Caustic Scrubbing for high concentrations of H2S with mercaptans is shown to be an effective and economic pretreatment for H2S removal. Final treatment of the low concentration H2S exit stream can then be made using other technology which is more efficient at low H2S concentrations.
Caustic scrubbing has not normally been used in gas streams which contain large amounts of CO2 due to the reaction with caustic which interferes somewhat with the absorption of the H2S and thus reduces the reaction rate. It is shown that caustic scrubbing can be used in streams containing as much as 85% by weight of CO2 if the residence time is marginally increased. In the commercial design it is shown that caustic scrubbing can be used to pretreat high H2S with mercaptans gas streams to approximately 250 ppm total sulfur. Final treatment can be accomplished using a nonhazardous, nonregenerative product.
The field test results show that the reaction is first order in H2S concentration. The reaction time with Na2CO3/NaHCO3 is shown to be about 80% of that with NaOH which indicates that extending the contact time is beneficial for optimum reduction of H2S in high CO2 content gases.
Many commercially available processes for the removal of H2S have their limitations. Some are limited by the level of H2S or CO2 in the gas streams. Others do not adequately remove mercaptans. Still others are not suitable because of their complexity of operation, initial capital investment and/or high maintenance costs. The challenge is to match the process or the binary process with the local conditions and constraints.
The binary process discussed in this paper involve two major parts:
Pretreatment with caustic scrubbing and
Final treatment with a nonhazardous, nonregenerative product.
It has met the challenge by being reliable, flexible and cost effective.
A process developed by Heitz and Rocklin proposed the removal of H2S simply by reaction with a caustic solution. Hohlfeld further described this process. The essential concept of the process is that the reaction between NaOH and H2S in solution to form NaSH is faster than the reaction between NaOH and CO2. They conclude that the reaction contact time is held at 0.02 seconds or less there will be little CO2 absorbed and the reaction with H2S will be more efficient. These conclusions were obtained for systems that were relatively low in CO2. For example, Heitz and Rocklin used gases where the flows of CO2 and H2S were equal. Hohlfeld only shows a single CO2 absorption curve even though the concentration of CO2 in the gas phase strongly affects the absorption. The range of data that Hohlfeld uses seems consistent with the 1% CO2 limit he mentions.
As stated in the referenced R.W. Hohlfeld paper, removal of H2S from sour gas can be achieved from gas streams of virtually any level of H2S concentration. As one would expect, it is much easier to remove a mole of H2S from a gas containing 20,000 ppm of H2S than it would be to remove that same mole from a gas containing 200 ppm H2S. It simply is easier for a mole of caustic to find the H2S when the H2S is more concentrated. Field tests results shown on Fig. 1 confirmed that the costs per pound of sulfur removed went up significantly as the inlet H2S concentration went down. In turn, the tests concluded that an alternate process for the final gas treatment stages was desirable.
Scrubber Chemistry. The present design calls for the treatment of a sour gas with CO2 concentration of 40-70 mole percent (up to 85% by weight CO2). Recently the new design was placed in service after several years of testing and working with a 4-stage prototype. In the final design only two stages are used. The prototype system was used to identify the key reaction parameters.
The basic scrubber contacts the sour gas with liquid containing either NaOH or NaOH/NaHCO3/Na2CO3 through a series of stages. The temperature rises to about 100 F due to the exothermic nature of the reactions and the pressure is about 90 psig. The contact time in each stage is on the order of 0.02 second. The general configuration is shown in Fig. 2.
The chemistry in the aqueous phase between CO2, H2O, H2S and NaOH can become quite complex. P. 203^