Severe Process Conditions

Processing under less severe conditions, close to ambient temperature and pressure, increases the inherent safety of a chemical process. Some examples include  

Catalyst improvements allow methanol plants and plants using the Oxo process for aldehyde production to operate at lower pressures. The process also has a higher yield and produces a better quality product (Dale, 1987). 

Improvements in polyolefin manufacturing technology have resulted in lower operating pressures (Althaus and Mahalingam, 1992 Dale, 1987). 

Use of a higher boiling solvent may reduce the normal operating pressure of a process, and will also reduce the maximum pressure resulting from an uncontrolled or runaway reaction (Wilday, 1991). 

Advances in catalysis will result in the development of high yield, low waste manufacturing processes. Catalysts frequently allow the use of less reactive raw materials and intermediates, and less severe processing conditions. High yields and improved 

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These primary particles also contain smaller internal stmctures. Electron microscopy reveals a domain stmcture at about 0.1-p.m dia (8,15,16). The origin and consequences of this stmcture is not weU understood. PVC polymerized in the water phase and deposited on the skin may be the source of some of the domain-sized stmctures. Also, domain-sized flow units may be generated by certain unusual and severe processing conditions, such as high temperature melting at 205°C followed by lower temperature mechanical work at 140—150°C (17), which break down the primary particles further. 

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 ... 

FIG. 33. The depletion of silane and the corresponding production of hydrogen for several process conditions, covering both the a- and the ) -regime. The solid line represents the case where all the consumed silane is converted into a-Si H() and 1.95H2. The dashed line represents the case where 30% of the consumed silane is converted into disilane instead of being deposited. (From E. A. G. Hamers, Ph.D. Thesis. Universiteit Utrecht. Utrecht, the Netherlands, 1998. with permission.)... 

While minimization possibilities are being investigated, substitutions should also be considered as an alternative or companion concept that is, safer materials should be used in place of hazardous ones. This can be accomplished by using alternative chemistry that allows the use of less hazardous materials or less severe processing conditions. When possible, toxic or flammable solvents should be replaced with less hazardous solvents (for example, water-based paints and adhesives and aqueous or dry flowable formulations for agricultural chemicals). 

The modeling of residual stress development during cure can be used to optimize the processing conditions to reduce or control residual stresses. The current process model is used next to assess the effects of several processing conditions on residual stresses. Reduced cure temperature, longer dwell times, slower cool down rate, and the use of novel cure cycles are all feasible for the reduction of residual stresses. 

Several processing conditions were, unfortunately, unavailable to calculate enthalpy and exergy due to the contractor s restrictions on proprietary data. In particular, pressures, temperatures and compositions of several streams associated with preheating and hydrogenation were not reported, so that several numerical values of their streams were estimated approximately. 

Dihydrate process maintenance costs are substantially less than those for hemi processes due to less severe process conditions. The on-stream factor is also higher for the average dihydrate facility. 

Important biomass fuel properties for thermochemical conversion processes are reported as proximate and ultimate analyses. The proximate and ultimate analyses for selected biomass feedstocks are presented in Table 33.5. For comparison, the analyses from two selected coal samples are also presented. Biomass generally has a lower energy density than coal, oils, and natural gas it also has higher oxygen content. The higher volatiles and oxygen content of biomass translate into a higher reactivity compared to traditional fossil fuels. In terms of thermochemical conversions, this means that less severe process conditions (lower temperature and shorter residence time) are required for bio-... 

Although severe process conditions are required (low pressure 1-2 mbar high temperature up to 250°C), steam refining is the most suitable and economical process to lower the cholesterol content of animal fats (e.g., milkfat and tallow). 

Moderate Reduce the hazards of a process by handling materials in a less hazardous form, or under less hazardous conditions, for example at lower temperatures and pressures. Dilution is one of the key words. Aqueous ammonia should be used instead of anhydrous NH3. Aqueous HCl in place of anhydrous HCl. Sulfuric acid in place of oleum. Figure 1.15 shows an example of the relevant effects observed for the concentration of ammonia measured in air as a function of distance from the place of rupture of a tank containing anhydrous or diluted ammonia solution. Less severe processing conditions are also another keyword. The use of improved catalysts is a critical element in reaching this objective and will be discussed extensively in the following chapters. 

In the first case, the reactions of interest are those which are intrinsically fast and exothermic, but which are currently limited by the poor heat and mass transfer for rates achievable in a stirred pot. Existing technology routinely entails substantial hazardous process inventories, possible reactor runaway and indifferent product selectivity. Fast response reactors open up the possibility of switching to more severe process conditions which would be prohibited in conventional reactors in view of the tendency to degrade the product. It may therefore be possible to exploit a virtuous circle - short residence time -higher temperature - faster kinetics - smaller reactor - shorter residence time. 

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