Reactor design With the catalyst protected and stored correctly, attention turns to ensuring the reactor itself is ready to make the most of its potential. Several design parameters help obtain and maintain good catalyst performance, including dimensions, distribution, access, thermocouples, and catalyst dumping. Bed diameter and height impact performance. Too large a diameter (to reduce pressure drop across the bed) and an excessively tall bed will lead to a higher level of maldistribu- tion. Even a good distributor cannot completely fix the bed, and maldistribution worsens with bed height, especially with mixed-phase reactors. Hydrocracking bed heights are limited to control the heat rise in each bed. Hydrotreating beds can be much higher, but above about 6-7 metres, the maldistribution risk increases, regardless of distributor design. Gas-liquid distributors and quench systems need proper installation and maintenance. Leak checks should be straightforward, even without full internal access. In one hydrocracker case study, properly maintained and upgraded internals increased effective catalyst utilisation by 29%. Poor performance due to leakage has halved diesel unit activity. A good tray design should allow for easy leak test- ing and observation, especially if maldistribution is detected on the previous cycle. Maldistribution is better assessed by looking at the ratio of radial over axial ‘delta T’ (ΔT) rather than just radial delta, especially when the axial ΔT is not large (such as, access). Access via side-entry manways in a multibed reactor is significantly easier for loading, monitoring, and mainte - nance. It is also much safer to work in emergencies, such as when exiting rapidly, when dealing with an injury. The side-entry manway flanges are not unevenly stressed, such as with the inlet and outlet, so leakage concerns are lower. It will add cost to the fabrication, which is why it is not so common in many companies. Thermocouple location and thermowell design, if used, can impact bypassing if it is not done carefully. Thermowells penetrating the support trays are not preferred, and hori- zontal thermowells are considered much better. Flexible thermocouples are now more common to give significantly better bed coverage, but maintenance needs to be prepared for a higher frequency of repair need. Skin temperatures, usually placed near the bottom of each bed, where maldistribution may be severe, are required for certain beds. This is particularly the case in hydrocrackers due to the exotherm risk, such as when recycle gas is lost or if there is severe flow maldistribution. A good practice at the bottom dump tube is to provide the option for a pressure gauge to be fitted. This allows the ΔP of the outlet collector to be checked. Angled catalyst side dump nozzles can replace inter-bed dump tubes, which often contribute to maldistribution. Two dump tubes at each bed reduce the risk of localised plug- ging in one dump tube. Support grids are frequently based on fine grid screens rather than bars to keep inerts from migrating. However, they can be more easily damaged, so they should still have some inert ball grading below the catalyst.
Operations and maintenance checks before loading Before loading, operations and maintenance checks should confirm that all reactor internals are installed and secured, including any tray gasketing or materials used to fill gaps. Tray level should be verified, and trays should be inspected for damage. If maldistribution is a concern or trays are newly installed, tray leak tests should be conducted. Where the economics justify it, changeout time can often be reduced significantly through effective planning, clear procedures, good communication, and coordinated teamwork. Many other design considerations for preheat systems and recycle gas systems also contribute to max- imising catalyst utilisation. Poor preparation will cause problems from the start, especially in relation to reduced activity, channelling, hot spots, and earlier colour problems. Best practices for reactor loading Reactor loading is critical to avoid maldistribution, pressure drop, or catalyst migration. Poor loading can quickly under- mine the best design and catalyst choice. A good loading procedure avoids fines by limiting freefall when loading. Keep track of the catalyst amounts used and clearly mark bed heights. This information, along with the associated catalyst density checks, confirms an even loading. Bed grading in the guard bed is also important to protect against feed contaminants and migration. It is essential at the bot- tom of any bed to have a correct layer-to-layer particle size change to prevent catalyst migration. There are several ways to transport catalyst to the reac- tor top, including one bag or two to three bags if the top platforms can handle the load. Hydraulic methods are also in use at some locations. Other key equipment to consider includes access to deliver catalysts to the site, dust extrac- tion facilities, and procedures to minimise catalyst and inerts freefall. Dense loading is now used on most units, except when putting in bed grading to open void fraction to manage fouling materials, remove metals, or if there are specific issues with highly viscous feeds. Several dense loading procedures are now used to achieve adequate packing, and dense loading should be considered unless special pres- sure drop constraints exist. Dense loading reduces channelling, whereas sock load- ing often allows preferential unbalanced flow, as density is not uniform. Most of the different dense loading technolo- gies work if there is a good technician and the site keeps in close contact. They will have different loading capacities and access needs, being able to load anything from approx- imately 8 to 20+ tonnes/hr. Dense loading also allows more catalyst to be put in the reactor. If the density increase is less than 15% more than sock loaded then there is a problem with the dense load- ing process. A more densely packed bed will increase the pressure drop across the reactor bed, so dense loading is not practised in the guard bed. Some contractors also use fluidised continuous loading, though experience is lower and should be first tested in a less critical service. While the various unloading options for spent catalyst (and the associated positives and negatives) are beyond
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PTQ Q4 2025
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