The petrochemical industry is made up of seven building blocks: synthesis gas, the three olefins— ethylene, propylene and C4 olefins—and the three aromatics— benzene, toluene and the xylenes. Among the olefins is propylene, one of the most significant chemical building blocks produced industrially for polypropylene (PP), acrylonitrile, propylene oxide derivatives and other uses.

Two-thirds of the world’s propylene goes to PP production. Demand for PP has been high for the past decade due to its versatility and reasonable price. Since its invention in 1954, PP has evolved into one of the most widely used products of the olefins industry. It is used in day-to-day products such as plastic parts, carpeting, paper and material found in loudspeakers and similar electronics. PP is also used in thermoplastic fiber, reinforced composites and laboratory equipment.

At present, global propylene demand is roughly 90 million metric tons (MMT) and is estimated to rise to 130 MMT by 2023, approximately 30% of which will be on-purpose production based according to IHS Chemical.

Propylene is largely produced in traditional processes such as steam cracking and fluid-catalytic-cracking (FCC) units. In the U.S., refinery-based production makes up a large portion of the supply given the many FCC units located in the country, says Chuck Carr, Global Olefins director at IHS Chemical. The split for producing finished grades of propylene is roughly divided as follows: polymer grade (PG) and chemical grade (CG) is about 60% from refinery splitters, 30% from steam crackers, 5% from propane dehydrogenation (PDH) and 5% from metathesis.

Currently, obtaining propylene from lighter feedstock instead of crude oil is growing in interest due to the shift from naphtha, which yields a large quantity of propylene, to natural gas liquids (NGLs), which do not, tapering propylene supply and opening opportunities for on-purpose production technologies.

The Market

As published in the October 2014 issue of IHS Chemical Week’s Basic Chemicals and Plastics, propylene prices have increased steadily in the U.S. for a number of months. In particular, a wave of plant outages, which have since been resolved, worsened the situation and helped to drive the price of refinery grade (RG) propylene to a three-year high. PG and CG have also been selling at high rates.

“All of the (outage) issues have been resolved and were basically short-term problems,” says Carr. “However, when propylene supply becomes tight due to these types of issues, prices rise in accordance and cause demand to shift lower." He says that higher prices can also open arbitrage windows with other regions, allowing both propylene and polypropylene to move towards North America. The higher prices close off export arbitrage opportunities for other derivatives such as acrylonitrile and oxo-alcohols.

In recent weeks, propylene prices have been correcting lower, according to Carr, allowing demand to improve and import arbitrage opportunities to close.

Several key players are working on increasing propylene production via on-purpose technologies. For example, Dow is currently building a PDH that will start production in mid-2015, says Carr. Enterprise is constructing a PDH that will start production in mid-2016. Formosa, Ascend and REXTac are all in various phases of engineering that are also considering adding PDH units in Texas. One of the most recent announcements is from Sunoco Logistics, which is considering building a PDH unit in Marcus Hook, Penn., Carr says.

The U.S. and the Middle East are leading this shift from traditional processes to PDH, largely due to the low propane cost. China is also starting to be a hotspot as it is home to numerous projects involving PDH plants. Canada, too, is embracing this change: Williams is working towards building a PDH unit in western Canada to take advantage of stranded propane, according to Carr.

“In all, we anticipate about 4 to 4.5 MMT of PDH propylene capacity being built by the year 2020 in North America,” says Carr.

On-purpose Propylene Technologies

Traditionally, propylene has been generated as a co-product either in an olefins plant or in a crude oil refinery’s FCC unit. However, the market reached a stage where by-product production can no longer keep up with propylene demand. Several on-purpose technologies are currently operational to meet the supply gap. These include PDH, olefin metathesis (also known as disproportionation metathesis), methanol to olefins (MTO), and methanol to propylene (MTP).

Among the on-purpose technologies, PDH has gained the most commercial interest with numerous plants currently operating or under construction. There are five licensed technologies to date: CATOFIN from Lummus Technology (now part of CB&I), Oleflex from UOP, Fluidized Bed Dehydrogenation (FBD) from Snamprogetti/Yarsintez, STAR process from Krupp Uhde, and PDH from Linde/BASF. All differ based on catalyst type, regeneration methods used, reactor design and operating conditions to achieve high conversion rates.

The concept behind PDH is relatively simple: propane goes through a catalytic reaction that removes hydrogen, leaving the recovered liquids to be fed to distillation units for propylene retrieval. For example, the CATOFIN process is divided into four sections: hydrocarbon processing, low temperature recovery section, hydrogen stripping and product recovery. It uses catalytic fixed bed reactors (FBR) under optimal operating conditions to achieve high conversion rates, selectivity and realistic energy consumption reductions.

During hydrocarbon processing, fresh feed and recycle feed from C3 splitter bottoms are vaporized through exchange with a number of process streams, and then increased to reaction temperature in the charge heater. Later, the stream is fed to a reactor and directed to pass through a high pressure steam generator, feed-effluent exchanger and trim cooler to the compressor. The compressor discharge is then cooled and transferred to the low-temperature recovery section. In that section, the stream, rich in hydrogen gas, is sent to the pressure swing adsorption (PSA) unit for hydrogen purification, while the remaining stream is fed to distillation facilities for product recovery.

Future Prospects: Counter Diffusion

Since the 1980s, dehydrogenation of propane has been a subject of commercial and scientific interest as better understanding of involved reactions that allows for utilization of cheap and abundant resources and selective production of propylene.

Researcher Hae-Kwon Jeong from Texas A&M University found an optimal method of isolating propylene from propane for commercial use. This proposed method (counter-diffusion) is based on preparing well-intergrown Zeolitic-Imidazolate Framework (ZIF)-8 membranes with significantly enhanced microstructure in order to achieve high separation performance toward propylene over propane.

“Counter diffusion method is a new synthesis method to prepare high quality nanoporous Metal-Organic Framework (MOF) molecular sieve membranes in a commercially viable manner,” says Jeong. In particular, the membranes are based on so-called ZIFs, which is a subclass of a family of MOFs. The method enables the synthesis of high-quality membranes of many different MOFs for other separations such as H₂/N₂, CO₂/CH₄, N₂/CH₄, and others.

Olefin/paraffin separation, such as propylene/propane has always been quite challenging due to the close physical properties. The current technology used for the separation of such mixtures is cryogenic distillation, according to Jeong. In this method, propylene/propane gases first will be liquefied under low temperature and high pressures and then distilled and separated by distillation.

“Due to the small difference in the boiling points of these gases, the process requires large number of trays, which corresponds to the energy per capital cost in addition to the liquefaction process,” he adds. “In 2006, the U.S. Department of Energy (DOE) estimates 1.2*10^14 Btu per year for olefin/paraffin separations (mainly ethylene/ethane and propylene/propane)”.

Jeong says that if separation of propylene/propane or ethylene/ethane is done based on the molecular sieving mechanism, then the chemical/petrochemical industries will advance due to energy savings since ethylene and propylene are highly consumed chemicals and starting materials for many products.

However, progress from lab-scale to commercialization takes a long time to occur according to IHS's Carr. For example, “the methanol based olefin production has only recently been commercialized, with operating units currently in China and many more, set to start production over the next 5 years”, he states.

“Technology and energy/chemical companies have been working for decades to bring this technology to a commercial scale. So any type of new technology being considered typically takes many years or decades to begin having an impact on the market. PDH technology has been around for a long time, but propane supply and economics have only recently pushed this technology into an economically viable situation,” Carr indicates.

On the other hand, Jeong observes that the industry should be more willing to support new technologies. “Companies are too conservative; once research has been demonstrated in the lab, then industries should fund it.”

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