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Recovering sulphur

We are developing methods to recover and reuse the sulphur in hydrogen sulphide (H₂S) from natural gas.

Technology

At our laboratories, we are studying new processes for separating and reusing sulphur, which is often found in natural gas, in the form of hydrogen sulphide. The complete recovery cycle consists of three phases: firstly, the hydrogen sulphide must be separated from natural gas, it is then converted to sulphur and finally, to sulphuric acid (with a yield of about 90%): one of the basic products of the chemical industry. We are testing alternative processes that could be more efficient than the existing ones, for all of these phases. For example, we are assessing the use of ionic liquids for the separation, these are absorbent substances with innovative and more advantageous properties than those currently used for the purification of natural gas. For the next phase of the conversion, we are studying a process called "HydroClaus", which is simpler than the current technologies. Finally, we are studying the use of sulphur to produce polymers through "inverse vulcanisation", which can produce plastic materials with a sulphur content of up to 80% by weight.

Context

H2S (hydrogen sulphide, also known as hydrosulphuric acid) is one of the components of natural gas. With a characteristic smell of rotten eggs, it derives from the bacterial decomposition of organic substances, that’s why it is a molecule that is always present in hydrocarbon formation. Unlike methane and other useful hydrocarbons, hydrogen sulphide is a toxic, corrosive acid gas, so it is essential that it is removed and prevented from being released into the environment. The treatments to do this are, however, complex and expensive, and have a high energy and environmental impact. In the energy industry, H₂S is currently converted to elemental sulphur using the "Claus" process. Among other things, the sulphur market is subject to excess production in relation to demand, which lowers its price. With a view to decarbonisation and the circular economy, we are working on H₂S separation processes that are more efficient and less expensive than the existing ones and on new uses of sulphur, to be used in products with higher added value.

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How to capture and convert H₂S - Energy Transition | Eni Video Channel

Technological challenge

The aim of our research into recovering sulphur is to find new ways of using this element. We are using three technologies to do this, each focused on one phase of the process. For the first step, the separation of hydrogen sulphide from natural gas, we are studying the innovative absorbent properties of ionic liquids. Due to their chemical and physical characteristics, these substances can absorb a greater quantity of acid gas at the same volume and need less energy to be regenerated, therefore less CO2 emissions are generated. In addition, compared to conventional technology, ionic liquids are non-corrosive, do not produce foams or foul-smelling volatile substances, have low toxicity and do not require the subsequent treatment of water contaminated by organic products. For the phase of converting H2S into elemental sulphur, we are using the “HydroClaus” process, which is simpler than the conventional “Claus” process. Here, the advantage comes from the fact that the reaction takes place in an aqueous environment, at low pressure and temperatures, producing a hydrophilic sulphur that is particularly suitable for use in agriculture, as a soil improver or fertiliser. This new product has also been shown to possess herbicide properties, suitable for use in organic crops. For the final phase, use, we are thinking of alternative uses of sulphur, were it becomes a base for producing polymers. Here, the innovation involves a process called “inverse vulcanisation”: whereas classic vulcanisation adds a small dose of sulphur to rubber to strengthen its structure, inverse vulcanisation uses a large amount of this element, along with a small amount of organic component that acts as a cross-linker and stabilises the polymer structure. This enables us to produce a polymer made with up to 80% sulphur.

Industrial integration

For technological projects on recovering sulphur, the goal is to turn a problem into an opportunity, in this case, reusing the pollutant, hydrogen sulphide, as raw material for producing useful, safe and sustainable new products for the market. Given the frequent presence of H₂S in the Upstream fields in which we operate, the management of this compound is very significant for our business. The technologies available for separating and converting it into sulphur have been proven, but our aim is to identify new solutions that will simplify the treatment of acid gases while maintaining safety, efficiency and sustainability, both economic and environmental. In line with the principles of the circular economy, we want to exploit this waste component to create new commercial opportunities. The products we are focusing on are innovative polymers, as well as fertilisers and soil improvers for agriculture. Based on hydrophilic sulphur, the fertilisers and soil improvers are particularly suited to improving saline or alkaline land that is prone to desertification, which is typical in the areas we work in.   Consisting largely of sulphur, the polymers could be used as substitutes for commonly used plastics (thermoplastic and elastomeric polymers), flame retardants and for applications in high energy density batteries.

Environmental impact

Natural gas, a mix of several hydrocarbons among which methane is prevalent, is fundamental to our decarbonisation strategy because it produces about 60% less CO2 than coal when burned. However, before it can be processed, natural gas must first be purified of any polluting substances, like hydrogen sulphide, or H2S, a toxic, corrosive acid gas. Thanks to the technologies we are developing, our treatment plants for acid gases will be less cumbersome and more versatile, as well as more efficient and environmentally sustainable. By focusing on ionic liquids, we can get greater absorption capacity than with traditional amines and therefore reduce the economic and environmental impact of the treatment system. Being able to produce elemental sulphur from hydrogen sulphide, and above all being able to use it for useful new products, makes these technologies even more interesting for the circular economy. Agricultural fertilisers and soil improvers based on hydrophilic sulphur, could make a big contribution to treating saline and excessively alkaline soil, and reclaiming areas where desertification is happening.