Economical control of equipment

An important further constraint is the fact that economic considerations ia the constmction of deposition equipment normally lead to a preference for an ambient temperature deposition chamber. Control of deposition temperature is possible, but it adds both equipment expense and operational complexity. 

Generally, it is more economical to prevent explosive atmospheres ia rooms than to try to provide explosion-proof electrical equipment. Personnel should never be allowed to work ia a ha2ardous atmosphere. Where such an atmosphere cannot be avoided through control of flammable Hquids, gases, and dusts, access to the area iavolved should be limited and the area segregated by hoods or special ventilation. Electrical equipment on open, outdoor stmctures more than 8 m above-ground usually is considered free from exposure to more than temporary, local explosive mixtures near leaks (86). 

Additional operations essential to commercial bauxite processing are steam and power generation, heat recovery to minimise energy consumption, process liquor evaporation to maintain a water balance, impurity removal from process liquor streams, classification and washing of ttihydrate, lime caustication of sodium carbonate [497-19-8] to sodium hydroxide [1310-73-2] repair and maintenance of equipment, rehabiUtation of mine and residue disposal sites, and quaUty and process control. Each operation in the process can be carried out in a variety of ways depending upon bauxite properties and optimum economic tradeoffs. 

More recentiy, sulfuric acid mists have been satisfactorily controlled by passing gas streams through equipment containing beds or mats of small-diameter glass or Teflon fibers. Such units are called mist eliminators (see Airpollution control methods). Use of this type of equipment has been a significant factor in making the double absorption process economical and in reducing stack emissions of acid mist to tolerably low levels. 

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. 

Specifying operating conditions, control methods, and auxiliary equipment to meet the technological and economic needs of the reaction process... 

Factors that enter into any economic analysis of handhng-warehousing systems are (1) expected mechanical and economic life of the system (2) annual maintenance cost (3) capital requirements and expected return on investment (4) building-construction cost and land v ue (5) detailed analysis of each work position (to determine trade-offs of labor and equipment expected future costs and availability of labor are important) (6) relation of system control and personnel used in system (trade-offs of people versus mechanical control) (7) type of information system (computerized or manual) and (8) expected changed in product, container, unit pallet loads, and customer preferences during the life of the system. 

In many situations, the most desirable control of vapor-type discharges can be accomplished by condensation. Condensers may also be used ahead of other air pollution control equipment to remove condensable components. The reasons for using condensers include (1) recovery of economically valuable products, (2) removal of components that might be corrosive or damaging to other portions of the system, and (3) reduction of the volume of the effluent gases. 

Control of atmospheric emissions from petroleum refining can be accomplished by process change, installation of control equipment, and improved housekeeping and maintenance. In many cases, recovery of the pollutants will result in economic benefits. Table 30-26 lists some of the control measures that can be used at petroleum refineries. 

Although the first industrial application of anodic protection was as recent as 1954, it is now widely used, particularly in the USA and USSR. This has been made possible by the recent development of equipment capable of the control of precise potentials at high current outputs. It has been applied to protect mild-steel vessels containing sulphuric acid as large as 49 m in diameter and 15 m high, and commercial equipment is available for use with tanks of capacities from 38 000 to 7 600000 litre . A properly designed anodic-protection system has been shown to be both effective and economically viable, but care must be taken to avoid power failure or the formation of local active-passive cells which lead to the breakdown of passivity and intense corrosion. 

Item 1 is the major factor in choosing continuous systems rather than batch systems for improved SCP production. Economics are improved by lower capital cost for the bioreactor (a conomica major equipment cost, see Section 4.10) and by a higher output rate. Item 3 leads to greater control of product quality. 

Some additional very specific types of corrosion may also occur in the economizer, feed pumps, and other items of pre-boiler equipment, the mechanisms of which must be firmly understood if they are to be properly controlled. Some of the waterside problems affecting these items of equipment are discussed in the following sections. 

However, very economical measurement of acceleration is possible when the washing machine is equipped with a high-resolution analog pressure sensor with microprocessor control. These pressure sensors come as a separate part or are integrated directly on to the PCB of the washing machine control. They work as differential pressure sensors and measure the static pressure via a hose in the lower part of the suds container, thus measuring the water level in the suds container (Fig. 5.57). 

First of all, before we compare flow reactors, let us mention the batch reactor briefly. The batch reactor has the advantage of small instrumentation cost and flexibility of operation (may be shut down easily and quickly). It has the disadvantage of high labor and handling cost, often considerable shutdown time to empty, clean out, and refill, and poorer quality control of the product. Hence we may generalize to state that the batch reactor is well suited to produce small amounts of material and to produce many different products from one piece of equipment. On the other hand, for the chemical treatment of materials in large amounts the continuous process is nearly always found to be more economical. 

For normal extraction times, two manifolds with a corresponding set of valves located on the top and bottom of each extractor are adequate. For short extraction times three manifolds may be necessary. Small and medium-sized plants are equipped with control valves for discontinuous operation steps. From the technical- and economic points of view, high-pressure control valves are limited with KVs values between 6 and 10, especially for pneumatically driven valves. If such valve sizes are too small, high-pressure ball valves must be used, thereby substantially increasing the costs for the interlocking system and the safety requirements. 

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

The general background of process design, flowsheets, and process control is reviewed in the introductory chapters. The major kinds of operations and equipment are treated in individual chapters. Information about peripheral and less widely employed equipment in chemical plants is concentrated in Chapter 19 with references to key works of as much practical value as possible. Because decisions often must be based on economic grounds, Chapter 20, on costs of equipment, rounds out the book. Appendixes provide examples of equipment rating forms and manufacturers questionnaires. 

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