Process Engineering for Pollution Control and Waste Minimization_10 pot

pollution prevention follows the axiom, “an ounce of prevention is worth a pound of cure.” The U.S. Pollution Prevention Act of 1990 and pollution prevention experts conclude that it makes far more sense for a waste generator not to produce waste in the first place, rather than developing extensive, never-ending treatment schemes (1). For industrial pollution prevention, two general approaches are used to characterize processes and waste generation. The first approach involves gather- ing information on releases to all media (air, water, and land) by looking at the output end of each process, then backtracking the material flows to determine the various waste sources. The other approach tracks materials from the point where they enter a facility, or plant, until they exit as wastes or products. Both approaches provide a baseline for understanding where and why wastes are generated, as well as a basis for measuring waste reduction progress. The steps involved in these characterizations are similar and include gathering background information, defining a production unit, general process characterization, under- standing unit processes, and completing a material balance. These steps, when performed systematically, provide the basis for a pollu- tion prevention opportunity assessment. It begins with a complete understanding of the various unit processes and points in these processes where waste is being generated and ends with the implementation of the most economically and technically viable options. It may be necessary to gather information to demon- strate that pollution prevention opportunities exist and should be explored. Often, an assessment team is established to perform the steps along the way (2). A preliminary assessment of a facility is conducted before beginning a more detailed assessment. The preliminary assessment consists of a review of data that are already available in order to establish priorities and procedures. The goal of this exercise is to target the more important waste problems, moving on to lower-priority problems as resources permit. The preliminary assessment phase provides information that is needed to accomplish this prioritization and to assemble the appropriate assessment team (3). A subsequent detailed assessment focuses on the specific areas targeted by the preliminary assessment. Analyzing process information involves preparing a material and energy balance as a means of analyzing pollution sources and opportunities for eliminating them. Such a balance is an organized system of accounting for flow, generation, consumption, and accumulation of mass and energy in a process. In its simplest form, a material balance is drawn up according to the mass conservation principle: Mass in = mass out – (generation + consumption + accumulation) If no chemical or nuclear reactions occur and the process progresses in a steady state, the material balance for any specific compound or constituent is as follows: Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. Mass out = mass in A process flow diagram may be helpful by providing a visual means of organizing data on the material and energy flows and on the composition of streams entering and leaving the system (see Figure 1). Such a diagram shows the system boundaries, all stream flows, and points where wastes are generated. Boundaries should be selected according to the factors that are important for measuring the type and quantity of pollution prevented, the quality of the product, and the economics of the process. The amount of material input should equal the amount exiting, corrected for accumulation and creation or destruction. FIGURE 1 Example flow diagram (3). Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. A material balance should be calculated for each component entering and leaving the process, or other system being studied. A suggested approach for making these calculations is offered in Section 3. Once the sources and nature of wastes generated have been described, the assessment team enters the creative phase. Pollution prevention options are proposed and then screened for feasibility. In this environmental evaluation step, pollution prevention options are assessed for their advantages and disadvantages with regard to the environment. Often the environmental advantage is obvious— the toxicity of a waste stream will be reduced without generating a new waste stream. Most housekeeping and direct efficiency improvements have this advan- tage. With such options, the environmental situation in the company improves without new environmental problems arising (3). Along with assessing the technical and environmental effectiveness in preventing pollution, options are evaluated for the estimated cost of purchasing, installing, and operating the system. Pollution prevention can save a company money, often substantial amounts, through more efficient use of valuable resource materials and reduced waste treatment and disposal costs. Estimating the costs and benefits of some options is straightforward, while for others it is more complex. If a project has no significant capital costs, the decision is relatively simple. Its profitability can be judged by determining whether it reduces operating costs or prevents pollution. Installation of flow controls and improvement of operating practices will not require extensive analysis before implementation. However, projects with significant capital costs require detailed analysis. Several techniques are available, such as payback period, net present value, or return on investment. These approaches are also described in Section 3. At times, the environmental evaluation of pollution prevention options is not always straightforward. Some options require a thorough environmental evaluation, especially if they involve product or process changes or the sub- stitution of raw materials. For example, the engine rebuilding industry is no longer using chlorinated solvents and alkaline cleaners to remove grease and dirt from engines before disassembly. Instead, high-temperature baking followed by shot blasting is being used. This shift eliminates waste cleaner but requires additional energy use for the shot blasting. It also presents a risk of atmo- spheric release because small quantities of components from the grease can vaporize. (3) Others are moving toward the use of aqueous cleaners as substitutes for solvents in an attempt to avoid using toxic materials. However, while the less toxic aqueous cleaners offer a suitable substitution for chlorinated cleaning solvents from a performance standpoint, their use may be resulting in increased environmental impacts in other areas. Most obvious is the increased energy use Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. that occurs from needing to heat the parts to be cleaned in order to get a satisfactory level of cleaning performance. To make a sound evaluation, the team should gather information on all the environmental aspects of the product or process being assessed. This information would consider the environmental effects not only of the production phase but of the acquisition of raw materials, transportation, product use, and final disposal as well. This type of holistic evaluation is called a life cycle assessment (LCA). The stages that are included within the boundary of an LCA are shown in Figure 2. LCA’s origins in mass and energy balance sheets have led to several important accounting conventions, including the following. A system-wide perspective embodied in the term “cradle-to-grave” that implies efforts to assess the multiple operations and activities involved in providing a product or service. This includes, for example, resource extraction, manufacturing and assembly, energy supplies and transporta- tion for all operations, use, and disposal. A multimedia perspective that suggests that the account balance include resource inputs as well as wastes and emissions to most common environmental media, e.g., air, water, and land. A functional unit accounting normalizes energy, materials, emissions, and wastes across the system and media to the service or product provided. Notably, this calculation allows the analysis of different ways to provide a function or service, for example, one can compare sending a letter via e-mail or via regular mail. Additionally, this approach entails allocation procedures so that only those portions or percentages of an operation specifically used to produce a particular product are included in the final balance sheet (4). The functional unit approach of LCA takes the assessment beyond looking at the environmental impacts associated with a specific location or operation. The value of LCA lies in its broad, relative approach to analyzing a system and factoring in global as well as regional and local environmental impacts. This general, macro approach makes it theoretically feasible to frame numerous potential issues and environmental considerations, identify possible trade-offs between different parts of the life cycle, and make these possible issues and trade-offs apparent to decision makers. These attributes enable the user to understand complex and previously hidden relationships among the many system operations in the life cycle and the potential repercussions of changes in an operation on distant operations and other media. This is particularly true where unanticipated or unrecognized issues on the life cycle of a product or service are revealed to decision makers. This leads to a more complete and thorough evaluation for making decisions, including applications in strategic planning, Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. FIGURE 2 Input/output flows in a product life cycle. Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. environmental management, product development and R&D, and liability assess- ment, as well as pollution prevention. 3 CALCULATION 3.1 Measurement of Pollution Prevention Accurate and meaningful measurement systems are vital to ensure long-term successful implementation of pollution prevention (5). To implement pollu- tion prevention, industrial facilities must first measure the environmental im- pacts of their facilities, beginning with the accounting of the inputs and outputs across the facility’s boundaries. This process is captured in a material and energy balance. 3.1.1 Material and Energy Balance Analyzing process information involves preparing material and energy bal- ances as a means of analyzing pollution sources and opportunities for eliminating them. Such a balance is an organized system of accounting for the flow, generation, consumption, and accumulation of mass and energy in a process. In its simplest form, a material balance is drawn up according to the mass conser- vation principle: Mass in = mass out – (generation + consumption + accumulation) The first step in preparing a balance is to draw a process diagram, which is a visual means of organizing data on the energy and material flows and on the composition of the streams entering and leaving the system. A flow diagram, such as Figure 1, shows the system boundaries, all streams entering and leaving the process, and points at which wastes are generated. The goal is to account for all streams so that the the mass equation balances. The boundaries around the flow diagram should be based on what is important for measuring the type and quality of pollution prevented, the quality of the product, and the economics of the process. Again, the amount of material input should equal the amount exiting, corrected for accumulation and creation or destruction. In addition to an overall balance, a material balance should be calculated for each individual component entering and leaving the process. When chemical reactions take place in a system, there is an advantage to performing the material balance on the elements involved. Material and energy balances do have limitations. They are useful for organizing and extending pollution prevention data and should be used whenever Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. possible. However, the user should recognize that most balance diagrams will be incomplete, approximate, or both (6). Most processes have numerous process streams, many of which affect various environmental media. The exact composition of many streams is unknown and cannot be easily analyzed. Phase changes occur within the process, requiring multimedia analysis and correlation. Plant operations or product mix change frequently, so the material and energy flows cannot be accurately characterized by a single balance diagram. Many sites lack sufficient historical data to characterize all streams. These are examples of the complexities that will recur in the analysis of real-world processes. Despite the limitations, material balances are essential for organizing data and identifying data gaps and other missing information. They can help calculate concentrations of waste constituents where quantitative composition data are limited. They are particularly useful if there are points in the production process where it is difficult or uneconomical to collect or analyze samples. Data gaps, such as an unmeasured release, can also indicate that fugitive emissions are occurring. For example, solvent evaporation from a parts cleaning tank can be estimated as the difference between solvent added to the tank and solvent that is removed by disposal, recycling, or dragout (6). It is an essential characteristic of a mass balance that unmeasured flows are used to balance the equation. 3.1.2 Industrial Production and Waste Generation Tracking System The Industrial Production and Waste Generation Tracking System shown in Figure 3 (7) establishes a framework for the determination of the main parameters for industrial production and waste generation. It is based on the following main production process variables: 1. Raw materials (rm) 2. Other materials entering production process (v) 3. Produced products (P) 4. Generated waste (y) The generated waste may be: 1. Managed (g) by applying waste management 2. Released (z) into the environment, causing environmental pollution Managed waste (g) may be further processed to be Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. 3. Used as secondary raw material and/or energy (s) 4. Finally disposed of as processed waste (residues) in a special (or secure) landfill (d). The development of the Industrial Production and Waste Generation Track- ing System model was based on the work of Baetz et al. (8). A model enabling calculation of quantities of waste generated in an industrial production was developed and defined as shown in Table 2. During an industrial process at time t, a production factor U, correlating quantity of raw and other materials r and products P, has a value of 0 ≤ U ≤ 1 and is defined as U = P r Note that raw materials r includes “other materials” not typically defined as raw materials entering a production process. For example, paints and lacquers in “white goods” and furniture manufacture are usually not defined as raw materials but are still input materials. Converting the last expression, the quantity of products may then be expressed as P = Ur FIGURE 3 Industrial production and waste generation tracking system (7). Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. TABLE 2 Definition of IPWGTS Model Parameters (7) Parameter Definition t Industrial production time in which system was observed rm Quantity of raw material entering industrial pro- duction in time t v Quantity of other materials (not defined as raw materials) entering industrial production in time t r = rm + v Quantity of raw and other materials entering indus- trial production in time t U = P/r Production factor after time t; U = 1 represents total production, while U = 0 represents zero level of production and therefore total waste generation P = Ur Quantity of products produced in time t y =(1 − U)r Quantity of solid, liquid, and/or gaseous waste generated in time t M = g/y = g/(1 − U)r Waste management factor after time t; M = 1 rep- resents total waste management, while M = 0 represents zero level of waste management and therefore total release (emission, spill, and/or discharge) into the environment g = M(1 − U)r Quantity of solid, liquid, and/or gaseous waste managed by waste management (temporary stor- age, collection, transportation, processing, and final disposal) z =(1 − M)(1 − U)r Quantity of solid, liquid, and/or gaseous waste released to air, water, and/or soil/land, causing environmental pollution R = s/g = s/M(1 − U)r Waste recycling factor after time t; R = 1 repre- sents total waste recycling by physical, chemi- cal, thermal, and/or biological process as to recover secondary materials and/or energy, while R = 0 represents total waste processing by physical, chemical, thermal, and/or biological process for final disposal in the environment s = MR(1 −U)r Quantity of secondary raw materials and/or energy recovered from solid, liquid, and/or gaseous waste by waste recycling d = M(1 − R)(1 − U)r Quantity of processed waste for final disposal Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. while quantity of waste generated is y =(1 − U)r Managed waste is further quantified by a waste management factor M, which is defined as the ratio between waste managed and waste generated: M = g y and can have a value 0 ≤ M ≤ 1. The quantity of waste managed by storage, collection, transportation, processing, and final disposition is then g = M(1 − U)r while the quantity of waste released (emitted, discharged, and/or spilled) into the environment is z =(1 − M)(1 − U)r If waste is further processed by physical, chemical, thermal, and/or biolog- ical processing to recover secondary raw materials and/or energy, then processed waste is determined by the waste recycling factor R, R = s g having the value 0 ≤ R ≤1. The quantity of waste recycled into secondary raw materials and/or energy by waste processing is S = MR(1 − U)r while the quantity of waste to be finally disposed is d = M(1 − R)(1 − U)r Knowing quantities of raw and other materials (rm + v) entering the observed system and quantities of products (P) produced, quantities of waste generated (y) can be calculated. If the quantity of waste managed (g) by the waste generator is known, it is possible to predict quantities of material lost (z) through release (emission, discharge, and/or spill). Finally, if the waste generator recycles managed waste into secondary raw materials and/or energy (s), then the quantity of waste to be disposed (d) can be determined. 3.1.3 Production-Adjusted Pollution Prevention After a pollution prevention activity has been implemented, adjusted figures from the process flow diagram should show a decrease in waste generation. This decrease is often approached in one of two ways (5). The first way is to look at the change in the quantity of chemicals or raw materials that are purchased or Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved. [...]... wastes, and solid wastes These environmental wastes are generated by both the actual manufacturing processes and the use of fuels in transport vehicles or process operations Waste management practices Depending on the nature of the product, a variety of waste management alternatives may be considered: landfilling, incineration, recycling, and composting Allocation of waste or energy among primary and. .. particular pollution prevention effort Production-adjusted measures of pollution prevention account for changes resulting from pollution prevention efforts For production-adjusted pollution prevention measures, a unit of product is the factor used to adjust gross quantities of waste or chemical use to infer the amount of pollution prevention progress (see Table 3) Using units of product to calculate pollution. .. PRODUCTS If the process or procedural change changes the production rate of the process, then the revenues before and after the change should be included MARKETABLE BY-PRODUCTS AND RECOVERED MATERIAL An increase in the amount of marketable by-products and materials that are recovered and reused should be included According to Humphreys and Wellman (9), the most widely used methods for calculating potential... successful 3.2.2 Inventory—Calculating Energy and Material Inputs and Environmental Releases The second activity of a life cycle assessment is the identification and quantification of energy and resource use and environmental releases to air, water, and land (18) This inventory component is a technical, data-based process with a goal of achieving a mass and energy balance for the life cycle system being studied... inventories of raw materials, in -process inventories, materials and supplies) not already included as charges for chemicals and catalysts for spare parts Working capital may also include personnel costs for operations start-up DISPOSAL COSTS The disposal cost includes all the direct costs associated with waste disposal, including solid waste disposal, hazardous waste disposal, and off-site recycling RAW... selected for study This in turn determines the scope of the study, which sets the boundaries and determines which unit processes will be included And, because the system is a physical system, it obeys the law of conservation of mass and energy Mass and energy balances are the goal of a life cycle inventory and perform a useful check on the completeness and validity of the data The selection of inputs and. .. as well as by in-house staff ENGINEERING AND PROCUREMENT This item includes the costs incurred to design the process equipment or process change and to purchase any new equipment Charges for consultants used in designing and procuring equipment are also included Estimate Expenses The costs in this category include both one-time costs and ongoing costs that are deductible for income tax purposes START-UP... need for a product and make purchasing choices Environmentally responsible choices need reliable information based on the life cycle characteristics of the alternative products or processes being considered LCA considers the environmental aspects and the potential impacts of a product or service system throughout its life—from raw material acquisition through production, use, and disposal This information... develop LCA methodology and tools Especially, the International Standards Organization (ISO) has developed a series of standards and technical reports that cover the various stages involved in LCA, from scoping through impact assessment and interpretation (10–13) Using agreed-upon principles, an LCA study can be done responsibly, transparently, and consistently Before the ISO efforts in LCA, early research... lifetime Include the value of working capital and catalysts and chemicals that will remain at the completion of the equipment’s life TRAINING COSTS Training costs include the costs for on-site and off-site training related to the use of the new equipment or for making sure the process change achieves its goal INITIAL CHEMICALS The initial charges for chemicals and catalysts can be considered a capital . thermal, and/ or biological process as to recover secondary materials and/ or energy, while R = 0 represents total waste processing by physical, chemical, thermal, and/ or biological process for final. U)r If waste is further processed by physical, chemical, thermal, and/ or biolog- ical processing to recover secondary raw materials and/ or energy, then processed waste is determined by the waste. particular pollution prevention effort. Production-adjusted measures of pollution prevention account for changes result- ing from pollution prevention efforts. For production-adjusted pollution