VDU vacuum system revamp Revamping a VDU’s vacuum system delivered operational improvements in a refinery, but also led to safer and more efficient handling of non-condensed gases AHMET BEBEK Turkish Petroleum Refineries Corporation
T
he non-condensed gases from a vacuum distillation unit (VDU) are considered to be waste gas. Since waste gas has some H2S and H2O content, the disposal of this waste gas poses serious safety and environmental problems. In this respect, engineers at Tüpras Kirikkale refinery redesigned the VDU tower top outlet downstream section to treat the overhead gases (noncondensables) leaving the vacuum column. In place of an underground atmospheric pit, the installation of a barometric seal system for a VDU vacuum system was achieved. The existing vacuum unit operation was not affected by the erection, commissioning and startup of the revamped vacuum system. As a result of the installation of a new barometric seal system, gaseous releases to the atmosphere, exposure of personnel to gaseous releases and the risk of back-flow of air into the vacuum unit have been eliminated. The seal system also helped in the disengagement of liquid and non-condensables, leading to improvements in reliability (for instance, a lower level of plug-
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ging in the non-condensables line and improved vacuum in the tower). Lastly, waste gas is burned by a dedicated burner in the vacuum heater furnace, which represents a significant saving in energy costs.
Vacuum system before revamp
The VDU vacuum system is a booster type (in four stages) with a condensate leg for each condenser (see Figure 1). Each stage has two ejectors. The system achieves a pressure of approximately 13.3 mbara top pressure and a flash zone pressure of 66.6 mbara. The ejectors are driven by mediumpressure steam (12.75 kg/cm2 and 192°C). Condensers downstream of the ejectors use water as a cooling medium. The non-condensed vapour from the last-stage ejector was passed to the VDU’s furnace via a small knock-out vessel. This stream had a high wet H2S content. It was common (once a week) for this line to become plugged. When this occurred, the vacuum level in the VDU decreased, resulting in a significant disturbance to operations. While the plugged line was
steamed out, the non-condensables from the last-stage ejector were directed to atmosphere, giving rise to offensive smells in the area. The condensed steam and hydrocarbons from each ejector condenser were passed directly to an underground atmospheric pit. Overflow separation of hydrocarbons took place in this pit. The liquid hydrocarbon was sent to a slops system and the aqueous phase was directed for further treatment to the sour water stripper unit. Even when waste gas was burned in the furnace, there was still airborne H2S in the area surrounding the vacuum column’s underground atmospheric pit, causing offensive smells. This was due to fact that the atmospheric pit was not sealed and waste gas dissolved in the condensate was being released to the atmosphere. Clearly, there had to be an improvement in the management of health, safety and environment in the area.
The approach
Tüpras Kirikkale refinery constantly invests to minimise
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Vent to atmosphere Non-condensed vapour to furnace
Steam H-1B
Vacuum unit vapour
Knock-out vessel
H-1A
Vacuum unit vapour
H-2B
H-2A E-1A
H-3B
H-3A
H-4B
H-4A Drain
E-1B
E-2
E-3
E-4
Atmospheric pit
SW to SWS feed drum P1
HC to slops P2
Figure 1 Vacuum system with atmospheric pit
the adverse impact of refining activities on the environment, employees and the public, to offer the safest possible working environment. In this respect, the existing vacuum unit was revamped to prevent waste gas release to the atmosphere, which was causing safety problems for personnel. The other basis for this work was to prevent the risk of air backflow into the vacuum unit to deal with the case of failure of ejectors. Lastly, the revamp was designed to eliminate operational disturbances resulting from plugging of the waste gas line, which was leading to
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a decrease in unit capacity and loss of operating margin. The major component of the condensed stream originated from the vacuum tower’s noncondensed vapour and from medium-pressure steam as a utility. The installation of a barometric seal system for the VDU vacuum system seemed the best option to solve the problems mentioned, especially the health, safety and environmental issues. The existing vacuum unit’s operation should not be adversely affected by the erection, commissioning and startup of the revamped
vacuum system. The revamp should not give rise to any negative impact on the column’s vacuum level.
New vacuum system
The underground atmospheric pit was replaced with a barometric seal system (see Figure 2). The purpose of the barometric seal system was to achieve good separation of noncondensed vacuum tower top outlets after they are condensed by the ejectors. In addition to this change, the final-stage ejector nozzles were replaced in order to provide a pressure high enough to send waste gas
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Non-condensed vapour to furnace
Steam H-1B
Vacuum unit vapour H-1A
Vacuum unit vapour
H-2B
H-2A E-1A
H-3B
H-4B
H-3A
Vent to flare
H-4A
Vent to safe location
E-1B
V4
E-2
E-3
E-4
V3
V1
V2
P3 SW to SWS feed drum
P1
MC to slops
P2
Figure 2 Vacuum system with barometric seal vessels
to the furnace. There was no other design change to either the ejectors or the condensers. The configuration of the present barometric seal system is described below. The condensed hydrocarbons and water from the first-, secondand third-stage condensers pass to the first seal vessel via a barometric seal leg. The condensed hydrocarbons and water from the fourth-stage condenser pass to the second seal vessel. The non-condensed vapour from the fourth-stage condenser passes to a waste gas seal vessel via a seal pipe that is about 17m high, to prevent
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ingress of air. The last vessel is the knockout vessel. The primary purpose of this vessel is to remove entrained liquid droplets in the offgas stream before it is sent for disposal. The offgas from this vessel then passes to three separate locations: the vacuum furnace (a dedicated waste gas burner), the flare (when there are problems sending to furnace) and the safe location (only during startup). The routes to the furnace or safe location are provided with flame arresters. Water in the water compartment of the first seal vessel is circulated between the other
vessels at all times to prevent any breakthrough of H2S and thus prevent build-up of corrosive and fouling material. Excess sour water is pumped to the sour water stripper unit. Since separation of water and hydrocarbons is improved in comparison to the old underground pit design (in which hydrocarbon was sent to the slops system), the hydrocarbon from the seal vessels can be pumped to the feed pool of the deep diesel desulphurisation unit.
Results Tüpras
Kirikkale
refinery
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redesigned the vacuum system of the vacuum distillation unit. An atmospheric pit was replaced with a barometric seal system. This change resulted in good separation of noncondensed gases from the vacuum tower’s top outlets after condensing by ejectors. The achievements of this revamp are: • Wet H2S release to atmosphere from both the atmospheric pit and the knockout vessel has been prevented • The possibility of exposure of personnel to wet H2S release has been avoided • The risk of a back-flow of air into the vacuum unit has been eliminated • The heating value of hydrocarbons in the waste gas can be
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used. 300 kg/h of waste gas is consumed in a dedicated special burner in the VDU’s furnace. This results in a saving in fuel of 1.4 Gcal/h • The reliability of the vacuum system has been improved because the seal system has helped in disengagement of liquids and non-condensables. There has been less plugging of the noncondensables line to the furnace and an improved vacuum level in the tower. In this way, a decrease in capacity leading to margin loss has also been prevented • Separation of hydrocarbons and water has been improved. As a result, hydrocarbons are being sent to the diesel pool instead of the slops system,
thereby avoiding reprocessing in the crude unit and so decreasing operational costs. Ahmet Bebek is an Operations Superintendent in the Production Department of Tüpras Kirikkale Refinery, Turkey. He holds a BS in chemical engineering from Middle East Technical University, Ankara, Turkey, a MS in process systems engineering from Technical University of Dortmund, Germany, and a MS in industrial engineering from Middle East Technical University, Ankara. Email:
[email protected]
LINKS More articles from the following categories: Crude Vacuum Units Emissions Control Gas Processing/Treatment
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