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Catalytic dewaxing is a hydrocracking process which requires the use of molecular sieve catalysts, such as zeolites, in order to convert large molecules into smaller molecules. This selective hydrocracking process is carried out in a high pressure and temperature environment, and in the presence of hydrogen (IRM, n.d.).
Catalytic dewaxing is used to produce commercially useful products, such as, base and lube oils, and naphtha; by converting heavier crude oil fractions, such as, wax, lubricating oil and asphalt. (Wansbrough, n.d.).
The catalytic dewaxing process has been slowly replacing the solvent dewaxing process ever since its first introduction by Mobil Oil in 1978 (van Veen et al., 2008). The importance of the catalytic dewaxing process to industry is that it is cheaper, has fewer environmental concerns and it is more efficient than solvent dewaxing.
Some of the advantages of the catalytic dewaxing process over the solvent dewaxing process as stated by ExxonMobil (n.d.) are:
Half the capital investment of solvent dewaxing for grass roots units
Significant operational savings - up to $7 per barrel less than solvent dewaxing
Dramatically lower utility costs than solvent dewaxing
Simpler, safer operations - no volatile, toxic organic solvents
Shorter maintenance downtimes with reduced maintenance costs
Since this new process was needed, new catalysts had to be developed and discovered. The first catalytic dewaxing catalysts were based on the zeolite mordenite and were developed by British Petroleum. However, the key starting point of all the developments in catalytic dewaxing processes and catalysts was the discovery of zeolite, ZSM-5, by Mobil in the 1960s (van Veen et al., 2008).
A brief history of the key events in the catalytic dewaxing industry have been summarised in a timeline (van Veen et al., 2008).
Mobil discover zeolite ZSM-5
Mobil introduce first ever ZSM-5 catalysed dewaxing process
Akzo and Fina jointly develop and introduce on the market the CFI process
Chevron become the first to combine catalytic dewaxing with hydrocracking and hydro¬nishing
First modern wax isomerisation dewaxing process commercialized by Chevron
Catalytic dewaxing is normally carried out using a medium-pore zeolite catalyst, for example, ZSM-5, under a hydrogen flow (van Veen et al. 2008). Both Huve (n.d.) and van Veen et al. (2008) mentioned that dewaxing catalysts usually contain a base metal (nickel) supported on a medium-pore zeolite (ZSM-5) and an alumina binder.
According to Mortier et al. (2009), dewaxing catalysts such as zeolites have a highly porous structure based on a regular arrangement of channels. These channels have openings with a diameter between 50 - 70 nm; therefore they only allow linear or slightly branched alkanes to enter.
The molecules which do manage to enter will be isomerised or cracked into smaller molecules. For example, when alkanes enter the zeolite they will be selectively cracked into lighter products, such as, LPG; or selectively isomerised into products which have more value, such as, diesel or lube oil (van Veen et al., 2008).
On the other hand bulky molecules will not be able to enter the zeolite; therefore, they bypass the channel opening of the zeolite.
Source: Huve (n.d.)
A possible way for any of the molecules to be catalytically dewaxed would be to have the feed make contact with a fixed stationary bed of catalyst. This can be achieved by using a trickle-bed reactor which allows the feed to trickle through a stationary fixed bed (Miller, 1988).
The dewaxing reaction has to operate at a high temperature and pressure and in the presence of hydrogen. The crude oil fractions which will undergo catalytic dewaxing and the hydrogen required make up the feed. The actual conditions the process operates on can vary, as can be seen in Table 1. (The temperature and pressure have been converted in order to give a better sense of the conditions used.)
Table 1 - Conditions at which catalytic dewaxing can occur
249 - 288
300 - 500
Kozlowski & Olson (1971)
399 - 482
34 - 170
0.3 - 3
2000 - 10000
260 - 371
102 - 204
0.5 - 5
Adams & Farcasiu
343 - 427
27 - 170
0.5 - 5
500 - 3000
A catalyst used in any process will always need replacing as catalysts do not last forever. The cost of the catalyst used, ZSM-5, is 60,000Pt dollars per ton, however this value doesn't include the price of the recoverable platinum (Garforth, 2010a). Due to the cost of catalysts, one has to be weary of the reasons why a catalyst life is reduced in order to extend the life of the catalyst used.
A factor which can reduce the life of a catalyst is the type of feedstock used. Bekkum (2001) has stated, 'depending on the type of feedstock (i.e. whether or not hydrotreated) the catalyst life can vary from weeks to years'.
One of the main problems for medium-pore zeolites is that coke build-up can occur on the external surface of the crystallites and at the pore mouths (van Veen et al., 2008). The build-up of coke will gradually decreases the activity of the catalyst.
The formation of coke on a zeolite catalyst has been explained by Mortier et al. (2009):
'The cracking of alkanes initially produces unsaturated, low molecular weight by-products which can polymerise and, through coke formation, cause a rapid loss of catalyst activity.'
Other factors which can cause a catalyst to die are (Garforth, 2010b):
Exhaustion: caused by active sites dealing with too many molecules of the reactant.
Poisoning: the reactants have brought in an alien atom or molecule which sits on the active site; therefore prevents access to the active site.
Fouling: the reaction generates a by-product that sits on the active site which prevents access to the active site.
Mechanical collapse: a liquid appears accidentally in the gas stream, and the thermal shock or the capillary effect blows the catalyst apart physically.
Thermal shock: rapid temperature change causes the ceramic support to shatter.
There has been a great deal of research done in catalytic dewaxing. An important factor which is usually discussed is the type of catalyst which is used and the appropriate properties which it has. One of the properties which have been looked into is the size of the pores and the effect it can on selectivity.
Huve (n.d.) refers to this where he mentions that in catalytic dewaxing, they usually use ZSM-5, a medium-pore zeolite which means that it is suitable for a process that requires high selectivity. However, if the feed is highly paraffinic (it has a hydrocarbon content of more than 25%) then a large-pore zeolite should be used.
Research has been carried out by Menoufy et al. (2009), on the use of a medium-pore zeolite (ZSM-5) and a large-pore zeolite (mordenite) in the production of lube oil. In addition to zeolites types, Menoufy et al. (2009) have also investigated the influences of metal loading and operating reactor temperatures on the two types of zeolites.
Their findings have shown that the conversion of the waxy molecules increased as the reaction temperature was increased, while the pour point (the lowest temperature at which it will flow) decreased. The process is based on selective cracking so they observed the selectivity for producing dewaxed oil.
Menoufy et al. (2009) found that, 'the selectivity for producing DWO was observed to be more selective on ZSM-5 than NiMo-H-mordenite at the operating temperatures. The difference was due to their difference in pore size member ring opening (MR) (i.e., 10 MR and 12 MR for ZSM-5 and H-mordenite, respectively) and the Si/Al ratio.'
The shape selectivity property of a catalyst is given in terms of the pour point of the product. If the pour point is low then it implies that there has been a greater selective removal of the product. The pour point was lower for ZSM-5 compared to mordenite, thus ZSM-5 exhibited a higher degree of selectivity (Menoufy et al., 2009).
Menoufy et al. (2009) further went on to compare which catalyst had a higher percentage of dewaxing when the temperature was changed. When the temperature was between 290oC - 375oC, ZSM-5 had the higher percentage of dewaxing. However, mordenite was more selective for wax removal when the temperature was above 375oC.
Mordenite has a high deactivation rate because of its one-dimensional pore system. With the addition of molybdenum and nickel it was found that the activity of mordenite increased and the deactivation rate decreased (Menoufy et al., 2009).
The product quality of the dewaxed lube oil was compared. Mordenite produced the higher product quality when compared to ZSM-5. In addition, the viscosity index of the dewaxed oil was also improved by mordenite catalyst compared to ZSM-5 (Menoufy et al., 2009).
To conclude, the data that was obtained by Menoufy et al. (2009) has shown that for lube oil production a large-pore catalyst was more suited. Outside of a laboratory, different companies use different feeds, operating conditions and use different metals for metal loading. The above study establishes that the conditions dictate whether a small-pore, medium-pore or large-pore catalyst needs to be used.