Chemical Process Engineering

Design And Economics

Harry Silla

• Éditeur: CRC Press
• Année: 2003
• ISBN:
  9781135527532
• Édition: 1
• Sujet: Physical Sciences
• Sous-sujet: Chemistry

This illustrative reference presents a systematic approach to solving design problems by listing the needed equations, calculating degrees-of-freedom, developing calculation procedures to generate process specifications, and sizing equipment. Containing over thirty detailed examples of calculation procedures, the book tabulates numerous easy-to-fol

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The Structure of Processes and Process Engineering

The activities of most engineering disciplines are easily identiflable by the public, but the activities of chemical engineers are less understood. The public recognizes that the chemical engineer is somehow associated with the production of chemicals, but often does not know the difference between chemists and chemical engineers. What is the distinguishing feature of chemical engineering? Briefly, chemical engineering is the development, design, and operation of various kinds of processes. Most chemical engineering activities, in one way or another, are process oriented.

The chemical engineer may work in three types of organizations. One is the operating company, such as DuPont and Dow Chemical, whose main concern is to produce products. These companies are also engaged in developing new processes. If a new plant for an old improved process, or a plant for a recently developed process is being considered, a plant construction organization, the second company type, such as the C.E.Lummus Corp. or the Forster Wheeler Corp., will be contacted. Finally, numerous small and large companies support the activities of the operating and plant construction companies by providing consulting services and by manufacturing equipment such as pumps, heat exchangers, and distillation columns. Because many companies are involved in more than one activity, classifying them may be difficult.

Table 1.1 Selected Process Types

PROCESS TYPES

There are numerous types of processes and any attempt to classify processes will meet difficulties. Nevertheless, attempts at classification should be made to achieve a better understanding of the process industries. Wei, et al. [1] discuss the structure of the chemical process industries. A classification is also given by Chemical Engineering magazine, and the North American Industry Classification System (NAICS) is provided by the U.S. Bureau of Budget. A selected list of process types, classified according to the product type, is given in Table 1.1, illustrating the variety and diversity of processes.

Chemical intermediates are listed first in Table 1.1. These are the chemicals that are used to synthesize other chemicals, and are generally not sold to the public. For example, ethlyene is an intermediate produced from hydrocarbons by cracking natural gas derived ethane or petroleum derived gas oil, either thermally using steam or catalytically. Ethlyene is then used to produce polyethylene (45%), a polymer; and ethlyene oxide (10%), vinyl chloride (15%), styrene (10%), and other uses (20%) [2]. The number of chemicals that are classified as intermediates is considerable.

Examples of energy processes are the production of fuels from petroleum or electricity in a steam power plant. A steam power plant is not ordinarily considered a process, but, nevertheless, it is a special case of a process. The plant contains a combustion reactor, the furnace; pumps; fans; heat exchangers; a water treatment facility, consisting of separation and purification steps; and most likely flue gas treatment to remove particulates and sulfur dioxide. Because of the mechanical and electrical equipment used, mainly mechanical and electrical engineers operate power plants. However, all chemical plants contain more or less mechanical and electrical equipment. For example, the methanol-synthesis process, discussed later, contains steam turbines for energy recovery. Chemical engineers have the necessary background to work in power plants as well, complementing the skills of both mechanical and electrical engineers.

Bread making, an example of a food process, is almost entirely mechanical, but it also contains fermentation steps where flour is converted into bread by yeast [3]. Thus, this process can also be classified as a biochemical process. Another well known biochemical process that removes organic matter in both municipal and industrial wastewater streams is the activated sludge process. In this process, microorganisms feed on organic pollutants, converting them into carbon dioxide, water, and new microorganisms. The microorganisms are then separated from most of the water. Some of the microorganisms are recycled to sustain the process, and the rest is disposed of.

Aspirin, one of the oldest pharmceutical products, has been produced for over a hundred of years [4]. A chemist, Felix Hoffmann, who worked for the Bayer Co. in Elberfeld, Germany, discovered aspirin. He was searching for a medication for pain relief for his father who suffered from the pain of rheumatism. Besides pain relief, physicians have recently found that aspirin helps prevent heart attacks and strokes.

Vitamin C, classified as either a pharmaceutical [5] or a food additive [6], has annual sales of 325 million dollars, the largest of all pharmaceuticals produced [7]. Pharmaceuticals, in general, lead in profitability for all industries [6]. Although vitamin C can be extracted from natural sources, it is primarily synthesized. In fact, it was the first vitamin to be produced in commercial quantities [6]. Jaffe [8] outlines the synthesis. Starting with D-glucose, vitamin C is produced in five chemical steps, one of which is a biochemical oxidation using the bacterium Acetobacter suboxydans. D-glucose is obtained from cornstarch in a process, which will be described later.

The personal products industries, which also includes toiletries, is a large industry, accounting for $10.6 billion in sales in the United States in 1983 [9]. The operation required for manufacturing cosmetics is mainly the mixing of various ingredients such as emollients (softening and smoothing agents), surfactants, solvents, thickeners, humectants (moistening agents), preservatives, perfumes, colors, flavors and other special additives.

Over a period of many years polymeric materials have gradually replaced metals in many applications. Among the five leading thermoplastics; low and high density polyethylene, polyvinyl chloride, polypropylene, and polystyrene; polyethylene is the largest volume plastic in the world. Polyethylene was initially made in the United States in 1943. In 1997, the estimated combined worldwide production of both low and high-density polyethylene was 1.230×1010 kg (2.712 ×1010 lb) [10]. Low density polyethylene is produced at pressures of 1030 to 3450 bar (1020 to 3400 atm) whereas high density polyethylene is produced at pressures of 103 to 345 bar (102 to 340 atm) [11].

Explosives are most noted for their military, rather than civilian uses, but they are also a valuable tool for man in construction and mining. Interestingly, as described by Mark [12], the first synthetic polymer, although it is only partially synthetic, was nitrocellulose or guncotton, a base for smokeless powder. Nitrocellulose was discovered accidentally in 1846 when a Swiss chemist, Christian Schoenbein, wiped a spilled mixture of sulfuric and nitric acids using his wife’s cotton apron. After washing the apron, he attempted to dry it in front of a strove, but instead the apron burst into flames. Although the first application of modified cellulose was in explosives, it was subsequently found that cellulose could be chemically modified to make it soluble, moldable, and also castable into film, which was important in the development of photography. Nitrocellulose is still used today as an ingredient in gunpowder and solid propellants for rockets.

Nitrogen is an essential element for life, required for synthesizing proteins and other biological molecules. Although the earth’s atmosphere contains 79% nitrogen, it is a relatively inert gas and therefore not readily available to plants and animals. Nitrogen must be “fixed”, i.e., combined in some compound that can be more readily absorbed by plants. The natural supply of fixed nitrogen is limited, and it is consumed faster than it is produced. This led to a prediction of an eventual world famine until 1909 in Germany, when Badische Anilin and Soda Fabrik (BASF) initiated the development of a process for ammonia synthesis [13]. In 1910, the United States issued a patent to Haber and Le Rossignol of BASF for their process [14]. The first plant was started up in 1913 in Ludwigshafen, Germany, expanded in the 1960’s, and only shut down in 1982 after seventy years of production [15]. This is certainly an outstanding engineering achievement. Although the fixed nitrogen supply is no longer limited by production from natural sources, they are still major sources. Agricultural land produces 38%; forested or unused land, 25%; combustion, resulting in air pollution, 9%; lightning, 4%; and industrial fixation, 24% [16]. The oceans produce an unknown amount.

Processes could be subdivided according to the type of reaction occurring, as illustrated by bread making and the activated sludge process, by also classifying them as biochemical processes. Similarly, we could also have electrochemical, photochemical, and thermochemical processes and so on, but this subclassification could lead to difficulties because in some processes more than one type of reaction occurs, such as in the vitamin C process.

CHEMICAL ENGINEERING ACTIVITIES

It is useful to delineate the various activities of a chemical engineer, from the conception of a project to its final implementation. Companies will assign a variety of job titles to these activities. In some companies, these activities will be subdivided, but in other companies many activities may be included under one job title, according to company policy. In this discussion, the engineering activity is of more concern than any particular job title assigned by a company. We will use the most frequently employed job title, keeping in mind that any particular company must be consulted for its definition of the job.

A project is initiated by determining if there is a market for a product, which may be a chemical, a processed food, a metal, a polymer or one of the many other products produced by the process industries. For example, a chemist first synthesizes a new drug in the laboratory, which after many tests is approved by the Food and Drug Administration (FDA) of the federal government. Then, chemical engineers develop and design the process for producing the drug in large quantities. The steps required to accomplish this task are outlined in Table 1.2. Under some circumstances, where knowledge of the process is highly developed and sufficient data exists, the research or pilot phase of the process, or both, may be omitted. In order to cover all aspects of a project, we will assume that a new chemical, which is marketable, has just been synthesized in the laboratory by a chemist.

Next, the technical, economic, and financial feasibility of proposed processes must be demonstrated. Unless the project shows considerable promise when matched against other potential projects, it may be abandoned. Any particular company will have several projects to invest in but limited financial resources so that only the most promising projects will be continued. The research engineer should estimate the capital investment required and the production cost of the product. No matter how crude or incomplete the process data may be, the research engineer must estimate the profitability of the process to determine if further process development is economically worth the effort. This analysis will also uncover those areas requiring further research to obtain more information for a more accurate economic evaluation.

If the project analysis shows sufficient uncertainty or the need for design data, the research engineer will plan experiments, design an experimental setup and correlate the resulting data. After completing the experiments, the research engineer, or more likely a cost engineer, revises the flow diagram and reevaluates the project. Again, he must show that the project is still economically feasible.

After completion of the research ph...