Foam control in crude units
Installation of vortex tube clusters in crude unit preflash drums has eliminated foam carry-over, increasing diesel and atmospheric gasoil product yields
Tony Barletta and Edward Hartman, Process Consulting Services
David J Leake, EGS Systems Inc
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When revamping the crude unit to increase feed rate, improve distillate yield or quality, the preflash drum must be checked to ensure that foam will be contained inside the drum. Preflash drums are notorious foaming systems. When foam is entrained into the atmospheric column with the preflash vapour, many unforeseen consequences have occurred. Some of these unforeseen consequences include low distillate yield, poor kerosene and diesel product quality, and high carbon residue and metals in atmospheric gasoil (AGO). Total diesel and AGO product yield losses as high as 5 vol% on crude have been measured following revamps (Figure 1).
By retrofitting preflash drums with vortex tube clusters, incoming foam has been destroyed even at higher charge rates. In many instances these devices can eliminate the high cost of preflash drum replacement.
Preflash drums are used in crude units to “flash off” light boiling range hydrocarbons from the crude oil so that vaporisation is minimised or eliminated in the hot end of the exchanger train. In the back end of the hot exchanger train the temperature is high and pressure is reduced. The combination of high temperature and low pressure can cause the crude/water mixture to partially vaporise.
Vaporisation in the preheat train exchangers can cause very high pressure drop. For many crude units, crude hydraulics is one of the most costly revamp constraints that must be overcome. Therefore, preflash drums can be an effective debottlenecking tool if they are sized adequately to contain the foam.
However, many existing preflash drums are undersized and care needs to be taken to ensure that the foam is contained inside the drum (Figure 2) by making the drum larger. Otherwise, separating devices specifically designed for foam prevention should be installed when a revamp is being contemplated. Without these separating devices the preflash drum becomes very large.
Foam is created in the preflash drum when pressure is reduced across the upstream control valve. As long as the foam is contained inside the drum, it typically causes few problems other than difficulty with level measurements and occasional flashed-crude pump cavitation. However, when the drum is too small, foam is entrained with the preflash drum vapour and carried into the atmospheric crude column. Even small quantities of foam can cause off-specification distillate. Large quantities of foam carryover can significantly reduce diesel and AGO product yields. In some cases foam carryover will significantly increase vacuum unit feed rate and overload the vacuum jets.
As indicated in the previous section, foaming not contained within the preflash drum causes distillate yield loss in the crude unit. In fact, foaming in any separator, be it in upstream or downstream applications has been the cause of operational problems for many years, whether as foam exiting with the gas stream and/or with the liquid stream as gas carry-under.
The vortex tube cluster (VTC) system was developed for use in the upstream industry almost 20 years ago specifically to combat foaming in production separ-ators. Following its success there, it has been applied in refinery applications including hydrotreater hot high-pressure separator and more recently in preflash drums and columns.
The VTC is connected to the drum’s inlet nozzle, as shown in Figure 3. The two-phase flow is fed to the vortex tubes by a central duct or manifold that is designed to distribute the flow equally to each tube. A side opening, at the top of each tube, admits the stream tangentially. The phases are separated from each other by the enhanced gravitational effect generated. The gas migrates to the centre and exits through the holes at the vortex tube tops. The liquid exits via the peripheral openings at the bottom of the vortex tube (Figure 4).
In actual operation, the tubes are partially immersed in the liquid, which provides an effective seal, preventing gas from blowing out of the vortex tube bottom openings. Adequate immersion is required to overcome the pressure difference generated over the vortex tubes. The liquid level is controlled in the same manner as with any conventional drum. Since there is no splashing or bubbling in the vessel, and any incoming foam is destroyed in the vortex tubes, the separ-ator operates free of foam.
The VTC system can be designed to suit almost any vessel configuration and application.
Design of the optimum VTC for a particular application is done with software developed in tandem with the cluster and continually updated to reflect knowledge gained from those VTC systems in operation.
Foam is carried out with the overhead vapour stream when the preflash drum is undersized. Foam is a mixture of gas bubbles and liquid. Because foam contains flashed crude that is low temperature, black in colour and has a large amount of 600ºF minus boiling range material that vaporises at flash zone conditions, the entrained foam causes problems when it enters the atmospheric crude column. When the preflash drum vapour stream enters the atmos-pheric column above the flash zone, all product streams below the entry nozzle will contain flashed crude if foam is contained in the vapour stream.
Flashed crude is black and has an endpoint greater than 1500°F. Even small amounts of foam carryover will cause colour and endpoint problems with kerosene and diesel product quality. When the preflash drum vapour stream containing foam enters the atmospheric column in the flash zone, distillate and AGO yields are reduced because the low temperature liquid quenches the flash zone.
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