Chapter 4: Column Distillation: Internal Stage-by-Stage Balances In Chapter 3 (Introduction to Column Distillation), we performed the external balances or the balances around the distillation column To obtain the answers for both design and specification problems, just doing the external balances is NOT enough, and, therefore, the internal stage-by-stage balances are required We can perform the internal stage-by-stage balances from either the top (i.e. a condenser) or the bottom (i.e. a re-boiler) of the column 1 In the enriching (rectifying) section (i.e. the section above the feed stage), it is convenient to perform the balances for the area that goes around the desired stage and the condenser For stages 1, 2, and j, the balance envelopes (boundaries) can be drawn as follows Stage 1 QC D V1 Lo 1 V2 L1 Figure 4.1: An internal balance around stage 1 2 Stage 2 QC D V1 Lo 1 2 V3 L2 Figure 4.2: An internal balance for stages 1 and 2 Note that, in each stage, the vapour and the liquid phases are assumed to be in equilibrium with each other Thus, in each stage, the temperature and pressure can be assumed to be constant 3 Stage j QC D V1 Lo 1 2 j Vj+1 Lj Figure 4.3: An internal balance for stages 1, 2, 3,..., and j The balances for stage 1 (Figure 4.1) can be written as follows Overall balance V2 = L1 + D 4 (4.1) Species balance (for a more volatile species) y2V2 = x 1L1 + x D D (4.2) Energy balance H 2V2 + QC = h1L1 + hD D (4.3) Note that, from now on, to avoid any confusion, an enthalpy of the vapour phase is denoted as upper-case (capital letter) H, while the enthalpy of the liquid phase is still denoted as lower-case h It is also noteworthy that, in most of the distillation problems, the pressure (P ) of the system or the column is normally specified; thus the corresponding equilibrium data or curve can be obtained 5 If it is assumed that the vapour phase is a saturated vapour and that the liquid phase is a saturated liquid, we obtain the facts that and h1 = h1 (x 1 ) (4.4a) H 2 = H 2 (y2 ) (4.4b) Additionally, as it is assumed that the liquid and the vapour phases are in equilibrium with each other, the mole fraction of the vapour stream leaving stage 1 (y1 ) is proportional to that of the liquid stream leaving the same stage (x ) as follows 1 or x 1 = x 1 (y1 ) (4.4c) y1 = y1 (x 1 ) (4.4d) 6 For stage 2, the balance equations are as follows V3 = L2 + D (4.5) y 3V3 = x 2L2 + x D D (4.6) H 3V3 + QC = h2L2 + hD D (4.7) h2 = h2 (x 2 ) (4.8a) H 3 = H 3 (y 3 ) (4.8b) x 2 = x 2 (y2 ) (4.8c) Likewise, for stage j, the balance equations can generally be written as follows Vj +1 = Lj + D (4.9) y j +1Vj +1 = x j Lj + x D D (4.10) H j +1Vj +1 + QC = h j Lj + hD D (4.11) h j = h j (x j ) (4.12a) 7 H j +1 = H j +1 (y j +1 ) (4.12b) x j = x j (y j ) (4.12c) The known values (which were obtained previously from the external column balances – see the details in Chapter 3) are y j , D, QC , and hD Thus, there are 6 unknowns; i.e. Lj , Vj 1 , x j , y j 1 , H j 1 , and h j Since we have 6 equations, we can solve for these 6 unknowns 8 For the stripping section or the section below the feed stage, the balances for the stage just below the feed plate (stage) or stage f+1 can be depicted as follows (Figure 4.4) f Vf +1 F, hF, z Lf f+1 N VN +1 QR B, hB, xB Figure 4.4: The internal balances for the stage just below the feeding stage (stage f + 1) 9 From Figure 4.4, the balance equations can be written as follows Lf = Vf +1 + B (note that, to distinguish between the flow rate above and below the feed stage, a bar is added to the flow rates of the liquid and the vapour phases below the feed stage) or Vf +1 = Lf - B (4.13) y f +1Vf +1 = x f Lf - x B B (4.14) h f Lf + QR = H f +1Vf +1 + hB B or H f +1Vf +1 = h f Lf - hB B + QR 10 (4.15) and h f = h f (x f ) (4.16a) H f +1 = H f +1 (y f +1 ) (4.16b) x f = x f (y f ) (4.16c) Again, there are 6 unknowns, with 6 equations available; thus, the problem can be solved For stage k, which is the stage below the feed stage, the balances are carried out from the reboiler to the desired stage (i.e. stage k), which can be illustrated in Figure 4.5 11 f F k Vk Lk -1 N VN +1 QR B, hB, xB Figure 4.5: The internal balances for a stage below the feed stage (e.g., stage k) The balance equations for Figure 4.5 or the balance equations for a stage below the feed stage can be formulated as follows Vk = Lk -1 - B (4.17) ykVk = x k -1Lk -1 - x B B (4.18) H kVk = hk -1Lk -1 - hB B + QR (4.19) 12 hk -1 = hk -1 (x k -1 ) (4.20a) H k = H k (yk ) (4.20b) x k -1 = x k -1 (yk -1 ) (4.20c) 4.1 Stage-by-Stage Solution Methods for a Binary Mixture As the stage-by-stage balances are rather complicated, before solving for the unknowns, the following assumptions are made: 1) The column is adiabatic 2) The enthalpy changes between the adjacent stages are small, compared to the latent heat changes (i.e. the changes in phase between the vapour and the liquid phases); i.e. 13 H j +1 - H j l and h j +1 - h j l 3) The latent heat of vaporisation (or condensation) per mole, l , is relatively constant throughout the column By making the above assumptions (especially # 3), it yields the following facts: L1 » L2 » L3 .... » Lj ... » Lf -1 » L (4.21) V1 » V2 » V3 .... » Vj ... » Vf -1 » V (4.22) Lf » Lf +1... » Lk -1 » Lk ... » LN = L (4.23) Vf » Vf +1... » Vk -1 » Vk ... » VN » V (4.24) 14 However, it is important to note that L¹L and that V ¹V This technique is developed by W.K. Lewis (1922) and called a constant molal overflow (CMO) For stage j in the enriching section, or Vj +1 = Lj + D (4.9) D = Vj +1 - Lj (4.25) y j +1Vj +1 = x j Lj + x D D (4.10) Combining Eq. 4.10 with Eq. 4.25 yields y j +1Vj +1 = x j Lj + x D (Vj +1 - Lj ) (4.26) 15 Re-arranging Eq. 4.26 for y j +1 gives y j +1 æ ö L ç j ÷ ÷÷ x ç x j + ç1 = Vj +1 çè Vj +1 ÷÷ø D Lj (4.27) but, from Eqs. 4.21 and 4.22, for the enriching section, L and V are constant Hence, Eq. 4.27 becomes y j +1 æ ö L L = x j + çç1 - ÷÷÷ x D çè V ÷ø V (4.28) Eq. 4.28 is the operating equation for the enriching section By doing the same for the stripping section, the following equation is obtained 16 æL ö L ç yk = x k -1 - ç - 1÷÷÷ x B çèV ÷ø V (4.29) Eq. 4.29 is the operating equation for the stripping section For the feed stage, the balances can be performed as follows (see the balance boundary in Figure 4.6) V, Hf, yf F, hF, z L, hf-1, xf-1 f V , Hf+1, yf+1 L , hf, xf Figure 4.6: The balances around the feed stage 17 F +V + L = L +V (4.30) hF F + H f +1V + h f -1L = h f L + H fV (4.31) but, from the assumption # 2, H f +1 » H f and h f » h f -1 Thus, Eq. 4.31 becomes hF F + HV + hL » hL + HV hF F + HV - HV » hL - hL and h f F + (V -V ) H » (L - L ) h (4.32) Re-arranging Eq. 4.30 yields V -V = L - L - F 18 (4.33) Substituting Eq. 4.33 into Eq. 4.32 gives h f F + H (L - L - F ) » h (L - L ) (4.34) Re-arranging Eq. 4.34 results in h f F + H (L - L ) - HF » h (L - L ) and (H - h )(L - L ) » (H - h ) F F (4.35) which can be re-arranged once again to L - L H - hF » F H -h (4.36a) Note that L - L is the amount of the liquid phase that goes down from the enriching section into the stripping section 19 L -L Hence, the ratio is essentially q in the F flash distillation (i.e. the fraction of the feed that becomes liquid and thus goes down to the bottom part of the tank); accordingly, Eq. 4.36a can also be written as L - L H - hF q= » F H -h which can be in a narrative form as (4.36b) é Liquid flow rate ù é Liquid flow rate ù ê ú-ê ú ê below the feed stageú êabove the feed stageú ê úû êë úû q=ë Feed flow rate (4.36c) or é Enthalpy of the vapourù ê ú - é Enthalpy of the feedù úû ê phase at the feed stage ú êë ê úû q= ë é Enthalpy of the vapourù é Enthalpy of the liquid ù ê ú-ê ú ê phase at the feed stage ú ê phase at the feed stageú êë úû êë úû (4.36d) 20 Re-arranging Eq. 4.36b: L -L q= F gives L - L = qF (4.37a) L = L + qF (4.37b) or Combining Eq. 4.37a with Eq. 4.33: V -V = (L - L ) - F (4.33) and re-arranging the resulting equation yields V -V = qF - F V = V - (1 - q ) F (4.38) The use of the CMO technique can be illustrated in the following Example 21 Example A steady-state, counter-current, staged distillation column is used to separate ethanol (EtOH) from water The feed is 30 wt% ethanol and is a saturated liquid The mass flow rate of the feed is 10,000 kg/h, and the column is operated at the atmospheric pressure (i.e. 1 atm) The reflux is assumed to be a saturated liquid and with the reflux ratio of 3.0 If the desired x B and x D are 0.05 (mass fraction of EtOH) and 0.80 (mass fraction of EtOH), respectively, use the Lewis method to find the number of stages required; given that the feed is fed on the 2nd stage from the top 22 The problem statement can be depicted as follows QC D, 80 wt% H1, y1,V1 EtOH 1 10,000 kg/h V2 (sat. liq.) L1 2 30 wt% EtOH (sat. liq.) Lo, xo V3 L3 N VN +1 QR B, 5 wt% EtOH From the Figure above, for EtOH, z F = 0.30 x D = xo = y1 = 0.80 x B = 0.05 23 In order to utilise the Lewis method, all the assumptions (see Pages 13-14) must be satisfied, and the most important one is the assumption # 3 [i.e. the heat of vaporisation (or condensation) per mole is relatively constant] Since we are dealing with 2 species, and the composition of the mixture varies from stage to stage, in order for the heat of vaporisation (or condensation) per mole to be relatively constant, the heats of vaporisation (or condensation) per mole of both components (i.e. EtOH and water in this Example) should be relatively close to each other 24 The latent heats of vaporisation of EtOH = 9.2 kcal/mol water = 9.7 kcal/mol which are relatively close to each other Thus, in this Example, it is safe to assume that the heat of vaporisation/condensation per mole of the mixture is nearly constant Accordingly, the CMO (constant molal overflow) technique (or the Lewis method) can be employed to solve this Example Hence, we have to deal with this Example on “mole” basis 25 Thus, all the given data, which are on “mass” basis, must be converted to “mole” basis Note that the molecular weights (MW) of EtOH » 46 water » 18 The mole fraction of EtOH of each stream can be computed as follows (note that the mole fraction is denoted with bar) 0.30 46 zF = = 0.144 0.30 0.70 + 46 18 0.80 46 xD = = 0.61 0.80 0.20 + 46 18 26 0.05 46 xB = = 0.02 0.05 0.95 + 46 18 The feed flow rate is 10,000 kg/h, with the concentration of EtOH of 30 wt% Thus, the mass flow rate of each component in the feed is æ 30 ö EtOH = çç ÷÷÷ (10, 000) = 3, 000 kg/h çè100 ÷ø æ 70 ö water = çç ÷÷÷ (10, 000) = 7, 000 kg/h çè100 ÷ø which can be converted to the molar flow rate as follows 27 3, 000 EtOH: = 65.2 kmol/h 46 7, 000 water: = 388.9 kmol/h 18 Hence, in total, the molar flow rate of the feed is 65.2 + 388.9 = 454.1 kmol/h Since there is no reaction in the distillation column, we can perform a mole balance in the similar manner as we do for the mass balance, as follows Overall mole balance F = D +B 454.1 = D + B 28 (4.39) EtOH mole balance zF F = xDD + xBB (0.144)(454.1) = (0.61) D + (0.02) B (4.40) Solving Eqs. 4.39 and 4.40 simultaneously for D and B yields D = 95.3 kmol/h B = 358.8 kmol/h æL ö It is given that the reflux ratio ççç o ÷÷÷ is 3.0 çè D ÷ø Thus, æL ö Lo = ççç o ÷÷÷ ´ D çè D ÷ø Lo = (3.0)(95.3) Lo = 285.9 kmol/h 29 (4.41) Since the CMO technique is employed (Eq. 4.21), Lo » L1 » ...Lf -1 = L = 285.9 kmol/h By performing a mole balance around the condenser, we obtain V1 = Lo + D (4.42) V1 = 285.9 + 95.3 = 381.2 kmol/h Once again, since the CMO technique is used (Eq. 4.22), V1 » V2 » ...Vf = V = 381.2 kmol/h Accordingly, L 285.9 (for the rectifying section) = 381.2 V = 0.75 30 Consider the feed stage (i.e. the 2nd stage) Since the feed is a saturated liquid, it does not add any flow rate to the vapour phase (all of the feed is added to the liquid phase) Thus, L2 = L1 + F (4.43) L2 = 285.9 + 454.1 = 740.0 kmol/h With the CMO technique, we obtain the fact that (Eq. 4.23) L2 » L3 » L4 » ...LN = L = 740.0 kmol/h 31 Again, by applying the CMO technique, the flow rate of the vapour phase below the feed stage is constant: i.e. (Eq. 4.24) V2 = V3 » V4 » ...VN +1 = V = 381.2 kmol/h Hence, L 740.0 (for the stripping section) = V 381.2 = 1.94 In order to perform the stage-by-stage calculations, the equilibrium data [between liquid phase (x ) and vapour phase (y )] is needed 32 Since the operating pressure is at 1 atm, we can use the following equilibrium data (curve) for the EtOH-water mixture (note that this is an x-y plot for EtOH) 1.0 yEtOH 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 xEtOH Let’s start from the top of the column, where y1 = x D = x o = 0.61 33 Since the equilibrium is assumed, the mole fractions of the liquid phase (x n ) and the vapour phase (yn ) leaving any stage n are in equilibrium with each other; for example, x 1 is in equilibrium with y1 Reading from the equilibrium curve, when y1 = 0.61, we obtain x 1 = 0.40 Then, the value of y2 can be calculated using Eq. 4.28: y j +1 æ L L ö÷ ç = x j + ç1 - ÷÷ x D çè V ÷ø V (4.28) as follows y2 = éê(0.75)(0.40)ùú + éê(1 - 0.75)(0.61)ùú = 0.45 ë û ë û 34 Once again, as x 2 is in equilibrium with y2 , the value of x 2 can be read from the equilibrium curve when y2 is known From the equilibrium curve, when y2 = 0.45 , we obtain x 2 = 0.11 Since stage 2 is the feed stage, to compute the value of y 3 , the mole fraction of the vapour phase leaving stage 3, which is the stage below the feed stage (i.e. the stripping section), Eq. 4.29 (the operating equation for the stripping section): æL ö L yk = x k -1 - çç - 1÷÷÷ x B çèV ÷ø V 35 (4.29) must be used as follows y 3 = éê(1.94)(0.11)ùú - éê(1.94 - 1)(0.02)ùú ë û ë û y 3 = 0.195 » 0.20 Once again, from the equilibrium curve, when y 3 = 0.20 , the value of the corresponding x 3 is x 3 = 0.02 Since x 3 = x B = 0.02 , our calculations are now finished This means that this column has 3 stages Actually, it has only 2 stages plus with a re- boiler, which can be considered as an additional stage 36 4.2 McCabe-Thiele Method From the Lewis (CMO) technique illustrated in the recent Example, the stage-by-stage solution involves the equilibrium curve and the operating lines Note that there are 2 operating lines: The operating line for enriching section (Eq. 4.28) – the TOP operating line, as shown in Figure 4.7: æ ö L L y j +1 = x j + çç1 - ÷÷÷ x D çè V ÷ø V æ L L ö÷ ç º slope and ç1 - ÷÷ x D º in which çè V ÷ø V Y-intercept 37 Figure 4.7: The top operating line (from “Separation Process Engineering” by Wankat, 2007) The operating line for stripping section (Eq. 4.29) – the BOTTOM operating line, as depicted in Figure 4.8: æL ö L ç yk = x k -1 - ç - 1÷÷÷ x B çèV ÷ø V æL ö L in which º slope and - çç - 1÷÷÷ x B º çèV ÷ø V Y-intercept 38 Figure 4.8: The bottom operating line (from “Separation Process Engineering” by Wankat, 2007) W.L. McCabe and E.W. Thiele (1925) proposed a graphical technique to provide the solutions for the distillation problems as illustrated in Figures 4.7 and 4.8 Note that the McCabe-Thiele method is, theoretically, based on the technique developed by W.K. Lewis 39 To start solving a distillation problem from the top of the column, we follow the following procedure: 1) x D must be specified, and since x D = y1 , the value of y1 can then be specified 2) Once y1 is specified, the corresponding value of x 1 can be read from the equilibrium curve 3) Since x 1 and y2 are related to each other through Eq. 4.28 (the operating line for the rectifying section), the value of y2 can be obtained from the top operating line once x 1 is specified 4) When y2 is obtained, x 2 is, once again, can obtained through the use of the equilibrium curve 40 5) Repeat 1) – 4), as illustrated in Figure 4.9, until we reach the feed stage After we enter the stripping section, the bottom operating line must be used (see Figure 4.8) We, then, follow the same procedure until we obtain x N +1 £ x B The McCabe-Thiele diagram for the whole column (i.e. both rectifying and stripping sections) can be illustrated in Figure 4.10 41 Figure 4.9: An illustration of the use of McCabeThiele technique to solve the distillation problems for the rectifying/enriching section (from “Separation Process Engineering” by Wankat, 2007) The number of steps or staircases (ขั้นบันได) from x D to x B is the number of stages of the distillation column 42 Figure 4.10: An illustration of the use of McCabe-Thiele technique to solve the distillation problems for the whole column (note that, in this Figure, the feed stage is stage 2 from the top) (from “Separation Process Engineering” by Wankat, 2007) From Figure 4.10, we obtain the facts that, in this Example, the total number of stages = 6 the feed stage = stage 3 (from the top) 43 Note that the feed stage/tray is the stage that crosses from the rectifying or enriching section into the stripping section, as depicted in Figure 4.10; note that, in Figure 4.10, it is the stage the covers the intersection of the top and the bottom operating lines However, it is NOT always the case that the feed stage must be the stage that includes the intersection of the top and bottom operating lines, as shown in Figures 4.11 and 4.12 For the cases of Figures 4.11 and 4.12, the 3 total number of stages are 6 » 7 stages, which 4 are more than that for the case of Figure 4.10 (in which the total number of stages = 6) 44 Figure 4.11 Figure 4.12 (Feed stage = # 2) (Feed stage = # 5) (from “Separation Process Engineering” by Wankat, 2007) For the binary mixtures, when the feed stage includes the intersection of the top and bottom operating lines, that feed stage is called the “optimal feed stage”, which yields the minimum number of stages of the distillation column 45 To draw the top and bottom operating lines, L L and , respectively, as the slopes it requires V V Practically, however, it is difficult to obtain L L the values of and , as these are the internal V V flow rates (within the distillation column) It would be much more convenient or more practical if we can obtain the values of L L and from the external flow rates, which V V can be derived as follows Since V = L +D 46 (4.44) Thus, L L = V L +D which can be re-arranged to L D L = V L +1 D (4.45) L where = reflux ratio D L For the value of , which is used for drawV ing the bottom operating line, since the boil-up æV ö ratio çç ÷÷÷ is practically NOT specified, we cannot çè B ÷ø calculate the value of L in the same manner that V L we do for V 47 L In order to obtain the value of , the feed V quality: L - L H - hF » q= F H -h (4.36b) is employed Eq. 4.36b can be re-arranged to L - L = qF (4.37a) L = L + qF (4.37b) and The overall material balance at the re-boiler (see Page 12) is L = B +V (4.46) which can be re-arranged to V = L -B 48 (4.47) By re-arranging the external overall and species balance equations (in Chapter 3): F = B +D (3.4) z F F = Bx B + Dx D (3.5) and we obtain the following equations æz - x ö B ÷ ÷÷ F D = ççç F çè x D - x B ÷ø (4.48) and æx - z ö F ÷ ÷F B = F - D = ççç D ÷ çè x - x ÷ø D (4.49) B Combining Eqs. 4.37b and 4.47 with Eqs. 4.48 and 4.49 and re-arranging the resulting equation for L results in V 49 L z F - x B ) + q (x D - x B ) ( D L = V L z F - x B ) + q (x D - x B ) - (x D - z F ) ( D (4.50) 4.3 Feed Line When it is assumed that the CMO (which is proposed by W.K. Lewis) is valid, the species balance (for a more volatile component) equations for the enriching (e.g., Eq. 4.2, 4.6, or 4.10) and the stripping (e.g., Eq. 4.14 or 4.18) sections can, respectively, be formulated as follows yV = xL + x D D (4.51) yV = xL - x B B (4.52) and 50 Since Eqs. 4.51 and 4.52 represent the TOP and BOTTOM operating lines, respectively, the intersection of these two lines means y top op = y bot op (4.53) x top op = x bot op (4.54) and As we have learned previously, the optimal feed point is the feed stage that covers the intersection of the two operating lines; thus, to solve for the value of yi at the point where y top op = y bot op , we do the following (4.52) – (4.51) results in y (V -V ) = (L - L ) x - (x D D + x B B ) (4.55) 51 Substituting the external balance equation for a more volatile species or Eq. 3.5: z F F = Bx B + Dx D (3.5) into Eq. 4.55 yields y (V -V ) = (L - L ) x - z F F (4.56) Re-arranging Eq. 4.56 gives (L - L ) z F y= x(V -V ) (V -V ) (4.57a) (L - L ) z F y =x+ (V -V ) (V -V ) (4.57b) F or F Consider the feed stage as shown in Figure 4.13, in which the feed (F ) is divided into the liquid phase (LF ) and the vapour phase (VF ) 52 Figure 4.13: A feed stage (from “Separation Process Engineering” by Wankat, 2007) Performing material (either mole or mass) balances at the feed stage for the liquid and the vapour phases gives L + LF = L (4.58a) L - L = LF (4.58b) or and 53 V +VF = V (4.59a) V -V = VF (4.59b) or Substituting Eqs. 4.58b and 4.59b into Eq. 4.57b results in LF F y =- x + zF VF VF (4.60) Dividing the numerator (เศษ) and the denominator (ส่วน) of the terms on the RHS of Eq. 4.60 by F yields LF / F F /F y =x+ zF VF / F VF / F (4.61) When combining Eq. 4.61 with the following definitions previously defined in Chapter 2: 54 VF =f (4.62) = 1- f (4.63) F LF F we obtain 1- f 1 y =x + zF f f (4.64) Alternatively, as F = LF +VF (4.65) by dividing Eq. 4.65 with F we obtain LF VF F = + F F F LF F + VF F =1 (4.66a) or q +f =1 55 (4.66b) Accordingly, q = 1- f = 1- VF F (4.67) or VF F = 1-q = 1- LF F (4.68) Thus, Eq. 4.61 can be written in another form as follows LF / F F /F y =x+ zF VF / F VF / F q 1 y =x+ zF 1-q 1-q (4.69) In addition to the fact that Eq. 4.69 is equivalent to Eqs. 4.61 and 4.64, Eqs. 4.64, and 4.69 are, in fact, Eq. 2.8: 56 zi 1- f yi = xi + f f (2.8) æ 1 ö q yi = x i + çç ÷÷÷ z i çè1 - q ÷ø 1-q (2.11) and Eq. 2.11: in Chapter 2, respectively Eq. 4.69: q 1 y =x+ zF 1-q 1-q (4.69) is commonly called the “feed line” equation (although Eq. 4.64 is equivalent to this equation, the feed-line equation is commonly written in the form of Eq. 4.69) 57 Let’s consider Eq. 4.36b: L - L H - hF » q= F H -h (4.36b) once again When the feed enters the distillation column as a saturated liquid; i.e. F = LF = L - L (4.70) (see Eq. 4.58b on Page 53) it results in the fact that L -L F = =1 q= F F With q = 1, the slope of Eq. 4.69 becomes (1) 1 q q = = = =¥ 1 - q q - 1 (1) - 1 0 which means, mathematically, that the feed line is a vertical line (เส้นแนวตั้ง) 58 In the case that the feed enters the column as a saturated vapour; it results in the fact that q = 0 as there is no liquid in the feed With q = 0 , the slope of Eq. 4.69 becomes ( 0) q q = = =0 1 - q q - 1 ( 0) - 1 which implies, mathematically, that the feed line is a horizontal line (เส้นแนวนอน) Note that, as we have already learned from Chapter 2, the feed line (either it is a vertical or horizontal line) intersects with the y = x line at y = x = zF 59 The feed line for other conditions of the feed, which include a saturated liquid-vapour mixture (0 < q < 1) a superheated vapour (q < 0) a compressed or sub-cooled liquid (q > 1) are shown, along with the case of a saturated liquid and a saturated vapour, in Figure 4.14 Figure 4.14: The feed line for various conditions of the feed (from “Separation Process Engineering” by Wankat, 2007) 60 Note that for a superheated vapour and a compressed liquid, the slopes are positive There are two major ways to operate the distillation column: 1) The feed condition is fixed, while the reæL ö flux ratio ççç o ÷÷÷ is varied çè D ÷ø 2) The reflux ratio is fixed, while the feed condition is varied æL ö For the first case, when the reflux ratio ççç o ÷÷÷ çè D ÷ø is varied, L and L are also varied, while the feed is fixed, which means that the feed line is fixed, it results in the changes in the slopes of 61 the top operating line the bottom operating line during the operation of the distillation column as illustrated in Figure 4.15 Figure 4.15: The top & bottom operating lines and the feed line for the operation that the feed condition is fixed, while the reflux ratio is varied (from “Separation Process Engineering” by Wankat, 2007) 62 For the second case, when the distillation column is operated in the manner that the reflux ratio is fixed, while the operating condition is varied, the top operating line is constant, while the feed line and the bottom operating line are varied, as illustrated in Figure 4.16 Figure 4.16: The top & bottom operating lines and the feed line for the operation that the reflux ratio is fixed, while the feed condition is varied (from “Separation Process Engineering” by Wankat, 2007) 63 In either operating condition, it should be emphasised once again that the optimal feed stage (which yields the minimum number of stages) is the stage in covers the intersection of the top and bottom operating lines 64 æ q q ö÷ ç = Example Calculate the slope ç÷÷ çè 1 - q q - 1÷ø of the feed lines for the following cases a) A two-phase feed where 80% of the feed is vaporised when it enters the column b) A superheated vapour feed where 1 mole of liquid will vaporise at the feed stage for each of every 9 moles of the feed c) A sub-cooled liquid feed that is fed to the column at 35 oF below its boiling-point temperature Given that the specific heat (cp ) of the feed (F ) is 30 Btu/(lb-mol–oF) and latent heat of vaporisation/condensation (l ) of the feed is 15,000 Btu/lb-mol 67 a) It is given that 80% of the feed is vaporised when it enters the column, which can be written as follows VF = 0.8F Thus, it results in the fact that LF = 0.2F Therefore, the value of q can be computed using Eqs. 4.36b and 4.70 (see Page 58) as follows 0.2F L - L LF q= = = F F F q = 0.2 68 The slope of the feed line can, then, be calculated as follows q 0.2 Slope of the feed line = = = -0.25 q - 1 0.2 - 1 This complies (agrees) with Figure 4.14 (for the two-phase mixture case), as the slope is negative b) Since the feed is a “superheated vapour” (i.e. it is hotter than a “saturated vapour”), some amount of liquid that flows down from rectifying section (with the flow rate of L ) is boiled and vaporised, and the amount of liquid vaporised is 69 L - L = (amount of liquid vaporised: lv ) (4.73a) or L - L = - (amount of liquid vaporised, lv ) (4.73b) It is given, in the problem statement, that liquid in the amount of 1 mol is vaporised per 9 mol of the feed Hence, the amount of liquid that is vaporised can be written mathematically as 1 (amount of liquid vaporised, lv ) = 9 F Accordingly, 1 L -L = - F 9 70 and, when combined with Eq. 4.36b, we obtain the value of q as follows 1 - F L -L 1 q= = 9 =F F 9 Eventually, the slope of the feed line for this feed condition (a superheated vapour) can be computed as follows æ 1ö çç- ÷÷ çè 9 ø÷÷ 1 q Slope = = = 10 q - 1 æç 1 ÷ö çç- ÷÷÷ - 1 è 9ø Note that the slope is positive, which agrees with Figure 4.14 (when the feed is a superheated vapour) 71 c) In this case, the feed is a “sub-cooled” liquid (i.e. it is cooler than a “saturated liquid”), when it enters the column, it causes a condensation of the vapour that flows up from the stripping section (with the flow rate of V ) The amount of vapour that is condensed when it is cooled by the sub-cooled liquid feed is V -V = (amount of vapour condensed: c ) Thus, in addition to the compressed (or subcooled) liquid feed, there is also the liquid that comes from the condensation of the vapour that flows up from the stripping section 72 Hence, in total, the liquid flow rate below the feed stage (L ) is L = L + F + (amount of vapour condensed: c ) (4.73) Since, normally, the distillation column is assumed to be adiabatic (see the assumptions for the CMO technique on Pages 13-14) or there is no heat moving in and out of the column, the heat generated from the condensation of the vapour phase is used to heat up the feed (which is a sub-cooled liquid) The amount of heat generated from the condensation of the vapour phase (Qcondensation ) is 73 Qcondensation = cl (4.74) where l is the latent heat of vaporisation/condensation In parallel, the amount of heat required to heat up the sub-cooled liquid feed (to its boiling point) (Qheat up ) is Qheat up = Fcp DT (4.75) where F = the feed flow rate cp = a specific heat of the feed DT = a difference between the boiling temperature of the feed and its actual (feeding) temperature 74 By employing the assumption that the distillation column is adiabatic, we obtain the fact that Qcondensation = Qheat up cl = Fcp DT (4.76) Substituting corresponding numerical values into Eq. 4.76 and solving for c yields F (30)(35) = c (15, 000) (30)(35) c= F = 0.07F (15, 000) Thus, in this Example, by using Eq. 4.73, the flow rate of the liquid phase below the feed stage is L = L + F + 0.07F 75 which can be re-arranged to L - L = F + 0.07F = 1.07F Accordingly, the value of q can be computed using Eq. 4.36b as follows L - L 1.07F q= = = 1.07 F F With the value of q of 1.07, the slope of the feed line can then be calculated as follows (1.07) q Slope = = = 15.3 q - 1 (1.07 ) - 1 Note, once again, that the slope is positive, which agrees with Figure 4.14 (for the case that the feed is a sub-cooled liquid) 76 An illustration of the use of the graphical McCabe-Thiele method is exemplified in the following Example Example A distillation column, at 1 atm, with a total condenser and a partial re-boiler is used to separate an ethanol (EtOH)-water mixture The feed is 20 mol% EtOH with a feed rate of 1,000 kmol/h (hF = 25 Btu/lb) A distillate composition of 80 mol% EtOH and a bottom composition of 2 mol% EtOH are required The external reflux ratio is 5/3, and the reflux is a saturated liquid If the CMO is assumed, find the number of equilibrium stages and the optimal feed stage 77 Given the enthalpies for a saturated vapour (H ) and a saturated liquid (h ) of 880 and 125 Btu/lb, respectively The problem statement can be described as the following flow chart: (from “Separation Process Engineering” by Wankat, 2007) 78 To solve this distillation problem, the following standard procedure is to be carried out 1) Find equilibrium data for a given mixture and pressure and then draw a y-x diagram (normally for a more volatile component: MVC) – e.g., the y-x diagram of EtOH on Page 33 2) Check if the CMO is valid The easiest way is to check the latent heat of vaporisation of each component and determine that whether or not their values are close to each other (normally 10% difference is acceptable) In this Example, lEtOH = 9.2 kcal/mol and lwater = 9.7 kcal/mol – close enough ( ~5-6%) to assume that CMO is valid 79 3) Locate the y1 = x D point (using the y = x line) L (for the enrich4) Calculate the value of V ing/rectifying section) 5) Draw the top operating line (in which the L values of or the Y-intercept of the top V operating line and x D are required) 6) Calculate/determine the value of q 7) Draw the feed line (note that, in some textbooks, this line is called the “q” line) 8) Draw the bottom operating line 9) Step off the stages [either bottom up–i.e. from the re-boiler (x B ) or top down–i.e. from the condenser (x D )] 80 It is given, in the problem statement, that x D = 0.80 (80 mol% EtOH) Thus, the point y1 = x D = 0.80 can be located using the y = x line (this y1 = x D = 0.80 point is on the y = x line) L can be computed using Eq. The value of V æL L ö÷ ç o or ÷÷ is 4.45, as the value of reflux ratio çç çè D D ÷ø given, as follows L D 5 3 L 5 = = = V L 5 8 +1 +1 D 3 81 L As is the slope of the top operating line, V æL ö with this slope çç ÷÷÷ and the value of x D , the top çèV ÷ø operating line can be drawn Alternatively, the Y-intercept of the top op- erating line: æ L L ö÷ ç y = x + ç1 - ÷÷ x D çè V ø÷ V (4.28) is used along with the value of x D to draw the top operating line In this Example, æ æ L ÷ö 5 ö÷ ç ç Y-intercept = ç1 - ÷÷ x D = ç1 - ÷÷ (0.80) = 0.30 çè V ÷ø çè 8 ÷ø 82 From this Y-intercept (y = 0.30) along with the point on the y = x line where y1 = x D = 0.80 , the top operating line can also be drawn To calculate the value of q for this Example, Eq. 4.36b is used as follows H - hF 880 - 25 q= = = 1.13 H -h 880 - 125 æ ö q ÷÷ Thus, the slope of the feed line ççi.e. çè q - 1÷÷ø can be calculated as follows q q 1.13 Slope = = = = 8.7 1 - q q - 1 1.13 - 1 To draw the feed line, in addition to its slope, the origin point of the feed line is also required 83 As we have learned previously, the origin point of the feed line is on the y = x line at the point where y = x = zF In this Example, the origin point of the feed line is at the point where y = x = z F = 0.20 If the feeding point is the optimal feed stage, it must include the intersection of the top and bottom operating lines By employing this principle, the feed line should pass through the intersection of the top and bottom operating lines 84 Accordingly, one point of the bottom operating line is now located (or fixed) – i.e. at the intersection of the top and the feed lines Another point through which the bottom operating line is passed is on the y = x line at the point where y = x = x B In this Example, x B = 0.02 (2 mol% EtOH) Then, as we put together: the equilibrium curve the top operating line the feed line the bottom operating line 85 on the same co-ordinate (i.e. the McCabe-Thiele diagram), as illustrated in Figure 4.18 Figure 4.18: An illustration of use of the graphical McCabeThiele technique to obtain the solutions for the distillation problem (from “Separation Process Engineering” by Wankat, 2007) 86 we obtain the following answers: the number of equilibrium stages required the optimal feed stage/location In this Example, the total number of equilibrium stages = 12 + a re-boiler = 13 stages the optimal feed stage/location = stage 1 or 2 (from the re-boiler) 87 4.4 Profiles for Binary Distillation From Figure 4.18, the values of x EtOH and y EtOH at each stage can be read from the graph; for example, the values of x EtOH and y EtOH for the stream leaving the re-boiler are 0.02 and 0.18, respectively Since x water = 1 - x EtOH and y water = 1 - y EtOH the value of x water and y water of each stage can also be obtained 88 Therefore, the concentration profiles for the vapour phase (yi of both EtOH and water) and the liquid phase (x i of both EtOH and water) can be plotted and depicted in Figure 4.19 Figure 4.19: The concentration, temperature, and flow rate profiles of the distillation column (from “Separation Process Engineering” by Wankat, 2007) 89 The temperature profile of the distillation column, also shown in Figure 4.19, is obtained when the values of x i and yi of each species for each stage are known Once the values of x i and yi can be specified, we can use a Txy diagram (e.g., Figure 4.20) to determine the temperature of each stage; for Example, for x EtOH = 0.02 and y EtOH = 0.18, the temperature can be read, from Figure 4.20, as ~95.5 oC The flow rate profile for the rectifying and stripping sections can also be plotted and included in Figure 4.19 90 100 90 Tx Ty o T ( C) 95 85 80 75 0.0 0.2 0.4 0.6 0.8 1.0 xEtOH or yEtOH Figure 4.20: A Txy diagram for ethanol (EtOH) for an EtOH-water binary mixture Note that, as the CMO is assumed, the flow rates of the liquid and the vapour phases for each section (either rectifying or stripping) are constant 91 The values of L , V , L , and V can be obtained by performing the material balances, as described and exemplified previously From the above explanations and Figure 4.19, it illustrates that the McCabe-Thiele technique is a powerful tool to deal with the distillation problem, as it is easy to use and also provides an insight into the distillation column (on the stage by stage basis) without having to measure the values of the variables (e.g., concentration, temperature, flow rate) internally (or stage by stage) 92 4.5 Open Steam Heating The open steam heating is another technique used to heat up and vaporise the stream coming out of the bottom part of the distillation column; the open steam heating is used in liu (ทดแทน) of the re-boiler Let’s consider the following Example, which illustrates the calculations of the distillation column using the open steam heating in replace of the re-boiler 93 Example The feed is 60 mol% MeOH and 40% water and is input as a 2-phase mixture that flashes at 1 atm such that VF F = 0.3 The feed flow rate is 350 kmol/h. The column is well insulated and has a total condenser The reflux is returned to the column as a saturated liquid, and the external reflux ratio æL ö çç ÷÷ is 3.0 çè D ÷÷ø The desired distillate and bottom concentrations are 95 mol% and 8 mol% MeOH Instead of using a re-boiler, saturated steam at 1 atm is sparged (พ่น) into the bottom of the column to provide the boil-up Calculate the total number of stages and the optimal feed location 94 To check if the CMO is valid for this Example, we consider the latent heat of vaporisation of both components, which are as follows lMeOH = 8.4 kcal/mol lwater = 9.7 kcal/mol The difference is quite large (15-20%); thus the CMO should be used with cautions The flow chart of this Example is shown in Figure 4.20 95 Figure 4.20: A distillation column with the use of the open steam heating in liu of the re-boiler (from “Separation Process Engineering” by Wankat, 2007) æL ö From the given reflux ratio çç ÷÷÷ , the value of çè D ø÷ L can be calculated, using Eq. 4.45 as follows V 96 L D L 3.0 = = = 0.75 V L 3.0 + 1 +1 D Additionally, the Y-intercept of the top operating line can be computed as follows æ ö L Y-intercept = çç1 - ÷÷÷ x D çè V ÷ø = (1 - 0.75)(0.90) Y-intercept = 0.24 It is given, in the problem statement, that VF F = f = 0.3 ; thus LF F = q = 1 - f = 0.7 97 Hence, the slope of the feed line is q 0.7 Slope = = = -2.33 q - 1 0.7 - 1 In addition, the feed line must intersect with the y = x line at the point where x = y = z F = 0.60 With the slope (= -2.33) and the point of x = y = z F = 0.60 , the feed line can be drawn Then, the bottom operating line can be drawn from the following two points: the intersection of the top operating line and the feed line the point on the y = x line at x B = 0.08 98 Eventually, the McCabe-Thiele diagram can be drawn as shown in Figure 4.21 The total number of stages = 5, and the optimal feed stage is the stage 3 Figure 4.21: The McCabe-Thiele diagram for the open steam heating problem (from “Separation Process Engineering” by Wankat, 2007) 99 It is evident that the calculations for the open steam heating is similar to that of the distillation column using a re-boiler Note that the total number of stages from the computer simulation is found to be 6 (the difference is due mainly to the CMO assumption) 4.6 General McCabe-Thiele Analysis Procedure From the previous Examples, it is evident that, for any kinds/types of distillation problems, the column is normally divided into 2 sections: the enriching/rectifying section or the section above the feed the stripping section or the section below the feed 100 and each section can be treated separately or is independent of another section In fact, it is not necessary that the column has to be divided into 2 sections only; in practical, it is common that the distillation column is divided into several (more than 2) sections This means that the column can have more than single (one) feed points Additionally, this also means that the column can have more output (exit) streams than the distillate (D ) and the bottom (B ) 101 Consider Figure 4.22, which illustrates the distillation column with 2 feed streams (F1 and F2 ) and 3 output streams [distillate (D ), bottom (B ), and side stream (S )] Note that the distillation column in Figure 4.22 is divided into 4 sections: As mentioned earlier, each of these 4 sections can be treated separately A standard procedure (or algorithm) for solving any distillation problems is as follows 102 Figure 4.22: The distillation column with 2 feed streams and an additional side-stream exit stream (from “Separation Process Engineering” by Wankat, 2007) 103 1) Draw a flow chart, in which all known variables are labelled 2) Establish the equilibrium data (in the form of either data points or equation) and draw/plot the equilibrium x-y diagram (curve) 3) Check if the CMO is valid (by checking the latent heat of vaporisation of each component) 4) For each section (either enriching or stripping section), a. locate a material balance envelope – keep in mind that the fewer the streams/unknowns involved, the easier the material balances would be 104 b. establish the overall and species balance equations c. write the operating equations by calculating all known slopes, intercepts, and intersections (mainly the intersections with the y = x line), and then draw the operating line for each section 5) Develop the feed equations: calculate q values, slopes, and y = x intersections, and then draw the feed line for each feed stream 6) For the operating and feed lines, a. try to plot as many operating and feed lines as possible 105 b. if necessary, perform external material and energy balances (to obtain, e.g., the values of D and/or B) 7) When all (or almost all) lines are in place (i.e. already drawn), step off the ้ บันได), and determine the stages (ลากขัน total number of stages and the optimal feed location The following Example illustrates the calculations of the distillation problem using the standard procedure 106 Example Ethanol (EtOH) and water are to be separated in a distillation column, operating 1 atm, with a total condenser and partial re-boiler There are 2 feed streams: Feed 1 is a saturated vapour and of the flow rate of 200 kmol/h, with 30 mol% EtOH Feed 2 is a sub-cooled liquid, in which 1 mole of vapour must be condensed inside the column in order to heat up 4 moles of the feed to its boiling point; the flow rate of Feed 2 is 300 kmol/h, with 40 mol% EtOH The desired products are the distillate and the bottom with 72 and 2 mol% EtOH, respectively The external reflux ratio is 1.0 107 If all the feeds are to be input at their optimal locations, find the total number of stages and the optimal feed stages The flow chart with all of known variables is depicted as Figure 4.23 Note that, in Figure 4.23, since the concentration of EtOH of Feed 2 is higher than that of Feed 1, we place Feed 2 higher than Feed 1 (do you know why?) The top operating line or the operating line for the top section – the section between the top of the column and Feed 2 – is as follows æ ö L L y = x + çç1 - ÷÷÷ x D çè V ø÷ V 108 (4.77) Figure 4.23: The distillation column with two feed streams (from “Separation Process Engineering” by Wankat, 2007) L The value of can be computed from the V æL ö given value of the reflux ratio çç ÷÷÷ as follows çè D ÷ø 109 L D L 1.0 = = = 0.5 V L 1.0 + 1 +1 D The Y-intercept of the top operating line (Eq. 4.77) can then be calculated as follows æ ö L çç1 - ÷÷ x = (1 - 0.5)(0.72) = 0.36 çè V ø÷÷ D L = 0.5 and Y-intercept of With the slope of V 0.36, the top operating line can be drawn Alternatively, this top operating line can be drawn when we have 2 points: the Y-intercept and the other point 110 Normally, the other point is the intersection of the operating line and the y = x line We have learned that the top operating line crosses the y = x line at the point where y = x = xD Thus, in this Example, the other point is the point on the y = x line where y = x = x D = 0.72 To draw the operating line for the middle section (or the middle operating line) – note that the middle section is the section between the two feed streams, we perform the material balances from the top of the column to the boarder 111 between the top and the middle sections (the balance envelope can be seen as the larger envelope of the top section in Figure 4.23) as follows Overall balance F2 +V ¢ = L ¢ + D (4.78) EtOH balance z F F2 + yV ¢ = xL ¢ + x D D 2 (4.79) Note that L ¢ and V ¢ are the flow rates of the liquid and vapour streams of the middle section, respectively 112 Re-arranging Eq. 4.79 gives ( x D D - z F F2 L¢ 2 y= x+ V¢ V¢ ) (4.80) Eq. 4.80 is the operating line for the middle section (for this Example) Feed 2 (see the feed location of Feed 2 in Figure 4.23 on Page 109) is a sub-cooled liquid, in which 1 mol of vapour is to be condensed to heat up 4 mol of the feed to its boiling point Let the amount of vapour condensed be denoted as c 113 V L c Feed, F2 L V Figure 4.24: The condensation of vapour flowing from the middle section to the top section due to the fact that the Feed 2 is a sub-cooled liquid In Figure 4.24, we obtain the following equations: V ¢- c = V L + F2 + c = L ¢ (4.81) (4.82) Then, the value of qF (the q value for Feed 2) 2 can be obtained by adapting Eq. 4.36b to suit this situation as follows 114 L¢ - L qF = 2 F2 (4.83) It is given that 1 mol of vapour is to be condensed to heat up 4 mol of the feed to its boiling point, which can be written in an equation form as follows c= F2 4 Thus, Eq. 4.82 becomes, L + F2 + F2 4 which can be re-arranged to = L¢ F2 5 L ¢ - L = F2 + = F2 4 4 Substituting L ¢ - L = 115 5 F2 into Eq. 4.83 yields 4 5 F2 qF = 4 = 1.25 2 F2 Accordingly, the slope of the feed line for Feed 2 can be calculated as follows SlopeFeed 2 = q 1.25 = = 5.0 q - 1 1.25 - 1 Additionally, the feed line of Feed 2 crosses the y = x line at the point where y = x = z F = 2 0.4 The value of L ¢ can be computed using Eq. 4.83: L¢ - L qF = 2 F2 116 (4.83) as follows L ¢ = L + qF F2 2 (4.84) The liquid flow rate of the top section, L, æL ö can be calculated when the reflux ratio çç ÷÷÷ and çè D ÷ø the distillate flow rate (D ) are known as follows æL ö L = çç ÷÷÷ D çè D ÷ø (4.85) To obtain the value of D , we must perform an external material balance, which results in F1 + F2 = D + B (200) + (300) = 500 = D + B (4.86) 117 z F F1 + z F F2 = x D D + x B B 1 2 é(0.30)(200)ù + é(0.40)(300)ù = 180 = (0.72) D + (0.08) B êë úû êë úû (4.87) Solving Eq. 4.86 & 4.87 simultaneously gives D = 242.9 kmol/h Thus, the value of L can be computed using Eq. 4.85 as follows æL ö L = çç ÷÷÷ D çè D ÷ø = (1.0)(242.9) L = 242.9 kmol/h Eventually, the value of L ¢ can be computed using Eq. 4.84: L ¢ = L + qF F2 2 118 (4.84) as follows L ¢ = (242.9) + éê(1.25)(300)ùú = 617.9 kmol/h ë û Alternatively, L ¢ can be calculated using Eq. 4.82: L + F2 + c = L ¢ (4.82) as follows L ¢ = L + F2 + c L + F2 + F2 4 L ¢ = 242.9 + 300 + 300 4 L ¢ = 617.9 kmol/h The value of V ¢ can then be calculated from Eq. 4.78: F2 +V ¢ = L ¢ + D 119 (4.78) as follows V ¢ = L ¢ + D - F2 = 617.9 + 242.9 - 300 V ¢ = 560.8 kmol/h L¢ or the slope of Accordingly, the value of V¢ the middle operating line (Eq. 4.80) is L¢ 617.9 = = 1.10 V ¢ 560.8 ( é x D -z F ê D F2 2 Additionally, the Y-intercept ê ê V¢ êë of Eq. 4.80 can be computed as follows (x D - z F F2 D 2 V¢ ) = {éêë(0.72)(242.9)ùúû - éêë(0.4)(300)ùúû} (560.8) Y-intercept (middle op. line) = 0.098 120 ) ù ú ú ú úû With the slope and Y-intercept, the middle operating line can be drawn Alternatively, since the middle operating line must pass through the intersection of the top operating line and the feed line of Feed 2 (in order for Feed 2 to be fed at the optimal location) Thus, from the following two points: the Y-intercept of the middle operating line the intersection of the top operating line and the feed line of Feed 2 the middle operating line can be drawn 121 To draw the feed line for Feed 1 is rather straightforward Since Feed 1 is a saturated vapour, its feed line is a horizontal line Additionally, the feed line of Feed 1 must cross the y = x line at the point where y = x = z F = 0.30 1 The bottom operating line, which can be written in an equation form as follows: æL ö L ç y = x - ç - 1÷÷÷ x B çèV ÷ø V (4.88) must pass through the intersection of the middle operating line and the feed line for Feed 1 122 Additionally, it crosses the y = x line at the point where y = x = x B = 0.02 Eventually, all operating and feed lines can be established, as shown in Figure 4.25 Figure 4.25: The McCabe-Thiele diagram for the distillation problem with 2 feed streams (from “Separation Process Engineering” by Wankat, 2007) 123 From Figure 4.25, the number of stages is found be 6.5 stages or 5.5 stages + 1 re-boiler, and the optimal locations are stage 1 (above the re-boiler) for Feed 1 stage 2 (above the re-boiler) for Feed 2 4.7 Other Distillation Column Situations Some special cases or situations of the operation of the distillation column are summarised as follows 4.7.1 Partial condenser In the normal distillation, the condenser used in the operation is a total condenser, at which all vapour from the top of the column is condensed to liquid 124 In the total condenser, both distillate (D ) and reflux (Lo ) are liquid On the contrary, a partial condenser con- denses only a fraction of vapour from the top of the column In the partial condenser, the distillate is vapour, while the reflux is liquid, as illustrated in Figure 4.26 In Figure 4.26, it is evident that there is an equilibrium between vapour (distillate: D , with the concentration of yD ) and liquid (reflux: Lo , with the concentration of x o ) at the condenser 125 Partial Condenser yD y1 x1 xo Figure 4.26: A partial condenser (from “Separation Process Engineering” by Wankat, 2007) Thus, the concentration of the distillate (yD ) is no longer equal to the concentration of the reflux (xo ), but x o can be read from the equilibrium curve when yD is known the partial condenser is considered an additional equilibrium stage 126 4.7.2 Total Re-boiler The re-boiler commonly used in the distillation operation is a partial re-boiler, in which only a fraction of liquid flowing out of the last stage of the distillation column is vaporised (see Figure 4.4 on Page 9) Hence, there is an equilibrium between the liquid (that flowing out of the last stage) and vapour (that is vaporised in the re-boiler and thus returned back to the column); this is why a re-boiler is considered an additional stage In the total re-boiler, all liquid flowing into the re-boiler is completely vaporised, as depicted in Figure 4.27 127 Figure 4.27: A total re-boiler (from “Separation Process Engineering” by Wankat, 2007) Note that the bottom (B ) with the concentration of x B is separated from the liquid stream from the last stage of the column before entering the re-boiler; it is not separated at the re-boiler as per the partial re-boiler 128 Since all the liquid flowing into the re-boiler is vaporised and there is no reaction at the reboiler, the concentration of the vapour coming out of the re-boiler is equal to that of the liquid flowing in This means that there is no equilibrium between the vapour and the liquid phases at the reboiler Accordingly, the total re-boiler cannot be considered as an additional stage as per the case of partial re-boiler 129 4.7.3 Side streams or withdrawal lines As mentioned earlier, in the real distillation operation, there are more than the distillate (D ) and the bottom (B ); there are also additional output streams, which are commonly called “side streams” and “withdrawal lines” When there are side streams or withdrawal streams, for each stream, there are 3 additional variables associated with that stream, comprising its flow rate, S its composition (concentration, x S ) its withdrawal stage Consider the distillation column with a side stream and the corresponding McCabe-Thiele diagram in Figure 4.28 130 Figure 4.28: The distillation column with a side stream and the corresponding McCabe-Thiele diagram (from “Separation Process Engineering” by Wankat, 2007) In Figure 4.28, the column comprises 3 sections: the top section: the section above the side stream (including the condenser) the middle section: the section between the side stream and the feed 131 the bottom section: the section below the feed stage To obtain the operating equation (line) for each section, we have to perform material balances with the appropriate balance envelope Note that the side stream can be treated as a feed, but in the opposite direction The middle operating line equation of Figure 4.28, which involves the side stream, can be written as follows (try to derive it yourself) xDD + xS S L¢ y= x+ V¢ V¢ 132 (4.89) Note that the middle operating line crosses the y = x line at the point where y =x = xDD + xS S D +S (4.90) As mentioned recently, the side stream can be treated as a feed; hence, if the side stream is a saturated liquid, the operating line of the side stream is a vertical line with concentration of x S the side stream is a saturated vapour, the operating line of the side stream is a horizontal line with the concentration of yS 133 4.7.4 Intermediate re-boiler and intermediate condenser As there are side streams, in addition to the condenser at the top and the re-boiler at the bottom of the column, there may be condensers and re-boilers associated with the side streams, which is generally called intermediate condensers and intermediate re-boilers, respectively An intermediate re-boiler removes a liquid side stream from the column, boils (or vaporises), and re-injects it (as a vapour) back to the same column, as illustrated in Figure 4.29 134 Figure 4.29: A distillation column with the intermediate re-boiler (from “Separation Process Engineering” by Wankat, 2007) Since there is an additional re-boiler added into the column, an additional energy requirement (i.e. heating load) of QI is required 135 As x S > x B (do you know why?), it is easier for this liquid side stream to be vaporised; thus, the heat requirement (or the heating load) is lower than that of the bottom re-boiler Additionally, by having an intermediate reboiler, it reduces the heating load of the bottom re-boiler An intermediate condenser removes a vapour side stream from the column, condenses it, and re-injects it (as a liquid) back to the same column Since the vapour side stream contains more amounts of less volatile components than does the distillate 136 These less volatile components can be condensed to liquid easier than the more volatile components Thus, for the intermediate condenser, the coolant with a higher temperature than that used at the top condenser can be used; in other words, by having the intermediate condenser, it reduces the cooling load at the top condenser It is important to note that the optimal feed stage for the re-injecting stream (for either intermediate re-boiler or intermediate condenser) is the stage adjacent (ที่อยู่ติดกัน) to the withdrawal stage 137 4.7.5 Stripping and enriching columns A stripping column is a distillation column that has only a re-boiler with the feed fed into the column from the top For a stripping column, the feed must be liquid (can be either saturated or sub-cooled) An enriching column is a distillation column that has only the condenser with the feed introduced to the column from the bottom The feed of the enriching column has to be vapour, which can be either saturated or superheated vapour 138 Figure 4.30 depicts the stripping (A) and enriching (B) columns Figure 4.30: A stripping column (A) and an enriching column (B) (from “Separation Process Engineering” by Wankat, 2007) In a stripping column, the bottom can be very pure, but the vapour product is not 139 Hence, the main purpose of using the stripping column is to obtain the bottom with as high concentration of a less volatile component as possible On the other hand, in the enriching column, the distillate is very pure, but the liquid product (the bottom) may not be so Accordingly, the enriching column is used when we want to obtain the distillate with the high concentration of a more volatile component The operating line equations of the stripping column are as follows: 140 The top operating line equation: y = yD The bottom operating line equation: æL ö L y = x - çç - 1÷÷÷ x B çèV ÷ø V (4.88) which is, in fact, the operating equation for the stripping section of the full distillation column These operating lines are as illustrated in Figure 4.31 The operating line equation for the enriching column can be written as follows: The top operating line: æ L L ö÷ ç y = x + ç1 - ÷÷ x D çè V ø÷ V 141 (4.77) Figure 4.31: The operating lines for a stripping column (from “Separation Process Engineering” by Wankat, 2007) The bottom operating line: x = xB Note the top operating line equation of the enriching column is, in fact, the operating line equation of the enriching section of the full distillation column 142 4.8 Limiting Operating Conditions There are 2 limiting conditions for distillation operations: æL ö Minimum reflux çç ÷÷÷ çè D ÷ø , which is the min condition that the external reflux ratio is as minimal as possible; in this condition, we need an infinite (¥) number of stages to obtain a desired separation Total reflux, which is the condition that all overhead vapour is condensed and returned to the column as a reflux (i.e. no distillate), and all bottom liquid is boiled at the re-boiler and re-turned to the column (i.e. no bottom product) 143 For the case of total reflux, as illustrated in Figure 4.32, the steady-state feed rate must be L L zero, with = = 1.0 (do you know why?) V V Also for this case, the operating lines of both sections (enriching and stripping) are y =x The total reflux condition is used for starting up the column or for determining the column efficiency With this condition, it yields the minimum number of stages 144 Figure 4.32: The total reflux distillation condition (from “Separation Process Engineering” by Wankat, 2007) For the case of minimum reflux, the reflux and the reflux ratio is minimal, and the number of stage required is infinite (¥) To obtain these, the top operating line must touch the equilibrium line, as illustrated in Figure 4.33 145 Figure 4.33: The McCabe-Thiele diagram and the concentration profile of the minimum reflux ratio condition (from “Separation Process Engineering” by Wankat, 2007) Normally, the touching point is the point where the top operating line intersects with the feed line, and that intersection has to be on the equilibrium line 146 However, it is impossible that the operating line (of the more volatile component) be higher than the equilibrium line; thus, in some cases (generally, the case that the mixture has an azeotropic point), the touching point is as illustrated in the right Figure in Figure 4.33 Note that, the concentration profile for the minimum reflux case is as shown in the lower Figure in Figure 4.33; the concentration around the touching point is found to be nearly constant The point where the operating line(s) touch the equilibrium line or the touching point is commonly called the “pinch” point 147 æL ö The slope çç ÷÷÷ of the top operating line in çèV ÷ø the case of minimum reflux ratio is minimal and æL ö denoted as çç ÷÷÷ çèV ÷ø min æL ö Once çç ÷÷÷ çèV ÷ø min æL ö is obtained, the value of çç ÷÷÷ çè D ÷ø min or the minimum reflux ratio can then be calcuæL ö lated from the value of çç ÷÷÷ çèV ÷ø as follows min æL ö çç ÷÷ çè D ÷÷ø min = æL ö çç ÷÷ çèV ÷÷ø (4.91) min æL ö 1 - çç ÷÷÷ çèV ÷ø min æL ö The minimum reflux ratio or çç ÷÷÷ çè D ÷ø is used min to determine the optimal operating reflux ratio 148 Normally, the optimal operating reflux ratio æL ö is between 1.05-1.25 times çç ÷÷÷ çè D ÷ø min 4.9 Efficiencies Commonly, there are 2 types of efficiencies: Overall efficiency Murphree efficiency The overall efficiency, denoted as Eo , is defined as the ratio of the number of theoretical equilibrium stages obtained from the calculations (using, e.g., the McCabe-Thiele diagram) to the actual number of stages used in the real distillation operation, which can be written in an equa- tion form as follows: 149 Eo = N equilibrium N actual (4.92) This type of efficiency (i.e. overall efficiency) is based on the concept that, in fact, each stage has not reached its equilibrium yet, and more stages are required (to reach the equilibrium) Hence, the meaning of N actual is the actual number of stages needed to reach the equilibrium between the liquid and the vapour phases For example, if the overall efficiency is 50%, it means that we need 2 actual stages for each theoretical equilibrium stage to allow the liquid phase and the vapour phase to reach the equili- brium 150 For the Murphree efficiency, it can be divided into 2 types: Murphree vapour efficiency (E MV ): E MV = yi - yi +1 * i y - yi +1 (4.93) Murphree liquid efficiency (EML ): E ML = x i - x i -1 * i x - x i-1 (4.94) where x i* and yi* are the concentration of species i (normally the more volatile components) at equilibrium of the liquid and vapour phases, respectively 151 By considering both Eqs. 4.93 and 4.94, the concept of the Murphree efficiency is similar to that of the overall efficiency, but it consider the efficiency in each phase (i.e. either liquid or vapour phase) separately The similarity of both efficiencies is that it based on the concept that, in the real operation, each stage does not reach the equilibrium of the liquid and the vapour phases (which results in the fact that x i < x i* and yi < yi* ); thus, additional stages are needed 152