Alkali purification is an assisted technology to be used by R&P Co. Grasys as a final desulfurization unit to remove residual content, most particularly mercaptans from gas that has previously been purified from hydrogen sulfide using liquid-phase hydrogen sulfide oxidation technology based on iron chelate aqueous solutions.
The liquid-phase desulfurization plant with alkali post-treatment unit is manufactured and supplied as process units in shipping dimensions, skid-mounted or in a container configuration (subject to the compressor and pump equipment placement solution), highly prefabricated.
R&P Co. Grasys liquid-phase desulfurization plant is not typical but subject to development in view of the Customer's specifications set forth in Terms of Reference.
Alkali Purification Process Description
The technology is based on alkali and mercaptans reaction resulting in mercaptides formation as follows:
RSH + NaOH → RSNa +H2O
The reaction is reversible: when solution is heated, mercaptides are destroyed releasing alkali and mercaptans, which enables to perform alkali recovery and recycle. In this case, mercaptans can be disposed of by combustion together with exhaust air produced at the iron chelate regeneration stage of the liquid-phase oxidation plant with ambient oxygen, with coolant generation for heating and process needs.
Furthermore, the gas is post-treated from hydrogen sulfide and carbon dioxide using the following reactions:
H2S + NaOH → NaHS + H2O
CO2 + 2NaOH → Na2CO3 + H2O
Na2CO3 + H2S → NaHS + NaHCO3
The process is performed at indoor temperature.
Mercaptan purification degree depends on their absorption capacity; the higher absorption temperature and molecular mercaptan weight is, the lower is their solubility.
Alkali solution flowrate as well as alkali concentration in such solution are determined by the total mercaptan content and single compound content of the mercaptans (thiols) homologous series in feed gas.
A rough reagent flowrate: 1 kg of dry matter / 1 kg of removable mercaptans.
Merits of Using Alkali Post-Treatment Unit
- Small size and low flowrate of reagent and water due to using alkali desulfurization unit for post-treatment of gas that has previously been purified from hydrogen sulfide by liquid-phase oxidation
- Meeting all treated gas requirements in terms of residual content of sulfur-containing impurities
- The plant is plain in terms of design and operation, similar in design and performance to the liquid phase oxidation plant, which is instrumental in enhancing reagent flowrate and keeping liquid waste to a minimum
- Easy customization to any operating conditions, including fluctuations in flowrate, pressure, component composition of the purified gas (except for₂)
- Low operational and maintenance expenses
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