Oct-2006

Increasing deisobutaniser capacity

Insight into procedures and details involving a distillation revamp can be gained by looking at a project carried out on a deisobutaniser unit separating isobutane from normal butane and heavier hydrocarbons

Guiseppe Mosca, Gitta Van Hemel and Roberto Tocco, Sulzer Chemtech
Bruno Lestrade Total Group

Article Summary

In many process industries and geographic locations today, chemical-process companies feel a strong motivation to increase their throughput capacity without resorting to grass-roots construction. Hence, the popularity of revamp projects. But for a revamp to be successful, the engineers planning and conducting the project need not only a deep knowledge of the process and its characteristics, but also a full understanding of the operation, limitations and capabilities of the equipment in which the process takes place. Such understanding is especially important for relatively complex systems such as distillation units, involving not only the column itself, but also the column internals, reboiler and the condenser, pumps and control hardware.

Insight into the procedures and details that a distillation revamp involves can be gained by looking at a project carried out several years ago on a deisobutaniser unit at the Total Group petroleum refinery in Antwerp, Belgium. This fractionation unit separates isobutane (iso-C4) from normal butane (n-C4) and heavier hydrocarbons. The revamp project increased the capacity of the unit by installing new, high-performance distillation trays of advanced design.
 
Deisobutaniser process description
The deisobutaniser unit (Figure 1) may receive feed from any or all of three different sources, which differ substantially from each other in iso-C4 content. The richest stream is fed only when the downstream alkylation plant poses a high demand on iso-C4.

The three feed streams are accumulated in a feed drum. The mixture is pumped, preheated by means of a stream coming from an iso-stripper and finally enters the distillation column above tray 29, which is shown in Figure 1. The feed flow rate is controlled “in cascade” with the level of the accumulator, whereas the temperature is not controlled automatically.

The deisobutaniser overhead vapour is cooled, condensed by water coolers and accumulated in the overhead accumulator. The reflux flow-rate control setpoint is in cascade with the temperature control of tray 10. The flow rate of the iso-C4 product is controlled in cascade with the control of the level in the overhead condensate accumulator. A common pump serves both the iso-C4 product, which becomes the feedstock for the refinery’s alkylation unit, and the overhead reflux. The heating duty at the kettle reboiler is provided with a hot fluid coming from another refinery unit with which it is integrated.

The bottom product is pumped and sent to refinery hydrocarbon stock after cooling at an ambient temperature, its flow rate being controlled in remote setpoint with the level control of the kettle reboiler.

Doubling isobutane output
Before the revamp, the unit was able to produce, at most, 150 metric tons/day of iso-C4. Refinery management commissioned a feasibility study to find out how this output could be increased. The study showed that the iso-C4 output could, in fact, be doubled by modifying some of the existing equipment and adding some new equipment, but keeping the existing distillation-tower vessel, the feed accumulator and the overhead accumulator.

The study evaluated two cases with different compositions (Table 1): design feed (the first and principal case) and alternate feed. For the latter case, the ultimate capacity was determined by fully utilising the reboiler and condenser duties of the first case. Only the design of the bottom product pump and cooler  was based on the controlling alternate-feed case.

Since the deisobutaniser is integrated with another process unit that furnishes the heat needed for the reboiler, the study was performed with a maximum available duty of 13.5 million Kcal/h. The condenser duty and the reflux ratio were calculated from the required degree of iso-C4 recovery and isobutene purity, as well as from the achievable mass-transfer efficiency at the existing tower.

A process simulation study was performed to identify the optimal feed inlet location, reflux ratio and number of trays to maximise the column capacity. For a given existing column (with fixed height), the only way to increase the number of theoretical stages is to reduce the tray spacing, provided that the fractionation trays are designed properly. For a given targeted degree of separation, an increase in the number of theoretical stages enables a decrease in the reflux rate. At reduced reflux, both the vapour and liquid traffic through the column decrease, so the hydraulic capacity of the tower should increase. However, a decrease in tray spacing (to achieve more separation stages) decreases the hydraulic capacity to a degree approximately proportional to the square root of the tray spacing. Therefore, for a given column (of fixed internal diameter and height) and a given set of separation goals, there is an optimal operating point, which can be determined from a proper analysis of the number of stages, reflux ratio and tray spacing (Figures 2 to 4).

For this deisobutaniser, the minimum flooding factor (maximum capacity) was found for a number of actual trays very close to the existing one, providing approximately 48 theoretical stages at 86% tray efficiency, for a total of 50 stages (assuming one stage each for the reboiler and the condenser). Therefore, the decision was made to retrofit the tower by means of a straightforward one-by-one tray replacement. The hydraulic results with the advanced-design trays are shown in Table 2.

This selected revamp option was notably cost-effective to implement. Since additional trays were not needed, the job could be done without adding new tower attachments or welding to the vessel. Furthermore, this option allowed the project to be carried out quickly, thus minimising the downtime. Note than in many revamp projects, the column may instead require an increased number of actual trays, in which case new support rings are required. For such cases, modern mechanical procedures, outside the scope of this article, are available to minimise the required onsite work.

Unit modifications
Since the debottlenecking significantly increased iso-C4 production, virtually all other equipment components had to be modified and/or replaced, with the exception of the distillation tower, feed accumulator and overhead accumulator (Figure 5).