RETRAYING AND REVAMP DOUBLE BIG LPG FRACTIONATOR'S CAPACITY

Richard Sasson
Independent Engineering Consultant
Friendswood, Tex.

Robin Pate
Enterprise Products Co.
Houston

Retraying existing columns and making other process improvements allowed Enterprise Products Co., Houston, to raise capacity on its "West Texas" fractionator at Mont Belvieu from 35,000 b/d to 60,000 b/d for a lower cost than a new 25,000 b/d parallel train.

The expanded distillation unit uses no more fuel to distill 60,000 b/d than the old unit did to distill 35,000 b/d.

DEBOTTLENECKING JOB

Enterprise operates two LPG fractionation units at Mont Belvieu: the Seminole unit and the West Texas unit. In 1985, Nye Engineering Inc., Friendswood, Tex., designed improvements to expand the Seminole plant from 60, 000 b/d of C2 + feed to 90,000 b/d.

When favorable marketing conditions required additional capacity, Enterprise asked Nye Engineering to debottleneck the West Texas plant which had been built in 1979.

The primary modifications made to increase the West Texas plant's capacity and reduce fuel consumption were the following:

• Retraying the deethanizer and depropanizer columns with new High Capacity Nye Trays.

• Lowering the pressure in the deethanizer and depropanizer to improve the separating efficiency of the columns.

• Replacing the debutanizer with a high-pressure column that rejects its condensing heat as reboil for the deethanizer.

• Adjusting the feed temperature to balance the load in the top and bottom of the depropanizer column to prevent premature flooding in one section of the tower.

• Installing convection heaters to recover existing stack gas heat into the process.

In conjunction with the capacity expansion, there was a strong incentive to improve the fuel efficiency of the unit.

TOWER RETRAY

The capacity benefit of retraying each of the three towers with new High Capacity Nye Trays, manufactured by Glitsch Inc., was evaluated.

These patented trays had been used successfully in 12 other distillation columns at this site and 30 columns for other companies. Their installation was the key in inexpensive capacity improvements.

The calculated capacity improvements for each tower in the West Texas plant with Nye Trays was de-ethanizer-28%, depropanizer-23%, and debutanizer-14%.

Ordinary distillation trays operate as shown in Fig 1a. The liquid flows across each tray into the downcomer and down to the next tray. The vapor travels up the column, bubbles through holes in the active area on each tray, disengages from the liquid, and travels to the tray above.

As throughput is increased, the tower reaches its capacity at the flood point.

Flooding can occur in two ways: Too much vapor flow carries liquid to the tray above (entrainment flooding) or too much liquid flow overfills the downcomer and backs up onto the tray above (downcomer flooding).

When one tray floods, the flooding tends to spread throughout the column and one or both products become impure.

With the conventional tray shown in Fig. 1a, as the vapor approaches each tray from the bottom, it is squeezed into the perforated area with the space under each downcomer being dead area.

Nye Trays use a patented modification to enable the vapor to utilize the normally inactive area beneath the downcomer as increased vapor-liquid disengaging space. The Nye Tray is shown in Fig. 1b.

Vapor can flow through the insert area beneath the downcomer in addition to the normal sieve area. At a given rate, the velocity of the rising vapor is lower.

Vapor that enters the insert area under the downcomer crosses the tray through a perforated panel in the face of the insert.

The vapor breaks through the liquid as the liquid flows onto the tray.

With the extra disengaging space, the trays entrain less than ordinary trays at the same liquid and vapor load.

Alternatively, the trays can handle extra liquid-vapor traffic at the same level of entrainment.

To prevent premature downcomer flooding and because a larger bottom area of the downcomer has little or no adverse effect on Nye Tray capacity, Enterprise decided to increase the downcomer size in conjunction with the retray to Nye Trays.

Historically, high-capacity trays have had a lower tray efficiency than conventional trays. Field experience with Nye Trays, however, has consistently demonstrated an efficiency improvement.

The tower tray support modifications were designed with no hot work required in the de-ethanizer and minimal hot work in the depropanizer. This helped shorten the required downtime for retray.

REDUCED TOWER PRESSURE

A study of the effect of pressure on the operation of each tower revealed that lowering the operating pressure of the de-ethanizer and of the depropanizer led to the following:

• Lower reboil and reflux required to operate the tower, saving energy

• Increased capacity of the tower.

• Lower temperature throughout the tower.

The energy savings came as a result of changing the volatility of the ethane relative to the volatility of the propane. The more volatile the light component, relative to the heavy component, the easier it is to make the separation.

Fig. 2 shows how the pressure affects the ethane-propane volatility. As the pressure is lowered, the ethane becomes more volatile relative to the propane.

At 435 psig, the relative volatility is 1.92, but at 370 psig it increases to 2.17.

With an increase in the relative volatility, the ethane tends to concentrate even more in the vapor phase on each tray of the deethanizer. Less reboil and reflux are therefore required to separate the ethane from the propane and heavier components.

Lowering the pressure reduced the hot-oil requirement by 10% and the reflux by 22% for the same level of feed. This also directly increased capacity.

In addition, as the column's pressure drops, the difference in density between the liquid and vapor increases. This causes the liquid and vapor to easily disengage and also contributes to increased capacity.

Fig. 3 shows how the deethanizer bottoms' temperature decreases as the pressure is lowered. This becomes important later in discussion of the use of the high-pressure debutanizer to provide the majority of the reboil heat for the deethanizer.

The method for lowering pressure was different in each column. For the de-ethanizer, the ethane product was taken as a compressed vapor rather than a condensed and pumped liquid.

This, coupled with some refrigeration improvements, allowed the de-ethanizer's pressure to be lowered by 60 psi.

For the depropanizer, the air-cooled condensers were replaced with cooling-water exchangers. This, together with some control changes that took advantage of the available condensing, allowed the depropanizer's pressure to be reduced by 35 psi.

HIGH-PRESSURE COLUMN

The existing conventional debutanizer would have limited the capacity of the plant even with additional condensing and new trays. As a result, and to save fuel, Enterprise decided to replace the debutanizer with a high-pressure column.

The intent of the high-pressure debutanizer column was to operate the tower overhead at so high a temperature that it could reject its condensing heat as reboil for the de-ethanizer. This required raising the operating pressure substantially more than what it had been.

This patented design was used successfully in the 1986 expansion of the Seminole plant.

Because it operates at a higher pressure, the new tower required more reboil than the old one to make the separation. Because all the heat is recovered as de-ethanizer reboil, however, it essentially operates for free.

The new debutanizer tower was evaluated with conventional trays and with High Capacity Nye Trays. Offsetting the increased tray cost was a reduction in required tower diameter and a smaller foundation.

The net result was that the tower's installed cost was reduced by initially using Nye Trays. The old debutanizer was left idle but intact for future use.

PREHEAT; HEAT RECOVERY

Analyses of the columns showed the expanded plant limit normally to be flooding on the bottom trays of the depropanizer. An increase in capacity would be achieved by preheating the feed.

This causes two beneficial effects:

1. The amount of liquid and vapor flow on the bottom trays is reduced.

2. The reboiler heat duty is reduced.

With the optimum feed preheat, the system can be arranged so that the column is equally loaded in both the top and bottom. This loading prevents premature flooding in one section of the tower.

The source of the feed preheat was the hot finished products that were formerly going directly to air coolers.

The existing hot-oil heaters had stack temperatures of 500 and 390 F.

To recover some of this waste heat, additional convection sections were installed on top of the heaters.

Depropanizer bottoms at 220 F. were pumped through the convection sections as heat-recovery fluid. Some of the depropanizer bottoms were vaporized and the two-phase mixture returned to the depropanizer beneath the bottom tray.

This arrangement served two purposes: In addition to providing more than 50% of the required reboil heat for the depropanizer, it removed the need to modify or replace the existing hot-oil reboiler.

INSTALLATION; START-UP

The new equipment was set and as much piping as possible installed.

When all else had been done, the plant was shut down, the towers were retrayed, modifications to existing equipment were performed, and final tie-ins completed.

During the retray operation, two or three crews were usually working simultaneously in each tower.

It took 3 1/2 days to remove the old trays and 10 days to modify the supports and reinstall the new trays in each tower.

Two days after the modified plant was started up, record production runs were made. The 60,000-b/d design rate was achieved within a week.

Comparisons of feed rates and fuel utilization before and after the plant modification are shown in Figs. 4 and 5.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.