水乙二醇和甲醇 - 水和阶段的建模实验的分离效率

Available online at Chemical Engineering and Processing47(2008)209–214Separation efficiency of thin-film evaporators:Experiments withwater–ethylene glycol and methanol–water and stage-based modelingFrank Schaal,Klemens Schilling,Hans Hasse∗Institute of Thermodynamics and Thermal Process Engineering,Universit¨a t Stuttgart,D-70550Stuttgart,GermanyReceived23November2006;received in revised form22February2007;accepted26February2007Available online3March2007AbstractThin-film evaporators are often used for separations.Despite the long-standing research in this area,there is still a lack of reliable data on measurements on test systems that could be used for model development and validation.In this research,therefore,the partial evaporation of the binary mixtures of ethylene glycol and water as well as water and methanol is studied in a laboratory-scale thin-film evaporator operated in counter-current mode.Two types of equilibrium stage models of the separation process are tested,in which the process is described as open and closed evaporation,respectively.The results show that,especially at low vapor streams,a considerable rectification effect occurs in the thin-film evaporator that can only be modeled using the closed evaporation approach.The equilibrium stage approach is sufficient for describing the data from the present work.©2007Elsevier B.V.All rights reserved.Keywords:Thin-film evaporator;Separation;Modeling;Ethylene glycol;Water;Methanol1.IntroductionThis research focuses on the use of thin-film evaporators for separations.The general advantages of thin-film evapora-tors are short residence times and small temperature differences required for heating[1].Billet[2,3]and Skoczylas and Dziak[4] have studied the separation efficiency of thin-film evaporators using the binary systems methanol–ethanol,methanol–water, water–ethylene glycol,and acetone–n-butanol.However,the data basis provided in that research is not wide,and most of the results are only presented in graphical form.The present work aims at extending the available data by providing experimental data for the systems water–ethylene glycol and methanol–water. These non-reactive test systems were chosen to complement studies on reacting solutions containing formaldehyde,water, and methanol[5–7].Abbreviations:GC,gas chromatograph;GOX,glycerine oxethylate;HP, Hewlett-Packard;PTFE,polytetrafluoroethylene/teflon;TEG,tri-ethylene gly-col;TCD,thermal conductivity detector∗Corresponding author.Tel.:+4971168566105;fax:+4971168566140.E-mail address:hasse@itt.uni-stuttgart.de(H.Hasse).The second target of this investigation is to analyze how accurately stage-based models can describe the separation in thin-film evaporators.Rate-based modeling is not addressed in this short contribution.However,the experimental data are pre-sented in a way so that they can be used for comparisons with such more sophisticated models at any time.Two different stage-based modeling approaches are discussed:the open and closed evaporation.In open evaporation,it is assumed that the vapor is separated from the liquid immediately after the evaporation. Hence,there is no difference between counter-and co-current and no rectification effects can occur.In closed evaporation,the vapor remains in contact with the liquid so that counter-and co-current can be distinguished and rectification effects can be modeled.It turns out that the latter model allows a descrip-tion of the data from the present work.The question of the appropriate choice of the number of stages used in the model is addressed.2.ExperimentsAll experiments were carried out using a laboratory-scale thin-film evaporator,cf.Fig.1.The evaporator(QVF Engineer-ing GmbH,Mainz)consists of a vertical glass tube with a heating jacket,which is thermostated by a liquid.The heatingfluids0255-2701/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.cep.2007.02.018210F .Schaal et al./Chemical Engineering and Processing 47(2008)209–214Fig.1.Flow sheet of the thin-film evaporator.were chosen as follows:for evaporation temperatures below 100◦C distilled water,for temperatures between 100and 150◦C tri-ethylene glycol (TEG),for higher temperatures glycerine oxethylate (GOX).The total length of the evaporator is 732mm,the inner diameter is 50mm,the wiped length is 300mm.The stirrer inside consists of three PTFE rotor blades,a so-called Smith rotor.The blades have slanted teeth to enhance mixing and transport of the film.The liquid is distributed to the inner wall by centrifugal force using a turning ring device.There are temperature-measuring points on the product side in the gas phase in the bottom and the head of the evaporator.Temperature data were taken in the experiments but are believed to be inaccurate,due to low vapor flow rates and probably addi-tional problems with the processing unit.On the heating side only the temperature of the thermostat bath is known.There-fore,the present study does not supply useful information on heat transfer,and consequently no temperature data is reported at all.Temperatures can be estimated from the model presented below.The feed is continuously pumped into the evaporator (diaphragm metering pump by ProMinent,Heidelberg,model gamma-RS),the head product (after condensation)and the bot-tom product are pumped out level controlled (diaphragm pumps by Telab,Moers,model BF414).The flow rates of feed,head product and bottom product are measured by means of balances,with which the mass changes in feed and product containers are monitored.The ‘evaporation rate’(ER)is defined as the quotient of the head and the feed mass flow.The compositions of the inlet and outlet streams are investi-gated by gas chromatography.The gas chromatograph used is a Hewlett-Packard HP 6890with a HP-INNOWax Polyethylene Glycol HP 19091N-213capillary and a thermal conductivity detector (TCD).The samples of the head and bottom productswere taken in stationary state,which was attained in 15–20min in most cases.All experiments that are presented in the following were conducted at constant pressure (20and 40kPa,respectively for methanol–water,10kPa for ethylene glycol–water),which was controlled and measured by a unit from MKS Instru-ments,Munich.The rotation speed of the stirrer was adjusted to 300min −1.In test runs the rotation speed was varied between 200and 1500min −1without any significant influence on the results.The feed flow rate varied between 0.5and 1.1kg h −1,again without significant influence on the results.For each of the two studied systems,three feed compositions were stud-ied.For each composition,the evaporation rate ER was varied over the entire accessible range by changing the heating fluid temperature.For controlling the heating fluid temperature,a thermostat (type MV4of Julabo,Seelbach)was used.Only the bath temperature is known,but not reported here.The results are fully documented in Table 1(water–ethylene glycol)and Table 2(methanol–water).The overall mass balance was fulfilled with a relative accuracy of typically 1%.For the component mass balances that number is 1.5%.The estimated accuracies are 1%(relative)for the mass flows,1%(relative)for the pressure,and 0.3%(absolute)for the mass fractions.3.ModelingThe modeling of the thin-film evaporation is based on the simple concept of equilibrium stages in the present work.Two different approaches,open and closed evaporation,are used.They are introduced in Fig.2.In multi-stage open evaporation,it is assumed that the gas phase does not have any contact with the liquid phase in other stages—this would correspond to a fast gas flow in the center of the thin-film evaporator basically withoutF .Schaal et al./Chemical Engineering and Processing 47(2008)209–214211Table 1Experimental results for water–ethylene glycol (p =10kPa)x F W(g g −1)˙m H (kg h −1)˙m B (kg h −1)x H W (g g −1)y B W(g g −1)0.150.0670.4230.8810.0280.0770.4150.8820.0140.0940.5110.8800.0160.0970.5270.8970.0140.0990.5620.8780.0210.1070.9360.9540.0610.1230.9450.9390.0410.1390.8530.8880.0320.1410.8620.9050.0260.1580.8480.8520.0150.1610.8700.8940.0180.1760.7940.7780.0070.2110.7700.6860.0050.2590.4170.3820.0030.500.0960.9660.9940.4420.2240.8540.9920.3640.3180.7550.9900.2920.3670.6080.9840.1990.3960.5500.9740.1600.4180.5450.9750.1210.4380.5070.9670.0970.4540.4820.9580.0720.4630.5090.9350.1060.5200.5080.9300.0540.5400.4820.9010.0490.800.2980.6770.9970.7080.4250.4830.9960.6270.4330.4780.9960.6200.4500.5290.9970.6200.4790.5790.9970.6280.6340.3790.9950.4670.6460.3900.9950.4570.6940.3170.9950.3640.7160.2700.9750.2950.7500.2720.9930.2610.7550.2210.9820.1650.7590.2470.9720.2350.7700.2070.9690.1510.7780.2490.9820.2310.7940.2850.9880.2480.8160.2290.9750.171Fig.2.Flow sheet of four-stage open evaporation (left)and four-stage closed evaporation in counter-current (right).any interaction with the liquid film at the wall.In contrast,in multi-stage closed evaporation a contact between the upward flow of the gas and the downward flow of the liquid enhances the separation efficiency.Each stage is heated.Both the number of stages as well as the heat supplied to each stage or alternatively the evaporation rate in each stage need to be specified.The number of equilibrium stages was varied here to study its influence,since no infor-mation on the appropriate choice of that number is available in advance.The heat supplied to each stage was assumed to be the same for all stages for simplicity.After specifying these parameters together with the feed flow rate and composition,the only remaining parameter is the evaporation rate ER,for which the experimental value was used.The most important result from the simulation is the composition of the head and bottom product of the evaporator.Since the feed is supplied at the upper end of the wiped range,it is also supplied to the top stage of the model.Condensation in the top of the evaporator above the feed point may enhance the separation,especially if vapor streams are small.Some con-densation was observed in the experiments in the evaporator top,but the effect of that condensation on the separation in the experiments is believed to be small.It is not described by the model.The physical properties of the pure fluids and the binary mix-tures were obtained from the ASPEN database.The Wilson method was used for the description of the binary interac-tions.The vapor-liquid equilibria of the studied systems are well described by that resulting model,at given temperature and liq-uid phase composition,the mean deviation of the vapor phase composition does not exceed 0.01mol/parison of experiments and simulation 4.1.Water–ethylene glycolIn Figs.3–6,the head and bottom concentration of the low boiling substance,water,are shown as a function of the evap-oration rate.Fig.3is a magnified view of the most interesting part of Fig.5.Experimental results are represented bysym-Fig.3.Model vs.experimental results for water–ethylene glycol,enlarged.x F W=0.5g g −1,p =10kPa.212F .Schaal et al./Chemical Engineering and Processing 47(2008)209–214Table 2Experimental results for methanol–water p (kPa)x F M(g g −1)˙m H (kg h −1)˙m B (kg h −1)y H M (g g −1)x B M(g g −1)200.1880.1680.6860.5950.0910.1980.6660.5670.0670.2480.6060.5290.0440.3210.5310.4710.0190.3530.4970.4420.0120.4160.4230.3780.0040.4970.3480.3170.0010.5960.2480.2600.0000.6800.1850.2340.001400.2700.2000.5550.7020.1200.2810.4910.5830.0740.3440.4160.5670.0310.4910.2600.4350.002400.7060.2610.4920.8990.6040.3630.3960.8880.5320.4310.3180.8840.4610.5140.2420.8690.3390.5670.1790.8470.2530.6240.1230.8050.162bols,model predictions by lines.The wider the gap between head and bottom concentrations at a constant evaporation rate is,the better is the corresponding separation efficiency.The observed results are strongly influenced by the feedcomposi-Fig. 4.Model vs.experimental results for water–ethylene glycol,x F W=0.8g g −1,p =10kPa.Fig. 5.Model vs.experimental results for water–ethylene glycol,x F W=0.5g g −1,p =10kPa.tion,as can be understood by considering the mass balance.For evaporation rates in the range of the mass concentration of the low boiling substance in the feed,large changes are observed.The simulation results show the expected trends:the cal-culated separation efficiency increases with the number of equilibrium stages.In Fig.3,results for 1–30stages are ing more than eight stages leads only to little further change.Of course,there is no difference between open and closed evaporation for one stage.For the multi-stage process,the separation efficiency of the closed evaporation is better than for the open paring the results from the simu-lations with those from the experiments,it can be seen that,as expected,the experimental separation efficiency is better than that of a single equilibrium stage.These trends were observed for all experiments.For the further discussion only modeling results for an eight-stage open evaporation and an eight-stage closed evaporation are used,see Figs.4–6.The choice of that particular number should not be interpreted physically.It ismoreFig. 6.Model vs.experimentalresults for water–ethylene glycol,x F W=0.15g g −1,p =10kPa.F .Schaal et al./Chemical Engineering and Processing 47(2008)209–214213Fig.7.Model vs.experimental results for methanol–water,x F M=0.706g g −1,p =40kPa.a question of discretization,and the number 8was simply consid-ered here to be sufficiently large.Note that all stages are heated and the influence of the stage number on the separation is much smaller than in distillation.Figs.4–6show that the experimental separation efficiency depends on the evaporation rate.For very high or low evapora-tion rates (close to 1or 0,respectively),the process is determined by mass balances.The most interesting effects occur at evapo-ration rates close to the feed concentration of the low boiling substance.A comparison of experiments and simulations in that region shows that for high evaporation rates (0.8,cf.Fig.4),the separation is reasonably described by an open evaporation,whereas for low evaporation rates (0.15,cf.Fig.6)the closed evaporation model is more appropriate.This finding is not unex-pected as small evaporation rates correspond to small gas flow rates and,hence,to a longer contact between gas and liquid,resulting in a better separation.This effect has already been described by Ref.[3].4.2.Methanol–waterCompared to the water–ethylene glycol system,the sep-aration of methanol and water is more difficult.Hence,the difference between the open and closed evaporation is by far larger.As for the water–ethylene glycol system,onlymodelingFig.8.Model vs.experimental results for methanol–water,x F M=0.27g g −1,p =40kPa.Fig.9.Model vs.experimental results for methanol–water,x F M=0.188g g −1,p =20kPa.results for an eight-stage open evaporation and an eight-stage closed evaporation are shown,see Figs.7–9.As already seen for the water–ethylene glycol system,also for the methanol–water system the separation efficiency decreases with increasing evaporation rates.At high evaporation rates (0.706,cf.Fig.7),the experimental results are well described by the open evaporation model,at low evaporation rates (0.188,cf.Fig.9),the closed evaporation is the better model.Especially for practical purposes it is interesting to note that within the experimental accuracy,all results lie between the lines resulting from these two modeling concepts so that they can be used for defining the range in which experimental results are expected.It should also be noted that in contrast to the conclusions of Ref.[4,8,9],our results for the methanol–water system show a considerable rectification effect.5.ConclusionsExperimental results from the partial evaporation of the binary mixtures of ethylene glycol and water as well as water and methanol in a thin-film evaporator are reported.The results can be described using the simple equilibrium stage model.Modeling results only vary for evaporation rates close to the feed concentration of the low boiler.At low evaporation rates (0.1–0.2)a considerable rectification effect occurs that can only be modeled using the closed evaporation approach whereas at high evaporation rates (0.7–0.8)the open evaporation model is more appropriate.AcknowledgementThanks are due to Alexei Gortychov,Universit¨a t Stuttgart,for his contributions to the experimental part of this work.Appendix A.Nomenclature ER evaporation rate (g g −1)˙m mass flow (kg h −1)p pressure (Pa)x imass fraction of the component i in the liquid phase (g g −1)214F.Schaal et al./Chemical Engineering and Processing47(2008)209–214y i mass fraction of the component i in the gas phase(g g−1)SuperscriptsB bottomF feedH headSubscriptsi component iM methanolW waterReferences[1]B.Kaiser,M.Kranz,D¨u nnschichtverdampfer in der thermischen Trenntech-nik,Vakuum in 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