Dewaxing unit

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

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). 

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. 

Pour point The pour point measures the temperature at which a base stock no longer flows, and for paraffinic base stocks, pour points are usually between -12°C and -15°C, and are determined by operation of the dewaxing unit For specialty purposes, pour points can be much lower. The pour points of naphthenic base stocks, which can have very low wax content, may be much lower (-30°C to -50°C). For very viscous base stocks such as Bright stocks, pour points may actually reflect a viscosity limit Pour points are measured traditionally by ASTM D97,4 but three new automated equivalent test methods are the tilt method (ASTM D5950), the pulse method (ASTM D5949), and the rotational method (ASTM D5985). 

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. 

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). 

FIGURE 10.9 Schematic of MLDW lube dewaxing unit. 

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. 

Feedstocks to dewaxing units are generally waxy distillates intended for lube base stock production, but dewaxing to produce wax may be performed on crude distillates if the wax content is high enough. 

Methyl Ethyl Ketone or loose term for ketone dewaxing unit... 

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... 

MEK is used in dewaxing units to produce waxes for the rubber industry to be used for static ozone protection. 

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