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Distillation Control - Material & Energy Balance

A distillation column is controlled by regulating its material balance and the energy balance.

In essence, a material balance means that the sum of the products leaving the column must be equal (approximately) to the feed entering the column; and an energy balance means that the heat input to the column must equal (approximately) to heat removed from the system.

When a column is in material and energy balance, there is no accumulation or generation of material or heat within the column, i.e. the column is "stable".

The control system is dynamic, i.e. if a process variable changes, the control system reacts by adjusting the affected process variables until the system returns to normal condition. Sample plant picture

The term "steady state operation" describes the condition in a column when the process variables are changing in small amounts within prescribed limits.

When a column is in steady-state operation, the changes to the column's material balance and energy balance variables are minimal and are handled by the control system. As mentioned in the start of this chapter, one of the objectives of control is to maintain the products within the required specifications, or simply "specs". A "spec" is a value, or a range of values, for a physical property or a set of physical properties that is required for a product or products. A sample of typical properties of interest in petroleum refining is shown in Appendix A.

Product specifications are set by the demands of downstream processes and by the marketplace. Products must meet certain quality standards. For a product to be saleable, it must comply with certain pre-determined quality.

Products are routinely tested to ensure that the specifications are met. Testing can be done by direct composition measurement or by indirect measurement, according to prescribed standards, such as ASTM.

Direct composition measurements are analysis that allow personnel to directly observe the percentages of components in a product. An example is the process chromatograph. It provides a direct read-out of the component percentages. The readings of the chromatograph can be compared against the specifications to see if any adjustments are needed to ensure that the product meets specifications.

Indirect composition measurements are analysis in which one measured property is used as an indicator of another property. One common indirect measurement is the boiling temperature. For example, since the boiling points of the components in a feed mixture are known, the components in a product can be indirectly identified by their boiling points. When the product is tested, its composition can be indirectly measured by recording the temperatures at which the different components in the product boil.

If the composition of a product is outside of the normal limits for that product, the product is referred to as "off-specification".

Exceeding product specifications or producing better quality product than is required is known as product giveaway.


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