University of San Carlos School of Engineering Talamban, Talamban, Cebu City, Philippines
CHE 512L Chemical Engineering Laboratory 2
Evaporation The Climbing Film Evaporator
A laboratory report submitted to
Anonymous
by
Anonymous
1. Intr Introd oduc ucti tion on
One of the important unit operations that make use of heat transfer through boiling is evaporation. In this operation, a liquid solution is heated until it boils and the vapor is removed from the remaining solution, which is then more concentrated. This solution, referred to as concentrate, is usually the desired product in the cases of some aqueous solutions of sugar, milk, and orange juice (eankoplis, !""#$. %mong the factors considered during evaporation are the concentration in the liquid, solubility, foaming or frothing, deposition of scale, temperature sensitivity of the solution, and the temperature and pressure of the evaporator. In addition, there are many types of evaporators which generally vary on the configuration of the heat transfer surface and the agitation or circulation of the liquid (eankoplis, !""#$. &vaporators may have feed flowing through natural circulation, forced circulation or by gravity, and may flow in hori'ontal or vertical tubes. igure ) is a simplified diagram of a single*stage evaporator. The feed enters at a temperature of T, has a concentration of + , and carries an enthalpy of h . The temperature and pressure in the evaporator are the boiling point and saturation vapor pressure, respectively, of the concentrate , which leaves at concentration + and enthalpy h . The vapor leaves with an enthalpy of - . /oth vapor and concentrate have the same temperature and pressure as the evaporator. The steam enters at a temperature T 0 with enthalpy -0 and leaves with enthalpy h 0. %ssuming that the steam is saturated vapor and the only heat given off is latent heat, the difference in enthalpy is simply the latent heat of vapori'ation 1.
Vapor V
Feed F TF, xF, P1, T1
Condensate S TS, hS
Steam S Concentrate L
igure ). 2iagram of a single*stage evaporator
2
The overall and component mass balances, and the heat balance are as shown in the equations below. F = L + V
(! Fx F
=
Lx L
("! Fh F
+
S ( H S
−
hS !
= Lh L + VH V
(#!
The heating rate due to condensing steam can be e+pressed in the form of the heat flu+ equation in terms of the overall heat transfer coefficient3 Q
=
S ( H S
− hS ! =
S λ = UA∆T
($!
In the equation, 4T is the temperature difference between the steam and boiling liquid while % is the heat transfer area. The overall heat transfer coefficient 5 is composed of the steam*side condensing coefficient, the tube thermal conductivity, and the heat*side boiling coefficient. The resistance offered by the wall is often neglected (eankoplis, !""#$. The steam*side condensing coefficient can be estimated using 6usselt7s model assuming film condensation. 8orrelations for boiling coefficient are also provided, one of which is the 9ostinsky7s correlation (0erth, !"":$. In the actual operation, formation of scales and deposits due to decrease in solubility or decomposition of solute can affect the heat transfer coefficient and also lower the amount of solute in the concentrate.
3
2. Objectives of the Eperiment ). Investigate the effect of the feed rate on the evaporator product concentration. !. Investigate the effect of the operating steam pressure on the rate of evaporation. #% 2etermine the overall heat transfer coefficient of the calandria and compare with values predicted
from empirical correlations. $% ;erform steady*state mass balance over the evaporator system to estimate mass losses during the evaporation operations conducted. !. "ethodo#o$y 3.1. Methodological Framework Objectives ) and ! concerned the relationships between evaporator stream properties.
Information about parameters set constant, the number of variations, methods of determining the dependent variables and the hypothesi'ed relationships between independent and dependent variables are summari'ed in igure !.
Evaporator operating pressure Steam pressure Feed inet temperature Feed concentration
Constant
Evaporator operating pressure Feed rate Feed inet temperature Feed concentration
FEE" #$TE
P#%")CT C%&CE&T#$T!%& Method of evaluation: mass balance hypothesized to: decrease with increasing feed rate
!ndepende number of variations: 1 nt
"ependent
%PE#$T!&' STE$( P#ESS)#E values: 5 psig, 10 psig
#$TE %F EV$P%#$T!%& Method of evaluation: measurement hypothesized to: increase with increasing operating steam pressure
igure !.
*
e t c i d e r
Evauated using empirica correations $ssumed t+o resistances ,ased on phenomena osteam.side condensation and -eed.side ,oiing in a vertica tu,e
P
Evauated using the heat /ux e0uation 1 2 )$3T The heating rate is determined -rom experimenta data gathered
igure #. ramework for the determination of overall heat transfer coefficients Objective = saw the construction of steady*state mass balances, inputting the mass flow rates of streams which were determined using volume and density measurements. These were then used to evaluate the operability of the equipment. 3.2. Materials % sugar solution was prepared using ) kg of brown sugar and )> liters of distilled water. This solution served as the feed for the evaporator. 0team is supplied from a steam generator and tap water for the condenser is supplied from a tap water line. 3.3. Equipment The key equipment used in the e+periment is a climbing film evaporator. This evaporator is a long*tube vertical evaporator using vacuum to induce forced circulation of feed. This is used for heat* sensitive solutions. The climbing*film evaporator consists of the calandria, where the feed is heated by steam to vapori'e part of the liquid, a cyclone where the vapor and concentrated liquid separate, a condenser that cools the vapor to condensation, and receivers for both the concentrate and condensed vapor. The feed rate is controlled by the feed inlet valve. Operating steam pressure is manipulated by the steam supply valve at the steam line. The feed container and concentrate receiver are graduated to allow measurement of volume and a thermometer pocket at the top of the evaporator 4
allows the measurement of vapor temperature. The concentrate and the condensed vapor can be collected through drain valves at the bottom of their receivers and the condensed steam can be obtained through the steam discharge valve in the steam line. The operating pressure of the evaporator can be determined through the pressure gauge reading at the vacuum pump. 3.4. Procedures The feed was prepared by adding ) kg of brown sugar to )> liters of distilled water. The solution was heated to ?"@8 and its density was measured using a densitometer. /efore the evaporator was operated, start*up procedures were done. alves were lined up in their proper positions. The cooling water to the condenser was then allowed to flow through the cooling coil and the feed inlet to the evaporator was connected to feed receiver through a hose. %fter the start*up operation, the vacuum pump was started and steam was then allowed to flow through the steam jacket, flushing out any non*condensable vapor and water in the evaporator. inally, the feed was allowed to flow through the equipment. The evaporator was allowed to equilibrate for five minutes and then the height of the liquid in the calandria was measured. %fter the evaporator was running smoothly for some time, an arbitrary )>*minute period was set for the determination of the following quantities in the feed, concentrate, and vapor3 temperature, flow rate, and density. The flow rate was determined by measuring the volume collected Alost during the measurement period. Temperature data were obtained using an alcohol thermometer and the densities were measured using a densitometer.
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%. &esu#ts and 'iscussions
4.1 Effect of the feed rate on the evaporator product concentration % bar graph of feed flow rate versus product concentration was constructed to infer the effect of the former on the latter. The data gathered was a result of two )>*minute runs conducted with varying feed flow rate, . igure ! shows the bar graph of feed flow rate versus product concentration. ".)! ".)" "."B
product concentration, + (gAm$
"."? "."= "."! ".""
::.?
)").!!
feed flowrate, ( gAmin$
igure =. ;roduct concentration, + as a function of feed flowrate, rom igure !, it can be noted that at a higher feed flow rate, the product concentration increases. This trend is contrary to what was hypothesi'ed. %t higher feed flow rates, the concentration of the product should be less compared to the concentration of the product at the lower feed flow rate. This can be attributed to the mass and heat balance. If the feed flow rate is higher, more heat is needed to vapori'e the liquid in the solution. The deviation of the acquired data from the e+pected trend may be attributed to the following factors3 the delay in making the density measurements, insufficient washing of the concentrate receiver from the second run, the inaccurate measurements of the volumetric feed rate and the fluctuating steam operating pressure and feed temperature. There is a peculiar phenomenon, however, that may have been the main reason why the results are not as e+pected3 both runs are at > psig but the steam flow rate is :".?: mlAmin for the run with the lower feed flow rate and )!".?: mlAmin for the run with the higher feed flow rate. Cith this in effect, the heat rate was not kept constant throughout the e+periment. This large difference in steam flow rate may have resulted to more liquid being evaporated, thus giving
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a higher concentration for the run with the higher feed flow rate. Thus, whether or not the hypothesis holds true remains unclear. 4.2 Effect of the operating steam pressure on the rate of evaporation In investigating the effect of steam pressure setting on the rate of evaporation, two )>* minute runs were conducted. One of which was already done in the first objective. % bar graph showing the results of the two runs can be seen in igure >. #>."" #"."" !>."" !".""
rate of evaporation, (gAmin$
)>."" )"."" >."" ".""
>
)"
steam pressure setting (psig$
igure >.
4.3 Eperimental and Predicted !verall "eat #ransfer $oefficients: The overall heat transfer coefficient of the calandria wall was calculated using both e+perimental data and empirical correlations. The results are shown on the table below. Table ). %ctual and Theoretical Overall -eat Transfer 8oefficients
5act (CAm!DE$
5theo (CAm!DE$
F diff 7
) ! #
?,:)?.>B G,B>:.?B )",>?"."=
>!!!.>"
!B.?)
>:>#.#>
:).#=
>"!!.B"
))".!=
It can be noted that the estimated heat transfer coefficient is much lower than the actual heat transfer coefficient as evident from the percent differences. This means that the theoretical resistance to heat transfer was predicted to be higher than the actual resistance. In addition, the actual heat transfer coefficients varied widely between runs. The differences of 5 between theoretical and actual values may have been due to the correlations used and the differences of 5 between runs may have been due to large differences in values of liquid height and steam flow rate for "m compared to >"m and ".)B""m, respectively. This, including the difference in steam flow rate for
The heat transfer coefficient for the feed side was estimated using 8hen7s method (/utterworth, )G::$ which states that the coefficient is the sum of two heat transfer coefficients, the forced convection heat transfer coefficient (calculated as if only the liquid is flowing through the tube$ and a nucleate boiling heat transfer coefficient. % correction factor is applied to this component as forced convection suppresses boiling (;erth, !"":$. The forced convection heat transfer coefficient was calculated using 2ittus*/oelter7s equation while the nucleate boiling heat transfer coefficient was estimated using 9ostinsky7s correlation as provided by 0erth. The same author also provided 8hen7s equation relating these to the convective boiling heat transfer coefficient. 0erth also illustrated that different correlations for nucleate boiling heat transfer may lead to differing estimated heat transfer coefficients and could vary at factors as great as !". Thus, using different correlations aside from 9ostinsky7s may lead to different theoretical overall heat transfer coefficients and may result to estimates close or even higher than the actual overall heat transfer coefficient. 4.4 Mass losses during evaporator operation: %ssuming the evaporator has already been running on steady state conditions, it is e+pected that the amount of feed entering the evaporator is equal to the amount that e+its. These would be the condensate and concentrate. -owever, by doing mass balance using actual e+perimental data, the amount of feed entering minus the amount e+iting doesn7t equate to 'ero. The resulting value is the mass loss or gain in the system. mloss
= F − L − V
(&! Table ! shows the values calculated for the mass losses. or the first two runs, we lost
"."BG? kg and "."?": kg of the solution, respectively. The third run had a gain of ".)B#) kg of the solution.
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Table !. 9ass losses at different runs
9ass osses (kg$ "."BG? "."?": *".)B#)
The results may be e+plained by ()$ human and instrumental error, and (!$ sucrose losses across the evaporator. The data gathered during the run of the e+periment are more or less appro+imates. The instruments used for measuring volume are also not completely accurateJ the feed container uses a pail manually calibrated for every half liter. Chen determining the densities of the streams, they may have cooled off by a few degrees compared to the actual temperature of the stream when it e+ited or entered the system, adding to the error. This may lead to some losses or gains during the calculation of the masses. It is also possible that the losses or gains are due to the equipment K the operation themselves. %nother factor that could cause the mass loss is scaling in the evaporator. 0ucrose can form scales in the evaporator. %s a result, some of the sucrose are unable to e+it the evaporator becoming unaccounted for in the mass balance (&ggleston, 2amms, 9onge, K &ndres, !""=$. This is also affected by the quality of the sugar used. 5. Conc#usions The hypothesi'ed relation that increasing flow rate decreases product concentration is still
inconclusive due to the steam flow rate not being held constant between the runs. -owever, the hypothesis that increasing operating steam pressure increases rate of evaporation is supported by the results in the e+periment. The values of the actual overall heat transfer coefficient of the calandria in terms of CAm !DE were determined to be ?,:)?.>B, G,B>:.?B, and )",>?"."= for the first, second, and third run respectively. %ll of these coefficients were comparatively higher than those estimated using empirical correlations. rom the steady state mass balances, mass losses were estimated to be "."BG? kg for the first run and "."?": kg for the second run. The third run had a mass increase of ".)B#) kg.
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&eferences /utterworth, 2. ()G::$. Introduction to Heat Transfer. 2esign 8ouncil. 8ao, &. (!")"$. Heat Transfer in Processes Engineering. 9craw*-ill. &ggleston, ., 2amms, 9., 9onge, %., K &ndres, T. (!""=$. 6ew insights on sucrose losses across factory evaporators and juice and syrup classifiers. SPRI 2004 Conference on Sugar Processing Research, #=G* #:". eankoplis, 8. L. (!""#$. Principles of Transport Processes and Separation Processes (4th ed.). 6ew Lersey, 50%3 ;earson &ducation, Inc. 0erth, <. (!"":$. Processes Heat Transfer Principles and pplications. &lsevier td.
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A((E) 'ata *rocessin$ and Ana#ysis &eport &evisions &evisions (!ld "alues are enclosed in parentheses) Actual and Theoretical Heat transfer coefficients
5act (CAm!DE$
)
?,:)?.>B
!
G,B>:.?B
#
)",>?"."=
5theo (CAm!DE$
F diff
>!!!.>" (!=!).G!$ >:>#.#> (!===.:>$ >"!!.B" (!:=G.#=$
!B.?) ()::.#!$ :).#= (#"#.!!$ ))".!= (!B=."G$
A2.6 Theoretical overall heat transfer coefficient 0team operatin &vapora &ffecti < M8 T0 N8 g tor ve u (mA (@8 (kgAmD pressure ;ressur height n min$ $ s$ , gauge e (k;a$ (m$ (psig$ 33:59 ".)>> )"B !.:"&* ) > :".?: "= " .#= 33:59 ".)"> ))> !.>#&* ! )" B>.## "= " .!) 33:59 ".)B" )!?.? )"B !.:"&* # > "= " : .#= < un
)
!
hs (CAmD E$ B=:?." =
B)=).= #
cp (kLAkg DE$ =.!>):
=.!?!)
6;r
).?B "#
).>: ?:
h (CAm DE$ >).== G
#?.!) !
;A;c
hnb (CAmD E$
).>!&* "# (?.)?& *"#$ ).>!&* "# (:.:!& *"#$
)#,G?= .B" (!,##". !"$ !",=>! ."> (!,?:>. ?"$
Ns (kgAmD s$
8 (kgA m#$
s (kgA m#$
6
6
k8 (CA mDE$
).##&* ">
GG?. B"
".>B #G
!!).B !")
)G".# ="?
".?B! "
).#?&* ">
GG?. ""
".>: #G
!B>.> :?B
)!?.= :B:
".?B! G
).##&* ">
)"! ?.?"
".>B #G
="G.= BB?
)BG.G #G?
".?B! "
vapor mass fraction ".)=="G
".=#B##
Ptt
"."# B!
"."# !=
(Ptt $ !?.) =)G
!G.= B!!
08-
".B: B"
".G" ?B
hb (CAm DE$
5theo (CAm!D E$
)#?"> .>! (##G". B)$ )G?)= .!G (#=G#. G#$
>!!!.>" (!=!).G !$ >:>#.#> (!===.: >$
13
#
:"=?.> :
=.!>):
).?B "#
>).#? #
).>!&* "# (?.)?& *"#$
)B,?=# ."# (#,=:>. =G$
".>!=?G
"."# )B
!G.B ?>:
".B> >B
):=BB .G? (=>"B. #?$
>"!!.B" (!:=G.# =$
1*
