PTQ Q1 2023 Issue

Design configurations for lowering quenched coke drum venting pressure Compare the advantages when installing either an ejector or a water ring compressor for lowering coke drum venting pressure

Piotr Lorenc Gdansk Refinery Keith Magdoza and Virendra Manral Chevron Lummus Global

I ncreasingly stringent regulations around the world have caused refiners to review their current refinery opera - tions and search for methods to comply with such regula - tions. An example of this is the United States Environmental Protection Agency (US EPA) standard 1 for the depressurisa - tion of a coke drum to a closed blowdown system during the decoking cycle. According to this regulation, the pressure of the coke drum is to be lowered to 2 psig or less before vent - ing the drum to the atmosphere at the end of the cooling step. This will result in a lower pressure difference between the coke drum and the atmosphere right before venting, leading to fewer volatile organic compounds (VOC) emis - sions and quieter openings of the coke drum vent valves. In a traditional system, a coke drum is typically depres - surised to 5-9 psig before it is vented to the atmosphere, depending on the coke drum operating pressure. To achieve a coke drum venting pressure of 2 psig or lower, several design configurations are often considered when design - ing new grassroots units or integrating into existing units. Two of the most practised and proven design configurations include either the installation of an ejector or the installa- tion of a water ring compressor. The following discussion details the configuration of these two design options and their advantages. Traditional systems In a traditional system, the hydrocarbon vapours from the blowdown overhead receiver are sent to the flare or the overhead of the fractionator. In each of these routings, the lowest pressure that the quenched coke drum can depres - surise to before venting to the atmosphere is limited by the flare system back pressure or the main fractionator overhead pressure, respectively. The layout of the latter routing is shown in Figure 1 , in which the hydrocarbon vapours from the blowdown overhead receiver are sent to the overhead of the fractionator. This configuration serves as the baseline system for this article, with the capability to recover hydro - carbons from the blowdown vapour rather than flaring them. Another routing option using the traditional layout involves sending the blowdown vapour to a flare gas recovery sys - tem, if available. Like the fractionator routing shown in Figure 1, sending the blowdown vapours to a flare gas recovery system has the benefit of being able to recover and utilise hydrocarbons from the blowdown vapour rather than flaring


To are

To fractionator overhead

Fuel gas

Coke drum

Blowdown overhead receiver

Blowdown tower

them. In this routing, the lowest pressure that the quenched coke drum can depressurise to before venting to the atmo - sphere is limited by the suction pressure of the flare gas recovery system’s compressor. To achieve even lower depressurisation pressures, a dif - ferent blowdown system configuration can be integrated into the unit design. One such configuration incorporates an ejector, which utilises pressurised steam passing through an expanding nozzle to create a vacuum. Another such configu - ration incorporates a water ring compressor, which utilises a vaned impeller and a rotating ring of water to compress a gas. The implementation and advantages of both system design options are detailed in the subsequent sections. Blowdown system design option: ejector An ejector is a device using steam as a motive force to com- press a gas. The simple ejector design has no moving parts. Instead, pressurised steam is passed through an expanding nozzle to create suction in the ejector’s inlet. In the blow - down system, an ejector can be installed in the vapour line downstream of the blowdown overhead receiver shown in Figure 2 . The main purpose of the ejector is to lower blowdown system pressure, allowing coke drum depressurisation to a lower pressure before venting. The ejector also has the added benefit of being able to send the blowdown vapour to the fractionator overhead for recovery of hydrocarbons near the end of the coke drum cooling step. Figure 1 Traditional system with blowdown vapour routed to fractionator overhead


PTQ Q1 2023

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