Develop Distillation Controls During Process Design

Address basic process control issues during the engineering design phase to increase system capacity and reduce the frequency of product-quality incidents.

The design of high-purity hydrocarbon distillation systems has remained relatively unchanged over the past 40 years. Loads are simulated at the required product purities for one or more potential feed streams, design factors are applied, and datasheets are prepared.

Process control engineering is usually deferred until after startup. Piping and instrumentation diagrams (P&IDs) are populated with a standard process control strategy (Figure 1) so the procurement of instruments can be completed on schedule. The standard strategy is used as a temporary placeholder that can be modified in the future.

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▲Figure 1. Using a standard process control strategy, the operator increases purity of the primary distillate product by increasing reflux.

However, it is rare to modify a basic regulatory control strategy after startup, or even after a hazard and operability (HAZOP) study. Operator training materials would have to be revised, operators would need to be retrained, and the HAZOP study would need to be repeated. Thus, the most practical time to perform basic process control engineering is during the design phase.

This article describes steps to address basic process control issues during the design phase of a distillation process, including how to:

• select an operating target using operating data from similar units
• consider changes to plant design in three major areas to reduce the impact of disturbances
• design regulatory controls to prioritize the primary product during constrained operation
• add a redundant analyzer, or create an inferential model, for analyzer downtime.