Control of Heat-Integrated Distillation Columns

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. 

Just as in the steady-state design situation, the heat duties must be equal (with opposite sign) at every point in time. 

There is only one reboiler duty to manipulate, and only one variable can be controlled by this input. As discussed later, we will use a control structure in which a tray temperature in the high-pressure column is controlled by manipulating the heat input to the reboiler in the high-pressure column (the only steam-heated reboiler). Then, we will vary the feed split (the fraction of the total feed fed to the low-pressure column) to control a temperature on a tray in the low-pressure column. 

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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. 

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. 

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. 

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. 

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. 

Whenever level control is to be effected on a boiling liquid or condensing vapor, properties more typical of thermal processes appear. Transfer of both heat and mass is involved, which, combined with the integration of flow into level, renders control surprisingly difficult. Level control in boilers and distillation columns is sufficiently problematic to warrant special consideration, which is given in Chaps. 8, 9, and 11. 

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 the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

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. 

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