Economics pressurized solvent process

Economical considerations are investigated in this section, with reference to high pressure extraction plants, to the production of polyethylene, and to the precipitation by supercritical anti-solvent process. 

Two feasible methods for removal of as much water as desired from the azeotrope are depicted on Figure 13.27. The dual pressure process takes advantage of the fact that the azeotropic composition is shifted by change of pressure operations at 100 and 760Torr result in the desired concentration of the mixture. In the other method, trichlorethylene serves as an entrainer for the water. A ternary azeotrope is formed that separates into two phases upon condensation. The aqueous layer is rejected, and the solvent layer is recycled to the tower. For economic reasons, some processing beyond that shown will be necessary since the aqueous layer contains some acetonitrile that is worth recovering or may be regarded as a pollutant. 

In the physical absorption process, the CO2 is absorbed in a solvent according to Henry s Law and then regenerated using heat, pressure reduction or both heat and pressure reduction. Typical solvents are Selexol (dimethylether of polyethylene glycol) and Rectisol (cold methanol) which are applied at high pressure. At lower pressures, the chemical absorption processes are more economical. The Selexol physical solvent process is frequently specified for coal gasification applications199. 

Dependent on economic pressure, the recycling ability of the solvent is becoming ever more important. Especially in case of production-scale processes the recycling of the mobile phase is essential for economic success. For solvent recycling the following rules should be taken into account ... 

Ultimately, chemical engineering methods and technology are needed to transform this molecular and empirical chemistry into a real process. This chemistry must be understood in terms of many scale-up, design and simulation issues that will influence the economics of the process. For example, process equipment and operating procedures in supercritical fluids require special attention to safety issues because of the high pressures involved. There is an economic penalty in achieving the compression required to reach operating pressures and to recover products at ambient conditions. Sometimes extensive pretreatment of feeds is required or a specialized co-solvent may have to be added. 

In their simplest form, physical solvent processes require little more than an absorber, an atmospheric flash vessel, and a recycle pump. No steam or other heat source is required. After the absorbed gases are desorbed from the solution by flashing at atmospheric pressure, the lean solution contains acid gas in an amount corresponding to equilibrium at I atm acid-gas partial pressure and this, therefore, represents the theoretical minimum partial pressure of acid gas in the purifled-gas stream. To obtain a higher degree of purification, vacuum or inert gas stripping or heating of the solvent must be employed. Other process modifications are used to minimize loss of valuable gas components, provide a relatively low temperature of operation, and otherwise improve process economics. 

The scope of chemical products can be very broad it certainly includes not only pure chemical compounds but also mixtures and macromolecules. This article is primarily concerned with design of pure components or solvents. The design (or at least systematic selection) for more complex mixtures is inevitable, because of the need for tighter design, analysis, and control of complex chemical processes arising from economic pressures and environmental restrictions. Where a single solvent gave satisfactory performance, a mixture of solvents may be pursued solely to enhance the economics of a process (or to provide a short-cut towards compliance with environmental restrictions). 

In the 1980s, the fluidized bed process was modified to produce a copolymer similar in density to polyethylene produced by the older, high-pressure process. Copolymerization of ethylene with 1-butene or other normal alpha olefins introduces short pendant chains that decrease crystallinity. In the plastics business, the terms high-density, or linear, polyethylene and LDPE (or branched polyethylene) are now supplemented by linear LDPE (LLDPE). While all the properties of the high-pressure polymer are not achieved by the newer material, the economics of the low-pressure, solvent-free process are such that LLDPE competes successfully in many of the markets once dominated by branched LDPE. The fluidized bed itself is only one of several gas-phase polyolefin processes. Stirred beds, arranged either horizontally or vertically, do not depend on gas velocity and are less sensitive to uniformity of gas flow. The stirred beds have been used primarily for polypropylene [19]. However, the fluidized bed has also been adapted for production of polypropylene and propylene copolymers. 

Solution Polymerization. Two solution polymerization technologies ate practiced. Processes of the first type utilize heavy solvents those of the second use molten PE as the polymerization medium (57). Polyethylene becomes soluble ia saturated C —hydrocarbons above 120—130°C. Because the viscosity of HDPE solutions rapidly iacrease with molecular weight, solution polymerization is employed primarily for the production of low mol wt resias. Solution process plants were first constmcted for the low pressure manufacture of PE resias ia the late 1950s they were later exteasively modified to make their operatioa economically competitive. 

Solution Polymerization. Two types of solution polymerization technologies are used for LLDPE synthesis. One process utilizes heavy solvents the other is carried out in mixtures of supercritical ethylene and molten PE as a polymerization medium. Original solution processes were introduced for low pressure manufacture of PE resins in the late 1950s subsequent improvements of these processes gradually made them economically competitive with later, more advanced technologies. 

Some alkylphenols in commercial production have low vapor pressures and/or low thermal decomposition temperatures. Eor these products, the economics of distillation are poor and other recovery processes are used. Crystallisation from a solvent is the most common nondistUlation method for the purification of these alkylphenols. 

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