Bojanna Shantheyanda, Dynalene Inc | Email: [email protected]
Ricardo Gonzalez, Gill Ranch Storage LLC
Salt-rich water infusion from underground storage to the dehydration system during gas withdrawal can increase the salt contamination in the triethylene glycol (TEG). The increase in salt contamination in TEG can lead to an increase in corrosion of the metal piping, create hot spots in the fire tubes and decrease the moisture removal efficiency of the dehydration system. These contaminants can be easily and economically removed from the TEG fluid using ion exchange based Contamination Removal System (CRS). This case study discusses the results of a contamination removal project carried out at a natural gas storage station. The report also includes discussion about the operational and economical benefits of using the ion exchange method for contamination removal.
Natural gas is usually stored underground, in large storage reservoirs. There are three main types of underground storage: depleted gas reservoirs, aquifers, and salt caverns. Though most of the storage in the United States is a depleted gas reservoir, the ability to perform several withdrawal and injection cycles each year provide salt caverns a unique advantage over rest of the storage techniques .
Natural gas, when withdrawn from the underground storage reservoirs, contains moisture; therefore, TEG dehydration towers are used to remove moisture from the natural gas. The moisture in the natural gas contains earthly salts and minerals such as chlorides, silica, sulfate, iron, and others. These salts and the minerals remain in glycol after water is removed through reboiling. Over time, the concentration of the salts increases in the glycol fluid leading to several efficiencies and technical related issues. Higher concentration of chloride in the TEG can cause corrosion of the equipment, create hot spots on the fire tube, increase glycol degradation, increase chances of clogging, lower pH and decrease the dehydration efficiency, leading to periodic glycol change-out .
The increase in salt concentration in the TEG can be predominantly noticed in salt cavern based natural gas storage stations. Storage sites also experience an increase in salt concentration in the TEG during an unexpected increase in moisture infusion during withdrawal. The moisture may travel all the way to the dehydration system due to malfunctioning of the slug catcher and coalition filters.
Ion exchange process can be used to remove the ionic salt contaminants and bring the contaminant level below the acceptable level. In an ion exchange process, a chemical reaction is used to remove the dissolved salt ions in the TEG and the unwanted ions in the fluid is replaced with similarly charged ions from ion exchange media. Due to simple operational technique, the ion exchange based glycol reclamation can be carried out while the plant is operational and does not require handling of the caustic solution.
3. Contamination removal project at Gill Ranch Storage, CA
The project was carried out at Gill Ranch Storage facility, which is a wholly owned by NW Natural and PG&E Corporation. The storage is an underground intra-state facility near Fresno, California and has the capacity to provide approximately 20 billion cubic feet (Bcf) of underground natural gas storage.
Table 1: lists of fluid parameters and site conditions during and before the project