Debottlenecking an FCC gas concentration unit

Component trapping by physical carry-over of water can cause capacity bottlenecks that are not easily predicted

J RAJESH and PAWAN GUPTA, Essar Oil Limited

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Article Summary

Component trapping in fractionation towers refers to a phenomenon wherein a component having intermediate volatility, as compared to the light and heavy key components, accumulates in a tower. This occurs when the tower top temperature is too low for the component to vaporise and leave the column along with top vapours, and the bottom temperature is too high to allow the component to condense and leave the column along with bottom liquid. The component tends to accumulate in the tower because its rate of removal from the tower is lower than its rate of influx into the tower. The accumulation continues until the intermediate component concentrations in the overhead and bottom allow their removal at the rate they enter, or a flooding limitation is reached. This phenomenon of a trapped component periodically exiting through the top or bottom in larger quantities results in unsteady tower operation, upsets, difficulties in controlling column temperature profile and tray flooding. If the trapped component is water, then it may also cause accelerated corrosion.

This article describes the investigation of and the solution to water trapping in the gas concentration unit (GCU) in Essar Oil’s Vadinar refinery. The FCC GCU typically has primary absorber, stripper, debutaniser and gasoline splitter towers for the separation of gas, LPG and gasoline. Water trapped in one or more of these towers can cause instability and reduction in capacity.

While the phenomenon of component trapping in fractionation towers and water trapping in the GCU of FCC units in refineries has been reported and discussed in earlier work,1 this case study demonstrates that looking outside the affected towers can be a critical factor when evaluating possible solutions.

Overview of FCC gas concentration unit
In Essar Oil’s refinery process, wet gas and raw gasoline from the FCC main fractionator overhead are routed to the GCU. In the GCU, C2 and lighter components are rejected into the fuel gas system from the primary absorber. In the debutaniser column, C3/C4 (LPG) are separated from gasoline (see Figure 1).

Vapours from the reflux drum of the main fractionator flow through a two-stage centrifugal compressor to the high pressure (HP) separator. Prior to entering the HP separator, the reflux drum vapours are joined by the liquid from the bottom of the primary absorber and the vapours from the stripper overhead and cooled in the trim cooler.The HP separator allows for liquid removal from the vapour streams. Hydrocarbon liquid is fed to the top of the stripper and the vapours are routed to the bottom of the primary absorber. A boot holds the interface level, creating the boundary between liquid phases such that the water can be removed from the separator. The primary absorber absorbs C3 and heavier components from fuel gas, and the stripper strips the C2 and lighter components from the liquid before it is fed to the debutaniser.

In the absorber-stripper configuration, it is envisaged that the dissolved water entering the column with the feed streams will get trapped. A water draw-off tray in the form of a chimney collector is typically installed near the top of these towers to facilitate water removal (see Figure 2).

Increased severity operation
Essar Oil proposed operating the FCC reactor in high severity mode to increase the yield of the lighter components. Process simulations were done by Essar Oil to predict the hydraulic loads in the GCU. Hydraulic ratings were performed by Koch-Glitsch with the simulated loads to check the adequacy of the GCU columns. The results indicated that the towers in the GCU had sufficient excess capacity to handle the future increased loads without any modifications to the existing internals.

When the severity in the FCC reactor was increased, problems were observed in operating the stripper and the debutaniser. The stripper had a higher and fluctuating pressure drop, and the debutaniser developed issues with boil-up. These symptoms forced the operators to lower the unit capacity.

During a troubleshooting review by Essar, it was observed that water was continuously being removed from the debutaniser reflux drum. The reflux drum was equipped with a control valve on the water drain, which normally did not open significantly at low severity reactor operation. At the higher loads and severity, the control valve opening was as high as 50-60% which corresponded to a water flow rate of around 450 l/h. This was much higher than the expected dissolved water in the feed to the GCU.

It was suspected that the water could be responsible for the issues with the towers. An attempt was made to increase the bottom temperature of the stripper to minimise water ingress into the debutaniser. Both the absorber and stripper were equipped with water draw-off trays; however, water was never available at the draw location of the absorber and stripper. When the bottom temperature of the stripper was increased, the operation became unsteady as pressure drop increased and fluctuated between 0.23 kg/cm2 and 0.35 kg/cm2. The stripper could not be stabilised with the higher bottom temperature, and capacity had to be reduced to ensure steady operation. Increasing the stripper reboiler duty also resulted in increased recirculation of the stripper overheads through the HP separator and into the absorber.

Cyclical slugging is the typical symptom of unsteady state water accumulation. Water builds in the column until the column floods or a slug leaves from the top or bottom of the column. It was clear that the amount of water entering the system could not be handled by the existing tower configuration. The focus then shifted to the HP separator of the GCU where there is a provision for separating free water in the boot of the vessel.

Performance improvement study of HP separator
Koch-Glitsch was asked to analyse the options to improve water separation in the HP separator. In Essar’s configuration, the HP separator is a horizontal drum with a water boot and no internals. It separates three phases: hydrocarbon vapours, hydrocarbon liquid and water.
Because superior collection of dispersed water was needed, Koch-Glitsch evaluated the use of  Ky-Flex liquid-liquid settling media to improve the separation (see Figure 3).

Ky-Flex media works by enhancing gravity separation. It minimises turbulence effects by dividing the stream into a number of separate chambers, which provides four primary benefits:
(1) Decreases the equivalent hydraulic diameter, thereby greatly reducing the Reynolds number of the flowing fluid and producing a deep laminar flow environment that is optimal for gravity settling
(2) Isolates the fluid in separate channels, thereby limiting how far droplets can ‘wander’ and reducing the negative impact of eddy currents
(3) Decreases the distance a droplet needs to rise or fall before reaching an interface, thereby greatly lowering the settling time requirement
(4) Provides multiple interfaces inside the equipment where droplets can coalesce, thereby positively improving the settling process.

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