Lec 6: Sulfur Production: Claus Process ​

Inorganic Chemical Industries: Production of Elemental Sulfur

Overview of the Module

• The lecture introduces the second module focusing on inorganic chemical industries, specifically sulfur and sulfuric acid production.

• Previous discussions included sources of sulfur and methods for producing sulfuric acid using the double contact process.

Sources of Sulfur Dioxide

• The primary source of sulfur dioxide is elemental sulfur, which is often produced from hydrogen sulfide (H2S).

• Elemental sulfur can exist in various forms (liquid, solid, vapor), with its structure denoted as S_n where n ranges from 1 to 8.

Hydrogen Sulfide as a Key Component

• The lecture emphasizes the importance of hydrogen sulfide as a raw material for producing elemental sulfur.

• It discusses two main processes for recovering sulfur: SCOT (Shell Claus Oil Treating Process) and SuperClaus process, both improving upon traditional Claus methods.

Acid Gas Treatment

• Definitions are provided distinguishing between acid gas and sour gas; acid gas includes components like carbon dioxide (CO2), hydrogen sulfide (H2S), and carbonyl sulfide (COS).

• Isoelectronic compounds such as CS2, CO2, and COS are discussed regarding their structural similarities despite differing elements.

Natural Gas Impurities

• Sour natural gas contains impurities like CO2 and H2S that must be removed before use; these impurities can serve as sources for elemental sulfur.

• Off-gases from hydrotreating units also contribute to H2S production; desulfurization processes yield H2S alongside linear alkanes through catalytic reactions.

Separation Techniques for Hydrogen Sulfide

• Various plant units generate H2S; it can be separated using chemical absorbents like alkanolamines or physical solvents such as methanol or propylene carbonate.

Chemical Solvents and Their Role in Hydrolysis

Importance of Chemical Solvents

• Chemical solvents should have high solubility in water, with common solutions ranging from 15% to 30%. This is crucial for hydrolyzing compounds like carbonyl sulfide (COS) into hydrogen sulfide (H2S).

• Ideal alkanolamines must be low volatile, reactive at a low cost, and flexible in design and operation.

Sources of Elemental Sulfur

• Elemental sulfur primarily originates from three sources: natural gas, oil refining units, and coal gasification. These are by-products that contribute to the formation of elemental sulfur.

Pre-treatment Process

• Pre-treatment is necessary to remove minor amounts of carbonyl sulfide present in gas streams to prevent adverse effects on subsequent processes.

• Catalytic hydrolysis converts carbonyl sulfide into hydrogen sulfide using water, resulting in CO2 as a by-product.

Acid Gas vs. Sour Gas

• The terms "acid gas" and "sour gas" are often used interchangeably; however, sour gas specifically refers to H2S content while acid gas includes both H2S and CO2.

Acid Gas Treatment Using Alkanolamines

Chemical Solvent Functionality

• Common chemical solvents include monoethanolamine (MEA), diethanolamine (DEA), or methyl-diethanolamine (MDEA), which exhibit high mass transfer rates for separating H2S from acid gases.

Absorption and Regeneration Cycle

• In the absorption process, acid gases pass through an alkanolamine solution where they are absorbed. The remaining purified gas can then be incinerated.

• The rich alkanolamine solution is heated before being sent to a stripper or regenerator where low-pressure steam separates H2S and CO2.

Claus Process for Sulfur Recovery

Overview of the Claus Process

• The Claus process involves oxidizing hydrogen sulfide to produce elemental sulfur and water but faces challenges due to high enthalpy leading to limited conversion rates (50%-60%).

Reaction Steps

• To achieve higher conversion rates (95%-97%), partial oxidation is employed instead of complete oxidation. Limited air or oxygen is introduced during the reaction.

• The first step combustes one-third of hydrogen sulfide into sulfur dioxide and water with a significant energy release (-529 kJ/mol).

Understanding the Claus Process

The Need for a Two-Step Reaction

• The Claus process involves two steps to optimize the conversion of H2S to sulfur, primarily due to temperature's significant impact on reaction efficiency.

• In the second step, residual H2S reacts with sulfur dioxide (SO2) to form elemental sulfur, necessitating an excess of H2S for maximum conversion.

Temperature Considerations

• Optimal furnace temperatures must be maintained between 200°C and 300°C; too low (<925°C) can hinder reaction rates while too high (>600°C) risks generating harmful byproducts like SOx and NOx.

• The first reaction is non-catalytic and irreversible, favoring high oxidation rates at elevated temperatures which thermodynamically favors SO2 formation.

Conversion Dynamics

• Research indicates that lower reaction temperatures enhance conversion percentages from H2S to sulfur but must remain above the sulfur dew point to prevent catalyst deactivation.

• If reactor temperatures drop below 500 K, liquid sulfur may condense on catalysts, leading to deactivation.

Hydrolysis Requirements

• Hydrolysis of carbonyl sulfide (COS) and carbon disulfide (CS2), crucial in catalytic stages, requires temperatures exceeding 230°C for effective conversion.

• Typical operating temperatures in Claus reactors range from 230°C to 250°C for optimal COS and CS2 conversion.

Reactor Configuration and Reactions

• The overall Claus process consists of two main reactions: partial combustion of H2S into SO2 followed by its reaction with remaining H2S to produce elemental sulfur and water.

• This second catalytic reaction is reversible and less exothermic than the first; thus, higher temperatures shift equilibrium towards reactants, reducing sulfur recovery.

Importance of Catalysts

• The Claus process relies heavily on catalysts that must possess high mechanical strength, activity at lower temperatures, cost-effectiveness, and resistance to attrition in humid conditions.

Catalysts in the Claus Process

Overview of Catalysts Used

• Both titania and vanadium pentoxide are highly reactive for converting hydrogen sulfide (H2S), but their limited lifespan restricts their use. Alumina-based catalysts are predominantly utilized in model plants.

Categories of Claus Catalysts

• The Claus catalyst can be categorized into three types: standard aluminas, Claus alumina with ultra-macro porosity, and aluminas promoted with alkaline or alkaline earth oxides like sodium oxide.

Role of Modifiers in Catalysts

• Certain modifiers such as sodium oxide, calcium oxide, magnesium oxide, iron, or nickel sulfur titania enhance the resistance of alumina to sulfation during reactions involving sulfur dioxide (SO2).

Importance of Resistance to Sulfation

• Modifiers like calcium oxide and magnesium oxide are added to improve the resistance of alumina against sulfation and cooking reactions that could degrade catalyst performance.

Summary of Catalyst Types

• The three categories discussed include:

• Alumina-based catalysts.

• Alumina with promoters.

• Non-promoted or calcium-promoted titania-based catalysts.

Understanding the Claus Process

Initial Reaction Setup

• The process begins in a burner where H2S reacts with oxygen to produce SO2 and water (H2O), releasing heat that is used to generate high-pressure steam.

Controlling Reaction Ratios

• The conversion ratio of H2S to SO2 can be controlled by adjusting the flow rate of air entering the burner, which influences reaction efficiency.

Complexity of Reactions Involved

• Multiple reactions occur simultaneously during the process; one example includes carbonyl sulfide reacting with water to form additional H2S and CO2.

Condensation and Sulfur Collection

• In the Claus reactor, H2S reacts with SO2 to produce elemental sulfur. The products condense at lower temperatures, allowing liquid sulfur to be collected in a sulfur pit.

Multi-Reactor System Efficiency

• A series of reactors ensures high conversion rates; while initial conversions yield about 50–60% efficiency, overall processes achieve up to 98% conversion through subsequent reactors.

Key Points on Tail Gas Management

Importance of Tail Gas Composition

• Effective management requires maintaining an optimal H2S to SO2 ratio (ideally 2:1). Deviations may lead to increased tail gas emissions beyond permissible limits.

Global Production Insights

Sulfur Recovery Processes: SCOT and Super Claus

Overview of Sulfur Recovery Techniques

• The Claus plant's tail gas contains about 5% sulfur, necessitating further treatment for improved sulfur recovery, targeting close to 200%.

• Shell has developed the Shell Claus Off-Gas Treating (SCOT) process, which achieves a sulfur recovery rate of 95-98%. This process includes a selective oxidation reactor in its final phase.

SCOT Process Details

• The SCOT process utilizes reducing gas (CO + H2 or syngas), produced from incomplete combustion, to facilitate reactions within the system.

• In the SCOT reactor, gases are converted primarily into H2S and CO2 through combustion processes. Other components like COS and CS2 are also processed here.

Absorption and Regeneration Steps

• After conversion in the SCOT reactor, gases are cooled down; any formed sulfur is removed before entering the absorber-regenerator section.

• An amine solution (ethanolamine or alkanolamine) is used in an absorption tower to selectively react with H2S while allowing CO2 to escape.

Regeneration Process

• In the regeneration section, low-pressure steam strips H2S from the amine solution. The lean amine then transfers heat back to incoming feed from the SCOT reactor.

• This cycle allows for nearly complete recovery of H2S and sulfur at rates close to 98%.

Comparison with Super Claus Process

• The Super Claus Process operates similarly but incorporates a reaction furnace and waste heat boiler. It maintains higher operational temperatures throughout its reactors.

• A key difference lies in controlling H2S concentration (0.8% - 3%) using airflow adjustments before it enters a selective oxidation reactor that converts it into elemental sulfur.

Efficiency Insights

Sulfur Recovery Processes

Overview of Small Scale Sulfur Production

• The discussion focuses on small-scale sulfur production processes, specifically those producing less than 10 tons per day.

• Unlike the Claus process, the gas flow rate and composition in these processes are not constant, making them less applicable for certain recovery methods.

• Despite their limitations, these processes are considered more economical compared to the Claus process.

• A catalyst is introduced into the amine solution to facilitate the direct conversion of H₂S into elemental sulfur.

Reaction Mechanism

• The primary reaction discussed is: 2 H₂S + O₂ → Elemental Sulfur + Water.

• Due to lower output flow rates, a Vanadium-based catalyst is utilized in Liquid Redox Sulfur Recovery systems.

SCOT Process Advantages

• The SCOT (Sulfur Compound Oxidation Technology) plants demonstrate superior sulfur recovery compared to other discussed processes.

• This superiority arises because other sulfur compounds like carbon disulfide (CS₂) and carbonyl sulfide (COS) can be converted back into hydrogen sulfide (H₂S), enhancing overall efficiency.

• In contrast, the Super Claus Process does not achieve this level of conversion, leading to potential waste in the Sulfur Recovery Unit.