. The proposed design of the baffles is illustrated below; the
slower the oil floated to the surface the better is its chances of being permanently
separated from water. The proposed separator was to be designed as a
rectangular channel with varied number of baffles. The separation channel has
the following assumptions.
- Oil droplet rise as discrete, free sizing
- Laminar Flow Reynolds Number < 1
- Separation is an ideal separation system with center-feed flow
- Steady state with even distribution of flow entering and leaving the channel
3.3 SEPARATOR AND BAFFLES DESIGN
The baffles were designed as a square 0.15m x 0.15m and were welded at
0.10m from top to the depth of the channel and 0.10m from bottom to the top,
and it was placed at 0.2m equal intervals, and the number of baffles was varied
for each of the channels. The intention of equal spacing is to provide flow passage
for effluent to the outlet. The size of the channels is 1.5m x 0.25m x 0.25m and
108
the capacity of the mixing tank is 200 liters. The pie that connects the mixing tank
to the channels is of diameter 0.12mm.
The required flow rates for each of the channels (Q)
Flow rate Q (m3/s) 1.6x10-4 1.4x10-4 1.05x10-4 0.9x10-4 0.3x10-4
3.4 EXPERIMENTAL PROCEDURE
Mixing Tank set up and Sample Collection
The primary function of the mixing tank in this experimental set up was to
mix oil and water to form water and free oil solution with known concentration,
the oil and water was mixed continuously with a mechanical stirrer to obtain a
good mixture and it flowed from the pipe connected from the mixing tank to each
of the varied baffled channels. An overhead tank was set up in order to maintain a
particular head in the mixing tank and the overhead tank was filled with a mixture
of oil and water with the same concentration in the mixing tank.
109
A volume of 200 liters of oil water mixture, 1.25% of oil was mixed with
water, that is to say that 2.5 liters of oil was mixed with 197.5 liters of water to
get a known concentration.
After stirring with mechanical stirrers for 5mins, the sample was collected
from the mixing tank, to test for the influent oil concentration. The valve was
opened at 5 different flow rates for each of the 5 varied baffled channels;
detention time was taken for each collection of samples, and the samples
collected was tested for effluent oil concentration.
However, 3 oils were used, 5 flow rates also used for each of the 5 channels
that was varied with no of baffles.
3.5 DETERMINATION OF OIL CONCENTRATION
When Engine Oil was used
Folch method of extraction, reagents (chloroform, methanol)
110
-50mls of the sample was measured with a measuring cylinder and poured in a
glass – stopped bottle.
-The sample macerated with 50mls of chloroform and 25mls of methanol and
was homogenized; it was allowed to stand overnight
- 25mls of 0.9% Nacl was added to the sample and thoroughly shaken and
allowed to separate, two layers were formed (upper and lower layer), then the
water forms an aqueous solution with Nacl at the upper layer.
- The two layers were separated using a separatory funnel, and the lower layer
was evaporated using a rotary evaporator at 500c to 600c.
- The residue was weighed with the beaker and after which the residue was
washed off, the beaker was oven dried and weighed.
- Then the mass of the residue was extrapolated
When hydraulic oil was used.
- 3mls of the sample was transferred into centrifuge tube containing 1.5mls of
20% of Ammonium sulphate and shaken vigorously.
111
- 2mls of 0.2% of fluorescence in n-hexane was added and the mixture shaken
vigorously for 5mins, and then centrifuges at 3000r.pm for 10mins, the
absorbance of the lower layer was taken at 470nm.
The concentration of the P.A.G (Poly-oxoakalyne Glycol) was extrapolated
from a standard curve, determined with poly-oxoakalyne Glycol as a standard.
When groundnut oil was used:
- - 1ml of the sample was transferred into test tubes - 2mls of conc. Sulphuric acid was added and boiled for 10mins and allowed
to cool - 0.3mls of the mixture was transferred into another tube - 2.5mls of phospho vandlence reagent was added mixed and allowed to
stand in the dark for 45mins. - The absorbent was taken at 450nm with spectophometer against plank.
3.6 Determination of Oil Removal Efficiency
Oil removal efficiency is mathematically calculated as
Oil removal efficiency = Influent concentration - effluent concentrationx
Influent concentration
3.7 Tracers Studies
X 100
112
Common salt was used as tracer for this research. 5g of common salt was
added to the sewage in the sewage tank and properly stirred. Samples were
collected at the outlet of the each pond consequent upon the outflow from the
sewage tank. Samples were collected at regular intervals while the first sample
was collected just before the theoretical detention time. The process was
continuous as equivalent inflow was simultaneously allowed from the overhead
tank into the sewage tank. A blank sample was usually collected before the
addition of the common salt. The above process was repeated for the other
values of discharges
3.7.1 Tracers Studies
Concentration of salt tracer for each pond can be calculated using the formula
below
Salt Concentration (mg/l) = (a - b) xNx3450
ml of sample
where, a = ml of Silver nitrate in blank sample titration = varies
b = ml of Silver nitrate in sample titration = varies
N = Normality of Silver nitrate = 0.0141
113
3.8 Dispersion Number
Dispersion Number for each pond can be calculated using the formula below
σƟ = [ ΣӨ 2C/ΣC – (Σ ӨC/ΣC)2] (ΣC/ΣӨC)2
δ = 1/8 [ √(1 + 8 σӨ2) – 1 ]
where, σӨ = Normalized variance = Varies
Ө = Time = Varies
C = Concentration of salt tracer = varies
δ = Dispersion number = Varies
114
CHAPTER FOUR: DATA COLLECTION AND ANALYSIS
The results obtained in this research show the variation of oil removal
efficiencies as a function five different flow rates and varied no baffles in the
channels.
GRAPHICAL REPRESENTATION OF RESULTS
0
20
40
60
80
100
0 1 2 3 4 5 6
Oil
Re
mo
va
l e
ffic
ien
cy %
No of Baffles
0.3 (m3/s)
0.9(m3/s)
1.05(m3/s)
1.4(m3/s)
1.6(m3/s)
Flow rate *10-4
Fig 4.1: Variation of oil removal efficiency with no of baffles for hydraulics oil
115
0
20
40
60
80
100
0 1 2 3 4 5 6
Oil
Re
mo
val
eff
icie
ncy
%
No of Baffles
0.3 (m3/s)
0.9(m3/s)
1.05(m3/s)
1.4(m3/s)
1.6(m3/s)
Flow rate *103
Fig 4.2: Variation of oil removal efficiency with no of baffles for groundnut oil
0
20
40
60
80
100
0 1 2 3 4 5 6
Oil
Re
mo
val
eff
icie
ncy
%
No of Baffles
0.3 (m3/s)
0.9(m3/s)
1.05(m3/s)
1.4(m3/s)
1.6(m3/s)
Flow rate *103
Fig 4.3: variation of oil removal efficiency with no of baffles for engine oil
116
Fig 4.4: Dispersion number against discharge
Dis
pe
rsio
n n
um
be
r (X
10
-4)
Discharge (l/s)
0.09
0.09
0.09
0.22
0.22
0.22
0.37
0.37
0.37
117
Fig.4.5: Graphs of correlation of oil removal efficiencies against no of baffles at various flowrates.( when hydraulics oil was used)
118
119
Fig.4.6: Graphs of correlation of oil removal efficiencies against no of baffles at various flowrates.( when groundnut oil was used)
y = 5x + 69.4 R² = 0.9874
0
10
20
30
40
50
60
70
80
90
100
0 5 10
Oil
Re
mo
val E
ffic
ien
cy,E
(%)
No of baffles
0.3 (m3/s)
Linear (0.3 (m3/s))
flowrate
y = 5.9x + 57.5 R² =
0.8233
0
10
20
30
40
50
60
70
80
90
100
0 5 10 Oil
Re
mo
val E
ffic
ien
cy,E
(%)
…
No of baffle
1.05(m3/s)
Linear (1.05(m3/s))
flowrate
y = 4.9x + 54.3 R² =
0.9604
0
10
20
30
40
50
60
70
80
90
0 5 10
Oil
Re
mo
val,E
ffic
ien
cy,E
(%)
No of baffle
1.4(m3/s)
flowrate y = 5.1x + 51.1 R² = 0.952
0
10
20
30
40
50
60
70
80
90
0 5 10
Oil
Re
mo
val E
ffic
ien
cy,E
(%)
No of baffles
1.6(m3/s)
flowrate𝑥〖10〗^(−4)
120
Fig.4.7: Graphs of correlation of oil removal efficiencies against no of baffles at various flowrates.( when engine oil was used)
4.1 DISCUSSION OF RESULTS
4.1.1 Effect of no of Baffles on Oil Removal Efficiency
There was a progressive increase in the oil removal efficiency in the various tables
above; it tends to increase with an increase in the number of baffled channels. In
the results, it was observed that there was a reasonable increased in the 4 and 5
baffled channels with the lowest flow rate Q. In Fig above, the oil removal
y = 5.7x + 61.9 R² = 0.7924
0
10
20
30
40
50
60
70
80
90
100
0 5 10
Oil
Re
mo
val E
ffic
ien
cy,E
(%)
No of baffle
0.9(m3/s)
Linear (0.9(m3/s))
flowrate
121
efficiencies in Fig.1, Fig.2 and Fig.3, baffled channels where not really significant
compare to the four and five baffled channel. In order words, a separator with up
to four baffled channels is said to be a fully baffled separator and it produces
better efficiency of oil removal. Figure 4.5 shows the plots of correlate oil
removal efficiency E, and flowrate (Q), for influent concentration for different oil
3 different oil at different influent oil concentration, hydraulics oil = 37500mg/h,
engine oil = 15300mg/l and groundnut oil = 13000 mg/l through curve fitting
practice. Table A.8, A.9 and A10 Shows the equations of the individual co-
relationship obtained from the curve fitting practice that based on the
relationship linears deemed as suitable to represent oil removal efficiency versus
flowrates. In Fig4.5 the value of Correlation coefficient increases when there is
low flowrate and . For example for COil of 37500mg/l at flowrate (0.3 x 10-4) and
their oil removal efficiencies, the value of R2 = 0.992, but at flowrate(0.9 x 10-4)
with their oil removal efficiencies, the R2 = 0.982, which tells us that there was
almost linear progression of oil removal efficiencies with no of baffles from higher
flowrates to the lower flowrates.
4.1.2 OIL REMOVAL EFFICIENCY AS A FUNCTION OF FLOW RATES (Q), AND
INITIAL CONCENTRATIONS
122
The tables illustrate that oil droplets removal efficiencies E, of the system
channel are directly proportional to the flow rates Q. For instance, in table 4.1.3,
influent oil concentration of 15300mg.l, oil removal efficiencies E was 58%, 61%,
58%, 62%, 74% at flow rates of 1.6x10-4, 1.4x10-4, 1.05x10-4,, 0.9x10-4, 0.3x10-4.
etc.
In the fig 4.1 also illustrate that oil removal efficiencies (E) of the separation
systems are directly proportional to the retention time (t), and inversely
proportional to flowrates,(Q)
However, oil removal efficiencies E increases with a decrease in flow rate Q,
an increase in retention time and invariably increase in number of baffles. For
instance, oil removal efficiencies 61%, 58%, 62%, 74%, at retention time t, 7
minutes, 12 minutes, 20 minutes, 24 minutes, 30 minutes.
The highest oil removal efficiency (E) achieved was 94% for influent concentration
of 15300mg/l at flow rate of 0.3x10-4, in the 5 baffled channel and the retention
time of 48 minutes.
4.2 Dispersion Number
123
From the results obtained from the tracers studies of the five channels with
different number of baffles, the 5 baffled channel (the one with the highest
number of baffles) was observed to record the lowest dispersion number for all
discharges studied indicating a high degree of axial dispersion. This therefore
implies that 5 baffled channel has the highest efficiency of treatment.
Furthermore,4 baffled channel recorded a low dispersion number however; its
dispersion number was higher than that of 5 baffled channel indicating a high
efficiency of treatment next to 5 baffled channel. 1 baffled channel recorded the
lowest efficiency of treatment compared to 4 and 5 baffled channel, which
produced the highest dispersion. All five channels were exposed to the same five
different discharges for each of the channels during tracers studies. The minimum
dispersion numbers was gotten when the lowest discharge was used for 1,2,3,4,5,
baffled channel were 0.000416,0.000309,0.000213,0.000128,0.000103
respectively. Also, the maximum dispersion numbers when the highest discharge
was used were 0.000909, 0.000771, 0.000520l,0.000421, 0.000273 respectively.
124
CHAPTER FIVE: CONCLUSION AND RECOMMENDATIONS
5.1 CONCLUSION
In this research work, the following conclusions were deduced
125
1. Number of baffles has a conspicuous effect on effluent oil concentration
and invariably on the oil removal efficiency. There was almost a linear
progression in the increase of the efficiencies.
2. It was also observed from this research work that the highest efficiency by
the separator was produced by the lowest flowrates, and longer retention
time (t).
3. Oil coagulates more when the flowrates was at the minimum.
5.2 RECOMMENDATIONS
1. Baffles should be introduced in our oil-water separators to enhance
efficient separation and improved oil removal in our oil industries
2. In further studies, effect of baffle configuration be ascertained so that a
better comparison could be done.
126
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137
APPENDIX 1
TABLE A.1 When engine oil was used for used for the experiment
No of baffles
Flowrate QX10-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time(mins)
1 baffled channel
1.6 15300 6500 58 7
,, ,, ,, 1.4 15300 6000 61 12
,, ,, ,, 1.05 15300 6400 58 20
,, ,, ,, 0.9 15300 5800 62 24
,, ,, ,, 0.3 15300 4000 74 30
No of baffles
Flowrates Qx(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time(mins)
2 baffled channel
1.6 15300 6200 59 10
,, ,, ,, 1.4 15300 5800 62 15
,, ,, ,, 1.05 15300 4000 74 25
,, ,, ,, 0.9 15300 3000 80 32
,, ,, ,, 0.3 15300 3200 79 37
No of baffles Flowrate Qx(m3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time(mins)
3 baffled channel
1.6
15300 5000 67 13
1.4 15300 5100 67 16
138
1.05 15300 3260 79 28
0.9 15300 2900 81 34
0.3 15300 2200 86 42
No of baffles
Flowrate Qx10-4(m
3/s)
Influent oil concentration(Mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time(mins)
4 baffled channel
1.6 15300 4800 69 15
,, ,, ,, 1.4 15300 4200 73 19
,, ,, ,, 1.05 15300 4000 74 29
,, ,, ,, 0.9 15300 2650 83 37
,, ,, ,, 0.3 15300 1700 89 45
APPENDIX 11 Table A.2 When hydraulics oil was used for the experiment
No of baffled channel
FlowrateQ x 10
-4
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time(mins)
5 baffled channel
1.6 15300 3300 78 17
,, ,, ,, ,, 1.4 15300 3100 80 23
,, ,, ,, ,, 1.05 15300 3250 79 32
,, ,, ,, ,, 0.9 15300 2400 84 39
,, ,, ,, ,, 0.3 15300 900 94 48
No of baffles
Flowrates Q x 10
-4 (m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/)
Oil Removal efficiency(%)
Retention time(mins)
139
No of baffles
Flowrates Q x 10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil Removal efficiency (%)
Retention
Time(mins)
2baffled channel
1.6 35790 21000 41 7
,, ,, .. 1.4 35790 20715 42 12
,, ,, ,, 1.05 35790 14420 60 17
,, ,, ,, 0.9 35790 14900 58 21
,, ,, ,, 0.3 35790 14820 69 25
No of baffles
Flowrate Q x 10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil Removal efficiency (%)
Retention time(mins)
3 baffled channel
1.6 35790 20700 42 10
,, ,, ,, 1.4 35790 20700 42 13
,, ,, ,, 1.05 35790 15200 58 18
,, ,, ,, 0.9 35790 12300 66 22
,, ,, ,, 0.3 35790 9100 75 27
No of baffles Flowrates Q x10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil Removal efficiency(%)
Retention time(mins)
4 baffled channel
1.6 35790 16800 53 12
,, ,, ,, 1.4 35790 14910 58 16
1 baffled channel
1.6 35790 25100 30 5
,, ,, ,, 1.4 35790 20630 42 9
,, ,, ,, 1.05 35790 18230 49 14
,, ,, ,, 0.9 35790 17122 52 19
0.3 35790 14820 59 23
140
,, ,, ,, 1.05 35790 11200 69 21
,, ,, ,, 0.9 35790 7650 79 25
,, ,, ,, 0.3 35790 5790 84 30
No of baffles
Flowrates Q x 10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil Removal efficiency(%)
Retention time(%)
5 baffled channel
1.6 35970 14900 58 15
,, ,, ,, 1.4 35970 15200 58 21
,, ,, ,, 1.05 35970 7020 80 26
,, ,, ,, 0.9 35970 5600 84 31
,, ,, ,, 0.3 35970 3510 90 34
APPENDIX 1V
Table A.3 When groundnut oil was used for the experiment
1 baffled channel Flowrate Q x 10
-4(m
3/s)
lnfluent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency (%)
Retention time (mins)
,, ,, ,, ,, 1.6 x 10-4
13000 7050 46% 5
,, ,, ,, ,, 1.4 x 10 13000 6300 51% 10
,, ,, ,, ,, 1.05x10-4
13000 4800 63% 17
,, ,, ,, ,, 0.9 x 10-4
13000 4900 62% 21
,, ,, ,, ,, 0.3x10-4
13000 3900 70 28
2 baffled channel Flowrate Q x 10
-4(m
3/s)
Lnfluent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time (mins)
,, ,, ,, ,, 1.6 x 10-4
13000 6300 52% 7
,, ,, ,, ,, 1.4 x 10 13000 5200 60% 13
,, ,, ,, ,, 1.05x10-4
13000 5320 59% 21
,, ,, ,, ,, 0.9 x 10-4
13000 3740 71% 29
141
,, ,, ,, ,, 0.3x10-4
13000 2800 78 34
3baffled channel Flowrate Q x10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil efficiency (%)
Oil removal efficiency (%)
Retention Time(mins)
,, ,, ,, 1.6 x 10-4
13000 6350 51% 17
,, ,, ,, 1.4 x 10 13000 4550 65% 14
,, ,, ,, 1.05x10-4
13000 3000 76% 24
,, ,, ,, 0.9 x 10-4
13000 2932 77% 30
,, ,, ,, 0.3x10-4
13000 3000 76% 38
4 baffled channel Flowrate Q x10
-4(m
3/s)
Influent oil concentratiom(mg/l)
Effluent oil concentration
Oil removal efficiency(%)
Retention time (mins)
,, ,, ,, ,, 1.6 x 10-4
13000 5220 59% 13
,, ,, ,, ,, 1.4 x 10 13000 3740 71% 16
,, ,, ,, ,, 1.05x10-4
13000 3000 76% 22
0.9 x 10-4
13000 2400 82% 32
0.3x10-4
13000 1750 86% 39
5 Baffled channel Flowrate Q x 10
-4(m
3/s)
Influent oil concentration(mg/l)
Effluent oil concentration(mg/l)
Oil removal efficiency(%)
Retention time (mins)
,, ,, ,, ,, 1.6 x 10-4
13000 3300 75% 15
,, ,, ,, ,, 1.4 x 10 13000 2740 78% 20
,, ,, ,, ,, 1.05x10-4
13000 2100 83% 28
,, ,, ,, ,, 0.9 x 10-4
13000 2000 84% 35
,, ,, ,, ,, 0.3x10-4
13000 1700 86% 43
142
APPENDIX V
Table A.5: OIL REMOVAL EFFICIENCIES AT DIFFERENT FLOWRATE AND VARIOUS NUMBERS OF BAFFLES (USING HYDRAULICS OIL)
No of baffles 0.3x10-4 (m3/s) 0.9x10-4(m3/s) 1.05x10-4(m3/s) 1.4x10-4(m3/s) 1.6x10-4(m3/s)
1 59 52 49 42 30
2 69 58 60 42 41
3 75 66 58 42 42
4 84 79 69 58 53
5 90 84 80 58 58
Table A.6: OIL REMOVAL EFFICIENCIES AT DIFFERENT FLOWRATE AND VARIOUS NUMBER OF BAFFLES (USING GROUNDNUT OIL)
No of baffles 0.3x10-4 (m3/s) 0.9x10-4(m3/s) 1.05x10-4(m3/s) 1.4x10-4(m3/s) 1.6x10-4(m3/s)
1 70 62 59 51 46
2 76 71 63 60 51
3 78 77 76 65 52
4 86 82 76 71 59
5 86 84 83 78 75
143
Table A.7: OIL REMOVAL EFFICIENCIES AT DIFFERENT FLOWRATE AND VARIOUS NUMBER OF BAFFLES (USING ENGINE OIL)
No of baffles 0.3x10-4 (m3/s) 0.9x10-4(m3/s) 1.05x10-4(m3/s) 1.4x10-4(m3/s) 1.6x10-4(m3/s)
1 74 62 58 61 58
2 79 80 74 62 59
3 86 81 79 69 67
4 89 83 81 73 70
5 94 89 84 80 78
APPENDIX VI
TABLE THAT SHOWS THE CORRELATIONSHIP OF OIL REMOVAL EFFICIENCIES AT VARIOUS FLOWRATES
Table A.8 When hydraulics oil was used
144
Table A.9 When groundnut oil was used
Influent concentration (mg/L)
Relationship Flowrates (M3/s)
Equation R2
13000 Linear
Linear
Linear
Linear
linear
0.3 x 10-4
0.9 x 10-4
1.05 x 10-4
1.4 x 10-4
1.6 x 10-4
Y = 4.2x + 66.6
Y = 5.5x + 58.7
Y = 6.1x + 53.1
Y = 6.5x + 45.5
Y = 6.6x + 36.8
0.934
0.918
0.927
0.991
0.855
Table A.10 When engine oil was used
Influent concentration (mg/L)
Relationship Flowrates
(M3/s)
Equation R2
15,300 Linear
Linear
Linear
0.3 x 10-4
0.9 x 10-4
1.05 x 10-4
Y = 5x + 69.4
Y = 5.7x + 61.9
Y = 5.9x + 57.5
0.987
0.882
0.822
Influent concentration (mg/L)
Relationship Flowrates (M3/s)
Equation R2
37,500 Linear
Linear
Linear
Linear
linear
0.3 x 10-4
0.9 x 10-4
1.05 x 10-4
1.4 x 10-4
1.6 x 10-4
Y = 7.7x + 52.3
Y = 8.5x + 42.3
Y = 7.1x + 41.9
Y = 4.8x + 34
Y = 6.8x + 24.4
0.992
0.980
0.908
0.750
0.957
145
Linear
linear
1.4 x 10-4
1.6 x 10-4
Y = 4.9x + 54.3
Y = 5.1x + 51.1
0.860
0.952
APPENDIX V11
Table A.11 shows the variation of dispersion at different flowrates with 5 different baffled channels
No of baffled (channel)
0.3 x 10-4
0.9 x 10-4
1.05 x 10-4
1.4 x 10-4
1.6 x 10-4
1.
2.
3.
4.
5
0.000416
0.000309
0.000213
0.000128
0.000103
0.000625
0.000421
0.000306
0.000307
0.000124
0.000800
0.000648
0.000413
0.000300
0.000148
0.000811
0.000721
0.000530
0.000323
0.000157
0.000909
0.000771
0.000520
0.000421
0.000273
146
