Jul-2007

Elimination of refinery sour gases and trace components

Case study concerning elimination of sour gases and trace components from gasification gases. The objective is to produce pure hydrogen for hydrotreating within a refinery, and a feed gas for energy production in an IGCC

Hans-Peter Kaballo, Ulvi Kerestecioglu and Harald Klein
Linde Engineering Division, Linde AG

Article Summary

The proprietary Rectisol wash process removes H2S/COS and CO2 from coal, asphalt, pitch or oil-derived synthesis gas. It is a physical gas wash system, using methanol as a solvent at operating temperatures below the water freezing point, to produce a synthesis gas with less than 0.1 vppm total sulphur. The CO2 content can be adjusted from several mol% down to a few ppm, as required by the specified application.

Its main advantages are the use of cheap and readily available methanol as a solvent, with a flexible process configuration, and low utility consumption compared with other wash processes, such as polyethylene glycol ether (PEGE)-based process or chemical washes. A modern concept of a Rectisol unit is described for treating shifted and unshifted gases in just one plant:
—   Shifted gas was used for hydrotreating in a refinery
—   Unshifted gas was used as fuel gas for power generation in an integrated gasification combined cycle (IGCC)
—   CO2 of the unshifted feed gas was only partially removed because the remaining CO2 was fed as inert gas together with the fuel gas to an IGCC
—    All sulphur compounds of both feed gases were concentrated in one stream with a high H2S concentration
—    Harmful impurities such as NH3, HCN or metal carbonyls were nearly eliminated.

The physical acid gas-removal process uses methanol (technical grade “A”) at operating temperatures below the water freezing point, as previously mentioned. By means of the process, it is possible to produce a purified synthesis gas with less than 0.1 vppm total sulphur (H2S plus COS) and an adjustable CO2 content of several mol% down to a few ppm. No additional sulphur guard beds (upstream synthesis catalysts) are necessary. Sulphur compounds and CO2 removed from the syngas can be concentrated in separate fractions. Table 1 shows the typical product specifications achievable with the process.

Basic principles
Chemical and physical wash processes are the two principal methods for removing H2S/COS and CO2 from a gas mixture. In a chemical wash, H2S and CO2 react with the alkaline compounds dissolved in the medium (eg, amines). The amount of circulated wash liquor, which is the dominant parameter for the investment and operating cost, is proportional to the amount of sour gas to be washed out and almost independent of the partial pressure. Normally, H2S and CO2 are removed simultaneously.

In physical wash processes, the different physical solubilities of gases in a solvent are the decisive factor. The minimum required solvent flow rate is almost independent of the concentration of the gas component to be removed from the feed gas. It is reverse-proportional with the feed gas pressure. This means the wash medium flow and, accordingly, the overall plant costs increase as the pressure decreases. In other words, the process economics improve with increasing gas pressure.

The solubility coefficients (m) of various components, as a function of the temperature, are shown for ambient pressure in Figure 1. The sour gas compounds H2S, COS and CO2 have much higher solubility coefficients than the valuable syngas components H2 and CO. This means methanol as the washing agent has a high selectivity for the removal of H2S/COS and CO2. This selectivity keeps the losses of valuable syngas components to a minimum. Figure 1 shows an increase in the solubility coefficients of the sour gas compounds with decreasing temperatures. Therefore, Rectisol wash columns are usually operated at low temperature of -35°C or -60°C, which requires external refrigeration. Another advantage is being able to decide which components of the raw gas, and to what extent, should be washed out. This can be done by adjusting the solvent flow according to the solubility coefficient of the gas component to be removed. Thus, it is possible to design a selective removal system.

As shown in the principal Rectisol flow scheme in Figure 2, the plant consists of an absorption section for the removal of sour gas compounds and a desorption section for the regeneration of lean solvent. The absorber column is operated at a high pressure and low temperatures, in which sour gas compounds (H2S/COS and CO2) are removed by methanol. The treated gas is routed to the downstream process units. It is absolutely free of water and impurities. To recover the valuable product compounds, such as H2 and CO, the loaded methanol is first depressurised to flash small amounts of the co-absorbed components. This flash gas is routed back to the feed gas line to recover the products. Due to a further pressure reduction, CO2 is flashed from the loaded methanol in the H2S enrichment column. Nitrogen is used as a stripping gas to improve the reduction of CO2 in the methanol. The flashed CO2 is routed to battery limit as CO2 product, while the stripped CO2 (including stripping N2) is vented to the atmosphere as tail gas. Methanol from the sump of the H2S enrichment column is routed to the hot regeneration column. The H2S and remaining CO2 are stripped by means of methanol steam generated in the reboiler. The H2S fraction is routed to battery limit (eg, as Claus gas to a Claus plant). Lean methanol is collected from the bottom of the column, cooled down and routed back to the absorber column as regenerated washing agent.

These wash units are used worldwide for the purification of hydrogen, ammonia and methanol syngases, and for the production of pure carbon monoxide (CO) or oxogases. As previously shown, the Rectisol process is operated at very low temperatures. Therefore, a combination of this process with a downstream cryogenic separation system (eg, cold box) is highly efficient when it comes to utility consumption.

For the acid gas removal of shifted syngas, emphasis has been placed on the one-stage Rectisol design. In this design, H2S and CO2 are removed selectively in different sections of the same wash column, with the shift conversion step upstream from the Rectisol wash. A sulphur-tolerant (sour) shift catalyst has to be used, which requires a minimum concentration of H2S to maintain the activity. Pressures of up to 80 bar are typical. As the one-stage unit requires only one wash column, the savings in required investment are obvious.

For some applications, these units are designed to treat a shifted gas as well as an unshifted gas. This is performed in two separate wash columns with one common regeneration system. A selective or simultaneous removal of sulphur components and CO2 is possible, depending on the specific project conditions.