Choice of Process

The most developed substrates are, of course. Si. 200 mm diameter substrates have been the Si standard for several years although substrate vendors and production facil- 

Sapphire substrates, for II-Vl or III-V applications, can be found through the 100 mm range. Whereas, ZnSe, ZnTe, and GaN are found in the 1 cm range in research quantities. Efforts on ZnSe and related substrates increased with the demonstration of blue lasers in this material system. However, those efforts are foreshadowed by the blue LED and laser results in GaN. Presently there are several major research efforts underway to produce large area GaN substrates. The nitride substrate development effort also is motivated by high power, temperature and frequency device applications. For superconductors, lanthanum aluminate, strontium titanate, magnesium oxide, and other related substrates are found in the few to tens of square centimeter ranges. 

An important aspect of any substrate that the reader should be familiar with is that it has a low defect concentration and good crystallinity. One measure of defects is the etch pit density or EPD, in which substrates are etched to highlight defects for counting. These defects subsequently often perpetuate themselves through the growing layer and become device failure loci. Crystallinity can be measured by X-ray diffracto-metry and is also a measure of material quality and hence defects. 

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The non-hydrocarbon components of crude oil may be small in volume percent, typically less than 1 %, but their influence on the product quality and the processing requirements can be considerable. It is therefore important to identify the presence of these components as early as possible, and certainly before the field development planning stage, to enable the appropriate choice of processing facilities and materials of construction to be made. 

Chemical Uses. In Europe, products such as ethylene, acetaldehyde, acetic acid, acetone, butadiene, and isoprene have been manufactured from acetylene at one time. Wartime shortages or raw material restrictions were the basis for the choice of process. Coking coal was readily available in Europe and acetylene was easily accessible via calcium carbide. 

Preparation of the vitamins in commercial quantities can involve isolation, chemical synthesis, fermentation, and mixed processes, including chemical and fermentation steps. The choice of process is economic, dictated by the need to obtain materials meeting specifications at the lowest cost. Current process technologies (ca 1997) employed for each vitamin are indicated in Table 9. 

Aerobic treatment is generally appHed to lower strength wastewaters, whereas anaerobic treatment is employed as a pretreatment for high strength wastewaters. The choice of process depends both on the concentration of organics and the volume of wastewater to be treated. 

MetaUic soaps are manufactured by one of three processes a fusion process, a double decomposition or precipitate process, or a direct metal reaction (DMR). The choices of process and solvent depend on the metal, the desired form of the product, the desired purity, raw material avadabihty, and cost. 

The use of any of the above techniques demands knowledge, experience, and flexibility. No prescriptive set of questions or key words or list is sufficient to cover all processes, hazards, and all impacted populations. As a research chemist reviews a chemistry and its potential application, there are advantages to maintaining an open mind when applying the various techniques designed to open up avenues of thought. The reader is referred to Chapter 7 for additional information and direction on the choice of process hazard review techniques. 

The life cycle cost of a process is the net total of all expenses incurred over the entire lifetime of a process. The choice of process chemistry can dramatically affect this life cycle cost. A quantitative life cycle cost cannot be estimated with sufficient accuracy to be of practical value. There is benefit, however, in making a qualitative estimate of the life cycle costs of competing chemistries. Implicit in any estimate of life cycle cost is the estimate of risk. One alternative may seem more attractive than another until the risks associated with product liability issues, environmental concerns, and process hazards are given due consideration. Value of life concepts and cost-benefit analyses (CCPS, 1995a, pp. 23-27 and Chapter 8) are useful in predicting and comparing the life cycle costs of alternatives. 

The choice of process is generally based on economic issues such as volume of seals to be produced elastomeric compound, cost and waste and product design. 

So far there is no cover-all recipe for remedying these undesirable phenomena yet recommendations have been made for some special cases which included proper choice of process conditions and methods, rational design of molds and articles allowing one to minimize the negative effect of filler and matrix orientation in the manufacturing process [370],... 

On completion of the first life cycle of plastics, various recycling processes are available for further utilisation of these valuable materials. The choice of process will depend upon the materials to be recycled. In chemical recycling polymers are degraded to basic chemical... 

GL 28] [R 2] [P 30] Ammonia absorption in dilute acidic solution containing Cresol Purple indicator was rapid, as expected [7]. By appropriate choice of processing parameters, neutralization was achieved close to the gas/liquid contacting zone or distributed over the full contact length. This is evidence for having controls by both solution and gas-phase transport. 

Development Chemistry— Characterization of materials Characterization of process alternatives Choice of process Suitability of process Screening for chemical reaction hazards... 

Edwards and Lawrence (1993) have developed a Prototype Index of Inherent Safety (PIIS) for process design. The inherent safety index is intended for analysing the choice of process route i.e. the raw materials used and the sequence of the reaction steps. This method is very reaction oriented and does not consider properly the other parts of the process even they usually represent the majority of equipment. 

The inherent safety is the pursuit of designing hazards out of a process, as opposed to using engineering or procedural controls to mitigate risk. Therefore inherent safety strives to avoid and remove hazards rather than to control them by added-on systems. The inherent safety is best considered in the initial stages of design, when the choice of process route and concept is made. 

Material Composition Effects on Choice of Processing Method..273... 

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