Paraffin crystals

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. 

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 ... 

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. 

Pake spectrum 26, 38, 44 Paraffin crystals 129 Parallel model 130 Partially ordered systems 27... 

If the oil contains paraffin crystals it may be necessary to clean the loam occasionally with a paraffin solvent. 

Stein, R. S. Interaction of methylene deformation frequencies in paraffin crystals. II. A theoretical treatment of the interaction of vibrations. J. chem. Phys. 23, 734—736 (1955). 

Solubility in Alcohol Determine as directed under Solubility in Alcohol, Appendix VI. One milliliter of sample dissolves in 0.5 mL of 90% alcohol, but the solution becomes cloudy upon further dilution, and paraffin crystals may occasionally separate. 

The exact temperature at which the cloud point is reached depends on the total n-alkane content of the fuel, the average size of the n-alkane molecules, their size distribution and chain structure (e.g. degree of branching). Conventional diesels contain as much as 20% of long-chain n-alkanes of limited solubility in the fuel. Pyrolysis-diesels from PE feedstocks can contain more than 40% long-chain n-alkanes. Paraffins crystallize at low temperature into very thin rhombic plates which can clog filters, transfer lines, and pumps, and can lead to engine failure at low temperatures. 

Diesel made predominantly from polyethylene has a higher proportion of paraffins and therefore a higher cloud point. The paraffins crystallize as the temperature is lowered, leading to small visible crystals forming in the diesel. It is these wax crystals that can plug filters at low temperatures. 

The best activity was observed for PCMEDDAC dissolved in n-hexane. Initial waxy crude oil behaves hke a viscoplastic fluid. Doped by PCMEDDAC, waxy oil approaches a Newtonian liquid, and the shear stress decreases considerably due to the modification of the paraffin crystals by the hydropho-bized macromolecules, hi oily environments, PCMEDDAC forms micelles consisting of a hydrophihc core (made of the betaine groups) and a hydrophobic corona (made of the dodecyl groups). The PPD mechanism of PCMEDDAC with respect to waxy crude oil suggests the adsorption of definite fractions of paraffin molecules on the surface of micelles and further retardation of agglomeration. 

Emulsions stabilized by paraffin are usually restricted to light crude oils in oil-field production. If paraffin deposition that restricts production is occurring upstream of an oil-treating facility, it may be feasible to apply a paraffin crystal modifier to the crude oil to prevent paraffin deposition and to eliminate paraffin as an emulsifying agent. A paraffin crystal modifier must enter an oil system at a temperature greater than the cloud point of the crude oil and upstream of the problem area. 

On the other hand, microhardness studies of short paraffin crystals reveal the influence of the molecular packing and hence of the chain ends on b (Ania et al, 1986). 

The trapping of polycyclic aromatic molecules in n-heptane monocrystalline matrices, prepared by slow cooling, leads to oriented molecules inside the paraffinic crystal lattice, and it appears that the trapping sites for coronene, perylene, and pyrene are similar to those observed earlier for quickly frozen solutions of coronene and designated pseudo-liquid sites and substitutional sites .28 From the former studies it seems that aggregation does not play a dominant role. A nitrogen-pumped laser has been used to excite perylene in n-octane at 4.2 in order to investigate non-linear dependence of fluorescence on excitation intensity in a Shpol skii system. The approximately square dependence of one of the vibronic emission lines on the excitation intensity is consistent with super-radiant emission.80... 

Thermal analysis, infrared. X-ray and iH NMR data were collected and interpreted as follows At about 390 K a sharp endothermic transition with most of the overall entropy-change indicates the transition into the condis phase of the paraffin chains. There is a second, small transition at about 408 K that may involve only a minor readjustment of the remaining crystal. At 7 K, finally the clearing is observed in a capillary melting point experiment. Only a minor endotherm corresponds to this transition to the isotropic phase. The low temperature phase is identified as fully ordered through X-ray diffraction, and the infrared spectrum shows the typical paraffinic crystal-splitting of the absorption lines. NMR-linewidth studies reveal at 77 K a second moment of about 27 that gradually narrows, and... 

Paraffin crystals held by a one-dimensional macroscopic rigid mesogen were observed for poly(L-glutamate)s with long n-alkyl side chains... 

Possibly the most direct route to resolving discrepancies of the magnitude in question would be a theoretical calculation of the surface tension of one of the solids under discussion. For this purpose a calculation for the case of paraffin would offer distinct advantages. These arise as the result of considerable prior effort which has been devoted to carrying out such calculations for the energy of sublimation of paraffin crystals. Recently, these calculations have been reviewed and extended by Salem [57], who obtained good agreement with experiment [15]. 

Complications such as these extend also to the case of polytetra-fluoroethylene. The large difference in estimated solid-vacuum tensions between this polymer and polyethylene is not imexpected, since a proportionately large difference exists for the liquid surface tensions of hydrocarbons and fluorocarbons having five to eight carbon atoms [58]. The underlying cause of this difference is, however, more obscure. The inter molecular forces for fluorocarbons apparently have features wuich lead to anomalous behavior, at least from the point of view of solubility parameter theory [59]. Thus, theoretical calculations of the surface tension for the bare solid in the case of polytetrafluoroethylene would face a number of difficulties not encountered with paraffin crystals. 

Verification of the estimates presented will be aided by further experimental studies, particularly direct measurements of film pressures. Also of distinct interest will be theoretical calculations of the solid-vacuum tension for a paraffin crystal. 

It is intermediate between the extreme values of 67 x 10 and 15 x 10 which are theoretically calculated by Keedy et al. using the Denbigh and the Bunn-Daubeny values of bond polarisability, respectively. This is reasonable since the internal field within a polypropylene crystal might be expected to be intermediate between that in a normal paraffin crystal (Bunn-Daubeny) and a normal paraffin vapour (Denbigh). 

The X-ray diffraction pattern of low molecular paraffin crystals shows interesting changes when the temperature is raised, indicating that, before the melting point is reached, the lateral spacings of the chains become less differentiated and it is assumed that the chains then become free to rotate around their long axes. In macro-molecular hydrocarbons the crystallites disappear beyond a certain temperature and the chains then coil up to form randomly kinked configurations. This is accompanied by the contraction of the macroscopic object if it was initially anisotropic. 

Kane, M., Djabourov, M., Voile, J-L., Lechaire, J-P., and Frebourg, G. (2003). Morphology of paraffin crystals in waxy crude oils cooled in quiescent conditions and imder flow. 

Morphology of paraffin crystals in waxy crude oils cooled in quiescent conditions and xmder... 

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