Dow Chemical has developed a new process for the production of propylene from gas feedstocks, which it says could be up to 20% cheaper than existing methods. Ros Davidson finds out more
Propylene is a vital intermediary in the production of plastics and countless other everyday products, and second only to ethylene in terms of global production volume. In 2016 just over 100 million tonnes were produced worldwide, and demand is still rising, with IHS Markit recording that this growth rate has been sustained at about 4% for the past 5 years.
Typically propylene is produced by steam-reformation of naphtha and LPGs, and as a by-product of catalytic cracking, but it can also be made on demand using propane dehydrogenation or olefin metathesis. The latter has become more favourable as feedstocks have shifted from naphtha (sourced from crude) to ethane (derived from gas) in light of the US unconventionals boom.
A new downstream technology developed by Dow Chemical can manufacture propylene directly from shale gas, potentially offering savings of more than 20% versus other commercial processes. Dow’s fluidised catalytic dehydrogenation (FCDh) process also reduces the energy requirements per pound of propylene produced. Moreover, its manufacture can be integrated into existing or new production processes for propylene, ethylene and other valuable hydrocarbon intermediates – meaning uptake could be fast.
On November 17, FCDh was named as one of the winners of the annual R&D 100 Awards, which recognise the best new technologies of the prior year. The awards are the most prestigious acknowledgement of invention and innovation globally.
FCDh uses a similar process to Dow’s already commercially available fluidised catalytic cracking (FCC) technology. But FCDh uses a different catalyst and a different set of process conditions, said Andy Arthur, R&D director at Dow’s hydrocarbons & energy and ethylene oxide-ethylene glycol businesses. “It uses a different feedstock that allows selective dehydrogenation to a specific olefin,” he told InnovOil. Olefins, or alkenes, are produced during cracking.
As the production of shale gas has surged in the US, an imbalance has emerged in the supply and demand of propylene. That is because of the shift to lighter cracker feedstocks – because of shale gas – and slowing gasoline demand for refineries.
Historically, propylene was an abundant co-product from ethylene production via refinery FCC and non-ethane steam cracking processes. “Since the shale revolution caused increased production of raw gas that contains more natural gas liquids, like ethane and propane, this has lowered global propylene production, creating a propylene shortage,” explained Arthur.
Dow said that it recognised the importance and limitations of existing propane dehydrogenation technology, and so moved to develop FCDh. The technology had originally been devised for the production of ethylene and styrene during 2000-2006, but was then adapted to propane dehydrogenation during 2008-2016.
The savings come from higher efficiencies and a lower overall cost. Existing propane dehydrogenation (PDH) technologies – in which propane is selectively dehydrogenated to propylene – work by slowly moving deactivating catalysts between reactors and a regenerator. This is the PDH technology already implemented around the world. “The FCDh technology takes a very different approach, using more active catalysts with shorter times between regenerations,” said Arthur. “This leads to lower overall capital investment, fewer limitations on increasing scale, higher efficiency and better operability.”
He added: “Dow’s FCDh technology manages the fundamental constraints of the chemistry with a simple, efficient and multifunctional process design. The new technology enables shorter contact times for the feedstock over the fluidised bed with close-coupled cyclones for catalyst/product separation. Frequent catalyst regeneration creates higher effective catalyst activity while in service.”
The lower energy requirement, per pound of olefin produced, is because less heat is needed to drive the conversion for PDH. “We applied a concept of using the catalyst to carry heat to the reactor,” said Matt Pretz, one of the FCDh technology developers who worked alongside colleagues Lin Luo, Mark Stewart, Brien Stears, Isa Mbaraka, Richard Pierce and Susan Domke, at a Dow lab in Freeport, Texas.
“This approach really improves the energy efficiency, but makes very little coke,” said Pretz in reference to the side reactions and coke formation which can occur during PDH conversion. “To satisfy the required energy balance in FCDh, we use the catalyst to independently control the reactor temperature and add supplemental fuel to the regenerator. The challenge lies in balancing the chemistry and process parameters to optimise yield and overall energy input.”
Moreover, the FCDh technology is more flexible, Dow said, and can be integrated into existing or new ethylene crackers either to increase production, or so that facilities can be tailored to produce a specific amount of ethylene and propylene. “This enables producers to “flex” their production ratios using shale gas resources in their existing processes, allowing them to quickly respond to changing market dynamics, while saving capital and energy versus existing commercial processes,” explained Arthur.
In addition, FCDh can be used in other aliphatic and alkyl aromatic dehydrogenation processes, he added, because the FCDh catalyst can perform the dehydrogenation of paraffin and alkyl aromatic compounds such as ethane, propane, butane, isobutane and ethylbenzene.
Given that there appears to be no shortage of unconventional feedstocks, and a solid demand forecast, process technologies like FCDh could have a fairly swift impact on the bottom line of chemical producers in the next few years. Dow is currently exploring commercial options to advance the technology and says it has received favourable interest from potential customers. It may not be long before FCDh begins to make its mark in the marketplace.