Types of Process

Types of processes—evaporation at reduced pressures, heating or cooling of fluids calcinations to remove volatiles, drying of wet products to remove moisture, evaporation of heat-sensitive products, and batchwise concentration or continuous evaporation at ambient pressures. 

Any conceivable process is either spontaneous, reversible, or impossible. These three possibilities were discussed in Sec. 3.2 and are summarized below. 

The spontaneous processes relevant to chemistry are irreversible. An irreversible process is a spontaneous process whose reverse is an impossible process. 

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Influence of the type of process fluid coking", compared to delayed coking" makes a harder coke that contains less volatile matter and forms finer grains. 

Though the type of processing required is largely dependent upon fluid composition at the wellhead, the equipment employed is significantly influenced by location whether for example the facilities are based on land or offshore, in tropical or arctic environments. Sometimes conditions are such that a process which is difficult or expensive to perform offshore can be exported to the coast and handled much more easily on land. 

In the case of ion exchangers, the primary ions are chemically bonded into the ftamework of the polymer, and the exchange is between ions in the secondary layer. A few illustrations of these various types of processes follow. 

We take up here some aspects of the thermodynamics of adsorption that are of special relevance to gas adsorption. Two types of processes are of interest ... 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

There is hardly a metal that cannot, or has not, been joined by some welding process. From a practical standpoint, however, the range of alloy systems that may be welded is more restricted. The term weldability specifies the capacity of a metal, or combination of metals, to be welded under fabrication conditions into a suitable stmcture that provides satisfactory service. It is not a precisely defined concept, but encompasses a range of conditions, eg, base- and filler-metal combinations, type of process, procedures, surface conditions, and joint geometries of the base metals (12). A number of tests have been developed to measure weldabiHty. These tests generally are intended to determine the susceptibiHty of welds to cracking. 

Change the type of process step to eliminate the pollutant. 

A broad comparison of the main types of processes, the strength and quaUty of phosphoric acid, and the form and quaUty of by-product calcium sulfate are summarized in Table 7. Because the dihydrate process is the most widely used, the quaUty of its acid and calcium sulfate and its P2O3 recovery are taken as reference for performance comparisons. Illustrative flow diagrams of the principal variations in process types have been pubUshed (39). Numerous other variations in process details ar also used (40—42). The majority of plants use a dihydrate process and some of these have production capacity up to 2100 of P2O3 per day. 

Type of process Operating temperature, °C Extraction Crystal conversion Acid concentration, % P3O3 Acid impurity level vs dihydrate acid P3O3 recovery, %... 

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

There are a large number of standard methods suitable for each stage in the hazard analysis and risk assessment procedure. The selection of the proper method depends on several factors. Some of these are the type of process, the stage in the lifetime of the process, the experience and capabiUties of the participants, and the step in the procedure that is being examined. Information regarding the selection of the proper procedure is available in an excellent and comprehensive reference (1). 

Table 3 lists typical failure rate data for a variety of types of process equipment. Large variations between these numbers and specific equipment can be expected. However, this table demonstrates a very fundamental principle the more compHcated the device, the higher the failure rate. Thus switches and thermocouples have low failure rates gas—Hquid chromatographs have high failure rates. 

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Relatively new methods for separating helium from natural gas use pressure swing adsorption (PSA) processes to recover helium at better than 99.99% purity. This type of process is probably less costiy for the production of gaseous helium but might be uneconomical for liquefied helium production. The PSA process is widely used to produce specification pure helium from 85+% cmde helium in conjunction with cryogenic enrichment of the ca 50% helium raffinate. 

A considerable amount of carbon is formed in the reactor in an arc process, but this can be gready reduced by using an auxiUary gas as a heat carrier. Hydrogen is a most suitable vehicle because of its abiUty to dissociate into very mobile reactive atoms. This type of processing is referred to as a plasma process and it has been developed to industrial scale, eg, the Hoechst WLP process. A very important feature of a plasma process is its abiUty to produce acetylene from heavy feedstocks (even from cmde oil), without the excessive carbon formation of a straight arc process. The speed of mixing plasma and feedstock is critical (6). 

Most Type A processes might be classified as chemical processes, whereas most Type B processes are classified as mechanical processes. Representative examples of both types of processes foUow. Type B processes tend to be promoted by organizations that seU and service equipment for producing microcapsules. Most Type A processes are not promoted by equipment manufacturers, but are developed and used by organizations that produce microcapsules. 

Purification. The method used to recover the desired alkylphenol product from the reactor output is highly dependent on the downstream use of the product and the physical properties of the alkylphenol. The downstream uses vary enormously some require no refining of the alkylphenol feedstock others require very high purity materials. Physical property differences affect both the basic type of process used for recovery and the operating conditions used within that process. 

Dual-Enzyme Processes. In some cases, especially in symp production in Europe, a Hquefaction process is used that incorporates both a thermostable enzyme and a high temperature heat treatment. This type of process provides better hydrolyzate tilterabiHty than that attained in an acid Hquefaction process (9). Consequendy, dual-enzyme processes were developed that utilized multiple additions of either B. licheniformis or B. stearothermophilus a-amylase and a heat treatment step (see Eig. 1). 

The main by-products of this type of process are sulfides and disulfides. The disulfides are formed by the inclusion of an oxidizing agent (generally oxygen) that may be present in the reaction mixture or upon purification. Some of the sulfides formed in this fashion are useful as intermediates for the production of antioxidants. Other mercaptopropionates can be made in similar fashion, if the alkyl acrylate is available. 

Oxidation. Disulfides are prepared commercially by two types of reactions. The first is an oxidation reaction uti1i2ing the thiol and a suitable oxidant as in equation 18 for 2,2,5,5-tetramethyl-3,4-dithiahexane. The most common oxidants are chlorine, oxygen (29), elemental sulfur, or hydrogen peroxide. Carbon tetrachloride (30) has also been used. This type of reaction is extremely exothermic. Some thiols, notably tertiary thiols and long-chain thiols, are resistant to oxidation, primarily because of steric hindrance or poor solubiUty of the oxidant in the thiol. This type of process is used in the preparation of symmetric disulfides, RSSR. The second type of reaction is the reaction of a sulfenyl haUde with a thiol (eq. 19). This process is used to prepare unsymmetric disulfides, RSSR such as 4,4-dimethyl-2,3-dithiahexane. Other methods may be found in the Hterature (28). 

There are several types of processes depending on the nature of the phases, the character of the extracted species, and the direction of extraction. 

All lation of Garbanions. Concentrated N a OH—hen syl triethyl amm onium chloride is the base/catalyst system normally used for this type of process (20). Classes of compounds alkylated in this way include phenylacetonitriles, ben2ylketones, simple aUphatic ketones, certain aldehydes, aryl sulfones, P-ketosulfones, P-ketoesters, malonic esters and nitriles, phenylacetic esters, indene, and fluorene (see Alkylation). 

In some liquid-phase processes, catalyst components are slowly leached from the catalyst bed and eventually the catalyst must be replaced. The feasibility of this type of process involves economics, ie, the costs of catalyst maintenance and keeping a unit out of service for catalyst replacement, and product quality and safety, ie, the effects of having catalyst components in the product and their ease of removal. 

Partially Reversible Processes. In a partially reversible type of process, exemplified by chemical exchange, the reflux system is generally derived from a chemical process and involves the consumption of chemicals needed to transfer the components from the upflow into the downflow at the top of the cascade, and to accomplish the reverse at the bottom. Therefore, although the separation process itself may be reversible, the entire process is not, if the reflux is not accompHshed reversibly. 

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