Wax crystal modifiers

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

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. 

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. 

Copolymers of ethylene vinylacetate are the most commonly utilized fuel wax crystal modifiers. Other compounds such as vinyl acetate-fumarate copolymers, styrene-ester copolymers, diester-alphaolefin copolymers, as well as alkyl carbamate compounds are effective wax crystal modifiers. These compounds differ in both chemical structure and in the extent of performance provided. See FIGURES 6-7 and 6-8. 

The different chemical compounds used as wax crystal modifiers do not all provide ideal performance under every circumstance. Various tests have been designed to help differentiate the performance of one wax crystal modifier over another. For example, a modifier may be quite effective at controlling wax crystal formation to enable a fuel to flow by gravity from a storage tank to a pump. However, once past the pump, the modifier may not effectively reduce the wax crystal size and shape to allow cold fuel to flow effectively through a line filter. The result is wax accumulation on the filter media, plugging of the fuel filter, and halting of fuel flow. A different wax crystal modifier or a product with wax dispersant properties may be required to permit effective fuel filtration. 

See FIGURES 6-9 through 6-11 for examples of wax crystal modifier performance in different refined fractions. 

Fuel problems which typically can be solved by a wax crystal modifier ... 

Distillate fuel filtration temperatures can be reduced by 10°F to 18°F (about 5°C to 10°C) by most wax crystal modifiers. Greater response is possible, but additive treating rate can be quite high. Also, fuels with low initial filtration temperatures often respond more effectively to wax crystal modifier treatment than high-filtration-temperature fuels. 

Low-temperature distillate fuel pumpability and viscosity can be reduced below that of an untreated fuel. Below the cloud point, fuel which does not contain a wax crystal modifier is difficult to handle and pump effectively. Wax crystal modifiers help minimize pumpability problems related to yield stress and viscosity. 

The pour point and low-temperature viscosity of a residual fuel or heavy fuel oil can be reduced by using a heavy fuel wax crystal modifier. Often, pour point reversion can be prevented by using the correct wax crystal modifier. 

FIGURE 6-10. Typical Response of a Heavy Fuel Wax Crystal Modifier in a... 

Pour Point Improvers—Wax Crystal Modifiers (WCM)—Cloud Point Improvers... 

In order to achieve very low pour points and filtration temperatures in some fuels, high levels of wax crystal modifiers must be used. On occasion, high concentrations of a WCM can cause the pour point or filtration temperature to reverse. This is due to the influence of the high viscosity of these inhibitors at the low test temperatures. 

Fuels treated with a cloud point improver (CPI) may require additional CPI treatment whenever a wax crystal modifier is used to reduce the pour point of the fuel. Often, the cloud point of a CPI-treated fuel will increase whenever a pour point improver is used. To compensate for this phenomenon, additional CPI must be added to recover the lost performance. 

Another common method which is used to improve the handling characteristics of heavy oils is to treat the oils with a wax crystal modifier. The process is similar to that used in diesel fuel treatment. Wax crystal modifiers for use in heavy oils are typically higher in molecular weight than those used in diesel fuel applications. The pour point method ASTM D-97 is also used to evaluate crude oil and heavy oils. 

This procedure can be utilized to determine whether heavy fuel wax crystal modifiers will lose their performance properties after long-term storage at fluctuating temperatures. Daily heating and overnight cooling may interfere with the ability of some wax crystal modifiers to maintain their performance properties in some residual oils and crude oils. This loss of performance is frequently termed pour point reversion. The British Admiralty Pour Point Test can be utilized to help predict these reversion tendencies. 

An oil sample treated with a wax crystal modifier is heated to 200°F (93°C) and poured immediately into pour point test jars fitted with a thermometer capable of reading temperatures from -35°F (-37°C) to 210°F (100°C). 

Treat with wax crystal modifier to control the growth of wax crystals. 

The distillation profile will provide some indication of the low-temperature performance of diesel fuel. IBP values >350°F (>176.7°C) and EP temperatures >700°F (>371.1°C) indicate that the fuel viscosity will probably be high at low temperatures. Even fuel treated with a wax crystal modifier may be difficult to pump due to the high viscosity of the fuel components. 

Often, these fuels respond poorly to treatment with a wax crystal modifier or may require high rates of a wax crystal modifier to improve filtration and pumpability performance. This problem seems to occur whenever two situations exist ... 

Treat with a wax crystal modifier effective at modifying high-carbon-number or narrow-boiling-point temperature range wax. 

Upon storage, it is also possible that the asphaltenes may interact with the wax crystal modifier to mask the performance of the modifier and inhibit the ability of the modifier to fully disperse in the oil. Wax crystal modifier performance may be minimized. 

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