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.
Table of Contents
Page
I.
Introduction
3
II.
Theory
3
III.
Industrial Applications
5
IV.
Apparatus and Procedures
V.
Results
9
VI.
Discussion
14
VII.
Conclusions and Recommendations
15
VIII.
References
16
IX.
Appendices
6-7
A.
Signed Data Sheet
17-18
B.
Calibration
19
C.
Sample Calculations
20
2
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)
Where,
F = Feed flow rate (kg/min)
Hf = Enthalpy of the feed (kJ/kg)
3
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
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.
4
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
usage and making its use more efficient. Through the application of such things as
mechanical vapor recompression, evaporators are becoming more and more efficient.
5
Reserved for apparatus
See Apparatus.jpg
IV. Apparatus and Procedure
6
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
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
7
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.
8
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
Inlet
Trial
1
2
3
Inlet
Steam
Pressure
(psig)
10
5
2.5
Inlet
Water
Flow
(gal/h)
4
4
4
Outlet
Steam
Volume
(ml)
550
355
61
Outlet
Liquid
Volume
(ml)
622
930
1059
Trials
timed
for(min)
5
Effect
Pressure
gauge
0
Cooling
Water Flow
(gal/min)
Cooling
Water T
(Deg F)
6
6
6
70
70
70
Outlet
Trial
1
2
3
Condensed
Vapor
Volume (ml)
705
338
130
Overall Data
Boiler T
(Deg F)
205
9
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
Inlet
Trial
1
2
Inlet
Steam
Pressure
(psig)
4
4
Inlet
Water
Flow
(gal/h)
6
4
Outlet
Steam
Volume
(ml)
385
344
Outlet
Liquid
Volume
(ml)
1646
1060
Cooling
Water T
(Deg F)
70
Trials
timed for
(min)
5
Cooling Water
Flow (gal/min)
6
6
Outlet
Trial
1
2
Condensed
Vapor Volume
(ml)
388
300
Overall Data
Effect
Pressure
gauge (inHg)
3
Boiler T
(Deg F)
200
10
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
Inlet
Trial
1
2
3
Inlet Steam
Temperature
TS (K)
388.35
381.55
377.65
Feed Water
Flow
FCalc (kg/min)
0.2654
0.2536
0.2378
Outlet Steam Outlet Liquid
Volume
Volume
Sact (kg/min)
L (kg/min)
Condensed
Vapor Volume
V (kg/min)
Latent Heat
l S(kJ/kg)
0.141
0.0676
0.026
2215.5
2234.2
2244.8
Inlet Steam
Pressure
(kPa)
170.272993
135.799193
118.562293
Tfeed (K)
298.15
298.15
298.15
Outlet
Trial
1
2
3
0.11
0.071
0.0122
0.1244
0.186
0.2118
Calculated Outlet
Trial
1
2
3
Outlet Steam
Volume
SCalc
(kg/min)
Heat Loss
(kJ/min)
0.180
0.103
0.059
N/A
N/A
N/A
110.701
64.043
36.483
38.05
34.19
28.53
Area A(m2)
Cp (kJ/kg K)
HL (kJ/kg)
Hv (kJ/kg)
Hf (kJ/kg)
0.152
4.18
0
2266.92
297.24
Heat
transferred
(W)
Overall Heat
Transfer
Q
Coefficient
U (kJ/min m2 K)
Overall Data
Trial
1
2
3
Capacity
(kg V / min)
0.141
0.0676
0.026
Steam
Economy
(kg V / kg S)
0.784
0.655
0.444
TB (K)
369.26
Consumption
(kg/min)
0.180
0.103
0.059
11
Figure 2 describes the relationship between inlet steam pressure and the outlet
liquid, vapor, and steam mass flow rates at atmospheric effect pressure.
Mass Flow Rate (kg/min)
Steam pressure vs Outlet Recordings
Atmospheric Pressure
12
10
8
Pressure vs Liquid Out
6
Pressure vs Vapor Out
4
Pressure vs Steam out
2
0
0
0.05
0.1
0.15
0.2
0.25
Steam pressure (psig)
Figure 2
Figure 3 illustrates the relationship between the inlet steam pressure and system
heat transfer and heat transfer coefficient.
heat transfer (W) Heat
transfer coefficent
(W/m2 K)
Heat Transfer and Heat Transfer Coefficent vs.
Steam pressure
120.000
100.000
80.000
Q (W)
60.000
U (W/m^2 K)
40.000
20.000
0.000
0
5
10
15
Pressure (psig)
Figure 3
12
Table 4 Shows in detail the calculated data obtained during the experiments run
under a vacuum.
Table 4: Calculated data for vacuum experiments
Inlet
Inlet Steam
Pressure
(kPa)
128.904433
128.904433
Inlet Steam
Temperature
TS (K)
380.05
380.05
Outlet Steam
Volume
Sact (kg/min)
Outlet Liquid
Condensed
Volume
Vapor Volume
L (kg/min)
V (kg/min)
Trial
1
2
Feed Water
Flow
FCalc (kg/min)
0.3968
0.272
Tfeed (K)
298.15
298.15
Outlet
Trial
1
2
0.077
0.0688
0.3292
0.212
0.0676
0.06
Outlet Steam
Volume
SCalc (kg/min)
Heat Loss
(kJ/min)
Heat
transferred
Q (W)
0.121
0.097
N/A
N/A
75.452
60.348
Area A(m2)
0.152
Cp (kJ/kg K)
4.18
HL (kJ/kg)
Latent Heat
l S(kJ/kg)
2237.6
2237.6
Calculated Outlet
Trial
1
2
Overall Heat
Transfer
Coefficient
U
2
(W/m K)
45.885
36.700
Overall Data
Trial
1
2
Capacity (kg
V / min)
0.0676
0.06
Steam
Economy
(kg V / kg S)
0.557
0.618
0
Hv (kJ/kg)
2273.4
Hf (kJ/kg)
297.24
TB (K)
369.26
Consumption
(kg/min)
0.121391935
0.097092074
13
VI. Discussion
Heat Loss:
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
Sact
(kg/min)
0.11
0.071
0.058
Scalc
(kg/min)
0.181
0.104
0.068
%Error
39.2
31.7
14.7
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.
14
Economy:
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).
15
VIII. References
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>.
16
Reserved for Signed Data Sheet
See Signed Data Sheet1.jpg
17
Reserved for Signed Data Sheet
See Signed Data Sheet2.jpg
18
IX. Appendices
B. Calibration
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.
19
IX. Appendices
C. Sample Calculations
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) +
20
.141kg/min*2266.92kJ/kg)
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)
21