Apr-2007

Hydrogen from refinery offgas

Utilisation of refinery offgases for hydrogen production. Three basic schemes of ROG integration into a steam reformer plant were investigated. These schemes are considered at different hydrogen concentrations

Harald Klein
Linde Engineering Division

Article Summary

In the past, hydrogen containing refinery offgas (ROG) has been routed into the refinery fuel gas system where only the heating value of the gas has been used. Since the hydrogen demand for refinery operations is growing, these gases become more and more attractive as a source for hydrogen production. This requires purification steps such as pressure swing adsorption (PSA) or membrane systems. However, if the ROG hydrogen content is too low, these options become less economical and the direct usage of ROG as steam reformer feed is an attractive alternative. This presentation outlines the key features of the described processes and introduces a guideline for optimised process selection.

The hydrogen content in ROG varies from 5-10 per cent, up to values of
90 per cent. Therefore, their utilisation in the hydrogen production scheme of a refinery must be evaluated on a case-by-case basis. Hydrogen recovery from ROG using PSA, membrane technology or cryogenic processes can be applied
to generate hydrogen streams of any required purity. The offgas streams
from these processes can be routed to the refinery fuel gas system. Another option is using the offgas as (supplementary) feedstock to a steam reformer plant, which also generates hydrogen from hydrocarbons such as natural gas, LPG or naphtha. Three basic schemes of ROG integration into a steam reformer plant were investigated within Linde Engineering. As a basis for all comparisons, a natural gas-based steam reformer plant producing 100,000Nm3/h of pure hydrogen was selected. The purge gas from the steam reformer plant PSA system is routed to the burner system of the steam reformer firing. It was assumed that fuel export to the refinery fuel system was not feasible. ROG containing 80 per cent and 50 per cent hydrogen were investigated.

Common PSA system
In the process scheme configured with a PSA system, the ROG is routed directly to the PSA system of the steam reformer plant. Depending on the pressure of the ROG, it may be required to compress it to the required PSA pressure. However, in the present study it was assumed that ROG is available at a pressure high enough to meet the required PSA pressure, which is usually fixed by the desired hydrogen product pressure.

Due to the additional feed stream to the steam reformer plant PSA system, the purge gas steam from the PSA increases. Since all purge gas must be utilised in the steam reformer firing system, the amount of ROG, which can be routed to the common PSA, is limited. With this type of scheme, it can be shown how the design capacity of the steam reformer can be reduced by increasing the amount of ROG routed to the PSA, which cannot be decreased in size. This process scheme is a very economical option to utilise ROG in a steam reformer plant. However, due to the purge gas increase along with reduced reformer firing, the amount of ROG must be limited to
12 per cent if the ROG contains 80 per cent of hydrogen. If the hydrogen content is 50 per cent the ROG contribution must limited to 8 per cent. The savings of this process scheme for an amortisation time of five years show approximately $1.6 million/yr at 80 per cent hydrogen contribution and approximately $0.5 million/yr at 50 per cent hydrogen contribution. It was assumed that the heat value-based price for the ROG is the same as the natural gas price (1 $/MMBtu).

Direct feed to reformer
If the hydrogen content of the refinery fuel gas becomes as low as 50 per cent, the usage of ROG as direct reformer feed is the preferred option. It can be seen that the reformer size can be reduced by more than 10  per cent such that the savings can add up to a significant value of $4 million. If the hydrogen content of the ROG is as high as 80 per cent, the contribution must be limited to approximately 40 per cent: too much hydrogen is routed to the reformer along with the ROG feed, such that the required reformer heating drops below the available heat supply via purge gas from the PSA system.

Dedicated PSA system for ROG
A dedicated PSA system for the refinery offgas upstream of the steam reformer plant is shown in Figure 1. No upper limitation concerning the contribution of ROG must be considered (Figure 2). However, the option is not economical for refinery offgases showing the lower hydrogen content of 50 per cent resulting in negative savings (Figure 3). The re-compression of the offgas from the ROG PSA requires additional investment as well as a significant amount of electrical compression power. Therefore, the implementation of a dedicated ROG PSA system pays off only if the hydrogen content is as high as 80 per cent and the ROG contribution is above 40 per cent.

Summary
Table 1 summarises the described process option of ROG utilisation in a steam reformer hydrogen plant. The recommendations are rough guidelines, the actual process selection must be evaluated case-by-case, depending on the actual economical figures of the project.