Zinc oxide reacts with hydrogen sulphide according to the following equation:
ZnO + H2S  ZnS + H2O (This is a reverse reaction)
Based upon detailed analyses of catalysts downstream the sulphur absorption section, It is
Topsoe’s experience that a system of hydrogenation followed by zinc oxide absorbers can bring
the sulphur concentration down to a level around 0.005 vol ppm. In order to achieve this,
Topsoe has developed a series of zinc oxide absorbents, which provide a very high absorption
capacity for all applications.
When a zinc oxide bed has been in operation for some time, the axial distribution of sulphur in
the bed will ideally have a sulphur profile as illustrated in the graphic below:
Bulk saturation
In top of Zone 1, the zinc oxide is bulk saturated and completely converted into zinc sulphide.
Solid state diffusion
Below the saturated zinc oxide there is a layer which has not been completely saturated, and
the rate of absorption will in this layer be determined by the rate of diffusion in the solid state
from the surface into the interior of the material.
Gas diffusion
Further down in the bed, the absorption rate is mainly determined by gas diffusion restriction in
the pores (Zone 2). The gas diffusion is a much faster process giving a sharper declining profile.
This part of the bed is normally referred to as the “front”.
Chemisorption
Zinc oxide further has the ability to chemisorb hydrogen sulphur on the active surface area.
Small quantities of hydrogen sulphide passing the front will consequently be removed by
chemisorptions (Zone 3). In practice, this means that the hydrogen sulphide slip may be lower
than the zinc oxide/ hydrogen sulphide equilibrium. However, the chemisorptions capacity is
very low compared to the bulk absorption capacity of zinc oxide, so in most cases the slip of
hydrogen sulphide below the equilibrium may be sustained only during a fraction of the entire
lifetime of the zinc oxide charge.
For more details about this part, find them in the link below:
http://www.topsoe.com/business_areas/synthesis_gas/Processes/~/media/PDF%20files/Feed_
purification/Topsoe_feed_purification_cat.ashx
Topsoe offers a series of zinc oxide absorbents designated HTZ-3, HTZ-4 and HTZ-5. The HTZ
Series has made Topsoe a leading supplier of zinc oxide absorbents to the industry providing
very high absorption capacity for all applications.
HTZ-3 is standard sulphur absorbent used for removal of hydrogen sulphide and to some extent
also certain organic sulphur components depending on operating temperature. HTZ-3 is
characterized by its very high purity (> 99% zinc oxide), which together with a high bulk density
ensures an absorption capacity up to 510 kg/m3 (31.5 IBS/CUFT). As a result of the high density
and purity, HTZ-3 typically provides 20-30% more zinc oxide for a given volume purchased than
other commercial zinc oxide products corresponding to a 20-30% longer lifetime operating at
the same conditions.
HTZ-3 is recommended for operating in the temperature range 300-400 0C (570-750 0F) and for
gases containing more than a few ppm sulphur.
For the details of HTZ-4 and HTZ-5, find them in the PDF.
http://www.topsoe.com/business_areas/synthesis_gas/Processes/~/media/PDF%20files/Feed_
purification/Topsoe_feed_purification_cat_htz.ashx
Pressure Range
The operating pressure for the HTZ absorbents is determined by the reformer and is typically in
the range of 10-50 kg/cm2 (150-750 psig).
Topsoe’s ST-101 is developed for final purification of sulphur in hydrocarbon feedstocks. ST -101
is based on the combined action of copper and zinc oxide with a very high surface area and will
remove hydrogen sulphide as well as organic sulphur compounds which may pass through the
conventional desulphurization section. ST -101 will pick-up sulphur even in the ansence of
hydrogen recycle. ST -101should placed as a bottom layer in the final sulphur absorption
reactor. The optimum temperature range is 150-360 0C (300-680 0F) but it can operate down to
80 0C (175 0F).
When cleaning up the ppb levels of sulphur leaking through the zinc oxide, the absorption
capacity is 1 wt%, but if exposed to several ppm of sulphur, ST-101 will be saturated to a level of
as much as 18 wt%.
At certain conditions as much as 100ppb of hydrogen sulphide may leak through hot zinc oxide
due to the equilibrium conditions. When ST-101 is placed downstream the zinc oxide absorbent,
the sulphur leakage can be reduced to a few ppb.
More details about, find the link below:
http://www.topsoe.com/business_areas/synthesis_gas/Processes/~/media/PDF%20files/Feed_
purification/Topsoe_feed_purification_cat_st%20101.ashx