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Sep-2010

A balanced approach to vacuum tower flash zone/wash section design

Revamping an existing vacuum column to operate at a higher feed rate, higher flash zone temperature, lower pressure, and different feed stock characterisation is a complex task.

Stefano Costanzo, S M Wong and Mark Pilling
Sulzer Chemtech USA, Inc.
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Article Summary
It requires reviewing the column design with respect to the revamp conditions and identifying suitable solutions to reliably operate at the new conditions. Unless all of the design and operating aspects work together in unison, the net result will be at best, less than optimum, and at worst, a very costly problem. To get a complete view of the flash zone operation, we must also take into account all the peripheral regions, including the heater, transfer line, inlet nozzle, inlet feed device, stripping section, and the overflash (slop wax) collector tray. Also, it is fundamentally important that we take into account the type and characteristics of the crude oil being processed.  All these aspects are integrally involved in the design revision; the correct balance among them identifies the optimum solution.

The operating requirements that determine the optimum, balanced design are yield recovery, product quality, operating flexibility, and reliability. An important aspect affecting recovery and product quality is the level of entrainment (heavier residue liquids) carried in the vapour in the flash zone and wash section. Entrainment generated from the flashing feed and carried to the upper sections can be a source of product quality deterioration as well as operational reliability. Therefore it is important to understand the sources of entrainment generation as well as the methods used to reduce entrainment. All good vacuum tower designs must take these factors into account to balance capacity, performance, and operational reliability (i.e. coking resistance).

Column Process and Equipment

Figure 1 shows the bottom portion of a vacuum column along with the heater and the transfer line. This defines the area of focus when reviewing the flash zone and wash section. Revamping the vacuum unit will likely create significant changes to the operating conditions in the vacuum column as well as the heater and the transfer line. Changes in vacuum tower products generally require changes to the tower flash zone temperature and pressure which are directly dependant on the heater outlet conditions and the transfer line hydraulics. All of this equipment is linked from a process performance standpoint.  Any complete process study needs to include these components.

The vacuum heater must supply the appropriate duty to the vacuum tower feed without exceeding tube wall temperature limits where excessive coking occurs. Aside from the feed composition, heater coking is mainly a function of tube wall temperature and residence time which are controlled by feed flow rate and the heater outlet temperature. Steam can be injected in heater passes to mitigate coking, but this extra volume must then be handled in the transfer line and the column. The heater outlet stream is fed to the transfer line typically from the various passes of a multiple-pass heater. Since this stream is continually flashing, the transfer line hydraulics, which are quite complex1, also affect the heater and column operation. Extra pressure drop creates additional flashing, which lowers the fluid temperature, requiring a higher heater outlet temperature to maintain the desired flash zone temperature.

The transfer line transports feed from the heater to the column flash zone with the pipe typically increasing in diameter as it approaches the column inlet. There is a significant pressure profile determined by the increasing velocity of the stream due to flashing which progressively reduces the fluid (mixed phase) density along the transfer line. Reusing an existing transfer line for operating conditions other than for which it was originally designed may adversely affect the new mixed phase fluid behaviour traveling through the pipe. The final result is that the flashing behaviour within the pipe creates additional entrainment. An undersized transfer line will increase entrainment in the vacuum column feed stream. Therefore, the transfer line should be designed to minimise the pressure drop as is practical, and avoid flow regimes which create excessive quantities of very small liquid droplets which are more difficult to de-entrain.

Inside the column, we have to review the flash zone, the stripping section, and the wash section.

The flash zone serves to transition the high velocity two-phase feed from the transfer line into the column, separating the liquid and routing it to the bottom of the column while providing the initial distribution of vapour upward to the wash section of the column. The column inlet can have a variety of arrangements with single or multiple inlet nozzles oriented tangentially or radially. The feed inlet distributor functions as a vapour/liquid disengagement device as well as a vapour distributor. It uses the feed inertia to redirect the feed stream to contact and remove dispersed liquid particles. Liquids that remain entrained in the upward flowing vapour portion of the feed stream need to be minimised and/or removed because they contain high amounts of heavy end contaminants such as metals and hydrogen deficient molecules. These contaminants can poison downstream catalyst, form coke, and adversely affect the distillate product end point and colour.

If no modifications are made to the transfer line and inlet nozzles during a revamp, the transfer line will discharge into the flash zone via an inlet nozzle that was originally designed for less aggressive conditions. In this case, the inlet device within the column becomes very important since it has to correct for the higher momentum and increased entrainment from the transfer line and still provide good liquid disengagement and uniform vapour distribution to the wash section above. The conditions in the flash zone are critical as they affect the performance of the wash section with respect to de-entrainment capabilities, coking resistance, and conditioning of the vapour flowing to the section above.

The next device of importance is the slop wax collector chimney tray which separates the flash zone from the wash section. It serves three purposes within the column. First, it redistributes and equalises vapour flow on a large-scale basis due to its inherent pressure drop and chimney distribution. Next, it can provide a de-entrainment effect, depending on the chimney hat shape. Finally, it collects liquid leaving the bottom of the wash bed. The total liquid product, slop wax, collected on the tray consists of “overflash” (the heavy condensed component portion from the vapour feed), the heavy portion of the clean wash oil making it through the packed bed without being re-vaporised, and the coalesced entrainment from the flash zone. Process economics usually dictate that the clean wash oil rate to the wash section be minimised while maintaining a sufficient flow rate so that coking is avoided. The flow rate of this liquid is relatively low and, if kept on the tray for a prolonged period, is prone to cracking due to its composition and the high temperature operating conditions. Therefore, a specific sloped design is recommended to lower residence time and mitigate coking.

Vapour leaves the chimney tray with any remaining entrained liquids and enters the bottom of wash section, which is typically packed with a combination of grid in the bottom and structured packing in the top. The role of the wash section is critical for the ultimate performance of the vacuum column product quality. It must provide the lowest practicable amount of contaminants and entrained liquids to upper sections without coking. A small liquid gas oil stream (“clean wash oil”) is fed to the top of the wash section to wet the packing and prevent it from drying out and coking. The packing removes heavier components in the vapour flowing upward from the flash zone by condensation and by coalescing entrained liquid droplets. This lowers the heavy vacuum gasoil (HVGO) end point by removing heavier components that belong in the vacuum residue. This also serves to reduce other contaminants such as organic metals, carbon, and asphaltenes.
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