How to address precipitation of Wax deposits in Oil transmitting Pipilines etc., (original) (raw)

Paraffin wax produced from crude oil consists primarily of long chain, saturated hydrocarbons (linear alkanes/ n-paraffins) with carbon chain lengths of C18 to C75+, having individual melting points from 40 to 70°C. This wax material is referred to as " macrocrystalline wax. " Naphthenic hydrocarbons (C18 to C36) also deposit wax, which is referred to as " microcrystalline wax. " Macrocrystalline waxes lead to paraffin problems in production and transport operations; microcrystalline waxes contribute the most to tank-bottom sludges.[1] The n-heptane structure is an example of a " normal " paraffin; 2-methyloctane is an " iso " paraffin and n-butylcyclopentane is a naphthene. These specific n-paraffins and naphthenes are too small to crystallize as wax deposits The iso-paraffins of the 2-methyloctane type are more likely to be included in a wax deposit than the more highly branched alkanes. The solubility line of wax is hardly a function of pressure, so wax deposition is considered to be only temperature dependent. (Rønningsen, 2006) As the temperature of the crude drops below a critical level and/or as the low-molecular-weight hydrocarbons vaporize, the dissolved waxes begin to form insoluble crystals. The deposition process involves two distinct stages: nucleation and growth. Nucleation is the forming of paraffin clusters of a critical size (" nuclei ") that are stable in the hydrocarbon fluid. This insoluble wax itself tends to disperse in the crude. Nathan (1955) classified the carbonaceous fractions of paraffin deposits as: (1) soluble in cold acetone (generally the contained oil); (2) soluble in hot acetone (the soft wax portion); and (3)insoluble in acetone (hard wax). The hard wax material consists of straight chain material with molecular weights of 525 to 700 daltons. The proportions of each of these will vary from well to well, and even in a given well at different times. Burger et al. (1981) found that deposits in the trans-Alaska pipeline consisted of 14 to 17% solid (wax), 86 to 83% whole oil. Wright, (1951) remarked that the number of different compounds and types of waxes found in paraffin are dependent upon the character of the crude oil.