The state of fluids in pipelines, and the physical arrangementof equiprnent around the distillation tower, establish the designparameters for Ineeting hydraulic and piping conditions of the system.Robert Kern, Hoffmann - La Roche Inc.*.,The required distance, H, between fractionator inletand exchanger centerline can be calculated from Eg.(1) as:(3)(1)(2)The vapor column can be neglected by assumingP2 =0 in Eq. (1) and (3). All pressures are in psi;densities, P, in Ib/ft3; and dimensions, H, in ft.As Eq. (3) shows, for a minimum of elevation differ-ence between the top of the column and the exchangercenterline, the piping and components resistances mustalso be minimal.M s must be equal to or greater than the sum of (1)the pipe-system resistance, tipp , between referencepoints A and B; (a) exchanger pressure drop, tipe; and(3) required pressure difference across the control valve,tipcv:Gravity-flow reflll.xHorizontal condensers-A condenser in"gravity-flow ar-rangements is located above the level of the terminatingpoint of the condenser's outlet line, as shown in FIIand F12. For the horizontal condenser in FlIb, vaporenters the exchanger at the top, and subcooled liquidleaves at the bottom. The looped-outlet pipe ensuresa permanent liquid level in the condenser. This liquidlevel is controlled through the reflux branch andthrough the takeoff line to storage.The static-head pressure difference, tiPs, between thevertical overhead line and the condenser's outlet linefor the arrangements in FII can be written as:o The state of fluid in the pipelines, and the hydraulicand thermal conditions in condensing systems of dis-tillation columns, are the reverse of those in reboilercircuits.The inlet line to condensers can carry superheatedor saturated vapor, or dispersed vapor-liquid mixtures.Fluid is cooled in the exchanger, and partial or fullcondensation takes place. The condenser's outlet linecan have stratified and dispersed two-phase flow, satu-rated liquid, or subcoolcd liquid. In addition, the flow"ing fluid can be a mixture of two substances. Thus, thistype of condensation offers a wide range of classificationfrom a thermodynamic standpoint [1].In contrast, saturated liquid normally flows in thedowncomer of reboilcrs. The liquid is vaporized whilepassing through the exchanger. The reboiler's outletline carries a dispersed vapor-liquid mixture having avapor content of 30 to 90% of total flow. (See Part 9of this series for more details, Chern. Eng., Aug. 4, 1975,p. 107.)In this article, we \vill examine the hydraulics forthe following:1. Condensers with gravity-flow return lines.2. Condensers with pumped-reflux lines.3. Two-stage condensation.Within these groups, hydraulic-design and plpmg-design conditions vary, depending on the state of fluidin the lines and the physical arrangement of the instal-lation.Vacuum technology has its own systems,equipmentand terminology. Piping design of vacuum-condensingsystems are outside the Euler-Bernoulli-Darcy theoriesand are not included in this article."For biography of author, see Ch,m. Eng., Aug. 4, 1975. p. 113.I~II,129CHEMICAL ENGINEERING SEPTEMBER 15. 19751

J!,1i]i1IIj!-i, '2)Vertical condensers with gravity-flow reflux condense vapors on the sheilside or tuheside of exchanger e;::,'Q : To Point 8-DrainGenerally, in condensing systems, the unit loss in thepiping is low-about a tenth or a hundredth of 1psi/lOO ft. Inlet and outlet resistances to process equip-ment usually take a considerable portion of the pipelineresistance and should not be ignored in the calculations.(Using three decimal places in the calculations is notunusual.)In horizontal condensers, condensation takes place inthe sheIl. This gives lower resistance than the tubeside.A baffle (or baffles) in the exchanger is in the horizontalplane through the exchanger's centerline. If necessary,two inlet and two outlet nozzles can halv.e the total flow,and reduce entrance and exit resistances considerably.In this case, the inlet and outlet piping should be sym-metrical.The subcooled liquid for the arrangement in F/1bcan be drained or pumped directly to storage. Theproduct stream for F/la is usually directed through acooler before storage.a.Saturated liquid {sheiiside condensation}Horizontal condensers with gravity-flow reflux VaporVaporII130CHEMICAL ENGINEERING 15. IY75.-----------._._.._----------------------------------,\/entSeal loops prevent a reversed flow of vapor in gravity-flow condensing systemswhere PI is usually vapor density, and P2 is vapor-liquidFor the dimensions given in F/5, the static-headdifference will be:iIIIt\(9)(8)leg (Zz dimension in F/3), the piping design should besuch as to prevent siphoning that can empty the seal.If dimension ZI is smaller than Z2' and the pressurejust before the seal loop and at the terminating pointafter the seal loop is identical (for example, with greatlyreduced flow), liquid can be siphoned out of the seal;and intermittently, the condenser will not operate well.This can be prevented if ZI is designed to be longerthan Zz (see F/3a).For the arrangement in F/3b, the final vertical leghas a larger diameter than the gravity-flow reflux line.Again, this can prevent the siphoning of liquid fromthe seal loop.Another arrangement (F/3c) has a closed vent line.This can be opened at reduced condensate flow to keepthe seal loop filled with liquid. With this type of vent-ing, the pressure difference across the vent valve shouldbe zero. Therefore, it is essential to connect the end-points of the vent line to locations where pressures areexpected to be about equal.Pumped-reflux arrangementsTypical overhead lines for hydrocarbon distillationcolumns are shown in F/4. Fluid circulation in thepiping is the result of the thermosiphon effect in grav-ity-reflux condensation. For the systems shown in F/4,there is (and most of the time must be) a pressuredifference between the top of the tower and refluxdrum. The reflux drum has a pump, which returns theliquid to the top of the tower or sends it to storage.In _these arrangements (besides the sum of the staticheads), actual pressure differences, 6.Pp , also enter intothe calculations:(7)Control valves in these svstems should be located ata low point of the r('tun; line and product stream.Sufficient static head before the valve inlet will preventvaporization across the valve. A product cooler shouldnot receive a liquid-vapor mixture.Vertical condensers-Arrangements for these condenserswith gravity-flow outlets are shown in F/2. Conden-sation can take place in the shell (F/2a), or tubes(F/2b). A single-pass vertical condenser is more suitablefor liquid subcooling than a horizontal one. The seal-loop height can be adjusted within a greater range thanwith horizontal condensers (F/2c). The required liquidlevel in the exchanger shell is determined by the ex-changer's designer.The hydraulic balance for the arrangements shownin F/2 is:(1/144)(HIPI + H2i'2) > (1/144)H3P3 + 6.P (4)HI > (l/PI)(f!;Ii'3 - H2P2 + 1446.P) (6)where PI is the density of condensate in the reflux line,P3 is the vapor density in the overhead line, and P2 isthe average density in the vertical exchanger:Seal loop _preve_nts flow reversalIn gravity-flow condensing systems, a seal loop is-provided to prevent a re"'ersed flow of vapor in thecondenser's outlet line. This loop can be used for hold-ing a liquid level in the condenser, as shown in F/laand F/2c.If the gravity-flow reflux line terminates in a vertical...here 6.P is the sum of piping, 6.Pp , exchanger, 6.Pe'and control-valve (if any), 6.Pcv' resistances:6.P =6.Pp + t:..Pe + 6.pcv (5)The elevation difference, as expressed from Eq. (4),between the condenser's outlet and the reflux inlet noz-zle will be:131Cm;II11CAL ENGINEEIUNG SEPTEMBER jj. 197jiOverhead lineIII!I

y,'jRefluxdrum Thesimp!cst overheadc', line gh:es thesmaUest pipe SiZ9.b. Subcooied liquidin theg:"avit"/-frovoJclitiet !ine.Overhead condensers for distillation columns 'Dimensional relations for a condenser at grademixture (or liquid) density. The overall available D.Pis the sum of Eg. (8) and (9), or:tiP =!:J.Ps + tiPp . (10)If the righthand side of Eg. (10) becomes negative,the condensc'!" 1l111St be placed at an elevation closer to,or above, tlte reilllx drum. A negative value indicatesthat the static-head backpressure (pzHz) in Eg. (9) isgreater titan tlte sum of (1) pressure difference betweenthe top of the (ower and the reflux drum plus (2) thevapor static !Jc;ld ((,;Ill) in the overhead line. Thegreater the condC'!Is,JtiOll, the heavier the mixture be- in the condcnser's outlet line which results in, ,a grealfT bad.plc,sure. Of course, pzHz becomes posi-tive when the li!!e has a gravity-flow arrangement be-tween the C()Iltlc-ll't'! oudet and the reflux-drum inlet,as shown ill F .Ja and F/4b.Eq. (Ill) slHm's the driving force in the overheadsystem. This must bc' greater than the sum of piping,inlet and exi ( lus'c's, 0.l'P' and exchanger resistance, tiPe:0.P >tipp + tiPe (11)!:J.Pl' lISlIallv is rWlween 2 to 6 psi, and condenser tipe froIll 0.:) to 5 psi.The IllaxiIlllIlll possible condenser-centerline locationbelov; tlte drum (dimension Hz in F15) can becalclilakd llOlll th.. combination of Eg. (9), (10) and(II), to give:(1/144)(p/ll - p}J2 ) +tiPp '- (tipp + Upe) = 0 (12)As a safety factor, the positive static-head column pres-132.__ ...' SEPTEMBER 1\ I!Ur, P,I!!iIILIPI\P1H1 = o.Expressing H2 from Eq. (12) in feet, we get:H2 =:: (144/P2)(Mp - D.pp ~ D.Pe) (13)In layout design, usually the reflux drum is elevatedfirst in accordance with the required NPSH (net posi-tive suction head) of the reflux pump. The dimensionsshown in Detail A of F/5 will establish the condenserelevation from grade.Elevated condensers and details of reflux drumSlug flow is undesirableSlug flow can develop in the pocketed condenser-outlet line shown in F/5, depending on vapor-liquidproportion and fluid velocity. Slug flow should beavoided because it can cause undesirable pressuresurges.An empirical relation can be used to estimate theslug-flow region. If the velocity (calculated with two-phase density) in the pipeline is smaller than (5Pl/Pv)1/2, slug flow is possible. The type of flow can alsosure of the overhead vapor line' can be neglected. Con-sider:Froth flowo\J53. Liquid and vapor velocities determine type of flow133CHEMICAL ENfiINEERING SEPTEMBER IS, 1975Detai! CWestern Supply Co.,Ii42jn dia.

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