Extraction Circuit Psepl Psep2

Figure 2. Flowsheet of SFE with two-stage separation 4.3. Design criteria

Design of a plant for high pressure extraction must be based on the required rate of production and the nature of employed feed. In principle, a distinction must be drawn between the following possibilities :-

1) Purification of the feedstock: The feed raw material is regarded as the product from which certain components must be removed during the extraction process. Examples being the production of decaffeinated coffee and of nicotine-free tobacco.

2) Recovery of an extract: In this case, bulk of the feedstock is regarded as a practically valueless carrier source of the extract. Examples are the production of hops extract, cocoa butter and various spice extracts.

Requirements of plant design differ only in relation to the mechanical treatment of the raw material, e.g., its feed into the extraction equipment or agitation. In the case of solids recovery mechanical effects should be avoided as far as possible, whereas in the production of a soluble extract such effects may be altogether desirable.

Furthermore, it may be required to recover both the solids as well as certain extracted components. In addition, there are cases where the plant must be operated in two stages; extracting one component from the feedstock, storing it temporarily and, after the extraction of second constituent, returning it to the bulk material. This procedure is adopted when a certain component A is to be selectively removed while retaining a component B in the feedstock which is more volatile than A. For example, in the high pressure extraction of tea, it is essential to remove the tea aroma from a batch fed into an extraction vessel in order to add this aroma to an already decaffeinated batch in another vessel. A similar situation exists in the case of tobacco, where nicotine and aroma are extracted separately and the aroma is subsequently blended back into the tobacco. In a multistage plant, a fractional extraction can be operated under precisely defined process conditions for the individual stages.

4.3.1. Parameters to be determined in process design

As a basis for sizing of the plant and of individual items of equipment, a prediction or prior knowledge of some process data is required. Information on the following is necessary:

1) Required rate of production.

2) Mode of operation of the plant (continuous or batchwise).

3) Bulk density of the solid feed. The bulk density depends not only on the density of the solids but also, to a large extent, on the form and consistency of the material and on its moisture content. It must, therefore, be determined in the course of preliminary experiments.

4) Ratio of solvent mass flow rate to the mass of solids treated.

5) The extraction time.

Out of these parameters, the required rate of production is specified. Depending upon this rate, the mode of operation i.e., whether batch or continuous, is decided. Bulk density of solids feed, as explained earlier, must be experimentally determined. These three parameters enable the volume of the extraction vessel to be calculated.

The quantity of solvent to be introduced into the plant is determined precisely from the required operating temperature and pressure. Then, by knowing the time of extraction the mass flow rate of solvent can be calculated.

As yet there is no model for a mathematical description of the predominantly non-steady state processes of extraction with supercritical fluids. Hence, one cannot predict the time of extraction from any model and one must rely on the results obtained in laboratory experiments. For a particular feed, the extraction time will differ for different solvents and entrainers as well as with different operating conditions. For predicting this extraction time or residence time of the solvent in the extraction vessel, mass transfer coefficients will have to be predicted. These values are not available at the present time.

Further, basic parameters required for the design are the operating pressures and temperatures. In addition to process data, e.g., the required supply and removal of energy, structural characteristics of the plant also depend on these variables. Thus, wall thicknesses must be determined and sealing problems solved, and this depends on the ranges of pressures and temperatures applied and on the associated load cycles in the plant.

Accurate knowledge of the physical properties of the solvent, the raw material and the extract are also required. For example, density and viscosity are required for knowing flow characteristics. The decomposition temperature of the raw material is particularly important from the point of view of determining operating temperature.

Data on the flow resistances and pressure losses in beds of the solid material and parts of the plant through which the solvent flows are also important with regard to choice of the conveying system for the solvent. These can be obtained either from theoretical considerations or from preliminary experiments.

Finally, mass and energy balances must be drawn up for the design of many plant components, e.g., for all equipments performing heat transfer function.

4.3.2. Thermodynamic considerations

To determine if an extraction of interest is technically and economically feasible, it is necessary to have an adequate quantitative representation of the phase equilibria between the solute(s) and the solvent(s) involved. Without this information, process models can not be made, and few conclusions can be drawn concerning equipment size, operating conditions, solvent flow rates, and extraction yields. Mixtures of materials at supercritical conditions exhibit highly non-ideal behaviour and do not lend themselves easily to quantitative data correlation. This is particularly true for materials of biological origin (King and Bott, 1982).

However, in simpler situations some progress can be made. The well known Soave-Redlich-Kwong and the Peng-Robinson equations of state are most frequently used for phase equilibrium calculations. Due to the difficulty in predicting phase equilibria, especially for natural products, only a crude estimate of operating pressures and temperatures can be made. For design, one must then rely upon the results obtained from experiments carried out with the material to be extracted.

4.4. Applications to food industry

Some of the applications of supercritical fluid extraction are discussed.

4.4.1. Oilseeds extraction

The extraction of various oilseeds like avocado, castor beans, corn germ (wet milled), cottonseed, peanut, rapeseed, sesame, soyabean and sunflower using supercritical carbon dioxide have produced good quality oil and undegraded meal in high yield. Figure 3 gives the solubility of corn oil in supercritical carbon dioxide as a function of pressure at different temperatures. It can be seen that high solubility of corn oil is obtained at higher pressures. The rate of increase of solubility with pressure is lower in the high pressure region. This is considered for the value of pressure to be used for extraction.

The overall yield of extraction with supercritical carbon dioxide compares well with that obtained by hot hexane extraction. The carbon dioxide-extracted oil has a very low phosphatide content. Therefore, it has the advantage of being equivalent to a degummed, hexane-extracted crude oil (Friedrich and List, 1982).

Some work using carbon dioxide at lower pressures has attracted a lot of attention. Various advantages are obtained by extracting the oil at lower or near critical conditions.

Figure 3. Solubility of corn oil in C02

4.4.2. Cholesterol extraction (Anon., 1988, 1989, 1990; Froning et al, 1990)

A high cholesterol diet can accelarate the development of atherosclerosis with its dual sequelae of thrombosis and infarction. In view of this, production of cholesterol-free butter, cheese, ice cream and egg yolk is drawing the attention of the food processing industry. NutraSweet (Deerfield, IL„ USA) is commercially producing low fat, low cholesterol egg yolk powder. Various companies (Phasex Corp., USA; Supercritical Processing, USA; SKW, Germany) have set up pilot plant facilities for cholesterol extraction using supercritical fluids.

The process for cholesterol-free dairy products involves separation of fats from the milk in a centrifugal separator. The cholesterol is then removed from the fats by extraction with supercritical carbon dioxide. Use of methanol as entrainer increases the solubility of cholesterol in fluid phase by an order of magnitude. The cholesterol-free fats are reblended into the milk by conventional methods.

A typical plant for supercritical fluid extraction of cholesterol from butter oil is shown in Figure 4. Butter oil is fed into the extraction column between two packed sections. The supercritical carbon dioxide at 40°C and 175 bar is fed at the bottom of the column for countercurrent extraction. The cholesterol-laden carbon dioxide flowing up through the top packed section is contacted with the cholesterol-rich extract which is refluxed at the top. The cholesterol-rich extract is obtained as the top product and low-cholesterol butter oil is obtained as the bottom product. The carbon dioxide is recycled from the

CO, Recycle

CO, Recycle

Low Cholesterol Butter Oil

Figure 4. Process for supercritical CO2 extraction of cholesterol from butter oil

Low Cholesterol Butter Oil

Figure 4. Process for supercritical CO2 extraction of cholesterol from butter oil separator to the bottom of the column. Conditions in the separator are subcritical for carbon dioxide and require the carbon dioxide to be recompressed with make up carbon dioxide for reuse in the process. In case entrainer is used for the separation, it is fed with the butter oil. Two columns may be used in place of two packed sections in one column. Some plants use sieve plate columns for the extraction process.

Froning et al (1990) have reported removal of approximately two-third of the cholesterol from spray dried egg yolk using supercritical carbon dioxide at 306 atm, 45°C and 374 atm, 55°C.

4.4.3. Decaffeination of coffee

A commercial process for removing caffeine from green coffee beans has been in commercial production at Hag AG (a unit of General Foods) in Bremen, Germany, for many years. The process was developed and commercialised under licence from SGK of Mullheim, Germany.

Cleaned, moisturized coffee beans are extracted to a caffeine level of about 0.1% with supercritical carbon dioxide at 120 bar and 40°C. The carbon dioxide after extraction passes over an activated carbon bed where the extracted caffeine is adsorbed. The carbon dioxide is then recycled to the extractor. Solids are charged and discharged in a batch manner.

4.4.4. Spices extraction

The standardization in the food industry has led to the spice extracts or oleoresins to be used in place of the spices. Hubert and Vitzthum (1978) have obtained extracts of spices like pepper, nutmeg and chillies. The degree of extraction of piperine from pepper was almost 98% and that of essential oil was 81%. The extract was yellow as compared to the olive-green with methylene dichloride as solvent. Similarly, the extraction of capsaicine was 97% from chillies. RAPS & Co. in Germany is commercially marketing oleoresins of a large number of spices (RAPS Bulletin, 1994; Hartmann, 1993).

Tables 3, 4, and 5 give results of the extraction of cumin, celery and ajowan seeds, respectively (Mishra and Tiwari, 1994). The extraction with supercritical carbon dioxide was performed at conditions: temperature, 35-55°C; pressure, 80-450 bar; batch time, 3 hours; and flow rate of carbon dioxide, 5-20 kg CCVCkg spice)(hour). Comparison is presented with hexane extraction (Soxhlet apparatus, 5 hours batch time) and hydrodistillation. Commercially available steam distilled volatile oil was also analysed. These results are presented in the tables. In all cases, the volatile oil could be obtained at 80-100 bar pressure and supercritical temperature. Good quality oleoresin was obtained at 200 bar and 35°C.

Table 3

Comparison of extracts of cumin seed extracted by different methods

Table 3

Comparison of extracts of cumin seed extracted by different methods

Method of

Oleoresin

Volatile

Composition of volatile oil

Color

Color

extraction

extracted

oil

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