Mass transfer and separation technology Massöverföring och separationsteknik ( MÖF-ST ) , 7 sp

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1 42432 Mass transfer and separation technology Massöverföring och separationsteknik ( MÖF-ST ) 4432, 7 sp. Batch distillation Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory tel ; ron.zevenhoven@abo.fi Batch distillation principle mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 2

2 Distillation, principle Liquid with composition a boils at temperature T 2, giving vapour with composition a 2 which is enriched in component A Taking out and cooling vapour a 2 gives liquid a 4 at temperature T 4 In equilibrium with liquid a 4 is vapour a 4, again further enriched in component A Picture: A83 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 3/23 Batch distillation A useful method for separating liquid mitures, if: Small amounts must be processed ( batches ) Different fractions are to be produced from one feed in one unit Upstream AND/OR downstream operations are batchwise Solids or tars can be formed mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 4/23

3 Differential distillation mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik Differential distillation / Distillation without reflu Batch process Constant rate heat-up Remaining liquid becomes steadily weaker in the volatile component Miture of A+B, with A most volatile, gives more pure A + a residue enriched in B, but not (or very seldom) pure B Picture: Ö96 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 6/23

4 Differential distillation /2 Picture: SH6 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 7/23 Differential distillation /3 Moles in bottom still = = (t), mol fraction of volatile component A = in liquid, y in vapour Mass balance for A for vaporisation of d: y d = d( ) = d + d ( = y D D) integrates to d d y d y Requires equilibrium data y = f() mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 8/23

5 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 9/23 Differential distillation /4 If a straight line can be assumed, y = m + c (where presumably c = ) or: m m y y y y or y y m c m c m m mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik /23 Differential distillation /5 Alternatively, with relative volatility α constant over the range concerned y = α / (+(α-) )) and : or ) ( ) ( ( ) or ) ( -

6 Eample: differential distillation REMEMBER: K B = y B / B = p B /p K T = y T / T = p T /p = (-y B )/(- B ) = (-K B )/(- B ) Source: SH6 T boil toluene T boil benzene B (K B -K T ) = - K T B = (-K T )/(K B -K T ) y B = K B B T gives K T and K B mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik /23 Eample: differential distillation answer At the start = = kmol, = =.5. Using for =.45, =.4,... =.5 gives values of corresponding to time t (h) = ( -) / y The instantaneous vapour composition follows from y = 2.4 / ( +.4 ) mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik Source: SH6 2/23

7 Eample: differential distillation answer For the average composition of the distillate the total mass balance gives For the temperature, the T,,y data for benzene toluene at.3 kpa is used. The results plotted Source: SH6 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 3/ Task ö3.5 The question in Swedish mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 4/23

8 Task ö3.5 / Relative volatility α = 2.6 Equilibrium y = 2.6 /(+.6 ) = f(α, ) t = : L = o = kmol, A,L,o = 5 kmol, D = Volatile component = A = n-heptan A, L = L A,L and y A = 2.6 A,L /(+.6 A,L ) Differential for A: d A = y A d, or Δ A = y A Δ hen L L Δ then A,L A,L - Δ y A ; A,L = A,L / L and y A = f(α, A,L ) D D + Δ; A,D = A,L,o - A,L and A,D, = A,D / D mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 5/23 Task ö3.5 / = 4.658, etc. produced a) b) c) _L _D _A in L _A in L y_a eq with _A _A in D _A in D Note: the result depends on step-size Δ mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 6/23

9 Batch distillation with reflu mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 7 Constant or variable reflu / For sharper distillations (and/or less low-purity distillate products) column with several stages can be used, with reflu: part of the distillate is sent back into the colum Options: Constant reflu Constant distillate composition Both reflu and distillate compostion vary: Optimal control mode (comple!) V L D Picture: SH6 Reflu ratio R = L/D = L/(V-L) L/V = R/(R+) mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 8/23

10 Constant or variable reflu /2 Eample of batch distillation with constant reflu Two theoretical stages Initial (Time ) composition in bottom still and distillate quality D After some time (Time ) the distillate composion is D, in bottom still Picture: SH6 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 9/23 Locating flows in,y diagrams Heat y D. n-heane - n-octane atm.8 y 2 y y n-heane (mol/mol).6.4 2,y 2 2,y 3,y,y 2 d,y d n-heane (mol/mol) mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 2/23

11 Constant or variable reflu /3 Eample of batch distillation with variable reflu (n-octane, n-heane) Three theoretical stages Constant distillate quality D (here =.9) Reflu increases while bottom still composition changes (here =.4.6 at total reflu) Less by-products, more process control needed Total reflu: L = V D = Picture: SH6 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 2/23 Liquid holdup If much material (= several % of the input batch) liquid is present in the column (i.e. large liquid holdup ), it will affect the batch distillation process less good separation, lower distillate purity, higher energy input requirements More problematic for tray columns than for packed bed columns For high pressures also gas holdup must be taken into account The comple design calculations require computers mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik Picture: 22/23

12 Sources # A83 P.. Atkins Physical chemistry 2nd Edition, Oford University Press (983) SH6 J.D. Seader, E.J Henley Separation process principles John iley, 2nd edition (26) Chapter 3-5 Ö96 G. Öhman Massöverföring Åbo Akademi Värmeteknik (996) 3.5 mars 25 Åbo Akademi - kemiteknik Värme- och strömningsteknik 23/23