Claus Sulfur Recovery Unit (SRU) Calculator
Design refinery and natural gas Claus sulfur plants. Calculate reaction furnace thermal conversion, 2-stage/3-stage catalytic bed recovery, molten sulfur production rate (metric tons/day), combustion air demand, and waste heat boiler steam generation.
1. Acid Gas Feed Composition
2. Operating Parameters & Stages
Sulfur Recovery Yield & Plant Performance
Combustion Air & Mass Balance
Stage-by-Stage Sulfur Production (t/d)
-- t/d
-- t/d
-- t/d
Interactive Claus Sulfur Recovery Unit (SRU) Process Flow Diagram
Complete schematic showing thermal reaction furnace, waste heat boiler (HP steam), 1st sulfur condenser, catalytic converters with interstage reheaters, and liquid sulfur collection pit.
In-Depth Process Chemistry: The Modified Claus Reaction Mechanism
The modified Claus process converts toxic hydrogen sulfide ($H_2S$) into elemental liquid sulfur via a dual-stage thermal and catalytic sequence:
1. Thermal Stage: Reaction Furnace ($T > 1000^circ ext{C}$)
Exactly one-third of the total entering $H_2S$ is oxidized to sulfur dioxide ($SO_2$) with controlled combustion air:
$$ ext{Claus Reaction:} quad 2 H_2S + SO_2 ightleftharpoons rac{3}{2} S_2 + 2 H_2O quad ( ext{Thermal endothermic at } > 1000^circ ext{C})$$
Approximately 60% to 70% of the inlet sulfur is converted directly into elemental sulfur in the furnace and condensed out in the 1st sulfur condenser after the Waste Heat Boiler (WHB).
2. Catalytic Stage: Claus Equilibrium Beds ($200^circ ext{C} - 320^circ ext{C}$)
The remaining unreacted $H_2S$ and $SO_2$ in an exact $2:1$ ratio pass over activated alumina or titanium dioxide catalysts. Lower temperatures favor the exothermic condensation of sulfur vapor ($S_6, S_8$):
$$ ext{Hydrolysis of side-products:} quad COS + H_2O ightleftharpoons H_2S + CO_2$$
$$CS_2 + 2 H_2O ightleftharpoons 2 H_2S + CO_2$$
Overall recovery across 2 catalytic stages reaches 95–96%, and across 3 catalytic stages reaches 97–98.5%. The remaining sulfur passes to a Tail Gas Treating Unit (TGTU) to achieve 99.8%+ environmental compliance.
5 Fatal Engineering Pitfalls in Claus Sulfur Plants
Operating a catalytic converter below the sulfur dew point ($< 180^circ ext{C}$) causes liquid sulfur to condense directly inside catalyst micro-pores. Liquid sulfur completely caps active alumina sites, reducing catalytic activity to zero and creating a massive solid sulfur cake that requires an emergency high-temperature heat soak to desorb.
When treating lean acid gas or sour water stripper gas containing ammonia (NH₃), reaction furnace flame temperature must exceed 1250°C. If furnace temperature drops below 1150°C, ammonia slips unburned through the WHB, reacting with SO₂ to form solid ammonium salts (sulfites/bisulfates). These salts clog downstream condenser tubes within hours, causing sudden plant shutdowns.
Claus catalytic conversion relies strictly on maintaining an exact $2:1$ ratio of $H_2S$ to $SO_2$ in the process gas entering catalytic converters. A mere 1% drift in combustion air flow shifts the ratio away from stoichiometry ($2.0$), causing tail gas emissions to spike and cutting overall sulfur recovery by 3% to 6%. Continuous feedback trim control from tail gas UV analyzers is mandatory.
The inlet tube sheet of the Waste Heat Boiler (WHB) is exposed to 1200°C furnace gas on one side and 250°C boiling water on the other. Ceramic ferrule inserts protect the tube sheet from catastrophic thermal fatigue. If ferrules crack or slip out of place, direct 1200°C gas impingement causes instant tube-to-tubesheet weld failure and boiler water blowout into the furnace.
Liquid sulfur in rundown lines and concrete storage pits continuously degasses trapped toxic and explosive $H_2S$ (and hydrogen polysulfides $H_2S_x$). If pit sweep air fails, $H_2S$ vapor exceeds the lower explosive limit (4.3 vol%). Reaction with rust forms pyrophoric iron sulfide ($FeS$), which auto-ignites upon air ingress, causing massive underground sulfur pit explosions.