Chemical Component Inventories

Three basic features of integrated chemical processes lie at the root of our need to consider the entire plant s control system (1) the effect of material recycle, (2) the effect of energy integration, and (3) the need to account for chemical component inventories. If we did not have to worry about these issues, then we would not have to deal with a complex plantwide control problem. However, there are fundamental reasons why each of these exists in virtually all real processes. 

As illustrated in the previous chapter, the need for a plantwide control perspective arises from three important features of integrated processes the effects of material recycle, of chemical component inventories, and of energy integration. We have shown several control strategies that highlight important general issues. However, we did not describe how we arrived at these strategies, and many of our choices may seem mysterious at this point. Why, for instance, did we choose... 

Component balances can often be quite subtle, but they are particularly important in processes with recycle streams because of their integrating effect. They depend upon the specific kinetics and reaction paths in the system. They often affect what variable can be used to set production rate or reaction rate in the reactor. The buildup of chemical components in recycle streams must be prevented by keeping track of chemical component inventories (reactants, products, and inerts) inside the system. 

Estimating the inventory of reactants and anticipating their dynamic effects is fundamental in the design and control of chemical plants. The occurrence of nonlinear phenomena is often interrelated with the method of controlling the makeup of fresh reactants [8]. There are two methods for controlling the component inventory in a plant. By self-regulation the fresh reactant is set on flow control at a value given by the desired production rate. No attempt is made to measure or... 

We recall that in process dynamics the material content of a given volume is usually called inventory . In recycle systems with chemical reactions the control of the components inventory is a central issue in developing a plantwide control strategy. From this point of view we can distinguish two important aspects ... 

Identify how chemical components enter, leave and are generated (or consumed) in the process. Accumulation of chemical components in recycle streams can be a major reason of failure in control. A preventing method consists in tracing the paths and evaluating inventories for reactants, products and inert, as well as for sub-products and main impurities. This accounting operation can be done by means of a component table where input, generation, output and accumulation region are noticed. 

The inventory of impurities is a plantwide control problem, because it involves both the reaction and separation subsystems through recycles. Ideally, the inventory of each component should be traced from the source to its final destination. Recent systematic studies on the dynamics and control of the recycle systems have been started, as described in the Chapter 13. Luyben and Tyreus (1998) proposed a ten steps plantwide control design procedure (section 13.7). The step 7 consists of Checking component balances, identify how chemical components enter, leave, and are generated or consumed in the process. At this stage it is necessary to find the specific mechanism or control loop to guarantee that there will be no uncontrollable build-up of any chemical component within the process . 

Make sure that the overall component balances for all chemical components can be satisfied. Light, heavy, and intermediate inert components must have a way to exit the system. Reactant components must be consumed in the reaction section or leave the system as impurities in product streams. Therefore, either reaction rates (temperature, pressure, catalyst addition rate, etc.) must be changed or the flow rates of the fresh feed makeup streams must be manipulated somehow. Makeups can be used to control compositions in the reactor or in recycle streams, or to control inventories that reflect the amount of the specific components contained in the process. For example, bring in a gaseous fresh feed to hold the pressure somewhere in the system, or bring in a liquid fresh feed to hold the level in a reflux drum or column base where the component is in fairly high concentration (typically in a recycle stream). 

The preceding chapter deals with methods describing the behaviour of product flows in chemical production plants to model the core components of chemical production networks in detail. The next step in modelling product flows in chemical production networks is to describe product flows between chemical production sites and plants. At a site, intermediate chemicals are produced by some plants and consumed by some other plants whereby raw chemicals are only consumed and final chemicals are only produced. To buffer temporal imbalances of chemical flows, inventories are hold at the sites. Figure 3.1 shows the schematic chemical production network with added inventories symbols and highlighted chemical flows. 

Table 10.5. Comparison of inventories of chemical components in the two landfill altema-...

The components of Bisoflex 124 are listed in the European Inventory of Existing Commercial Chemical Substances (EINECS) and the US Toxic Substances Control Act (TSCA) Chemical Substances Inventory. 

Fugitive air emissions of volatile components released during hydrotreating may also be toxic components. These include toluene, benzene, xylenes, and other volatiles that are reported as toxic chemical releases under the EPA Toxics Release Inventory. 

European Inventory of Existing Chemical Substances (ENMECS) This is a list of all chemicals either alone or as components in preparations supplied to a person in a Community Member State at any time between 1st January 1971 and 18th September 1981. 

Absorption of a component of a gas stream into a liquid is a common practice in the chemical industry to affect cleanup of vent gases, conduct chemical reactions, purify products, or to recover products from process streams. The enhanced mass transfer capability of RPBs provides the opportunity to perform absorption processes in smaller equipment, to lower inventories, to shorten startup and shutdown times, and to lower pressure drop (48). Figure 8 provides a visual comparison of the size of a conventional absorber tower next to three RPBs that handle the equivalent gas and liquid flows (9). 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

An estimate of the toxicity or intrinsic hazard is needed for each material identified in the inventory. Such information for many chemicals in the form of a Material Safety Data Sheet (MSDS) are required by the OSHA Hazard Communication Standard. (Other countries have similar requirements.) Standard hazard-data sources may need to be consulted for those chemical compounds for which no MSDSs are presently available. Adequate hazard data may be lacking for various mixtures that are unique to the plant. For such mixtures, it may be necessary to analyze the contents and then estimate the overall hazard based on the individual components. 

Plasma conditions and wall materials must also enable a sufficient lifetime of the first wall components for economic reasons. Chemical erosion of graphite leads to significant erosion yields even under low-temperature, cold plasma conditions and can seriously limit the lifetime. Since the tokamak is a fairly closed system, most of the eroded material will be re-deposited somewhere inside the machine. The question of tritium retention and overall inventory in the device is closely connected to the chemical erosion and to possible co-deposition as well [6,7]. In order to minimize the net-erosion and optimize the lifetime of wall components, the re-deposition should be concentrated in areas of major erosion. Another way to minimize chemical erosion is the use of mixed materials, which - in laboratory experiments - display a reduced erosion yield in comparison to pure graphite. 

TE problem. As discussed previously, all chemical inventories must be regulated it cannot be left to chance. Unless setpoints for key internal concentrations are provided, MPC allows reactant partial pressures to drift to unfavorable values. Our design procedure considers the concept of component balances as an explicit step in the design. 

We can characterize a plant s chemical species into three types reactants, products, and inerts. A material balance for each of these components must be satisfied. This is typically not a problem for products and inerts. However, the real problem usually arises when we consider reactants (because of recycle) and account for their inventories within the entire process. Every molecule of reactants fed into the plant must either be consumed via reaction or leave as an impurity or purge. Because of their value, we want to minimize the loss of reactants exiting the process since this represents a yield penalty. So we prevent... 

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