Pour point increase

This problem differs from the problem of pour point increase after shearing and heating. In the case of pour point reversion, an increase in the pour point of a crude oil or heavy fuel oil occurs upon long-term storage. 

From the above lists, there is a natural trade-off between viscosity index and pour point, e.g. by increasing the linearity of the ester, the viscosity index improves but the pour point increases. Esters made from mixtures of normal and branched acids with the same carbon number have viscosity indices between those of the normal and branched acid esters. But their pour points are lower than those esters formed separately from either branched or normal acids. 

The effects of molecular mass and shape on viscosity and VI are similar to that of other base fluid types. Increasing the length of straight-chain alkyl substiments increases viscosity and VI but has a negative effect on low-temperature performance with increased pour points. Increased branching of alkyl substituents, for constant molecular weight, gives lower viscosities and Vis but improved pour point. 

At low n-paraffin concentrations, the pour point increase for all the n-paraffin additions is about 3°C per percent n-paraffin addition, and beyond 10% paraffin content, this increase is reduced to about 0.25°C per percent. These curves can be linearized by recasting the equation in the form... 

At lower temperatures, the crystals increase in size, and form networks that trap the liquid and hinder its ability to flow. The pour point is attained which can, depending on the diesel fuel, vary between -15 and -30°C. This characteristic (NF T 60-105) is determined, like the cloud point, with a very rudimentary device (maintaining a test tube in the horizontal position without apparent movement of the diesel fuel inside). 

The pour point is the lowest temperature at which an oil can still pour while it is cooled, without agitation, under standardized conditions. The pour point of paraffinic bases is linked to the crystallization of n-paraffins. The pour point of naphthenic bases is related to a significant viscosity increase at low temperatures. This property can be improved by additives. 

Viscosity (Viscosity-Index) Improvers. Oils of high viscosity index (VI) can be attained by adding a few percent of ahnear polymer similar to those used for pour-point depressants. The most common are polyisobutylenes, polymethacrylates, and polyalkylstyrenes they are used in the molecular weight range of about 10,000 to 100,000 (18). A convenient measure for the viscosity-increasing efficiency of various polymers is the intrinsic viscosity Tj, as given by the function... 

Shale oil contains large quantities of olefinic hydrocarbons (see Table 8), which cause gumming and constitute an increased hydrogen requirement for upgrading. Properties for cmde shale oil are compared with petroleum cmde in Table 10. High pour points prevent pipeline transportation of the cmde shale oil (see Pipelines). Arsenic and iron can cause catalyst poisoning. 

The increase in fuel viscosity with temperature decrease is shown for several fuels in Figure 9. The departure from linearity as temperatures approach the pour point illustrates the non-Newtonian behavior created by wax matrices. The freezing point appears before the curves depart from linearity. It is apparent that the low temperature properties of fuel are closely related to its distillation range as well as to hydrocarbon composition. Wide-cut fuels have lower viscosities and freezing points than kerosenes, whereas heavier fuels used in ground turbines exhibit much higher viscosities and freezing points. 

Paraffin crystalline waxes Apart from asphaltenes, a number of differing molecular weight paraffinic waxes are also present. These progressively crystallize at lowering temperatures (their respective pour points). These waxes increase friction and resistance to flow, so that the viscosity of the fuel is raised. This type of problem is controlled by the use of pour-point depressants (viscosity improvers), which limit the growth of the crystals at their nucleation sites within the fuel. They also have a dispersing effect. 

On the other hand, in order to preserve the cold properties of the fuel (Cloud Point, Pour Point and low-temperature filterability), it is mandatory not to increase the melting point, that in turn depends on both the saturated compound (stearic acid, C18 0) content and the extent of cis/trans and positional isomerization as the difference in melting point between the cis and trans isomer is at least 15°C according to double bond position as shown in Table 1. 

It is recommended that any higher-viscosity product such as residual oil or heavy distillate fuel be evaluated for changes in low-temperature handling properties over time. Testing for reversion in pour point by the Shell Amsterdam Reversion Test or the British Admiralty Pour Point Reversion Test are recommended. Also, viscosity increase versus temperature decrease determinations are recommended for products stored at low temperatures for extended periods of time. 

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. 

Pour point reversion can occur in oils treated with a pour point improver. Often, pour point reversion can be overcome by increasing the cold flow improver treat rate by about 25% to 50%. 

If possible, measure viscosity vs. temperature between cloud and pour points look for a dramatic increase rather than a gradual increase in slope. 

PROBLEM INCREASE IN POUR POINT OF RESIDUAL FUEL OIL OR CRUDE OIL AFTER HEATING OR SHEARING... 

An unusual behavior can be seen in some crude oils and residual fuel oils after they have been heated or have been sheared through a high-rpm pump. When sampled at the pump outlet, some oils can experience an increase in pour point above that of the same oil prior to heating or shearing by pumping. 

Check for an increase in the pour point of a heated vs. unheated sample. 

PROBLEM REVERSION AND ACTUAL INCREASE IN THE POUR POINT OF A CRUDE OIL OR RESIDUAL FUEL OIL... 

The chilled oil-naphtha solution containing crystallized wax and the filter aid is then filtered by means of a continuous vacuum filter or an intermittent pressure leaf filter. Increased dewaxed oil yields are realized by applying a cold naphtha displacement wash to the wax cake on the filter. The pour point of the dewaxed oil is usually 25° to 35° F. higher than the filtering temperature. 

Before 1925, there were a few compounded oils made for special purposes, such as lubrication of marine engines and steam cylinders, but additives were not used in automotive crankcase oils. In the 1930 s, chemical compounds made by condensation of chlorinated paraffin wax with naphthalene were found to lower the pour points of oils. Pour depressants (9) apparently are adsorbed on small wax crystals which separate from oils when they are chilled. The protective adsorbed layer of additive prevents the normal interlacing of larger wax crystals which forms a gel. In 1934 polymerized unsaturated hydrocarbons first came into large scale commercial use to lower the temperature coefficient of viscosity of oils. Other compounds for increasing the viscosity index of oils have since become common. 

Ignition-Quality Improvers. Diesel fuels have found greatly increased use in recent years—so much so that refiners have had to look to cracked distillates from catalytic cracking operations for their extra Diesel fuels. While these cracked distillates have the advantages of relatively high heat content and low pour point, they are inferior in ignition quality (cetane number) to straight-run distillates from the same crudes. 

Other factors indicated m the data of Tables 1 and 2 include Pour Point—defined as the lowest temperature at which the material will pour and a function of the composition of the oil in terms of waxiness and bitumen content Salt Content—which is not confined to sodium chloride, but usually is interpreted in terms of NaCl Salt is undesirable because of the tendency to obstruct fluid flow, to accumulate as an undesirable constituent of residual oils and asphalts, and a tendency of certain salt compounds to decompose when heated, causing corrosion of refining equipment Metals Content—heavy metals, such as vanadium, nickel, and iron, tend to accumulate in the heavier gas oil and residuum fractions where the metals may interfere with refining operations, particularly by poisoning catalysts. The heavy metals also contribute to the formation of deposits on heated surfaces in furnaces and boiler fireboxes, leading to permanent failure of equrpment, interference with heat-transfer efficiency, and increased maintenance. 

POUR POINT DEPRESSANT. An additive for lubricating and automotive oils that lowers the pour point (or increases the flow point) by 11.0°C. The agents now generally used are polymerized higher esters of acrylic add derivatives. They are most effective with low-viscosity oils. See also Petroleum. 

Yields and properties for high-severity desulfurization of Kuwait atmospheric tower bottoms are contained in Table XV. Residuum conversion to lighter products is apparent by the higher distillate yields relative to low-severity yields indicated in Table XIV. The LSFO product properties affected to the greatest extent with increasing severity are sulfur, viscosity, and pour point. 

A test was made with 2,3-dimethylbutane as the supercritical solvent it has a lower critical temperature than 2,2,4-trimethyl-pentane. Operating at a temperature of 508-512 K, a pressure of 4.10-4.37 MPa, a molecular sieve/oil ratio of 6.39, and a solvent/ oil ratio of 21.3, the molecular sieve capacity attained is 5.73 g/100 g of molecular sieves (as compared to 3.2 g/100 g of molecular sieves with 2,2,4-trimethylpentane at 550 K). The n-paraffin content of the wax distillate was reduced by 88% to a level of 2 wt %, giving a pour point of 266 K. The yield of denormal oil was lower (63%) and the n-paraffin content of the desorbate was lower (44%) at this lower temperature level. This is probably due to increased capillary condensation. Conversely, operation at temperatures greater than 550 K should produce less capillary condensation and purer n-paraffin product. It would be interesting to try supercritical solvents with critical temperatures in the 600-670 K range. 

---