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Sulfur Recovery Unit (SRU) - Analyzers

sulfur recovery unit plant diagram

Overview

Hydrogen sulfide (H2S) is a byproduct of processing natural gas and high-sulfur crude oil. At low concentrations, H2S has an odor much like rotten eggs. H2S is heavier than air, and exposures to H2S with a concentration greater than 500 ppmv can be fatal. This extremely dangerous byproduct must be closely monitored and the amount of H2S released to the atmosphere must be minimized.

A Sulfur Recovery Plant serves to convert hydrogen sulfide (H2S) in an incoming gas stream into elemental sulfur. Sulfur recovery plants are required to recover 95 to 99.99% of the total sulfur that is introduced into the plant. Emissions regulations are becoming more stringent with each year that passes, which is why high-quality industrial analysis equipment in Sulfur Recovery Plants is of major importance.

AT1
SOUR GAS

measuring H2S in sour gas in sulfur recovery unit sru plant

Incoming “sour gas” (moderate H2S concentration, low CO2 concentration) is sent to an amine treatment process (absorber). The “sour gas” is run through an absorption tower containing an aqueous amine solution. The amine selectively absorbs the H2S from the gas stream, which results in a treated “sweet gas”. To achieve efficient control of the amine unit cycle time, plant operators must know the concentration of the H2S in the “sour gas”. This analyzer is usually used in conjunction with the analyzer at AT2.

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AT2
SWEET GAS

measuring H2S in sweet gas in sulfur recovery unit sru plant

The gas leaving the absorber is known as “sweet gas” due to its low concentration of H2S and other sulfur-containing compounds. The “sweet gas” must be monitored in order to ensure that the amine treatment process is running efficiently. This analyzer is usually used in conjunction with the analyzer AT1 to control the amine unit cycle time. H2S concentration in the “sweet gas” must be below a certain level in order for the gas to be considered “sweet” (usually below 4 ppmv).

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AT3
ACID GAS

measuring H2S, CO2, Hydrocarbons in acid gas in sulfur recovery unit sru plant

This analyzer is used as a feed forward analysis system to measure the level of H2S in the “acid gas” (high H2S concentration) before it reaches the Claus process. The feed forward analysis allows for the preemptive adjustment of air demand to the furnace in the Claus process based on the real-time level of H2S in the feed gas. While tail gas analysis (AT6) provides the most accurate air demand calculation, this measurement occurs after the furnace. Feed forward analysis allows for air demand control with no process lag by immediately detecting sudden changes in acid feed gas composition and preventing any losses in Claus process efficiency. CO2 (from “sour gas” feed) is also analyzed here in order to keep side reactions inside the Claus process involving CO2 to a minimum.

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AT4
TAIL GAS/AIR DEMAND

measuring H2S, SO2, COS, CS2, tail gas, air demand in sulfur recovery unit sru plant, in Claus, Euroclaus and Superclaus

The effluent leaving the Claus process is termed “tail” gas. The level of H2S and SO2 must be monitored in order to ensure that the Claus process is running efficiently, and to provide an accurate air demand calculation. A set H2S/SO2 ratio must be maintained in the Claus reaction to efficiently convert H2S in the acid gas stream into elemental sulfur. The H2S/SO2 ratio is maintained by monitoring the tail gas stream, and outputting an air demand signal to the Claus process furnace (more air means less H2S in the tail gas and more SO2). Additionally, operators sometimes require online measurement of COS and CS2 due to side reactions in the reactor.

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AT5
SULFUR PIT

measuring H2S, SO2 in sulfur pit in sulfur recovery unit sru plant

The liquid elemental sulfur that is produced in the Claus process is directed into a sulfur storage pit. This stream of sulfur can contain upwards of 300 ppmw of dissolved H2S. Over time, the dissolved H2S will off-gas into the headspace of the sulfur storage pit, creating an equilibrium between the gas and the liquid. The H2S that accumulates in the sulfur storage pit must be monitored because H2S has a lower explosive limit of 4% by volume, and can ignite when in contact with mechanical equipment such as pumps or compressors.

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AT6
TAIL GAS TREATMENT

measuring H2S, H2 in tail gas treatment unit TGTU in sulfur recovery unit sru plant

The Sulfur Cleanup process receives tail gas from the Claus process, and converts the small amount of sulfur compounds which were not converted there (usually less than 5%) into H2S using a catalytic hydrogenation reduction stage and an amine absorber. The treated gas is sent to the incinerator and the rich amine is recycled back to the amine stripper. H2S and H2 must be monitored in order to validate the reduction reaction taking place in the Sulfur Cleanup process. In addition, the H2S measurement is also used to identify the sulfur load heading to the amine absorber.

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