As the world population increases and the demand for plastics skyrockets, the process of propane dehydrogenation has gained widespread momentum. Propane Dehydrogenation or PDH is a process that converts a propane feedstock to propylene which is used commonly in various petrochemical applications.
As the global demand for propylene increases in the automobile sector, manufacturing of bottle caps, fabrics, packaging material, and production of various derivative chemicals the industry is moving more towards on-purpose production and away from co-production. This is mainly achieved through dehydrogenation of propane where propane is selectively dehydrogenated (removal of hydrogen from the propane stream) to convert propylene in the presence of a platinum or chromium oxide-based catalyst depending upon the type of the process.
Paraffin dehydrogenation reaction chemistry is accompanied by a strong endothermic reaction and the reaction must be maintained in strong thermodynamic equilibrium. The high risk of side reaction occurring can cause coking, cracking of hydrocarbons, formation of tar laydown on the reaction catalyst, and hydrogenation of olefins. PDH is a revolutionary process commercialized in the 1990s that converts propane-rich streams to high purity polymer-grade propylene.
Propylene is a vital intermediate petrochemical whose derivatives include:
Although its demand has been steadily increasing, the supply has been constrained. This is due to steam crackers focusing more on producing ethane than propylene. As well as due to the shale revolution in the US. The USA produces more of the lighter crude which does not produce as much propane as heavy crude.
Most of the global capacity was co-production, which could not keep up with the increasing demand for propylene. This was further complicated by massive investments in US LPG and LNG transportation terminals. Shipping the refrigerated ethane and LPG to the booming economies of China and India further reduced the co-product co-production of propylene.
This situation has been solved by the “on-purpose” production of propylene. There are several other factors that make “on-purpose” production of propylene make economic sense e.g., demand for propylene has outpaced the demand for ethylene in recent years and is forecasted to do so in years to come.
Most of the “on-purpose” production of propylene is attributed to the dehydrogenation of propane because recent advancements and resultant technical maturity in the process have made the PDH competitive with the co-production technologies in the market.
One example closer to home is the Inter pipeline new PDH production facility in the heartland of Alberta. The “on-purpose” propane dehydrogenation facility uses propane generated by the Inter pipeline Redwater petrochemical complex. It converts 20,000 barrels per day of locally produced propane thanks to massive heavy crude oil deposits in the province.
The project economics have been helped due to the higher propylene-propane price spread in Alberta. Propane is oversupplied in the province and is a hard sell due to the pipeline capacity taken to transport the heavy crude to refineries in the US Gulf Coast. This makes it a cheap and stranded product. The propane cannot be transferred by rail using the similar methodology of “crude by rail” because it significantly reduced the propylene-propane spread.
There are three commercial PDH process technologies available on the market. CATOFIN PDH process is licensed to Lummus Technology, OLEFLEX is licensed to UOP, and STAR is licensed to ThyssenKrupp Uhde.
CATOFIN uses a chromium oxide-based catalyst arranged in a horizontal fixed bed parallel reactor. The process can be operated at an optimum reactor pressure and temperature thus maximizing the yield of propylene. The capital cost of installing a CATOFIN based plant, operating expenditure, energy consumption, and catalyst cost are lower compared to other technologies, reducing the production cost.
The conversion efficiency from propane to propylene is 45%-50% while there is no production of the H2 recycle gas and there is no need for a continuous catalyst regeneration unit. Thus reducing the complexity of operation and cost.
OLEFEX uses a platinum oxide-based catalyst in a moving bed reactor and is a continuous catalytic dehydrogenation process to convert propane feedstock to propylene. The catalyst maintains high activity which allows for high productivity and reduces the time in between catalyst change out. One flexibility OLEFEX offers is that it can also dehydrogenate isobutane, isopentane as a mixture or separately.
Although the process is more reliable and safer compared to the CATOFIN but needs continuous catalyst regeneration which increases the cost, has a more complicated internal design due to moving bed reactor and has higher operating expenditure with a conversion efficiency of 35% -45%.
The STAR process is newer and has some advantages on the OLEFEX and CATOFIN. For instance, it has no moving bed catalyst which makes the process reliable and safer with reduced maintenance costs. The feedstock is transferred to the feed preparation unit to remove contaminants and then fed to a reactor where it is heated and mixed with the process stream. The feedstock is then fed to an external heater reformer with catalyst filled reformer tubes.
STAR uses higher pressure compared to other technologies in the reaction section which increases the suction pressure at the inlet of the downstream raw gas compressor thus the lower compression ratio and lower inlet volume flow offers significant energy savings for the raw gas compression.
Both processes take place at temperatures above 650 ˚C with relatively low pressure and the reaction is endothermic that means it absorbs heat from the surroundings. The difference in the three technologies of OLEFLEX, STARS, and CATOFIN stems from the catalyst used, operating pressure and temperature, type of reactor bed, performance, and technology process.
While STARS is comparatively new, OLEFIN and CATOFIN have been mature processes implanted worldwide in several “on-demand” PDH plants.
The three PDH technologies mentioned work at the temperature of 600˚C-650˚C because at temperatures higher than these will result in significant side reactions like the formation of coke which will reduce the conversion efficiency. With more “on-demand” PDH process plants coming online around the globe it is expected that rising demand will keep the propylene prices at $800/ton. Higher oil prices especially after lack of investment in new crude oil exploration and developments will drive the prices up for propylene and its derivatives.