Multistage

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

Single-stage evaporators tend only to be used when the capacity needed is small. It is more usual to employ multistage systems which recover and reuse the latent heat of the vaporized material. Three... 

An alternative type of downhole mud motor is the mud turbine, (multistage axial flow turbine) which directly drives the bit. The tool consists of an upper section containing the turbine blades and lower section with bearings. As mud is pumped through the upper section the blades are turned. Turbines are designed to rotate at higher speed than the displacement motor. The higher rotation speed requires diamond or composite bits. 

Valleau J P and Card D N 1972 Monte Carlo estimation of the free energy by multistage sampling J. Chem. Phys. 57 5457-62... 

Fig. 13. Multistage spout-fluid-bed reactor. 1, spouted bed 2, perforated plate 3, spray no22le 4, air header 5, fluidi2ed bed.

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

The need for a continuous countercurrent process arises because the selectivity of available adsorbents in a number of commercially important separations is not high. In the -xylene system, for instance, if the Hquid around the adsorbent particles contains 1% -xylene, the Hquid in the pores contains about 2% xylene at equiHbrium. Therefore, one stage of contacting cannot provide a good separation, and multistage contacting must be provided in the same way that multiple trays are required in fractionating materials with relatively low volatiHties. 

The exact order of the production steps may vary widely in addition, some parts of the process may also vary. Metal formate removal may occur immediately after the reaction (62) following formaldehyde and water removal, or by separation from the mother Hquor of the first-stage crystallization (63). The metal formate may be recovered to hydroxide and/or formic acid by ion exchange or used as is for deicing or other commercial appHcations. Similarly, crystallization may include sophisticated techniques such as multistage fractional crystallization, which allows a wider choice of composition of the final product(s) (64,65). 

Fig. 5. Arrangement of multistage contactors where F = feed flow (A-rich), R = raffinate flow, 5 = solvent flow (B-rich), and E = extract flow, (a)...

Other types of RPC have been proposed but are not in industrial use as of this writing. These include a reciprocated wire-mesh packing (188), a reciprocating screen-plate (205), and the multistage vibrating disk column (MVE)C) developed in Japan (189,206,207). These types of RPC may be useful for gas—hquid contact as well as Hquid-Hquid contact. 

Solution Filtration. The polymer solution, free of unacetylated ceUulose, rigid particle contaminants, and dirt, must pass through spinnerets with holes of 30—80 ]lni diameter. Multistage filtration, usuaUy through plate-and-frame filter presses with fabric and paper filter media, removes the extraneous matter before extmsion. Undesirable gelatinous particles, such as the hemiceUulose acetates from ceUulose impurities, tend to be sheared into smaller particles rather than removed. The solution is also aUowed to degas in hoi ding tanks after each state of filtration. 

Washing of Solids. Washing is a process designed to replace the mother liquor in the solids stream with a wash Hquid. The growing importance of this process is due to demands for increased purity of the products combined with the increasingly poorer raw materials available. Washing often may represent a dominant portion of the installation cost because it is usually multistaged and often countercurrent. 

Horizontal belt filters are well suited to either fast or slowly draining soHds, especially where washing requirements are critical. Multistage countercurrent washing can be effectively carried out due to the sharp separation of filtrates available. Horizontal belt vacuum filters are classified according to the method employed to support the filter medium. 

Since its conception, the dynamic filter has been widely reported and further developed. Most European designs are comprised of a multistage disk arrangement (Fig. 28) with both the rotating and stationary elements covered with filter cloth, thus utilising the space inside the pressure vessel. Such filters have been found (29) to be from 5 to 25 times more productive in mass of dry cake per unit area and time than filter presses for the same moisture content of the final slurry. In some cases, the moisture content with the dynamic filter was actually lower than with a filter press. The maximum productivity was achieved with peripheral disk speeds from 2.8 to 4.5 m/s. 

Compressors and blowers for gaseous fluorine service vary in design from multistage centrifugal compressors to diaphragm and piston types. 

Pentafluorobenzene. Pentafluoroben2ene has been prepared by several routes multistage saturation—rearomati2ation process based on fluorination of ben2ene with cobalt trifluoride reductive dechlorination of chloropentafluoroben2ene with 10% pabadium-on-carbon in 82% yield (226,227) and oxidation of penta uorophenylbydra2ine in aqueous copper sulfate at 80°C in 77% yield (228). Its ioni2ation potential is 9.37 V. One measure of toxicity is LD q = 710 mg/kg (oral, mouse) (127). 

---