Adsorption Systems
A second important group of gas treatment processes are based on the adsorption of impurities onto a solid carrier bed. Some of these processes, such as molecular sieve driers or pressure swing, allow in situ regeneration of the bed. Others, such as H2S chemisorption onto zinc oxide, cannot be regenerated economically in situ, and the beds require regular exchange.
The quantity of a gaseous component, which can be carried by any particular adsorbent, depends not only on the characteristics of component and sorbent but also on the temperature and pressure under which it takes place. This increase in loading capacity with higher pressures and lower temperatures is illustrated in Figure 8-10 and is utilized for the in situ regeneration of such sorbents as activated carbon, activated alumina, silica gel, and molecular sieves.
The classic adsorption-desorption cycle uses both the temperature and pressure effect “swinging” between high pressure and low temperature for adsorption (point 1 in Figure 8-10) and low pressure and high temperature (point 2) for desorption. The differential loading (Lj — L2) is extremely high. The pressure swing cycle operates at constant temperature T0 between points 3 and 4. A temperature swing process operating at constant pressure between points 1 and 5 is possible but unusual in practice.
о! < і Q!
Molecular Sieves
The most common application of molecular sieves in connection with gasification plants is the removal of water and C02 upstream of cryogenic units. Processes working at cryogenic temperatures, such as air separation or cryogenic gas separation, require a feed gas completely free of these components, which would otherwise freeze and deposit on the inlet heat exchangers and finally block them.
The classic cycle described above is usually employed. In air separation duty, water and C02 are not the only considerations. The prepurification unit also prevents the ingress of hydrocarbons into the cold box as a safety measure. Recently the ingress of NOx into the cold box has also become an issue of concern. For air separation. a combination of molecular sieve and silica gel is often used.
Pressure Swing Adsorption
Pressure swing adsorption (PSA) operates on an isothermal cycle, adsorbing at high pressure and desorbing at low pressure. The principle application of PSA is for hydrogen purification, although there are a number of others including air separation (see Section 8-1).
The optimum pressure for hydrogen purification lies in the range 15-30 bar. At higher pressures the hydrogen yield falls off, a point to be considered when integrating a hydrogen off-take from a gasification plant optimized for a different application.
The hydrogen yield of a modern PSA unit usually lies between 80% and 92%. Apart from the matter of pressure already mentioned, other influences are the quality
|
Table 8-3 Relative Strength of Adsorption of Typical Impurities
Source: Miller and Stoecker 1989 |
of the feed gas (the higher the quantity of impurities to be removed, the more hydrogen is lost with them) and the tail gas pressure. Where the tail gas is burned in dedicated burners, as for instance in a steam reformer hydrogen plant, the typical tail gas pressure is 0.3-0.4 bar gauge. Where the tail gas pressure is higher (e. g., 3-5 bar gauge), the drop in hydrogen yield can become very significant.
Additionally, the hydrogen purity can affect the yield, though only to a small degree. Typical purities range from 99 to 99.999 mol%. An additional common hydrogen specification is a limit on the amounts of carbon oxides (CO and C02). Levels of 0.1 to lOppmv are easily achieved. In the design of an overall gasification — to-hydrogen system, it is useful to have an idea about the performance of likely impurities in the PSA unit. A comparison of a number of components is shown in Table 8-3. In this connection it is important to note that although water is strongly adsorbed and so will not contaminate the product. It is disadvantageous to have large quantities in the feed gas since this requires excessively large beds. Usually, cooling to below 40°C with subsequent condensate separation is sufficient to provide an economic design.
A further design consideration is the number of adsorber vessels. Early plants used four beds, as is still the practice on smaller plants. Larger modern plants use as many as twelve adsorbers. Sophisticated cycles have been developed to minimize the loss of hydrogen on depressurization from the adsorption step to the desorption step, by using this hydrogen to repressurize a bed that has just completed its desorption step. Thus there can be a trade-off between a higher investment for an increased number of vessels (and valves) and operating savings from an increased hydrogen yield.
Adsorption of H2S onto zinc oxide is an effective method for removing trace quantities of sulfur from gas to achieve a purity of less than O. lppmv, as is required by copper or nickel catalysts. It is therefore the standard method of desulfurization upstream of natural gas steam reformers. The adsorption takes place via the reaction of hydrogen sulfide with zinc oxide to form zinc sulfide. In situ regeneration is not possible, and this places a limitation on the amount of sulfur that the process can accept in the inlet gas.
There are two generally accepted designs for zinc oxide desulfurization units. In a guard bed function or where the sulfur load is low, a single bed is provided, sized to adsorb the total quantity of sulfur to be expected between planned turnarounds, say one or two years. Where the sulfur load is higher and a single bed would become unmanageably large, a two-vessel series arrangement is provided and provision is made for exchanging the adsorbent online. With this arrangement, the individual bed can be sized smaller, such as for a six-month interval between bed replacement.
Zinc oxide can adsorb sulfur present as H2S almost completely. Performance with other sulfur compounds (COS, mercaptans) is not as good. In cases where sulfur is present other than as H2S, it is necessary to hydrogenate these components to H2S upstream of the zinc-oxide bed. This is normally done over a cobalt-molybdenum (CoMox) or nickel-molybdenum (NiMox) catalyst.
Zinc oxide adsorption is essentially a process for polishing or guard bed duty. This becomes clear when considering a zinc-oxide bed for the carbon monoxide plant described in Section 7.1.4. Operating in its optimum temperature range of 350 to 400°C, zinc oxide has a pick-up capacity of around 20% by weight. Assuming a sulfur content of 100 ppmv in the natural gas, the total sulfur intake is about 10 tons/ year, requiring replacement of about 50 tons/year zinc oxide. Compare this with the nearly 30t/d sulfur intake of the lOOOt/d methanol plant of Section 7.1.2, and the limitations become very apparent.
Given these numbers, zinc oxide in the gasification environment is limited either to guard bed duty, for example, upstream of a low temperature shift or methanator catalyst or to natural gas feeds. As discussed in Section 7.1.4, there are arguments for desulfurizing either upstream or downstream of the partial oxidation reactor.
Where extreme sulfur cleanliness is required, copper oxide can be used for final desulfurization down to lOppbv. Commercial adsorbents are available for this purpose, either in a mixed ZnO/CuO formulation or as a separate polishing bed.