Dewaxing
Another important separation process in petroleum refining is removal of wax. The process of dewaxing is introduced and discussed in the next sub-section.
Figure 1 locates the dewaxing process in the refinery landscape. The feedstocks to dewaxing include DAO from deasphalting, and HVGO from vacuum distillation as shown in Figure 1 along with some compositional characteristics of the feedstock and the dewaxing product. Note that wax (long-chain paraffins) obtained in dewaxing is a marketable by-product. Lubricating oil base stock is the principal product of interest. The main purpose of dewaxing is to remove hydrocarbons that solidify readily (i.e., wax) for making lubricating oil base stock with low pour points (-9 to 14°F).
In addition to low pour points, other important properties of lube oil base stocks include:
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Volatility – should be low to keep oil in the liquid phase during engine operation. Vapors are not good lubricants.
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Viscosity– important to control because of lubrication and heat transfer considerations. Moderate viscosities are desired. Low viscosity may not provide the required lubrication and lead to high friction between metal parts. High viscosity causes loss of energy.
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Viscosity Index (change in viscosity with temperature) – small change in viscosity is desired over a wide temperature oil, i.e., high viscosity index (HVI). HVI ensures that the lube oil functions well at both cold start and at high temperatures generated by the engines.
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Thermal Stability – High thermal stability (or small degree of thermal degradation at high temperatures) is necessary to minimize viscosity loss and coke deposition on metal surfaces.
All of these properties depend on the molecular composition of the hydrocarbons constituting the lubricating oil base stocks. Commercial engine oils and other commercial lube oils are formulated with chemical additives that would enhance the performance of the base stocks.
Two commercial methods of dewaxing are:
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Solvent dewaxing - physical process; separation of wax by freezing and solvent transport.
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Catalytic dewaxing - chemical process; removal of wax by selective reaction of long chain n-alkanes (wax).
1. Solvent Dewaxing
Figure 2 shows a general scheme of solvent dewaxing that uses stage-wise refrigeration of the feedstock after it is mixed with the solvent. The lowest temperature used in the refrigeration cascade depends on the desired pour point of the lube oils base stock product. Upon refrigeration, wax compounds solidify to form crystals. Wax crystals are carried in the solvent to a rotary filter where wax is separated on a filter cloth covering the rotating drum. The layer of wax (filter cake) on the drum is scraped from the filter by a blade and carried away in a solvent stream to a steam-stripping unit to recover and recycle the solvent separated from the wax product. The wax product, called slack wax, can be used to make paraffin wax for candles, microwax used in the cosmetics industry, and petrolatum for petroleum jelly. The dewaxed oil from the filtration unit is also steam stripped to recover the solvent to produce the lube oil base stock.
The two principal solvents used in solvent dewaxing units are methyl ethyl ketone (MEK) and propane. Although the majority of dewaxing units in the U.S. refineries use MEK), some advantages of using propane as a solvent compared to MEK include the following:
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Propane is used both as a diluent and as a refrigerant
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Lower capital investment
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Refrigeration energy savings
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Higher filtration rates
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Rejection of asphaltenes and resins in the feed
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Higher VI than ketone dewaxing
2. Catalytic Dewaxing
Although included under the separation processes, catalytic dewaxing is actually a low- severity conversion process involving a selective catalytic cracking of n-paraffins. Because of removing wax (long chain n-paraffins) by chemical reaction, the process is called dewaxing. The selective cracking of n-alkanes takes place in the pores of molecular sieve catalysts (zeolites) with pore openings in the order of 0.6nm, which keep i-paraffins out because of their larger size due to branching in the hydrocarbon skeleton, as shown in Figure 3. This selective cracking increases the ratio of i-paraffins to n-paraffins in the product and lowers its pour point. Hydrogen is introduced along with the feed to prevent coking on the catalyst surfaces (Figure 4). The cracking of n-paraffins produces distillate fuels such as gasoline as a by-product from catalytic dewaxing.
The advantages of catalytic dewaxing include:
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production of lube base stock with lower pour point and in higher yield compared to the product obtained from solvent dewaxing. Low yield from solvent dewaxing results from the difficulty of separating the oil from the wax;
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lower capital investment;
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good product stability;
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flexibility to produce both lube oil base stock and light distillates.
