Supercritical pump process

Figure 8.5. Schematic diagram of pilot-scale supercritical CO2 processing system. 1. anhydrous milk fat (AMF) flow meter 2. AMF pump 3. CO2 pump 4. CO2 loop 5. flow loop 6. entrainment vessels 7. view cell 8. separator 1 9. separator 2 10. separator 3 11. separator 4 12. CO2 meter 13. dry test meter (Reproduced with permission from Bhaskar et al., 1993).

For a pump process with throttling to the two-phase area of the solvent, the necessary heat and electric energy have been calculated, assuming that the liquid is subcooled. The amounts of energy are listed in Table 4. For a pump process with throttling in the supercritical state of the solvent the necessary heat and electric energy have been calculated. The amounts are listed in Table 5. 

TABLE 4. Energy consumption of a pump process. Throttling to the subcritical state. Isobaric heating after the pump. Supercritical solvent CO . Conditions of the extraction 40 MPa, 339 K Conditions of the regeneration 6 MPa, 301 K. 

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. 

FIG. 20 22 Schematic of supercritical antisolvent with enhanced mass-transfer process to produce nanoparticles of controllable size. R, precipitation chamber SCF pump, supply of supercritical COg I, inline filter H, ultrasonic horn P, pump for drug solution G, pressure gauge. 

Waste water treatment. Supercritical CO2 has been put to use in a variety of industrial waste treatment applications. Clean Harbors Environmental Services, Inc., has used SCCO2 in Baltimore since 1989 to treat wastewater from chemical and pharmaceutical manufacturers. In the process the wastewater is pumped into the top of a 32-ft-high, 2-ft-diameter column, while the CO2 is pumped in from the bottom and percolates up. As the CO2 trowels up it dissolves the organics. CO2 contaminated with organics is at the top of the column, and clean water is at the bottom. The contaminants are incinerated off-site after separation from the CO2 which is recycled. 

Of the various extraction processes the decaffeination with supercritical CO2 exhibits the most commercial advantages for bulk production. The process is a discontinuous one. Fig. 1.4-3 shows a number of serially arranged extractors (5) charged with the supercritical CO2 feed by the centrifugal circulation pump (1). 

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. 

Supercritical fluid extraction system - Hewlett Packard Model 7680A totally automated system with unlimited-capacity reciprocating pump, specially designed extraction chamber with safety interlocks, a variable restrictor nozzle and analyte collection trap. The operation of the extractor is controlled by a personal computer which is a Microsoft Windows-based system. An animated status screen provides real-time monitoring of the extraction process. Table II gives the SFE conditions for the HP extractor. 

The overhead from the second stage is heated by an exchange with hot solvent. The fired heater further raises the temperature of the solvent/demetallized oil mixture to a point above the critical temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam-stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used for process heat exchange. The subcritical solvent recovery techniques, including multiple effect systems, allow much less heat recovery. Most of the low grade heat in the solvent vapors from the subcritical flash vaporization must be released to the atmosphere requiring additional heat input to the process. 

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. 

For purposes of comparison, some supercritical fluid extraction processes have been calculated in which the extract is separated at the subcritical pressure p = 60 bar (Process 4). Such a process corresponds to that in Fig. 1 with the difference that a pump is employed to increase the pressure from state 1 to state 2, since the CO2 is cooled down to 17°C after separation, i.e. is present in the liquid state before the pressure is increased. Even for the most favourable variant with K = 0.062 DM/kg hop extract, the operating costs for this process are significantly higher than for processes employing supercritical separation. They can be reduced significantly by heat recovery with a heat pump as published by Sievers and Eggers 3. ... 

Food-grade butane in a supercritical, low-pressure, liquefied gas extraction procedure has also been described for oil extraction from peanuts (21). The extraction process consists of mixing the liquefied butane with the material to form a slurry. The liquefied gas and oil are moved to a solvent recovery system where the oil is removed from the butane. The oil is pumped from the solvent recovery system to a holding tank, and the butane is then transformed into a gas in the solvent recovery system and transported back to the butane storage tank for reuse. 

---