Heat integration control structure

Synthesis is the step in design where one conjectures the building blocks and their interconnection to create a structure which can meet stated design requirements. This review paper first defines chemical process synthesis and indicates the nature of the research problems—to find representations, evaluation functions and search strategies for a potentially nearly infinite problem. It then discusses synthesis research and the most significant results in each of six areas—heat exchanger networks, separation systems, separation systems with heat integration, reaction paths, total flowsheets and control systems. 

Figure 4.11 present the complete flowsheet together with the control structure. The reaction takes place in an adiabatic tubular reactor. To avoid fouling, the temperature of the reactor-outlet stream is reduced by quenching. A feed-effluent heat exchanger (FEHE) recovers part of the reaction heat. For control purposes, a furnace is included in the loop. The heat-integrated reaction system is stabilized... 

The final concern we have about the control structure in Fig. 5.16 is how to start up and turn down the plant. For example, how would we start up the columns without running the furnace and the reactor Also, how could we turn off the heat to any of the reboilers when the reactor and the furnace are running The bypass valves may not be designed to take the full gas stream when fully opened. This implies that we need two control valves working in tandem around each reboiler or a three-way valve. Neither of those options is particularly attractive. See Jones and Wilson (1997) for further discussions on process flexibility related to heat integrated designs. 

In this section we present more complex distillation column processes that go beyond the plain vanilla variety. Industry uses columns with multiple feeds, sidestreams, combinations of columns, and heat integration to improve the efficiency of the separation process. Very significant reductions in energy consumption are possible with these more complex configurations. However, they also present more challenging control problems. We briefly discuss some common control structures for these systems. 

The tenfold increase in energy prices in the 1970s spurred efforts to reduce energy consumption in chemical and petroleum plants. Heat integration was extensively applied to achieve very significant reductions in energy consumption in distillation columns. There are a host of alternative configurations that have been built in industry. We discuss below several of the most widely used process structures and their control schemes. 

In this way another structure is obtained, as presented in Fig. 13.27, which is more general for industrial applications. This will be called heat-integrated PFR. This generic structure has been studied by Bildea Dimian (1998, 2000) from the point of state-multiplicity, stability, and controllability. The investigation has put in evidence the close link between design and controllability. 

Figure 13.30 Control structure for forward heat-integration.

Perform a degrees-of-ffeedom analysis for the noninteracting exothermic reactor shown in Figure 20.3a. Suggest an appropriate control structure. Carry out the same exercise for the heat-integrated reactor shown in Figure 20.3b. Compare the results. 

The heat-integrated process provides an excellent example of the power and usefidness of dynamic simulation of distillation column systems. Alternative control structures can be easily and quickly evaluated. 

The issue of this kind of control configuration has been investigated using frequency dependent formulations of measures such as the condition number, the Relative Gain Array by Bristol (1966) and the Relative Disturbance Gain by Stanley et al (1985). This paper will focus on discussing the dynamic control structure on the heat pump section and how each dynamic control structure affects on the stability of the integrated distillation column. 

The plant-wide control system developed before is finally applied to the modelled process in HYSYS.PLANT and evaluated based on its close-loop dynamic behaviour and disturbance rejections performance. Once the control system of the non-heat integrated plant is validated, the favoured HEN design, based on its operational performance, is integrated within the entire plant and its control system is linked with the proposed plant-wide control structure mainly through cascade control configurations. 

The goal of plantwide control structure synthesis is to develop feasible control structures that address the objectives of the entire chemical plant and account for the interactions associated with complex recycle and heat integration schemes, and the expected multivariate nature of the plant. Many strategies have been proposed for accomplishing this task, and the majority of them have been demonstrated using dynamic process simulations. However, none have been accepted as the universal approach, in a manner similar to the steady-state process design synthesis hierarchy of Douglas [1]. 

Figure 6.12 Heat-integrated flowsheet and control structure.

Figure 6.30 Control structure for acetone-methanol with partial heat integration.

Figure 6.30 gives the control structure developed for the heat-integrated pressure-swing process. Notice that the temperatures shown in the faceplates for TC2 are in degrees Centigrade. 

Figure 11.13 Control structure for extractive distillation with heat integration.

Figure 11.13 shows the control structure for the heat-integrated extractive system. The three block arrows point to the changes that have been made from the base-case control stmcture. 

The effectiveness of this control structure on the heat-integrated extractive distillation system is compared with that of the partially heat-integrated pressure-swing distillation system discussed in Chapter 6 in Figure 11.14. The controllability is quite similar. 

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