High-Pressure Operation

Most methanol plants operating before 1965 were small, producing about 150 tonnes per day at 250-260 bar pressrae. The small catalyst volume of up to 4.5- 

TABLE 10.9. High-Pressure Methanol Process (150 Tonne Day , 260 Bar). 

0 cubic meters was divided into five beds, with inter-bed cooling to limit the temperature rise, in a one-meter diameter reactor. The recycle loop, similar to that in ammonia synthesis, had a high circnlation rate of 150,000 m hr so that space velocity in the catalyst was np to 30,000 hr. A typical set of operating conditions is shown in Table 10.9 for a plant of this kind, which also shows the temperatme increase in each bed, the methanol concentration in effluent leaving the final bed and the composition of synthesis gas entering the reactor. 

About 2-5wt% of organic byproducts including about 1% of dimethyl ether, apart from water, formed during the reaction and the amount usually increased as the catalyst aged (a list of by-product formation is shown in Table 10.14.) A significant amount of methane was also formed, particularly at temperatures in excess of 390°C. Much of this was formed on metallic iron, deposited on the catalyst by the thermal decomposition of iron pentacaibonyl. Thus, the maximum operating temperature of the catalyst was severely limited. And the temperature of the reactor was controlled by inter-bed quench cooling with cold synthesis gas. 

TABLE 10.10. Surface Area of Samples Used for Two Years. 

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Bi-Gas process A high-pressure operation for the conversion of solid fuel into substitute natural gas (SNG) using two stages of gasification. 

Figure Bl.27.9. High-temperature heat-leak calorimeter. (Reproduced by pemiission from Cliristensen J J and Izatt R M 1984 An isothemial flow calorimeter designed for high-temperature, high-pressure operation...

A third factor is the ease with which various membrane materials can be fabricated into a particular module design. Almost ah membranes can be formed into plate-and-frame, spiral, and tubular modules, but many membrane materials caimot be fabricated into hollow-fine fibers or capihary fibers. Finahy, the suitabiHty of the module design for high pressure operation and the relative magnitude of pressure drops on the feed and permeate sides of the membrane can sometimes be important considerations. 

The large variety of displacement-type flmd-transport devices makes it difficult to list characteristics common to each. However, for most types it is correct to state that (1) they are adaptable to high-pressure operation, (2) the flow rate through the pump is variable (auxiliary damping systems may be employed to reduce the magnitude of pressure pulsation and flow variation), (3) mechanical considerations limit maximum throughputs, and (4) the devices are capable of efficient performance at extremely low-volume throughput rates. 

Aromatic polyamide (aramid) membranes are a copolymer of 1-3 diaminobenzene with 1-3 and 1-4 benzenedicarboxylic acid chlorides. They are usually made into fine hollow fibers, 93 [Lm outer diameter by 43 [Lm inner diameter. Some flat sheet is made for spirals. These membranes are widely used for seawater desalination and to some extent for other process applications. The hollow fibers are capable of veiy high-pressure operation and have considerably greater hydrolytic resistance than does CA. Their packing density in hoUow-fiber form makes them veiy susceptible to colloidal fouling (a permeator 8 inches in diameter contains 3 M fibers), and they have essentially no resistance to chlorine. 

Special versions are available, such as hazard-proof and for high-pressure operation, but these are not normally applicable to the field of air conditioning. Fans are available with fewer blades (part-solidity) to allow the use of smaller higher-speed motors. There are also fans with short casings having a length little more than that to accommodate the blades for use in confined spaces. 

Suitability for high-pressure operation Yes No Yes Marginal Marginal... 

The most widely known of these tracer polymers are the Trasar products from Nalco, of which the latest offering is NexGuard , a polyacrylic sulfonate (PAS) designed for high-pressure operation. Other tracer polymer systems include Optidose (from Rohm and Haas Company) and Bel-Trak (Biolab/Water Additives Division of Great Lakes Chemical Corp.). 

The development of ammonia synthesis represents a landmark in chemical engineering, as it was the start of large-scale, continuous high-pressure operation in flow reactors, and in catalysis, because the numerous tests of Mittasch provided a systematic overview of the catalytic activity of many substances. 

External recycle reactor Polymerizations, catalytic reactions Very useful for viscous mixtures Equipment cost can be high (for viscous systems and for high pressure operations)... 

With high-pressure operation, the problem of leaks becomes much more serious, since this increases the mass flowrate of fluid that can leak out through a given hole. This is particularly so when the fluid is a flashing liquid. 

For high-pressure operation, safety considerations are extremely important and care must be taken to ensure proper mechanical design. Closures must be designed to withstand the same maximum pressure as the rest of the autoclave. Various authors have treated the problems involved in designing medium- and high-pressure batch reactors (2-4). 

Careful attention to such detail is necessary as a second line of defence against the effects of reactive hazards. The level of protection considered necessary may range from the essential and absolute minimum of effective eye protection, via the safety screen, fume cupboard or enclosed reactor, up to the ultimate of a remotely controlled and blast-resistant isolation cell (usually for high-pressure operations). In the absence of facilities appropriate to the assessed level of hazard, operations must be deferred until such facilities are available. 

Measurements with the latter real-case solvent system give a more complex picture. Even for the one-plate design, achieving uniform flow conditions in each channel is an open issue. Preliminary experiments show that high-temperature, high-pressure operation gives better flow uniformity, which is also advantageous to speed up the polycondensation reaction (see Fig. 10). 

Experimental and pilot scale work has been carried out on pressurised operation and plants have been operated up to 600 kN/m2, and in at least one case up to 1 MN/m2 pressure. High pressure operation permits the use of smaller beds. The fluidising velocity required to produce a given condition in the bed is largely independent of pressure,... 

Thus when ri,f and L are constant, shortening the analysis time can be achieved by increasing the pressure drop and the k° value. Increasing k° can be done by decreasing s and/or increasing the particle size, dp, of the stationary phase material. Increasing the particle size decreases the resolution, Rs therefore, small particle-size stationary phase materials must be densely packed. As a result, a high-pressure operation is unavoidable. 

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