Alternative chemicals separation technologies are worthwhile investments

Improved approaches to chemical separations, if applied to the US petroleum, chemical and paper manufacturing sectors alone, could save $ 4 billion in annual energy costs and 100 million tonnes of CO2 emissions. Improvements in membranes and sorbents technologies have the potential to impact the industry. They could make 80 percent of separations methods ten times more energy efficient and open up endless opportunities in the chemical industry. Three areas could benefit from such improvements - hydrocarbons extraction, olefins purification, and benzene derivatives separation.

Hydrocarbons extraction 

Hydrocarbons are the main ingredients for production of fuels, polymers and plastics. Global refineries process around 90 million barrels of crude (2 liter per person) per day, and consume about 230 Gigawatts (equivalent to the UK energy consumption). Modern distillation to extract hydrocarbons from crude oil involves pumping oil through pipes in hot furnaces and separating light hydrocarbon molecules from heavy ones in downstream distillation towers. These are the tall, narrow columns that give refineries their distinctive skylines. The use of membranes based separation technology can, in a number of circumstances, accomplish separations with considerable less energy (by an order of magnitude) than other methods. 

Finding an alternative to distillation has been difficult because crude oil contains many complex molecules with various viscosities, some impurities, such as sulfur, and metals. Separation processes involving membranes allow one or more species to pass through in preference to others. Active research is focusing on enhancing their efficiency in separating many types of molecules and their life time during operation at high temperatures to keep heavy oils flowing without becoming blocked by contaminants. Others are exploring the possibility of combining membrane separation and distillation, membrane processes performing part of the separation.

Olefins purification

Olefins such as ethylene and propylene are important building blocks for a large number of chemicals. Global annual production of ethylene and propylene exceeds 200 million tonnes, which represents 30 kilograms per person. Ethylene is produced from natural gas and crude oil by cracking. It is separated from a gas mixture containing ethane at high pressures and temperatures. Generally, olefins separation from the associated paraffins (ethane and propane) is performed in a distillation column. Separation of propylene and ethylene is among the most expensive industrial separations, alone accounting for 0.3 percent of global energy use, roughly equivalent to Singapore’s annual energy consumption. 

Like in the case of crude oil, finding separation systems that do not require changes from one phase to another could reduce by a factor of ten the energy intensity of the process, and offset carbon emissions by a similar amount. Researchers have been exploring the use of membranes. Polymeric membranes are limited by the selectivity of the polymers. Inorganic membranes, such as carbon membranes, are also being explored to separate gaseous olefins at room temperature and moderate pressures. But, they have not achieved the effectiveness that would provide more than 99% pure olefins. High pressure sorbent technologies for olefins - paraffin separations are also being explored, but they have not been scalable either.

Benzene derivatives separation

Benzene, a cyclic hydrocarbon, is a key component in the chemical industry. Its derivatives, such as toluene, ethylbenzene and xylene isomers, are all separated in distillation columns, consuming globally 50 Gigawatts, enough to power 40 million US homes. One important isomer, p-xylene, is used in the production of polyethylene terephthalate (PET), which is used as polyester fiber, film, and resin for a variety of applications. Production of p-xylene is a significant portion of the US chemical industry’s annual output. There is an increased pressure to produce p-xylene with purities of 99.9 percent or more, instead of the typical purity of 99.5 to 99.7 percent. Separating xylene isomers is expensive because of their very close boiling points.

Improved processes are needed to reduce energy consumption in the purification of p-Xylene. Advances in membranes and sorbents technologies could help to reduce the energy required in these processes. Researchers are exploring inorganic membranes that will separate p-xylene more efficiently than by conventional methods. Membrane separation of p-xylene would require over 75 percent less energy than current technology. While UOP has pioneered the principle of adsorption separation in industrial scale for the production of p-xylene, scientific literature reports numerous patents and periodical articles on the use of several principles to separate xylene isomers. 

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Dr Mosongo Moukwa is director of technology at PolyOne, USA, and was recently an independent consultant based in Chapel Hill, USA, and vice president - technology at Asian Paints Ltd, Mumbai, India. He is a member of the American Chemical Society and Product Development Management Association. 

Email: mosongo@mosongomoukwa.com

Opinions expressed here by the author are his own and do not represent the views of the company