BATCH DISTILLATIONExperiment #1: To determine the pressure drop over the distillation columnfor various boil up rates.Theory:Batch distiationType of distillation where the entire batch ofliquid feed is placed into the stillat the start of the process and then heated. The vapor is collected and condensedto a distillate some of which is as reflux. The composition of the remainingliquid and the distillate are functions of time.Description o! the co"mn:The batch distillation column consists of a still at the base, which is chargedwithethanol - water mixtureat thestart oftheexperiment, aglasscolumnpacked with small glass cylinders, a condenser positioned above the column, arefluxvalve, aproductcoolerandacollectioncylinder HeatersThe roundbottomedflasksits in anelectric heater,which iscontrolledfrom the heatercontrol panel. The heatersare turned onand off usingthe switchesnext tothedials ontheheater control panel.There is no temperature measuring instruments connected to the heater controlso the dial will read zero.Thermoco"pe DispayThermocouplesareusedtomeasurethetemperatureat variouspointsinthecolumn. Thethermocouplesthat areconnectedtothedisplayhaveblueandwhite wires. They are numbered from 1-. !umbers 1-" measure temperaturesin the column, with " being the temperature of the top product vapor. !umbers#- measure the cooling water temperature at the inlet to the product cooler, theinlet to the condenser and the outlet of the condenser respectively. The dial isused to select which thermocouple outlet is displayed.#e!"x $a%e ControThe reflux ratio is controlled using a solenoid valve which has two positions$%pen - all the condensed liquid is returned to the column&losed - all the condensed liquid goes to the overhead productThe reflux ratio is determined by the ratio of the time the valve is open to thetime the valve is closed. 'f the valve is open for twice as long as it is closed thenthe reflux ratio is (. The dial on the reflux valve control box is used to set ratioof the time the valve is open to the time the valve is closed and therefore the numberson the dial represent the actual reflux ratio. )hen the dial is at zero the valve will be open allthe time. The switch on the control box turns the reflux valve on and off.Sampe $a%esThere are 1* sample valves. !umber 1 is a syringe which allows a sample to betaken from the round bottomed flask. !umbers ( -give samples of the liquidflow within the packing. !umber 1* samples the overhead liquid product.Types o! cross !o& traysThree basic types of cross flow trays used are +ieve ,late -,erforated ,late. /ubble &ap ,lates 0alve plates -floating cap plates.Operation at constant prod"ct composition:'n many instances processes are carried out in batches, and it is more convenientto distil each batch separately. 'n these cases the whole of a batch is run into theboiler of thestill and, onheating, thevapor is passedintoafractionationcolumn, as shown in 1igure 11.22. 3s with continuous distillation, thecomposition of the top product depends on the still composition, the number ofplates in the column and on the reflux ratio used. )hen the still is operating,since the top product will be relatively rich in the more volatile component, theliquid remaining in the still will become steadily weaker in this component. 3s aresult, thepurityofthetopproduct willsteadilyfall. Thus, thestill maybecharged with S1 mols of a mixture containing a mole fraction xs1 of the morevolatilecomponent. 'nitially, witharefluxratioR1, thetopproduct has acomposition xd1. 'f after a certain interval of time the composition of the topproduct starts to fall, then, if the reflux ratio is increased to a new value R(, itwill be possible to obtain the same composition at the top as before, althoughthe composition in the still is weakened to xs(. This method of operating a batchstill requiresacontinuousincreaseintherefluxratiotomaintainaconstantquality of the top product. 3n alternative method of operation is to work with aconstant reflux ratio and allow the composition of the top product to fall.%ne of the added merits of batch distillation is that more than one product maybe obtained.Thus, a binary mixture of alcohol and water may be distilled toobtain initially a high quality alcohol. 3s the composition in the still weakenswith respect to alcohol, a second product may be removed from the top with areduced concentration of alcohol. 'n this way it is possible to obtain not onlytwo differentqualityproducts,butalsoto reducethealcoholinthestilltoaminimumvalue.Thismethodofoperationisparticularlyuseful forhandlingsmall quantitiesofmulti-component organicmixtures, sinceit ispossibletoobtainthedifferent componentsat reasonabledegreesof purity,inturn. Toobtain the maximum recovery of a valuable component, the charge remaining inthe still after the first distillation may be added to the next batch.Operation at constant prod"ct compositionThe case of a column with four ideal plates used to separate a mixture of ethylalcohol and water may be considered. 'nitially there are S1moles of liquor ofmole fraction xs1with respect to the more volatile component, alcohol, in thestill.Thetopproductistocontainamolefractionxd,andthisnecessitatesareflux ratio R1. 'f the distillation is to be continued until there are S( moles in thestill, of mole fraction xs(, then, for the same number of plates the reflux ratiowill have been increased to R(. 'f the amount of product obtained is Dbmoles,then a material balance gives4S1xs1 5 S(xs( 6 Db xdS1 5 S( 6 DbS1xs1 5 (S1 5 Db)xs(6 Db xdS1xs1 5 S1xs( 6 Dbxd5 Dbxs(Db6 S17xs1 5 xs(8xd5 xs(96 -a/b)S1)here a and b are as shown in 1igure. 'f is the intercept on the Y -axis for anyoperating line, equation 11.:;, then4xd/(R < 1. 6 , or R 6 - xd/)5 1These equations enable the final reflux ratio to be determined for any desiredend concentration in the still, and they also give the total quantity of distillateobtained. )hat is important, in comparing the operation at constant reflux ratiowith that at constant product composition, is the difference in the total amountofsteamusedinthedistillation, foragivenquantityofproduct,Db. 'fthereflux ratio R is assumed to be ad=usted continuously to keep the top product atconstant quality, then at any moment the reflux ratio is given by R 6 dLb/dDb.>uringthe course of the distillation, the total reflux liquor flowing down the column isgiven by40LbdLb=R 1R 2RdDbTo provide the reflux dLb the removal of a quantity of heat equal to dLb in thecondenser is required, where is the latent heat per mole. Thus, the heat to besupplied in the boiler to provide this reflux during the total distillationQR isgiven by4Q=0LbdLb=R1R2RdDbThis equation may be integrated graphically if the relation between R and Dbisknown. 1or any desired value ofR,xsmay be obtainedby drawing theoperating line, and marking off the steps corresponding to the given number ofstages.The amount of productDbis then obtained from equation and, if thecorresponding values of R and Db are plotted, graphical integration will give thevalue of R dDb. The minimum reflux ratio Rm may be found for any given stillconcentration xs from equation 11.?".Operation at constant re!"x ratio'f the same column is operated at a constant reflux ratio R, the concentration ofthe more volatile component in the top product will continuously fall. %ver asmall interval of time dt, the top-product composition with respect to the morevolatile component will change from xdto xd< dxd, where dxdis negative for themore volatile component. 'f in this time the amount of product obtained is dDb,then a material balance on the more volatile component gives4@ore volatile component removed in product 6 dDb [xd< dxd/( 9which, neglecting second-order terms, gives4 6 xddDband4 xddDb6 5d(Sxs )/ut dDb6 5dS,and hence4 5xddS6 5S dxs5 xsdSand4 S dxs6 dS(xd5 xs)Thus4S1S2dSS =xs1xs2dxsxdxsThe right-hand side of this equation may be integrated by plotting 1/(xd5 xs)against xs. This enables the ratio of the initial to final quantity in the still to befound for any desired change in xs, and hence the amount of distillate Db. Theheat to be supplied to provide the reflux is QR 6 RDb and hence the reboil heatrequired per mole of product may be compared with that from the first method.Limitin' Conditions ( #e!"x #atiosThe external reflux ratio, A8>, is often a parameter that is specified for columnoperation since it is perhaps the easiest to change under column operation andchanging A8> often has a substantial effect on the overall column behavior andseparation.There are two limiting conditions with respect to the reflux ratio, A8>, that oneobtains4Tota re!"x ratio, > 6 *, A80 6 1, and A8> B )inim"m re!"x ratio, -A8>.min. %ne will often see an external reflux specification as a multiple of the minimumreflux ratio, for example, A8> 6 (-A8>.min, etc.3columnwhichhas total refluxandboil-uphas nofeedCthe distillate,bottoms, and feed flow rates are zero.This is a limiting condition which provides one with the minimum number ofequilibrium stages that can be obtained for a given column.)hile one would not typically operate under such conditions, total reflux andboilup are used for starting up a column, for operating the column off-line whenother unit operations are off-line, and for testing column efficiency.1.* 0 8 A0 A1.* A80A8> D=== = =3t total reflux4/oth operating lines, T%A and /%A, become the y 6 x auxiliary line.%perating a column under total reflux yields the minimumnumber ofequilibrium stages for a particular column.Total Deflux C@inimum !umber of Equilibrium +tages)inim"m #e!"x@inimum reflux, Dmin or -A8>.min, is defined as the external reflux ratio at whichthe specified separation could =ust be obtained with an infinite number of stages.)e obtain an infinite number of stages if our operating lines touch theequilibrium curve.The point on the equilibrium curve that this occurs is called a FpinchG point.T&o Types o! *inch *oints1or many systems, the pinch point will occur where the feed line crosses theequilibrium curve.3secondtype of pinchpoint canoccur inFnon-idealG systemwhere theoperating line touches the equilibrium curve before the intersection of the feedline and the equilibrium curve.@inimum Deflux C'nfinite !umber of Equilibrium +tages#"e o! Th"m+The best operating condition lies between minimum and total reflux.3s a ruleof thumb, theoptimumrefluxratiolies between1.*?and1.(?-A8>.min.Too low of an A8> results in a large number of equilibrium stages, while toohigh of an A8> increases the re-boiler duty as well as the required diameter ofthe column.,ACTO#S A,,ECTIN- DISTILLATION COL.)NO*E#ATIONTheperformanceof adistillationcolumnisdeterminedbymanyfactors, for example4feed conditionsstate of feedcomposition of feedtrace elements that can severely affect the 0AE of liquidmixturesinternal liquid and fluid flow conditionsstate of trays -packings.weather conditions+ome of these will be discussedbelowto give anidea of thecomplexity of the distillation process.,eed Conditions The state of the feed mixture and feed composition affects the operating linesand hence the number of stages required for separation. 't also affects thelocation of feed tray. >uring operation, if the deviations fromdesignspecifications are excessive, then the column may no longer be able handletheseparationtask. Toovercometheproblems associatedwiththefeed,somecolumnaredesignedtohavemultiplefeedpointswhenthefeedisexpected to containing varying amounts of components. Deflux &onditions 3s the reflux ratio is increased, the gradient of operatingline for therectification section moves towards a maximum value of 1. ,hysically, whatthismeansisthat moreandmoreliquidthat isrichinthemorevolatilecomponents are being recycled back into the column. +eparation thenbecomes better and thus less trays are needed to achieve the same degree ofseparation. @inimumtraysarerequiredunder total refluxconditions, i.e.there is no withdrawal of distillate.%n the other hand, as reflux is decreased, the operating line for therectification section moves towards the equilibriumline. The HpinchIbetweenoperatingandequilibriumlines becomes more pronouncedandmore and more trays are required. This is easy to verify using the @c&abe-Thiele method.The limiting conditionoccurs at minimumreflux ration, whenan infinite number oftrays will be required to effectseparation. @ost columns are designed tooperate between 1.( to 1.? times theminimum reflux ratio because this isapproximately the region of minimumoperatingcosts -morerefluxmeans higherre-boiler duty..0apor 1low &onditions3dverse vapor flow conditions can cause foaming entrainment weeping8dumping flooding,oamin'1oaming refers to the expansion of liquid due to passage of vapor or gas. 3lthough itprovideshighinterfacial liquid-vaporcontact, excessivefoamingoftenleadstoliquidbuildup on trays. 'n some cases, foaming may be so bad that the foam mixes with liquidon the tray above. )hether foaming will occur depends primarily on physical propertiesof the liquid mixtures, but is sometimes due to tray designs and condition. )hatever thecause, separation efficiency is always reduced.EntrainmentEntrainment refers to the liquid carried by vapor up to the tray above and is again causedbyhighvapor flowrates. 't isdetrimental becausetrayefficiencyisreduced4 lowervolatilematerial iscarriedtoaplateholdingliquidofhighervolatility.'t couldalsocontaminate high purity distillate. Excessive entrainment can lead to flooding./eepin'0D"mpin'Thisphenomenoniscausedbylowvaporflow.Thepressureexertedbythevaporisinsufficient toholduptheliquidonthetray.Therefore, liquidstartstoleakthroughperforations. Excessive weeping will lead to dumping. That is the liquid on all trays willcrash -dump. through to the base of the column -via a domino effect. and the column willhave to be re-started. )eeping is indicated by a sharp pressure drop in the column andreduced separation efficiency.,oodin'1loodingisbrought about byexcessivevapor flow,causingliquidtobeentrained in the vapor up the column. The increased pressure from excessivevaporalsobacksuptheliquidinthedown-comer,causinganincreaseinliquid holdup on the plate above.>epending on the degree of flooding, themaximumcapacityof thecolumnmaybeseverelyreduced. 1loodingisdetectedby sharp increasesin columndifferentialpressureand significantdecrease in separation efficiency.&olumn >iameter@ost of the above factors that affect column operation are due to vapor flowconditions4 either excessive or too low. 0apor flow velocity is dependent oncolumndiameter. )eepingdeterminestheminimumvapor flowrequiredwhile flooding determines the maximum vapor flow allowed, hence columncapacity. Thus, if the column diameter is not sized properly, the column willnot performwell. !ot onlywill operational problems occur, thedesiredseparation duties may not be achieved.+tate of Trays and ,ackingDemember that the actual number of trays required for a particularseparation duty is determined by the efficiency of the plate, and the packingif packing are used. Thus, any factors that cause a decrease in tray efficiencywill also change the performance of the column. Tray efficiencies areaffected by fouling, wear and tear and corrosion, and the rates at which theseoccur depends on the properties of the liquids being processed. Thusappropriate materials should be specified for tray construction.)eather &onditions@ost distillation columns are open to the atmosphere. 3lthough many of thecolumns are insulated, changing weather conditions can still affect columnoperation. Thus there-boiler must be appropriatelysizedtoensurethatenough vapor can be generated during cold and windy spells and that it canbe turned down sufficiently during hot seasons. The same applies tocondensers.These are some of the more important factors that can cause poor distillationcolumn performance. %ther factors include changing operating conditions andthroughputs, brought about by changes in upstream conditions and changes inthe demand for the products. 3ll these factors, including the associated controlsystem, should be considered at the design stages because once a column isbuilt and installed, nothing much can be done to rectify the situation withoutincurring significant costs. The control of distillation columns is a field in itsown right, but thatJs another story.Ad%anta'es 1lexible 3ccurate implementation of recipe specific to a given mixture +everal components separated using one column Dequires least amount of capital*roced"re:/efore starting make sure that all valves on the distillation column are closed.%pen valve 01* on the reflux pipe. 1ill the boiler with 1* A of the solution.Turn on the power from the control panel. +elect the temperature to T on thepanel. open the valve 0? to enter the cold water to the condenser on the flowrate of 2A8min. ad=ust the heater power to *.#? k). The heating of the feed willstart andtemperaturewill riseoneachplate. %penthevalve0"and0#tomeasure the pressure drop. The 1stand ?thplates are not insulated to observedistillation. Eventually vapor will rise and will enter the condenser which willbe observed entering the receiver. The distillate will overflow the receiver andwill return to the column by reflux valve because column is operating at totalreflux. Then liquid will reach each plate and vapor will pass through the liquid.Eventuallyequilibriumwill beachievedwhenall platetemperaturebecomeconstant. Then take the samples and note the boil up rates. Then measure thepressure across the column using manometer and valve 0" and 0# note downthe The pressure and boil up rate value. Depeat the process and note down thevalues and build a graph showing the relationship of boil up rate and pressuredrop.O+ser%ations:#es"ts: 3s the boil up rate increases the vapor flow rate from still to the column also increases so pressure drop also increases.