Jan-2008

Advanced propane dehydrogenation

Oxdehydrogenation-based on-purpose propane dehydrogenation can close the propylene supply-demand gap. Reactor design and the effect of thermodynamic equilibrium on conversion, volume and required compression are discussed

Max Heinritz-Adrian and Sascha Wenzel, Uhde GmbH
Fekry Youssef, Egyptian Propylene & Polypropylene Company

Article Summary

The world is currently witnessing a significant change in the propylene market as it moves away from co-production and towards more on-purpose production and the supply of propylene. By applying the principle of oxydehydrogenation, the feasibility of on-purpose propylene production by propane dehydrogenation (PDH) is further improved. The first plant based on this principle is now under realisation in Port Said, Egypt.

Propylene market
Propylene is one of the most important intermediate petrochemical products. Over many years, it has maintained a remarkable growth in demand that is mainly attributed to the significant growth in polypropylene (PP) demand, which absorbs more than 60% of all the propylene produced worldwide. PP has grown into one of the most widely applied plastic products, and its broadness in application — which is continuously expanding — and its superior features in many applications continue to drive demand. Other important propylene derivatives include acrylic acid, acrylonitrile, cumene/phenol and propylene oxide.

Although most propylene has been produced as a co-product to ethylene in steam crackers and as a byproduct in refineries, we are currently witnessing a significant change in the propylene supply chain with a move away from co-production and towards on-purpose production. There are several reasons for this change:
— The growth in demand for propylene has outpaced the growth in demand for ethylene for many years and will continue to do so
— Due to significantly better feasibility, a large share of new cracker projects for ethylene production is based on ethane feedstock yielding no propylene co-production
— Several on-purpose propylene production technologies such as propane dehydrogenation (PDH) and metathesis have achieved technical maturity and acceptance, and significant developmental efforts have made them competitive with co-production technologies in the market
— Private-sector companies specialising in specific value chains, such as the propylene value chain, are growing in the marketplace
— The tight propylene market and high oil prices have continuously driven up prices for propylene and propylene derivatives.

As a result, while in 2003 more than 97% of all propylene was produced from steam crackers and refineries, and only 3% by on-purpose technologies, 10% of worldwide propylene production by 2012 will be provided by on-purpose technologies, mainly PDH and metathesis plants.

Metathesis was first commercially applied to propylene production at the BASF/FINA Port Arthur Cracker, USA, and several plants have been built in Asia since then. PDH was commercialised in the 1990s, with six PDH plants having been started up (one is co-production) in Thailand, Malaysia, Korea and Belgium, out of which five apply UOP technology and one (owned and operated by Borealis in Kallo near Antwerp, Belgium) employs Lummus technology. In the first five years of the 21st century, two PDH projects started up; namely, the SPC plant in Al-Jubail, Saudi Arabia, employing Lummus technology and the Basell/Sonatrach joint venture in Tarragona, Spain, employing UOP technology. A total of five PDH projects are currently under realisation in Egypt, Saudi Arabia and Thailand. Development for most of these projects had already begun in the 1990s. However, only after the long-expected tightening of the propylene market materialised did they finally begin to move forward.

Although additional propylene production capacity will come on stream in the next few years, propylene prices are expected to remain above $800/ton.1 The EPPC PDH/PP project at Port Said, Egypt, is the first project in the world to apply the Uhde STAR process with oxydehydrogenation.

PDH chemistry
Paraffin dehydrogenation reaction chemistry, although quite simple from a stochiometric point of view, is very complex due to its strongly endothermic nature and significant conversion limitations caused by thermodynamic equilibrium. Side reactions include the cracking of hydrocarbons and hydro-genolysis, as well as oligomerisation, cyclisation, hydrogenation of olefins, deep dehydrogenation and eventually the formation of coke and tar laydown on the catalyst, which requires frequent catalyst regeneration (ie, burning off of coke and tar laydown on the catalyst with oxygen or air).

Two main catalytic systems have been identified and commercialised for light paraffin dehydrogenation, the first being supported chromia catalysts (ie, chromia on alumina support), which are doped with alkali metals to add alkalinity and suppress unwanted side reactions, and the second supported platinum or platinum-tin catalysts, with different support materials. These materials include alumina or zinc-/calcium-aluminate, and potentially further modifiers such as alkali metals, again to reduce the acidity of the support and suppress side reactions. While chromia catalysts were already used commercially for butane dehydrogenation in the 1940s (UOP), platinum catalysts have only been used since the 1960s, when they were first applied in UOP’s proprietary Pacol process. Today, both catalyst systems are used in commercially available PDH processes (ie, chromia catalyst in Lummus CATOFIN and platinum-tin catalysts in UOP Oleflex and Uhde’s STAR process).

With the dehydrogenation reaction being favoured by high temperatures and low partial pressures, all commercially available technologies keep the process temperature at or below approximately 650°C, because higher temperatures would lead to a significant increase in side reactions and therefore a decrease in the selectivity of the main reaction for propylene production. Furthermore, the most significant side reactions are not limited by thermodynamic equilibrium. As a result, the higher retention times required to achieve higher per-pass conversions also have a strongly negative impact on propylene selectivity. At the same time, partial pressure is kept low by applying absolute pressures slightly above atmospheric pressure or even under vacuum, or by using a diluent to reduce the hydrocarbon partial pressure. As a result of these considerations, all the technical processes for PDH have limited conversion in the range of 32–55%, with selectivities to propylene in the range of 87–91 mole%.