Control volume, environmental

Tittlebaum, Marty E., Roger Seals, Frank Cartledge, Stephanie Engels, Louisiana State University. State of the Art on Stabilization of Hazardous Organic Liquid Wastes and Sludges. Critical Reviews in Environmental Control, Volume 15, Issue 2,1985. 

Export processes are often more complicated than the expression given in Equation 7, for many chemicals can escape across the air/water interface (volatilize) or, in rapidly depositing environments, be buried for indeterminate periods in deep sediment beds. Still, the majority of environmental models are simply variations on the mass-balance theme expressed by Equation 7. Some codes solve Equation 7 directly for relatively large control volumes, that is, they operate on "compartment" or "box" models of the environment. Models of aquatic systems can also be phrased in terms of continuous space, as opposed to the "compartment" approach of discrete spatial zones. In this case, the partial differential equations (which arise, for example, by taking the limit of Equation 7 as the control volume goes to zero) can be solved by finite difference or finite element numerical integration techniques. 

The discussion above provides a brief qualitative introduction to the transport and fate of chemicals in the environment. The goal of most fate chemists and engineers is to translate this qualitative picture into a conceptual model and ultimately into a quantitative description that can be used to predict or reconstruct the fate of a chemical in the environment (Figure 27.1). This quantitative description usually takes the form of a mass balance model. The idea is to compartmentalize the environment into defined units (control volumes) and to write a mathematical expression for the mass balance within the compartment. As with pharmacokinetic models, transfer between compartments can be included as the complexity of the model increases. There is a great deal of subjectivity to assembling a mass balance model. However, each decision to include or exclude a process or compartment is based on one or more assumptions—most of which can be tested at some level. Over time the applicability of various assumptions for particular chemicals and environmental conditions become known and model standardization becomes possible. 

The construction of a mass balance model follows the general outline of this chapter. First, one defines the spatial and temporal scales to be considered and establishes the environmental compartments or control volumes. Second, the source emissions are identified and quantified. Third, the mathematical expressions for advective and diffusive transport processes are written. And last, chemical transformation processes are quantified. This model-building process is illustrated in Figure 27.4. In this example we simply equate the change in chemical inventory (total mass in the system) with the difference between chemical inputs and outputs to the system. The inputs could include numerous point and nonpoint sources or could be a single estimate of total chemical load to the system. The outputs include all of the loss mechanisms transport... 

The scope definition is similar to the definition of the control volume in the thermodynamic analysis or the battery limits in process design, and for the LCA in terms of space and time (e.g., we follow the use of product X in the process from the raw materials to the time it is disposed by the consumer. Throughout the lifetime of the product, we analyze the environmental burden). The reasons for the study are also clearly defined (e.g., is the study necessary to make a decision about a process ), as well as an answer must be given as to who is performing the study and for whom. Consider the following hypothetical example ... 

We can determine the specific enthalpies and the specific entropies from the temperature, pressure, and composition of the environment. Once we specify the environmental conditions, all enthalpy and entropy terms are fully defined regardless of the process within the control volume. The term 7 0Chemical exergy, Exch, is... 

Control volumes are chosen to be convenient and useful. While the choice of a good control volume is somewhat of an art and depends on both the chemicals and the environmental locations that are of interest, the control volume boundaries are almost always chosen to simplify the problem of determining chemical transport into and out of the control volume. 

A little reflection on a variety of other environmental pollution situations suggests any number of relevant control volumes having convenient, useful, and well-defined boundaries. Three typical examples are shown in Fig. 1-3. If... 

FIGURE 1-3 a Examples of useful control volumes for three principal environmental media. Control volume (a) would be practical if we were studying the various processes that remove a contaminant from a river the difference between the input and output fluxes would represent internal sinks in the river or volatilization loss to the air (Figure continues). 

Although the forms of the mass conservation equation shown in Eqs. [1-5] and [1-6] may not appear to be directly applicable to large-scale environmental situations, they actually are very powerful tools. These equations can be integrated to yield mathematical solutions to chemical distributions in many physical systems. Given information on the inflow rates and chemical concentrations at the boundaries of a control volume, the chemical concentrations throughout the control volume may be determined by invoking solutions to Eqs. [1-5] and [1-6]. 

Equilibrium, by contrast, describes the final expected chemical composition in a control volume. In chemical parlance, a control volume with its chemical contents is often referred to as a system. Equilibrium is relevant in the case of reactions that are rapid compared with other environmental processes of interest. For example, if potassium hydroxide (KOE1) is added to municipal drinking water to decrease its acidity, the reaction of the potassium hydroxide and the acids present in the drinking water may be assumed to be instantaneous compared with the time it takes to transport water to customers. A... 

Both quality and process control should be able to produce a product of known and consistent quality, suitable for use as a lubricant base stock. In past practice, however, oils that can meet the required performance characteristics have tended not to be readily available and have historically been expensive. For these reasons, the use of re-refined oils as a lubricant base oil was not considered to represent a viable alternative to virgin mineral base oils of consistent quality when these are readily available. More recently, the quality of re-refined base oils has increased to a high level, for some cases up to Gp. II quality, and in volume quantity. In addition, the substantially increased price of crude oil, and its associated products, has made re-refining of used lubricant much more economically viable. The alternative route of using the used lubricant as a fuel directly or as a fuel extender is now much more restricted and controlled by environmental legislative authorisations and is also much less viable economically. 

It should be understood that to develop process metrics based on a credible risk assessment will require a considerable number of tests to assess appropriately the potential environmental hazards associated with the process and the materials used in the process. One typically needs to screen compounds to assess their environmental hazard and their tendency for persistence, bioaccumulation, and toxicity. Depending on how a compound is ultimately used, environmental testing might lead you to the conclusion that it is best to avoid the commercial production of a particular compound. Alternatively, you might devise a process that uses the compound but controls the environmental risk to acceptable levels. For the latter case, performing a process-specific risk assessment would be imperative to assess the impact of the inherent hazard and environmental fate and effects of a given chemical or set of chemicals. In addition, the unique characteristics of the process, available treatment, and volumes will need to be taken into consideration. 

Control," Volume I, Report Eo. EPA-6OO/2-8O-I56, Industrial Environmental Research Laboratory, Office of Research and Development, U. S, EPA, Cincinnati, Ohio 45268 i4T-199. 

In accordance with CEN EN 13302 (2010), for unmodified or modified bitumens, a small volume of heated sample (specified for the spindle to be used) is placed in the sample cylindrical container, which is then placed in the temperature-controlled device (environmental chamber). The sample together with the appropriate size spindle is left for a certain period to reach uniform testing temperature. Figure 4.5 shows a rotational viscometer. 

A key concept in describing environmental chemical fate is the principle of mass preservation. A chemical in a par-ticirlar location at a specific time can remain at that location, can be transported elsewhere, or can be transformed into another chemical. A mass balance can be formulated for a specific subsection of the environment, called the system or control volume. Defining a system boundary involves a decision of what is considered a part of the system and what is part of its srrnoundings. The mass balance then accormts for how much chemical crosses the system botmdary and how much chemical is generated and lost within the system dtuing a particular time interval (Fig. 5) ... 

Cesium and iodine atoms which are released from fuel specimens into a high-temperature steam-hydrogen environment are thermodynamically unstable and will be rapidly converted into species that are stable under these conditions. Since the chemical form of iodine in particular will considerably influence its transport and retention behavior within the reactor pressure vessel and the primary system, it is important to know the kinetics of these conversion reactions. A kinetics assessment of the most essential reactions (Cronenberg and Osetek, 1988) has shown that for extremely low concentrations of iodine and cesium in steam (e. g. mole ratio I H2O < 10" ), the predominant form of iodine is HI and that of cesium is CsOH. This is due to the fact that because the concentrations of iodine and cesium are so dilute, the elements are much more likely to collide and react with H2O and H2 than with each other. Low concentrations of iodine and cesium increase the time for thermochemical equilibrium to be established for their reaction products. For mixtures which are so dilute in fission products, the reaction times may approach tens of seconds or longer, so that for high effluent rates the environmental conditions may change (e. g. by transport into the next control volume showing other conditions) before thermochemical equilibrium has been achieved. Under such conditions, certain limitations caused by reaction kinetics may exist. 

Best Practicable Environmental Option Assessments for Integrated Pollution Control, Volume 1 - Principles and Methodology, and Volume 2 - Technical Data, The Stationery Office, London (ISBN 011310126 0)... 

Biofiltration works to degrade a diversity of airborne contaminants, including industrial chemicals like styrene (Arnold et al. 1997), pentane and isobutane mixtures (Barton et al. 1997), toluene (Matteau and Ramsay 1997), chlorinated benzenes (Oh and Bartha 1994), dimethylsulfide (Pol et al. 1994), ethylene (Elsgaard 1998), and other volatile organic compounds (VOCs Leson and Winer 1991). Maintenance of good degrada-tive activity of biofilter microbial communities sometimes requires the addition of nutrients to the bioliltration matrix, since materials like peat or wood chips are generally nutrient poor. Adjustments and careful control of environmental variables such as temperature, pH, and availability of moisture (humidity) also are often required (Arnold et al. 1997 Matteau and Ramsay 1997). Removal rates for contaminants by biofilters can be impressive. For example, removal of vapors of chlorinated compounds (chlorinated benzenes, in one instance) was measured at 300 g of solvent vapor h m of filter volume (Oh and Bartha 1994). 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

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