Positive displacement pumps for CO2-extraction
In the extraction of active and valuable substances, aromatic and flavouring substances, the food and pharmaceutical industry is forced to make high demands concerning quality and economy of the extraction process. The CO2-method is therefore preferably used in the extraction of primary vegetable products. The optimal sequence of the process depends on the quality and the controllability of process data in the high pressure section. Efficient industrial utilization of this method requires high reliability and functionality of the most important technical system components. The pumps for increasing the pressure of the liquid CO2 therefore play a key role in the process, because these pumps can be reliably adapted to the CO2-conditions.
The main advantages of extraction with the help of CO2 are, on the one hand, the gentle treatment of the substances and, on the other hand, the cleanliness of the end product, i.e. extracts do not contain any solvent residues. Apart from this, the method enables high selectivity of the extraction. By using suitable process parameters (pressure and temperature), even different active substances can be extracted from the same raw material. This property of CO2 demands that the parameters process pressure, process temperature and flow volume of the extraction system are maintained almost exactly at a level that matches the required nominal values.
Pressure generation and volumetric flow control is preferably accomplished by a positive displacement pump, which is able to meet the high demands concerning controllability and constant flow volume and can therefore be very well adapted to the properties of CO2. By minimizing the dead space, an oscillating positive displacement pump is able to achieve a very good volumetric efficiency, despite the high compressibility of liquid carbon dioxide. Due to the high process pressures and the relatively low flow volumes, this pump type is particularly suitable for CO2-extraction. Positive displacement pumps therefore are the only pump type, which, in combination with a good rate of efficiency, achieve the demanded process data and guarantee exact compliance with the parameters most economically. This stability results from the excellent proportionality of rotary speed and flow volume and its almost complete independence from the process pressure.
The following requirements concerning the rating of the pump, which is intended to deliver the CO2 for the extraction process, must be met. The pump must first be compressed to the required supercritical pressure to be able to transform the CO2 into the supercritical condition. Compared with water, liquid carbon dioxide has a considerably higher compressibility. Density value essential for the flow quantity (suction and pressure side) can be taken from the table of thermodynamic state variables.
Special attention must additionally be paid to the requirements concerning high durability of components and cavitation free delivery at a high overall efficiency of the pump. High demands are thereby placed on sealing of the plunger and the leak tightness of suction and pressure valves.
The requirements on fluid contacting parts can only be met by highly specialized designs in connection with the use of special materials. The only standard element of such a pump is the power end with crosshead and crank drive.
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