Caustic Treating – GT-Treat™ for Sulfur Removal
GT-TREAT technology utilizes GTC’s proprietary high contact mass transfer trays to improve the removal of sulfur impurities found in gas and liquid streams when they are contacted with a caustic solution. GT-TREAT will efficiently remove hydrogen sulfide (H2S), carbonyl sulfide (COS), sulfur dioxide (SO2) and mercaptans (RSH) to meet today’s low sulfur specifications.
The process can be used in gas streams with low or no CO2 in it since CO2 will consume sodium hydroxide (NaOH) and make it uneconomical. Natural gas, associated gas and gases in refineries containing H2S and RSH can be caustic treated to remove sulfur impurities. In liquid streams, Propane, Butane, LPG, NGL, pentane and light naphtha streams can be easily treated in a GT-TREAT process to remove all of the previously mentioned sulfur impurities.
The treating process consists of a reactor where COS, H2S and RSH are extracted with a circulating caustic solution. Depending on the amounts of H2S and COS present in the feed stream, the reactor may include a pre-wash section to hydrolyze the COS and remove the H2S and CO2 formed or present in the feed stream. An in-line mixer and caustic circulation will be utilized to increase the contact of the two phases and extract the impurities out of the hydrocarbon phase.
The reactor will utilize high contact mass transfer trays to extract the acidic impurities from the hydrocarbon phase to the aqueous phase and, once in the aqueous phase, the acidic impurities will react with NaOH to form RSNa (sodium mercaptides), Na2S, Na2CO3 and Na2SO3. The spent caustic solution out of the GT-TREAT reactor is sent to a caustic regenerator (GT-RENEWTM) to regenerate the caustic solution before it is returned back to the reactor to remove more sulfur impurities. Depending on the amounts of H2S and COS present in the feed stream, a small split stream can be withdrawn out of the treating system and fresh caustic added to keep the concentration of NaOH above a certain level. The reactor in the GT-TREAT process is designed in such a way that a low caustic entrainment can be guaranteed without the use of a water wash unit and/or a sand filter.
Caustic Treating Process Advantages
- Guaranteed minimum caustic carryover.
- High utilization of caustic solution and high efficiency of impurities extraction due to the high performance contacting trays.
- Ability to operate the treating reactor over a wide range of hydrocarbon throughputs, and hydrocarbon-to-caustic ratios for both operating and treating flexibility.
- Excellent turndown and overdesign operating capabilities.
- Minimal plot space required. One reactor will contain several ideal stages in one vertical reactor. Other technologies utilize co-current treating packed filters (one ideal stage) making the unit cumbersome and increasing the plot space requirements.
- Ability to maintain 100% service factor with scheduled turnarounds every five years.
- Simplicity of operation.
- Low capital investment.
- Lower operating costs than other processes where high caustic circulation is needed.