Another solvent extraction process that has been used employs phenol as the solvent. The capacity and the number of phenol extraction units in operation (if Duo-Sol units are included with phenol) exceed those available for furfural extraction. However, recent trends indicate that the use of phenol extraction will decline. ExxonMobil’s recent solvent extraction units are based on furfural extraction. The action of these solvents on the oil charge is quite similar, although different operation conditions are used with each solvent. Some lube refineries use N-methylpyrrolidone (NMP) as the extraction solvent.
E. Hybrid Processing
Certain lubricants, used in critical applications, require base stocks with high viscosity indices (⬎ 105 VI) and very low sulfur contents. While solvent extraction can achieve this VI level with some crudes, raffinate yields are typically very low and sulfur removal is limited to about 80%. An alternative processing route for high VI is to couple hydrotreat- ing with extraction. This is commonly called hybrid processing. While hydrotreating helps to preserve yield, the chemical reactions that occur result in some additional viscosity loss. By properly targeting the correct distillate viscosity, extraction severity, and hydrotreating severity, high VI base stocks can be produced at economic yields. Another application of
hybrid processing consists of mild furfural extraction and lube hydrocracking to produce
base stocks from crudes of marginal quality.
F. MEK Dewaxing
The next process in lube base stock manufacture is the removal of wax to reduce the pour point of the base stock.Figure 2.19, a simple representation of the process, shows the waxy oil being mixed with MEK–toluene solvent. The mixture is then cooled to a tempera- ture between 10⬚F (ⳮ12⬚C) and 20⬚F (ⳮ6⬚C) below the desired pour point. The wax crystals that form are kept in suspension by stirring during the cooling. The wax is then
Figure 2.19 Simplified MEK–toluene dewaxing for lubes.
removed from the oil by filtration. Solvent is removed from both the oil and the wax and recycled for reuse.
A simplified flow diagram for a commercial dewaxing unit is shown in Figure 2.20. In this unit, the waxy oil is mixed with solvent and heated sufficiently to dissolve all the oil and wax. The purpose of this step is to destroy all the crystals that are in the oil so that the crystals that will be separated at the filter are formed under carefully controlled conditions. The solution is then cooled, first with cooling water, then by heat exchange with cold product, and finally by a refrigerant. In some cases, more solvent is added at various points in the heat exchange train.
One other distinctive feature of this cooling train is the use of scraped-wall, double- pipe heat exchangers, which consist of a pipe inside a pipe. The inner pipe carries the solvent–oil–wax mixture; the outer pipe the cooling medium, either cold product or refri- gerant. The inner pipe is equipped with a set of scraper blades that rotate and scrape away any wax that plates out on the walls of the inner pipe. This action is necessary to maintain a reasonable rate of heat transfer. Although the method is efficient, scraped-wall, double- pipe heat exchangers are expensive and costly to maintain.
The cooled slurry is passed to filter feed surge drum and then to the filter itself, where the actual separation is accomplished. Rotary vacuum filters used in dewaxing plants are large drums covered with a filter cloth, which prevents the wax crystals from passing through to the inside of the drum as the drum rotates in a vat containing the slurry of wax, oil, and solvent. A vacuum applied inside the drum pulls the solvent–oil mixture (filtrate) through the cloth, thus separating the oil from the wax.
Figures 2.21–2.24 illustrate the operation of this type of filter: by looking at the end of the drum in each one, we can follow a single segment as the drum rotates through one revolution. In Figure 2.21, the segment is submerged in the slurry vat and is building up a wax cake on the filter cloth. The filtrate is being pulled into the interior of the drum. As the drum rotates through the slurry, the wax cake will increase in thickness and, as the segment leaves the vat, the vacuum state is maintained for a short period to dry the cake and remove as much oil and solvent as possible for the wax cake.Figure 2.22shows the cold wash solvent being applied to the cake, displacing more of the oil in the cake. The wash portion of the cycle is followed by another short period of drying. The wax cake (Figure 2.23)is lifted from the filter by means of flue gas. This is accomplished by applying a positive pressure to the inside of the drum. As the drum rotates, the wax cake is guided from the drum by means of a blade (Figure 2.24), which directs it to a conveyor and then to the solvent recovery system. This segment of the drum is then ready to reenter the vat and continue with another cycle of pickup, dry, wash, dry, cake lift, and wax removal.
Figure 2.22 Dewaxing cycle wash.
The oil and solvent are removed continuously from the inside of the drum through a complicated valving system on one end of the drum, and then are pumped to a solvent recovery system. After removal of solvent for recycle, the base stock is ready for use in many applications. The wax from the dewaxing filter, after its solvent is removed, is the starting material for wax manufacture.
In addition to MEK–toluene, several other dewaxing solvents are used. Propane, the same solvent used for deasphalting, may be used for dewaxing. The solubility character- istics of propane are unique, since at the temperatures needed for dewaxing, wax is insolu-
Figure 2.24 Dewaxing cycle wax cake discharge.
ble and may be filtered from the propane–oil mixture. Methyl isobutyl ketone (MIBK) also has been used as a dewaxing solvent. The operation of these solvents is generally similar to that of MEK–toluene.
G. Catalytic Dewaxing
Catalytic dewaxing is a cost-effective and flexible altnerative to solvent dewaxing for lube base stock manufacture. Unlike solvent dewaxing, which physically separates wax crystals, catalytic dewaxing transforms the wax to either nonwaxy isoparaffinic lube molecules or light fuels by-products, depending on the dewaxing catalyst. The first-generation process, Mobil Lube DeWaxing Process (MLDW), produced specification pour point lubes by shape selectively cracking waxy feed molecules to naphtha and LPG. The second-genera- tion process and catalyst developed by ExxonMobil, Mobil Selective DeWaxing (MSDW), is specifically targeted for hydrocracked and severely hydrotreated stocks. Figure 2.25
shows the schematic of the catalytic dewaxing process developed and used at ExxonMobil.
H. Wax Isomerization
Was isomerization is a process for making base stocks of extrahigh viscosity index (140Ⳮ VI) from concentrated wax streams such as slack wax. The process sequence typically includes a hydrocracking step followed by a hydroisomerization step.
I. Hydrofinishing
For many base stocks, dewaxing is the final process. The stocks are then shipped to blending plants, where the final products are made by blending base stocks with additives. Some stocks—particularly premium stocks—require a finishing process to improve color, oxidation, or thermal stability of the base stock.
As shown inFigure 2.26,the hydrofinishing process consists of a bed of catalyst through which hot oil and hydrogen are passed. The catalyst slightly changes the molecular
Figure 2.25 Catalytic dewaxing process flow diagram.
structure of the color bodies and unstable components in the oil, resulting in a lighter colored oil that is improved in certain performance qualities. The process operates similarly to processes that are used to desulfurize kerosenes and diesel fuels.
Hydrofinishing represents relatively mild operation at relatively low temperatures and pressures. It will also saturate residual olefins to form paraffins. This process stabilizes base stock color and improves demulsibility and air release characteristics. Slight improve- ment in oxidation resistance may also result.
J. Hydrotreating
Hydrotreating represents a more severe set of operating conditions than hydrofinishing. At higher pressures and with selected catalysts, the aromatic rings become saturated to become naphthenes. In addition to converting aromatics, hydrotreating can remove most of the sulfur and nitrogen. This also has a positive effect on oxidation stability and deposit control. ExxonMobil uses a proprietary hydrotreating process to manufacture very high quality (VHQ) stocks for severe turbine applications. This process allows retention of selected aromatic compounds to provide the desired additive and contaminant solvency in the finished product, while maintaining good oxidative stability.
K. Hydrocracking
Lube hydrocracking is an alternative to solvent refining for producing lube base stocks. In solvent lube processing, the main objective is to remove undesirable low VI components in crude via liquid–liquid extraction. Lube hydrocracking may be employed to convert the undesirable components into valuable lube molecules.
Hydrocracking is the most severe hydroprocessing operation and is less dependent on feedstock than solvent refining. However, feedstock can have a significant impact on the product properties. Hydrocracking vacuum distillate usually targets base stocks in the 95–105 VI range. Higher operating severities can increase this up to 115Ⳮ VI, but with loss of yield. Use of a high wax (paraffin) content feed will result in an even higher VI (130Ⳮ). Hydrocracking produces predominantly lower viscosity base stocks (80–500 SUS) owing to the cracking of larger, heavier molecules. Thus hydrocracked base stocks cannot be used in many heavy industrial and engine oil products. They must be blended with solvent-refined base stocks and/or other thickening agents to achieve the higher viscosity required in some products.
Hydrocracking is a well-established process in the refining industry. It has been widely used for many years in fuels manufacture and its use in lube manufacture is currently expanding. This growing interest for lube manufacture stems from several advantages hydrocracking offers versus solvent processing:
1. Higher lube yields: converts undesirable components to lubes.
2. Broader feedstock flexibility: permits production from lower quality, cheaper
crudes.
3. Higher quality base oils: can produce base stocks meeting emerging standards