Sidestep side-draw control surprises

The simplest approach to control side-draw distillation columns uses flow control on the side product. This solution, however, is not ideal for all side-draw control situations.

By Andrew Sloley, contributing editor

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The simplest approach to control side-draw distillation columns uses flow control on the side product. This typically works best when the side-draw rate is relatively small compared to the rates of the overhead and bottoms products. In other situations, however, side-draw control can present severe challenges.

During a period of seven months, a plant had struggled on a daily basis to operate a solvent recovery column. The packed column was supposed to yield a pure solvent product as a vapor side-draw. The feed consisted of a mix of water (~27%), solvent (~70%) and a range of heavy hydrocarbons (~3%). Trace contaminants included light hydrocarbons and a mix of other, mostly polar, light-ends. The heavy hydrocarbons were the bottoms product, the solvent was the vapor side-draw, and the water and some light-ends were supposed to leave overhead.

Unfortunately, solvent product concentration had never exceeded 88%, versus 95% required, and throughput had never surpassed 20% of design.

 

Adding a collector-distributor allowed direct measurement of vapor flow, which was critical to achieving stable control.

A variety of factors complicated the control problem:

* An uncontrolled hot-oil system with large variations in hot-oil temperature was causing large heat-input disturbances;

* The column was grossly oversized;

* Internals had been selected to allow for future vacuum service, even though current operation was at atmospheric pressure;

* The differential pressure between the column and the vapor side-draw condenser was very low;

* The temperature profile in the column was extremely sharp;

* Temperature control points were located incorrectly for inferential control;

* The liquid inventory in the column bottoms was too low for the recirculating heater control;

* The feed preheat was unstable;

* No control valve existed on the vapor side-draw, so vapor draw rate was instead controlled by operating pressure;

* The overhead condenser had an insufficient liquid-level range (6 in.); and

* The side-draw condenser had an insufficient liquid-level range (6 in.).

The problem here was that we had to control a vapor draw that was a large fraction of the total column feed using highly unstable heat input. Not only was the heat input unstable, but constraints on the hot-oil system made measuring it essentially impossible except by back-calculating from the tower performance.

To arrive at a fix, we did not restrict ourselves to the information we already had. Instead, we asked ourselves what information the process had to generate for a controllable tower. We needed a measure of vapor traffic up the column , so we could make sure that the vapor side-draw rate was sufficiently less than the column vapor traffic to allow for proper distillation. Vapor traffic in a distillation column directly depends upon heat input into the column. Most units can measure heat input; we could not.

The solution was to directly measure vapor rate (Figure). A collector-distributor was installed in the column that was specially designed to impose a significant back-pressure. Essentially, we added an orifice plate inside the column. The differential pressure across the collector enabled direct measurement of vapor flow. The collector was sized to allow both for current atmospheric-pressure service and future vacuum operation without flooding.

Due to massive oversizing of the column, the packing pressure drop was nearly constant at atmospheric pressure operation. Without the added restriction at the collector, pressure drop did not provide enough signal gain to allow for control.

Yes, this system is highly interactive, but it works for this case. The results: capacity increased to about 70% of design rates and product purity increased to more than 98%. Capacity is now restricted by too small a reboiler and hot-oil loop problems (more on these another time).

A partial list of supporting changes included a complete redesign of the column internals; relocation of temperature control points; switching to a dynamic temperature threshold for composition control; use of an on/off level control for bottoms; installing a control valve in the vapor draw line; and adding level controls to the overhead condenser and the side-draw condenser.

We will discuss this troubled column several more times, because there are many other things we can learn from its host of problems.

 

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