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Term Paper on Industrial Ecology
Term Paper # 1. Introduction to Industrial Ecology:
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The phrase ‘Industrial Ecology’ evokes powerful images and strong reactions, both positive and negative. To some, the phrase conjures images of industrial systems that mimic the mass conservation properties of natural ecosystems. Powerful analogies can be drawn between the evolution of natural ecosystems and the potential evolution of industrial systems.
Billions of years ago, the Earth’s life forms consumed the planet’s stocks of materials and changed the composition of the atmosphere. Our natural ecosystems evolved slowly to the intricately balanced, mass-conserving networks that exist today. Can our industrial systems evolve in the same way, but much more quickly? These are interesting visions and thought-provoking concepts. But is industrial ecology merely a metaphor for these concepts? Is there any engineering substance to the emerging field of industrial ecology?
Industrial ecology is much more than a metaphor and it is a field where engineers can make significant contributions. At the heart of industrial ecology is the knowledge of how to reuse or chemically modify and recycle wastes—making wastes into raw materials. Chemical engineers have practiced this art for decades.
The history of the chemical manufacturing industries provides numerous examples of waste streams finding productive uses. Nonetheless, even though the chemical manufacturing industries now provide excellent case studies of industrial ecology in practice — tightly networked and mass-efficient processes—there is much left to be done. While the chemical manufacturing industries are internally integrated, there is relatively little integration between chemical manufacturing and other industrial sectors and between chemical manufacturers and their customers.
Engineers could take on design tasks such as managing the heat integration between a power plant and an oil refinery or integrating water use between semiconductor and commodity material manufacturing. The goal is to create even more intricately networked and efficient industrial processes—an industrial ecology. Not all of the tools needed to accomplish these goals are available yet, but the basic concepts and suggests the types of tools that the next generation of process engineers will require.
Term Paper # 2. Environmental Performance of Chemical Processes:
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The environmental performance of chemical processes is governed not only by the design of the process, but also by how the process integrates with other processes and material flows. Consider a classic example—the manufacture of vinyl chloride.
Billions of tons of vinyl chloride are produced annually. Approximately half of this production occurs through the direct chlorination of ethylene. Ethylene reacts with molecular chlorine to produce ethylene dichloride (EDC). The EDC is then pyrolysed, producing vinyl chloride and hydrochloric acid.
In this synthesis route, one mole of hydrochloric acid is produced for every mole of vinyl chloride. Considered in isolation, this process might be considered wasteful. Half of the original chlorine winds up, not in the desired product, but in a waste acid. But the process is not operated in isolation. The waste hydrochloric acid from the direct chlorination of ethylene can be used as a raw material in the oxychlorination of ethylene. In this process, hydrochloric acid, ethylene, and oxygen are used to manufacture vinyl chloride.
By operating both the oxychlorination pathway and the direct chlorination pathway, as shown in Fig. 8.1, the waste hydrochloric acid can be used as a raw material and essentially all of the molecular chlorine originally reacted with ethylene is incorporated into vinyl chloride. The two processes operate synergistically and an efficient design for the manufacture of vinyl chloride involves both processes.
Additional efficiencies in the use of chlorine can be obtained by expanding the number of processes included in the network. In the network involving direct chlorination and oxychlorination processes, both processes incorporate chlorine into the final product. Recently, more extensive chlorine networks have emerged linking several isocyanate producers into vinyl chloride manufacturing networks. In isocyanate manufacturing, molecular chlorine is reacted with carbon monoxide to produce phosgene –
CO + Cl2 → COCl2
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The phosgene is then reacted with an amine to produce an isocyanate and by-product hydrochloric acid –
RNH2 + COCl2 → RNCO + 2HCl
The isocyanate is subsequently used in urethane production, and the hydrochloric acid is recycled. The key feature of the isocyanate process chemistry is that chlorine does not appear in the final product.
Thus, chlorine can be processed through the system without being consumed. It may be transformed from molecular chlorine to hydrochloric acid, but the chlorine is still available for incorporation into final products, such as vinyl chloride, that contain chlorine. A chlorine-hydrogen chloride network incorporating both isocyanate and vinyl chloride has developed in the Gulf Coast of the United States. The network is shown in Fig. 8.2.
Molecular chlorine is manufactured by Pioneer and Vulcan Mitsui. The molecular chlorine is sent to both direct chlorination processes and to isocyanate manufacturing. The by-product hydrochloric acid is sent to oxychlorination processes or calcium chloride manufacturing. The network has redundancy in chlorine flows, such that most processes could rely on either molecular chlorine or hydrogen chloride.
Consider the advantages of this network to the various companies:
1. Vulcan/Mitsui effectively rents chlorine to BASF and Rubicon for their isocyanate manufacturing; the chlorine is then returned in the form of hydrochloric acid for ethylene dichloride/vinyl chloride manufacturing.
2. BASF and Rubicon have guaranteed supplies of chlorine and guaranteed markets for their by-product HCl.
Even more complex networks could, in principle be constructed. As shown in Table 8.1, chlorine is used in manufacturing a number of non-chlorinated products. Table 8.1 lists, for selected reaction pathways, the pounds of chlorinated intermediates used along the supply chain, per pound of finished product. This ranking provides one indication of the potential for networking these processes with processes for manufacturing chlorinated products.
An examination of individual processes, such as those listed in Table 8.1, can be useful in building process networks, but the individual process data do not reveal whether efficient use of chlorine is a major or a minor issue in chemical manufacturing. To determine the overall importance of these flows, it is useful to consider an overall chlorine balance for the chemical industry. The overall flows of chlorine into products and wastes, as well as the recycling of chlorine in the chemical manufacturing sector, is shown in Fig. 8.3. The data indicate that roughly a third of the total chlorine eventually winds up in wastes. By employing the types of networks shown in Figs. 8.1 and 8.2, the total consumption of chlorine could be reduced.
Identifying which processes could be most efficiently integrated is not simple and the design of the ideal network depends on available markets, what suppliers and markets for materials are nearby, and other factors. What is clear, however, is that the chemical process designers must understand not only their process, but also processes that could supply materials, and processes that could use their by-products.
And the analysis should not be limited to chemical manufacturing. Continuing with our example of waste hydrochloric acid and the manufacture of vinyl chloride, by-product hydrochloric acid could be used in steel making or by-product hydrochloric acid from semiconductor manufacturing might be used in manufacturing chemicals.
Finding productive uses for by-products is a principle that has been used for decades in chemical manufacturing. What is relatively new, however, is the search for chemical by-product uses in industries that extend far beyond chemical manufacturing. Here both of these topics will be examined—the overall flows of raw materials, products and by-products in chemical manufacturing industries—as well as the potential for combining material and energy flows in chemical manufacturing with material and energy flows in other industrial sectors. Variously called by-product synergy, zero waste systems, or even industrial ecology, the goal of this design activity is to create industrial systems that are as mass-efficient as possible.
One provides an overview of material flows in chemical manufacturing and describes analysis methods that can be used to optimise flows of materials. Another one examines case studies of exchanges of materials and energy across industrial sectors and the emerging concept of eco-industrial parks and briefly attempts to assess the potential benefits of by-product synergies.
Term Paper # 3. Material Flows in Chemical Manufacturing:
The chemical manufacturing industries are a complex network of interrelated processes. An individual process typically relies on other chemical manufacturing processes for raw materials and as markets for its products. Take the manufacture of styrene as an example. Styrene manufacturing relies on ethylene and benzene, manufactured in other processes, for raw materials. The styrene is not sold as a consumer product; rather, it is used as a raw material for polystyrene manufacturing. Additional complexity arises from the fact that most sequences of chemical manufacturing process are not unique. There are generally a variety of pathways available for manufacturing products.
As a relatively simple example of the multiple pathways available in chemical synthesis, again consider styrene. Styrene is produced from ethylene and benzene, but the source of the ethylene might be naptha, or refinery gases. Benzene might be produced by dehydrogenation of cyclohexane, dealkylation of toluene or separation from crude oil. These options provide multiple pathways from raw materials to styrene. Each route has raw material requirements, energy requirements, water usage, and rates of emissions and waste generation.
Selecting the most environmentally benign and most economical route is a difficult proposition. It is made even more difficult when the entire chemical supply chain is considered. For example, in methanol production, the methanol is produced using carbon monoxide. The carbon monoxide in turn may be produced through a partial oxidation of a material that is currently wasted by another process. On the other hand, to convert the carbon monoxide into methanol requires hydrogen, which is an energy-intensive material.
Evaluations of the environmental features of producing a chemical product should examine the entire chemical raw material supply chain, but to realistically examine these supply chains requires comprehensive, integrated models of material flows in the chemical process industries. Fortunately, such models have been developed. Rudd and coworkers have developed basic material and energy flow models of over 400 chemical processes associated with the production of more than 200 chemical products, describing a complex web of chemical manufacturing technologies.
An understanding of material flows in these networks can be used at a variety of levels. First, the material flow networks can be used simply to identify potential users and suppliers of materials, and to identify networks of processes that are strategically related. For example, for the types of networks, it would be useful to have lists of processes that produce and consume hydrochloric acid. A partial list is given in Table 8.2. 
Once consumers and producers of targeted chemicals are identified, material and energy flow models can be used to construct networks. The network that makes the most sense depends on the features that are to be optimised. Analyses have been performed to identify networks that minimise energy consumption, the use of toxic intermediates, and chlorine use. Other analyses have considered the response of networks to perturbations in energy supplies and restrictions on the use of toxic substances. Regardless of the application, however, the material flow model of the chemical manufacturing web provides the basic information necessary to identify and optimise networks of processes.
Yet another use of comprehensive material flow models is in the evaluation of new technologies. Consider once again the case of chlorine use in chemical manufacturing. Rather than generating complex networks involving HCl and molecular chlorine, it might be preferable to use a chemistry that converts waste HCl into molecular chlorine. Several processes have been proposed and are listed in Table 8.3.
These processes will only be successful if they can compete with the reuse of by-product HCl. Data on material and energy flows in the chemical manufacturing web can again be used to assess the competitiveness of new chemical pathways, such as the technologies listed in Table 8.3.
Term Paper # 4. Assessing Opportunities for Waste Exchanges and By-Product Synergies:
Productive uses can be found for selected waste streams. Are these anomalies, or are there large quantities of waste materials that can be productively used? This question is difficult to answer with certainty, but a few simple examples may illustrate the potential for finding new uses for waste.
One estimate of the potential for industrial exchanges of materials and energy can be drawn from a simple examination of energy flows in the United States. Approximately a third of the 80-100 quadrillion BTU of energy consumed annually in the United States is used for electric power generation.
Of the energy used in electricity generation, roughly % is lost as waste heat. This means that roughly a quarter of all energy demand in the United States could be met through the utilisation of lost heat. Combined heat and power systems are emerging throughout the country to take advantage of such opportunities, but much remains to be done.
A second example of the potential for conservation through material exchanges involves another ubiquitous material—water. Water is used in virtually all industrial processes and major opportunities exist for reuse since, in general, only a small amount of water is consumed; most water in industrial applications is used for cooling, heating, or processing of materials, not as a reactant.
Further, different industrial processes and industrial sectors have widely varying demands for water quality. For example, waste-water from a semiconductor manufacturing facility that requires ultrapure water may be suitable for a variety of other industrial applications. Thus, water exchanges and reuse provide a significant opportunity.