Abstract
Paraffin deposition from several relined wax-solvent systems was studied by using a coldspot test apparatus The purpose of the investigation was to determine the effect of surface roughness, plastic coatings and solution composition on the plastic coatings and solution composition on the amount of deposition. No correlation was observed between surface roughness and deposit weight. The wax composition determined whether or not a given deposit remained on a given surface. Normal paraffin waxes of lower molecular weight formed paraffin waxes of lower molecular weight formed deposits that sloughed or slid off smooth steel or plastic coated surfaces and flaked off roughened plastic coated surfaces and flaked off roughened steel or plastic coated surfaces. Wax of a higher molecular weight, containing more microcrystalline material, formed deposits that did not slide or flake off either smooth or roughened steel or plastic coated surfaces. Plastic coatings decreased deposit weights by 30 percent or more for the higher molecular weight wax due to thermal insulation. Deposit weight decreased with stirring rate and increased with time and temperature differential.
Introduction
Why paraffin adheres to a surface has long been a point of discussion. Two theories that have evolved are summarized as follows.
WETTABILITY OR FREE SURFACE ENERGY
"As paraffin is deposited on a surface, it is held in place by adsorption forces. These adsorption forces are dependent upon the bee surface energy possessed by both the paraffin and the surface. As possessed by both the paraffin and the surface. As the free surface energy of the plate or surface is reduced, a resultant decrease in the adsorption force holding the paraffin to the plate or surface takes place. This causes a decrease in the amount of paraffin which can be retained on the plate surface for the flow conditions present."
SURFACE ROUGHNESS
"The results obtained in this investigation indicate that wax does not adhere to the pipe wall but is held in place by surface roughness and/or irregularities. As a wax particle grows along the pipe wall and out into the oil stream, the force pipe wall and out into the oil stream, the force tending to move it out of the tubing would increase. The smoother the surface, the more easily the embryonic deposit would be removed from the surface… Thus, it is concluded that wax does not adhere to steel but that deposits which form on steel surfaces are held in place by surface roughness." Or stated more concisely, "Paraffin deposition on metallic or nonparaffinic plastic surfaces at a given temperature is governed by surface roughness." The purpose of this investigation was to determine the effect of surface roughness, plastic coatings and solution composition on paraffin deposition from refined wax-solvent systems. Transparent refined systems were used so that the process could be studied visually. A cold spot test apparatus similar to that used by Hunt and Jorda was used.
EXPERIMENTAL
DESCRIPTION OF APPARATUS
The deposition tests were carried out and a coldspot tester shown in Fig. 1. This apparatus consists of a double-walled glass test cell, cell lid, cold-spot probe, magnetic stirrer and two constant temperature water circulation systems. The probe face plate temperature was measured throughout each test with a 1/16-in. OD thermocouple that was inserted through the edge of the plate so that the thermocouple bead was positioned at its center. The thermocouple used was a shielded iron-constantan couple with a floating bead. A thermometer was used to measure solution temperature.
SPEJ
P. 17
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