Plantwide Energy Management

In this section we outline the approach we would take in controlling a complex heat-integrated scheme like Alternative 6 of the HDA process. The first step is to identify the three heat pathways. 

Path 1 is intended to carry heat from exothermic reactions that must he dissipated from the process. This path, shown in Fig. 5.12, goes through all three distillation column reboilers as well as the three preheaters before it terminates in the water cooler. An enthalpy disturbance in the reactor propagates through the entire plant. 

Path 2, shown in Fig. 5.13, conveys heat to the distillation columns. This heat covers the thermodynamic work requirement for the separations. In Alternative 6 for the HDA process all the heat needed to run the process is supplied by the furnace. Heat intended for the columns must travel through the reactor and the preheaters before it reaches its destinations in the three condensers. The columns can indirectly upset the reactor. 

Path 3 is the heating and cooling circuit that starts from the reactor exit and goes through the preheaters and column reboilers. In the preheaters the hot streams give up most of their enthalpy to the incoming cold feed streams that travel back to the reactor. This path is shown 

It should be clear from Figs. 5.12 to 5.14 that it would be impossible to build a process so that the flow of heat would balance itself along the three paths for all operating conditions. We could not even bank on this happening for the design conditions because of imperfections in our basic data. We need a control system to help direct the flow of heat just as we need controllers to manage material flow and inventory. 

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We have discussed in detail each of the nine steps in our plantwide control design procedure. The first two steps establish the control objectives and control degrees of freedom for the plant. In the third step we discuss how the plantwide energy management problem can be converted to a local unit operation energy management problem by using the plant utility system. 

The nine steps of the design procedure center around the fundamental principles of plantwide control energy management production... 

In term of plantwide control the energy management has to fulfil two functions ... 

The implementation of a plantwide control structure on a given design can follow a step-wise procedure (Luyben Tyreus, 1999). The actions with plantwide character regard energy management, production rate, product quality, safety and environmental protection, and control of impurities. 

Based on the steady-state optimization for each module, only one active control constraint is identified for the HDA process - the reactor inlet temperature. The nine-step procedure to generate a plantwide control structure developed by Luyben et al.[7] is now applied to each module. These steps are (i) establish the control objectives, (ii) determine the control degrees of freedom, (iii) establish energy management, (iv) set the production rate, (v) control the product quality, (vi) fix a flow in every recycle loop and control inventories, (vii) check component balances and (viii) control individual imit operations, and (ix) optimize the economics or improve the dynamic controllability. The number of control degrees of freedom identified for each module (referred to by their respective dominant unit operation) are as follows reactor 10, product column 10, and recycle column 5. 

The procedure essentially decomposes the plantwide control problem into various levels. It forces us to focus on the unique features and issues associated with a control strategy for an entire plant. We highlighted some of these questions in Chap. 1 in discussing the HDA process. How do we manage energy How is production rate controlled How do we control product quality How do we determine the amounts of fresh reactants to add ... 

Plantwide control problems arise in the context of intricate recycles of mass and energy that characterise modem process plants. Positive feedback effects complicate the dynamics and control because of interactions and non-linear phenomena. Managing the production rate and the formation of waste are important plantwide control problems that originate primarily from the control of component inventory. In this paper we will make the distinction between the inventory of main components and of impurities. Main components designate reactants, intermediates and products that ensure the targeted production rate and the economic efficiency. Although much less as material amount, the inventory of impurities is equally important since they may be harmful for the environment, affect the product quality and lower the price, plug the equipment and lead to troubles in operation, etc. 

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