Design, Construction, and Operation of a Multipurpose Plant for

Chapter 31

Design, Construction, and Operation of a Multipurpose Plant for Commercial Supercritical Gas Extraction 1

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F. Böhm , R. Heinisch , S. Peter , and E. Weidner l

Norac Technologies, Inc., 4222-97 Street, Greystone Pavillion, Edmonton, Alberta T6E 5Z9, Canada Kasyco GmbH, Dammannstraβe 61, D-4300 Essen 1, Federal Republic of Germany Lehrstuhl für Technische Chemie Π Egerlandstraβe 3, D-8520 Erlangen, Federal Republic of Germany 2

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The design of a multi-purpose plant for the continuous extraction of liquids with supercriticalfluidsis presented. To provide flexibility in order to treat different feedstocks, a modular concept was developed based on experience gained in the operation of bench-scale and pilot plants. Four test systems were chosen in order to determine the proper dimensions for the equipment. Based on experimental data, e.g. measurements offloodingpoints and maximum flows for various column internals, the design pressure and temperature and heat exchange requirements were determined. The plant was built by a German manufacturer and was operated successfully by a Canadian company in Edmonton, Alberta. Asreportedin a lot of reviews, extraction with supercritical solvents has very promising commercial potential. Until now the commercialization has been restricted mainly to batchwise extraction of solids with carbon dioxide (e.g. decaffeination of coffee and tea, extraction of hops). Although laboratory and pilotplant experiments have indicated very good economics for continuous extraction of liquids with carbon dioxide and other gases, so far this technique has been applied industrially only for the production of 2-butanol by Idemitsu Petrochemical Corp. in Japan. A Canadian group of entrepreneurs and scientists decided in 1985 to install an industrial extraction centre in Edmonton, Alberta, forresearch,product development and plant design. The centre is equipped with 5 units from laboratory size to commercial plants. Not only are batch extractors installed, but also columns for treating liquid materials are available. The maximum capacity for continuous extraction is about 50 tons/yr. In order to achieve a maximum degree offlexibility,a modular concept was developed for the column, based on experience gained at the University Erlangen in Germany. (X)97^156/89A>40&-O499$06.00A) © 1989 American Chemical Society

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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y

Balances and Height Calculations In the last ten years about 12 separations of technical interest were studied extensively both by doing phase equilibrium measurements and continuous operations. Four typical separation problems were chosen as basis for designing the plant: a) Deacidification of palm oil with ethane as extradant and acetone as entraîner. b) Separation of monoglycerides (CI8:1) from a mixture with Di- and Triglycerides with carbon dioxide as extractant and propane as entraîner. c) Separation under point b) where the column is operated with a temperature gradient. d) Deoiling of lecithin with a mixture of C 0 and propane. The mass flows and purities obtained by experiments in a pilot plant with a height of 4 m were used as a basis. By applying a modified Redlich-Kwong equation of state (1.2). the heat and mass balances were calculated for the new plant, which contains an extractor with an inner diameter of 7 cm and a height of 6.5 m. In the course of process design and operation, it was found that phase equilibrium measurements and pilot plant data were required in order to develop a reliable computer simulation. The theoretical prediction or even a description of the mass transfer was often impossible, because of a lack of knowledge of the thermodynamic properties of the co-existing phases, especially if large molecules were involved. A practical method for process design and scaleup is to determine HETP values by stagewise computer simulation of pilot plant experiments, based on phase equilibrium calculations. If the range of the HETP values for a certain set of experiments is known, an optimization of the separation can be earned out by varying pressure, temperature, solvent to feed ratio, reflux and so on. The results of the optimization must be verified by experiment. This optimization strategy was applied to the separation problems listed above. Based on these results the extraction column was designed. In Figure 1 a typical flow sheet including the mass and heat flows is given for the separation of Monoglycerides from Di- and Triglycerides using a mixture of carbon dioxide (45 wt.%) and propane as extractant (55 wt.%). As shown in the figure, a product with 99% purity of Monoglycerides can be obtained. Based on the heat and mass balances the required dimensions of pumps, heat exchangers, piping were defined. Additionally the energy and solvent consumption were calculated. 2

Layout In general, a multi-purpose plant should be able to treat many kinds of different feedstocks. In order to achieve this requirement, a flexible design of the mechanical and electrical parts is required. Nevertheless it is not possible to run every feedstock with a fixed arrangement of apparatus and control units. Therefore a modular

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BOHM ET A L

Commercial Supercritical Gas Extraction

SOI

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SUPERCRITICAL FLUID SCIENCE A N D T E C H N O L O G Y

concept was developed, so that the plant could be adapted quickly to a given feedstock. Column Design The extraction column consists of six modules with a length of 1 m each (Fig. 2). The packing is fixed in each module by supports. The modules are linked together with connecting elements with a length of 0.25 m each. Each connecting element has 6 ports in two levels. These ports can be used for the feed, for temperature and pressure measurement, for the reflux, and for taking samples from the extraction column (Figure 3). The modular concept with this type of connecting elements allows for relatively easy changes in the column height and in the feed position along the length of the column. This is important for the flexibility because different separation problems can require different lengths of the stripping and enriching sections. Additionally it can be necessary, e.g. in the separation of Glycerides, to apply a reflux in order to improve the efficiency of the column. In this case, a part of the bottom product collected in thefirstregenerator is pumped to the connecting element situated at the top of the column. Packings and Flooding As pointed out above, optimized mass and heat balances have been derived from a combination of experimental results with a computer simulation of the process. The optimized balances can be used for the layout of a production plant A multi-purpose plant should be able not only to produce samples, but also to determine scaleup parameters. The scaleup parameters depend on the type of packing anditsspecific flooding point. The ability to measure flooding points or to test different packings is restricted mainly by the range of flow rates. For sizing the gas circulating pump, feed pump and reflux pump the results of the optimizations were used. The determination of the maximum flows was based on a flooding point diagram. As an example, the optimization of the Monoglyceride process required a gas flow of the regenerated CO^C^Hg mixture of 97 kg/h. The flooding point occurs at about 130 kg/h. From comparison with other separation processes, it was determined that a gas circulation pump with a capacity of about 140 kg/h should meet most of the requirements of a multi-purpose plant. Naturally it must be taken into account, that normal metering pumps work volumetrically. It the density of the media changes, the delivered mass flow changes too. A pump that is designed for 130 kg/h of a certain solvent delivers a different mass flow if the density of the extraction solvent is changed. The flooding point and the column efficiency are functions of the type of packing. In order to choose a useful packing for different feedstocks, a comparison was carried out in two pilot plants with inner diameters of 2.5 cm and 6.5 cm. As indicated in Figure 4, the HETP values for different packings decrease with increasing liquid flow. The endpoint of the plotted curves are the flooding points where liquid droplets are withdrawn with the gas. The best separation

31. BOHM ET AL.


Figure 2. Column Module and Connecting Element

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Figure 3. Detail of Connecting Element (cut Α-A, B-B)

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SUPERCRITICAL FLUID SCIENCE AND TECHNOLOGY

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Figure 4.

Separation Efficiency of Several Packings

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efficiency was found for wire spirals, followed by a wire mesh packing (Sulzer CY) and drop dispensers (Figure 5). From this diagram it can be deduced that a wire mesh packing should provide a good combination of capacity (very high flooding point) and separation efficiency (1). The test system for the packing was a glyceride separation with acetone and C 0 . The results for the Sulzer packings were confirmed by three further systems. All the investigated separations have a feature in common, that is the packing material is not wetted by the liquid phase under the applied conditions. There are some indications by other authors (2) and our own experiments that the HETP values might change if the packing is wetted. 2

Regeneration For fractionating the extract three regenerators were designed for the new plant. In pilot plant experiments it was found that the regeneration efficiency has a very strong influence on the quality of the bottom product of the extractor. The reason is that in supercritical fluid extraction relative high solvent to feed ratios (5 to 25) must frequently be applied. In such cases already small concentrations of extracted product in the regenerated solvent cause a non-negligible backmixing. At the bottom of the extraction column the regenerated gas comes in contact with the raffinate. If the regeneration quality is not sufficient, contamination of the raffinate occurs. A sufficient regeneration quality is essential to obtain a pure raffinate. The main parameters influencing the regeneration are pressure and temperature (phase equilibrium) and the complete separation of liquid droplets. In Figure 6 the residual concentration of Triglycerides in a mixture of propane and C 0 is plotted versus mean residencetimeof the gas in the regenerator. The regenerator was equipped with a wire mesh for improving the droplet separation. Therighthand points are the inlet concentration of the Triglycerides in the gas. The points on the left side indicate the remaining outlet concentration. It can be seen from the plotted curve that the regeneration efficiency is improved by increasing the mean residence time. The influence gets smaller with higher residence times, which correspond to lower linear gas velocities. The inlet concentration has no significant influence on the outlet concentration. Reducing the pressure to 54 bar at 100°C gave about half the residual concentration in the regenerated gas. Removing the wire mesh from the regenerator doubled the residual outlet concentration at constant pressure and temperature. The regenerator and separators in the new plant were sized for a residencetimeof about 400 sec at 45 bar, 80°C. The inner diameter was increased in order to reduce the linear gas velocity at a given mean residence time. 2

Pressure. Temperature. Extraction Solvents The plant was designed for extracting a variety of natural products. For most of these products a temperature of 120°C

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SUPERCRITICAL FLUID SCIENCE AND T E C H N O L O G Y

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B Ô H M ET A L .


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should meet all requirements. At higher temperatures these products can easily degrade. The maximum pressure is notrestrictedby issues such as thermal lability but by the investment costs. The maximum operation pressure wasfixedat 350 bar. This value seems rather high. For economical extractions the pressure should be reduced to an optimum value. This can be done by adding entrainers. As an example the pressure for extracting lecithin from soya oil can bereducedfrom350 bar with G 0 to 80 bar by using propane as an entraîner (2). Supercritical extraction should not berestrictedto C 0 as solvent Some separation problems, e.g. the glyceride separation cannot be solved economically with C 0 as a pure extractant Therefore the plant was designed also for other solvents and for the use of entrainers. Design characteristics are: - explosion-proof installation - sandwich membrane pumps to avoid leakage - installation of a flare - sufficient air circulation - installation of gas sensors 2

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Heat exchangers Arelativelydifficult task is the calculation of heat transfer coefficients for near-critical fluids, especially when entrainers are used. Normally the heat exchangers are designed for heating, cooling or condensing carbon dioxide. The difficulties in calculating the heat transferresultsmainlyfromthe large changes in thermo-physical properties in the critical region. With the generally acknowledged design methods, this effect is not treated in a satisfactory manner. Thereforerelativelylarge safety factors of up to 100% are required. The question is, whether the calculated heat transfer area is sufficient, when entrainers are used. Fig. 7 shows measured heat transfer coefficients in an existing shell and tube heat exchanger for a mixture of propane and C 0 . It can be seen, that the behavior, especially at pressures around the critical pressure of C 0 , is relatively complex and needs further experimental study. A conclusionfromFigure 7 is that a heat exchanger designed for pure C 0 cannot meet all requirements when an entraîner is used. Some additional considerations have been made concerning the heat exchanger between the extraction column and theregenerator.Some products which are dissolved in a supercritical gas can be precipitated by increasing the temperature andreducingthe pressure, which is equal toreducingthe density and thus the solvent power. Often at higher pressure, the situation isreversed.Here temperature reduction isrequiredto precipitate the solid. Therefore the heat exchanger infrontof a regenerator should be designed for heating and cooling. This demand causes additional investment costs. In the case of the plant under consideration the heat exchanger is only a heater. This limits the flexibility only a small amount, since cooling may be obtained by expansion. Normally, the pressurereductionof a 2

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Figure 7.

Heat Transfer Coefficients for CO^CsHg-Mixtures

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supercritical fluid in an expansion valve causes a significant temperature reduction due to the Joule-Thompson effect. Automation Total automation is not favorable for a multi-purpose plant, because the operation conditions for various feedstocks can be quite different. Each control circuit has only a certain dynamic range which cannot cover all the requirements. Therefore complete automation is recommended only for a production plant, when the properties of the treated product change only over a relative small range. Different feedstocks and/or solvents require different solvent to feed ratios. Each packing has a specific flooding point These two effects can lead to a great difference in mass flow. Because of this, pressures and temperatures should be controlled automatically. The flow must be measured exactly, preferably with a mass flow meter, but automatic flow control is not suitable for a multi-purpose plant. The valves which are used for pressure and temperature control should be installed so that they can be replaced easily or adapted to new conditions. A very important requirement for the continuous operation of the plant is liquid level indication and control in the extractor and regenerator. Therefore the plant is equipped with capacitive level sensors which are part of a control circuit. The suitability of these sensors for measuring the level of oily products, vitamins and some type of hydrocarbons in supercritical systems have been tested in the lab previously. Measurement of the loading of the gas at the top of the column and at the top of the regenerator is required for establishing the mass balance. The ability to monitor the concentration-profile along the separation column is desirable but not necessary for technological process development. Construction and Operation The plant was built by UHDE in Germany in late 1986. Before shipping, test runs were carried out by circulating pure C 0 to ensure that the heaters, coolers and pumps were sized properly. The skid-mounted plant was partially disassembled and shipped to Edmonton, Alberta. It was built at the NORAC extraction centre. It was in operation by mid-1987 and tested with two systems. Atfirstit was attempted to deacidify and deodorize natural, non-esterified fish oil. The main components of the feedstock were Triglycerides, which are quite similar to the products studied during the plant design stage. Hie first objective was to prove that all the components work sufficiently well at the maximum operation conditions. To do this,fishoil Triglycerides are a good test system due to theirrelativelyhigh flooding point. The plant was operated at 300 bar with a pure C 0 flowof 110 kg/h, corresponding to 85% of the maximum capacity of the gas pump. The components of the plant fulfilled the requirements. As an example of some minor problems, theremovalof the bottom 2

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product is mentioned. This product is enriched in Triglyceride and the free fatty acid concentration is reduced. If the level sensor indicated an upper limit of the liquid level the product removal valve was opened automatically until the lower switch point of the sensor is reached by the liquid. Then the valve was closed. Due to the gas that dissolved in the Triglycerides (about 35 wt.%) a relative large Joule-Thompson effect occurred. The temperatures in the expansion valve became so low, that the crystallization point of the Triglycerides was reached (5 -10 °C). The solid Triglycerides in the expansion valve caused damage to the stem during closing of the valve. This problem could be solved by reducing the distance between the upper and lower switch points of the level sensor and by heating the product removal pipe and the valve body. In a second set of experimentsfishoil esters were used to test the plant behavior. In this case the flooding point is significantly lower. The plant was operated at C 0 flows between 40 and 50 kg/h. Evaluation of phase equilibria measurements indicated that an extraction temperature between 60°C and 70°C is required. The reduction in flow and the increase in temperature resulted in problems with the preheating of the gas before the extraction column. At a temperature of 90°C for the heating fluid, the lower end of the dynamic range of the temperature regulating valve was reached. The gas temperature after the preheater was no longer constant but fluctuated by about ± 4°C. Replacing the stem of the regulation valve at the heating fluid inlet with a larger stem solved the problem. These examples described the kind of problems that may occur in the operation of multi-purpose pilot plants. 2

Literature Cited 1. 2. 3. 4. 5.

Tiegs, C., Ph.D. Thesis, University Erlangen 1984. Seibert, A. F., Bravo, J. L., Johnston, K. J., Int. Symp. on Supercritical Fluids, Nice 1988, Vol. 2. Weidner, E., Ph.D. Thesis, University Erlangen 1985. S. Peter, C. Tiegs, Preprints of the International Symposium High Pressure Chemical Engineering October 8.-10. 1984 Erlangen West Germany. S. Peter, M . Schneider, E. Weidner, R. Ziegelitz; Chem. Eng. Technol. 10 (1987) 37-42.

RECEIVED May 1, 1989

Design, Construction, and Operation of a Multipurpose Plant for

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