557
Solvent Processes
from the liquid. However, the centrifuge method has now been largely displaced by solvent dewaxing methods and by more modern methods of wax removal.
There are also later generation dewaxing processes that are being brought on-stream in various
refineries (Hargrove, 1992; Genis, 1997). For example, British Petroleum has developed a hydrocatalytic dewaxing process that is reputed to overcome some of the disadvantages of the solvent
dewaxing processes. In the process of dewaxing, waxy molecules are removed from heavy distillate
fuel cuts or lube distillates.
Catalytic dewaxing is a hydrocracking process and is therefore operated at elevated temperatures
(280°C–400°C, 550°F–750°F) and pressures (300/1500 psi). However, the conditions for a particular
dewaxing operation depend upon the nature of the feedstock and the product pour point required.
The catalyst employed for the process is a mordenite-type catalyst that has the correct pore structure
to be selective for n-paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive
intermediates so that further paraffin degradation is limited to the initial thermal reactions. The process has been employed to successfully dewax a wide range of naphthenic feedstocks (Hargrove
et al., 1979), but it may not be suitable to replace solvent dewaxing in all cases. The process has the
flexibility to fit into normal refinery operations and can be adapted for prolonged periods on-stream.
Other processes include the ExxonMobil distillate dewaxing (MDDW) process (Smith et al.,
1980) by which dewaxing is achieved by selective cracking in which the long paraffin chains are
cracked to form shorter chains using a shape-selective zeolite that rejects ring compounds and
isoparaffins. In a related process (MIDW process), the paraffins are selectively isomerized using
low-pressure conditions (Smith et al., 1980). This process also uses a zeolite catalyst to convert lowquality gas oil into diesel fuel.
In the process, the proprietary catalyst can be reactivated to fresh activity by relatively mild nonoxidative treatment. Of course, the time allowed between reactivation is a function of the feedstock,
but after numerous reactivation, it is possible that there will be coke buildup on the catalyst. The
process can be used to dewax a full range of lubricating base stocks and, as such, has the potential
to completely replace solvent dewaxing or can even be used in combination with solvent dewaxing.
This latter option, of course, serves to de-bottleneck existing solvent dewaxing facilities.
Both the catalytic dewaxing processes have the potential to change the conventional thoughts
about dewaxing insofar as they are not solvent processes and may be looked upon (more correctly) as
thermal processes rather than treatment processes. However, both provide viable alternatives to the
solvent processes and offer a further advance in the science and technology of refinery operations.
Catalytic dewaxing yields various grades of lube oils and fuel components suitable for extreme
winter conditions. Paraffinic (waxy) components that precipitate out at low temperatures are
removed. In the UOP catalytic dewaxing process, the first stage saturates olefins and desulfurizes and denitrifies the feed via hydrotreating (Genis, 1997). In the second stage, a dual-function,
non-noble-metal zeolite catalyst selectively adsorbs and then selectively hydrocracks the normal
and near-normal long-chain paraffins to form short-chain (nonwaxy) molecules.
Alternatively, in the Chevron Isodewaxing process, the dewaxing results from isomerizing the
linear paraffins to branched paraffins by using a molecular sieve catalyst containing platinum
(Miller, 1994a,b). In the Isodewaxing process, which is followed by a hydrofinishing step (Figure 20.8),
waxy feedstock from a hydrocracker or hydrotreater, together with the hydrogen-containing gas,
is heated and fed to the Isodewaxing reactor. The conditions in the reactor cause isomerization of
n-paraffins to iso-paraffins. Other paraffins are cracked to highly saturated low-boiling products
such as jet fuel and diesel fuel. The effluent from the Isodewaxing reactor is then sent to the hydrofinisher where hydrofinishing, including aromatics saturation provide the product. The catalysts
used in the Isodewaxing and hydrofinishing units are selective for dewaxing and hydrogenation. The
catalysts are at their maximum efficiency with low-sulfur and low-nitrogen feedstocks. The process
generally uses a high-pressure recycle loop. Because of the conversion of wax constituents to other
usable products, the process has obvious benefits over solvent dewaxing insofar as quality of the
product is increased.
Solvent Processes
from the liquid. However, the centrifuge method has now been largely displaced by solvent dewaxing methods and by more modern methods of wax removal.
There are also later generation dewaxing processes that are being brought on-stream in various
refineries (Hargrove, 1992; Genis, 1997). For example, British Petroleum has developed a hydrocatalytic dewaxing process that is reputed to overcome some of the disadvantages of the solvent
dewaxing processes. In the process of dewaxing, waxy molecules are removed from heavy distillate
fuel cuts or lube distillates.
Catalytic dewaxing is a hydrocracking process and is therefore operated at elevated temperatures
(280°C–400°C, 550°F–750°F) and pressures (300/1500 psi). However, the conditions for a particular
dewaxing operation depend upon the nature of the feedstock and the product pour point required.
The catalyst employed for the process is a mordenite-type catalyst that has the correct pore structure
to be selective for n-paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive
intermediates so that further paraffin degradation is limited to the initial thermal reactions. The process has been employed to successfully dewax a wide range of naphthenic feedstocks (Hargrove
et al., 1979), but it may not be suitable to replace solvent dewaxing in all cases. The process has the
flexibility to fit into normal refinery operations and can be adapted for prolonged periods on-stream.
Other processes include the ExxonMobil distillate dewaxing (MDDW) process (Smith et al.,
1980) by which dewaxing is achieved by selective cracking in which the long paraffin chains are
cracked to form shorter chains using a shape-selective zeolite that rejects ring compounds and
isoparaffins. In a related process (MIDW process), the paraffins are selectively isomerized using
low-pressure conditions (Smith et al., 1980). This process also uses a zeolite catalyst to convert lowquality gas oil into diesel fuel.
In the process, the proprietary catalyst can be reactivated to fresh activity by relatively mild nonoxidative treatment. Of course, the time allowed between reactivation is a function of the feedstock,
but after numerous reactivation, it is possible that there will be coke buildup on the catalyst. The
process can be used to dewax a full range of lubricating base stocks and, as such, has the potential
to completely replace solvent dewaxing or can even be used in combination with solvent dewaxing.
This latter option, of course, serves to de-bottleneck existing solvent dewaxing facilities.
Both the catalytic dewaxing processes have the potential to change the conventional thoughts
about dewaxing insofar as they are not solvent processes and may be looked upon (more correctly) as
thermal processes rather than treatment processes. However, both provide viable alternatives to the
solvent processes and offer a further advance in the science and technology of refinery operations.
Catalytic dewaxing yields various grades of lube oils and fuel components suitable for extreme
winter conditions. Paraffinic (waxy) components that precipitate out at low temperatures are
removed. In the UOP catalytic dewaxing process, the first stage saturates olefins and desulfurizes and denitrifies the feed via hydrotreating (Genis, 1997). In the second stage, a dual-function,
non-noble-metal zeolite catalyst selectively adsorbs and then selectively hydrocracks the normal
and near-normal long-chain paraffins to form short-chain (nonwaxy) molecules.
Alternatively, in the Chevron Isodewaxing process, the dewaxing results from isomerizing the
linear paraffins to branched paraffins by using a molecular sieve catalyst containing platinum
(Miller, 1994a,b). In the Isodewaxing process, which is followed by a hydrofinishing step (Figure 20.8),
waxy feedstock from a hydrocracker or hydrotreater, together with the hydrogen-containing gas,
is heated and fed to the Isodewaxing reactor. The conditions in the reactor cause isomerization of
n-paraffins to iso-paraffins. Other paraffins are cracked to highly saturated low-boiling products
such as jet fuel and diesel fuel. The effluent from the Isodewaxing reactor is then sent to the hydrofinisher where hydrofinishing, including aromatics saturation provide the product. The catalysts
used in the Isodewaxing and hydrofinishing units are selective for dewaxing and hydrogenation. The
catalysts are at their maximum efficiency with low-sulfur and low-nitrogen feedstocks. The process
generally uses a high-pressure recycle loop. Because of the conversion of wax constituents to other
usable products, the process has obvious benefits over solvent dewaxing insofar as quality of the
product is increased.