Olefins Plant Debutanizer

INSTALLATION An olefins plant C, splitter (Figure 12.19.1). Feed ! to the column was a vapor ethylene/ethane mixture with minor | quantities of other components. Top product was polymer-grace j ethylene, wnile bottom product was ethane, which was recycled to the j plant's cracking furnaces as a cracking feedstock. The main requirement of the column was to produce on-spec ethylene. There was an economic incentive to minimize the amount of ethylene in the column bottom stream, but in this case the bottom flow rate was small and minimizing the loss of ethylene to that stream was not critical. The rectifying section of the column contained about 3-4 times as many trays as the stripping section.

PROBLEM Following previous revamps, the plant was operated at 13 5 j percent of its initial design capacity. Field experience indicated ( that at this rate, the C. splitter operated right at its hydraulic capacity limit, both in the top and bottom sections. This was confirmed by calculation.

The plant capacity was to be further raised to 150 percent of its original capacity, the C- splitter being one of the major bottlenecks. Since downtime and lost capacity were extremely costly, the proposed solution had to positively assure that the desired capacity increase would be achieved, and that ethylene purity would be maintained. A preliminary revamp study concluded that replacing the column internals alone could not positively assure that both these objectives would be simultaneously met. The only solution that appeared capable of positively achieving both objectives was to add a 40-tray section in series with the existing column, which would enable reflux and reboil to be reduced and allow for greater i throughput. This solution required large capital expenditure, and had a negative impact on the payout of the planned revamp.

SOLUTION The idea that solved the problem with relatively little expense is shown on Figure 12.19.1. A feed condenser was added, which lowered the vapor and liquid loads in the rectifying section sufficiently to ensure this section was capable of processing the increased throughput. This, however, considerably loaded up the bottom section of the column. To accommodate the greater loads, the sieve trays in the bottom section were replaced by dual flow trays (i.e., sieve trays without downcomers). This type of tray is capable of achieving significantly greater capacity than a normal sieve tray, often at the penalty of a slightly lower efficiency and a somewhat lower turndown. The loss in efficiency, however, only occurred in the small stripping section, and could be tolerated, since it did not affect the purity of the ethylene product.

POST MORTEM The revamped column (Figure 12.19.1) achieved 150 percent of its initial design capacity while producing on-spec ethylene product. Ethylene losses in the bottom stream increased from about 1 percent to 1.8 percent, which represented a minor economic loss, especially when one considers the small bottoms flow rate.

pr025y-0025 .0.0

Polymer Grade Ethylene Product


Condenser (Added)

Feed c3 Refng, LiQUid

Vapor Feed (Initiai) Liquid Feed (Post-Revamp)

c3 Refng Liquid

Ca Splitter c

Reflux Drum

This Section Retrayed

Ethane Recycle to Furnaces

C3 Refng Vapor

FIGURE 11.19,1 Olefins Plant C2 Splitter

iA new plant was shut down for its first scheduled shutdown after a j period of successful operation. When one of the large-diameter I columns was opened up, the maintenance crew found a carpenter's sawhorse in the bottom, probably left there after construction. The sawhorse was removed, and the column was closed up.

When the plant was re-started, it was found that the column bottom pump lost suction at low levels. At high levels, little problem was observed.

It appears that when the sawhorse was in the column, it served as a vortex breaker. Once removed, vortexing became a problem. When the column was shut down again, a bona-fide vortex breaker was installed and the problem was eliminated.

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