Experimental study on multicomponent distillation in packed columns

Chemical Engineering and Processing 40 (2001) 235– 243 www.elsevier.com/locate/cep

Experimental study on multicomponent distillation in packed columns Sami Pelkonen a,*, Andrzej Go´rak b, Andre´ Ohligschla¨ger c, Ruth Kaesemann d b

a Krupp Uhde GmbH, Friedrich-Uhde-Strasse 15, D-44141 Dortmund, Germany Uni6ersita¨t Dortmund, Lehrstuhl Thermische Verfahrenstechnik, D-44221 Dortmund, Germany c Axi6a GmbH, Industriepark Ho¨chst, D-65926 Frankfurt-am-Main, Germany d Uni6ersita¨t Dortmund, Lehrstuhl Urnwelttechnik, D-44221 Dortmund, Germany

Received 10 June 1999; received in revised form 24 March 2000; accepted 26 May 2000 Professor Alfons Vogelpohl

Abstract An extensive set of experimental data on multicomponent distillation with non-ideal systems in structured packed columns is presented to fill the gap of such data in literature. Experimental composition profiles along the column height obtained by distillation experiments with nonideal systems, methanol– 2-propanol– water, methanol– acetonitrile– water, and acetone– methanol–2-propanol–water are reported. The distillation columns and the sampling technique are discussed in detail. An example how to use these data is shown by comparing the experimental data with simulation results when applying different mathematical models. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Multicomponent systems; Distillation; Experiment; Structured packings; Simulation models

1. Introduction The existing experimental distillation data are either used for the determination of model parameters (HETP’s, VLE-parameters, and mass transfer correlations) or for the validation of the mathematical models. Most of the experimental data published are concerned with binary systems and contain compositions measured only at the column top and the bottom of the column. The problems of using such data for model validation are: 1. The model parameters or the models verified by binary mixtures are not necessarily valid for multicomponent systems. 2. The conclusions based only on the measurements at the top of the column and at the bottom of the column provide little information about the behaviour inside the column. Such information may be of essential importance when investigating the

* Corresponding author. Tel.: +49-231-5472424. E-mail address: [email protected] (S. Pelkonen).

distillation of non-ideal mixtures with distillation boundaries. The publications known to us illustrating experimental composition profiles along the column height on multicomponent distillation with structured packed columns are summarised in Table 1. These experiments have been used for the investigation of the mass transfer mechanism [15,17], for the steady-state and dynamic model validation [3,4,8 –13,16] and for the model parameter determination [2]. Recently, Baur et al. [1] have used experimental data to examine the behaviour of distillation boundaries in multicomponent azeotropic mixtures.

2. Set-up of the distillation columns Fig. 1 illustrates the distillation column used at the University of Clausthal. The column consists of a reboiler (2), two packed section (3) and a total condenser (5). The column was equipped with Sulzer BX and Platte K. Both sections have a diameter of 100 mm and

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Table 1 Experimental multicomponent distillation data on structured packings with compositions along the column height Source

Packing

Column dimensions D, h

Column operation

Test systems

[3,4,10,11]

70 mm, 2.5 m

[8]

Sulzer DX, Sulzer BX Sulzer CY

100 mm, 0.8 m

20 mbar, Partial reflux, dynamic operation 1000 mbar, Total reflux

Six-component-fatty-alcohol mixtures Methanol/2-propanol/water

[9]

Sulzer BX

100 mm, 0.8 m

1000 mbar, Total reflux

[2]

Sulzer BX

10 mbar, Partial reflux

[17]

RomboPak 6M

70 mm, 1 m, 70 mm, 2 m 150 mm, 2.2 m

950 mbar, Total reflux

[12]

Mitsubishi MC-250T Montzpak A3-500 Rombopak 6M

210 mm, 3 m

1000 mbar, Partial reflux

100 mm, 3 m

1000 mbar, Total reflux

100 mm, 3 m

1000 mbar, Partial reflux, dynamic operation

[13] [16]

Composition data

Five compositions along the column height Ten compositions along the column height Methanol/2-propanol/water/ Ten compositions along the acetone column height Ternary fatty alcohol Five compositions along the mixtures column height Cyclohexane/toluene/chloro- 11 compositions along the benzene column height methanol/acetonitrile/water Methanol/ethanol/water Six compositions along the column height Methanol/acetonitrile/water 16 compositions along the column height Extractive distillation of 16 compositions along the acetone/methanol with column height water

Fig. 1. Flow-diagram of the pilot plant column constructed at the University of Clausthal.

S. Pelkonen et al. / Chemical Engineering and Processing 40 (2001) 235–243

Fig. 2. P&I-diagram of the pilot plant column constructed at the University of Dortmund.

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S. Pelkonen et al. / Chemical Engineering and Processing 40 (2001) 235–243

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Fig. 3. Metal gauze wire packing Montzpak A3-500 with a liquid collector basin.

Fig. 4. Device used for measurement of the liquid maldistribution below the lowest packing section.

The liquid is taken out of the column with a teflon tube and led into a sampling flask. Additional samples are

taken from the reflux and the condensate at the top of the column. When the column is operated at finite reflux the distillate is introduced into the reboiler so that no additional feed is needed. The reflux is fed back into the column at its boiling point. The volume stream is measured with a rotameter (7) and a damming basin (8). Fig. 2 shows the column set-up used at the University of Dortmund. The column has been constructed following the criteria of Goedecke et al [6] and has an inner diameter of 100 mm. In the experiments the gauze wire packing Montzpak A3-500 and the lamella packing type Rombopak 6M from Ku¨hni AG were investigated. The effective packing height amounts to 3 m divided into three sections, each 1 m high. The liquid flowing down the column is collected between each section and is redistributed by means of liquid distributors from Julius Montz GmbH. The column is made of glass surrounded by an electrically heated insulation jacket in order to reduce heat losses. The feed(s), which may be pre-heated, can be introduced into the column at the packing heights of 1 or 2 m. A thermosyphon reboiler and a total condenser with a reflux divider are used. A condensate accumulator is provided in order to guarantee a steady reflux flow. The operational variables such as column top pressure, pressure drop, column temperature profile, feed flow rates, feed temperature, reboiler heat duty, distillate mass stream and reflux ratio are measured or controlled by a process control unit. The distillate mass flow rate is measured by a coriolis massflow meter. Composition and temperature can be measured at 18 different locations that are placed at the same column height. Small stainless steel collector basins are installed directly into the packing for liquid sampling (Fig. 3). The collector basins are placed at the heights of 0.1, 0.5 and 0.9 m in each packing section. The liquid samples are taken with a special syringe out of the packing. Samples are also taken above and below the packing sections from the liquid collectors, and the feed, reflux and product streams.

3. Accomplishment of the experiments The experiments at the University of Clausthal and at the University of Dortmund were conducted in simi-

Table 2 Mixtures, packings and column dimensions used in the experiments Source

[7]

[14]

[14]

Mixture Packing Column diameter (mm) Packing height (m)

Methanol/2-propanol/ water Sulzer BX, Platte K 100 1.79

Methanol/acetonitrile/water Montzpak A3-500 100 3

Acetone/methanol/2-propanol/water Rombopak 6M 100 3

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Table 3 Experimental results for the ternary experiments with methanol–2propanol–watera

Table 3 (Continued) h

XMeOH

XIP

h

XMeOH

XIP

MIW 9 0 0.1 0.205 0.320 0.405 0.505 0.705 0.805 0.810 0.942 1.055 1.168 1.281 1.413 1.526 1.790 TTop LTop pTop

0.0377 0.0573 0.1120 0.1259 0.1683 0.2153 0.4229 0.4610 0.4870 0.4987 0.5762 0.5766 0.6234 0.6329 0.6507 0.8175 55.47 43.83 960

0.5156 0.5827 0.5889 0.5827 0.5608 0.5326 0.3952 0.3749 0.3576 0.3483 0.2982 0.2960 0.2652 0.2613 0.2504 0.1357

1.281 1.413 1.526 1.790 TTop LTop pTop

0.9625 0.9694 0.9734 0.9799 54.55 43.25 960

0.0243 0.0237 0.0199 0.0146

MIW 10 0.1 0.205 0.320 0.405 0.505 0.605 0.705 0.805 0.905 1.795 TTop LTop pTop

0.0566 0.1304 0.1499 0.1998 0.2365 0.3273 0.8591 55.70 45.59 960

0.5835 0.5701 0.5616 0.5339 0.4958 0.4533 0.1097

MIW 13 0 0.1 0.205 0.320 0.405 0.505 0.605 0.705 0.805 0.810 0.942 1.055 1.168 1.281 1.413 1.526 1.790 TTop LTop pTop

0.1777 0.3055 0.3758 0.4589 0.5352 0.6551 0.7892 0.8152 0.8468 0.8508 0.8539 0.8542 0.8818 0.9072 0.9052 0.9475 53.62 42.91 960

0.4786 0.4447 0.4087 0.3595 0.3136 0.2387 0.1483 0.1370 0.1131 0.1127 0.1116 0.1084 0.0917 0.0709 0.0704 0.0402

MIW 11 0.1 0.205 0.320 0.405 0.505 0.605 0.705 0.805 1.324 1.795 TTop LTop pTop

0.0114 0.0442 0.0522 0.0923 0.1378 0.1651 0.2046 0.2590 0.4304 0.5508 51.05 55.86 960

0.6093 0.6302 0.6320 0.6100 0.5869 0.5674 0.5376 0.5110 0.3956 0.3169

MIW 14 0 0.1 0.205 0.320 0.405 0.505 0.605 0.705 0.805 0.810 0.942 1.055 1.168 1.281 1.413 1.526 1.790 TTop LTop pTop

0.0585 0.0911 0.1722 0.1986 0.2577 0.3279 0.3904 0.5521 0.6424 0.6637 0.6693 0.7442 0.7447 0.8033 0.8076 0.8403 0.9245 53.38 38.98 960

0.4558 0.5507 0.5357 0.5279 0.4884 0.4534 0.4412 0.3078 0.2560 0.2382 0.2238 0.1854 0.1831 0.1445 0.1430 0.1157 0.0595

MIW 12 0 0.1 0.205 0.320 0.405 0.505 0.605 0.810 0.942 1.055 1.168

0.2199 0.2968 0.4986 0.5694 0.6247 0.7249 0.8610 0.9023 0.9244 0.9304 0.9590

0.3704 0.4099 0.3204 0.2813 0.2492 0.1787 0.0990 0.0735 0.0554 0.0499 0.0316

MIW 15 0 0.1 0.205 0.320 0.405 0.505 0.605 0.705 0.805 0.810 0.942 1.055

0.0580 0.0779 0.1342 0.1660 0.1860 0.2803 0.3595 0.4885 0.5864 0.5934 0.5942 0.6812

0.4748 0.5610 0.5621 0.5509 0.5429 0.4863 0.4366 0.3513 0.2865 0.2862 0.2928 0.2283

S. Pelkonen et al. / Chemical Engineering and Processing 40 (2001) 235–243

240 Table 3 (Continued) . h

XMeOH

XIP

1.168 1.281 1.413 1.526 1.790 TTop LTop pTop

0.6877 0.7451 0.7602 0.7817 0.8910 54.08 40.31 960

0.2222 0.1825 0.1740 0.1593 0.0825

a

The packing height 0 m corresponds to the sampling place below the lowest packing section.

lar way. The column was first filled with the mixture under investigation. The steady-state composition profiles at total reflux experiments are very sensitive to the initial composition of the mixture in the reboiler. To obtain a composition profile that undergoes major changes in the column the composition profile was shifted to a preferred direction after having reached the steady-state by adding the component(s) in too less extent. After obtaining a desired composition profile a set of experiments were carried out at total reflux and at atmospheric pressure by varying the reboiler heat duty. The temperature profile along the column height was observed and stored in a process control unit. After having reached a constant temperature profile 60–120 min were needed to reach a steady-state operation of the column. This is important due to the possibly flat temperature changes in spite of large composition variations. Before sampling the temperature profile, the column top pressure and the pressure drop were measured and stored. After sampling the volume streams at the top of the column and below the packing were measured. The measured streams were compared with those obtained from the energy balances around the condenser and the reboiler, so that the heat losses from the column could be calculated. The compositions of the samples were analysed with a gas chromatograph both at the University of Clausthal and at the University of Dortmund. The water content was determined with TCD and the organic substances with FID. The experiments are performed in such a way that the errors caused by heat losses (insulation), extra heat input (column wall temperature control) or unsteadystate behaviour can be ignored. The volume and surface area of the collector basins installed directly into the packing was kept at its minimum in order to prevent a disturbance of plug flow. The technique was tested in laboratory with water/air and no disturbance of plug flow caused by the collector basins could be noticed. Since maldistribution is present in every packed distillation column, its influence was investigated with a device

Table 4 Experimental results for the ternary experiments with methanol–acetonitrile–watera H

XMeOH

XACN

MAW 1 Cond. 2 1.9 1.5 1.1 1 0.9 0 TTop VL(Top) pTop

0.923 0.995 0.997 1 0.833 0.325 0.053 0 54 0.009 1001

0.077 0.005 0.003 5E-04 0 0 0 0

MAW 2 Cond. 2.5 2.1 2 1.9 1.5 1.1 TTop VL(Top) pTop

0.922 0.972 0.96 0.942 0.893 0.062 0 57 0.009 999

0.078 0.028 0.006 0.004 0.002 0 0

MAW 3 Cond. 2 1.9 1.5 1.1 1 0.9 0 TTop VL(Top) pTop

0.807 0.72 0.66 0.178 0.004 0.002 0 0 52 0.01 1000

0.193 0.28 0.308 0.618 0.729 0.734 0.736 0.032

MAW 4 Cond. 3 2.9 2.5 2.1 2 1.9 TTop VL(Top) pTop

0.789 0.791 0.773 0.493 0.025 0.008 0 52 0.008 1021

0.21 0.209 0.227 0.407 0.42 0.276 0.01

MAW 5 Cond. 2.5 2.1 2 1.9 1.5 1.1 TTop VL(Top) pTop

0.759 0.766 0.61 0.467 0.255 3E-04 0.017 51 0.01 1021

0.241 0.229 0.337 0.421 0.492 0.01 0.005

MAW 6

S. Pelkonen et al. / Chemical Engineering and Processing 40 (2001) 235–243 Table 4 (Continued) H

XMeOH

XACN

Cond. 2.1 2 1.9 1.5 1.1 TTop VL(Top) pTop

0.823 0.773 0.743 0.726 0.349 0 50 0.007 1019

0.16 0.214 0.238 0.249 0.475 0

MAW 7 Cond. 2 1.1 1 0.9 0.5 TTop VL(Top) pTop

0.838 0.81 0.77 0.747 0.727 0.108 50 0.01 1026

0.162 0.181 0.207 0.217 0.221 0.027

a The packing height 0 m corresponds to the sampling place below the lowest packing section.

illustrated in Fig. 4 installed below the lowest packing section. The differences in the measured volume flow rates and the compositions in the six cross sectional slices were found to be negligible. The maximal relative error of the composition analysis with the GC was measured to be 2% and that of the molar liquid load 10%.

241

number of theoretical stages for the equilibrium stage model, the mass transfer correlations for the equal diffusivity model and the non-equilibrium stage model were determined from distillation experiments with a binary ideally behaving system chloro-/ethlybenzene [14]. In the simulations the activity coefficients were calculated with the UNIQUAC-model with parameter values obtained on the basis of binary VLE-experiments [5]. The system acetone –methanol –2-propanol –water exhibits a distillation boundary due to an azeotrope between 2propanol and water. The experimental composition in the liquid distributor at the column height of 2 m was fixed in the simulations. This composition point was chosen because the liquid phase is fully mixed (no maldistribution) and because the composition of 2propanol and water are close to the distillation boundary. The results show, that the composition profiles end up to completely different reboiler products depending on which model is applied: the non-equilibrium model showing a quite good accordance to the experiment, whereas the equilibrium stage and equal diffusivity model predict that the reboiler is filled with 2-propanol. This result caused us to investigate the theory of distillation boundaries more closely. We found out that the diffusional interactions between the components need to be considered when dealing with mixtures exhibiting distillation boundaries [15]. This result has been recently validated by Baur et al. [1].

6. Conclusions 4. Listing of experimental data The parameters that were constant in the experiments are listed in Table 2. The experimental composition profiles, column pressure as well as reflux flow rates and temperatures are summarised in Tables 3 – 5 for methanol –2-propanol – water, methanol – acetonitrile – water and acetone – methanol – 2-propanol – water. The pressure drop over the column height is not reported because of its insignificant influence on the composition profiles in the conditions of the experiments.

The essential content of this paper is a listing of experimental composition profiles obtained with multicomponent non-ideal systems, in structured packed columns, at total reflux operation of the column and at atmospheric pressure. Also valuable technical information is provided for construction of pilot scale distillation columns. The authors recommend the composition data to be used for the investigation of the phase equilibria, mass transfer effects and behaviour of distillation boundaries.

5. Example: simulation versus experiment

7. Notation

The following example shows how important such data as presented in this paper may be when investigating the behaviour inside a distillation column. As an example Fig. 5 shows the composition profiles along the column height for each component for an experiment with acetone –methanol – 2-propanol – water, AMIW3 in Table 5. The simulated composition profiles with equilibrium stage model [18], equal diffusivity model [17] and non-equilibrium stage model [19] are presented, too. The

AC ACN D h IP LTop

acetone acetontirile column diameter, (mm) packing height, (m) 2-propanol (isopropanol) liquid molar flux at the column top, (mol/ m2 s) MeOH methanol pTop pressure at the column top, (mbar)

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Table 5 Experimental result for the quaternary experiments with acetone–methanol–2-propanol–watera AMIW1

h

Cond. 3 2.9 2.5 2.1 2 1.9 1.5 1.1 1 0.9 0.5 0.1 0 TTop LTop pTop a

AMIW2

AMIW3

AMIW4

XAC

XMeOH

XW

XAC

XMeOH

XW

XAC

XMeOH

XW

XAC

XMeOH

XW

0.6255 0.6255 0.5937 0.4384 0.1787 0.1240 0.0927 0.0246 0 0 0 0 0 0 47 16.5 1013

0.3745 0.3745 0.4062 0.5428 0.7338 0.7531 0.7493 0.6241 0.3798 0.2951 0.2340 0.1340 0 0

0.0078 0.0073 0.0083 0.0104 0.0310 0.0423 0.0549 0.1095 0.2380 0.2650 0.2835 0.4925 0.9963 1.0000

0.5350 0.5231 0.4665 0.2445 0.0652 0.0398 0.0310 0.0084 0 0 0 0 0 0 55.8 40.7 1017

0.4164 0.4249 0.4567 0.5444 0.4705 0.4212 0.3952 0.2551 0.1459 0.1105 0.0787 0.0391 0 0

0.0198 0.0182 0.0238 0.0612 0.1321 0.1596 0.1602 0.2062 0.3042 0.3239 0.3529 0.4898 0.9963 1.0000

0.5215 0.5171 0.4598 0.2185 0.0572 0.0360 0.0261 0.0079 0 0 0 0 0 0 55.9 36.9 1018

0.4182 0.4208 0.4509 0.5290 0.4341 0.3874 0.3574 0.2339 0.1281 0.0969 0.0708 0.0381 0 0

0.0197 0.0201 0.0262 0.0725 0.1417 0.1655 0.1771 0.2120 0.2988 0.3118 0.3231 0.3582 0.5994 0.9831

0.5689 0.5646 0.5267 0.3363 0.1034 0.0648 0.0434 0.0104 0 0 0 0 0 0 52.8 24.2 1018

0.4122 0.4158 0.4452 0.5743 0.6127 0.5733 0.5289 0.3497 0.2003 0.1540 0.1147 0.0611 0.0243 0

0.0137 0.0097 0.0124 0.0290 0.0854 0.1127 0.1303 0.1823 0.2826 0.2969 0.3125 0.3339 0.5235 0.9261

The packing height 0 m corresponds to the sampling place below the lowest packing section.

Fig. 5. Experiment with acetone/methanol/2-propanol/water at total reflux operation of the column. Presented are the simulated composition profiles with non-equilibrium stage model (thick dashed line), equal diffusivity model (thin dashed line) and equilibrium stage model (full line). The circles indicate the measured compositions.

S. Pelkonen et al. / Chemical Engineering and Processing 40 (2001) 235–243

TTop VL(Top) W x

temperature of the reflux stream at the column top, (°C) liquid volume flow rate at the column top, (l/s) water mole fraction (−)

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