Chapter 1

Introduction

Evaporators are kind of heat transfer equipments where the transfer mechanism is controlled by

natural convection or forced convection. A solution containing a desired product is fed into the

evaporator and it is heated by a heat source like steam. Because of the applied heat, the water in the

solution is converted into vapour and is condensed while the concentrated solution is either removed

or fed into a second evaporator for further concentration. If a single evaporator is used for the

concentration of any solution, it is called a single effect evaporator system and if more than one

evaporator is used for the concentration of any solution, it is called a multiple effect evaporator

system. In a multiple effect evaporator the vapour from one evaporator is fed into the steam chest of

the other evaporator. In such a system, the heat from the original steam fed into the system is reused

in the successive effects.

1.1) Multi Effect Evaporator

A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for

efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator,

water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because

the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel

can be used to heat the next, and only the first vessel (at the highest pressure) requires an

external source of heat. While in theory, evaporators may be built with an arbitrarily large

number of stages, evaporators with more than four stages are rarely practical except in

systems where the liquor is the desired product such as in chemical recovery systems where

up to seven effects are used.

The multiple-effect evaporator was invented by the African-American engineer

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Norbert Rillieux. Although he may have designed the apparatus during the 1820s and

constructed a prototype in 1834, he did not build the first industrially practical evaporator

until 1845 . Originally designed for concentrating sugar in sugar cane juice, it has since

become widely used in all industrial applications where large volumes of water must be

evaporated, such as salt production and water desalination. Multiple-effect evaporation

plants in sugar beet factories have up to eight effects.

In the pulp and paper industry, multi-effect evaporators are mainly used to evaporate water

from black liquor solutions to allow its recycle as chemicals and fuel for the process.

1.2) Application of evaporators

Evaporators are integral part of a number of process industries namely Pulp and Paper, Chlor-alkali,

Sugar, pharmaceuticals, Desalination, Dairy and Food processing, etc (Bhargava et al., 2010).

Evaporators find one of their most important applications in the food and drink industry. In these

industries, evaporators are used to convert food like coffee to a certain consistency in order to make

them last for considerable period of time. Evaporation is also used in laboratories as a drying

process where preservation of long time activity is required. It is also used for the recovery of

expensive solvents and prevents their wastage like hexane. Another important application of

evaporation is cutting down the waste handling cost. If most of the wastes can be vapourized, the

industry can greatly reduce the money spent on waste handling (Bhargava et al., 2010).

The multiple effect evaporator system considered in the present work is used for the concentration

of weak black liquor. It consists of seven effects. The feed flow sequence considered is backward

and the system is supplied with live steam in the first two effects.

In the system, feed and condensate flashing is incorporated to generate auxiliary vapour to be

used in vapour bodies in order to improve the overall steam economy of the system.

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1.3) Overview of Kraft Recovery Cycle

Kraft pulping and the chemical recovery process is composed of the following

units: cooking, washing, evaporation, burning, causticizing and calcining .

Wood chips are cooked with white liquor (NaOH + Na2S) in a digester at about 170 ºC,

to produce kraft pulp and weak black liquor. Weak Black liquor (WBL) ,

the by-product of the chemical recovery cycle in the pulp and paper industry, is

composed of water, lignin, cellulose and inorganic sodium salts . These chemicals

need to be recovered for the pulping process to be economically feasible. In order to do

that, weak black liquor is separated from pulp in a washing unit. The black liquor is

diluted by the wash water and generally contains 14-17% solids. 95-98% of chemicals are

recovered in modern pulp washing units . For each ton of pulp, 8-10 tons of weak

black liquor is produced. Weak black liquor is concentrated in a series of evaporators.

The resulting concentrated black liquor is burned in the recovery furnace to produce an

inorganic smelt of Na2CO3 and Na2S. The smelt is then dissolved in water to yield green

liquor, an aqueous solution of Na2CO3 and Na2S, as shown in Reaction . The green

liquor undergoes the causticizing process where Na2CO3 is converted into NaOH by

reacting with Ca(OH)2, as in Reaction 3. At this point, the original white liquor required

for pulping is recovered. In order to provide lime for the causticizing process, lime mud

(or precipitated CaCO3) is dewatered, dried and burned in a lime kiln to produce lime for

the causticizing reaction.

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The main reactions in the kraft process are listed below:

Pulping

Wood + NaOH + Na2S Pulp + Weak Black Liquor (Reaction. 1)

Combustion

Black Liquor + O2 Na2CO3 + Na2S + CO2 + H2O (Reaction. 2)

Causticizing

H2O + CaO Ca(OH)2 (Reaction. 3)

Na2CO3 + Ca(OH)2 CaCO3↓+ 2NaOH

Calcining (lime kiln @ 800ºC)

CaCO3 CaO + CO2 (Reaction. 4)

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1.4) Problems associated with multiple effect evaporators

The problems associated with a multiple effect evaporator system are that it is an energy intensive

system and therefore any measure to reduce the energy consumption by reducing the steam

consumption will help in improving the profitability of the plant. In order to cater to this problem,

efforts to propose new operating strategies have been made by many researchers to minimize the

consumption of live steam in a multiple effect evaporator system in order to improve the steam

economy of the system. Some of these operating strategies are feed-, product- and condensate-

flashing, feed- and steam- splitting and using an optimum feed flow sequence.

One of the earliest works on optimizing a multiple effect evaporator by modifying the feed flow

sequence was by Harper and Tsao in 1972. They developed a model for optimizing a multiple effect

evaporator system by considering forward and backward feed flow sequence. This work was extended

by Nishitani and Kunugita (1979) in which they considered all possible feed flow sequences to

optimize a multiple effect evaporator system for generating a non inferior feed flow sequence. All

these mathematical models are generally based on a set of linear or non- linear equations and when the

operating strategy was changed, a whole new set of model equations were required for the new

operating strategy. This problem was addressed by Stewart and Beveridge (1977) and Ayangbile,

Okeke and Beveridge (1984). The developed a generalized cascade algorithm which could be

solved again and again for the different operating strategies of a multiple effect evaporator system.

In the present work, in extension to the modeling technique proposed by Ayangbile et al,(1984) feed

and condensate flashing has also been included and it also considers the variations in the boiling point

elevation and overall heat transfer coefficient.

1.5) Classification of Evaporators

Weak black liquor leaving the washers contains 13 to 17% dissolved

solids. In order to safely and effectively burn the black liquor to recover chemicals and

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heat, the solids content must be at least 60%. Increasing the solids content improves the

recovery boiler thermal efficiency, stabilizes boiler operation and reduces sulfur

emissions. Heating value of black liquor ranges between 5800-6600 Btu/lb of dry

solids, which is low in comparison to other fuels, such as gas and oil .Thus, a large

amount of water must be evaporated in order to increase the net fuel value of black

liquor. To accomplish this task, the most common types of evaporators presently used in

the industry are the rising film long tube vertical evaporators (LTV) and the falling film

evaporators (FF).

1.5.1) Rising film long tube vertical evaporators (LTV)

A LTV evaporator is composed of two parts: a single pass shell-and-tube heat

exchanger at the bottom and a vapour dome at the top. The tubes are typically 5 cm (2”

OD), 6.7 – 9.1 meters long, held in place by a tube sheet at the top and bottom . Black

liquor enters the tubes from the bottom of the unit, where it is heated by the steam on the

shell side of the tubes. As the heated black liquor boils, the resulting water vapour helps

push the black liquor upward, until it reaches a deflector at the top where it is separated

from the vapour. All vapour domes have a deflector directly over the heating element to

break foam and initiate downward flow to the liquor. The concentrated black liquor exits

the unit through a liquor outlet at the bottom of the vapour dome. The fine black liquor

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droplets entrained in the water vapour are separated by means of a demister as the vapour

passes through it at the top of the dome .

Vapour

Demister Vapour

Deflector

Condensate

Steam Inlet

Condensate Outlet

Liquor Inlet

Dome

Liquor Manufacturer:Outlet Universal Process Engineers

Private Ltd.(INDIA)

Hangzhou Semya Machinery Co., Ltd.(CHINA)

Vent

Tubes

Rising film long tube vertical evaporator

1.5.2) Tubular falling film evaporator

A tubular falling film evaporator is composed of a heating element similar to LTV

and a vapour body at the bottom. Liquor is fed to the bottom of the evaporator where a

fixed level is maintained. Liquor rises to the top by means of a recirculation pump, and

flows down the tubes with gravity . The liquor and vapour mixture leaving the

tubes enters the dome at the bottom of the unit. The vapour is then separated from the

liquor and is cleaned by a drop separator.

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Falling film evaporators have a significantly smaller risk of scaling than the LTV

evaporators because no bulk boiling occurs inside the tubes; consequently, no dry spot

formation exists where scaling can initiate . Falling film evaporators run at lower

steam pressures than the LTV evaporators, since steam does not have to push the liquor

upwards in the tubes. Consuming weaker steam minimizes the scaling caused by reverse

solubility of compounds such as sodium sulphate or temperature-sensitive calcium

complexes.

Recirculating Liquor

Steam Inlet

Vapour

Vent

Condensate Outlet

Liquor Inlet

Liquor

Outlet

Falling film tubular evaporator

Manufacturers :

Universal Process Engineers Private Ltd.

(INDIA)

Anhui OECH Mechanical Equipment Co., Ltd. (CHINA)

Outlet

Drop Separator

Dome

Recirculation Pump

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Chapter 2

Literature Survey

Performance of parallel feed multiple effect evaporation system for seawater desalination.

Hisham T. El-Dessoukya, Hisham M. Ettouneya, Faisal Mandanib

a) Department of Chemical Engineering, College of Engineering and Petroleum, Kuwait University, P.O. Box 5969,Safat 13060, Kuwait

b) College of Technological Studies, P.O. Box 42325, Shuwaikh 70654, KuwaitReceived 9 December 1998; accepted 14 November 1999

Abstract

Performance analysis is presented for the parallel feed multiple effect evaporation system. Two

operating modes are considered in the analysis, which includes the parallel and the parallel/cross flow

systems. Analysis is performed as a function of the heating steam temperature, salinity of the intake

seawater, and number of effects. Results are presented as a function of parameters controlling the unit

product cost, which includes the specific heat transfer area, the thermal performance ratio, the

conversion ratio, and the specific flow rate of the cooling water. Results indicate that better performance

is obtained for the parallel/cross flow system. However, the parallel feed system has similar

characteristics and simpler design and operation procedures. Performance of both systems is consistent

with literature data. Comparison of the two parallel feed systems versus conventional multistage flash

desalination and the forward feed multiple effect evaporation schemes show that the forward feed

system has better performance characteristics than the other three systems.

All rights reserved.

Keywords: Seawater desalination; Multiple effect evaporation; Parallel feed; Modelling

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Chapter 3

Working of Multi Effect Evaporators (MEE)

To use steam efficiently, a series of evaporators are connected to each other so

that the latent heat of vapour is used multiple times. Each evaporator is called an effect in

this system. Live steam is only fed to the first effect, while the vapour generated in the

first effect is the heating medium in the second effect, and so on. In kraft pulp mills,

evaporation occurs in multiple effect evaporators (MEE), where steam and black liquor

flow counter-currently . Steam economy, defined as the ton of water evaporated over the ton of

steam used, increases as the number of effects increases lists measured steam economies for a

practical operation shows specific heat consumption decreases as the number of evaporators in a

series increases. It is therefore desirable to have more evaporators connected in a series; however,

in practice this number is limited to six to eight evaporators. ΔT of an evaporator body is defined as the

temperature difference between the saturated vapour temperature and the liquor temperature. ΔT is

critical for long tube vertical evaporators, where a ΔT of less than 9.5 ºC will often cause

poor performance behaviour.

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The evaporators operate at different pressures and are connected so that the produced vapour in one is

used as heating steam for the next one. Live steam is only fed to the first evaporator body/effect. The

weak black liquor feed usually splits between the last two effects, where the liquor boils at lower

temperature under vacuum. As the liquor flows through the evaporators (from sixth to first), the

pressure of the evaporator, boiling temperature and % solids increase, while the volume of liquor

decreases.

240 KPa - 80 KPa

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The evaporation system starts with weak black liquor (WBL) from the brown

stock washers entering No. 1 and No. 2 WBL storage tanks. Weak black liquor is

received from the digesters after the pulp fibres have been washed in the brown stock

washers. WBL contains about 14 - 16% dried solids, which contains the inorganic

compounds sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium sulphide

(Na2S), sodium sulphate (Na2SO4), calcium carbonate (CaCO3) and silicates. These

chemicals are the result of chemical reactions, which take place in the digester cooking

process. Disposing of these chemicals is undesirable both environmentally and

economically because they are costly to replace. By utilizing the evaporation process the

liquor can be concentrated to a density suitable for burning in the recovery boiler, where

chemicals and energy are recovered.

Black liquor is useful as a fuel because it also contains organic compounds such

as lignin and tannins. These compounds give the liquor a heating value of about 14.5

MJ/kg. The liquor is concentrated most efficiently using a series of evaporators or

multiple effects. These effects are shell and tube heat exchangers, which are connected by

vapour piping so that the water boiled off the liquor in the first effect, acts as heating

steam in the steam chest of the following effect. The liquor basically follows a reverse

flow to the vapour.

3.1) Black Liquor Flow:

One of two transfer pumps to the weak liquor flash tank, where solids

concentration is increased slightly, pumps the weak black liquor from the storage tanks.

The liquor flows to the 5th effect vapour body where it is concentrated to

17% solids. The liquor flows through without a pump (by gravity) as the 5th effect

vapour body is at a lower pressure than the flash tank. The 5th effect recirculation pump

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continuously circulates liquor through the 5th effect. From the 5th effect, the liquor is

pumped through a level control valve on the 5th effect to the suction line of 4th effect

recirculation pump.

Black liquor flow path through evaporation system at Mill A

4th effect concentrates the liquor to 21% solids. A transfer pump draws liquor off

the suction line of the 4th effect recirculation pump and pumps it through either the

secondary reflux condenser or through the secondary reflux condenser bypass line to the

2nd effect vapour body. The secondary reflux condenser heats the liquor to 91°C. The

2nd effect concentrates the liquor to 27% solids. Liquor is recirculated through the 2nd

effect by the 2nd effect recirculation pump. From the 2nd effect the liquor flows to No. 2

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product flash tank where it is concentrated to 28% solids.

The liquor is pumped from the flash tank to the soap skimming tank. The purpose

of the evaporator soap system is to recover tall oil soap from various points in the black

liquor system and to deliver this soap to the recovery boiler for incineration. This is

necessary to maintain the efficiency of the evaporator and concentrator heating surfaces

which would otherwise foul, if soap is not removed.

After soap removal, the liquor is pumped to the 3rd effect vapour body. Liquor is

recirculated through the effect with 3rd effect recirculation pump, concentrating the liquor

to 39% solids. The 3rd effect transfer pump draws liquor off the recirculation pump

suction line and pumps it through the primary reflux condenser to the 1st effect vapour

body. The condenser heats the liquor to 112°C.

The 1st effect concentrates the liquor to 58 - 62% solids. The liquor is circulated

through the 1st effect by the 1st effect recirculation pump. Liquor, from the 1st effect,

flows into the 58% flash tank where the liquor temperature is reduced to 115°C. Liquor is

pumped from the flash tank by the 1st effect transfer pump to the 58% storage tank. The

name of the flash tank is an indication of approximate solids content of the liquor inside

it.

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After the 58% storage tank, black liquor is further concentrated in the High Solids

Concentrator. HSC is composed of two heathers and a flash tank. Liquor concentration

rises after HSC due to decrease in pressure in its flash tank.

3.2) Steam Flow:

Saturated steam enters the system at 325 kPa (143°C) through the HSC and the

1st effect. The vapour from the 1st effect is used as a steam source to concentrate the

liquor in the 2nd effect. The resulting vapour from the 2nd effect is, in turn, used to

concentrate the liquor in the 3rd effect, and so on. The same process principle is carried

through the 4th effect and the 5th effect. Steam pressure and temperature decreases as it

travels through the effects. The final vapour from the 5th effect is condensed in the

surface condenser to create a vacuum of –70 kPa to help drive steam and vapour through

the system (Figure 14).

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At the outlet of the vapour system, there is a surface condenser, a shell and tube

heat exchanger that has cold mill water on the tube side and vapour out of the 5th effect

on the shell side. In this case the primary purpose of the surface condenser is to cool and

condense the vapour from the 5th effect and, in doing so, create a vacuum of

approximately -70 kPa in the 5th effect. The other effects have progressively higher

pressure where 1st effect operates at about 105 kPa of pressure. As the pressure decreases

through the evaporators (first to last), so does the boiling point of the water in the liquor,

therefore, water will boil at temperatures significantly lower than 100oC.

3.3) Performance Measures:

There are three main measures of evaporator performance:

1. Capacity (kg vaporized / time) 2. Economy (kg vaporized / kg steam input) 3. Steam Consumption (kg / hr)

Note that the measures are related, since Consumption = Capacity/Economy.

Economy calculations are determined using enthalpy balances.

The key factor in determining the economy of an evaporator is the number of effects. The

economy of a single effect evaporator is always less than 1.0. Multiple effect evaporators have

higher economy but lower capacity than single effect.

The thermal condition of the evaporator feed has an important impact on economy and

performance. If the feed is not already at its boiling point, heat effects must be considered. If the

feed is cold (below boiling) some of the heat going into the evaporator must be used to raise the

feed to boiling before evaporation can begin; this reduces the capacity. If the feed is above the

boiling point, some flash evaporation occurs on entry.

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3.4) Boiling Point Elevation

Since evaporators dealing with boiling solutions, and in particular with solutions with non-

volatile solutes, any calculations must account for the effect of boiling point elevation.

The vapour pressure of an aqueous solution is less than that of pure water at the same

temperature; so the boiling point of the solution will be higher than that of the water. This is called

Boiling Point Elevation (BPE) or vapour pressure lowering.

The boiling point of a solution is a colligative property -- it depends on the concentration of

solute in the solution, but not on what the solute and solvent are.

When working problems involving heat transfer to or from boiling solutions, it is necessary to

adjust the temperature difference driving force for the boiling point elevation.

The equilibrium vapour rising from a solution exhibiting boiling point elevation will

exist at a temperature and pressure such that it is superheated with respect to pure vapour. The

vapour rises at the solution boiling point, elevated with respect to the pure component boiling point.

The vapour, however, is solute free, so it won't condense until the extra heat corresponding to the

elevation is removed, thus it is superheated.

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3.5) Methods of feeding in Multi Effect Evaporators

There are three feed operations - backward feed , forward feed and mixed feed operations. A brief

explanation of these operations:

3.5.1) Backward Feed :

In the backward operation, the raw feed enters the last (coldest) effect and the discharge from

This effect becomes a feed for the next to last effect.This technique of evaporations is advantageous, in

case the feed is cold, as much less liquid must be heated to the higher temperature existing in the

early effects. The procedure is also used if the product is viscous and high temperatures are required

to keep the viscosity low enough to produce good heat transfer coefficients.

3.5.2) Forward Feed:

In the case of a forward feed operation, the raw feed is introduced in the first effect and is

passed from effect to effect parallel to steam flow. The product is withdrawn from the last effect. This

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procedure is highly advantageous if the feed is hot. The method is also used if the concentrated

product may be damaged or may deposit scale at high temperature.

3.5.3) Mixed Feed :

In mixed feed the dilute liquid enters an intermediate effect , flows in forward feed to the end of the

series , and is then pumped back to the first effects for final concentration. This eliminates some of the

pumps needed in backward feed and yet permits the final evaporation to be done at the highest

temperature.

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Chapter 4

Mass And Energy Balances

4.1) Single Effect Evaporators

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F : Feed flow rate (kg\hr)

L: Product glow rate (kg\hr)

S: Steam flow rate (kg\hr)

V: Vapour flow rate (kg\hr)

C: Condensate flow rate (kg\hr)

Tf : Temperature of feed

Ts : Condensing temperature of steam

TBP : Boiling temperature of the liquid in evaporator

qs : rate of heat transfer through heating surface from steam

Hs : Specific enthalpy of steam

Hc : Specific enthalpy of condensate

Hv : Specific enthalpy of vapour

Hf : Specific enthalpy of thin liquor

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HL : Specific enthalpy of thick liquor

λ : ΔHvap: latent heat of condensation of steam.

ASSUMPTIONS MADE:

1) There is no leakage or entrainment , that the flow of noncondensables is negligible, and that heat losses from the evaporator need not be considered.

2) The steam entering the steam chest may be superheated, and the condensate usually leaves the steam chest somewhat subcooled below its boiling point. Both the superheat and subcooling of

the condensate are small, however , and it is acceptable to neglect them in making an enthalpy

balance.

4.2) Multi Effect Evaporators

Typically, multiple effect evaporator calculations require an iterative solution procedure because so

many of the required properties, etc., depend on unknown intermediate temperatures. Fortunately,

the overall approach is basically the same for the majority of problems, requiring only minor

adjustments to compensate for problem quirks.

In a typical evaporator problem, you are given the steam supply pressure, the operating

pressure of the final effect, values for the overall heat transfer coefficient in each effect, the feed

pattern, and the feed and product compositions. You also know that the effects are all to have the

same heat transfer area.

You typically want to find the steam consumption and the heat transfer area, and one or

more of the temperatures, flows, and compositions from within the system.

The overall strategy is to estimate intermediate temperatures, solve the material balances

for the solven t vapor flow rates, use these to determine the heat transferred in each effect, and from

that information find the heat transfer area. If the areas are not equal, you revise the temperature

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estimates and repeat the procedure.

The steps in the procedure can be summarized as:

1) Use the overall component balance to completely determine the feeds and product streams.

these numbers are fixed and are not changed by iteration.

2) Calculate the total amount of solvent vaporized (another fixed number). Divide this up into

estimated amounts for each effect; usually it is convenient to split it equally.

3) Use component and material balance to get estimates for the remaining flowrates within the

system and the compositions of the intermediate streams. These (and all the estimated quantities)

will change each iteration.

4) Use the compositions to estimate BPEs and other properties. Be sure to keep track of which

properties depend on composition, temperature, or both.

5) Determine the overall temperature drop between the steam and the saturation temperature of the

last effect (remember to subtract off the BPEs).

Note that the BPE values may depend on the concentrations, so the overall Delta T can vary with

each iteration.

6) Allocate the overall drop among the various effects. Since the areas are the same, the

temperature difference in each effect is roughly proportional to the overall transfer coefficients.

7) Use the Delta T and BPE values to obtain estimates for all the temperatures in the system.

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Typically, you do this starting with the steam to the first effect, subtracting a Delta T, adding a

BPE, etc.

You can use the saturation temperature of the last effect as a check -- it should match the value for

your final effect operating pressure.

8) Use the temperature and composition estimates to get enthalpy values. You can get these from

specific heat calculations or from data.

Be sure to use the same reference temperatures for all streams, including those taken from steam

tables, etc.

9) Set up the process side enthalpy balances. Use material balances to eliminate the liquid flows

from the enthalpy equations. Do enough algebra so that the only unknowns left in the balances

are the vapor flow rates and the steam to the first effect.

10) Solve the set of equations that is made up of one enthalpy balance for each effect and the total

vapor material balance for the unknown vapor flows (one off each effect and the steam to the

first).

11) Use heat transfer equations to calculate the heat transfer area for each effect.

12) Compare the areas. If they are not equal, you need to repeat the calculation. Begin by

using the areas you obtained to revise the temperature estimates. The recommended approach is

to use the ratio of the calculated heat transfer area for an effect to the arithmetic mean of the

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calculated areas.

13) Repeat the calculations (from step 7) until the system converges. If your BPEs, enthalpy data,

etc., depends on composition, you will need to include steps 3 and 4 in each cycle as well.

14) Once the system has converged, answer questions. Be sure to use values from the final iteration

to calculate your answers.

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Chapter 5

Advantages and Limitations of Multiple Effect Evaporators

5.1) Advantages

Two or more evaporator units can be run in sequence to produce a multiple effect evaporator.

Each effect would consist a heat transfer surface, a vapour separator, as well as a vacuum source

and a condenser. The vapours from the preceding effect are used as the heat source in the next

effect. There are two advantages to multiple effect evaporators:

Economy - they evaporate more water per kg steam by re-using vapours as heat sources in

subsequent effects

Improve heat transfer - due to the viscous effects of the products as they become more

concentrated

Each effect operates at a lower pressure and temperature than the effect preceding it so as to

maintain a temperature difference and continue the evaporation procedure. The vapours are

removed from the preceding effect at the boiling temperature of the product at that effect so that

no temperature difference would exist if the vacuum were not increased. The operating costs of

evaporation are relative to the number of effects and the temperature at which they operate.

5.2) Limitations in Multi Effect Evaporators

Scaling Problem

Scaling is a persistent problem in evaporators in the kraft pulp mills. As black

liquor is concentrated, dissolved salts begin to precipitate from the system as they reach

solubility. Precipitated solids may deposit on the heat transfer surfaces, forming a layer

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of scale. Severe scaling can interrupt black liquor flow, creating a bottleneck in pulp

production. Due to its low thermal conductivity, scale greatly reduces the heat transfer

efficiency, lowering the evaporator performance .

The scaling in a kraft black liquor evaporator is of the following types: calcium

carbonate scaling, burkeite scaling, soap or fibre scaling, aluminum silicate and oxalate

scaling. A brief description of each type is given below.

5.2.1) Calcium scaling:

Calcium scales form mainly in the first effect. The rate of scaling strongly

depends on temperature. Calcium binds to organic compounds such as lignin complexes,

oxalate and soap. Calcium ions become free when temperature reaches 90 – 130 ºC,

therefore causing calcium carbonate to form and precipitate on heating surfaces. Since

calcium compounds are less soluble at higher temperatures, calcium scaling increases

rapidly as temperature increases.

5.2.2) Sodium carbonate and sulphate scaling:

Sodium carbonate and sodium sulphate precipitate as a double salt, burkeite,

(2Na2SO4.Na2CO3). The solubility of sodium carbonate and sulphate decreases slightly

when the liquor temperature is above 40 ºC . Heat transfer surfaces can be the host for

nuclei formation as they have the highest temperature in the evaporator body. Studies

show a great influence of calcium ions in the solubility of Na2CO3-Na2SO4-H2O system.

Calcium ions restrain the nucleation of burkeite and dicarbonate, resulting in a higher

degree of super-saturation . This type of scale is easily washable by circulating weak

liquor or vapour condensate through the evaporators.

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5.2.3) Fibre and Soap scaling:

Black liquor soap is a mixture of resin and fatty acids that is separated from weak

and intermediate black liquors to avoid scaling and foaming in the evaporators and

concentrators. High fibre content makes the separation of soap harder, since soap adheres

to the fibre surface. This type of scaling is common in the 2nd, 3rd and 4th effects. To

reduce this type of scaling, soap is usually removed from the evaporators at the 3rd

effect.

5.2.4) Aluminum silicate scaling:

Sodium aluminum silicate scales are hard, glassy and persistent. This type of scale

is usually found in first effect and final concentrators, and its amount is determined by

aluminum and silicate concentrations [4]. Generally, in North American mills, silicate

scaling is not a common problem due to its small input in the recovery cycle.

5.2.5) Oxalate scaling:

The oxalate ions are formed in the cooking and bleaching process. Sodium

oxalate particles can form in black liquor when the concentration exceeds 45% solids.

Also when evaporation is performed under vacuum at about 90 ºC sodium oxalate can

precipitate at 30-40% solids. To avoid this precipitation, the process temperature is raised

to about 110 ºC . Calcium oxalate deposition is not a concern, since calcium is

removed in the form of calcium carbonate which is less soluble.

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Scale Samples collected from the 1st Effect, 58% flash tank outlet pipe, and high solids concentrator tube

The mill’s initiative toward the scaling problem is to clean the evaporators and

flash tanks with water or weak black liquor, a process called “boiling out”. Scaling is

costly due to the price of cleaning and loss of efficiency, i.e. more steam is consumed in case of

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scaling due to loss of heat transfer area. Cleaning the 1st effect during shut down costs

$137,000, with the additional cost of lost efficiency.

Chapter 6

Future Aspects in Multi Effect evaporators

In the present work different energy reduction schemes (ERSs), used to reduce the

consumption of steam for a multiple effect evaporator (MEE) system, are developed. These

ERSs are condensate-, feed- and product- flashing and vapor bleeding. Further, a new

scheme is proposed where condensate of vapor chest of an effect is used to preheat the

liquor, which is entering into that effect using a counter current heat exchanger. This work

also presents a comparative study between existing ERSs and selects the best ERS amongst

these based on steam consumption as well as number of units involved. Further, in the

present paper a simple graphical approach named “Modified Temperature Path (MTP)” is

developed for the analysis of different feed flow sequences of a MEE system to screen best

possible feed flow sequence. To study the effect of different ERSs on steam consumption

and MTP analysis an example of septuple effect flat falling film evaporator (SEFFFE)

system, employed for concentrating weak black liquor in an Indian Kraft Paper Mill, is

considered. The results show that ERSs reduce the steam consumption up to 24.6%.

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Conclusion

Liquor evaporation is an important energy consumer in a pulp and paper mill. The study

focuses on the identification of actions to reduce the energy cost related to the evaporator

section of wood pulping mill. The future energy saving methods concern the modification of the

operation conditions of the decrease of the ∆Tmin (Temperature Drop) and increasing or

decreasing pressures of evaporation effects allowed one to reduce by 20% the minimum energy

requirement of the evaporation system with an associated utility cost reduction of 23%.

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Bibliography:-

1) Multivariate Analysis of Variables Affecting Thermal Performance of Black Liquor Evaporators

By - Hamideh HajihaA thesis submitted in conformity with the requirements for the degree of Master of Applied Science - Graduate Department of Chemical Engineering and Applied Chemistry, Faculty of Applied Science and Engineering, University of Toronto.

2) Energy integration study of a multi effect evaporator by Zoe Perin-Levasseur,Vanessa Palese, France

3) Energy reduction schemes for multiple effect evaporator systemsBy :- Shabina Khanam & Bikash Mohanty

* Department of Chemical Engineering, National Institute of Technology Rourkela, Rourkela, India

* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-, India.

4) Wikipedia – “Multi effect evaporators”.

5) Unit operations of chemical Engineering – 7th EditionWarren L. McCabe, Julian C.Smith, Peter HarriottMcGraw – Hill international edition

6) Through Net, www.google.comOn google – “RMP lecture notes”

7) Through Net, www.google.comOn google – “Energy balance in multi effect evaporations”.

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