PTQ Q4 2022 Issue

Lastly, critical equipment should be evaluated for needle coking operation. Critical equipment evaluations Critical equipment of the coker must be evaluated for needle coking operations considering the lower feedstock capac- ity, higher unit recycle, and higher operating pressures and temperatures. These adequacy checks will identify what is required to achieve a reliable operation for both campaign mode and long-term, sustainable operations. Critical equip- ment includes the coker heater, coke drums, coke cutting pump, coker fractionator, and wet gas compressor. Specific items of concern include: Coker heater Needle coke is a niche market, and as such typical unit throughput is much lower than a fuel or anode coker. While capacity and duty should not be an issue processing the lower amount of feedstock (even at higher recycle ratios), an increase in vaporisation is expected, affecting the furnace and transfer line hydraulics. Coke drums As shown in Table 3, the operating pressure of the drums is significantly higher for needle coke opera - tion. The design pressure of the drums must be checked to ensure they can withstand increased operating pressure. Since the pressure drop between the coke drums and frac- tionator is fixed, this will require the fractionator to also operate at a higher pressure. However, increasing fraction- ator pressure proportionally to maintain the same differen- tial as in fuel-grade coke mode may not always be possible due to limitations in the fractionator design pressure and vapour/liquid loadings. As the unit is normally at reduced capacity during needle coke production, the fractionator may have to be operated at lower than desired pressures to maintain minimum vapour/liquid loading to avoid weep- ing. Rigorous process and mechanical simulations will be required to confirm maximum fractionator operating pres - sure. Operating the fractionator at the maximum possible pressure may still result in higher differential pressure between the coke drum and fractionator than fuel-grade mode. This will pose a challenge in maintaining the pres- sure differential as throttling of the coke drum overhead vapour may be required. Typically, isolation valves used in the coke drum overhead circuit to the fractionator are either gate or ball type, which are unsuitable for throt- tling. Continuous throttling by pinching these valves will wear down the gate seals even when hard-faced, caus- ing chattering or dismantling of the gate. Another option is to replace the gate or ball valves with a butterfly valve. However, these types of valves are prone to coke accumu- lation and may require frequent cleaning. Options such as ring valves or other valve types can be considered based on vendor offering. Coker fractionator As mentioned in the previous section, the fractionator trays must be evaluated for the change in vapour/liquid flow rate and operating conditions associ - ated with needle coking operation. In addition, since needle coke operation is accomplished using much higher pressure

and recycle compared to fuel-grade coke operation, there is very little or zero heavy coker gasoil produced. Careful attention must be paid to rebalancing the heat and product/ pumparound flow rates around the tower. Options to increase the unit recycle must also be stud- ied. In a typical coker, recycle is generated using a) wash oil in the fractionator wash zone and b) quenching the coke drum overhead. Since coke drum temperatures are higher for needle coke operation, advantage can be taken to gen- erate more overhead quench by increasing the temperature differential between quenched and unquenched vapours. Typically, two layers of wash oil spray headers, each designed to produce 100% of the required wash rate, are provided. Increased recycle could be achieved by operat- ing both spray headers at maximum capacity to maximise wash oil rate. If that does not provide the required recycle, routing a portion of the feed above the sheds could also be considered. Since coke drum overhead vapours enter the fractionator below the shed decks, the introduction of relatively cold feed above the sheds will condense the coke drum overhead vapour by counter-current contact, thereby producing more recycle. Finally, product circuits must be analysed for the lower production rates and significant reduction or zero produc - tion of heavy coker gas oil. Draw rates can be adjusted such that the heavy fraction of light coker gas oil takes the place of heavy coker gas oil. Wet gas compressor The gas make during needle coking operation is expected to be significantly less than that of the fuel-grade coke operation. Therefore, the compres- sor loads may be too low, which will require continuous recirculation of flow to reutilise the compressor. Along with recycle being an inefficient operation, there are addi - tional issues to consider, including recycle flow rates, siz - ing of anti-surge valve, and pressure ratio due to higher operating pressures. The problem is more pronounced in a motor-driven compressor compared to a steam-driven compressor. Due to the higher operating pressure of the fractionator (including the fractionator condenser and overhead drum), it may not be possible to route the anti- surge vapour from the first-stage discharge to upstream of the condenser without significant modifications to the circuit. Surge control and pressure levels for the needle coking operation must be evaluated and confirmed with evaluation from the compressor vendor. Coke cutting pump The cutting pressure required to cut needle coke out of the coke drums is typically 15-20% higher than what is required for fuel-grade coke. Modifications will be required to the coke cutting tool nozzles. This should be confirmed based on input from the cutting pump vendor regarding the exit velocity, force, spread, and cutting time. Coke drum feed entry Typical coke drum feed entry for needle coking operation is through the centre of the drum to promote the orderly growth of the mesophase. A single or dual feed inlet may require changing to a centre feed

PTQ Q4 2022