" It seems that in any hydrogenation reaction (say with the fix-bed solid catalyst) gas-phase process should be preferred"

What about a hydrogenation of molecules with a very high boiling points such as vegetable oils?

Dear Yuri,

thank you for your comment!

Of course we can not apply gas-phase process everywhere. But the question is: if we could, would we?

I know two technologies of hydrogenation of 2-ethylhexenal to the corresponding isooctyl alcohol. One process is conducted in gas phase, another - in liquid. The feed is relatively high-boiling, but still could be vaporised without degradation. Now I'm trying to understand, what are pros and cons from the theoretical point of view. Thank you!

The catalyst may be porous or not. Its particle may be smaller or larger. The average pore size may vary. Mass transfer may change the selectivity or not. Deactivation may be smaller in gas-solid system or not. Hot spots are more likely in the gas-solid system. It does not have to be a tubular reactor. Mixing can then be more or less intense. etc., etc. Regards,

boiling solvent may be needed to carry away heat.

In the refining industry Hydrotreating is used to remove sulfur (Hydrodesulfurization or HDS) Nitrogen (HDN) and to saturate olefins and aromatics. The lighter cuts consisting of hydrocarbons boiling below 200-250 C are usually 100% vapor phase all other cuts are usually mixed phase reactors the hydrocarbon is primarily liquid phase and H2 is of course vapor phase. As a result, the vast majority of commercial hydrotreating units are mixed phase (liquid Hydrocarbon, Vapor H2 and lt Hydrocarbon cracking products and H2s) reactors operating at high mass velocities to assure proper catalyst contacting. H2 must diffuse through the liquid phase to reach the catalyst sites but this isn't a significant problem because H2 can diffuse through almost anything.

In short, many different factors affect how the process is run and in what system: gas-solid, liquid-solid or gas-liquid-solid. And refining industry is a very good example. Regards,

Try to chaining your catalysts whit diffident porous. particle may be smaller or larger. The average pore size may vary. Mass transfer may change the selectivity or not.

I'm currently working on an adsorption experiment of several metals remove from water solutions

In literature, the two most popular kinetic models are Lagergren's pseudo-first-ord

In the specialty chemicals area, most production rates are very small compared to petrochemical production. Many processes are multi-batch processes and dedicated vessels are used for each chemical transformation. The process design, for a single reaction chemistry is based more on the length of time to complete the step, and the required material of construction (MOC) of the vessel. To produce the yellow ink intermediate DCB, the following reactions are required; hydrogenation, a reaction in sulfuric acid, precipitation, decanting, and solids separation. Vessel sizes, and number of vessels vary due to required reaction times versus each other to maintain productivity in the train. The vessel MOC, and dimensions vary due to process and agitation requirements. For example the hydrogenation vessel is larger, L/D is 1.5, made of Stainless, and has agitation to slurry the catalyst in the batch. This batch goes to 4 smaller vessels which are glass-lined with a L/D of about 2. The precipitation vessel has an L/D of >3, with a non-metallic MOC compatible with a HCl / Sulfuric mix.

Vaporizing the hydrogenation mixture would be to costly and require a continuous process with a condenser, scrubber and high energy costs.

There is another way to hydrogenate indirectly Exxon used to call it the "Exxon Donor Solvent" process. Naphthalene was partially hydrogenated to form Tetralin which would "donate" its hydrogen to the material being hydrogenated and reform Naphthalene. The Naphthalene formed would be recovered and sent to a hydrogenation reactor to form Tetralin once again and be recycled back to the donor solvent reactor. The process worked reasonably well in a pilot/lab scale reactor but not nearly as well in a larger unit.

Dear colleagues,

thank you very much for your comments. However, I'm still in search for an insight relating my question. I can see that all who gave answers have great level of expertise in hydrogenation reactions applications.

But please let me pose the question once more. Let us imagine that we have a compound to be hydrogenated. This virtual compound has moderate boiling point providing a possibility to be fed into reactor as liquid as well as vapor (we could take 2-ethylhexenal as an example).

What are the differences between gas- and vapour-phase reactions? What are advantages and disadvantages of each approach? Am I right when I suggest that in principle gas-phase process is preferred just because of faster diffusion?

I'll be grateful to everyone who will give a responce, thank you!

2-ethylhexenal boils around 360 F (180 C). It is Heavy naphtha/kerosene boiling range. It would most likely be hydrotreated in a vapor phase reactor at around 500 F (250 C), with a significant excess of H2 (5-10 times consumption). The primary product will be Octane (likely a mixture of all Isomers if the catalyst has an acid functionality) but some hydrocracking to lighter products may also occur,

As a practical matter, I would say it depends on the material balance, but also on the reaction rate. I once designed to gas-phase reactor for a reaction that occurred at a very slow rate. I mixed the two gases on the inlet line to the reactor, and the reactor had very little if any mixing. I needed 51% conversion to be successful. I oversize the container (With little impact on cost) and was able to achieve about 85% conversion. But in this case there were no side reactions are other species being created, so purity was not a consideration.

In practice consideration would be related to a final concentration leaving the reactor. For small volume products batch reactors are best in liquid phase, but for a gas, it would have to balance final concentrations of species with cost of separation and recovery of the hydrogen or the primary product. It is relatively easy to mix gases, as long as there is no severe corrosion concern. As a process engineer, I would calculate the time in the reactor versus the amount of conversion, And weigh that against the cost of separating and recovering hydrogen.

For polysilicon production, gas phase hydrogenation is the preferred method. A large excess of hydrogen is used, and diffusion is not an issue, the large excess of hydrogen needs to be recovered and returned to the process. But in the analysis presented in the paper, the final cost to produce a kilogram is the overriding factor. Energy requirements and the number of process steps (equipment costs) are the key factors.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860017211.pdf