PTQ Q1 2023 Issue

To are

To fractionator overhead

From blowdown overhead condenser

To fractionator overhead

From blowdown overhead condenser

Ejector

Steam

Water ring compressor

Water ring compressor package

Water ring separator

Fuel gas

Blowdown overhead receiver

Ring water cooler

Recirculation pump

Figure 2 Ejector implementation

Figure 3 Water ring compressor implementation

The operation of the ejector is as follows: during the begin- ning of the coke drum cooling step, the ejector is offline and bypassed, as there is a high enough pressure from the vaporisation of the coke drum cooling water to send the blowdown vapours directly to the fractionator overhead. As the coke drum cooling progresses, the ejector is manually taken online, wherein the ejector inlet valve is opened, while the valve to the fractionator is closed. This typically occurs about 3-4 hours into the coke drum cooling step. By the end of the cooling step, the coke drum can achieve pressures as low as 2 psig or less, depending on the blowdown system layout and the system hydraulics. The ejector is then taken offline at the end of the cooling step, right after the closing of the coke drum-to-blowdown isolation valves. As for its incorporation into the design of the blowdown system, the ejector is a static piece of equipment with no moving parts and is, therefore, easy to maintain. An ejector is also relatively small, which is advantageous when con- sidering plot space availability, and has a lower capital cost when compared to other design options. However, the ejec- tor does require pressurised steam to operate, and because this steam enters the process directly, the amount of sour water from the coker unit will increase during the ejector’s operation. The above points must be taken into consider- ation when determining the design of the blowdown system. Blowdown system design option: water ring compressor Another option for the blowdown system design is the water ring compressor. A water ring compressor utilises a rotating, vaned impeller to accelerate water such that an outer ring of water is formed inside a cylindrical casing. Because the impeller is eccentrically placed within the casing, the impel- ler vanes and the outer water ring create spaces of varying volume. Gas drawn into the inlet of the compressor enters a space of larger volume and experiences compression as the impeller turns, causing the volume of the space to be reduced. The compressed gas then exits at the discharge of the compressor. In the blowdown system, a water ring compressor pack- age can be installed in the vapour line downstream of the blowdown overhead receiver shown in Figure 3 . Aside from the compressor itself, the water ring compressor package consists of multiple equipment, including the water ring

separator, the recirculation pump, and the ring water cooler. The water ring separator removes any working water from the compressed gas. This water is then pumped by the recir- culation pump to the ring water cooler to remove the heat of compression from the water. The working water is then reused in the water ring compressor. A spillback valve at the outlet of the water ring separator controls the suction pres- sure of the water ring compressor. Like the ejector, the main purpose of the water ring com- pressor is to allow the coke drum pressure to be lowered even further before venting the coke drum to the atmosphere after the quenching step. The blowdown vapours from this process can be sent via the water ring compressor to the fractionator for recovery. The operation of the water ring compressor is similar to that of the ejector. At the start of the coke drum cooling step, the water ring compressor is bypassed, and blowdown vapours are sent directly to the fractionator overhead. As the coke drum cooling progresses and the blowdown system pressure lowers, the water ring compressor is taken online. This typically occurs about 2-3 hours into the fast water cooling step. By the end of the cooling step, the coke drum can achieve pressures as low as 2 psig or less, depending on the blowdown system layout and the system hydraulics. The water ring compressor is then taken offline at the end of the cooling step, right after the closing of the coke drum-to- blowdown isolation valves. The benefits of selecting a water ring compressor over other blowdown system design options such as an ejector are its cost-effective operation and robustness. In terms of utility consumption, the water ring compressor package uses electric power rather than the pressurised steam of an ejec- tor. Furthermore, because most of the water is separated from the compressed gas in the water ring separator, there is minimal impact on the sour water generation of the unit when compared to the operation of an ejector. The compressor package also recycles this water via the recirculation pump and ring water cooler shown in Figure 3 and requires only a small amount of make-up water. In terms of robustness, if a refinery does not already have a flare gas recovery system, the water ring compressor can also be used as a flare gas recovery compressor by send - ing flare gases to the coker fractionator overhead. However, utilising the water ring compressor in this service will require

PTQ Q1 2023