Troubleshooting identifies, solves operating problems. (Petrochemicals).(Brief Article)

Successful troubleshooting combines a thorough understanding of the process and equipment, application of chemical engineering basics to problem solving, and good field techniques and data interpretation skills. Troubleshooting often fails due to faulty incomplete, and misleading numbers.

This article reviews three case histories that demonstrate successful troubleshooting:

* Operational instability in a petrochemical tower due to a faulty reboiler conversion.

* Off-specification petrochemical tower operations due to fouling and an out-of-level distributor.

* Control instability on a stab-in reboiler on a petrochemical plant deisobutanizer due to improper design.

Malfunctioning reboiler loop

A petrochemical plant operator revamped a high-pressure (>100 psig) column with new trays. The column used a once-through thermosyphon reboiler for column boil-up. The prerevamp column arrangement had a feed pipe inside the column that came from a sump through the boot and then out the tower bottom to the reboiler feed line (Fig. 1).

Postrevamp

After the revamp, the operator commissioned the column and stabilized operations. The column, however, did not meet project objectives due to insufficient boil-up. We identified the problem by analyzing field data and comparing simulation work done before the column was revamped.

When reviewing the column revamp design and inspection records, we found that the internal pipe feeding the reboiler and tray sump had been removed. The liquid-level driving force through the reboiler was substantially reduced, which created flow instability problems in the reboiler loop (Fig. 2).

The thermosyphon converted from a once-through to a recirculating operation when vessel internals were removed. The tower lost approximately one theoretical stage during the revamp. The tower had sufficient stages to operate, however; the loss of one stage was not significant, therefore.

Thermosyphon reboilers

A once-through thermosyphon reboiler is classified as a natural-circulation reboiler. Natural circulation draws liquid from the bottom tray and circulates it directly to the reboiler. The stream that returns to the column is partially vaporized and net return liquid is recovered as bottom product (Fig. 1).(1)

A reboiler can have different heating zones:

* Liquid sensibly heating to its bubble point.

* Liquid vaporization.

* Vapor superheating.

At low liquid rates, liquid feed entering the reboiler is essentially vaporized. The liquid heating zone is small, and nucleate boiling occurs almost immediately.

Mist flow develops and gas superheating occurs above the nucleate boiling zone in the reboiler. The nucleate boiling heat-transfer coefficient is high; but the gas-superheating coefficient is low. The net effect is that high vaporization reduces the effective reboiler heat-transfer coefficient.

The temperature difference in the liquid heating and nucleate boiling zones is high and relatively constant because the low two-phase static head, and hydraulic losses in the tubes and outlet piping, does not significantly raise the boiling point.

In the mist-flow zone, gas heating due to a severe temperature pinch at the outlet for low liquid levels can lead to a significant temperature rise. This temperature rise results in a low overall integrated temperature difference. The temperature rise, combined with the low overall heat-transfer coefficient, explains poor reboiler performance at low liquid rates. (2)

An increased liquid driving head leads to more …