Level 2: Input output structure

Transcription

1 Level : Input output structure Cheng-Ching Yu Dept of Chem. Eng. National Taiwan University ccyu@ntu.edu.tw

2 Input/output Structure Hierarchy of decisions 1. batch versus continuous. Input-output structure of the flowsheet 3. Recycle structure of the flowsheet 4. General structure of the separation system a. Vapor recover system b. Liquid recovery system 5. Heat-exchanger network Flowsheet Alternatives rule: desirable to recover >99% of all valuable materials but #1: might be cheaper to lose inexpensive reactants (e.g., air, water etc) but #: recycle gaseous reactant and purge a gaseous impurity or by-product Remarks: If the cost of separation is cheap, the gas recycle and purge might not be necessary.

3 Level- decisions 3

4 Purification of feeds -not inert and is present in significant amount (remove it) - in a gas feed (feed to process) - in a liquid feed is also a product or by-product (feed to process) - is present in a large amount (remove it) - is present as an azeotrope with a reactant (feed to process) - inert but is easier to separate from the product (feed to process) - catalyst poison (remove it) Recover or recycle by-products Toluene + H Benzene + CH Benzene Diphenyl + H recycle: oversize all equipment in the recycle loops remove: increase raw material costs 4 4

5 Gas recycle and purge If we have light reactant, feed impurity or by-product has a normal b.p. less than propylene (-48 C), use a gas recycle and purge stream. What not to recover water, air Remark: We should always consider the cost of pollutant treatment. Number of product streams 5

6 1. order by bp s. group neighboring components with the same destination. 3. no. of groups - recycle streams= product streams Ex. How many product streams? 6

7 Ex. How many product streams for HDA process? component b.p. Destination code Hydrogen -53C recycle and purge methane -161C recycle and purge benzene 80C primary product Toluene 111C recycle Diphenyl 53C fuel Results: three product streams (4-1=3) 7

8 Design Variables, Balances and Costs Design Variables Degrees of freedom Typical design variables: Overall material balances Focus on input/output flows 8

9 HDA example Toluene + H Benzene + CH 4 Benzene Diphenyl + H 9

10 HDA example-cont 5 Given: PB = 65 mol/hr benzene = ton/yr, x = 0.75, y = 0.4, S= (Douglas, 1988; p.51) PH PB 65 Freshfeed Toluene: FFT = = = 73.4 mol/hr S Diphenyl produced: P = P (1 S)/S = 4. mol/hr D Extents: ξ = 73.4 (P /S, toluene consumed) ξ Makeup Gas: 1 B B 1 B B =4. (P (1-S)/(S), diphenyl produced; ξ - ξ =P ) PB PB(1 S) H : yfhfg + = yphpg S S PB CH 4 : (1- yfh) FG + = (1 yph) PG S (1 S) P [1 (1 )(1 )/] & B yph S PG = FG + PB FG = S S( y y ) Balance on H : F = y P = E PH G FH PB PB(1 S) reacted = 69. S S PH 10

11 Stream Tables 11

12 Stream Costs Economic potential (EP) at level. EP= product value + by-product value -raw material costs 1

13 Remark: We have not considered the recycle cost yet. Summary: Flowsheet Alternatives - purify the feed: probably not desirable -recycle diphenyl: We must oversize all the equipment in the diphenyl-recycle loop. -recover some hydrogen: Is justified by determining the cost of the recovery system. 13

14 Level 3: Recycle structure Cheng-Ching Yu Dept of Chem. Eng. National Taiwan University

15 Decisions Reactor and Recycle Structure No. of reactors HDA Toluene + H Benzene + CH Benzene Diphenyl + H F and 500 psia Remark: One reactor will do. 15

16 Acetone Ketene + CH 1 Ketene CO + C H 4, 700 C and1atm Ketene + Acetic Acid Acetic Anhydride, 4 80 C and1atm Remark: Two reactors are required for this system. 16

17 No. of recycle streams Do not separate two components and remix them at a reactor inlet! groups: 7 product streams: 4 (A+B, D+E, F and J) recycle streams: 7-4=3 (C, G+H and I) 17

18 18

19 HDA reactor: 1 product streams: 3 recycle streams: gas compressor: 1 19

20 Excess reactants -improve selectivity Butene + Butene + Isobutane Isooctane Isooctane C 1 -convert undesirable reactant CO + Cl COCl -shift equilibrium Benzene + 3H Cyclehexane Remark: Unfortunately, there is no rule of thumb available to make a reasonable guess of the optimum amount of excess. 0

21 Recycle material balances Make balance on the limiting reactant. For x = 0.75, P = 65 and F = 73. Toluene: FFT PB 73 FT = = = = 365 x Sx 0.75 Recycle Gas: R B P MR y FH SyPH x yfh yph = = (0.4) B G = FT (from balabces: y FH G + y = PH G F FT Design heuristics F G F R MR x PB ) S( y y ) FH PH Again, no rules of thumb available for selecting x. A reasonable first guess: For single reaction, choose x= x eq. 1

22 Reactor Heat Effects Reactor heat load reactor heat load= heat of reaction*fresh feed flow Ex. Heat load for HDA process. For x = 0.75, P B = 65 and F = QR = H R FFT = ( 1530)(73) = But / hr FT

23 Adiabatic temperature rise 3

24 Heuristics Since the availability of the hear transfer area is limited. The heat load is limited to 6-8 million Btu/hr. For a heat load of Q 1 10 A = = = 1000 ft U T (0)(50) The maximum heat trans fer area that fits into the shell of a floating - head heat exchanger is in the range of 6000 to 8000 ft. 6 Btu/hr, Heat carrier Since heat load depends on fresh feed flow rate and Tout is also a function of recycle flow rate. We can moderate the temperature change by increasing recycle flow rate. 4

25 Equilibrium Limitations exothermic 5

26 P tot MR T 6

27 Reactor/Separator 7

28 Compressor design and costs The design equation for a centrifugal gas compressor is: hp = γ 5 PinQ in P P out in γ 1 Remarks: 1. This is an expensive equipment and normally we do not have spares.. Heuristic for multistage compressor: P/P1=P3/P=... 8

29 Choice of Reactor Decisions -type -concentration -temperature -pressure -phase -catalyst Recation Path Path 1 Path Path 3 C C C C C H H H H H HCl + Cl Cl + 1/ O Cl C H + HCl heat C C C 3 Cl + HCl H H H Cl Cl C Cl + HCl H 4 Cl + H O EP = -$11kmolVCM EP = $8.89 kmolvcm EP = -$1.4 kmolvcm Remark: EP= values of products-raw materials costs 9

30 Types of recation systems Single Reaction : k A R k A + B R (irreversible) (irreversible) k1 f, k1 b A R (reversible) Parallel Recations : k1 A R (desired) k A S (waste) k1 A + B R (desired) k A + B S (waste) Consecutive Recations : k1 A R (desired) k R S (waste) Remark: There are a lot of more reaction systems, e.g., mixed parallel and consecutive reactions. 30

31 Reactor concentration and temperature The objectives to design the reactor concentration and temperature profiles are: O1. to improve selectivity (minimize the generation of byproducts) O. to increase economic potential (minimize reactor cost) O3. to facilitate downstream separation (decrease separation cost) O4. to possess operability (handle production rate changes) Remark: Certainly, there are cases which are important to ensure complete conversion of hazardous or corrosive material. 31

32 Reactor concentration - single reaction Single Reaction : A -use PFR (O) k R Single Reaction : A + B k R -A/B=50/50 is most economic but with little operability -if R is HK, make LK excess (Cheng and Yu) -if B is HK and R is IK, make B excess (Cheng and Yu) -degree of excess depends on the relative reactor/separator costs (Cheng and Yu, AIChE J003, 49, 68.) Remark: Note that: Total reaction rate : k ( T ) C A C B V R 3

33 Reactor concentration - parallel reactions For the followings reactions orders of reactions become important. Parallel Recations : A + B A + B 1 k1 k The selectivity is related to to maximize. r r = k k 1 C R (desired) S (waste) a1 a A C b1 b B r 1 / r r 1 r = = k 1 k C a1 A C C a A b1 B C b B which we want a 1 >a & b 1 >b : keep both C A and C B high a 1 >a & b 1 <b : keep C A high and C B low a 1 <a & b 1 <b : keep both C A and C B low a 1 <a & b 1 >b : keep C A low and C B high Ref: Ward et al. (IEC&R 004, 43, 3957) discuss operating policies for parallel reactions in planwide control. 33

34 Reactor type - parallel reactions Remark: Normally we set the temperature at the highest and yet acceptable level. 34

35 Reactor type - parallel reactions 35

36 Reactor temperature - reversible reaction A k f, kb R 1- x x x K eq = = 1 x (reversible) r k k f b = k k f 0 b0 e E f Eb RT = k f (1 x) k b x endothermic (E f >E b ): - high temperature favors equilibrium conversion and also gives higher reaction rate -set the temperature as high as possible exothermic (E f <E b ): - low temperature favors equilibrium conversion but high temperature gives higher reaction rate -set the temperature high initially and decrease the temperature as equilibrium approaches 36

37 Reversible reaction- remember physical chemistry A B + heat A + heat B 37

38 Implication in reactor design - reversible and exothermic Design: series of reactors with cold shot or intermediate heat exchangers with cold shot 38

39 Reactor heat removal 39

40 More reactor heat removal 40

41 Reactor pressure- vapor phase reaction Irreversible single reactions: -high pressure increases vapor density and thus gives higher reaction rate (smaller reactor volume if given conversion) Reversible single reactions: A==B -an increase in the pressure shifts the reaction toward compensating the pressure increase (RHS) and thus increases the equilibrium conversion (Le Chatelier s principle). A==B -an increase in the pressure shifts the reaction toward compensating the pressure increase (LHS) and thus decreases the equilibrium conversion. 41

42 Summary - heat removal 4

43 Summary - reactor design A--B A=B A--R A--S A+B--R A+B--S A--R--S 43

44 Recycle Economics input/output: favors zero conversion and no purge recycle: favor large conversion and purge 44