Hydrogen consumption is higher than we would like in our raw diesel hydrotreater. Can we lower it without loss of throughput?
Responses to a question in the Q1 2021 issues Q&A feature
Various from Shell Catalysts & Technologies, Axens, Haldor Topsoe, Unicat and Magma Ceramics & Catalysts
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Ihsan Raad, Senior Technical Services Engineer, Shell Catalysts & Technologies - Ihsan.Raad@shell.com
Hydrogen consumption for a raw diesel hydrotreater can be optimised either during catalyst selection or during the run. We have covered the impact of catalyst selection on hydrogen consumption in an earlier question. Here we will discuss possible variables to manage H2 consumption during the run:
- Feed management: maximise straight run feed and avoid or minimise cracked feeds with high olefins and aromatic content.
- Pressure: lowering unit pressure is an effective method to reduce hydrogen partial pressure which will reduce aromatic saturation and thus hydrogen consumption.
Temperature may need to be increased to compensate for the loss of activity due to lower H2 partial pressure, and there may be an increase in deactivation rate during this lower pressure period which will impact overall unit cycle life.
- Hydrogen purity: Increasing make-up hydrogen purity will deliver more H2 to the unit for a given compressor capacity and also lower purge requirements, thus minimising loss of H2.
- Solution losses: a large percentage of solution loss is from the hot separator liquid. Reducing the separator temperature lowers the solubility of H2 in the liquid phase, thus keeping more of it in the gas phase. Separate temperature needs to be optimised, keeping in mind heat requirements in downstream fractionation.
- Wash oil: a wash oil system can be deployed to recover H2 lost via solution losses.
Marion Guillevic, Technologist, Axens - email@example.com
Hydrogen consumption in a hydrotreater comes from hydrotreatment reactions but also from losses within the process.
Hydrogen consumption related to the hydrotreatment process is of course inevitable since it is directly linked to hydrogenation reactions targeted to achieve product specifications (hydrodesulphurisation or aromatics hydrogenation). One way to reduce catalytic hydrogen consumption is to use a specific catalyst adapted to the product specification targeted. For instance, for hydrodesulphurisation two paths are possible: hydrogenolysis or hydrogenation. Hydrogenolysis is favoured by CoMo catalysts where hydrogenation is favoured by NiMo catalysts. A CoMo type catalyst will be chosen to minimise hydrogen consumption when the hydrodesulphurisation target is driving. It should be noted that it is not possible to perform hydrodesulphurisation reactions without simultaneous minimum hydrogenation of aromatics at the same time.
Hydrogen consumption related to dissolution losses within the process can be minimised by adapting the operating temperature of high pressure separator drums to minimise hydrogen dissolution loss and hydrogen sent to the separation unit. If some minor revamping is envisioned for the unit, hydrogen losses can be minimised by minor modifications such as modification of the scheme (hot scheme versus cold scheme), or addition of membrane on the off-gas of the MP drum (if any). Moreover, the addition of an amine absorber on the recycling loop can be envisioned to increase recycle gas purging and to avoid the need for purging.
If other diesel hydrotreatment units are present on site, or envisioned, optimisation of the feedstock blend routed to the raw unit can also be a solution to lower the aromatics content of the blend and reduce hydrogen consumption within the unit.
Torkil Ottesen Hansen, Fellow, Haldor Topsoe - firstname.lastname@example.org
Yes, there are potentially several ways but they all come at a cost.
When the processed gasoil consumes less hydrogen then the product, quality always drops, when the target is ULSD. Normally, a hydrotreater is measured on its product sulphur level. The main consumer of hydrogen is however the aromatics in the feed. They saturate, which reduces the density and increases the cetane level. If these product specifications have some margin, then you may try one of the following ways to reduce hydrogen consumption.
The aromatics saturation is to a large degree decided from equilibrium, thus if you force a higher temperature in the reactor you most likely consume less hydrogen. The penalty is a faster catalyst deactivation and the sulphur product level will be too good.
It is also a thought to reduce the total pressure in the reactor loop. Also, here there is a penalty. The oil let-down valve will be the bottleneck, so it is only possible as long as it has sufficient capacity. Furthermore, the catalyst deactivation will also increase.
If you purge gas from the loop, then it is a possibility to reduce the purge. The penalty here is that the hydrogen partial pressure gets lowered and the catalyst deactivation rate increases.
If stream management of the feed streams can reduce the aromatics content of the combined feed, then it will also reduce the hydrogen consumption in this unit.
Tom Ventham, Sales & Technical Europe and Africa, Unicat - email@example.com, Xavier Llorente, Technical Sales Engineer, Asia, Europe, MEA, Unicat - Xavier.firstname.lastname@example.org and Gary Bennington, Business Development Manager, Magma Ceramics & Catalysts - email@example.com
Increased hydrogen demand is a growing trend throughout refining. On top of future requirements to meet more stringent fuel and product specs, alternative feedstocks, such as bio based materials, tend to necessitate high hydrogen input to enhance fluid properties and meet final product specifications.
Instead of attempting to thrift hydrogen input to the hydrotreater and sacrifice product quality, a better approach is to increase the net availability of hydrogen from the steam methane reformer (SMR). Various options exist to increase production from the hydrogen plant. Unfortunately, options such as adding a pre-reformer or post-reformer come at extremely high capital expense, need for local plot space, high cost of catalyst replacement, and a need to guarantee ppb levels of contaminants following the purification section. Further, considerations for pre-reforming and post-reforming include: future catalyst change costs and increased complexity of operations as a result of adding new process equipment into the flowsheet, which means increased operator training and an increased risk of errors and mishaps when operating unfamiliar equipment.
A preferable approach would be to maximise output, whilst retaining product purity, of the existing hydrogen plant. Two main areas can be focused on here: SMR which is the productive heart of the plant, and a pressure swing absorption (PSA) unit at the end of the system to ensure the purity of the expanded quantity of hydrogen meets the quality required by downstream consumers. In the SMR, step-out catalyst technology exists in the form of MagCat from Magma Catalysts to increase hydrogen output in most settings by at least 5-10% to satisfy additional hydrotreating demands. Unicat provides expertise in PSA, supplying advanced absorbents and logic control upgrades to enhance yields of recovered hydrogen and maintain purity specifications. This brief answer gives an alternative, pragmatic solution to the challenge faced. Operators in this situation should contact Unicat and Magma for further information on how these improvements can be introduced at their plants.
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