Wax crystallization

Wax blends Wax cracking Wax crystal modifiers Wax emulsions Waxes... 

Pour-Point Depressants. The pour point of alow viscosity paraffinic oil may be lowered by as much as 30—40°C by adding 1.0% or less of polymethacrylates, polymers formed by Eriedel-Crafts condensation of wax with alkylnaphthalene or phenols, or styrene esters (22). As wax crystallizes out of solution from the Hquid oil as it cools below its normal pour point, the additive molecules appear to adsorb on crystal faces so as to prevent growth of an interlocking wax network which would otherwise immobilize the oil. Pour-point depressants become less effective with nonparaffinic and higher viscosity petroleum oils where high viscosity plays a dominant role in immobilizing the oil in a pour-point test. 

Insofar as they are used to purify other products, several processes used in the refinery fall under the classification of dewaxing processes however, such processes must also be classified as wax production processes (2). Most commercial dewaxing processes utilize solvent dilution, chilling to crystallize the wax, and filtration (28). The MEK process (methyl ethyl ketone—toluene solvent) is widely used. Wax crystals are formed by chilling through the walls of scraped surface chillers, and wax is separated from the resultant wax—oil—solvent slurry by using fliUy enclosed rotary vacuum filters. 

The crystallization of wax from lubricating oil fractions makes better oil. This is done by adding a solvent (often a mixture of benzene and methyl ethyl ketone) to the oil at a temperature of about -5 F. The benzene keeps the oil in solution and maintains fluidity at low temperature the methyl ethyl ketone acts to precipitate the wax. Rotary filters deposit the wax crystals on a sp woven cloth stretched over a perforated cylindrical drum. A vacuum in the drum draws the oil through the perforations. The wax crystals are removed from the cloth by metal scrapers and ol vent-washed to remove oil followed by solvent distillation to remove oil for reuse. 

This condition is of concern only when equipment operates in subzero ambient temperatures. Since diesel fuel extracted from crude oil contains a quantity of paraffin wax, at some low ambient temperatures this paraffin will precipitate and create wax crystals in the fuel. This can result in plugging of the fuel filters, resulting in a hard or no-start condition. Any moisture in the fuel can also form ice ciystals. Cloud point temperatures for various grades of diesel and other fuels should be at least 12°C (21.6°F) below the ambient temperature. In cases where cloud point becomes a problem, a fuel water separator and a heater are employed. 

Cold flow improvers (pour point depressants) These viscosity improvers are often specified in cold climates for unheated gas oil or where existing residual oil heaters are inadequate. The use of these paraffin crystal modifiers permits fuel to continue to flow at temperatures of 30 to 40 °F lower than the point at which wax crystallization would normally occur. 

Solvent dewaxing (Figure 3.14), in which the feedstock is mixed with one or more solvents, then the mixture is cooled down to allow the formation of wax crystals, and the solid phase is separated from the liquid phase by filtration. 

Wax Crystal Control Nanocomposites Stimuli-Responsive Polymers... 

This value is important to help ensure that wax crystals do not form and interfere... 

These wax formations are not as symmetrical as plate crystals. Mal-shaped wax is composed of both linear and branched paraffin molecules. The branched paraffin molecules change the confirmation of the wax so that stacking and alignment typical in plate wax formation does not occur. The result is a wax crystal which is smaller than plate crystal wax. The temperature at which mal-shaped wax forms is also lower than plate crystal formation temperature. 

The shape of these wax crystals resembles a hollow cylinder or cone. Needle wax crystals form at the lowest temperature of all wax crystals and are quite small. Cycloparaffins and branched paraffins comprise the needle wax crystal structure. 

Further cooling of the fuel leads to wax crystal formation throughout the fuel matrix. The growing wax crystals develop into a larger latticelike network encompassing the bulk fuel volume. This latticelike network eventually causes the fuel to become highly viscous and to eventually gel into a semisolid mass. The lowest temperature at which fuel remains in the liquid state just prior to gellation is called the pour point. 

It is possible to dilute diesel fuel such as 2-D low sulfur with kerosene, 1 fuel oil, or jet fuel to reduce the fuel cloud point. Also, additives are also marketed which have the ability to inhibit nucleation of wax crystals in some fuels, thereby lowering the cloud point of the fuel. These products are called cloud point improvers. 

During winter and under other low-temperature operating conditions, fuel cannot be effectively filtered at temperatures much below its cloud point unless the fuel wax is diluted with kerosene or treated with a wax crystal modifier. 

Operability additives must often be used at high treat rates, 1,000 ppm or higher, to obtain a reduction in LTFT temperature greater than 10°F (5.6°C). Some wax crystal modifiers provide LTFT performance. However, as with operability additives, the performance should always be evaluated before use. 

The addition of a wax crystal modifier to diesel fuel is a common and well-accepted alternative to kerosene dilution. Wax crystal modifiers are typically polymeric compounds which have the ability to co-crystallize with wax to alter the size, shape, and structure of the wax crystal lattice. 

Two possibilities exist to explain how wax crystal modifiers work. They are summarized as follows ... 

This concept deals with the possibility of the wax crystal modifier acting as the seed crystal onto which fuel wax crystallizes. This explanation requires that the modifier crystallize at the same temperature as the fuel wax. Once crystallized, the wax crystal modifier thus controls the conformation and structure of the resultant wax crystal. 

The most common type of wax crystal modifier used to reduce the pour point and filtration temperature of distillate fuel is based on ethylene vinylacetate (EVA) copolymer chemistry. These compounds are quite common throughout the fuel additive industry. The differences, however, are found in the variation in the molecular weight and the acetate ratio of the copolymer. 

On occasion, the performance of an EVA copolymer can be enhanced by blending with a wax crystal modifier of a different chemical type. Wax crystal modifiers used to modify the crystal structure of lubricant, residual fuel, and crude oil waxes can be blended at low concentrations with EVA copolymers to improve their performance. However, the performance enhancement is usually fuel specific and not broad ranged. Also, the low-temperature handling properties of the EVA may be impaired when blended with other wax crystal modifiers. 

Problems associated with the use of wax crystal modifiers do not pertain so much to the ability of the modifier to perform, but to the proper application technique. These copolymers are quite viscous in nature and must be diluted in solvent in order to be handleable. Even after dilution, they are still quite viscous and have relatively high pour points. 

Whenever a wax crystal modifier does not perform as expected, there are several possible explanations. Some of the possibilities are as follows ... 

In order for a wax crystal modifier to function properly, it must be present to cocrystallize with fuel wax. This requires that the modifier be added to fuel well before wax crystal formation begins. 

It is known that wax can begin the process of organization into a crystal structure above the actual, observable cloud point temperature. Because of this fact, the wax crystal modifier should be added at a temperature at least 20°F (11.1°C) above the cloud point of the fuel. Addition at this higher temperature helps to ensure that the modifier is completely solubilized in the fuel prior to the formation of the wax crystals. 

Wax crystal modifiers added after wax crystals begin to form will have only minimal affect at modifying the size, shape, and structure of wax crystals. Consequently, little improvement in the low temperature handling characteristics of the fuel will be obtained. 

When wax crystal modifiers are added to cold fuel, even to fuel well above its cloud point, modifiers may not dissolve properly. The polymeric nature of wax crystal modifiers makes them quite viscous at low temperatures. Additive suppliers will often provide modifiers in a highly dilute form (i.e., 10% or 20% solution), so they will remain fluid at low temperatures. 

However, if a typical, nondiluted wax crystal modifier is added to fuel which is at a cold temperature of+10°F to +20°F (-12.2°C to -6.7°C), it may not dissolve completely in this fuel. The result will be additive accumulation as a viscous layer at the bottom of a fuel or storage tank. Ultimately, the additive will be trapped by a filter as it flows from the tank. 

A second, and even worse possibility, would be the addition of cold additive to cold fuel. In this case, the additive would not dissolve at all and would set as a gelled mass at the bottom of the fuel tank. When the gelled wax crystal modifier does move from the tank, it may plug a cold fuel line or filter. If allowed to reach a pump, the gelled additive could cause sticking of pistons or other pump parts. 

FUEL WAS PREVIOUSLY TREATED WITH A WAX CRYSTAL MODIFIER... 

Occasionally, wax crystal modifiers will not provide the performance anticipated. Either the response to additive treatment was much less than expected or no response was obtained. When this occurs, it is quite possible that the fuel was previously treated with a wax crystal modifier. Under these circumstances, the expected performance of secondary treatment with wax crystal modifier is minimal. 

It is quite time consuming and expensive to analyze for the presence of a wax crystal modifier in fuel. However, it is possible to determine whether a fuel already contains a wax crystal modifier by analyzing the following test information ... 

Fuel which does not contain a wax crystal modifier will have temperature differences between the cloud and pour points typically from 15°F to 20°F (8.3°C to 11.1°C). If the difference between the cloud and pour point values is greater than 25°F (13.9°C), it is quite reasonable to believe that the fuel contains a wax crystal modifier. 

The cloud point and the cold filter plugging point temperatures for fuel which does not contain a wax crystal modifier can often be the same. Typically, untreated cloud point and CFPP values will be within 2°F to 4°F (about 1°C to 2°C) of each other. If the temperature difference between an untreated fuel s cloud point and CFPP differ by 10°F (5.6°C) or more, the fuel probably contains a wax crystal modifier. 

When pour point testing is performed on crude oil, little to no shear is applied to the oil in the pour point tube. Under these conditions, the wax crystal lattice matrix which forms in the crude oil normally remains intact and the oil gels at the pour point. 

However, when disturbed by pumping, mixing, or agitation, the loosely formed wax crystal lattice can sometimes be broken with applied shear. If this occurs, some crude oils may again begin to pour and continue to flow at temperatures below the initial reported pour point. 

At low temperatures, usually less than 20°F (-6.7°C), waxy paraffins may begin to crystallize in highly paraffinic diesel fuel. These paraffin wax crystals can cause a variety of fuel handling and performance problems. Examples include ... 

Some chemical additives such as corrosion inhibitors, wax crystal modifiers, detergents, and demulsifiers provide performance which is difficult to duplicate through refining without adversely affecting some other fuel property. Other additives such as metal chelators, fuel sweeteners, biocides, lubricity improvers, foam control agents and combustion enhancers can also be used to solve fuel performance problems. 

In addition to refining techniques, compounds identified as wax crystal modifiers are available for use in contending with the effects of wax in fuels. Wax crystal modifiers, also called pour point depressants or cold flow improvers, are typically polymeric compounds which have the ability to crystallize with fuel wax as it forms. By co-crystallizing with wax, the modifiers typically effect a change in the size, shape, and conformation of wax crystals. Other wax crystal modifiers function by dispersing or inhibiting the nucleation or growth of wax crystals within a fuel or oil. 

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