Distillation Column System Design, Operation, and

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Two-Stripper/Flash/Distillation Column System Design, Operation and Control for Separating 2-Pentanone/4-Heptanone/ Water Azeotropic Mixture via Navigating Residue Curve Maps and Balancing Total Annual Cost and Product Loss Ming Xia, Hui Shi, Congbiao Chen, Zhongyi Ma, Yong Xiao, Bo Hou, Litao Jia, and Debao Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02658 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Industrial & Engineering Chemistry Research

Two-Stripper/Flash/Distillation Column System Design, Operation and Control for Separating 2-Pentanone/4-Heptanone/Water Azeotropic Mixture via Navigating Residue Curve Maps and Balancing Total Annual Cost and Product Loss Ming Xia,† Hui Shi,† Congbiao Chen,† Zhongyi Ma,† Yong Xiao,† Bo Hou,† Litao Jia,† and Debao Li†, *

†State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

*E-mail: [email protected]; *Tel: +86 0351-4121793 Abstract: A novel method for the synergetic production of 2-pentanone and 4-heptanone has been explored in Institute of Coal Chemistry, Chinese Academy of Sciences (ICC, CAS) recently. The collected mixture, containing mainly 2-pentanone/4-heptanone/water/carbon dioxide, presents heterogeneous azeotropes. The separation of this quaternary mixture is the main problem urgent to be effectively solved. In this work, a two-stripper/flash/column flowsheet is proposed to achieve the separation, in which the heterogeneity of the system is fully utilized by using an overhead VLL flash. A general total annual cost (GTAC) for balancing total annual 1 ACS Paragon Plus Environment


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cost (TAC) and product loss (PL) is proposed as an optimization function and several optimization sequences are presented to facilitate the optimization. Four candidate sequences (S1, 2, 3, and 4) are derived from navigating the ternary and residue curve maps, of which the S4 cuts GTAC with a marked margin (83.9 %) compared with the S1. The rigorous economic design and optimum operation of the flowsheet are further investigated in details. Moreover, several control structures (TCS1, TCS2, and DTCS, CTCS, C&TCS) are successively explored with consideration of ordinary- and high-purity separation. Dynamic control shows that whereas the flowsheet can be controlled by controlling one tray temperature in each stripper/column for ordinary-purity separation, an expensive and high-maintenance online composition control should be considered for high-purity separation.

1 Introduction 2-Pentanone and 4-heptanone are important compounds extensively used in chemical, pharmaceutical and food industries. For instance, 2-pentanone is used as de-waxing agent, organic and pharmaceutical intermediate, and as painting solvent1. 4-Heptanone is commonly used as flavoring agent for tropical fruit, as nitrocellulose lacquer solvent, and as organic synthetic raw material1. Generally, 2-pentanone is produced via two main methods: dehydrogenation of 2-amyl alcohol and co-heat of butyryl ethyl acetate and water2. The coupling reaction of butyric acid over an oxide catalyst generates 4-heptanone, and the direct catalyzing route from 1-butanol to 4-heptanone has also been explored in recent decades for 2 ACS Paragon Plus Environment

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upstream extending3. In consideration of completely different producing methods and rather high prices of the two ketones4, 5 (11300 and 8870 USD/t for 2-pentanone and 4-heptanone), a novel method for the synergetic production of 2-pentanone and 4-heptanone via heterogeneous catalytic fixed-bed reactor has been explored by Group 610 at ICC, CAS. The reaction product after pre-separation has a composition of 19.5/20.45/60/0.05 mol % 2-pentanone/4-heptanone/water/carbon dioxide. Since there are quaternary components, in which 2-pentanone and 4-heptanone forms heterogeneous azeotropes with water at atmospheric pressure separately, the separation of this quaternary mixture is the main problem urgent to be effectively solved. A lot of papers have concentrated on the separation of azeotropic systems, mainly emphasizing on the design and control. Generally, pressure swing6-16, azeotropic17-23 and extractive distillation8, 9, 15, 21, 24 have been studied for various azeotropic systems, such as THF/water, isopropyl alcohol/water, ethanol/water, TAME/methanol/C5s for their industrial importance. However, pressure swing distillation requires that azeotropic composition should be changed significantly with pressure. Azeotropic distillation commonly needs effective light entrainer with azeotropic column overhead being condensated and further separated in a decanter. Extractive distillation needs introducing effective heavy entrainer, which is capable of altering the relative volatility of azeotropic components. Obviously, the above-mentioned distillation technologies possess their own scope of application, but it seems quite tough and unwieldy to apply these technologies in this quaternary separation. 3 ACS Paragon Plus Environment


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Fortunately, the heterogeneity of the system should be fully utilized to get the separation complete due to the formation of heterogeneous azeotropes. It has been found that rare papers deal with two binary heterogeneous azeotropic systems presenting the so-called ‘self-entrained’ behavior. This feature enables one azeotropic component to entrain the other azeotropic component, and the entrained vapor is further condensated and split to vapor, liquid 1 and liquid 2 phase in an overhead VLL flash. Hence, the heterogeneity of the system provides a promising and convenient approach to complete the separation by using the ‘self-entrained’ behavior. Literature on this distillation technique could be retrieved in Doherty and Malone25, in which a two-column distillation with a decanter was employed for the separation of a heterogeneous butanol/water azeotrope. For the ternary acetone/water/butanol system, Sticklmair and Fair26 discussed a two-column system. Pucci et al.27 proposed a single three-phase column with a side decanter used to remove water. In recent decade, Luyben28 studied the control of the two-column/decanter flowsheet suggested by Doherty and Malone. Wu et al.29 considered a two-stripper/decanter flowsheet for the separation of the C5/methanol azeotropes that takes advantage of the heterogeneity. However, these works are subjected to binary/ternary systems containing alcohol/lower ketone/water. Studies on system of higher ketones/water are limited, and the economic design, optimum operation and effective control are not systematically considered. This is the main motivation of this paper. In

this

work,

a

two-stripper/flash/column

flowsheet

for

separating

2-pentanone/4-heptanone/water/carbon dioxide system is investigated, with focus on 4 ACS Paragon Plus Environment

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the economic design, optimum operation and effective control. The proposed flowsheet takes advantage of the heterogeneity featuring ‘self-entrained’ behavior, and thereby no other entrainer is added. The VLL flash instead of decanter is employed to cross the distillation boundary, with the carbon dioxide being escaped through vapor stream. Firstly, the economic design of this flowsheet is screened and optimized via navigating residue curve map based on TAC ant GTAC. Furthermore, a trade-off between operation energy cost and PL in terms of GTAC is made to obtain the optimum recovery of 2-pentanone. At last, several control structures are explored and dynamic control of the optimum design is tested via introducing feed throughput and composition disturbances.

2 Process Studied A fresh feed from upstream preliminary separation has a flowrate of 12500 kg/h, with

a

composition

of

19.5/20.45/60/0.05

mol

%

2-pentanone/4-heptanone/water/carbon dioxide. The commonly market purity specification of 2-pentanone and 4-heptanone are both 99.0 wt %, which is also the desired specification. Thus we specify 99.5 wt % of the two products for overdesign. Because 2-pentanone, 4-heptanone and water form a minimum boiling binary heterogeneous azeotrope separately, conventional distillation systems cannot achieve complete separation. Fortunately, we can take advantage of the heterogeneity of 2-pentanone/4-heptanone/water to cross distillation boundary using an overhead VLL flash. As depicted in Figure 1, we initially adopt the two-stripper/flash flowsheet to 5 ACS Paragon Plus Environment


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achieve the separation of ketones and water, derived from the concept of Doherty & Malone25, Luyben28, and Wu29. Then 2-pentanone and 4-heptanone shall be easily separated via conventional distillation. Obviously, carbon dioxide is mainly escaped from vapor of the flash through pressure reduction and the system involves higher ketones/water/carbon dioxide. This is the main differences from these previous contributions.

V

M ainly C O 2

V1

O rg.Phase

A qu.Phase

V2

D3 2－P Feed 2－P/4－H /W /C O 2

4－H

B3

T

M ainly 2－P & 4－H M ixture B1

T

T

B2

Figure 1. Two-stripper/flash process flowsheet diagram.

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Pure W ater

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(a) Alternative 1 S1

S2 V V

V1

Org. Phase

Aqu. Phase

V1

Org. Phase

R2

R1

R1

Feed

V2

R2 Feed

2－H /4－P/W /C

B1

Aqu. Phase

2－H /4－P/W /C

B1

T

T

B2

T

(b)

Alternative 2 S3

S4 V

V1

Org. Phase

2－H /4－P/W /C

V1

Aqu. Phase

B1

T

Org. Phase

Feed

2－H /4－P/W /C

Aqu. Phase

R1

R2

R1

B1

V

Feed

V2

R2

T

T

B2

Figure 2. Alternative sequences for 2-pentanone/4-heptaone/water system: (a) Alternative 1: S1 & S2 and (b) Alternative 2: S3 & S4

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2.1 Process Alternative Statement Two process alternatives are explored for the ketones/water separation as described in Figure 2, which are one-stripper/flash scheme (A1) and two-stripper/flash scheme (A2). The two schemes differ on whether ketones in aqueous phase 1 (water-rich phase) from a decanter needs to be recovered. That is to say, S1 discards the aqueous liquid 1 for capital reduction and energy saving at the expense of ketones in aqueous liquid 1 while S2 increases the recovery of ketones by adding the other stripper. Moreover, the feed can either enter to a stripper, or enter to a decanter/flash since its composition is positioned in the immiscible field at the state studied. Therefore the potential feasible process shall be extended to four types of sequence (S1, 2, 3, and 4), as shown in Figure 2. The proposed flowsheet involving any sequence (S1, 2, 3, and 4) and a conventional mixed-ketones distillation column offers two functional sections. The proposed sequences are used to achieve the separation of ketones/water while the column to separate the two ketones. For instance of S1, the fresh feed is firstly fed to a stripper 1 in which the mixed ketones as heavy components are obtained at the bottoms, with ketones/water mixture as light being out at the overhead. The overhead vapor is condensated and further decompressed into a VLL flash at 313.15 K and ~16 kPa, and then the condensate has a composition located at the immiscible field and thereby forms vapor-liquid1-liquid2 phase. While the carbon dioxide-rich vapor is escaped, the liquid 1 mainly containing ketones are pumped to the strippers 1. The aqueous liquid 2 is discarded. As for the S2, The liquid 2 mainly containing water is further fed to stripper 2, in which almost pure water is obtained at the bottoms and the ketones/water mixture as light is stripped at the overhead. Then the mixed ketones shall be easily separated in a conventional distillation column 3, with 2-pentanone and 4-heptaone being out at the top and bottoms respectively. 2.2 Thermodynamic Property


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It is of vital importance to select a suitable thermodynamic method to describe this system accurately and precisely. Azeotropic data comparison is the most effective approach to validate the prediction accuracy. The vapor-liquid-liquid (VLL) data of 2-pentanone/water and 4-heptanone/water involved in open literature is limited, but we get the azeotropic datum from chemical handbook. As for the prediction in this study, the NRTL equation30 is used to calculate the activity of liquid, with Hayden-O’Connell equation31 accounting for the dimerization among the ketones in vapor. The azeotropic datum at pressure of 101.33 kPa in Cheng’s book1 and Azeotrope Data-II by Horsley32 and those in Aspen databank are listed in Table S1 (see the Supporting Information). It is found that the predicted azeotropic data of 2-pentanone/water and 4-heptanone/water systems are fairly in agreement with those in handbook, with only -3.11 % and +4.39 % relative deviations respectively in ketone compositions. This suggests that the NRTL-HOC method is capable of predicting the two azeotropes. Furthermore, since the binary interactive parameters of 2-pentanone/4-heptanone are not available in the embedded databank of Aspen, these parameters are estimated by using UNIFAC equation. However, in simulations it is found that the computed results are similar to these without estimated parameters using the UNIFAC, and therefore we do not use these parameters in the following study. It should be anticipated that the NRTL-HOC method is able to describe the VLL behavior accurately. The binary interaction parameters of NRTL-HOC method embedded in Aspen Plus are listed in Table S2 (see the Supporting Information).


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2.3 x-y, T-x-y and Ternary Phase Diagram and Residue Curves Map Analyses xy for Water/2-pentanone

1

1 atm 5 atm 10 atm

0.9

0.7

Temperature K

Vapor Massfrac Water

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0.6 0.5 0.4 0.3 0.2 0.1 0 0

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Liquid Massfrac Water xy for Water/4-heptanone

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0.9 0.8 0.7

Temperature K

Vapor Massfrac Water

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0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Liquid Massfrac Water

Figure 3. x-y and T-x-y diagrams of 2-pentanone/water and 4-heptanone/water systems at 1, 5, 10 atm


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(a)

(b)

Figure 4. Ternary phase and residue curve map analyses for 2-pentanone/4-heptaone/water system at 1.2 atm: (a) S1 & S2 and (b) S3 & S4.

x-y and T-x-y diagrams are effective tools for the feasibility of distillation sequence. Figure 3 gives the x-y and T-x-y diagrams for the two systems at different pressure (1, 5 and 10 atm). It is found that the separation difficulty of 2-panatone/water is moderate since the x-y curves


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apart from the diagonal. The separation of 4-hepatanone/water is fairly easy since the x-y curves are rather far away from the diagonal. The pressure effects on the x-y behavior shows that increasing pressure increases the separation difficulty both in the opposite side of the azeotropic point for the two systems. Therefore, operation pressure has to be set nearly atmospheric pressure in each stripper with a compromised consideration of separation difficulty and avoiding the use of vacuum device. The ternary phase map incorporated in residue curve map is a fairly useful method for concept design of distillation sequence. Herein carbon dioxide is not included in this analysis since its composition is rather low and it acts as the lightest component, which can be easily removed. Figure 4 gives the ternary phase diagram with residue curve map for 2-pentanone/4-heptaone/water system at 0.15 and 1.2 atm. It is found that an immiscible liquid-liquid field occupies a large area of the ternary phase map, and there are two heterogeneous azeotropic points, of which 2-pentanone/water azeotrope acts as unstable point while 4-heptanone/water azeotropic as saddle point. Pure 2-pentanone presents a saddle point while both pure 4-hepatone and pure water are stable points. Connecting the unstable and saddle points generates distillation boundary, which separates the phase map into two regions 1 and 2. Feed in any region is separated to two products, whose compositions must be always restricted at the same region but cannot cross the distillation boundary. Specifically, as depicted in Figure 4, the feed F1 and overhead reflux R1 give pseudo-mixture M1, which shall be separated to bottom B1 and top vapor withdrawal V1 that are positioned in same distillation region 1 in stripper 1. At last, the stream B1 almost composed of 2-pentanone and 4-heptanone shall be conventional distillated to two pure products D3 and B3. The phase liquid 1 in the flash is drained and entered to the top of stripper 2 as an overhead reflux R2 that shall be stripped to bottom water B2 and top vapor withdrawal V2. B2 and V2 are in same distillation region 2. The two top withdrawals V1 and V2 is joint and condensated into


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a flash which is further split to vapor withdrawal, liquid 1 and liquid 2. Thereby the separation of mixed ketones and water is feasible via concept design analyses.

3 Steady-State Optimization There are several parameters, two stripper number of stages (NT1, NT2), and the number of stage and feed stage of column 3 (NT3, NF3), which are varied by minimizing the TAC to search for the optimum design at certain specifications. The NT1 and NT2 are both initially set at 10 and 20 for the moderate separation difficulty. NT3 and NF3 are initially set at 30 and 15. The two-stripper/flash system and the mixed-ketone column do not interaction with each other since there is no recycle between them. Therefore, the entire process, divided to two sections: two-stripper/flash system and mixed-ketones distillation column, shall be optimized separately and successively. The specifications of two-stripper/flash system are 0.01 wt % H2O at B1 and 0 % 2-pentanone at B2 for reducing treatment cost of the wastewater. The specifications of C3 column involve two CASEs: 99.5 wt % 2-pentanone at D3 and 99.5 wt % 4-heptanone at B3; and 99.5 wt % 2-pentanone at D3 and 99.99 % initial recovery of 2-pentanone at D3. 3.1 Optimization Function and Sequences It has been widely recognized that total annual cost (TAC) recommended by Douglas, accounting for both equipment capital cost (CC) and energy cost (EC), is utilized as an optimization function for screening the optimum chemical process. It can be simply expressed by the equation: TAC (103 $/a) = EC + CC/Payback Period In which CC mainly involves major pieces of equipment, specifically, including column vessel and heat exchangers (condenser, reboiler, and cooler). EC refers to the utility consumption cost, mostly stream costs. Both equations for estimating CC and EC are covered


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in Douglas33, Turton34, and Luyben35 (see the Supporting Information). A small 3-year payback period is used because the total capital cost is about 3 to 4 times the CC that is an alternative approach to increase the CC by 3 and use a 9-year payback period, as suggested by Luyben35. Because of the fairly high price of the two products, both economically design and optimum operation should be considered for the ketones distillation column. We roughly regard 100 % recovery of ketones as a base case for this column, and thereby the value of the lost product is determined as following formula. Product Loss =∑[(, – ) × Price ] In which Pbc, i and Pi are the base case and practical flowrate of product i respectively, with Pricei is the market price of the product i. The Product Loss (PL) can effectively reflect the value lost as impurities at the both ends. Meanwhile, the 2-pentanone product costs much higher than the 4-hepatone does. This indicates that increasing the recovery of the higher-price product should be more favorable. Hence that the PL adds to TAC produces GTAC, namely general total annual cost, shall be a reasonable criterion for the optimizations of design and operation of the ketones distillation column: GTAC = TAC + PL The optimization sequences for the two separate sections are listed in Figure S1 (see the Supporting Information). At design stage for the two-stripper/flash section, NT1 and NT2 are the inner and outer search variables, which are varied to reach the minimized TAC. For the mixed-ketone distillation column, two CASEs (1 and 2) are optimized with rather different optimization sequence featuring two CASEs. For CASE 1, NT3 and NF3 are varied to reach the minimized TAC and GTAC; for CASE 2, NT3 and NF3, as inner variables, are still changed to reach the minimized TAC, but the 2-pentanone recovery, placed as an outer variable, is changed to obtain GTAC. Once the two CASEs 1 and 2 have been optimized, the


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GTACs for the two CASEs are compared, and the one with the smaller GTAC gives the optimum design. At operation stage, the C3 column has been designed and built, with the NT3, NF3, diameter and height all fixed. The operation recovery should be varied (decreased strictly speaking) to get the minimized GTAC via balancing the TAC and PL. It should be noticed that the operation recovery must be lower than the design recovery theoretically. This is because a higher recovery brings about higher energy consumption, implying larger diameter of column. However, the diameter has been fixed on operation stage.

3.2 Preliminary Sequences Screening Among the four sequences for the S1 as shown in Figure 2a, the fresh feed is directly fed into a stripper and an overhead vapor V1 is distilled from the top of the stripper 1, with a composition lying on the distillation boundary near the hypotenuse. This vapor is condensated and fed into a VLL flash, in which the condensate is separated to vapor, aqueous liquid R2 and organic liquid R1. The organic liquid R1 is entered in the top tray of the stripper 1 as an overhead reflux, with the aqueous liquid R2 being discarded and vapor being vented. As for the S2, the composition of each stream (F1, B1, V1, R1 and R2) are positioned at almost the same point as the S1 but the aqueous liquid R2 is further pumped to the stripper 2, in which the ketones-rich vapor are escaped in the overhead, with pure water being obtained in the bottoms. It is noticed that the overhead vapor composition for each sequence is located near the distillation boundary at the vicinity of the 2-pentanone/water azeotrope (Figure 4). It can be intuitively inferred that the revised sequences S3 and S4 should cut energy consumption, which shall be proven by following simulations. After comparisons among the four sequences, the preliminary screen results are listed in Table S3 (see the Supporting Information). It is found that the S3 (2.111 GJ/h) requires much less energy consumption than the S1 requires (14.912 GJ/h) in spite of more capital


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investment since M1V1/B1V1 in Figure 4a is considerably less than M1V1/B1V1 in Figure 4b and flowrate of M1 in Figure 4a is less than that of M1 in Figure 4b. Similarly, the S4 cuts about total energy consumption of 12.935 GJ/h in spite of more capital investment compared with the S2. The S1 is with a minimum TAC of 1.186 million USD/a. Meanwhile, in consideration of the ketones recovery for their high market prices of the ketones, the PL of S1 and S2 are 1.657 and 0.259 million USD/a, while the PL of S3 and S4 are 0.861 and 0.160 million USD/a respectively. This indicates that there exists a quite large margin through increasing the recovery of 2-pentanone via converting sequence from S1 to S2, or converting sequence from S3 to S4. It is noticed that the potential margin in order of magnitude is comparable with the TAC. Hence by balancing the TAC and PL, S4 has the minimum GTAC of 0.457 million USD/a, and it is a preliminary screening optimum sequence that needs to be further optimized.

3.3 Two-Stripper/Flash System Optimization There actually exist multiple designs that meet the ketone specifications for the S4. On one hand, if V2 is located close to the distillation boundary in Figure 4b, the smaller the included angle between B1V1 and horizontal right-angle edge, the less the V1 required for stripper 1. On the other hand, this overhead vapor flowrate should not be too small to supply the liquid overhead reflux for each stripper. There exists a proper V2 that enable to balance the energy consumption and the overhead refluxes. Hence, the V2 is located and settled by navigating residue curve maps as marked in Figure 4b in a compromise way, rigorous simulation is performed to obtain the preliminary optimized case of the S4. The NT1 are varied to investigate their effect on the TAC for fixed NT2 = 20 based on the preliminary optimum case of S4. Interestingly, as shown in Table S4 (see the Supporting Information), the reboiler duty of stripper 1 appears nearly flat as the NT1 is increased from


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10 to 20, and thus NT1 = 10 is selected as an optimum stage of stripper 1 for NT2 =20. The reboiler duty of stripper 2 is so small compared with that of stripper 1 that it can be only reduced with increasing NT2. In fact, changing NT2 hardly reduces the reboiler duty of stripper 2. Therefore, NT2 is set at 20. Hence the optimized design of two-stripper/flash system is obtained as NT1 = 10 and NT2 = 20 with a TAC of about 0.297million USD/a.

3.4 Mixed-Ketone Distillation Column Optimization The market purity of 2-pentanone and 4-heptanone is both 99.0 wt %, which is also the desired specification. Herein, we specify 99.5 wt % of the two products for overdesign to deal with the changing marketing demand. After the optimum design of two-stripper/flash section has been obtained, there are two CASEs to perform the optimization of the mixed-ketone distillation column. The conventional one is to use ‘Design Spec/Vary’ to achieve 99.5 wt % for both products. The other one is to specify 99.5 wt % purity and 99.99 % recovery of 2-pentanone also using ‘Design Spec/Vary’ section. The optimizing results shall be evaluated based on GTAC throughout the simulations. In the latter CASE, we assume a recovery of 100 % as a base case for 2-pentanone in mixed-ketone distillation column optimization.


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TAC vs NT3 at various recovery

10 5

5.5

99.00 % 99.50 % 99.90 % 99.99 % 99.999 %

5.4 5.3

TAC $/a

5.2 5.1 5 4.9 4.8 4.7 20

22

24

26

28

30

32

34

36

38

40

NT3

TAC2 $/a

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Figure 5. the effect of NT3 on the TAC, PL, and GTAC at 99.5 wt % purity and various recovery (99.0 %, 99.5 %, 99.9 %, 99.99 % and 99.999 %) of 2-pentanone


18

GTAC $/a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EC $/a

Page 19 of 41

Figure 6. The effect of operation recovery on GTAC, EC and PL at NT3 = 25 and NF3 = 11 with a specification of 99.5 wt % 2-pentanone.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Feed

V

313 K ,16.7 kPa 24.37 kg/h 47.28 w t % 2－H V1 13.97 w t % 4－P 366 K ,121.6 kPa 16.61 w t % W 22.14 w t % C 253.64 kg/h 47.80 w t % 2－H 24.41 w t % 4－P 26.67 w t % W 1.13 w t % C

D3 375K ,101.3 kPa 5731.45 kg/h 4123.36 kg/h 99.50 w t % 2－H 4.76E -3 w t % 4－P 2.38E -2 w t % W 0 w t% C

Condenser Duty -101.66 kW

R eflux R atio = 1.246

B1

399K ,127.9 kPa 9830.14 kg/h 41.78 w t % 2－H 58.21 w t % 4－P 0.01 w t % W 3.14E -19 w t % C

B3

420 K ,111.1 kPa 5706.78 kg/h 7.17E -2 w t % 2－H 99.93 w t % 4－P 4.37E -39 w t % W 0 w t% C

T

Reboiler Duty 1024.41 kW

Page 20 of 41

313 K ,51.6 kPa 12500 kg/h 32.95 w t % 2－H 45.81 w t % 4－P 21.20 w t % W 4.32E -2 w t % C

Cooler Duty -221.92 kW

V2 374 K ,121.6 kPa 155.90 kg/h 38.68 w t % 2－H 3.66 w t % 4－P 57.58 w t % W 8.17E -2 w t % C

R1

313 K ,121.6 kPa 10083.77 kg/h 41.93 w t % 2－H 57.36 w t % 4－P 6.80E -1 w t % W 2.84E -2 w t % C

R2

313 K ,121.6 kPa 2801.18 kg/h 2.15 w t % 2－H 2.04E -1 w t % 4－P 97.64 w t % W 4.55E -3 w t % C

T


T

B2

381 K ,134.9 kPa 2645.29 kg/h 4.78E -31 w t % 2－H 4.92E -21 w t % 4－P 100.00 w t % W 3.66E -21 w t % C


Figure 7. The final optimized design and operation parameters of the proposed flowsheet. Table S5 gives the case study results with initial parameters from “DSTWU” block based on the CASE 1 with the specifications of 99.5 wt % 2-pentanone and 99.5 wt % 4-heptanone (see the Support Information). It is found that the optimized design based on the CASE 1 is NT3=25 and NF3=14 with a minimum GTAC of 0.493 million USD/a. Figure 5 depicts the case study results based on the CASE 2 with specifications of 99.5 wt % and various recoveries of 2-pentanone. It is revealed that the optimized design based on the CASE 2 is NT3=25 and NF3=11 with a minimum GTAC of 0.450 million USD/a and a slight higher TAC of 0.499 million USD/a. Note that the minimized GTAC and TAC of the CASE 2 are quite different from those (0.493 and 0.482 million USD/a) of the CASE 1 obtained. This indicates that a small increase in TAC reduces PL much more, and the GTAC balancing TAC and PL is a reasonable criterion to find the optimum design, especially on high-price product. The optimum design on the CASE 2 cut 8.7 % GTAC compared with the one on the CASE 1. This is because 2-pentanone cost higher than 4-heptaone, and thereby increasing the recovery of 2-pentanone can significantly reduce the PL in spite of a small increase in TAC. Thus the


20

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optimum design of mixed-ketone column is NT3=25 and NF3=11 with a minimum GTAC of

0.450 million USD/a. After the column has been designed and built, the capital cost is no longer relevant. In this case, reducing energy consumption saves the energy costs but decrease the recovery of the 2-pentanone, resulting in decrease in PL. Thus, energy costs and the PL should be balanced based on GTAC. Since the column has been designed and built at 99.9 % recovery of 2-pentanone, the operation recovery has to be investigated below 99.9 %. This is because higher operation recovery requires larger dimensions of the column and heat exchanger. As depicted in Figure 6, as the operation recovery is increased from 99.0 % to 99.9 %, the energy cost slightly increases in contrast to marked decreasing PL. Hence, the optimum operation recovery should be kept at 99.9 % on the upper boundary. Up to now, the optimum design of two-stripper/flash/column flowsheet has been obtained, with detailed information described in Figure 7. Because the economic design and optimum operation cannot justify the practical physical run if the controllability is not investigated, dynamic control should be the other important aspect to be examined. Hence, in the following section, several control structures are explored and dynamic controllability is tested by introducing disturbances in throughput and feed composition.

4 Dynamic Control Strategies Luyben18, Wu et al.19 have investigated the heterogeneous azeotropic distillation sequence without adding entrainer. Luyben18 studied the control of n-butanol/water system with three different feed compositions and developed a simple effective control structure that is capable of handling fairly large feed disturbances. Wu et al.19 considered the separation of the C5/methanol azeotrope that takes advantage of the heterogeneity of the azeotrope, in which


21


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Page 22 of 41

they found that the process is a factor of 4 less than that of the pressure-swing system, with an effective but ease control by using a tray temperature in each stripper. In this work, the proposed flowsheet owns two-stripper/flash section and a distillation column, in which the flash features VLL equilibrium throughout this study, and thereby the stripper 1 bottoms produces the mixed-ketones. This is the most important difference compared with the flowsheet by Luyben and Wu et al. Moreover, in light of high-price of the two target products, a tighter control is needed. Hence, based on their contributions, several control structures will be presented with dynamic controllability to be tested by introducing feed disturbances. Figure S2 gives the temperature and liquid composition profiles in each unit (see the Support Information). The slope criterion36, 37, consisting in selecting the tray where there are large changes in temperature from tray to tray, is employed to find the appropriate controlled-temperature tray. It is obvious that the tray 6 and tray 2 are the appropriate location in stripper 1 and 2, on which the temperatures need to be controlled. The tray 16 temperature shall be hold in ketone column 3. All of the loops established are proportional and integral (PI) controllers. Pressure and level controls are specified in conventional way. Temperature and composition are PI control, with deadtime of 1 and 3 minute respectively due to the existence of measurement and actuator lags in any real physical system. Relay-feedback tests are made on the temperature and composition controllers to obtain the ultimate gains and periods. Then Tyreus-Luyben tuning method is employed to obtain the gain Kc and integral time constant τI. Each disturbance is added at the time equal 0.2 h and is rerun to 10 h. The composition changes are introduced as following, e.g. the composition of 2-P is increased by 20 % with that of the 4-H being decreased by 20 % and the other two components are kept constant. As for the other composition changes, there is the similar case.


22

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4.1 Temperature Control Structure: TCS1 and TCS2

R3/F Ratio

Figure 8. Control structure with R3/F ratio: TCS1


23


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Page 24 of 41

(a) +20 % F

10 -4

3

-20 % F

1.01 1.005

2

1 1

0.995

0

0.99 0

5

10

0

1

5

10

5

10

1 0.9995

0.99

0.999 0.98

0.9985

0.97

0.998 0

5

10

0

Time(hr)

Time(hr)

(b) +20 % 2-P/-20 % 4-H

10 -4

3

-20 % 2-P/+20 % 4-H

1.01 1.005

2

1 1

0.995

0

0.99 0

5

10

0.996

0

5

10

5

10

1 0.9995

0.9955

0.999 0.995

0.9985

0.9945

0.998 0

5

Time(hr)

10

0

Time(hr)

Figure 9. Dynamic responses to disturbances in throughput and feed composition using TCS1: (a) 20 % F, (b) 20 % 2-P/4-H, (c) 20 % 4-H/W, and (d) 20 % W/2-P. solid lines denote increase, and dashed lines denote decrease.


24

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(c) +20 % 4-H/-20 % W

10 -4

2

-20 % 4-H/+20 % W

1.01 1.005

1.5

1 1

0.995

0.5

0.99 0

5

10

0

0.998

5

10

5

10

0.9994

0.996

0.9993

0.994 0.9992

0.992 0.99

0.9991 0

5

10

0

Time(hr)

Time(hr)

xB2(W) xB3(4-H)

xB1(W)

(d)

xD3(2-P)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Continued.


25


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Page 26 of 41

QR3/F Ratio Figure 10. Improved temperature control with schemes of QR3/F ratio and liquid level-flowrate cascade: TCS2


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(a) 10 -4

3

1.01 1.005

2

1 1

0.995

0

0.99 0

5

10

+20 % F,TCS1

0

-20 % F,TCS1

1

5

+20 % F,TCS2

10 -20 % F,TCS2

1 0.9995

0.99

0.999 0.98

0.9985

0.97

0.998 0

5

10

0

Time(hr)

5

10

Time(hr)

(b) 10 -4

3

1.01 1.005

2

1 1

0.995

0

0.99 0

5

+2-P/-4-H,TCS1

10 -2-P/+4-H,TCS1

0.996

0

5

+2-P/-4-H,TCS2

10 -2-P/+4-H,TCS2

1 0.9995

0.9955

0.999 0.995

0.9985

0.9945

0.998 0

5

Time(hr)

10

0

5

10

Time(hr)

Figure 11. Dynamic responses to the identical disturbances in throughput and feed composition using TCS2 in comparison with TCS1: (a) 20 % F, (b) 20 % 2-P/4-H, (c) 20 % 4-H/W, and (d) 20 % W/2-P. solid lines denote increase, and dashed lines denote decrease.


27


xB2(W) xB3(4-H)

xD3(2-P)

xB1(W)

(c)

xB2(W) xB3(4-H)

xB1(W)

(d)

xD3(2-P)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41



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Convention

control

experiences

suggest

that

one

temperature

control

with

reflux-to-distillate ratio and reflux-to-feed ratio is capable of stabilizing the ketone distillation column. More, the one with reflux-to-feed ratio usually do a better job than the one with reflux-to-distillate ratio. Therefore, the structure with reflux-to-feed ratio is initially considered as a basic one for the proposed flowsheet. Figure 8 gives the temperature control structure with reflux-to-feed ratio (TCS1), and the tuning results are listed in Table S6 (see the Supporting Information). Figure 9 gives the dynamic responses to feed disturbances in throughput and compositions using TCS1. It is observed that the TCS1 handle the feed changes well with small offsets except that the -20 % F and -20 % C7/+20 % W are with rather large dynamic deviations. In order to suppress the large transient dynamic deviations, a heat input-to-feed ratio scheme and liquid level/feed cascade to ketone column are inserted to offer quick and precise responses to throughput change. This improved structure denoted as TCS2 is shown in Figure 10, with tuning results listed in Table S7 (see the Supporting Information). Dynamic responses to the identical disturbances show that the large dynamic deviations are effectively suppressed, especially for the throughput change, as shown in Figure 11. 4.2

Dual-Temperature,

Composition/Temperature

Cascade,

and

Composition

&

Temperature Control Structures: DTCS, CTCS and C&TCS


29


(a) 10 -4

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10

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10 -20 % F,CTCS

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0.995

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xB2(W)

xB1(W)

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+2-P/-4-H,TCS2

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0

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5

+2-P/-4-H,CTCS

10 -2-P/+4-H,CTCS

1

xB3(4-H)

xD3(2-P)

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0.9955

0.995

0.9995 0.999 0.9985

0.9945

0.998 0

5

Time(hr)

10

0

5

10

Time(hr)

Figure 12. Dynamic responses to disturbances in throughput and feed composition using CTCS in comparison with TCS2: (a) 20 % F, (b) 20 % 2-P/4-H, (c) 20 % 4-H/W, and (d) 20 % W/2-P. solid lines denote increase, and dashed lines denote decrease.


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xB2(W) xB3(4-H)

xB1(W)

(c)

xD3(2-P)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(d) 10 -4

2

1.01 1.005

1.5

1 1

0.995

0.5

0.99 0

5

+2-P/-W,TCS2

10

0

-2-P/+W,TCS2

0.998

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+2-P/-W,CTCS

10 -2-P/+W,CTCS

1

0.996

0.995

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0.985 0

5

10

0

Time(hr)

5

10

Time(hr)



31


xD3(2-P)

xB2(W) xB3(4-H)

xB1(W)

(a)

xB2(W) xB3(4-H)

xB1(W)

(b)

xD3(2-P)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 41

Figure 13. Dynamic responses to disturbances in throughput and feed composition using C&TCS in comparison with TCS2: (a) 20 % F, (b) 20 % 2-P/4-H, (c) 20 % 4-H/W, and (d) 20 % W/2-P. solid lines denote increase, and dashed lines denote decrease.


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(c) 10 -4

1.4

1.01

1.2

1.005

1

1

0.8

0.995

0.6

0.99 0

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+4-H/-W,TCS2

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+4-H/-W,C&TCS

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10 -4-H/+W,C&TCS

1 0.9995

0.995

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0.9985

0.985

0.998 0

5

10

0

Time(hr)

5

10

Time(hr)

xB2(W) xB3(4-H)

xB1(W)

(d)

xD3(2-P)

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Though the TCS2 holds the two product purities fairly close to the desired values, there are some small offsets at the new steady state. It may be acceptable for 99.0 wt % desired specifications. However, if the need of 2-pentanone product with 99.5 wt % specification is soared, we have to explore improved structure to meet the increasing market demand. Hence, a dual-temperature control structure (DTCS) is firstly considered, with reflex-to-feed ratio being manipulated to control tray-8 temperature and with reboiler heat input-to-feed ratio being manipulated to hold tray-16 temperature. However, it is analyzed with SVD method in steady state that the condition number is rather large, which implies the two control loops may be interactive rather than independent. The control structure, dynamic responses and tuning results for DTCS are given in Figure S3, S4 and Table S8 (see the Supporting Information), it is apparent that the DTCS offers normal control for some of the disturbances, but there exist a little large new steady-state offsets in 2-pentanone purities for composition disturbances. Therefore, a composition/temperature cascade control structure have to be investigated to improve the product quality of 2-pentanone. The structure and tuning results of CTCS in detail are shown in Figure S5 and Table S9 (see the Supporting Information). Figure 12 gives the dynamic response comparison between the CTCS and the TCS2. It is observed that the structure CTCS has a similar settling time but offers fair good dynamic performance compared with the TCS2. The CTCS significantly suppresses the dynamic deviation of the product purities for throughput changes, except for the -20% F in 4-heptanone purity. As for the composition disturbances, both the dynamic deviations and new steady-state offsets essentially reduced with the product purities being hold fairly close to the specifications, except for the composition disturbances of +20% 4-H/-20%W and +20% 2-P/-20% W in the 4-heptanone purities. It is also interesting to investigate composition & temperature control structure (C&TCS) for effective control of column 3. The C&TCS features that the impurity of 4-heptanone in


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the distillate is directly control by adjusting R3/F ratio, and tray 16 temperature is controlled by adjusting QR3/F, as depicted in Figure S6 (see the Supporting Information). The tuning results is given in Table S10 (see the Supporting Information). The dynamic responses to feed disturbances using C&TCS in comparison with TCS2 are presented in Figure 13. Obviously, the dynamic deviation and new steady-state offsets are effectively suppressed and reduced respectively and the product purities reach fairly close to the specifications, implying that C&TCS provides fairly superior control to any structure above for the proposed flowsheet.

5 Conclusions In this work, a two-stripper/flash/column flowsheet is proposed for the separation of 2-pentanone/4-heptanone/water/carbon dioxide azeotropic mixture, with focus on the economic design, optimum operation and effective control. To cross the distillation boundary, the heterogeneity of the azeotropes of 2-pentanone/water and 4-heptanone/water is fully utilized by using an overhead VLL flash. Four sequences are economically compared to reach the preliminary optimized sequence (S4) by navigating the residue curve maps based on TAC and GTAC. Balancing TAC and PL generates the economic design, which are NT1 = 10, NT2 =20 and NT3 = 25. Furthermore, a trade-off between energy cost and PL is made to get the optimum operation, with an optimum recovery of 99.9 % for the 2-pentanone. With several control structures explored, dynamic control effectiveness are evaluated by introducing feed disturbances using dynamic simulations. The basic TCS1 handles the feed changes well with small offsets except that the -20 % F and -20 % C7/+20 % W are with large dynamic deviations. Further, an improved structure TCS2 proposed substantially suppresses the large dynamic deviations are for throughput changes, with comparable dynamics to TCS1 for composition changes. To meet marketing change and reduce new


35


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Page 36 of 41

steady-state offsets in composition changes, the structure DTCS, CTCS, and C&TCS are successively put forward. It is revealed that although DTCS and CTCS seem to handle disturbances, there still exist a little large new offsets for some composition changes. The C&TCS offers a superior control, with small offsets for almost all disturbances. The drawback of the C&TCS is that an expensive and high-maintenance online composition measurement is needed. Therefore, a detailed economic evaluation should be made for decision-making in production, whether produces ordinary-purity ketones with inexpensive temperature control or produces high-purity ketones with expensive composition control with carefully consideration of prices for the two specifications.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR IMFORMATION Corresponding Author *E-mail: [email protected]. Tel: +86 0351-4121793.

Funding Sources Coal Base Key Technologies R & D Program of Shanxi Province (Grant No. MH2014-13).

ACKNOWLEDGMENTS M. Xia et al. appreciate the assistance from the staff in Group 610 at State Key Laboratory of Coal Conversion, ICC, CAS and the invaluable advice from Prof. Chunjian Xu at Tianjin


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University. This work is financially supported from Coal Base Key Technologies R & D Program of Shanxi Province (Grant No. MH2014-13).

NOMENCLATURE 2-P = 2-pentanone component 4-H = 4-heptanone component Bi = mass flowrate of column/stripper i bottoms CD = carbon dioxide component CI = capital investment D3 = distillate mass flowrate of column 3 EC = energy cost Kc = controller gain NTi = number of stage for column/stripper i NFi = feed stage for column/stripper i PL = product loss Pricei = price of product i Pbc, i = mass flowrate of product i in base case Pi = practical mass flowrate of product i R1 = mass reflux rate of stripper 1/organic flowrate from flash R2 = mass reflux rate of stripper 2/aqueous flowrate from flash TAC = total annual cost GTAC = general total annual cost Vi = overhead vapor mass flowrate of stripper i W = water component τI = integral time constant


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LITERATURE REVIEWED 1.

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2.

WebBook, N. C. http://webbook.nist.gov. (2016.10.14),

3.

Plint, N.; Ghavalas, D.; Vally, T.; Sokolovski, V. D.; Coville, N. J., Catalysed Synthesis of 4-Heptanone from 1-Butanol. Catalysis Today 1999, 49, 71-77.

4.

Molbase, http://chanpin.molbase.cn/c107-87-9/. (2016.10.14),

5.

Molbase, http://chanpin.molbase.cn/c123-19-3/. (2016.10.14),

6.

Abu-Eishah, S. I.; Luyben, W. L., Design and Control of a Two-Column Azeotropic Distillation System. Industrial & Engineering Chemistry Process Design and

Development 1985, 24, 132-140. 7.

Al-Arfaj, M. A.; Luyben, W. L., Plantwide Control for Tame Production Using Reactive Distillation. AIChE Journal 2004, 50, 1462-1473.

8.

Luyben, W. L., Comparison of Pressure-Swing and Extractive-Distillation Methods for Methanol-Recovery Systems in the Tame Reactive-Distillation Process. Industrial &

Engineering Chemistry Research 2005, 44, 5715-5725. 9.

Luyben, W. L., Comparison of Extractive Distillation and Pressure-Swing Distillation for Acetone-Methanol Separation. Industrial & Engineering Chemistry Research 2008, 47, 2696-2707.

10. Luyben, W. L., Design and Control of a Fully Heat-Integrated Pressure-Swing Azeotropic Distillation System. Industrial & Engineering Chemistry Research 2008, 47, 2681-2695. 11. Yu, B.; Wang, Q.; Xu, C., Design and Control of Distillation System for Methylal/Methanol Separation. Part 2: Pressure Swing Distillation with Full Heat Integration. Industrial & Engineering Chemistry Research 2012, 51, 1293-1310. 12. Li, W.; Shi, L.; Yu, B.; Xia, M.; Luo, J.; Shi, H.; Xu, C., New Pressure-Swing Distillation for Separating Pressure-Insensitive Maximum Boiling Azeotrope Via Introducing a Heavy Entrainer: Design and Control. Industrial & Engineering Chemistry

Research 2013, 52, 7836-7853. 13. Wang, Y.; Cui, P.; Zhang, Z., Heat-Integrated Pressure-Swing-Distillation Process for Separation of Tetrahydrofuran/Methanol with Different Feed Compositions. Industrial

& Engineering Chemistry Research 2014, 53, 7186-7194. 14. Wang, Y.; Zhang, Z.; Zhang, H.; Zhang, Q., Control of Heat Integrated Pressure-Swing-Distillation

Process

for

Separating

Azeotropic

Mixture

of


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Distillation Column System Design, Operation, and

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