High-pressure processing description

We begin with a description of the high-pressure polymerization process since it is an authentic example of how the principles of thermodynamics and kinetics can be combined with creative engineering to develop an economically viable high-pressure process. These principles can be generalized and extended to other high-pressure processes. After describing the polyethylene process, we move on to more recent work on polyethylene and ethylene copolymers, followed by a discussion of other recent SCF studies with a variety of other polymers and monomers. 

The removal of precipitated polyethylene from the wall is an interesting operation. About once every 2-3 sec the expansion valve is opened more fully than required for the expansion/precipitation function this results in a rapid decrease in pressure in the reactor of as much as 300-600 bar. The concomitant rapid increase in the velocity of the gas phase in the tubular reactor shears the walls and strips off any deposited polyethylene so that a reasonably steady state heat transfer situation exists. This description of the operation of the polymerization process, the polyethylene precipitation step, and the accentuated expansion, which maintains a clean wall and a high heat transfer coefficient, help to illustrate the interesting SCF solubility behavior and they also supply some information on the commercial reality of high-pressure processing in what we consider to be an extreme case. 

The solution process is carried out in tertiary-butanol solution under medium pressures [6]. The maximum molecular weight reached is higher than that of the high-pressure process because the chain-transfer activity of the vinyl acetate molecule is reduced in solution. A more detailed description of the different processes is given by Gilby [2]. 

It has been a persistent characteristic of shock-compression science that the first-order picture of the processes yields readily to solution whereas second-order descriptions fail to confirm material models. For example, the high-pressure, pressure-volume relations and equation-of-state data yield pressure values close to that expected at a given volume compression. Mechanical yielding behavior is observed to follow behaviors that can be modeled on concepts developed to describe solids under less severe loadings. Phase transformations are observed to occur at pressures reasonably close to those obtained in static compression. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

For the description of the different processes that take advantage of the use of high-pressure conditions, the knowledge of thermodynamic properties is essential. 

Injection Molding. In injection molding a molten thermoplastic is injected under high pressure into a steel mold. After the plastic solidifies, the mold is opened and a part in the shape of the mold cavity is removed. General descriptions of the process and related equipment are given in References 29—34. 

CVD reactors operate at sufficiently high pressures and large characteristic dimensions (e.g., wafer spacing) such that Kn (Knudsen number) << 1, and a continuum description is appropriate. Exceptions are the recent vacuum CVD processes for Si (22, 23) and compound semiconductors (156, 157, 169) that work in the transition to the free molecular flow regime, that is, Kn > 1. Figure 7 gives an example of SiH4 trajectories in nearly free molecular flow (Kn 10) in a very low pressure CVD system for silicon epitaxy that is similar to that described by Meyerson et al. (22, 23 Meyerson and Jensen, manuscript in preparation). Wall collisions dominate, and be-... 

The best-developed method of solar electricity storage is to send it to an electrolyzer that splits water into Oz and H2. In this process, the 02 is released (used or sold), and the H2 is stored either as high-pressure gas or as a cryogenic liquid. This process will be described in the discussion of hydrogen processes after the forthcoming description of various solar collector designs in Section 1.5. 

A chemical company is considering the production of 1000 tons/day of high-purity anhydrous ammonia. The method selected is a high-pressure steam-methane reforming process. The process description is as follows ... 

Description Gas feedstock is compressed (if required), desulfurized (1) and sent to the optional saturator (2) where some process steam is generated. The saturator is used where maximum water recovery is important. Further process steam is added, and the mixture is preheated and sent to the pre-reformer (3), using the Catalytic-Rich-Gas process. Steam raised in the methanol converter is added, along with available C02, and the partially reformed mixture is preheated and sent to the reformer (4). High-grade heat in the reformed gas is recovered as high-pressure steam (5), boiler feedwater preheat, and for reboil heat in the distillation system (6). The high-pressure steam is used to drive the main compressors in the plant. 

Description The Vinnolit PVC process uses a new high-performance reactor (1), which is available in sizes up to 150 m3. A closed and clean reactor technology is applied thus, opening of the reactors is not necessary, except for occasional inspections. Equally important, high-pressure water cleaning is not necessary. All process operations of this unit are controlled by a distributed process control system (DCS). 

Description A single jacketed fixed-bed reactor removes the heat of the reaction by producing high-pressure steam. The process is carried out with a large ethylene excess. The flexibility of catalyst staging, reactor temperature profiles, and feed flowrates with EVC s single reactor system, produces maximum throughput with minimal byproducts. After condensation and separation of the reaction products (EDC and water), excess ethylene is compressed and recirculated. 

Description Feeds are sent to USC cracking furnaces (1). Contaminants removal may be installed upstream. A portion of the cracking heat may be supplied by gas turbine exhaust. Pyrolysis occurs within the temperature-time requirements specific to the feedstock and product requirements. Rapid quenching preserves high-olefin yield and the waste heat generates high-pressure steam. Lower temperature waste heat is recovered in the downstream quench oil and quench water towers (2) and used in the recovery process. Pyrolysis fuel oil and gaso-... 

---