Process Control

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The aim of the control system for a distillation column is to stabilize and to optimize the operation. The control system should also ensure the production of products in specification at the minimum energy consumption. Conventional control systems are often designed for stabilization only. They rely on the intervention of an operator for any optimization.

In Figure 7.4 a conventional control system for a two-component separation is shown [1]. In this conventional control system the product flows are level-controlled and the reflux is set at a fixed flow, which is usually too high. This conventional control system often leads to an operation in a comfort zone with less trouble for the operators but low energy efficiency.

Advanced process control (APC) aspires to establish best practice process conditions and to maintain them as rigorously as possible, regardless of disturbances, grade changes or load changes, while satisfying process constraints and produc-

Figure 7.4 Conventional control system for a two-component separation.

tion goals. The most efficient process conditions are often subject to process constraints, such as the purities at top and bottom, weeping and flooding. Since there is always process variability due to disturbances, a distance is kept from the efficient conditions in order to avoid constraint violations. Examples for external disturbances are feed flow rate and composition, feed temperature, reflux temperature (air coolers) and steam pressure. This motivates a useful strategy for energy saving as shown in Figure 7.5.

Advanced control systems comprise feed-forward control, material-balance control and frequently computers for optimization. These systems control the column at the optimum operating point, provide more stable operation and reduce the need for operator action. Additionally, they offer considerable potential for

reducing energy requirements and operating costs. An example for an advanced control system for a two-component separation is shown in Figure 7.6.

In addition, online product analyzers to measure the concentration or purity are recommended to allow a tight process operation. Examples for online product analyzers are NIR, UV, VIS, Raman spectroscopy, or gas chromatography [6, 7].

Improved Design for Single Columns 7.4.3.1 Column Internals

It is recommended that for the design of new columns a reflux ratio close to the minimum should be chosen. However, many existing columns were designed for higher reflux ratios. In these cases, energy consumption can be reduced by increasing the number of stages.

The number of stages and the stage efficiency or the total height of a packing and the height of a transfer unit for packed columns influences the performance of a distillation column. For tray columns the number of theoretical stages can be increased by increasing the number of trays, for example, by increasing the column height or reducing the tray spacing. However, increasing column height

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Figure 7.7 Comparison of temperature profiles in a distillation column.

usually leads to high investment costs. In addition, reducing tray spacing may be expensive. Generally, the most attractive measure is the implementation of more efficient trays or packing. An analysis of the energy savings for different trays and packing is recommended. If there is an attractive improvement, tray and packing suppliers can be contacted to determine the exact increase in efficiency.

The implementation of a packing with a higher efficiency can reduce the requirement for reflux, reduce the pressure drop and improve purities. With vacuum columns in particular, a reduction in the pressure drop can also increase the energy consumption of the vacuum pump.

Figure 7.7 shows two temperature profiles of a distillation column. The curve on the left hand side is the temperature profile of a tray column at atmospheric pressure. The condensation temperature at the top is approx. 65 °C. The vapors are condensed with cooling water. By a pressure increase and the installation of a high efficient packing reducing the pressure drop, the temperature profile gets steeper as shown in the curve on the right hand side in Figure 7.7. In addition, the condensation temperature at the condenser is approx. 130 °C. The more valuable condensation heat can now be used, for instance to generate low pressure steam or in an adsorption chiller to generate chilled water.

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