Equipment considerations

Despite their cost differences, the three types of system described above have many common features. Some features are expensive and will impact the initial capital investment. Others affect long term operating costs. A review of basic system features and their cost implications follows. 

Materials of Construction. For demanding SCF processes such as precision cleaning, the pressure vessel and most of the components are fabricated fi om 316 stainless steel. Process vessels 

The rest of the system, valves, separators, heat exchanges, filters, and storage and piping for demanding applications is also fabricated using stainless steel. These components are also cost sensitive to pressure and temperature. 

Recently, systems suppliers have developed subcritical cleaning systems (i.e., systems operating below the critical point for carbon dioxide) that are fabricated using low cost alloy steel coated on the inside with an appropriate plastic to protect it from attack. This approach promises to significantly reduce system costs and enhance the competitive position of carbon dioxide cleaning in less demanding applications. 

Controls. Process consistency is a function of equipment and process repeatability. Design and manufacturing quality is also very important. All systems should be supplied with a control strategy that allows automatic cycling. Automatic operation improves consistency, safety and reliability, and reduces the labor content, assuring efficient and cost effective operation. 

Pressure ratings of reactors range from atmospheric to 100 bar with temperature of operation in the range 5-200 °C. (in certain special cases sub-zero temperatures may be achieved through the use of anti-freeze agents in the emulsion). The most common temperature range for emulsion polymerization is 60-100 °C. 

To control the temperature of these highly exothermic reactions, reactors are fitted with a variety of cooling systems. This may be a jacket around the reactor (annular, dimpled, half-pipe) the reactor may be fitted with internal cooling coils or cooled baffles there may be supplemental cooling systems such as reflux condensers, or heat exchangers of various types through which the reaction mixture is circulated. 

For agitation, a wide variety of impeller types are in use both axial and radial flow turbines, propellers, paddles, anchor agitators and many other proprietary designs. The choice of impeller is very much dependent on the properties of the emulsion polymer being made (stability, viscosity etc.). 

As discussed in Sect 2.3.1, batch strippers, stripping columns, filtration equipment and mixing vessels are all in use to a varying extent downstream of the reaction stage. Because of the tendency of emulsion polymers to cause deposits in the process, the secret of successful operation is usually to keep equipment simple. Ease of cleaning is paramount and moving parts , particularly if high shear is imparted to the latex, should be avoided where possible. 

A major driving force for the change from batch to semi-batch polymerization processes, despite the loss in some polymeric properties which often accompanies this change, has been the inherently safer aspects of semi-batch, with smaller quantities of monomer in the reactor at any given time. 

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Another aspect of deflection is the foundation s natural frequency. The foundation must be tuned in such a way that any resonances are not in coincidence with any of the compressor train natural frequencies. Rela tive to equipment considerations, it would be desirable to have all foundation natural frequencies well above any equipment critical speeds... 

Thus, it can be said that the capillary itself is the most important CE equipment consideration when considering CE performance. Considerations must be given to capillary ID, OD, and length. Other considerations include the cleanliness of the window and the cutting of the capillary ends. A poorly selected or prepared capillary will cause poor results and much valuable time lost. 

Equipment Considerations When the solute has a large heat of solution and the feed gas contains a high concentration of solute, as in absorption of HCl in water, the effects of heat release during absorption may be so pronounced that the installation of heat-transfer surface to remove the heat of absorption may be as important as providing sufficient interfacial area for the mass-transfer process itself. The added heat-transfer area may consist of internal cooling coils on the trays, or the liquid may be withdrawn from the tower, cooled in an external heat exchanger, and then returned to the tower. 

Mehta, A. Processing and equipment considerations for aqueous coating. Drug Pharm. Sci. (Aq. Polym. Coat. Pharm. Dosage Forms), 36 267-302, 1989. 

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