Zinc oxide desulfurization Adsorbent

Eliminating hydrogen sulfide via ZnO adsorbent

There is a percentage of hydrogen sulfide in natural gas, which can adversely affect the performance of catalysts loaded in the reactor during the production of chemical fertilizers and other industries, and reduce the activity of catalysts during operation; since it is easily adsorbed by nickel metal and occupies the active sites of the catalyst. Hence, so as to eliminate hydrogen sulfide from the input feed in the industry, some measures must be taken. You should keep in mind that large amounts are produced in gas and oil industries. Hydrogen sulfide, is a colorless and odorless gas that is very dangerous, toxic and corrosive. When digging oil and gas wells, this gas can penetrate drilling fluids, so, to maintain the health of digging workers, preventing the corrosion of pipelines, equipment and reducing the environmental pollution, this dangerous gas must be removed. The odor of 0.002 mg/lit H2S in fresh air is completely perceptible and it is the main source of air pollution. However, the human sense of smell gets tired quickly and may not warn off to higher concentrations. This will lead to coma and even death by respiratory failure. Based on international environmental regulations, H2S in acid gases must be eliminated before releasing into the atmosphere.

There are several chemical methods for eliminating hydrogen sulfide such as phosphate methods, iron oxide, hydroxide, absorbing by activated carbon, molecular screening and zinc oxide method. Among all these methods, using ZnO adsorbent to eliminate sulfur compounds, especially H2S, has been more effective. After the reaction, ZnO nanoparticles can be recycled and reused without losing their appearance making this process cost-effective and therefore environmentally friendly.

Smart Catalyst; producer of zinc oxide  (ZnO) adsorbent

Zinc oxide adsorbent consists of a mixture of ZnO and alumina as a binder (binder, adhesive). ZnO has a high amounts of zinc metal (80%) and its reaction kinetics and adsorption capacity are well predictable. This adsorbent is considered as a =n appropriate candidate, since it is easily available compared with other adsorbents such as molecular screenings or zinc-titanium oxide. The absorption of H2S by ZnO (reaction of zinc oxide with hydrogen sulfide) during the desulfurization process is actually controlled by the following reaction, producing an inert insoluble zinc sulfide.

ZnO + H₂S → ZnS + H₂O

ZnO nanoparticles have a large surface area and high surface activity. These important key features provide a wide range of potential usages in various industries. The amount of H2S elimination by zinc oxide is higher than iron oxide. This is due to thermodynamic issues in sulfidation. The efficiency and performance of the catalyst depends on the chemical reaction between the catalyst and hydrogen sulfide to form zinc sulfide. Besides the amount of hydrogen sulfide absorbed by zinc oxide depends on its amount in the feed and the amount substrate containing ZnO.

It is worth mentioning that desulfurization method of zinc oxide (hollow window shape of the catalyst) has advantages over the traditional method, as the holes in the surface of the catalyst absorb sulfur from inside and outside simultaneously without affecting the power of the catalyst.

The most commonly methods of producing zinc oxide adsorbent

Various methods have been used to produce ZnO adsorbent , but the most common methods are thermal and chemical ones.

• Thermal methods

The thermal method involves the combustion of zinc metal with air, in which ZnO is obtained with an area of ​​10 square meters per gram. Khamas et al. prepared a zinc oxide adsorbent from heat-treated zinc oxide, then mixed ZnO with 95% purity with alumina and water and formed it. Subsequently, dried it at the temperature of 150-130 ° C and calciante it for 2 hours at the temperature of 700 ° C. after performing all these procedures, its performance was about 90%.

• Chemical method

Here, the chemical method is actually the sedimentary method. ZnO adsorbent is formed by the deposition of zinc from its salts (zinc salt) and then drying and calcinating of the sediments carried out at a temperature of 300-350 ° C.

Disadvantages of zinc oxide adsorbent

• Reducing ZnO in the procedure (for instance in the process of producing synthesis gas). Cause, vaporization of zinc at the temperatures above 600 ° C, leading to loss of adsorbent and frequently reducing removal capacity.
• Formation of sulfate during the desulfurization process

To fulfill this aim, doping zinc with metals such as Co, Mn and Cu can be used to increase its stability and performance. Besides, adopting Al2O3 or TiO2 bases can be used for achieving stability. So as to prevent the formation of sulfate, water vapor contact with ZnS should be averted, because when the oxygen which is required for this reaction is supplied from water vapor, it will form sulfate.