Mercaptans, commonly referred to as thiols, are organosulfur molecules composed of carbon, hydrogen, and sulfur that are known for having a pungent odor similar to rotten cabbage or garlic. In nature, these compounds can be found in living organisms as a waste product of metabolism, and also in oil and gas. While their particular odor is usually a nuisance, it can be used for gas detection purposes. For example, in natural gas, non-corrosive mercaptans are added to it in order to detect it.
The concentration and speciation of naturally occurring mercaptans in oil and gas can vary widely, and with this, the odor intensity and undesired side effects. For example, when these products contain high levels of the lower molecular weight mercaptans, the odor can negatively impact not only the oil and gas facilities but also the surrounding communities. Some examples of undesired side effects are the corrosion of pipelines and the poisoning or life-shortening effects on catalysts and solid adsorption beds, such as silica gel or alumina.
In order to control the mercaptans’ odor and adverse side effects in the oil and gas industry, a government agency usually establishes strict regulations to limit their concentration in the final products. Therefore, if the concentration is high, the mercaptans must be converted to non-odorous/non-corrosive compounds or removed to reduce the value to an acceptable limit.
One method for the removal of mercaptans is by the use of chemical scavengers. Scavengers react with the mercaptan and convert it into another compound that does not have an objectionable odor or corrosion capability. The sulfur atom remains and is incorporated into the reaction product as part of a new functional group.
The removal of mercaptans removal or purification can be accomplished through several methods. A highly efficient method is the use of acrolein as a scavenger compound. This molecule reacts quickly and efficiently with sulfides and mercaptans. However, it is an extremely toxic substance, thereby limiting its use commercially by field and plant operators. A very common method used today to remove mercaptans is reacting them with caustic (sodium hydroxide, NaOH), potash (potassium hydroxide, KOH), or some combination thereof. Additionally, oxidation using strong reagents, such as sodium hypochlorite (NaClO), O2, and hydrogen peroxide (H2O2) are also used. Other alternative methods for the removal of mercaptans are available and include oxidation using ozone, biological removal processes, and catalytic oxidation. The last one can be achieved with the use of a catalyst such as PERCO®, or a solid adsorbent media bed such as SULFURTRAP®. Residual or unreacted mercaptans from these treatment methods retain an odor, but an odor that is not unpleasant, and it doesn’t cause corrosion.
The decision of which chemistry is to be used is based on the sulfur speciation of the hydrocarbon to be treated, as well as the needs of the operator.
PERCO® Catalyst can quantitatively reduce Mercaptans in hydrocarbon streams to meet copper strip and odor specifications.
SULFURTRAP® Adsorbent can reduce both H2S and Mercaptans in hydrocarbon streams.
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