THE EFFECTS OF USING H2S SCAVENGER UPSTREAM OF GAS PROCESSING EQUIPMENT THE EFFECTS OF INJECTING EXCESSIVE METHANOL UPSTREAM OF GAS PROCESSING EQUIPMENT

摘要

When designing a high H2S content gas processing facility which utilizes an amine plant and a dehydration unit,several additional processes may be added upstream to mitigate issues with operation in cold ambient conditions and/or exceeding regulatory emission permits.The two main processes added to combat these circumstances are methanol injection and liquid H2S scavenger.The processes are typically implemented separately but can be used together; for this paper,both will be looked at independently.These processes solve the issues mentioned previously but can possibly lead to unforeseen consequences and costs associated with the operation of downstream amine plants and dehydration units. Methanol is injected into pipelines and upstream of pressure-reducing items(e.g.flow control valves,pressure control valve,etc.)to lower the freezing point of water,preventing ice plugs and hydrate formation in colder ambient conditions.In frequent cases,methanol is injected at quantities greater than needed and is seen as a"the more,the better"chemical.However,as oxygen is highly soluble in methanol,it can be carried into the pipeline and eventually into the amine unit.There it can potentially react adversely with H2S or the amine to form elemental sulfur,bicine,acetate,formate and heat stable amine salts. Liquid H2S scavenger is sometimes used to remove some or all the H2S upstream of an amine plant to curb emissions at the facility.The amine plant can then remove the C02 with a minimal amount of H2S in the acid gas stream.In some situations,to decrease the amount of scavenger consumed,the product stream from the scavenger vessel will be treated to a ppm H2S content above zero.This leads to a side reaction between the unreacted H2S and scavenger biproduct to form an amorphous solid.Further downstream,this solid can increase filter consumption and cause plugging of internal tower components.This plugging can affect process viability and lead to the high cost of a shutdown to clean out the tower. Both process scenarios were observed and recorded for several months at two facilities operating in the Eagle Ford Basin.Several mitigation techniques were brainstormed and then implemented to reduce the negative impact these upstream processes had on the amine and dehydration units.For the sites that had excessive methanol injection,the techniques proposed were: 1.Discontinue methanol injection and add valve heaters and heat tracing to susceptible areas of the process. 2.Inject a calculated amount of methanol and not to excess. 3.Implement a different hydrate inhibitor.4.Keep methanol tanks enclosed with nitrogen blanket on free space. 5.Analyze amine samples monthly during the winter to coincide with controlling methanol injection to ensure bicine,acetate,and formate levels do not increase drastically The techniques implemented for the sites that had upstream H2S scavenger were: 6.Ensure that the H2S is being treated to 0 ppm out of the scavenger system 7.Increase the filter efficiency(decreasing to 0.3 or 0.1 micron)downstream of the scavenger system to ensure particle pick-up. Upstream methanol injection and scavenger processes are important for adequate conditioning and operation of sour gas facilities,and if the effects of these processes are known and mitigated correctly,they can also be cost-effective.