Ahtesham JAVAID1, Costin Sorin BILDEA2

An integrated process for the production of γ-butyrolactone by coupling maleic anhydride hydrogenation and 1, 4-butanediol dehydrogenation in a fixed bed adiabatic reactor is studied. The reaction conditions realizing optimal hydrogen utilization and better energy efficiency are determined. Compared to stand-alone processes, the integrated process has several advantages, such as simpler reactor, easy temperature control, improved γ-butyrolactone yield, good energy efficiency and optimal hydrogen utilization. The integrated plant shows robust behavior with respect to disturbances and allows production rate change.

Keywords: γ-butyrolactone, reaction coupling, process design and control.

1. Introduction

γ-butyrolactone (GBL) is known as a versatile solvent and a raw material for the synthesis of pyrrolidone, N-methyl pyrrolidone, N-vinyl pyrrolidone, herbicides, and rubber additives. GBL is manufactured via two methods: hydrogenation of maleic anhydride (MA) and dehydrogenative cyclization of 1,4- butanediol (BDO). Both processes are typically performed in multi-tubular fixed- bed reactors [1]. The catalytic hydrogenation of MA or its derivatives such as maleic acid, succinic anhydride or succinic maleate to GBL has frequently been reported in the literature [2]. The reaction equation of MA hydrogenation to GBL can be represented as follows:

C4H2O3 + 3H2 → C4H6O2 + H2O ∆H = -211 kJ/mol (1)

It is obvious from equation (1) that production of 1 mole of GBL requires 1 mole of MA and 3 moles of H2, releasing 211 kJ of heat. Owing to the strong exothermic nature of the reaction, it is difficult to control the temperature of the process. In a tubular fixed-bed reactor, this might cause thermal runaway, lead to apparent hotspots and low selectivity to the desired product, GBL. A large excess of hydrogen is needed.

1 PhD student, Faculty of Applied Chemistry and Materials Science, University POLITEHNICA of Bucharest, e-mail: a_javaid@chim.upb.ro

2 Prof., Faculty of Applied Chemistry and Materials Science, University POLITEHNICA of Bucharest, e-mail: s_bildea@upb.ro

34 Ahtesham Javaid, Costin Sorin Bildea

The vapor phase catalytic dehydrogenation of BDO to GBL has also been described in the literature [3]. This reaction can be expressed as follows:

C4 H10O2 → C4 H6O2 + 2H2 ∆H = +61.6 kJ/mol (2)

From equation (2) it is evident that production of 1 mole of GBL requires 1 mole of BDO, releasing 2 moles of H2 and requiring 61.6 kJ of heat. The hydrogen released from reaction cannot be used properly in a single dehydrogenation process.

The new catalytic process proposed in this work combines reactions (1) and (2) into one coupled system which can significantly improve the yield of the catalytic hydrogenation of MA, apart from the better thermal balance and the effective usage of hydrogen through hydrogen transfer between the two reactants. In addition, this new integrated process produces one desired product (GBL) with a substantially increased hydrogenation yield. The new combined reaction can be expressed as follows:

C4H2O3 + 1.5 C4H10O2 → 2.5 C4H6O2 + H2O ∆H = -119 kJ/mol (3)

The MA to BDO ratio used in the process is 1:1.5 to balance the hydrogen consumed and produced in the system. Due to this perfect hydrogen balance it was expected to easier control of the reactor temperature due to less heat released as compared to individual MA hydrogenation. From equation (3), it can be noted that 2.5 mole of GBL requires 1 mole of MA and 1.5 moles of BDO.

The system can also be coupled on the basis of balancing heat. The combined reaction can be expressed as follows:

C4H2O3 + 3C4H10O2 → 4C4H6O2 + H2O + 3H2 ∆H=-25kJ/mol(4)

From equation (4) it can be noticed that 4 moles of GBL requires 1 mol of MA and 3 moles of BDO. The MA to BDO ratio used in the process is 1:3 to balance the heat consumed and produced in the system. Due to this almost perfect heat balance the adiabatic operation of the chemical reactor becomes an attractive option.

The combined system for hydrogenation of MA and the dehydrogenation of BDO could be carried out over the same Cu-based catalyst, and under similar reaction conditions. The reaction kinetics used for the simulations are extracted from the literature based on Cu-Zn-Al catalyst:

Maleic anhydride hydrogenation [4]:

k = 4.68×105 ⋅exp (-5800/T) kmol×m-3×s-1×Pa-1 (5) 1

Integrated process for γ-butyrolactone production 35

1, 4-butanediol dehydrogenation [5]:

k = 2.57×1017 ⋅exp (-262/T) kmol×m-3×s-1×Pa-1 (6) 1

The coupled reactions can be performed at temperatures which are 30–50 °C less as compared to individual reaction, because the active hydrogen species on the catalyst surface from BDO dehydrogenation promotes the MA hydrogenation reaction [6], [7]. The coupling may lead to advantages, such as good energy efficiency, the easy temperature control in an adiabatic reactor due to its moderate heat release compared with the single reaction processes which can avoid the formation of apparent hotspots or cold spots [6], [7].

The previous studies [6], [7] considered the possibility of coupling these reactions taking into account only the reactor. However, no plantwide design and control results have been reported. The goal of this paper is to fill this gap by presenting details of an integrated process for 1,4-butanediol (BDO) dehydrogenation and maleic anhydride (MA) hydrogenation, to produce one valuable product, γ-butyrolactone (GBL). The advantages of optimum hydrogen and energy utilization, reduced parametric sensitivity, dynamic stability and safe operation are emphasized. Our conclusions are supported by rigorous steady state and dynamic simulations performed in Aspen Plus and Aspen Dynamics. Conclusion and final remarks end the paper.

2. Integration by Hydrogen Balance

This section presents a process which couples dehydrogenation of 1,4- butanediol and hydrogenation of maleic anhydride in a single adiabatic reactor. The benefit of coupling the exothermic and endothermic reactions (hydrogenation and dehydrogenation) is that the heat generated by the first reaction is taken up by the second one, and no additional heating or cooling arrangements are required in the system. This leads to a much simpler reactor and the parametric sensitivity of the reactor is greatly reduced. The conversion of both maleic anhydride and 1, 4- butanediol is > 99.5%, eliminating recycling of reactants. Fig. 1 presents the flowsheet of the integrated maleic anhydride hydrogenation and 1, 4-butanediol dehydrogenation process. Reactants (MA – 20 kmol/h, BDO – 30 kmol/h and hydrogen – 5000 kmol/h) are mixed and brought to reaction temperature (210 oC). The reactions take place in a tubular reactor (5 m diameter, 3 m length) which is adiabatically operated. The reactor is designed for total conversion of reactants. Stream table of the process is given as Table 1. Fig. 2 shows mole fraction and temperature profiles along the reactor. The conversion of reactants into products is uniformly distributed along the reactor length and almost constant temperature should be remarked.

36 Ahtesham Javaid, Costin Sorin Bildea

The reactor outlet is cooled to 20 oC and vapor – liquid separation is performed. As the complete conversion (> 99.5%) of reactants is achieved, there is no recycling of reactants simplifying technical difficulties. A high boiling point difference between water and GBL results in easier separation by distillation. GBL is obtained as a product at the rate of 49.6 kmol/h with high purity > 99.5%. In theory, the plant is best operated at MA: BDO = 1: 1.5 with very high ratio of hydrogen: feed = 100:1 to 200:1. Perfect balancing of hydrogen consumption / generation in the two reactions is impossible in practice. Therefore, a small amount of fresh and purge hydrogen streams are provided for better control purposes.

1

PURGE

SPLITER

H2

H2MIXER

COMPRESR

ROUT

H2IN

MA BDO

10

H2REC

RIN

13

H2SEP

PURG

FLASH

MIXER

HEATER

WATER

GBL

2

SEPIN

250 240 230 220 210 200

0.01 0.008 0.006 0.004 0.002

0

REACTOR

COOLER

Fig. 1. Integrated Process

0 0.5 1 1.5 2 2.5 3

0 0.5 1 1.5 2 2.5 3

L / [m]
Fig. 2 Temperature and Molar Profile

L / [m]

BDO MA

GBL

T / [oC]

Mole Fraction

Integrated process for γ-butyrolactone production 37

Table 1

Stream Table (Hydrogen Balance)

MA BDO GBL H2REC RIN ROUT SEPIN WATER

Temperature (oC)
Pressure (bar)
VaporFraction 0 0 0 1 1 1 0

20 1.2 0.001 14.94

0.001 0.984 0.007 0.003 0.005

Mole Flow (kmol/hr) Mole Fraction Hydrogen Water
MA
GBL
BDO

20 30

0 0 0 0 1 0 0 0 0 1

49.62 4548

5098 5118

64.56

0 0.228 0.002 0.769 0.001

25 25 20 20 210 230.3 20 1.5 1.5 1.2 1.2 1.5 1.5 1.2

0 0.989
0 0.01
0 0
1 00 0.01 0 0 0.006 0

Reactants (MA – 20 kmol/h, BDO – 60 kmol/h and hydrogen – 3200 kmol/h) are mixed and brought to reaction temperature (220 oC). The reactions take place in a tubular reactor (4 m diameter, 3 m length) which is adiabatically operated. The reactor is designed for the total conversion of reactants. Stream table of the process is given as Table 2. Fig. 3 shows mole fraction and temperature profiles along the reactor. The conversion of reactants into products is uniformly distributed along the reactor length and almost constant temperature should be remarked.

280 260 240 220 200 180

0.025 0.02 0.015 0.01 0.005 0

0.981 0.977 0.009 0.013 0.004 0

3. Integration by Heat balance

BDO

MA

GBL

0 1 2 3

0 1 2 3

L / [m]
Fig. 3 Temperature and Molar Profile

L / [m]

T / [oC]

Mole Fraction

38

Temperature (oC)

Ahtesham Javaid, Costin Sorin Bildea

Stream Table (Heat Balance)

MA BDO GBL H2REC RIN 25 25 203 20 220 1.5 1.5 1 1.2 1.5

Pressure (bar)
VaporFraction 00011100

Mole Flow (kmol/hr) Mole Fraction Hydrogen Water
MA
GBL
BDO

20 60 79.84

0 0 0 0 0 0 1 0 0 0 0 1 0 1 0

3200 3310 3390

0.991 0.967 0.962 0.009 0.009 0.014 0 0.006 0

0 0 0.024 0 0.0018 0

64.56
0 0

Table 2

ROUT SEPIN
235 20 45

1.5 1.2 1

0.228
0.002 0

0.769
0.001 0

WATER

19.22

0.996

0.004

The reactor outlet is cooled to 20 oC, and vapor – liquid separation is performed. The hydrogen is recycled. The amount of hydrogen generated by the reactions is withdrawn and can be stored and used for other purposes. As the complete conversion (> 99.5%) of reactants is achieved, there is no recycling of reactants simplifying technical difficulties. A high boiling point difference between water and GBL result in easier separation by distillation. GBL is obtained as a product at the rate of 79.8 kmol/h with high purity > 99.5%. In heat coupled system, the plant can be operated at much lower ratio, hydrogen : feed (BDO + MA) = 40:1 as compared to 100:1 in hydrogen coupling. Even at this low ratio, the temperature is under control.

4. Parametric Sensitivity

The chemical reactor deserves a special discussion. To remove the risk of run-away in hydrogenation of maleic anhydride as an individual process a large excess of hydrogen to feed ratio (100 – 200: 1) is provided to the system. Despite this, the reaction is very difficult to control. The operation of the integrated reactor is simulated, at various process conditions, in order to emphasize the stability of reactor with reduced parametric sensitivity.

Fig. 4 and Fig. 5 show the effect of changing the reactor-inlet temperature and the hydrogen to feed ratio on the products mole fractions and temperature respectively.

280 260 240 220 200 180

Feed Tempearture: 220 oC 210 oC 200 oC

0.012 0.01 0.008 0.006 0.004 0.002

220 oC 210 oC 200 oC

Integrated process for γ-butyrolactone production

39

0.004 0.003 0.002 0.001

0.006 0.005 0.004 0.003 0.002 0.001

0

L / [m] L / [m] Fig. 4 Effect of Feed Temperature (Hydrogen Balance)

0
0 1 2 3 0 1 2 3

L / [m]

220 oC
0 1 2 3 0 1 2 3

L / [m]

0

210 oC

200 oC

220 oC

210 oC

200 oC

280 260 240 220 200 180

H2:Feed = 80:1 120:1

0.014 0.012 0.01 0.008 0.006 0.004 0.002 0

H2:Feed = 80:1 100:1 120:1

0.008 0.006 0.004 0.002

0

0.008 0.006 0.004 0.002

0

L / [m] L / [m] Fig. 5 Effect of Hydrogen to Feed Ratio (Hydrogen Balance)

0 1 2 3 0 1 2 3

L / [m]

L / [m]

H2:Feed = 80:1

H2:Feed = 80:1

120:1

120:1
0 1 2 3 0 1 2 3

Mole Fraction / [MA] T / [oC] Mole Fraction / [MA] T / [oC]

Mole Fraction / [BDO] Mole Fraction / [GBL]

Mole Fraction / [BDO] Mole Fraction / [GBL]

Fig. 6 and 7 shows the effect of changing the reactor inlet feed temperature and hydrogen to feed ratio on molar and temperature profiles for the heat-balance setup. The reduced reactor sensitivity with respect to disturbances is observed.

40

Ahtesham Javaid, Costin Sorin Bildea

280 260 240 220 200 180

Feed Tempearture: 230 oC

220 oC 210 oC 200 oC

0.0125 0.01 0.0075 0.005 0.0025

230 oC 200 oC

0.0025 0.002 0.0015 0.001 0.0005 0

280 260 240 220 200 180

0

0.0075 0.006 0.0045 0.003 0.0015 0

200 oC

0.008 0.006 0.004 0.002

0
0 1 2 3 0 1 2 3

230 oC

230 oC

L / [m] L / [m] Fig. 6 Effect of Feed Temperature (Heat Balance)

L / [m] H2:Feed = 40:1

L / [m]

L / [m]

L / [m]

200 oC
0 1 2 3 0 1 2 3

H2:Feed = 40:1 120:1

0.025 0.02 0.015 0.01 0.005

H2:Feed = 40:1

120:1

0.02 0.016 0.012 0.008 0.004

H2:Feed = 40:1

120:1
0 1 2 3 0 1 2 3

L / [m] L / [m] Fig. 7 Effect of Hydrogen to Feed Ratio (Heat Balance)

0

0
1 2 3 0 1 2 3

0

120:1

Mole Fraction / [MA]

T / [oC] Mole Fraction / [MA] T / [oC]

Mole Fraction / [BDO] Mole Fraction / [GBL] Mole Fraction / [BDO] Mole Fraction / [GBL]