Sulphur Recovery: A Game-Changer for the Energy Industry

As the world grapples with the challenges of climate change, industrial pollution control has become a critical concern. One of the most significant pollutants in industrial processes is hydrogen sulfide (H2S), a toxic and flammable gas that can have devastating environmental and health impacts.

The Problem of Hydrogen Sulfide Pollution

Hydrogen sulfide is a byproduct of various industrial processes, including oil refining, natural gas processing, and chemical manufacturing. When released into the atmosphere, H2S can react with oxygen to form sulfur dioxide (SO2), a potent greenhouse gas that contributes to acid rain, air pollution, and climate change.

The Solution: Sulphur Recovery

One of the most effective ways to mitigate H2S pollution is through sulphur recovery, a process that converts H2S into elemental sulphur. This process not only reduces the environmental impact of H2S but also produces a valuable commodity that can be used in various industries, including agriculture, pharmaceuticals, and construction.

The Process Flow Diagram

Here is a simplified process flow diagram (PFD) that illustrates the sulphur recovery process:

                         

                                     

                                     

 From Toxic to Treasure: The Art of Sulphur Recovery

                                     

Process Conditions and Operation Procedure

The sulphur recovery process involves several unit operations, including scrubbing, oxidation, reduction, condensation, and storage. The process conditions and operation procedure are critical to ensuring safe and efficient operation.

Block diagram of sulfur recovery process

Oxidation

2H2S + 3O2 ↔ 2SO2 + 2H2O   -247.89 Kcal

 Reduction

4H2S + 2SO2 ↔ S6 + 4H2O       -42.24 Kcal 

The above reactions are exothermic in nature. On the surface of the bauxite catalyst, the reaction is carried forward yielding about 70-80% conversion.

Benefits of Sulphur Recovery

The benefits of sulphur recovery are numerous:

- Reduced environmental impact: Sulphur recovery reduces the amount of H2S released into the atmosphere, minimizing the formation of SO2 and other pollutants.

- Valuable commodity: Elemental sulphur is a valuable commodity that can be used in various industries.

- Improved process safety: Sulphur recovery reduces the risk of H2S-related accidents and improves process safety.

- Compliance with regulations: Sulphur recovery helps industries comply with increasingly stringent environmental regulations.

Sulphur recovery is a critical process that can help industries reduce their environmental footprint, improve process safety, and comply with regulations. By investing in sulphur recovery technologies, industries can turn a pollutant into a valuable commodity, contributing to a more sustainable future.

Diesel Hydro-Desulphurisation Process Plant with Capacity 1MMTPA

Various process options used for desulphurisation technology:

1. SYN/SAT technology
2. Unisol
3. Hydro Desulphurization (HDS)

Diesel Hydro desulphurization (HDS) Process:

The purpose of HDS unit is :

1. To perform desulfurization of the reactor feed. The specification to meet is 500ppm wt.
2. The hydrogenation of eventual olefin structures (achieved easily using a special catalyst )

Main chemical reaction mechanisms that influence the process:
1. Refining reactions.
2. Hydrogenation reactions.

(a). Refining reactions

Desulfurisation: Mercaptans, sulfides and disulphide easily react, leading to the corresponding saturated or aromatic compounds. Sulphur combined into cycles of aromatic structure, like thiophene is more difficult to eliminate. These reactions lead to the H2S formation and hydrogen consumption.

Denitrification: The denitrification reaction rate is lower than that of desulphurization. it occurs mainly in case of heterocyclic compounds having an aromatic structure(pyridine for example). These reactions lead to NH3formation and H2 consumption.

(b). Hydrogenation reactions :

• These reactions affect the Diolefins, olefins and aromatics and are highly exothermic. Diolefins and olefins are converted into saturated compounds.

• The hydrogenation rate of aromatics is limited.

Process Descriptive Flow sheet of Diesel Hydrodesulphurisation Plant

• The feed to the DHDS (Diesel Hydro Desulphurisation section) unit is a blend of straight run and cracked gas oils. The feed blend is filtered through feed filter package and sent to feed surge drum. The pressure in the feed surge drum is maintained by fuel gas blanketing.

• The liquid phase feed is pumped under flow control by a feed pump, mixed with the hydrogen recycle compressor delivery stream and let in the heat exchanger train. This mixing of recycle hydrogen with feed ensures an adequate hydrogen partial pressure at the inlet of the reactor train. Polymerization inhibitor is injected.

• The hydrogen make-up gas coming from B/L is routed through chlorine adsorbent and is then compressed by the compressor. The part of the gas is sent as quench gas to the reactor. The mixed stream (feed + recycle hydrogen) is heated in first feed/effluent heat exchanger then in the second feed/effluent exchanger and finally in the reactor heater to the required reactor inlet temperature.

• The reactor inlet temperature is maintained by the controlling the fuel gas/fuel oil to the heater burners. The stream is then let in the reactor which includes a catalyst in three beds.

• Cold quenches of hydrogen coming from recycling compressor are added at the inlet of each new bed to control the bed inlet temperature.

• The reactor effluent is cooled in the heat exchangers. To avoid ammonium salt deposits and risk of corrosion, water is injected at the inlet of the effluent cooled.This mixture is collected in the cold separator where three phases are separated.

• The sour water containing ammonium salts is partly recycled to the washing.

• The gas phase from cold separator goes to amine absorber where H2S is removed. In the amine absorber, the gas is washed by a 25%wt DEA solution. The lean DEA solution is pumped by the DEA booster pump to the top of the absorber. The rich DEA solution is withdrawn from the bottom of the absorber to be fed to the regeneration section. The gas from the absorber contains hydrogen which is recycled back to the reactor.

• The hydrocarbon liquid is withdrawn from the separator and sent to storage which is our product.

Predesulphurization of Raw Naphtha Process Plant Flow Sheet

Predesulphurization of Naphtha process description:

Hydrogenation and stripping are the primary processes to remove bulk sulphur present in naphtha. Sulphur is a harmful component in raw naphtha when used as primary stock in ammonia production. Sulphur is treated as an impurity which has a huge impact on the production cost of ammonia. It is a poisoning element for steam naphtha reforming catalyst it should be removed from raw naphtha. The process involved in the removal of sulphur impurity is called as Pre-desulphurization since another final desulphurization system is been established in present process ammonia manufacturing plants along with PDS because of the availability of raw naphtha containing sulphur about 1500 ppm which is not desirable to present technology catalyst. This fraction is reduced to 10 to 5 ppm in Pre-desulphurization, and final desulphurization is used to absorb the remaining traces of sulphur.

Unit operation equipment used in this process are:

• Deaerator
• Recycle compressor
• Feed effluent heat exchangers
• Fired heater
• Hydrogenator (fixed packed bed reactor)
• Effluent separator
• Naphtha stripper 

The raw naphtha is passed through deaerator to remove oxygen content dissolved in the liquid naphtha using natural gas or naphtha vapors, this operation will prevent the risk of gum formation in heat exchange equipment. Stripped off-gas from deaerator is used as a fuel and the liquid naphtha from deaerator is pumped through a series of effluent heat exchangers to vaporize and then to the superheated fired heater. Sulphur present in the form of mercaptan, thioether, and disulphide is made to react with hydrogen and form hydrogen sulphide at Hydrogenator.The reaction is exothermic; heat obtained is used to preheat the inlet feed to fired heater before the reactor to attain the reaction temperature. Hydrogen gas is added at the compressor section to the liquid stream of deaerated naphtha. The mixture at 40 kg/sq.cm pressure is passed to the effluent heater and vaporized to a temperature of 300degC. This temperature is raised to 380 deg C by the fired heater and passed to the top side of fixed packed bed catalytic Hydrogenation reactor. Most of the sulphur is reacted with hydrogen as per equation 

Where R, is a radical of hydrocarbon.

Process flowsheet of predesulphurization plant

 The reaction mixture is cooled to separate the naphtha and hydrogenated gas. Liquid naphtha which is cooled is fed to a stripper to strip out dissolved water and hydrogen sulphide. Stripper feed preheater is used to preheat the feed naphtha by the naphtha stream obtained from the bottom of the stripper which is at a temperature about 200-250 deg C. Stripper is distillation column having around 20 trays and operates at 10kg/sq cm. Temperatures are maintained around 200 to 130 deg C at the bottom and 130 to 90 deg C at the top depending on feedstock. Off gases are used as fuel and naphtha obtained at the bottom is called sweet naphtha. Sweet naphtha should contain less than 15ppm of total sulphur.