Supercritical compressor process

For a compressor process with throttling in the supercritical state of the solvent the according numbers for the necessary heat and electric energy for cases of energy recovery and without energy recovery are listed in Table 3. 

TABLE 2. Energy consumption of a compressor process. Throttling to the subciitical state. Supercritical solvent CO2. Conditions of the extraction 40 MPa. 313 K Conditions of the regeneration 5 MPa, 299 K. 

TABLE 3. Energy consumption of solvent CO2. a compressor process. Throttling in the supercritical state. Supercritical ... 

A comparison of both cycle processes is presented in Figure 31. Energy consumption of both cycles increases with extraction pressure, without much difference between either if precipitation and regeneration is carried out in the supercritical state. Differences are small even for subcritical precipitation and regeneration for an extraction pressure of around 30 MPa. At higher pressures the compressor process needs more energy, at lower extraction pressures the pump process needs more energy. 

The supercritical solvent is expanded with the throttling valve (9) in order to remove the caffeine (separator 8) and to bring the solvent back to the liquid state (condenser 10). The gasrecycling (dry running) reciprocating compressor (7), the C02 and the co-solvent feed (2, 3 diaphragm pumps) represent variable process components if required. Heat exchangers (4) maintain the suitable thermodynamic conditions. 

The single-stage supercritical fluid extraction process for solid natural materials is shown schematically in Figure I. The solvent is conveyed from the low pressure to the high pressure by a pump or compressor V. Extraction is at pressure p and temperature t in extractor E, where the soluble substances are transferred from the natural material to the solvent. Normally, the extractor consists of several autoclaves connected in series in the solvent flow. In throttle valve D the solvent loaded with extract is relaxed to the lower pressure. The extract is separated from the solvent in separator A at separation pressure p and temperature t. Heat exchangers WI, W2 and W3 are installed to achieve the desired temperatures. 

To show the potential of SCF processing, we apply the ideas of Todd and Elgin in a hypothetical process. Figure 6.1 shows a schematic diagram of the supercritical fluid extraction process described by Todd and Elgin. Four major pieces of equipment are shown an extraction vessel, a pressure reduction valve, a separator, and a compressor. For simplicity and ease of discussion. 

An excellent review or LLE applications in the food industry is given by Hamm,17 Since solvent toxicity is a major consideration, supercritical solvents, such as CO, which are nomoxic and LLE from an aqueous leachate are two popalar means of deriling with this problem. Figure 7.8-11 is a cooceptual supercritical extraction flowsheet. Because Tc is much lower than either 7 or rB, the solvent (e.g., C02) simply is flashed from the solute by throttling (an isenthalpic process). Then the vapors are compressed (an isentropic process) and cooled (an isobaric step) to complete the solveni recycle. Usually, costs are determined by the compressor requirearerUs. Several potential applications of supercritical or near-critical solvents are discussed in more datail else where,... 

The flow method ts the simplest and the most straightforward. In the flow method, the solvent fluid is supplied to a compressor by a pressure cylinder. At the desired pressure, the fluid passes into the thermostatted extractor cell that contains the solute present in appropriate matrix (e.g., multiple layer of glass wool). The fluid dissolves the solute in the extractor and, on expansion through a heated metering valve, precipitates solute into a series of collection vessels to be measured gravimetrically. The volume of the decompressed fluid is totaled by a wet or dry gas test meter. Static or equilibrium solubility measurement methods are used to eliminate the need to sample the supercritical fluid solution. A high-pressure flow cell is placed in the flow circuit to monitor the dissolution process by spectrophotometry. 

The necessary operations for changing conditions of state and composition in the solvent circuit can be carried out in different ways which depend on the nature of the substances involved, the scale of the process unit, and the operating conditions of the processing unit. The main difference in solvent cycles is whether the solvent is cycled in the supercritical (gaseous) or subcritical (liquid) state. In both cycles the solvent can be driven by a pump or by a compressor. 

A Figure 11.21 Diagram of a supercritical fluid extraction process. The material to be processed is placed in the extractor. The desired material dissolves in supercritical CO2 at high pressure, then is precipitated in the separator when the CO2 pressure is reduced. The carbon dioxide is then recycled through the compressor with a fresh batch of material in the extractor. 

The SSTAR (24) and STAR-LM (25) lead cooled reactor concepts are based on nitride fuel and use a higher core outlet temperature to drive a supercritical CO2 Brayton cycle at 550 to 600°C, with a potential to gain energy conversion efficiencies of 43% at these temperatures. Moreover, the outlet temperature on the cool side of the recuperator can lie in the range of 70 to 125°C with only weak influence on the efficiency. As the inlet to the compressor is just above 31°C, these conditions facilitate installation of bottoming cycles for district heating, seawater desalination, or process heat production, using the heat otherwise rejected in thermodynamic cycle (see Annexes XXII and XXIII). The supercritical CO2 Brayton cycle lacks an industrial experience base this non-conventional Bra)don cycle will require R D. 

There are a number of other applications of pumps and compressors in supercritical extraction plant. The exact nature of these depends on the particular process. Examples are ... 

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