Controllability heat-integrated columns

Now, we want to look at the dynamics of this complex flowsheet. There are three major issues that must be addressed in designing a control system for a heat-integrated column process that is operating under neat conditions. Auxiliary reboUers or auxiliary condensers are not used to balance the vapor boUup needed at the base of the low-pressure column with vapor condensation needed at the top of the high-pressure column. 

W. L. Luyben, Heat-integrated columns, in Practical Distillation Control, van Nostrand-Reinhold, 1992, p. 492. 

The sketch below shows a distillation column that is heat-integrated with an evaporator. Draw a conirot concept diagram which accomplishes the following Directives (a) In the evaporator, temperature is controlled by steam, level by liquid product, and pressure by auxiliary cooling or vapor to the rcboiler. Level in the condensate receiver is controUed by condensate. 

While these techniques have been applied to energy-related processes such as heat-integrated distillation columns and fluid catalytic cracking reactors, there is still extensive research required before the concept of plant design/control is reduced to practice. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

The control performance of the heat-integrated reactor-column system shown in Fig.. 5.9 deteriorates as the auxiliary rehoiler provides less and less heat to the column. The reason is that uncontrolled variations in the steam pressure of the waste heat boiler affect the heat supplied to the column. When these variations are of the same order of magnitude as the total heat supplied by the auxiliary reboiler, the latter cannot compensate properly for the variations. Part of the prob-... 

Figure 5.10 Reactor/column heat integration with auxiliary reboiler in parallel and Q controller.

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. 

Heat integration of distillation columns can lead to significant energy saving. However, this should not penalise operability. In this section we will show how simultaneous design and control can solve conveniently this problem (Bildea and Dimian, 1999). 

In the second step we introduce cost elements sensitive to recycles. For gas recycle we should account for the cost of compression plus compressor depreciation, both on annual basis. For liquid recycle we should consider the operating costs of the distillation column plus the recovery of investment. Preliminary heat integration around the chemical reactor has a feed-effluent heat exchanger (FEHE), as well as a furnace, necessary for start-up and control (Figure 17.3). A rigorous analysis will be present in the next section. 

Sketch a control system for the two-column heat-integrated distillation system shown. 

The sketch below shows a distillation column that is heat integrated with an evaporator. Draw a control concept diagram that accomplishes the following objectives ... 

Buckley, P. S., "Control of Heat-Integrated Distillation Columns , in T.F. Edgar (ed.) Chemical Process Control 2 Proceedings of the Engineering Foundation Conference, The American Institute of Chemical Engineers, New York, 1982, p. 347. 

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 method of self-optimizing control is applied to a heat integrated prefractionator arrangement. This system has a total of eleven degrees of freedom with six DOF available for optimization when variables with no-steady state effects have been excluded and the duties of the two columns are matched. From the optimization it is found that there is one degree of freedom left for which there is not an obvious choice of control variable. The method of self- optimizing control will be used to find a suitable control variable that will keep the system close to optimum when there are disturbances. 

For the designed plant with no heat integration, there are 38 control degrees of freedom in this process. These degrees of freedom represents the available manipulated variables in the process and can be characterised as follow four feed valves, direct reaction and oxy-reaction coolers valves, direct reaction and oxy-reaction product valves, oxy quench cooler valve, three decanter product valves, pyrolysis preheater and heater valves, pyrolysis product valve, pyrolysis quench cooler valve, HCl heater valve, eight valves for the heating and cooling systems of the four distillation columns, thirteen valves for the base, top and reflux streams of the four distillation columns. 

---