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Dec-2020

Recovering hydrogen and LPG from off-gases

A practical investigation devised the most effective combination of available separation technologies to recover valuable products from refinery off-gases.

MARCELO TAGLIABUE
Air Liquide

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Article Summary

Some 66% of world production of hydrogen, estimated at 70 million tonnes, is used as an input to oil refining, ammonia production, methanol and, in recent years, in the development of engines that run on hydrogen. In the last case, the use of fuel cells completely eliminates polluting emissions. This circumstance makes hydrogen the cleanest existing fuel.

Currently, the best known and developed production methods are:
- Electrolysis of water: currently limited to 5% of hydrogen production. It is achieved by dissociating the water molecule into its components (hydrogen and oxygen) using electricity and is known as green hydrogen.
- Reformed with water steam from natural gas: this represents 95% of current production worldwide. It is a thermochemical process which requires high temperatures and subsequent purification of the final stream obtaining the so-called grey hydrogen. If the CO2 produced is captured, you are in the presence of blue hydrogen.   

Hydrogen is an important and expensive utility in oil refining and petrochemicals processing. It is required for many operations such as hydrotreating (where it is used to remove impurities such as sulphur from streams and to hydrogenate aromatics and olefins) and hydrocracking (where it breaks down large hydrocarbons into smaller, higher value molecules). The main consumers are the refinery processes that consume H2 such as hydrotreaters and hydrocrackers.

As refinery product specifications become stricter to meet environmental requirements, demand for hydrogen from the refinery has continuously increased to supply hydroprocessing units. Further improvements in combustion qualities, such as cetane, also require more hydrogen.

Global economic trends toward the use of heavier crudes results in a higher need for hydroprocessing and thus a higher need for hydrogen.1

For many refineries, the hydrogen byproduct produced by gasoline reforming has been able to supply enough, but restrictions to the addition of BTX to gasoline hydrogen mean that production by reforming gasoline has diminished.

Nowadays, full conversion refineries generally require more hydrogen.

This article explores processes used to recover hydrogen and hydrocarbons in refineries and petrochemical processes, and how these techniques can be optimised and adapted to different circumstances. In addition, some innovative techniques to improve recovery and increase the capacity of the units mentioned are described.

There are several possible sources of hydrogen. Typically, most make-up hydrogen in refineries is supplied from gasoline catalytic reforming. Catalytic reforming produces aromatic compounds from the cyclisation and dehydrogenation of hydrocarbon molecules and is used to increase the octane number of gasoline. At the same time, large amounts of hydrogen are produced as a byproduct. Since the amount of BTX added to gasoline is limited, the charges to these units are decreased in the same way that hydrogen production is decreased.

If the hydrogen from catalytic reforming is insufficient, additional hydrogen may be supplied by building a hydrogen plant that produces the gas either by steam reforming or partial oxidation of hydrocarbons. Alternatively, hydrogen may be imported via pipeline.

Finally, the most economical option and the one we are going to explore in depth is hydrogen recovery from the off-gas currents of refineries (ROG). These techniques provide the ability to recover hydrogen from streams that would otherwise end up in the fuel gas system or in the flare. And in the event that this recovered H2 is still insufficient to meet a refinery’s needs, the investment necessary for the acquisition of a steam methane reformer (SMR), a partial oxidation process (POX) or an over-the-fence contract with a third party would be considerably less.2

The amount of hydrogen from the main ROG stream in a refinery may be seen in Table 1. The remaining streams are hydrocarbons ranging from CH4 to C6+, some of which represent a benefit to the refinery and can also be recovered.

The amount of hydrogen in the fuel gas system of a refinery can reach 40%. In this way ,the recovery of hydrogen also contributes to a fuel gas with more kcal/Nm3, which enables better furnace operation.      

Units used to recover hydrogen and LPG
Membrane and pressure swing adsorption (PSA) units are mainly used to recover H2 from the various waste gases in the refinery. Cryogenic recovery is also feasible, but high complexity in the operation of cryogenic units makes them less desirable, unless recovery of light hydrocarbons (ethane, ethylene) is also desired.
Table 2 shows the factors and criteria that affect the selection of one technique or another.

As we can see, many factors influence the decision-making process. Without underestimating other factors, the pressure of the ROG, the percentage of hydrogen it contains, the need for pretreatment, and the pressure of the recovered hydrogen are the most significant.

PSA — adsorption mechanisms
Three mechanisms are used to carry out mixture separations in PSA. They are selective speed, particle size, and selective balance.

In the first case, the force that drives separation is the difference in the rate of adsorption, desorption, and diffusion of the components to be separated. In this way, the flow of the adsorbate inside and outside the adsorbent controls the process.

In the second mechanism, separation of the feed gas is based on the size of the molecular component relative to the pore of the adsorbent. Large molecules are simply excluded from the adsorbent, while small or narrow ones are adsorbed. The pore size is what determines which molecules are adsorbed and which are not.

In the last model, the affinity for the adsorbent or the greater or lesser adsorption force of one component with respect to another is what controls the separation.

To determine the efficiency of a PSA system, in addition to the mechanisms seen, the following aspects, among others, must be taken into account: installation capacity, cycling time, loss of charge, pressure ratio, adsorber geometry, and so on. It is clear that the optimal performance of a PSA does not obey a simple work guide or predetermined recipe.


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