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Jan-2013

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
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Article Summary
The 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 (non-condensables) 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 plugging 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 medium-
pressure 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 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 a decrease in unit capacity and loss of operating margin. 

The major component of the condensed stream originated from the vacuum tower’s non-condensed 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 non-condensed 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 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-, second- and 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 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.
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