experimental data for propyl propionate synthesis

Chemical Papers 62 (1) 65–69 (2008) DOI: 10.2478/s11696-007-0080-x

ORIGINAL PAPER

Reactive distillation – experimental data for propyl propionate synthesis‡ a

Marcel Kotora, b Carsten Buchaly, b Peter Kreis, b Andrzej Górak, a Jozef Markoš* a Institute

b Department

of Chemical and Environmental Engineering, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovak Republic of Biochemical and Chemical Engineering, Chair of Fluid Separation Processes, Emil-Figge-Str. 70, D-44227 Dortmund, Germany

Received 24 April 2007; Revised 6 August 2007; Accepted 10 August 2007

A set of experimental data for heterogeneously catalysed esterification of propan-1-ol and propionic acid to propyl propionate in a pilot scale reactive distillation column is presented. The catalytic section of the column was equipped with the structured packing Katapak SP-11. Both, rectifying and stripping, sections consisted of the non-reactive structured Sulzer BX packing. As catalyst, the strongly acidic ion-exchange resin Amberlyst 46 was used. The experimental results show concentration as well as temperature profiles along the column height and therefore exhibit reliable data for model validation purposes. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: reactive distillation, distillation, experiments, propyl propionate, esterification

Introduction The reactive distillation (RD) represents a reliable technology which became almost the standard for a variety of chemical processes. This fact is well documented in the open literature for a number of applications, e.g. esterification (Hanika et al., 1999, Saha et al., 2000, Górak & Hoffmann, 2001), etherification (Bravo et al., 1993), aldol condensation (Podrebarac et al., 1998), propylene oxide synthesis (Carr` a et al., 1979). Nevertheless, a successful design of RD columns still remains challenging. The reason is the complicated nature of reactive distillation, where simultaneously reaction and separation phenomena occur. A useful tool for designing the scale and optimal performance condition of an RD device are appropriate process models. So far, different modelling approaches can be found in literature; the equilib-

rium, non-equilibrium, cell models, etc. (Taylor & Krishna, 2000). Although the precision and complexity of these models improve, they still have to be confronted with reliable experimental data. This is emphasised by the fact that these mathematical models require several parameters for the description of reaction kinetics, phase equilibrium, multicomponent mass and heat transfer, etc. An “a priori” estimation of the parameters for the description of multicomponent mass transfer is usually done using packing specific HETP values (equilibrium stage models) or correlations for the specific interfacial area as well as the liquid and gas phase mass transfer coefficients (non-equilibrium models), which could be found in literature (Bravo et al., 1985, Rocha et al., 1996, Hoffmann et al., 2004). The aim of this paper is to provide reliable experimental data to validate process models for reactive distillation.

*Corresponding author, e-mail: [email protected] ‡ Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.

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M. Kotora et al./Chemical Papers 62 (1) 65–69 (2008)

Table 1. Azeotropic data of the investigated system at 101 kPa (Buchaly et al., 2006) T

Component

POH ProAc ProPro H2 O

Azeotrope

POH-ProPro-H2 O* POH-H2 O** ProPro-H2 O* ProAc-H2 O**

Tb

Component mole fraction

◦C

xPOH xProAc xProPro xH2 O

T WI

T

97.2 140.9 122.9 100.0

1 – – –

– 1 – –

– – 1 –

– – – 1

F

Q

Component mole fraction

◦C

xPOH xProAc xProPro xH2 O

Q

0.350 0.432 – –

– – – 0.050

0.130 – 0.350 –

F T

Tb

86.2 87.6 90.0 99.9

Q

0.520 0.568 0.650 0.950

F

T

T Q

F

Q

T

T Q

*Heterogeneous azeotrope. **Homogeneous azeotrope. T Q

Theoretical T

The heterogeneously catalysed esterification of propionic acid (ProAc) and propan-1-ol (POH) to propyl propionate (ProPro) and water (H2 O), which is an equilibrium reaction, was investigated. For this type of chemical reaction, reactive distillation is a wellknown alternative to conventional processes with sequential reaction and separation steps. The reaction scheme for the propyl propionate synthesis can be represented by the following expression

Q T F

Q

T

Fig. 1. Schematic overview of the experimental device arrangement; Q – sampling point, T – temperature indicator, F – flow indicator, WI – weight indicator.

H+

CH3 CH2 CH2 OH + CH3 CH2 COOH − ←− −− −− −→ − CH3 CH2 COOC3 H5 + H2 O

Q

(1)

As an acidic catalyst, the surface-sulfonated catalyst Amberlyst 46 was used. This ion-exchange resin is tailor-made for esterification as a competing side product formation, the etherification of propan-1-ol to dipropyl ether is suppressed. The information related to kinetics of the studied process is provided in manuscripts by Duarte et al. (2006) and Buchaly et al. (2007). The chemical system shows a complex thermodynamic behaviour, where the separation becomes more challenging due to the presence of several azeotropes and miscibility gaps. The azeotropic data are summarised in Table 1.

Experimental All experiments were conducted in a pilot-scale reactive distillation column with an inner diameter of 50 mm (DN 50 column) at the University of Dortmund. A schematic view of the column used is given in Fig. 1, while its specifications are provided in Table 2.

Table 2. Pilot plant distillation column specifications Characteristics Diameter Reactive section Stripping section Rectifying section Condenser type Pressure

50 mm 2.2 m (Katapak-SP 11) 1.0 m (Sulzer BX) 2.4 m (Sulzer BX) Total Atmospheric

The reactive function of the column was supplied by the KATAPAK-SP11 reactive packing with the catalyst Amberlyst 46 immobilised in catalyst bags. Process parameters, such as flow rates of the distillate, bottom, both feeds, and reflux streams as well as the vapour temperatures along the column, were measured continuously. Adiabatic process conditions could be achieved by the use of a heating jacket and a proper insulation with mineral wool. The purpose


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Table 3. Experimental conditions Experiment

χ

RR

D/F

E1 E2 E3

1.99 1.99 1.99

2.48 4.00 2.49

0.323 0.255 0.405

of the heating jacket was to maintain the temperature on the first layer of isolation equal to the temperature inside the column trough the control scheme. The practically negligible driving force assured thus the adiabatic conditions of the process. Samples were taken at 8 different sampling points along the column height. The organic components were analysed using Shimadzu GC-14Agas chromatograph equipped with FID. For the determination of water content, the Karl-Fischer titration was applied (Mettler Toledo DL 31). Experiment specifications All experiments were performed at the column load of 4 kg h−1 , with a constant feed mole ratio of χ = n˙ fPOH /n˙ fProAc = 2.0. The main product, propyl propionate, was removed from the column in the bottom stream. The excess of alcohol was chosen to maintain high conversion of propionic acid and to ensure that the column is operating only in the homogeneous region of its rectifying part. Additionally, the operational conditions were chosen to avoid L–L splitting at the top of the column as well as to guarantee that the temperature in the reaction section does not exceed the maximum operating temperature of the catalyst (Tmax,cat = 120 ◦C). These boundary conditions determine the range of the operational parameters such as the reflux ratio (RR) and the distillate to feed mass ratio (D/F). The latter is indirectly adjusted by the heat duty of the reboiler. A summary of the experimental operating conditions for each individual experiment is given in Table 3. Special attention was given to the determination whether the reactive distillation column is operating in the steady state. In order to fulfil this prerequisite, the following conditions had to be met:

– “flat” profiles of monitored parameters (e.g. a change of each measured temperature along the column in a range of ± 0.5 ◦C), – no time change of the distillate and bottom stream composition, – fulfilled mass and component balance of the column. Therefore, data reconciliation was applied. It adjusts the experimentally determined mass streams and concentrations so that both the mass and component balance as well as the reaction rates are satisfied. The experimental values are varied within their experimental error (Buchaly et al., 2006). All experimental values, given in Tables 3–5, are reconciled process data. The information on feed streams is given in Table 4.

Results and discussion The following set of diagrams documents the experimental data obtained from the reactive distillation device. The textured rectangles represent the reactive section. The height of the packing was measured from bottom to top. The reference level was above the reboiler at the point from where the stripping section started. Three independent column profiles were taken during each experiment. As shown in Fig. 2, remarkable changes occur at the top and bottom of the distillation device. The sharpness of the separation in the bottom part of the column is positively affected by the increasing value of D/F mass ratio. The light-boiling component propan-1-ol is being stripped out. Therefore, the propyl propionate concentration increases (see experiments E1→E3, with the exception E2). On the top of the column, this trend is contradictory, the separation efficiency being furthered by means of lowering the value of the D/F mass ratio. Accordingly, the concentration of propyl propionate decreases (E3→E1, with the exception E2) and together with the synergy effect of the increased reflux ratio the composition shifts from nearly ternary azeotrope (E1, E3) to a POH– H2 O binary one (E2). However, the experiments for various reflux ratio and constant D/F mass ratio are missing; we cannot conclude the detailed impact of the reflux ratio on the column behaviour.

Table 4. Summary of inlet streams ProAc feed

POH feed

Experiment

E1 E2 E3

m ˙F

T

wProAc

wH2 O

m ˙F

T

wPOH

wH2 O

kg h−1

◦C

kg kg−1

kg kg−1

kg h−1

◦C

kg kg−1

kg kg−1

1.52 1.53 1.53

90.8 89.7 89.6

0.997 0.997 0.998

0.003 0.003 0.002

2.46 2.47 2.47

88.2 87.0 87.0

0.997 0.997 0.997

0.003 0.003 0.003


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M. Kotora et al./Chemical Papers 62 (1) 65–69 (2008) 1,0 0,9

110

ProAc feed

E1

105

0,7 0,6

T/ C

0,5

100

O

mole fraction, x

0,8

E1 ProAc feed

0,4

POH feed

0,3

95

0,2

0,0

POH feed

90

0,1 0

1

2

3

4

0

5

1

packing height / m

E2

5

ProAc feed

105

0,7 0,6

100

T/ C

0,5

O

mole fraction, x

4

E2

ProAc feed

0,8

POH feed

0,4

95

0,3 0,2

POH feed

90

0,1 0,0

0

1

2

3

4

5

0

1

packing height / m

0,9 0,8

4

5

E3

110 105

T/ C

0,6 O

0,5 POH feed

0,3

ProAc feed

POH feed

0,7

0,4

3

115

ProAc feed

E3

2

packing height / m

1,0

mole fraction, x

3

110

1,0 0,9

2

packing height / m

100 95

0,2

90

0,1 0,0

0

1

2

3

4

0

5

1

2

3

4

5

packing height / m

packing height / m

Fig. 2. Liquid composition profile along the packing height (mole fraction of propan-1-ol , propyl propionate , and water ), and the vapour temperature profile for individual experiments E1, E2, and E3.

•, propionic acid

Table 5. Composition of distillate and bottom streams Distillate stream*

Bottom stream*

Experiment

E1 E2 E3

m ˙D

wProPro

wProAc

wPOH

m ˙B

wProPro

wProAc

wPOH

kg h−1

kg kg−1

kg kg−1

kg kg−1

kg h−1

kg kg−1

kg kg−1

kg kg−1

1.29 1.02 1.62

0.089 0.082 0.087

0.000 0.000 0.000

0.686 0.658 0.742

2.69 2.98 2.38

0.645 0.547 0.698

0.125 0.145 0.159

0.227 0.304 0.138

*The water mass fraction can be calculated within the summation condition.


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It can be observed that the temperature in the reactive section never exceedes the maximum operating temperature of the catalyst of Tmax,cat = 120 ◦C. It has to be mentioned that the distributor of the propionic acid feed does not allow for an accurate determination of the composition at this column feeding position due to construction features. Since for propan-1-ol feed, a different distributor was used, the composition accuracy at this feeding point has not been affected. The compositions of the distillate and bottom streams are given in Table 5. Acknowledgements. This work was supported by the Maria Currie office under the contract No. HPMT-CT-2001-00408 and the Science and Technology Assistance Agency under the contract No. APVT-20-000804.

Symbols D/F E1 m ˙B m ˙D m ˙F n˙ F POH ProAc ProPro RR T W w x

distillate to feed mass ratio ( = m ˙ D /m ˙ F) first experiment (analogically E2, E3) mass flow rate of the bottom stream kg mass flow rate of the distillate kg mass flow rate of the feed kg molar flow rate of the feed kmol propan-1-ol (CH3 CH2 CH2 OH) propionic acid (CH3 CH2 COOH) propyl propionate (CH3 CH2 COOC3 H5 ) reflux ratio temperature water (H2 O) component mass fraction component mole fraction

Greek Letters χ

feed mole ratio ( = n˙ fPOH /n˙ fProAc )

Superscripts f

feed

Subscripts D F

distillate feed

h−1 h−1 h−1 h−1

◦

C

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