Cut column energy consumption

Distillation units account for a significant fraction of most plants’ energy requirements. With increasing energy prices and public sensitivity to energy consumption, many plants are paying more attention to optimizing distillation. Energy improvements can result from changes in equipment, process and operations.

Of these, operational changes pay off most quickly and reliably. Usually because plants over time invariably stray from optimized operation many opportunities for improvement exist.

A number of factors can contribute to unnecessary energy consumption: improper control points; selection of the wrong control variables; unused system capabilities; excessive reflux or boil-up: poor equipment efficiency; and missing basic heat-integration steps. Nearly every plant can capture profits by paying attention to these basics. In many cases, improvements involve only simple operating changes and low investment.

So here we’ll look at simple steps and procedures to verify energy efficiency and to identify possible operating changes and minor projects and their benefits.

Is your control measurement in the right place?

First let’s consider control measurement on the bottom of a column, which commonly relies on temperature to infer bottoms composition (Figure 1).

Figure 1. The bottom tray usually is not the best place to make an inferential measurement of composition.

While it may seem obvious to take the temperature measurement at the bottom of the column, the proper measurement location depends upon many factors including control response, process gain, system composition and behavior [1], as well as energy costs.

Simple inferential composition control most often works best with the control point a number of trays — typically from three to 20 depending upon the system — away from the column bottoms. The same holds true for inferred control on the column overhead.

In one typical case, shifting the control point from the column bottoms to six trays up yielded a $60,000 annual energy savings, and required only a new thermowell and a minor instrumentation run. Column energy consumption dropped by 4.3%, with 3% due to reduced variation in heat duty at a constant target operation and the remainder from the tighter operation allowing for a reduced operating margin above minimum specification.

Are you controlling the right thing?

Thorough process analysis [2] to ensure you are measuring the right control variable is important in energy conservation, as Figure 2 helps illustrate. It shows a compound absorber/stripper using an absorption fluid to reduce product losses and a reboiler for stripping.

Figure 2. Common control scheme is susceptible to significant errors in inferred measurement.

It also depicts a common, but poorly chosen, control scheme — the temperature on a tray above the bottoms infers bottoms composition. In most stripping services small temperature changes produce large changes in bottoms composition.

Figure 3 charts a specific case of bottoms composition versus the control-point temperature. To keep the bottoms composition under the required operating point (line D) the temperature operating point needs to be set high enough to maintain purity specifications during all reasonable temperature excursions. Line F shows the resulting average composition for a 2°F offset. In comparison, using an alternative control specification, relative stripping vapor recycle flow allows operation at line E (a 2.5% flow offset).

Figure 3. Control should take into account that small temperature changes can cause large swings in composition.

The average energy saving is 3.6% of reboiler duty for the alternative control scheme. Additionally, that scheme cuts consumption by an extra 2.7% because of non-symmetrical response to control variations.

The control of the overhead gas rate is an excellent choice for strippers as long as feed composition is relatively constant. If feed composition varies occasional updates to the ratio of overhead gas to feed may be required.

Can you control to system limits?

Take advantage of the full capability of existing equipment. Don’t let specification-sheet design values restrict you. Make the most of the safety margin that invariably is included.

For instance, consider reducing column pressure to leverage potentially under-used capacity in the tower and the overhead system. Some authorities claim up to 25% savings in distillation energy consumption depending upon operating conditions [3].

For nearly all systems, decreasing the pressure increases the relative volatility. Higher relative volatility reduces the boilup and reflux requirements at a constant product split and so cuts energy needs. Factors favoring lower pressure are:

1. actual number of separation stages close to the theoretical minimum;
2. high purity requirements;
3. low relative volatility; and
4. relatively large changes in relative volatility with pressure.