Solvent dewaxing unit

B A solvent dewaxing unit in an oil refinery is separating 3000 bbl/day of a lubricating... 

Depending upon the specific needs, conventional plants may be converted to hybrid plants to improve capacity and/or product quality. An example of a hybrid configuration is used by Shell to manufacture their XHVI. In this particular case, the plant couples a lube hydrocracker with a solvent dewaxing unit. There are several modifications or improvements of known technologies that can be used as a Mix and Match to Needs to upgrade existing plants. 

Another variant of the severe hydrotreatment process is the substitution of wax for lubricant distillate as feedstock. The wax recovered from conventional solvent dewaxing units is essentially a pure alkane feedstock containing a high proportion of linear alkanes. With this type of feedstock and under appropriate operating conditions, the isomerisation reaction can be made to predominate over cracking reactions. Unconverted wax can be removed by conventional methods to yield a base oil that is exclusively composed of isoalkanes and that resembles synthetic polyal-phaolefin base fluids more closely than the hydrocracked base oils described in Section 1.5.2. A comparison of some of these base fluid properties is shown in Table 1.4. 

In their hybrid scheme (Figure 7.7), the light neutral fraction from the vacuum tower and solvent extraction steps goes directly to the solvent dewaxing unit, by-passing the hydrotreater, since their experience was that on such good quality streams, solvent extraction sufficed to make acceptable base stocks. The other two heavier waxy distillate streams are subjected to quality upgrading... 

This scheme maximizes the use of existing solvent refining equipment and the solvent dewax unit if wax production is profitable. Alternatively, solvent dewaxing can be replaced by wax isomerization, which improves dewaxing yields to 85% to 97% and gives a VI uplift of 4 to 10 units. 

In the case of paraffinic feeds, very low pour point products were most economically obtained by using partially solvent dewaxed feedstocks (Table 9.9). It can be seen as well that these are all relatively light lubes. The advantage cited for this approach is that this can debottleneck the solvent dewaxing unit, where throughput becomes very limited when attempting to produce very low pour products. 

Lubricants. Petroleum lubricants continue to be the mainstay for automotive, industrial, and process lubricants. Synthetic oils are used extensively in industry and for jet engines they, of course, are made from hydrocarbons. Since the viscosity index (a measure of the viscosity behavior of a lubricant with change in temperature) of lube oil fractions from different cmdes may vary from +140 to as low as —300, additional refining steps are needed. To improve the viscosity index (VI), lube oil fractions are subjected to solvent extraction, solvent dewaxing, solvent deasphalting, and hydrogenation. Furthermore, automotive lube oils typically contain about 12—14% additives. These additives maybe oxidation inhibitors to prevent formation of gum and varnish, corrosion inhibitors, or detergent dispersants, and viscosity index improvers. The United States consumption of lubricants is shown in Table 7. 

The dewaxed oil and wax phases discharged from the centrifuges are transferred through heat exchange equipment to individual solvent recovery units for continuous recovery of solvent. 

Figure 5.28 Simplified flow schemes of (a) a conventional and (b) Mobil Oil s membrane solvent dewaxing processes. Refrigeration economizers are not shown. The first 3 million gallon/day commercial unit was installed at Mobil s Beaumont refinery in 1998. Polyimide membranes in spiral-wound modules were used [41-43]...

The state of the art, today, is based on an all catalytic lube plant, which does not rely on solvent processing. An example of such a plant is Mobil s Jurong plant located in Singapore (11). The configuration of the plant relies on the use of a lube hydrocracker, coupled with a selective catalytic dewaxing unit (Fig. 8.2). 

BP found that the solvent extraction step used in traditional lubes manufacturing could be either before the catalytic dewax unit or downstream from it. They were able to solve the VI problem if solvent extraction followed catalytic dewaxing. In this configuration, the depth of extraction could be adjusted to produce base stock with the same viscosity and VI as solvent dewaxing—the extraction removes low VI, high viscosity material so both parameters are brought into line without further loss of yield (Table 9.11). 

As in the traditional acid extraction process, the feedstock is generally dewaxed solvent refined base stock, since levels of the aromatics, polynuclear aromatics, and nitrogen and sulfur compounds are already reduced relative to a straight-run gas oil. This facilitates hydroprocessing by lightening the load on the catalysts and extending their lives. Equally important is that this is an already dewaxed feed, so the white oil producer does not have to bear the capital costs of crude fractionation and dewax units. 

Figure 6 Membrane filtration unit for solvent dewaxing.

The primary goal of solvent dewaxing is to make the pour and cloud point requirements. This is accomplished by paraffin separation by solubility of non-paraffins in cold solvent, fractional crystallization, and filtering the solid paraffins from the slurry. This may be done in ketone units which use MEK, MEK/MIBK, MEK/Toluene solvents or in propane units which use liquefied propane as the solvent. Secondary effects include viscosity increase, density increase, sulfur increase, and reduction in VI. 

A colorless wax extracted from paraffin-base lubricating oils. Typically solid at room temperature. Propane Dewaxing Unit to remove wax from oil using propane as a solvent... 

Although the wax distillate and the cylinder stocks from vacuum units are usually dewaxed without difficulty, some refiners have found that an intermediate heavy wax distilliEite ciit must be produced to the amount of 3 to 6 per cent if the wax distillate and the cylinder stock are to have good dewaxing characteristics. The use of solvent dewaxing processes has eliminated most of these difficulties. 

Nine units have been installed using liquid propane as a dewaxing solvent. 

This dewaxing process is always installed in combination with Edeleanu solvent extraction because recovery of the solvents from the preceding step is not required. For example, if dewaxing follows the refining operation, the raffinate solution is taken directly from the extraction unit and the solvent composition is adjusted to that used for dewaxing by the addition of a benzene-rich solvent which is readily recovered from the dewaxed oil solution because of the wide difference in boiling points of the solvents. 

Tlie complex refinery has 8 Mt-y distillation capacity. It comprises an FCC, a catalytic reforming unit, a lube oil plant (NMP solvent, MEK dewaxing) and a bitumen plant among others. 

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