Economics in Material Selection

One of the most effective methods of preventing corrosion is the selection of the proper metal or alloy for a particular corrosive service. Once the conditions of service and environment have been determined that the equipment must withstand, there are several materials available commercially that can be selected to perform an effective service in a compatible environment. Some of the major problems arise from popular misconceptions for example, the use of stainless steel. Stainless steel is not stainless and is not the most corrosion-resistant material. Compatibility of material with service environment is therefore essential. For example, in a hydrogen sulfide environment, high-strength alloys (i.e., yield strength above 90,000 psi or Rc 20 to 22) should be avoided. In material selection some factors that are important to consider are material s physical and chemical properties, economics and availability. 

Economics of Material Selection. Cost is an overpowering consideration in material selection. The basic cost of a material depends upon ... 

Normally the materials used in the interior of a freezer or refrigerator are not selected to take this type of abuse. The main reason such materials as polystyrene and impact styrene are used is that they are relatively inexpensive and do not readily absorb food odors. Economics is a major factor in material selection and the use of higher cost materials with good resistance to abuse does not seem to be justified. Here the responsibility of giving proper instructions in use becomes apparent. Failure of a part in the interior of a refrigerator or freezer generally does not create a high risk situation. In most... 

Three important criteria in materials selection are in-service conditions to which the material will be subjected, any deterioration of material properties during operation, and economics or cost of the fabricated piece. 

In electrolytic processes, the anode is the positive terminal through which electrons pass from the electrolyte. Anode design and selection of anode materials of constmction have traditionally been the result of an optimisation of anode cost and operating economics, in addition to being dependent on the requirements of the process. Most materials used in metal anode fabrication are characteristically expensive use has, however, been justified by enhanced performance and reduced operating cost. An additional consideration that has had increasing influence on selection of the appropriate anode is concern for the environment (see Electrochemical processing). 

Heat Sensitivity. The heat sensitivity or polymerization tendencies of the materials being distilled influence the economics of distillation. Many materials caimot be distilled at their atmospheric boiling points because of high thermal degradation, polymerization, or other unfavorable reaction effects that are functions of temperature. These systems are distilled under vacuum in order to lower operating temperatures. For such systems, the pressure drop per theoretical stage is frequently the controlling factor in contactor selection. An exceUent discussion of equipment requirements and characteristics of vacuum distillation may be found in Reference 90. 

Although the source of raw materials may not be at the plant site, it is an e.xtremcly important factor in the ultimate location of the plant. Process development work and economic studies will indicate the minimum standards for raw materials selection. When these standards have been determined, all possible sources of acceptable raw materials can be located and a detailed analysis can proceed. 

Since pumps are the essential element in a flow system, the selection of the correct machine is crucial to the success of a plant. It must be constructed from the appropriate materials, run at the right speed, be compatible hydraulically with the system at all flows, economical in first cost and in operation, and user-friendly . 

The materials selection procedure for new or replacement plant is crucial to the safe and economic operation of that plant. There is no one correct way to select the appropriate construction material, since for small plant handling toxic or inflammable products the integrity of the plant is high in priority, whereas large plant producing bulk material in a competitive market is more likely to be made of cheaper constructional materials, and the occasional leak or failure may have fewer safety implications. 

In principle, the selection of dispersion equipment for a given purpose is very simple. The obvious choice is the one that will give the required degree of dispersion most economically. In practice it is not so easy. Availability of equipment, the nature of the raw materials, mill-base formulation, batch size, product type and the time available all influence the decision of which machine to use. 

When a metallic material of construction (MOC) is selected to contain, transport, and/or to be exposed to a specific chemical, unless we make a correct, viable, and optimum MOC selection, the hfe expectancy of those facihties, in a given chemical exposure, can be very short. For the inexperienced in this field, the direct capital costs of the MOC facet of the production of chemicals, the funds spent to maintain these facilities (sometimes several times those initial capital costs), the indirect costs that are associated with outages and loss of production, off-quahty product (because of equipment and facility maintenance) as well as from contamination of the product, etc., are many times not even considered, let alone used as one of the major criteria in the selection of that MOC as well as its costs to keep the plant running, i.e., a much overlooked cost figure in the CPI. To emphasize the magnitude and overall economic nature of the direct and indirect (nonproductive) costs/losses that result from the action of corrosion of our metallic facihties, equipment, and the infrastructures, within the United States, Congress has mandated that a survey of the costs of corrosion in the United States be conducted periodically. 

The targets of electrocatalysis are at the basis of recent developments in the field of water electrolysis. First, it is necessary to distinguish between materials evaluation and materials selection. The former is the search for materials with better and better properties for the wanted electrode process. The latter implies global considerations of applicability. This is probably what makes academic research differ from R D. The former is favored by scientifically exciting performance, in the latter it is necessary to find a compromise between, for instance, activity and stability or between efficiency and economic convenience. 

At this point, experiments must be performed. Experimental results for the erosive wear of the selected candidate ceramic materials in coal slurries are presented in Table 8.4. Notice that the wear rate has a very rough inverse correlation with which is consistent with some of the descriptions of erosive wear from the previous section. Any of these ceramic materials is suitable for the piping and pump components based solely on wear rate, with the lowest wear rate for SiC being the most attractive. Formability and economic criteria can be applied to assist in the final material selection. 

One of the primary variables which influences the recoveries of volatile flavor and aroma chemicals during spray drying is the wall material. Utilization of spray dried flavors in food systems presents further constraints on the wall material selection process. Of the food grade polymers available to the manufacturer of spray dried flavorings (i.e., gum acacia, lipophilic starches, maltodextrins, corn syrup solids), no single wall material exhibits the ideal traits deemed necessary for this economically important process. 

Perhaps the most important of these factors involves the raw material employed for this purpose and the by-product volumes and prices. In this connection we discuss the product distributions from potential various feedstocks and current trends in feedstock selection, illustrating the significant role feedstocks play in the ethylene commercial picture. In addition, the effects on production economics of the factors of plant size and severity of operation are investigated. 

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