Cloud point temperature

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. 

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). 

In this study we examined the influence of concentration conditions, acidity of solutions, and electrolytes inclusions on the liophilic properties of the surfactant-rich phases of polyethoxylated alkylphenols OP-7 and OP-10 at the cloud point temperature. The liophilic properties of micellar phases formed under different conditions were determined by the estimation of effective hydration values and solvatation free energy of methylene and carboxyl groups at cloud-point extraction of aliphatic acids. It was demonstrated that micellar phases formed from the low concentrated aqueous solutions of the surfactant have more hydrophobic properties than the phases resulting from highly concentrated solutions. The influence of media acidity on the liophilic properties of the surfactant phases was also exposed. 

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. 

Theta temperature (Flory temperature or ideal temperature) is the temperature at which, for a given polymer-solvent pair, the polymer exists in its unperturbed dimensions. The theta temperature, , can be determined by colligative property measurements, by determining the second virial coefficient. At theta temperature the second virial coefficient becomes zero. More rapid methods use turbidity and cloud point temperature measurements. In this method, the linearity of the reciprocal cloud point temperature (l/Tcp) against the logarithm of the polymer volume fraction (( )) is observed. Extrapolation to log ( ) = 0 gives the reciprocal theta temperature (Guner and Kara 1998). 

Turbidimetry is ideally suited to detect the temperature at which a transparent polymer solution turns opaque. The temperature corresponding to the onset of the increase of the scattered light intensity is usually taken as the cloud-point temperature, TcP, although some authors define the cloud point as the temperature for which the transmittance is 80% (or 90%) of the initial value. This technique is commonly known as the cloud-point method [199]. Turbidimetry was employed, for instance, to show that the cloud-point temperature of aqueous PNIPAM solutions does not depend significantly on the molar mass of the polymer [150]. 

In all cases, the cloud-point temperature was slightly dependent on polymer concentration for a given copolymer it increased with decreasing concentration. This effect is enhanced with increasing number of PEO grafts per chain. Also, the PNIPAM collapse seemed to be less abrupt with decreasing concentration. Upon dilution of the solution the distance between polymer chains increases, which favours intrapolymeric interactions over in-terpolymeric attractions. Dilution also enhances the surface stabilisation of the polymer particles by PEO. 

The surfactant selected for CPE technique should not have too high a cloud point temperature. In practice, it is possible to obtain almost any desired temperature by choosing an appropriate mixture of surfactants, as cloud point temperatures of mixtures of surfactants are intermediate between those of the two pure surfactants, or by the choice of an appropriate additive (i.e., salts, alcohols, organic compounds) [105]. 

Aqueous micellar solutions of many nonionic surfactants, with an increase in temperature, become turbid over a narrow temperature range, which is referred to as their cloud-point [17,277]. Above the cloud-point temperature, such solutions separate into two isotropic phases. Cloud-point extraction (CPE) is also referred to as a particular case of ATPE [278,279] and more specifically as aqueous micellar two-phase systems [10,277]. Very recently, in an extensive review, Quina and Hinze [280] have discussed in detail the emergence of CPE as an environmentally benign separation process, highlighting the basic features, experimental protocols, recent applications, and future trends in this area. 

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. 

Reduction of fuel viscosity at high temperatures. At temperatures above the cloud point, wax in fuel is not organized into a lattice-like network or into an organized crystalline form. Above the cloud point temperature, fuel viscosity is influenced primarily by the chemical composition and concentration of all fuel components. 

For most distillate fuels, cloud point temperatures can range from 50°F to -10°F (10.0°C to -23.3°C) or lower. However, typical cloud point temperatures fall between 6°F and 16°F (-14.4°C and -8.9°C). Distillate blends having a high paraffin content will often have cloud point and pour point values close together, sometimes within 5°F (2.8°C). Highly aromatic blends will usually have cloud and pour point values further apart in temperature. 

The cloud point test is one of the most commonly used methods to evaluate the low-temperature characteristics of distillate fuel. The cloud point temperature identifies the point when wax begins to form into crystals large enough to become visible in the fuel. At this temperature, wax can settle from fuel, deposit onto fuel filters, and interfere with the flow of fuel through small tubes and pipes. During cold weather months, distillate fuels with lower cloud point values are refined and blended to minimize the low-temperature problems associated with wax. 

Diesel fuel may have been stored for several days below its cloud point temperature wax settling has resulted... 

Suzuki et al. reported cloud-point temperatures as a function of pressure and composition in mixtures of poly(ethyl acrylate) and poly(vinylidene fluoride) [9], Their data in terms of p(T) curves at constant composition show that miscibility in the same system may either improve or decline with rising pressure, depending on the blend s composition. Important consequences for blend-processing ensue. A planned two-phase extrusion may easily be jeopardized by the pressure building up in the extruder. Conversely, a homogeneous melt may be turned into a two-phase system when the pressure on the blend increases. 

EOS models were derived for polymer blends that gave the first evidence of the severe pressure - dependence of the phase behaviour of such blends [41,42], First, experimental data under pressure were presented for the mixture of poly(ethyl acetate) and polyfvinylidene fluoride) [9], and later for in several other systems [27,43,44,45], However, the direction of the shift in cloud-point temperature with pressure proved to be system-dependent. In addition, the phase behaviour of mixtures containing random copolymers strongly depends on the exact chemical composition of both copolymers. In the production of reactor blends or copolymers a small variation of the reactor feed or process variables, such as temperature and pressure, may lead to demixing of the copolymer solution (or the blend) in the reactor. Fig. 9.7-1 shows some data collected in a laser-light-scattering autoclave on the blend PMMA/SAN [46],... 

The cloud point temperature passes through a maximum at ca. 20-30 EO units. For example, with increasing EO number in (EO)y-C12 14, the cloud point (°C) increased in the following series ... 

Caution should be exercised when considering temperature effects on solubilization by micelles, since the aqueous solubility of the solute and thus its micelle/water partition coefLcient can also change in response to temperature changes. For example, it has been reported that although tt solubility of benzoic acid in a series of polyoxyethylene nonionic surfactants increases with temperature, the micelle/water partition coefLci rt, shows a minimum at 2C, presumably due to the increase in the aqueous solubility of benzoic acid (Humphreys and Rhodes, 1968). The increasr in Km with increasing temperature was attributed to an increase in micellar size, as the cloud point temperature of the surfactant is approached (Humphreys and Rhodes, 1968). 

Figure 8.1 Cloud-point temperatures versus volume fraction of modifier, for mixtures of diglycidyl ether of bisphenol A, DGEBA (n = 0.15) with two CTBN copolymers with different acrylonitrile content 18 and 10 wt%. (Reprinted from Verchere et ai, 1989, Copyright 2001, with permission from Elsevier Science)...

Another new approach combines MAE with the use of an aqueous surfactant solution as the extracting phase. This new technique is called microwave-assisted micellar extraction (MAME). This procedure is based on the well-known solubilization capacity of aqueous micellar solutions toward water-insoluble or sparingly soluble organic compounds. As a general rule, nonionic surfactants are usually the most effective, showing greater solubilization capacities that rapidly increase with the solubilization kinetics as the cloud-point temperature of the solution is raised. 

Cloud point Temperature at which a surfactant solution of a given concentration becomes turbid (cloudy). 

The opposite behavior as sketched before was detected for solutions of PS in DOP [112], Again, the critical temperature (an UCST at Tc = 12 °C in the quiescent state) turned out to be a function of the shear rate to which the solution is subjected. But, in contrast to solutions of PS and PB in DOP, here enhancements of the UCST as large as 28 °C were recorded at a shear rate of 220 s-1. Similar results have been found for PS solutions in di(2-ethyl hexyl)phthalate or in a mixture of cis- and frans-decalin [113], The solutions demixed in a converging flow from a reservoir into a capillary tube. It has been observed that an increase in the deformation rate raised the UCST or reduced the region of miscibility. In both of these studies an increase of the cloud point temperature of the polymer solutions was used as an indication of phase separation. 

In a third set of experiments we compared cloud-point data of the polystyrene-cyclohexane-carbon dioxide system (9.6 wt% polystyrene, 9.6 wt% C02) to the data of de Loos (9.4 wt% polystyrene, 5.0 wt% and 9.1 wt% polystyrene, 10.0 wt% CO2) [7], This author used the identical polymer sample. The isopleths in Figure show an almost linear increase in pressure as the temperature is risen. Interpolation of the cloud-point temperature with carbon dioxide content and polystyrene content yields excellent agreement of the data. 

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