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Single Effect Evaporator ChE 3787L Unit Operations Lab I Dr. Sundar Vaidyaraman 9/25/2003 Group 3 Team Leader: Ashley King Team Members: Ricardo Cruz, Tony Koulianos, Courtney Morrison Abstract A single effect evaporator was utilized using water as feed and condensed water and liquid water as product. The evaporator was run with and without a vacuum at different steam pressures to determine the effects on the outlet liquid and vapor. The outlet vapor and liquid varied linearly with the steam pressure. Running the evaporator under a vacuum proved to be more efficient with respect to steam usage for evaporation.

Single Effect Evaporator 2

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Page 1: Single Effect Evaporator 2

Single Effect EvaporatorChE 3787L

Unit Operations Lab IDr. Sundar Vaidyaraman

9/25/2003Group 3

Team Leader: Ashley KingTeam Members: Ricardo Cruz, Tony Koulianos, Courtney Morrison

Abstract

A single effect evaporator was utilized using water as feed and condensed water

and liquid water as product. The evaporator was run with and without a vacuum at

different steam pressures to determine the effects on the outlet liquid and vapor. The

outlet vapor and liquid varied linearly with the steam pressure. Running the evaporator

under a vacuum proved to be more efficient with respect to steam usage for evaporation.

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Table of Contents

Page

I. Introduction 3

II. Theory 3

III. Industrial Applications 5

IV. Apparatus and Procedures 6-7

V. Results 9

VI. Discussion 14

VII. Conclusions and Recommendations 15

VIII. References 16

IX. Appendices

A. Signed Data Sheet 17-18

B. Calibration 19

C. Sample Calculations 20

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I. Introduction

The objective of the experiment was to utilize a single effect evaporator to study

the effect of steam pressure on the system both under atmospheric and vacuum

conditions. Under atmospheric pressure experiments were conducted with the steam

pressure at 2.5, 5, and 10 psig. For each steam pressure the volumetric flow rates of the

outlet steam, dilute liquid, and condensed water were measured. Under vacuum

conditions, steam pressure was held constant at 4psig and effect pressure at 3inHg. The

volumetric flow rates of the outlet steam, dilute liquid and condensed water were

measured for two different inlet flow rates, 6 and 4gal/min inlet feed. By observing and

operating the single effect evaporator it was important to understand the fundamentals of

liquid-liquid separations. Also observed was how the fundamental elements of pressure

and vacuum affects on heat transfer, capacity and economy.

II. Theory

The purpose of the evaporation process is the formation of a more concentrated

solution or product form a dilute feed. To obtain the concentrated product, the feed is

boiled to evaporate off water. The vapor and liquid located in the boiler are in

equilibrium therefore sharing equal outlet temperatures which is the boiler temperature.

The vapor then proceeds to the first effect to be condensed by cooling water and normally

considered a waste product or possibly purification worthy. The concentrated product

from the first effect is the final product or in large capacity operations is sent to multiple

effects. During the case study the volumetric flow rates of the outlet steam, condensed

vapor, and product liquid were recorded. The effect pressure and temperature, and the

inlet steam pressure were known. The inlet feed temperature was assumed to be room

temperature (298.15K). The above data was used to calculate the systems heat loss and

overall heat transfer. An overall energy balance for the system is shown in equation (1).

FHf + Sls = LHL + VHV (1)

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Where,

F = Feed flow rate (kg/min)

Hf = Enthalpy of the feed (kJ/kg)

S = Steam flow rate (kg/min)

L = Liquid product flow rate (kg/min)

HL = Enthalpy of the liquid product (kJ/kg)

V = Vapor flow rate (kg/min)

HV = Enthalpy of the vapor (kJ/kg)

It is then desired to calculate the theoretical output steam flow rate by rearranging

equation (1) to give equation (2).

Scalc = (1/ls) (FCp (TB – Tf) + VHV) (2)

Where,

ls = Latent heat of the steam (kJ/kg)

Cp = Heat capacity of the feed (kJ/kg K)

TB = Temperature of the boiler (K)

Tf = Temperature of the feed (K)

The outlet steam flow rate found above is used to theoretically find the overall heat

transferred of the system as seen in equation (3).

Q = Scalc*ls (3)

The amount of heat transferred from the system is then used in equations (4) and (5) to

solve for the heat transfer coefficient.

Q = U A (TS – TB) (4)

U = Q/A (TS – TB) (5)

Where,

A = Area of the boiler (m2)

The theoretical equations behind single effect can be viewed in more detail in section IX.

Under vacuum conditions, the vapor will boil off at a lower temperature; hence,

less amount of steam is needed to obtain the desired product. Under vacuum conditions

the same equations and theoretical principals at atmospheric pressure apply. However it

is expected that the vapor enthalpy will change. For industrial processes the steam

pressure calculations are based on the desired final product concentration. The higher the

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steam pressure leads to a higher product concentration. In this case study, a single effect

evaporator was observed. The use of single effect evaporators are cost efficient only

when the required capacity of operation is small2.

III. Industrial Applications

Evaporators are an important unit operation and find application in many different

industries. Evaporators find use in such applications as caustic soda processing in the

chemical industry, ammonium nitrate in the fertilizer industry, Bright dip acid

(phosphoric) in steel mills, as well as applications in the aluminum industry, paper mills,

distilleries, and others4. The different kinds of evaporators are as varied as their

applications. Forced-circulation evaporators are used for processes where crystals are

formed. Long tube vertical evaporators are used in concentrating liquids that have no

solids present4. Short tube vertical evaporators are natural circulation evaporators and are

good for non-crystallizing, clear and non-corrosive liquors1.

Pure Malt Products Ltd, which produce a wide range of malt extracts that find

many applications in the food and beverage industries, use single- and multiple-effect

evaporators3. In their Haddington factory near Edinburgh, they installed a single-effect

falling film tubular evaporator with mechanical vapor recompression. Mechanical vapor

recompression is an energy saving operation. In a steam-heated evaporator, all or part of

the evaporated vapor is discharged to a condenser, and the heat content is lost to the

system. In mechanical vapor recompression, the vapor is compressed to a suitable

pressure. It can then be condensed in the evaporator acting as the heating medium. The

steam supply is replaced by the mechanical energy input to the compressor, and the

energy input is largely reduced. This evaporator uses three falling film stages in one

body. The operation is under vacuum. The feed is heated by a plate heat exchanger. It

then passes to the vapor-liquid separator. From the separator it joins the circulating

product flow in the first stage of the evaporator. It is circulated through three falling film

stages3.

Evaporators find many applications in industry and are configured in many

different ways (single effect and multiple-effect). The main concern becomes energy

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usage and making its use more efficient. Through the application of such things as

mechanical vapor recompression, evaporators are becoming more and more efficient.

Reserved for apparatus

See Apparatus.jpg

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IV. Apparatus and Procedure

Procedure:

Following the calibration steps mentioned in the appendix, the system was

allowed to reach a steady state under an undetermined steam flow rate. To implement the

trials run under atmospheric pressure, the effect pressure, PI 3, was maintained at 0 gauge

pressure. The inlet feed flow, FCV 1, was set to a constant flow of 4gph and the cooling

water to the condenser, FCV2, was set to a constant 6gpm. The inlet steam, V11,

pressure, PCV 1, was then set to 10psig to commence the first experiment. The system

was then allowed 30 minutes to reach a steady state. When sufficient amounts of liquid

and condensed vapor were exiting the system, the flow rates of the streams could be

measured. To measure the liquid in the product receiver, V 8 was opened and the liquid

was allowed to flow into a graduated cylinder for 5 minutes and then measured in mL.

To measure the condensed vapor in the distillate receiver, V 26 was closed for 5 minutes.

The distillate receiver was then isolated from the system by closing V 27. To collect the

condensed vapor, V 26 was opened and the condensed vapor was allowed to flow into the

graduated cylinder and measured in mL. After the condensed vapor was collected V 27

was opened to introduce the distillate receiver back to the system. To measure the steam,

V 18 was opened for 5 minutes and the condensate was allowed to flow into a small

container. The condensate was then transferred to the graduated cylinder to be measured

in mL. The above procedure was repeated for steam flow rates of 5 and 2.5psig

After the atmospheric pressure experiments, it was desired to run the evaporator

under a vacuum. The system was set up in the same manner as stated above, however the

vacuum pump was turned on. The system was initially opened to the atmosphere with no

vacuum effect. The inlet feed rate, FCV 1, was initially set to 6gph. The cooling water,

FCV 2, was set to a constant 6gpm. The inlet steam, PCV 1, was then set to a constant

4psig. In order to create a vacuum on the system, the product receiver, V6, was closed

from the atmosphere by the use of valves V 8 and V 10. Next the distillate receiver was

closed to the atmosphere by the use of valves V 25 and V 26. The entire system was then

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placed under a vacuum of 3inHg gauge by slowly manipulating valve V 23. The system

was then allowed to reach steady state for 30 minutes. After a sufficient amount of

distillate and liquid product were noticed, recordings were taken under a time period of 5

minutes. To collect the liquid product, the product receiver was isolated from the system

by closing valve V 6. The product receiver was then isolated from the vacuum by closing

valve V 9. The receiver was then opened to the atmosphere by valve V 10 and the liquid

product was allowed to flow into a graduated cylinder by opening V8 and subsequently

measured in mL. The product receiver was reintroduced to the vacuum by the order of

closing valves V 8, V10 and opening valves V 9, V6. The distillate was measured in the

same manner by the order of closing V 27, V24 and opening V 25, V26. The distillate

was then collected and measured in a graduated cylinder. To reintroduce the distillate

receiver to the vacuum by the order of closing V 26, V 25 and opening V24, V 27. To

collect the outlet steam, V 18 was opened and collected in a small container then

transferred to the graduated cylinder to be measured in mL. Following the recordings

taken at an inlet feed flow rate of 6gph, the flow rate was decreased to 4gph to start the

next set of identical experiments.

To conclude the experiment, the system was shut down by slowly introducing the

entire system to the atmosphere by opening V 23. The product receiver and distillate

receiver were also opened to the atmosphere by opening valves V 10, V 8 and V 25, V26.

The pump was then turned off. Steam flow was discontinued by closing V11. Valve 15

was subsequently closed removing all remaining steam from the effect. The cooling

water and inlet feed water continued to flow for 10 minutes to allow the system to cool

down. The inlet feed and cooling water were discontinued by closing FCV 1 and FCV 2.

The above shut down procedure allowed for the equipment to be left safely.

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V. Results

The following shows in detail the raw data and calculated data obtained during

the experiment. The data presented in this will be discussed in the following section.

Sample calculations are shown in the sample calculations section.

Table 1 Shows in detail the raw data obtained during the experiments run under

atmospheric pressure. The recorded data for the inlet, outlet and overall system are

shown.

Table 1: Raw data for atmospheric effect pressure experiments

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Table 2 summarizes the raw data obtained during the experiments run under a

vacuum. The recorded data for the inlet, outlet and overall system are shown.

Table 2: Raw data for vacuum experiments

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Table 3 Shows in detail the calculated data obtained during the experiments run

under atmospheric effect pressure.

Table 3: Calculated data for atmospheric effect pressure experiments

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Figure 2 describes the relationship between inlet steam pressure and the outlet

liquid, vapor, and steam mass flow rates at atmospheric effect pressure.

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Steam pressure vs Outlet RecordingsAtmospheric Pressure

0

2

4

6

8

10

12

0 0.05 0.1 0.15 0.2 0.25

Steam pressure (psig)

Mas

s F

low

Rat

e (k

g/m

in)

Pressure vs Liquid Out

Pressure vs Vapor Out

Pressure vs Steam out

Figure 2

Figure 3 illustrates the relationship between the inlet steam pressure and system

heat transfer and heat transfer coefficient.

Heat Transfer and Heat Transfer Coefficent vs. Steam pressure

0.000

20.000

40.000

60.000

80.000

100.000

120.000

0 5 10 15

Pressure (psig)

hea

t tr

ansf

er (

W)

Hea

t tr

ansf

er c

oef

fice

nt

(W/m

2 K

)

Q (W)

U (W/m^2 K)

Figure 3

Table 4 Shows in detail the calculated data obtained during the experiments run

under a vacuum.

Table 4: Calculated data for vacuum experiments

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VI. Discussion

Heat Loss:

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Table 5 shows the percent error calculations on the outlet steam flows under

atmospheric conditions. In theory, it is expected that the calculated steam flow rates

should be smaller than the experimental flow rates. For this particular case study, the

experimental steam flow rates were lower than the calculated steam flow rates. As the

steam was being collected form the trap, it was noticed that the steam was evaporating

off. It was also noted that some of the steam that was condensed was not fully exiting the

pipe. Due to steam evaporating during the collection process, the transfer of the steam,

and losses in the exiting pipe the experimentally recorded flow rates for the outlet steam

were lower than the calculated steam flow rates. Since the actual steam flow rates were

lower, heat losses could not be calculated.

Table 5: % error calculations on steam flow rates for atmospheric conditions

Heat Transfer Coefficient:

The experiment run at 2.5psig and atmospheric effect pressure show deviations in

heat transfer coefficients. The experiment was run twice due to inadequate data collected

during the first trial. The liquid product collected was significantly lower then

theoretically expected. By examining Figure 3, the heat transfer coefficient, U,

noticeably decreased at 2.5psig steam pressure. In theory, the heat transfer coefficient

should remain relatively constant for the system even under steam pressure changes2.

The heat transfer coefficient is dependant upon system geometry, fluid properties, flow

viscosity, and temperature differences. In experimentation, it is expected that the heat

transfer coefficient will deviate slightly due to the temperature difference. By observing

the inconsistencies in the data described by Table 3 and Figure 3 it could be concluded

that running the system at a steam pressure of 2.5psig leads to inaccurate results.

Economy:

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The economy of a single effect evaporator, in theory, should be less than 1.

Under atmospheric conditions, as seen in Table 3, the steam economy is greater than 1.

This could be due to the system only evaporating water and not producing an actual

condensate product. Using the calculated steam flow rates, the appropriate values for

steam economy would be achieved. This again shows a large error in experimental steam

collection. However under vacuum conditions the steam economy was less than 1.

During experimentation, it was found that the evaporator could not function properly at

effect pressure higher then 3inHg. Effect pressures higher then 3inHg would result in

total vaporization of product. Under general conditions, multiple effects, the vacuum

would be used with a lower steam temperature and pressure.

VII. Conclusions and Recommendations

The physical process of running and maintaining a single effect evaporator under

atmospheric and vacuum conditions was established. A single effect evaporator was run

at three different pressures with the outlet liquid product, condensed vapor, and steam

volumetric flow rates being recorded. This allowed for a realistic observation of the

effects of steam pressure on the evaporation process. The theory behind heat transfer was

applied to real situations to compare expected values with actual results. The inlet steam

had more effect on the process than any other element. It was discovered that the system

should be ran at steam pressures above 3psig to maintain a level of accuracy in the

results. Running the evaporator under a vacuum allowed for the observation of boiler

temperature and inlet feed flow rate effects on the system. By successfully running the

evaporator under a vacuum, it can be deducted that the system can be run as a multiple

effect evaporator (i.e. lower steam pressure fed to the second effect).

VIII. References

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1.Confederation of Indian Industry. Energy Bulletin on Evaporators.

Alwarpet, Chennai.

<http://www.greenbusinesscentre.com/documents/Evaporator.pdf>. Pages 1-3.

2.Geankoplis, Christie J. Transport Processed and Unit Operations. Third

Edition. Prentice hall. Englewood Cliffs, New Jersey. 1993. Pages

858-859 and 494-498.

3.Pure Malt Products Ltd. Food industry, Haddington. United Kingdom –

Mechanical Vapor recompression

<http://www.heatpumpcentre.org/cases/ind_07.htm>.

Pages 1-3.

4.Swenson Technology, Inc. Energy Conservation Heat Exchangers /

Multiple Effect. Copyright 2002 Swenson Technology, Inc

<http://www.swenson-equip.com/energy.html>. Pages 1-3.

5.Times Food Processing Journal. Concentrating on concentrated milk .

Copyright © Bennett Coleman & Co. Ltd.

<http://www.timesb2b.com/foodprocessing/feb_mar03/tech.html>.

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Reserved for Signed Data Sheet

See Signed Data Sheet1.jpg

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Reserved for Signed Data Sheet

See Signed Data Sheet2.jpg

IX. Appendices B. Calibration

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Calibration of the system included turning on the system and setting the variables

to prepare the system for steady state. The feed vessel, V1, was filled to approximately

30 gallons with tap water supplied by valve V 1. The feed was introduced to the system

by opening valves FCV 1 and V 3. The feed was allowed to flow through the system to

fill the pipes and boiler. Cooling water to the condenser was then turned on by valve

FCV 2. Valve V 12 was closed to prevent any steam form entering the first effect. Valve

V 15 was opened to the second effect to allow the steam to enter only this effect. Inlet

steam was then introduced to the system by opening valve V 11. The pressure of the inlet

steam was controlled by PCV 1. Valves V 8 and V26 were opened to allow the distillate

and liquid product to flow into the drain till collection. The inlet feed flow, FCV 1, and

inlet steam, PCV 1, was set so that an adequate amount of liquid product and condensed

vapor was noticed. This step caused the flow rates of the inlet feed and inlet steam to be

relatively high in order to activate the system dynamics. Once the apparatus was stable,

the pressure and inlet feed flow rate could be set for the experiment.

IX. Appendices C. Sample Calculations

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The following sample calculations correspond to the trial run at 10psig. The

equations used were previously derived from the text and also the handout2. Steam tables

were utilized for latent heat and stream enthalpies2. The overall energy balance can be

written as follows.

FHf + Sls = LHL + VHV (1)

Where,

F = Feed flow rate (kg/min)

Hf = Enthalpy of the feed (kJ/min)

S = Steam flow rate (kg/min)

L = Liquid product flow rate (kg/min)

HL = Enthalpy of the liquid product (kJ/kg)

V = Vapor flow rate (kg/min)

HV = Enthalpy of the vapor (kJ/kg)

The individual terms for equation (1) are illustrated by equations (2)-(4).

F = L + V (2)

F = .1244 kg/min + .141 kg/min

-Hf = Cp (TB – Tf) (3)

-Hf = 4.14 kJ/kg K (369.26K – 298.15K)

ls = Hvapor – Hliquid @ saturation pressure and temperature.

By rearranging equation (1) the inlet steam flow rate could be theoretically

calculated. This Flow rate, in theory should be lower than the gathered steam during

experimentation.

Scalc = (1/ls) (FCp (TB – Tf) + VHV) (4)

Where,

ls = Latent heat of the steam (kJ/kg)

Cp = Heat capacity of the feed (kJ/kg K)

TB = Temperature of the boiler (K)

Tf = Temperature of the feed (K)

S = (1/2215.5kJ/kg) (.2654kg/min * 4.14 kJ/kg K (369.26K – 298.15K) +

.141kg/min*2266.92kJ/kg)

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Assuming the system is at datum2 of 373.15K, it can be stated that HL = 0. Next

the Heat loss from the effect could be calculated. Since the steam gathered during

atmospheric pressure and vacuum effect trials were smaller than theoretically calculated,

the heat loss could not be calculated in these cases.

Heat Loss = (Scalc – Sact) ls (5)

The overall heat transferred in the system could then be calculated by utilizing the

collected amount of the steam and its respective latent heat.

Q = Scalc*ls (6)

Q = .18kg/min*2215.15kJ/kg

The overall heat transfer equation is as follows where area, A, is given as 0.5m2

and TS and TB were the temperatures of inlet steam and boiler respectively.

Q = U A (TS – TB) (7)

By using the heat transfer value obtained in equation (6) and rearranging equation

(7) the overall heat transfer coefficient, U, could be calculated.

U = Q/A (TS – TB) (8)

U = 243.705kJ/min / 0.152m2(388.35K-369.26K)

Where,

A = Area of the boiler (m2)

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