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Chemical processing should form part of a sustainable industrial activity. For chemical processing, this means that processes should use raw materials as efficiently as is economic and practicable, both to prevent the production of waste that can be environmentally harmful and to preserve the reserves of raw materials as much as possible. Processes should use as little energy as economic and practicable, both to prevent the build-up of carbon dioxide in the atmosphere from burning fossil fuels and to preserve reserves of fossil fuels. Water must also be consumed in sustainable quantities that do not cause deterioration in the quality of the water source and the long-term quantity of the reserves. Aqueous and atmospheric emissions must not be environmentally harmful, and solid waste to landfill must be avoided. Finally, all aspects of chemical processing mustfeature good health and safety practice.
It is important for the designer to understand the limitations of the methods used in chemical process design. The best way to understand the limitations is to understand the derivations of the equations used and the assumptions on which the equations are based. Where practical, the derivation of the design equations has been included in the text.
The book is intended to provide a practical guide to chemical process design and integration for undergraduate and postgraduate students of chemical engineering, practicing process designers and chemical engineers and applied chemists working in process development. Examples have been included throughout the text. Most of these examples do not require specialist software and can be performed on spreadsheet software. Finally, a number of exercises have been added at the end of each chapter to allow the reader to practice the calculation procedures.
1.1 CHEMICAL PRODUCTS
Chemical products are essential to modern living standards. Almost all aspects of everyday life are supported by chemical products in one way or another. Yet, society tends to take these products for granted, even though a high quality of life fundamentally depends on them.
When considering the design of processes for the manufacture of chemical products, the market into which they are being sold fundamentally influences the objectives and priorities in the design. Chemical products can be divided into three broad classes:
1. Commodity or bulk chemicals: These are produced in large volumes and purchased on the basis of chemical composition, purity and price. Examples are sulfuric acid, nitrogen, oxygen, ethylene and chlorine.
2. Fine chemicals: These are produced in small volumes and purchased on the basis of chemical composition, purity and price. Examples are chloropropylene oxide (used for the manufacture of epoxy resins, ion-exchange resins and other products), dimethyl formamide (used, for example, as a solvent, reaction medium and intermediate in the manufacture of pharmaceuticals), n-butyric acid (used in beverages, flavorings, fragrances and other products) and barium titanate powder (used for the manufacture of electronic capacitors).
3. Specialty or effect or functional chemicals: These are purchased because of their effect (or function), rather than their chemical composition. Examples are pharmaceuticals, pesticides, dyestuffs, perfumes and flavorings.
Because commodity and fine chemicals tend to be purchased on the basis of their chemical composition alone, they are undifferentiated. For example, there is nothing to choose between 99.9% benzene made by one manufacturer and that made by another manufacturer, other than price and delivery issues. On the other hand, specialty chemicals tend to be purchased on the basis of their effect or function and are therefore differentiated. For example, competitive pharmaceutical products are differentiated according to the efficacy of the product, rather than chemical composition. An adhesive is purchased on the basis of its ability to stick things together, rather than its chemical composition and so on.
However, undifferentiated and differentiated should be thought of as relative terms rather than absolute terms for chemical products. In practice, chemicals do not tend to be completely undifferentiated or completely differentiated. Commodity and fine chemical products might have impurity specifications as well as purity specifications. Traces of impurities can, in some cases, give some differentiation between different manufacturers of commodity and fine chemicals. For example, 99.9% acrylic acid might be considered to be an undifferentiated product. However, traces of impurities, at concentrations of a few parts per million, can interfere with some of the reactions in which it is used and can have important implications for some of its uses. Such impurities might differ between different manufacturing processes. Not all specialty products are differentiated. For example, pharmaceutical products like aspirin (acetylsalicylic acid) are undifferentiated. Different manufacturers can produce aspirin and there is nothing to choose between these products, other than the price and differentiation created through marketing of the product.
Scale of production also differs between the three classes of chemical products. Fine and specialty chemicals tend to be produced in volumes less than 1000 [t*y.sup.-1]. On the other hand, commodity chemicals tend to be produced in much larger volumes than this. However, the distinction is again not so clear. Polymers are differentiated products because they are purchased on the basis of their mechanical properties, but can be produced in quantities significantly higher than 1000 [t*y.sup.-1].
When a new chemical product is first developed, it can often be protected by a patent in the early years of commercial exploitation. For a product to be eligible to be patented, it must be novel, useful and unobvious. If patent protection can be obtained, this effectively gives the producer a monopoly for commercial exploitation of the product until the patent expires. Patent protection lasts for 20 years from the filing date of the patent. Once the patent expires, competitors can join in and manufacture the product. If competitors cannot wait until the patent expires, then alternative competing products must be developed.
Another way to protect a competitive edge for a new product is to protect it by secrecy. The formula for Coca-Cola has been kept a secret for over 100 years. Potentially, there is no time limit on such protection. However, for the protection through secrecy to be viable, competitors must not be able to reproduce the product from chemical analysis. This is likely to be the case only for certain classes of specialty and food products for which the properties of the product depend on both the chemical composition and the method of manufacture.
Figure 1.1 illustrates different product life cycles. The general trend is that when a new product is introduced into the market, the sales grow slowly until the market is established and then more rapidly once the market is established. If there is patent protection, then competitors will not be able to exploit the same product commercially until the patent expires, when competitors can produce the same product and take market share. It is expected that competitive products will cause sales to diminish later in the product life cycle until sales become so low that a company would be expected to withdraw from the market. In Figure 1.1, Product A appears to be a poor product that has a short life with low sales volume. It might be that it cannot compete well with other competitive products, and alternative products quickly force the company out of that business. However, a low sales volume is not the main criterion to withdraw from the market. It might be that a product with low volume finds a market niche and can be sold for a high value. On the other hand, if it were competing with other products with similar functions in the same market sector, which keeps both the sale price and volume low, then it would seem wise to withdraw from the market. Product B in Figure 1.1 appears to be a better product, showing a longer life cycle and higher sales volume. This has patent protection but sales decrease rapidly after patent protection is lost, leading to loss of market through competition. Product ITLITL in Figure 1.1 is a still better product. This shows high sales volume with the life of the product extended through reformulation of the product1. Finally, Product D in Figure 1.1 shows a product life cycle that is typical of commodity chemicals. Commodity chemicals tend not to exhibit the same kind of life cycles as fine and specialty chemicals. In the early years of the commercial exploitation, the sales volume grows rapidly to a high volume, but then does not decline and enters a mature period of slow growth, or, in some exceptional cases, slow decline. This is because commodity chemicals tend to have a diverse range of uses. Even though competition might take away some end uses, new end uses are introduced, leading to an extended life cycle.
The different classes of chemical products will have very different added value (the difference between the selling price of the product and the purchase cost of raw materials). Commodity chemicals tend to have low added value, whereas fine and specialty chemicals tend to have high added value. Commodity chemicals tend to be produced in large volumes with low added value, while fine and specialty chemicals tend to be produced in small volumes with high added value.
Because of this, when designing a process for a commodity chemical, it is usually important to keep operating costs as low as possible. The capital cost of the process will tend to be high relative to a process for fine or specialty chemicals because of the scale of production.
When designing a process for specialty chemicals, priority tends to be given to the product, rather than to the process. This is because the unique function of the product must be protected. The process is likely to be small scale and operating costs tend to be less important than with commodity chemical processes. The capital cost of the process will be low relative to commodity chemical processes because of the scale. The time to market the product is also likely to be important with specialty chemicals, especially if there is patent protection. If this is the case, then anything that shortens the time from basic research, through product testing, pilot plant studies, process design, construction of the plant to product manufacture will have an important influence on the overall project profitability.
All this means that the priorities in process design are likely to differ significantly, depending on whether a process is being designed for the manufacture of a commodity, fine or specialty chemical. In commodity chemicals, there is likely to be relatively little product innovation, but intensive process innovation. Also, equipment will be designed for a specific process step. On the other hand, the manufacture of fine and specialty chemicals might involve:
selling into a market with low volume,
short product life cycle,
a demand for a short time to market, and therefore, less time is available for process development, with product and process development proceeding simultaneously.
Because of this, the manufacture of fine and specialty chemicals is often carried out in multipurpose equipment, perhaps with different chemicals being manufactured in the same equipment at different times during the year. The life of the equipment might greatly exceed the life of the product.
The development of pharmaceutical products is such that high-quality products must be manufactured during the development of the process to allow safety and clinical studies to be carried out before full-scale production. Pharmaceutical production represents an extreme case of process design in which the regulatory framework controlling production makes it difficult to make process changes, even during the development stage. Even if significant improvements to processes for pharmaceuticals can be suggested, it might not be feasible to implement them, as such changes might prevent or delay the process from being licensed for production.
1.2 FORMULATION OF THE DESIGN PROBLEM
Before a process design can be started, the design problem must be formulated. Formulation of the design problem requires a product specification. If a well-defined chemical product is to be manufactured, then the specification of the product might appear straightforward (e.g. a purify specification). However, if a specialty product is to be manufactured, it is the functional properties that are important, rather than the chemical properties, and this might require a product design stage in order to specify the product. The initial statement of the design problem is often ill defined. For example, the design team could be asked to expand the production capacity of an existing plant that produces a chemical that is a precursor to a polymer product, which is also produced by the company. This results from an increase in the demand for the polymer product and the plant producing the precursor currently being operated at its maximum capacity. The designer might well be given a specification for the expansion. For example, the marketing department might assess that the market could be expanded by 30% over a two-year period, which would justify a 30% expansion in the process for the precursor. However, the 30% projection can easily be wrong. The economic environment can change, leading to the projected increase being either too large or too small. It might also be possible to sell the polymer precursor in the market to other manufacturers of the polymer and justify an expansion even larger than 30%. If the polymer precursor can be sold in the marketplace, is the current purity specification of the company suitable for the marketplace? Perhaps the marketplace demands a higher purity than what is currently the company specification. Perhaps the current specification is acceptable, but if the specification could be improved, the product could be sold for a higher value and/or at a greater volume. An option might be to not expand the production of the polymer precursor to 30%, but instead to purchase it from the market. If it is purchased from the market, is it likely to be up to the company specifications, or will it need some purification before it is suitable for the company's polymer process? How reliable will the market source be? All these uncertainties are related more to market supply and demand issues than to specific process design issues.
Closer examination of the current process design might lead to the conclusion that the capacity can be expanded by 10% with a very modest capital investment. A further increase to 20% would require a significant capital investment, but an expansion to 30% would require an extremely large capital investment. This opens up further options. Should the plant be expanded by 10% and a market source identified for the balance? Should the plant be expanded to 20% similarly? If a real expansion in the market place is anticipated and expansion to 30% would be very expensive, why not be more aggressive and instead of expanding the existing process, build an entirely new process? If a new process is to be built, then what should be the process technology? New process technology might have been developed since the original plant was built that enables the same product to be manufactured at a much lower cost. If a new process is to be built, where should it be built? It might make more sense to build it in another country that would allow lower operating costs, and the product could be shipped back to be fed to the existing polymer process. At the same time, this might stimulate the development of new markets in other countries, in which case, what should be the capacity of the new plant?
From all of these options, the design team must formulate a number of plausible design options. Thus, from the initial ill-defined problem, the design team must create a series of very specific options and these should then be compared on the basis of a common set of assumptions regarding, for example, raw materials prices and product prices. Having specified an option, this gives the design team a well-defined problem to which the methods of engineering and economic analysis can be applied.
In examining a design option, the design team should start out by examining the problem at the highest level, in terms of its feasibility with the minimum of detail to ensure the design option is worth progressing. Is there a large difference between the value of the product and the cost of the raw materials? If the overall feasibility looks attractive, then more detail can be added, the option reevaluated, further detail added, and so on. Byproducts might play a particularly important role in the economics. It might be that the current process produces some byproducts that can be sold in small quantities to the market. But, as the process is expanded, there might be market constraints for the new scale of production. If the byproducts cannot be sold, how does this affect the economics?
Excerpted from Chemical Process Design and Integration by Robin Smith Copyright © 2005 by John Wiley & Sons, Ltd. . Excerpted by permission.
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|Ch. 1||The nature of chemical process design and integration||1|
|Ch. 2||Process economics||17|
|Ch. 4||Thermodynamic properties and phase equilibrium||57|
|Ch. 5||Choice of reactor I - reactor performance||77|
|Ch. 6||Choice of reactor II - reactor conditions||97|
|Ch. 7||Choice of reactor III - reactor configuration||121|
|Ch. 8||Choice of separator for heterogeneous mixtures||143|
|Ch. 9||Choice of separator for homogeneous fluid mixtures I - distillation||157|
|Ch. 10||Choice of separator of homogeneous fluid mixtures II - other methods||181|
|Ch. 11||Distillation sequencing||211|
|Ch. 12||Distillation sequencing for azeotropic distillation||235|
|Ch. 13||Reaction, separation and recycle systems for continuous processes||259|
|Ch. 14||Reaction, separation and recycle systems for batch processes||291|
|Ch. 15||Heat exchanger networks I - heat transfer equipment||317|
|Ch. 16||Heat exchanger networks II - energy targets||357|
|Ch. 17||Heat exchanger networks III - capital and total cost targets||387|
|Ch. 18||Heat exchanger networks IV - network design||399|
|Ch. 19||Heat exchanger networks V - stream data||429|
|Ch. 20||Heat integration of reactors||439|
|Ch. 21||Heat integration of distillation columns||445|
|Ch. 22||Heat integration of evaporators and dryers||459|
|Ch. 23||Steam systems and cogeneration||465|
|Ch. 24||Cooling and refrigeration systems||513|
|Ch. 25||Environmental design for atmospheric emissions||551|
|Ch. 26||Water system design||581|
|Ch. 27||Inherent safety||625|
|Ch. 28||Clean process technology||635|
|Ch. 29||Overall strategy for chemical process design and integration||649|
|App. A||Annualization of capital cost||653|
|App. B||Gas compression||655|
|App. C||Heat transfer coefficients and pressure drop in shell-and-tube heat exchangers||661|
|App. D||The maximum thermal effectiveness for 1-2 shell-and-tube heat exchangers||667|
|App. E||Expression for the minimum number of 1-2 shell-and-tube heat exchangers for a given unit||669|
|App. F||Algorithm for the heat exchanger network area target||671|
|App. G||Algorithm for the heat exchanger network number of shells target||673|
|App. H||Algorithm for heat exchanger network capital cost targets||677|