Chemical plant design

Chemical engineers use engineering principles to solve problems of a chemical nature. Nearly three-fourths of the approximately 33,000 chemical engineers in the United States work in chemical manufacturing. They design chemical plants, develop chemical processes, and optimize production methods. A primary concern of chemical engineers working in industry is to... 

Process design Chemical plants have inherent additional problems caused by reaction, etc. 

Design to exclude air. Oxygen reduction is one of the most common cathodic reactions during corrosion, and if oxygen is eliminated, corrosion can often be reduced or prevented. In designing chemical plant equipment, particular attention should be paid to agitators, liquid inlets, and other points where air entraimnent is a possibihty. Exceptions to this rule are active-passive metals and alloys. Titanium and stainless steels are more resistant to acids containing dissolved air or other oxidizers. 

Fig. 1. Fine chemicals plant design showing successive additions of processing equipment, where A represents the reaction vessel with agitator B, centrifuge C, dryer D, crystaUi2ation vessel E, raw material feed tanks F, centrifuge which may have an automatic discharge G, mother Hquor tank H,...

In the design of a fine chemicals plant equally important to the choice and positioning of the equipment is the selection of its size, especially the volume of the reaction vessels. Volumes of reactors vary quite widely, namely between 1,000 and 10,000 L, or ia rare cases 16,000 L. The cost of a production train ready for operation iacreases as a function of the 0.7 power. The personnel requirement iacreases at an even lower rate. Thus a large plant usiag large equipment would be expected to be more economical to mn than a small one. 

H. Tongue, The Design and Construction of High Pressure Chemical Plant, 2nd ed.. Chapman Had Ltd., London, 1959. 

Pressure Vessels and Piping. Some of the most critical components of a chemical plant involve pressure vessels. A thorough knowledge of the American Society of Mechanical Engineers (ASME) Pressure Vessel Code (75) is essential for design and maintenance of chemical plants. Some states have their own codes, which usually conform closely to the ASME version (see High pressure technology Tanks and pressure vessels). 

Normal Operation. The designer of a chemical plant must provide an adequate interface between the process and the operating employees. This is usually accompHshed by providing instmments to sense pressures, temperatures, flows, etc, and automatic or remote-operated valves to control the process and utility streams. Alarms and interlock systems provide warnings of process upsets and automatic shutdown for excessive deviations from the desired ranges of control, respectively. Periodic intermption of operations is necessary to ensure that instmments are properly caUbrated and that emergency devices would operate if needed (see Flow measurement Temperaturemeasurement). 

Fire and Explosion Prevention. Prevention of fire and explosion takes place in the design of chemical plants. Such prevention involves the study of material characteristics, such as those in Table 1, and processing conditions to determine appropriate ha2ard avoidance methods. Engineering techniques are available for preventing fires and explosions. Containment of flammable and combustible materials and control of processes which could develop high pressures are also important aspects of fire and explosion prevention. 

Process Plant Ha ard and Control Building Design, Chemical Industries Association (U.K.), 1979. 

The need for skill and experience on the part of sample designers and persoimel cannot be overemphasized in chemical plant sampling. Safety precautions are of the utmost importance. Necessary steps must be taken to document the hazards involved in an operation and to ensure that the staff are weU-trained, informed, protected, and capable. Except for bulk powder sampling, most chemical plant sampling is hazardous and difficult and must be designed with care. The following discussions are based on the assumptions that most of these decisions have been made and a satisfactory sampling procedure has been planned. 

Classification Process simulation refers to the activity in which mathematical models of chemical processes and refineries are modeled with equations, usually on the computer. The usual distinction must be made between steady-state models and transient models, following the ideas presented in the introduction to this sec tion. In a chemical process, of course, the process is nearly always in a transient mode, at some level of precision, but when the time-dependent fluctuations are below some value, a steady-state model can be formulated. This subsection presents briefly the ideas behind steady-state process simulation (also called flowsheeting), which are embodied in commercial codes. The transient simulations are important for designing startup of plants and are especially useful for the operating of chemical plants. 

Such mishaps can be worse if the chemistiy is not fuUy understood. A chemical plant can be inherently. safer if knowledge of the chemistiy of the process and the reactive chemicals systems involved is used in its design. 

Thus, a cogeneration system is designed from one of two perspectives it may Be sized to meet the process heat and other steam needs of a plant or community of industrial and institutional users, so that the electric power is treated as a by-produc t which must be either used on site or sold or it may be sized to meet electric power demand, and the rejected heat used to supply needs at or near the site. The latter approach is the likely one if a utility owns the system the former if a chemical plant is the owner. 

Wilday, A.J. 1991. The Safe Design of Chemical Plants with No Need for Pressure Relief Systems. Elazards IX—New Directions in Process Safety. IChemE Symposium Series. No. 124, pp. 243-253. Institute of Chemical Engineers, IChemE, Rugby, U.K. 

A cylindrical tube in a chemical plant is subjected to an excess internal pressure of 6 MN m , which leads to a circumferential stress in the tube wall. The tube wall is required to withstand this stress at a temperature of 510°C for 9 years. A designer has specified tubes of 40 mm bore and 2 mm wall thickness made from a stainless alloy of iron with 15% by weight of chromium. The manufacturer s specification for this alloy gives the following information ... 

An alloy tie bar in a chemical plant has been designed to withstand a stress, ct, of 25 MN m at 620°C. Creep tests carried out on specimens of the alloy under... 

Evans, F. L., Equipment Design Handbook for Refineries and Chemical Plants, Vol. 1, 2nd Ed., Gulf Publishing Co., 1979. 

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