Part Two of a five part series.
Feed gas analysis (AT1)
Gas streams entering the sulfur recovery unit (SRU) are often referred to as "SRU feed gas" and are complex as they can include many different components at varying concentrations. It should be remembered that the feed streams entering the SRU are a result of prior processes – stripping and hydrotreating. As such, they can be impacted by changes in the plant feedstocks and process upsets, resulting in significant changes to their make-up. Hydrogen sulfide (H2S), carbon dioxide (CO2), ammonia (NH3) and ‘hydrocarbons’ (THC) are the most common constituents, with H2S having the highest expected concentration.
In the reaction furnace, the SRU feed gas is mixed with air or pure oxygen and heated to produce sulfur dioxide (SO2). This is shown in reaction 1 below. The SO2 will then react with H2S in the converters and elemental sulfur will be produced, see reaction 2 below.
Modified Claus reaction (heat)
3H2S + 3/2O2 ---> SO2 + 2H2S + H2O (1)
SO2 + 2H2S ---> 3/x SX + 2H2O (2)
Historically, SRU operators attempted to maintain a strict 2:1 ratio of H2S:SO2 to ensure reliable operation and high SX recovery. With different licensors and users constantly modifying their equipment and processes, the actual H2S:SO2 ratio may be different than 2:1. In any case, designers and users do strive to maintain a proper ratio of H2S:SO2 throughout the converters and condensers. A process engineer can see how important it is to control the flow of feedstock into the SRU or the amount of air/oxygen injected to maintain a proper ratio. Experienced SRU engineers know that management of feedstock and oxygen flow rate is not too difficult when the incoming components and concentrations are consistent and known. Issues arise when the feedstock begins to vary.
When hydrocarbon concentrations are suddenly changing, the impact on the SRU can be significant. The hydrocarbons consume more of the available oxygen, reducing the amount of H2S being converted to SO2 in the reaction furnace. From Figure 1 and Table 1, it can be seen that the amount of oxygen being introduced to the reaction furnace needs to increase as more hydrocarbons enter the reaction furnace, and reduced as the hydrocarbons "go away."
Figure 1. The sudden increase in hydrocarbons entering the SRU results in an increase in H2S after the final condenser. SO2 concentrations would decline and the desired ratio would not be met.
|
Compound
|
Moles O2 per Mole HC
|
Ratio of O2 needed per mole HC compared to per mole H2S
|
|
Methane
|
2
|
4
|
|
Ethane
|
3.5
|
7
|
|
Propane
|
5
|
10
|
|
Butane
|
|
13
|
|
Pentane
|
8
|
16
|
|
Hexane
|
9.5
|
19
|
Table 1. Hydrocarbons "steal" the O2 intended for H2S to SO2 conversion
Some SRU professionals have said that problems arise not when the hydrocarbons come in, but rather when the hydrocarbons suddenly go away. What they have experienced is that the excess oxygen being introduced into the reaction furnace results in excess SO
2 formation. This results in SO
2 breakthrough to the TGTU, causing damage to the amine found in TGTU absorber. Costs of US$40 – 50/liter are common for specialized amines, so damages not only result in possible plant shutdown or TGTU bypass (higher SO
2 and H
2S emissions will need to be reported), but also increased operational expense.
Conversely, SRUs that have extremely efficient converters and condensers, or lower recovery requirements, may not have a TGTU present. For these applications, end users have noted that a sudden increase in hydrocarbons requires operators to get on the air in the incinerator (thermal oxidizer) to prevent a temperature condition at the emission stack.
AMETEK has a dual analytical bench analyzer that is specifically designed to measure the incoming SRU feed gas components and their concentrations. The IPS-4 analyzer utilizes both an ultraviolet (UV) and infrared (IR) bench to measure in real time the components and concentrations, and houses a fit for purpose sample conditioning system in one enclosure. Coupled with a double block heated acid gas (HAG) probe, the IPS-4 Feed Forward solution has been installed at SRUs around the world.
Part One - Introduction
Part Three - Tail Gas/Air Demand Analyzer
Part Four - Tail Gas Treatment
Part Five - Continuous Emission Monitoring Systems
Learn more about the IPS-4 analyzer.