A few sweet tricks for sour gas
Large volumes of acid gas (H2S and SO2-rich gas) are generated from various technologies used to sweeten sour off-gases, or acid gas.
Garrett Palmquist, Jeannie Branzaru, Steve Puricelli, and Yves Herssens
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Disposing of this acid gas in an environmentally responsible manner presents a significant challenge. Technologies typically used for off-gas treating are often viewed as a mandatory expense (non-revenue generating). The most common acid gas treatment solution is the Claus-based Sulphur Recovery Unit (SRU) in which elemental sulphur is produced from large concentrations of H2S off-gas.
This paper explores all of the available technology options for treatment of sour off-gases, including additions to a Claus SRU to improve acid gas removal efficiency. Key factors driving the technology selection shall be discussed, such as capital and operating costs, disposal of the resulting sulphur products (sulphur, sulphuric acid, and sulphate effluent), technology limitations, acid gas removal efficiency, and reliability.
The processing of sour oil and gas generates large volumes of sour gas (H2S and other sulphur component–containing gas) in order to meet the required sulphur specifications of the external markets. Disposing of this sour gas in a cost-effective and environmentally responsible manner presents a significant challenge. Processing facilities view the technologies utilised and the by-products generated as mandatory expenses contributing very little to the operation’s bottom line.
This paper discusses the by-product and technology options available for the treatment of sour gas, with a specific focus on several novel solutions and improvements to traditional technology options. Key issues to consider when selecting technology are presented, such as:
• Minimising capital expenditures while providing the flexibility to obtain very low sulphur emissions under a wide range of operating conditions by utilising a simple caustic scrubber;
• Overcoming H2S concentration limitations by processing acid gas in an efficient and reliable sulphuric acid plant; and
• Achieving ultra-low emissions while improving the performance of the base Claus SRU plant by utilising regenerative SO2 technology.
Sour oil and gas processing
For the purposes of this paper, the operating goals of oil and gas processing facilities are simplified into three categories:
1. Operate the facility continuously and reliably
2. Economically create products meeting the market’s quality standards
3. Sell the products to the market
Numerous configurations exist for oil refineries and gas production facilities, depending on the quality of the feed stocks and desired product mix (see Figures 1 and 2 for typical production facility flow diagrams). The importance of unit operations dealing with sour oil and gas within these facilities has increased as more unconventional oil and gas fields are brought into production, and as the product quality standards become more stringent. Hydroprocessing steps (hydrotreating) separate unwanted components from oil, resulting in sour gas comprised of hydrocarbons, H2S and other undesired components. Further separation and concentration of this sour gas occurs in an Amine Treatment Unit (ATU), which is also used in natural gas processing to remove undesirable components from the raw gas feed. The resulting concentrated acid gas stream (H2S-rich) along with off gas from the site’s sour water stripper (SWS) must be rejected in an environmentally acceptable manner.
Figure 31 shows the typical process steps for disposal of sour gas.
A Claus SRU process (Figure 42) converts the sulphur components in the amine acid gas and SWS acid gas into elemental sulphur, with the sulphur recovery efficiency and final gaseous sulphur emissions determined by local environmental regulations. By itself, the Claus SRU achieves up to about 97% sulphur recovery in a three stage system; well below typical environmental requirements of >99.9% sulphur recovery. Typically, one of many different Tail Gas Treating Unit (TGTU) technologies or a modification to the Claus SRU process itself ensures compliance with environmental regulations, allowing the overall facility to continue operating. Specific characteristics of the acid gas feed may require these modifications as well, if treatment by a Claus SRU alone is insufficient.
Improvements to the overall sulphur recovery of the facility generally classify into one of three categories: SCOT TGTU, direct oxidation or extended catalytic (EC) TGTU, and sub dew point Claus. Figure 53 shows the flow scheme of a SCOT-type TGTU process, which converts all remaining sulphur components to H2S, absorbs the resulting H2S from the tail gas using a solvent system, incinerates the residual sulphur components, and releases the clean tail gas to atmosphere. Regenerated solvent returns to the absorber, while the concentrated H2S recycles back to the Claus unit. In this configuration, sulphur recovery can be increased to >99.9%, with very low sulphur emissions.
Regardless of the technology choice, each addresses specific shortcomings of the Claus SRU process, such as:
• Increasing sulphur recovery from 99% to >99.9%
• Processing weaker H2S feed streams (<20 vol%) without significant modification to the upstream ATU
• Handling additional or changing components in the sour gas, such as ammonia, mercaptans, hydrocarbons, and carbon dioxide
• Improving reliability
Alternatives to traditional sulphur recovery methods
In addition to the traditional sulphur recovery methods described above, numerous alternatives exist in the market, several of which warrant a closer look as solutions for sour off gas treatment to address the overall facility’s goals.
Wet Gas Scrubbing
Wet gas scrubbing is a popular method for tail gas cleaning. Installed as a supplemental or standalone TGTU, the MECS® DynaWave® wet gas scrubbing technology enhances plant reliability and flexibility while achieving ultra-low sulphur emissions. The DynaWave technology minimises capital cost and plot area while generating a liquid sulphate effluent as a by-product. Two DynaWave applications are considered here, as a supplement to another upstream TGTU technology (Figure 6), or as the sole TGTU unit for small Claus applications (Figure 7).
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