Cloud-point

In the simple cloud point test method (ASTM D-2500), the sample is first heated to a temperature above the expected cloud point and then cooled at a specified rate and examined periodically. The temperature at which haziness is first observed at the bottom of the test jar is recorded as the cloud point. 

The chemical composition of diesel fuel is extremely complex, with an enormous number of compounds normally present (Table 8.2). For this reason, it usually is not practical to analyze diesel fuel for individual compounds but it is often advantageous to define the compounds present as broad classifications of compound types, such as aromatics, paraffins, naphthenes and olefins. 

ASTM D-6379). Mass spectrometry can provide more compositional detail than chromatographic analysis. However, this method requires that the sample be separated into saturate and aromatic fractions (ASTM D-2549) before mass spectrometric analysis. This separation is applicable to diesel fuel but not to jet fuel, because it is impossible to evaporate the solvent used in the separation without also losing the light ends of the jet fuel. 

The aromatic hydrocarbon content of diesel fuel affects the cetane number and exhaust emissions. One test method (ASTM D-5186) is applicable to diesel fuel and is unaffected by fuel coloration. Aromatics concentration in the range 1-75 mass% and polynuclear aromatic hydrocarbons in the range 0.5-50 mass% can be determined by this test method. In the method, a small aliquot of the fuel sample is injected onto a packed silica adsorption column and eluted with supercritical carbon dioxide mobile phase. Mono- and polynuclear aromatics in the sample are separated from nonaromatics and detected with a flame ionization detector. The detector response to hydrocarbons is recorded throughout the analysis time. The chromatographic areas corresponding to the mononuclear aromatic constituents, polynuclear aromatic constituents, and nonaromatic constituents are determined, and the mass-percent content of each of these groups is calculated by area normalization. 

Whereas nuclear magnetic resonance can be used to determine mass-percent hydrogen in diesel fuel (ASTM D-3701, ASTM D-4808), the percentage of aromatic hydrogen atoms and aromatic carbon atoms can be 

This is defined as the temperature at which a haze appears in a sample which is attributed to the formation of wax crystals. Cloud point data is used to determine the tendency of small orifices to plug in cold operating temperatures, normally measured on middle distillate cuts. This property can 

Aqueous solutions of nonionic surfactants become suddenly turbid when the temperature is raised. The temperature at which the sudden onset of turbidity occurs is called the cloud point. The cmc decreases with increasing temperature and the micellar weight increases [103-106]. At the cloud point, the aggregates have become so large that turbidity becomes perceptible even to the naked eye [98,106]. The surfactant solution separates into two coexistent phases a surfactant-rich phase and a phase in which the surfactant is depleted. The cloud point depends on the surfactant concentration, but the concentration effect is usually weak. 

The cloud point and the surfactant-water systems have been extensively studied for nonfluorinated nonionic surfactants. The phase separation at the cloud point has been explained by dehydration of the polyoxyethylene chain of the nonionic surfactant with increasing temperature [107]. The dehydration theory has 

A cloud-point curve determined by Mathis et al. [109] for a fluorinated surfactant is shown in Fig. 6.12. Aliquots (1-2 mL) of surfactant solutions were heated in capped vials and the temperature was gradually raised until the solution became turbid. The curve exhibits a sharp decline and a flat minimum, followed by a gradual increase when the mass fraction of the surfactant is increased from 0% to 1%. The lowest point on the curve is the cloud point of the surfactant in the absence of any additives. 

Critical micelle concentrations of fluorinated surfactants are approximately equal to those of their hydrocarbon analogs with a 1.5-1.8 times longer 

The cloud point is affected by the presence of other solutes. Salts may increase or decrease the cloud point, depending on the nature of the anion and cation. Alcohols, fatty acids, and phenols depress the cloud point [106]. 

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The characteristics of diesel fuel taken into account in this area are the cloud point, the pour point, and the cold filter plugging point (CFPP). 

The cloud point, usually between 0 and -10°C, is determined visually (as in NF T 07-105). It is equal to the temperature at which paraffin crystals normally dissolved in the solution of all other components, begin to separate and affect the product clarity. The cloud point can be determined more accurately by differential calorimetry since crystal formation is an exothermic phenomenon, but as of 1993 the methods had not been standardized. 

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). 

One remaining possibility that is less costly from an energy point of view but needs to be carefully controlled is to incorporate additives called flow improvers. These materials favor the dispersion of the paraffin crystals and in doing so prevent them from forming the large networks which cause the filter plugging. The conventional flow improvers essentially change the CFPP and pour point, but not the cloud point. They are usually copolymers, produced, for example, from ethylene and vinyl acetate monomers ... 

Figure 5.9 shows an example of the efficiency of these products. The reductions of CFPP and pour point can easily attain 6 to 12°C for concentrations between 200 and 600 ppm by weight. The treatment cost is relatively low, on the order of a few hundredths of a Franc per liter of diesel fuel. In practice, a diesel fuel containing a flow improver is recognized by the large difference (more than 10°C) between the cloud point and the CFPP. 

The properties of straight run diesel fuels depend on both nature of the crude oil and selected distillation range. Thus the paraffinic crudes give cuts of satisfactory cetane number but poorer cold characteristics the opposite will be observed with naphthenic or aromatic crudes. The increasing demand for diesel fuel could lead the refiner to increase the distillation end point, but that will result in a deterioration of the cloud point. It is generally accepted that a weight gain in yield of 0.5% could increase the cloud point by 1°C. The compromise between quantity and quality is particularly difficult to reconcile. 

It is mainly in cold behavior that the specifications differ between bome-heating oil and diesel fuel. In winter diesel fuel must have cloud points of -5 to -8°C, CFPPs from -15 to -18°C and pour points from -18 to 21°C according to whether the type of product is conventional or for severe cold. For home-heating oil the specifications are the same for all seasons. The required values are -l-2°C, -4°C and -9°C, which do not present particular problems in refining. 

The tendency to separate is expressed most often by the cloud point, the temperature at which the fuei-alcohol mixture loses its clarity, the first symptom of insolubility. Figure 5.17 gives an example of how the cloud-point temperature changes with the water content for different mixtures of gasoline and methanol. It appears that for a total water content of 500 ppm, that which can be easily observed considering the hydroscopic character of methanol, instability arrives when the temperature approaches 0°C. This situation is unacceptable and is the reason that incorporating methanol in a fuel implies that it be accompanied by a cosolvent. One of the most effective in this domain is tertiary butyl alcohol, TBA. Thus a mixture of 3% methanol and 2% TBA has been used for several years in Germany without noticeable incident. 

Cloud point Pour point Neutralization index Sediment content... 

The nature of these paraffins and their concentration in diesel fuel affect the three temperatures that characterize the cold behavior. The cloud point is the temperature at which crystals of paraffins appear when the temperature is lowered. The cold filter pluming point is defined as the temperature under which a suspension no ionger flows through a standard filter. Finally, the pour point is the temperature below which the diesel fuel no longer flows by simple gravity in a standard tube. These three temperatures are defined by regulations and the refiner has three types of additives to improve the quality of the diesel fuel of winter. 

Additives that affect the cloud point are no longer in frequent use however, it has been shown that certain polymers having branched paraffins can recognize paraffins of equivalent size and keep them in solution. It is therefore possible to complex the longest paraffins selectively and to decrease the cloud point by 3 to 4°C (Damin et al., 1986). 

Cloud point NFT 60-105 ISO 3016 ASTM D 2500 Observation during gradual cooling... 

Damin, B., A. Faure, J. Denis, B. Sillion, P. Claudy and J.M. Letoffe (1986), New additives for diesel fuels cloud point depressents . SAE paper No. 86-1527, International fuels and lubricants meeting and exposition, Philadelphia, PA. 

Many solutions of common nonionic surfactants and water separate into two phases when heated above a certain temperature (the cloud point), and some investigators call the phase of greater surfactant concentration, a microemulsion. Thus, there is not even universal agreement that a microemulsion must contain oil. 

Some additives have the ability to lower the pour point without lowering the cloud point. A number of laboratory scale flow tests have been developed to provide a better prediction of cold temperature operability. They include the cold filter plugging point (CFPP), used primarily in Europe, and the low temperature flow test (LTFT), used primarily in the United States. Both tests measure flow through filter materials under controlled conditions of temperature, pressure, etc, and are better predictors of cold temperature performance than either cloud or pour point for addithed fuels. 

Sodium lauryl sulfate is available in solution, paste, and soHd forms. As a solution its activity ranges between 28—30%, and as a paste it is 55% active. With this detergent in a shampoo, inorganic salts can affect viscosity. In addition, the limited solubiHty of sodium lauryl sulfate requires its judicious use in low cloud point clear shampoo systems. 

Viscosity—Temperature. Oil viscosity decreases with increa sing temperature in the general pattern shown in Eigure 8, an example of ASTM charts which are available in pad form (ASTM D341). A straight line drawn through viscosities of an oil at any two temperatures permits estimation of viscosity at any other temperature, down to just above the cloud point. Such a straight line relates kinematic viscosity V in mm /s(= cSt) to absolute temperature T (K) by the Walther equation. 

Fig. 6. Phase diagram showing the composition pathway traveled by the casting solution during precipitation by cooling. Point A represents the initial temperature and composition of the casting solution. The cloud point is the point of fast precipitation. In the two-phase region tie lines linking the...

Tempera.ture Effect. Near the boiling point of water, the solubiUty—temperature relationship undergoes an abmpt inversion. Over a narrow temperature range, solutions become cloudy and the polymer precipitates the polymer caimot dissolve in water above this precipitation temperature. In Figure 4, this limit or cloud point is shown as a function of polymer concentration for poly(ethylene oxide) of 2 x 10 molecular weight. 

Solubility. PPO polyols with a molecular weight below 700 are water soluble. The triol is slightly more water soluble than the diol. The solubihty in water decreases with increasing temperature. This inverse solubiUty causes a cloud point which is important in characteri2ing copolymers of propylene oxide and ethylene oxide. 

Miscible blends of high molecular weight polymers often exhibit LOST behavior (3) blends that are miscible only because of relatively low molecular weights may show UCST behavior (11). The cloud-point temperatures associated with Hquid—Hquid phase separation can often be adequately determined by simple visual observations (39) nevertheless, instmmented light transmission or scattering measurements frequendy are used (49). The cloud point observed maybe a sensitive function of the rate of temperature change used, owing to the kinetics of the phase-separation process (39). 

A melamine laminating resin used to saturate the print and overlay papers of a typical decorative laminate might contain two moles of formaldehyde for each mole of melamine. In order to inhibit crystallization of methylo1 melamines, the reaction is continued until about one-fourth of the reaction product has been converted to low molecular weight polymer. A simple deterrnination of free formaldehyde may be used to foUow the first stage of the reaction, and the build-up of polymer in the reaction mixture may be followed by cloud-point dilution or viscosity tests. 

Solubility. Poly(vinyl alcohol) is only soluble in highly polar solvents, such as water, dimethyl sulfoxide, acetamide, glycols, and dimethylformamide. The solubiUty in water is a function of degree of polymerization (DP) and hydrolysis (Fig. 4). Fully hydrolyzed poly(vinyl alcohol) is only completely soluble in hot to boiling water. However, once in solution, it remains soluble even at room temperature. Partially hydrolyzed grades are soluble at room temperature, although grades with a hydrolysis of 70—80% are only soluble at water temperatures of 10—40°C. Above 40°C, the solution first becomes cloudy (cloud point), followed by precipitation of poly(vinyl alcohol). 

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