Raw Materials and Chemical Reactions

Eventually, the two sections of the plant are combined and heuristics are used to set the purge-to-recycle ratio. Here, the simulator determines the recycle streams, which in the analysis heretofore were specified, once again using heuristic rules. Then the simulator provides an easy-to-use vehicle for studying the effect of the purge-to-recycle ratio on the performance of the process. 

Heuristic 1 Select raw materials and chemical reactions to avoid, or reduce, the handling and storage of hazardous and toxic chemicals. 

As discussed in Chapter 3, the selection of raw materials and chemical reactions is often suggested by chemists, biologists, biochemists, or other persons knowledgeable about the chemical conversions involved. In recent years, with the tremendous increase in awareness of the need to avoid handling hazardous and toxic chemicals, in connection with environmental and safety regulations (as discussed in Sections 1.3 and 1.4), raw materials and chemical reactions are often selected to protect the environment and avoid the safety problems that are evident in Material Safety Data Sheets (MSDSs). For example, recall that when the vinyl chloride 

For these reasons, societal needs are increasingly being formulated that call for new processes to avoid or sharply reduce the handling of hazardous chemicals. As an example, consider the manufacture of ethylene glycol, the principal ingredient of antifreeze. Ethylene glycol is produced commonly by two reactions in series  

The first reaction involves the partial oxidation of ethylene over a Ag-gauze catalyst. Since both reactions are highly exothermic, they need to be controlled carefully. More important from a safety point of view, a water spill into an ethylene oxide storage tank could lead to an accident similar to the Bhopal incident. Yet it is common in processes with two reaction steps to store the intermediates so as to permit the products to be generated continuously, even when maintenance problems shut down the first reaction operation. 

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Basic process chemistry using less hazardous materials and chemical reactions offers the greatest potential for improving inherent safety in the chemical industry. Alternate chemistry may use less hazardous raw material or intermediates, reduced inventories of hazardous materials, or less severe processing conditions. Identification of catalysts to enhance reaction selectivity or to allow desired reactions to be carried out at a lower temperature or pressure is often a key to development of inherently safer chemical synthesis routes. Some specific examples of innovations in process chemistry which result in inherently safer processes include ... 

Evaporated salt is of sufficient purity that it is used as a food ingredient for humans and animals, as well as a raw material for chemical reactions and polishing pharmaceutical tablets. Purified salt can also be used in food processing, and is frequently used in the pharmaceutical industry to make hemodialysis and intravenous fluids.6... 

A second very important use of petroleum is in the manufacture of plastics and other chemicals. The number of chemical compounds obtained from petroleum and used as raw materials in chemical reactions is almost endless. It includes compounds such as methane, ethane, propane, butane, ethene (ethylene), propene (propylene), butene (butylene), benzene, methanol (methyl alcohol), ethanol (ethyl alcohol), phenol, xylene, naphthalene, and anthracene, to mention only a few. 

Process technicians play a major role in the production and manufacturing of chemicals They operate and maintain the systems that combine raw materials and modern reaction technology to form new products. The foundation upon which this industry rests is modern chemistry. As noted earlier in this chapter, chemistry s the study of the characteristics or structure of elements and the changes that take place when they combine to form other substances. A reactor is designed to make or break chemical bonds, thereby changing the molecular structure of raw materials. In short, a reactor is a device used to convert raw materials into useful products through chemical reactions. Process operators are responsible for the safe and efficient operation of the reactor and its associated equipment. 

Using knowledge bases of the type mentioned in Section 4.2 these systems propose economic alternatives for chemical processes. The. systems start with basic information about the chemical reaction system, available raw materials and data about the desired products and their quality. Based on these data they allow development of a complete process including raw material preparation, chemical reaction and product separation. ... 

Source reduction includes any in-plant actions to reduce the quantity or the toxicity of the waste at the source. Examples include equipment modification, design and operational changes of the process, reformulation or redesign of products, substitution of raw materials, and use of environmentally benign chemical reactions. 

Since the first synthesis of ammonia, catalyst development and chemical reaction engineering have been instrumental in the creation of the chemical process industry. As a result, catalytic processes have contributed much to the realization of prosperous civilizaticm. In the future, catalytic processes are expected to fulfill important roles in petroleum refining, diemical processing and environmental preservation. However, at present, many catalytic processes discharge large amounts of byproducts and consume large amounts of auxiliary raw materials. 

Once the product specifications have been fixed, some decisions need to be made regarding the reaction path. There are sometimes different paths to the same product. For example, suppose ethanol is to be manufactured. Ethylene could be used as a raw material and reacted with water to produce ethanol. An alternative would be to start with methanol as a raw material and react it with synthesis gas (a mixture of carbon monoxide and hydrogen) to produce the same product. These two paths employ chemical reactor technology. A third path could employ a biochemical reaction (or fermentation) that exploits the metabolic processes of microorganisms in a biochemical reactor. Ethanol could therefore also be manufactured by fermentation of a carbohydrate. 

To do this, not only must he know the chemistry of the reactions but he must know the rates at which the reactions occur and what affects those rates. The study of this is called chemical kinetics. By the proper choice of raw materials and operating conditions for the reaction stage the process designer can manipulate the ratio of products formed. One major variable is the temperature. An increase in temperature usually causes the reaction rates to increase, but some increase faster than others. Thus, the product mix in the reactor is dependent on the temperature. The pressure and the time the material spends in the reactor also affects the results. In the gaseous phase ahigh pressure will impede those steps in which the number of moles is increased and assist those in which the number of moles is decreased. A... 

The plant is used to produce two chemically different EPS -types A and B in five grain size fractions each from raw materials FI, F2, F3. The polymerization reactions exhibit a selectivity of less than 100% with respect to the grain size fractions Besides one main fraction, they yield significant amounts of the other four fractions as by-products. The production processes are defined by recipes which specify the EPS-type (A or B) and the grain size distribution. For each EPS-type, five recipes are available with the grain size distributions shown in Figure 7.2 (bottom). The recipes exhibit the same structure as shown in Figure 7.2 (top) in state-task-network-representation (states in circles, tasks in squares). They differ in the parameters, e.g., the amounts of raw materials, and in the temperature profiles of the polymerization reactions. 

It should be noted that MSDSs are not required for all chemical intermediates and byproducts. These should be considered in addition to raw materials and products. Also, be aware that storage, handling or processing at higher temperatures or pressures may initiate an uncontrolled reaction in a material or mixture that is apparently unreactive closer to ambient conditions. 

Biofuels are produced in a sequence of large batch operations involving bio/chemical reactions, separation and purification steps, followed by formulation with specific additives. The final product must comply with multiple quality specifications despite the variability in raw materials and the complexity of unit operations used in their processing. 

Benzene and its derivatives are used widely throughout the chemical industry as solvents and raw materials. Mono-, di-, and trichlorobenzenes are used directly as pesticides for their insecticidal and fungicidal properties. Benzene, toluene, and chlorobenzene are used as raw materials in the synthesis of at least 15 pesticides, although their main use is as a carrier solvent in 76 processes. Additional priority pollutant aromatics and chlorinated aromatics exist as impurities or as reaction byproducts because of the reactions of the basic raw materials and solvent compounds. 

Neither of these methods is used today. Around 1970 the industry switched from C2 raw materials and classical organic chemical addition reactions to the ammoxidation of propylene. Now all acrylonitrile is made by this procedure, which involves reaction of propylene, ammonia, and oxygen at 400-450°C and 0.5-2 atm in a fluidized bed Bi203 nMo03 catalyst. The yield is approximately 70%. 

Let us study the sellers of chemical products, which are collectively called the CPI. These manufacturers are skilled in the use of chemical reactions and separations to make their products, and they employ many chemical engineers and chemists, often in highly responsible positions. Many of the firms in the CPI are also our suppliers of raw materials and intermediates, our customers for our products, and our competition in making and selling their products. 

Cysteine can be obtained by hydrolysis from cysteine-rich proteins in hair or feathers or from petrochemical sources. Cysteine is an important raw material in Maillard reactions for the preparation of process flavours, but it can also serve as a source of ammonia and hydrogen sulfide for the preparation of flavour chemicals, such as the terpene sulfur compounds mentioned in Sect. 13.2.4 and furfuryl mercaptan mentioned in Sect. 13.4.2.4. 

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