Chemisorbent Materials for Olefin/Paraffin Separation

Propylene and ethylene are commonly employed for food packaging, and chemicals amongst others.1-4 To keep pace with a greater demand for propylene and ethylene products, the production of light olefins must also therefore increase.3 Currently, the most feasible route is preparation via the refinement of crude oil. 5
The petrochemical industry produces olefins (ethylene and propylene) from crude oil.7 After the processing of the crude oil, there is a mixture of olefins and lower chained paraffins. Therefore, a separation process is required to separate the olefins and paraffins. A mixture is not useful to industry. In terms of propylene there are three different grades: polymer grade is a minimum of 99.5% purity, chemical grade a minimum of 93-94% purity, and a refinery grade with a minimum of 60% purity. The standard method used for the separation of olefin and paraffins is cryogenic distillation.6 For example, propylene and propane separation is difficult
as both gases have very similar boiling points, (-47.6 degree centrigrade for propylene and -42.1 degree centrigrade for propane),10 thus, distillation columns that can be maintained at low temperatures and high pressures are required. However, cryogenic distillation is an energy intensive process. Generally, cryogenic distillation columns operate with 150-200 trays at temperatures between
-40 degree centrigrade and -90 degree centrigrade with pressures ranging from 16 to 20 bar.13 To understand the quantity of energy, every metric ton of ethylene produced in 2016, 20 Gigajoules of energy was required. That, on average is the same value of energy required for an individual to live in the UK for one year in 2016.14
Even with this large energy requirement, cryogenic distillation is still the preferred method of separation since its introduction in the 1960s. Alternative separation methods have been reported in the literature, based on mass transfer systems instead of the energy transfer methods, however, there are still issues surrounding the implementation of these systems, resulting in the reluctance of companies to adopt them into their processes. Alternatives have included silver and copper salts, membranes, metal organic frameworks, zeolite imidazolate frameworks.15-19 The focus of this project will focus upon the ionic liquid alternative.
Ionic liquids have been the focus of great interest in the field of gas separation and capture.20 This is due to the unique properties they can bring such as negligible vapor pressure, high thermal stability, and large liquidus range.22-24 Furthermore, they can be modified and the physical-chemical properties of ionic liquids can be finely tuned, and so they have been named as "designer solvents". However, the limiting factor for some ionic liquids include their low gas solubilities, separation selectivities, and high viscosities.25
This project aims to develop and test new chemisorbent (forming strong interactions between the solute and the ionic liquid. These interactions are usually irreversibly bound to the surface) materials for the separation of light olefins and paraffins. Furthermore, this project will focus on the design, development, and benchmark of a gas rig to test the ability of these materials to separate olefin and paraffin gases, whilst mimicking industrial conditions. The equipment allows the gas mixture to come into contact with the ionic liquid, and the gas composition change over time being monitored by headspace gas chromatography, until equilibrium is reached (and equilibrium time determined). Experiments will be carried out with pure (single) gas, binary mixtures of olefin and paraffin, and subsequently with contaminates present, to mimic gas streams obtained after steam cracking.

References available.