Polymer membrane offers 'low-energy alternative' to crude oil distillation
A class of ultrathin polymer membranes could enable fast and selective fractionation of crude oil.
The team of international researchers from the UK, China, Saudi Arabia and Singapore claim to have created a new way to form the separating layers in polymer membranes for molecular separations. They use a Tröger’s-base polymer – a class of rigid, contorted and inherently microporous macromolecules.
Intrinsic microporosity means the material separates complex organic hydrocarbon mixtures into fractions by size and type.
This property stems from sub-nanometre pores that are locked in place during production, due to the way the crosslinking agent is added.
The in situ crosslinking enables the acyl chloride monomers to react and form a polyamide network. This stabilises the polymer structure while the membrane is being formed and overcomes the challenge of swelling when polymers are exposed to hydrocarbons, whereby pores expand and lose their selectivity.
'Stabilising the structure before [it has] a chance to swell preserves the tiny pores…while still allowing hydrocarbons to flow through very quickly,' says Assistant Professor Zhiwei Jiang from Nanyang Technological University, Singapore.
The sponge‑like membrane reportedly offers extremely high molecular selectivity with fast liquid transport. Membranes made from polymers of locked intrinsic microporosity (PLIMs) are found to show up to 10-fold higher permeance than existing membranes when tested with synthetic crude oil, discriminating between hydrocarbon molecules that differ in size.
Conventional crude oil refinement currently relies on energy-intensive thermal distillation and industrial uptake of membrane alternatives has been limited by fundamental materials challenges.
'The problem has been finding materials that are both fast and selective when exposed to real hydrocarbon mixtures,' explains Professor Andrew Livingston from Queen Mary University of London, UK.
When tested with real Arabian Extra Light crude oil, the material has removed 99.8% of hydrocarbons heavier than 15 carbon atoms and reduced sulphur-containing compounds by 93%, report the researchers.
The PLIM membranes are also found to perform well with refinery streams such as virgin naphtha. They separate light hydrocarbons – suitable for fuel upgrading – from heavier naphtha fractions used to produce plastics and chemicals.
This can be achieved at permeances comparable to commercial desalination measures, the team reveals.
At lab scale, the researchers made 6.5x6.5cm membranes, while at pilot scale they used roll-to-roll processing to manufacture sheets around 5m long and 0.3m wide. These can be tuned from a thickness of ~15nm to several hundred nanometres, with a fractional free volume of ~35% for transport.
Two-metre sheets create a spiral-wound membrane module. The team claims this can be integrated into existing infrastructure, and tests reportedly show stable performance over 30 days of continuous operation.
Jiang says the next challenge is 'scaling up production of the precursor polymer…there is still considerable scope to optimise the process for larger-scale and more cost-effective production'.
Ultimately, the researchers say their technology could help cut energy consumption, reduce carbon emissions, operate with smaller and more flexible processing units, and integrate selective desulphurisation earlier in the refining process.
They also suggest the same pore-locking concept could be extended to other liquid separation challenges, such as chemical manufacturing, solvent recovery and bio-based feedstocks, and potentially other polymers.
Jiang continues, 'Our membrane could replace or complement one of the [crude oil] separation stages. However, for membrane technology to replace an entire distillation process, we would need a portfolio of different membranes capable of performing different molecular separations.
'If researchers…can develop complementary membranes for different hydrocarbon separations, I believe membrane technology could play an increasingly important role in the oil and gas industry.'
