We’d like to produce less light naphtha and more liquid fuels from our hydrocracker. How do we go about it?Jun-2021
Steve DeLude, Becht, SDeLude@becht.com
For changes mid-run with an existing hydrocracker catalyst load, facilities and feed, the most effective way of lowering light naphtha yield is to reduce the conversion in the hydrocracking catalyst portion of the load. This is usually accomplished by reducing the hydrocracking catalyst temperature, reducing liquid recycle rate and/or increasing recycle cut point. Often the extent of de-conversion is limited by product specifications (sulphur, nitrogen, Cetane, PNAs) or equipment constraints within the unit (product pump rates, hydrocracking catalyst quench / cooling, product fractionator stripping rate). Some temperature adjustments to the hydrotreating catalyst may be required to meet product specifications while equipment constraint can often be determined by doing some planned rate and performance tests. Note that reducing the conversion will also likely reduce the total liquid product yield thereby leading to an economic trade-off with light naphtha yield.
For long term reduced light naphtha yield reduction with the same/ increased total liquid product yield, a combination of changes to the loaded catalyst system and debottlenecking of unit equipment is typically required. Working with the catalyst vendors can help identify catalyst systems that have reduced naphtha selectivity / increased diesel selectivity as well as increased liquid swell for a more desirable product yield slate. Some unit constraints may be known from past operations; however, typically a good project basis for identifying and relaxing equipment constraints can be accomplished by doing a unit performance test. The performance data can then be used in simulations and to check performance against equipment data sheets limits. Upgrades to product pumps, quench valves, fractionator internals and safety valves are common for modest upgrades to run in a de-converted mode. Experience shows that desired product yield profiles shift often with economics, so some flexibility to run in both low and high conversion modes is usually maintained.
Matt Lehr, KBC, Matt.Lehr@kbc.global
There are a variety of process variables available to reduce hydrocracker light naphtha production and increase heavier liquid fuels from the hydrocracker. These range from feedstock selection, catalyst selection, reactor operation and severity, use of liquid recycle within the unit (if available), and fractionator cutpoint changes. The selection of any of those variables, or a combination, to meet the yield objective is determined by existing unit constraints, and also consideration of the hydrocracker’s position and strategy within the refinery. While directional movements (increase/decrease) of yields are roughly understood for each variable above, this is not always the case, especially if multiple reactors and liquid recycle are involved. It’s recommended to use a simulation model tuned to the unit’s operation to both validate and quantify the directional yield movements expected from these variable changes.
Product quality will also be affected as the yield profile shifts towards heavier liquid fuels, which must also be considered in the final decision of hydrocracker yields.
Catalyst management must also be considered, as changes in reactor severity typically lead to longer/shorter catalyst life. Shorter catalyst cycles imply more frequent catalyst change outs and costs. Shorter catalyst cycles may be justified, but must first be considered against the expected economic benefit.
The final decision of optimal hydrocracker yields is typically determined via the LP model, which considers the economics of running various feeds and producing different amounts of each hydrocracker product. A robust and representative hydrocracker LP submodel, validated with plant data on a regular basis using a digital twin, is critical to reaching the optimal yield decision.
Bob Scheffer, Petrogenium, Bob.Scheffer@petrogenium.com
There are several handles to tune the product composition of a hydrocracker. I would like to talk about these on the basis of their timeframe, as operational, catalyst selection, and revamp:
- Feedstock selection: various types of crudes (aromatic, paraffinic and so on) give rise to different yield patterns in a hydrocracker. However, usually the crude diet is fixed (based on crude pricing) and little flexibility exists here. The feed to the hydrocracker can be complemented by other streams from the refinery that increase distillate yield, such as cycle oils or extracts. We recommended to study these options carefully as to not compromise the expected life of the catalysts.
- Combined feed ratio: in recycle operation, the CFR is a parameter that influences the yield structure: higher CFR (at the same overall conversion level) means that a lower conversion per pass is required and therefore a more middle distillate selective yield is achieved.
- Recycle cut point: an increase in the RCP reduces severity of operation, thereby producing a more distillate selective yield pattern.
- Increasing the gas-to-oil ratio can shift the yield pattern somewhat to the distillate range. The effect depends on the type of catalysts used, so this should be evaluated in pilot plant testing campaign. Supplying more (fresh and recycle) gas also reduces the risk of maldistribution and hotspots, which can be a cause of the formation of lighter products.
- A hydrocracker usually contains multiple catalyst beds, which contain different catalysts. There is some flexibility in setting the temperature of each catalyst bed by adjusting the quenches between the catalyst beds. Optimising this temperature profile can shift the product yield toward distillates by applying the higher temperatures to the catalyst beds with the most selective catalysts. Tuning a hydrocracker model and running different temperature profiles will support the refinery in finding the optimum settings, while achieving maximum run length.
- An effective way to maximise fuel production is by minimising the downtime of the hydrocracker. Here we can distinguish planned downtime (regular shutdown for maintenance) and unplanned downtime. Just reducing the downtime by a few days per hydrocracker cycle brings significant benefits, which go directly to the bottom line of the refinery. To achieve this, we would do a dedicate study investigating the causes of unplanned downtime, in conjunction with smart maintenance management. A detailed review of the regular shutdown procedures, while closely managing the contractors involved (optimising working in shifts, incentives for early delivery and so on), can yield strikingly good results.
- Key to optimisation of the hydrocracker is the selection of the catalysts that meet the objectives of the refinery best. There are many suppliers, each with a large portfolio of catalysts. Refiners do not always have the resources to rigorously evaluate the proposals from catalyst companies. We would enlist the support of an expert consultancy to support the decision making, which involves clever target setting, modelling of the hydrocracker, and – in some cases – pilot plant testing.
- During operation, the performance of the catalyst should be monitored, in order to intervene at the earliest possible time if some deviation from the expected performance occurs. At an early stage, fixes are still possible, but if the malperformance endures for too long, the catalysts may have been damaged irreparably and the desired product yields cannot be obtained.
- A major shift in yields is obtained if a hydrocracker is revamped from one-stage/series flow to a two-stage operation.
- Minor revamps can also be effective. If maldistribution of hotspots is persistent, then revamping the reactor internals can lead to better yields. Also re-traying columns in the work-up/distillation section can lead to better/sharper separation of products, maximising the desired products.