On-demand propylene from naphtha
A new catalytic process targets economic production of propylene from naphtha at relatively low capital and operating costs
BART DE GRAAF and RAY FLETCHER, Inovacat B.V.
ANGELOS LAPPAS, Chemical Process and Energy Resources Institute of the Center for Research and Technology
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Today, steam crackers are still the leading source of propylene. The next largest source of propylene comes from refineries. However, the share in propylene production by these sources is decreasing. On-purpose technologies have been gaining ground rapidly over the last 10 years. Current front runners among on-purpose technologies are propylene dehydrogenation (PDH) and coal-to-olefins (CTO). The main difference between conventional propylene production and on-purpose technologies is feedstock: conventional technologies use mostly oil or naphtha, whereas on-purpose production technologies use propane and coal.
The US market is experiencing a substantial increase in availability of naphtha with the advent of very light tight oils. This will be exacerbated by the implementation of CAFE standards. These aim to reduce US gasoline consumption by 2.5 million b/d. In Europe, export refineries are experiencing similar trends, combined with the fact that over the past five years US gasoline production has started to meet US gasoline demand. A substantial surplus in naphtha and gasoline is starting to emerge. Distressed naphthas can be a very cheap feedstock for petrochemical production.
Propylene production processes
Steam cracking utilises what is likely the most severe conditions of any chemical process in industry. When sufficient heat is supplied, molecules will start to ‘fall apart’ into free radicals. Steam cracking involves networks of many different free radical reactions, including initiation, propagation and termination steps. Cracking light naphtha feeds in a steam cracker produces high yields of ethylene due to the free radical chemistry involved. Selectivities towards propylene can be improved by reducing the severity of operation but steam crackers are not designed to operate at propylene-to-ethylene ratios much larger than one.
Catalytic cracking creates a pathway for molecules that allows for a lower energy barrier for the reactions to proceed. This requires an affinity of the reactant for the catalytic site and the creation of an intermediary product that will crack into the desired products. The lower energy barrier means that reactions already occur at a lower temperature. This reduces energy losses and helps to steer selectivity to the most desired products. Thermal cracking products show a higher selectivity towards ethylene, whereas catalytic cracking occurring at gentler temperatures favours larger olefins such as propylene and butylene.1 Selectivities can be optimised using zeolitic catalysts. Optimising the pore geometry and affinity to the intermediates may strongly influence the selectivity towards desired products.
Process design can help to increase selectivities further. Fluid catalytic cracking (FCC) of naphtha has been commercialised under various names and is sometimes also applied as a secondary riser added to a conventional FCC unit. One of the main challenges in this process is the heat imbalance: there is a gap between the highly endothermic deep conversion of naphtha needed to produce high propylene yields and the very low coke make produced by naphtha cracking.
There are a limited number of FCC units operating worldwide using these technologies. Whereas secondary riser technology benefits from the much more stable FCC heat balance, it pays a penalty in selectivities due to the presence of faujasites in the catalyst mix which boosts hydrogen transfer reactions. A second challenge in current fluidised cracking processes is the high degree of back-mixing reducing selectivities towards propylene.
Addition of ZSM-5 additive to the FCC unit is a simple and effective way to increase propylene yields. The propylene yield achievable utilising ZSM-5 is a function of feedstock, FCC design and base catalyst composition. ZSM-5 additives may be a relatively cheap option to incrementally increase propylene yields in various refineries.
At present, the conversion of methanol to propylene is commercially unattractive due to methanol and propylene prices. The process starting with the conversion of coal to methanol, followed by methanol to propylene, is commercially feasible. The CTO process comes at a high environmental price: 14-20 tonnes of CO2 is emitted for every tonne of propylene produced. Additionally, substantial amounts of wastewater are produced. The environmental challenges make this process less attractive compared to competitive processes.
PDH technology has in recent years begun to gain increasing importance. Many PDH units have been built over the past five years, especially in China. Typically, these units have a capacity between 300000 t/y and 660000 t/y of propylene. In the US recently, one 750000 t/y unit came online and a second one is expected to start up this summer.
The recent large surpluses in natural gas liquids have benefited both steam crackers and PDH units. Relatively inexpensive ethane has replaced naphtha in steam crackers which has resulted in a large decline in propylene production in these units. Inexpensive propane has also been a benefit to PDH operations.
Worldwide there are approximately 30 PDH units. The main challenges of PDH unit operation are catalyst breakage and transport from unit to unit, chromium content of catalyst, and limited possibility for turndown. These units tend to run either at full capacity or are idled.
Naphtha is an attractive feedstock for propylene production. Existing processes can convert naphtha into 15-20% propylene. Steam crackers cracking naphtha at high severity yield about 15% propylene. FCC processes can produce up to 20% propylene, with some claims of up to 22%. Significant optimisation of the catalyst and process conditions are required to achieve these propylene yields when cracking naphtha.
Cracking of olefinic feeds occurs readily via carbenium ion mechanisms. For paraffinic and naphthenic feeds, super acid cracking is required. This mechanism accurately describes the initial stages of cracking paraffins.2 However, cracking of light paraffinic feeds at temperatures below steam cracking has been proven to be rather challenging in existing processes and conversion is limited. Therefore, to enhance conversion of paraffinic and naphthenic feeds, the Gasolfin catalytic system has two distinct functionalities: in the first step a pre-conditioning of the feed molecules occurs, followed by cracking over a second component of the catalytic system.
Controlling reaction conditions are critical in this process. The graded bed promotes the initial cracking step while suppressing side reactions. Optimising temperature and hydrocarbon partial pressure are key to steering selectivities to maximum propylene or maximum aromatisation mode, or any selectivities in between.
Testing of a variety of feedstocks over the Gasolfin catalytic system has shown propylene selectivities between 25% and 45%. Selectivities depend on feed type. Different yield patterns are achieved while processing paraffinic vs olefinic feeds. Propylene or aromatic yields may be optimised for each feed stock modification of operating conditions and composition of the catalytic system.
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