1. Field of the Invention
The proposed invention relates to the field of pipeline transportation of liquid hydrocarbons, in particular to methods for decreasing their hydrodynamic resistance or method of drag reduction.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Recently, polymeric anti-turbulent additives (ATAs) or polymeric drag reducing additives (DRAs) are used for increasing capacity of oil and products pipelines. They represent a polymer solution or a polymer suspension in a liquid medium. A polymer should be soluble in a transported liquid and have a high molecular weight. Another pre-requisite for the effect of drag reduction (Toms Effect) is the turbulent mode of a hydrocarbon liquid flowing in a pipeline.
The introduction of a DRA into a crude oil flow in an amount from 10 to 50 grams per one ton results in increasing the pipeline capacity for 15-25%. The higher a polymer molecular weight, the lower is its concentration that is required for achieving said value of drag reduction.
Ultra-high molecular (co)polymers of higher α-olefins, which are synthesized on Ziegler-Natta catalysts, are most commonly used as a polymeric component of DRA. As to their price-quality relation, they have no match among other oil-soluble polymers. Monomers having from 6 to 16 carbon atoms are used.
Initially, DRAs were produced as a polymer solution in gasoline (kerosene). However, injection-related problems due to its high viscosity, especially in winter, caused the present use of suspension-type DRAs. A suspension is produced, as a rule, by comminuting a product of block-polymerization of higher α-olefins, which is a rubber-like material, at a temperature below its glass-transition point, and thus-obtained polymer particles are mixed with a liquid medium that does not dissolve the polymer. The liquid medium is selected so that its density does not greatly differ from that of the polymer, in order to avoid irreversible suspension layering during storage. The polymer content of a suspension-type DRA may be up to 25% and even more, which exceeds the polymer content of a solution-type additive greatly. The DRA solid phase may comprise surfactants or other anti-agglomerator agents that prevent polymer particles from agglomerating as well as additives for inhibiting polymer oxidative destruction.
Polymerization in a solution medium has been replaced by block polymerization also from the point of a polymer molecular weight that is significantly higher at block polymerization; consequently, a polymer thus produced is of higher quality.
The conventional technology of a suspension agent for decreasing hydrodynamic resistance may be divided into three steps:
1. Block polymerization of a monomer (co-monomers)
2. Comminution of a rubber-like co-polymer
3. Production of a suspension stable against layering (sedimentation)
Many patents relating to methods of producing suspension-type DRAs describe cryogenic comminution of polymers (U.S. Pat. No. 4,826,728, U.S. Pat. No. 4,720,397, U.S. Pat. No. 4,340,076). Since polymers and co-polymers of higher α-olefins are rubber-like materials with low glass-transition temperatures (for example, poly-1-octene has glass-transition temperature below −70° C.), their mechanical comminution is carried out in the liquid nitrogen medium, i.e. below their glass-transition temperatures.
It is well known, however, that operations employing liquefied gases are associated with higher risks and require special expensive equipment as well as certain safety measures for personnel. Furthermore, liquid nitrogen itself is an expensive material. Therefore, recently many companies started to work on creating non-cryogenic technologies of comminuting polyalphaolefins (U.S. Pat. No. 6,946,500, U.S. Pat. No. 6,894,088, U.S. Pat. No. 7,271,205, US Patent Application No. 0276566, US Patent Application No. 0287568). Preliminary impregnation with moistening agents is used for which purpose heavy alcohols, solid and liquid separating agents (derivatives of stearic acid, higher linear alcohols) as well as special equipment (homogenizers, abrasive mills, rotor-stator type grinders) are applied.
Here, it should be noted that a suspension-type additive must comprise a finely comminuted polymer, or the process of dissolving the polymer in an oil pipeline will be too long, which affects the resulting efficiency of a DRA. On the other hand, mechanical comminution of polymers, especially fine comminution to particles having a size of 100-300 microns, leads to partial mechanical destruction of macro molecules and poorer quality of the polymer.
Mechanical destruction can be avoided by, for example, capsulated polymerization of higher α-olefins (U.S. Pat. No. 6,126,872, U.S. Pat. No. 6,160,036, U.S. Pat. No. 4,693,321, US Patent Application No. 20030013783). It consists in that catalyst-comprising monomer droplets are enclosed in a polymeric coating and suspended in a medium that does not dissolve any capsulated component. Micro-block polymerization is carried out in this way, and a dispersed polymer is produced in one step in the synthesis process. A significant disadvantage is low productivity of this process.
An alternative method for producing fine dispersions, which does not affect the length of a polymer molecule, consists in precipitating a polymer from a solution by adding a precipitating agent (U.S. Pat. No. 5,733,953). This method is the closest one to the present invention and is taken as a prototype.
Said method consists in that a polymer is precipitated, which has a high molecular weight and is synthesized by co-polymerization of higher α-olefins with a Ziegler-Natta catalyst in a solvent medium.
The sequence of producing such a suspension is as follows (a citation): “a low-viscosity highly concentrated polymer suspension is produced by slowly adding a liquid not dissolving a polymer (e.g., isopropyl alcohol) to a polymer solution in a solvent (e.g., kerosene). When a non-solvent is added in a sufficient quantity, a polymer precipitates from the solution in the form of small particles. A liquid is separated from the precipitate, the latter is once again washed with the non-solvent. The resulting concentrated suspension, when introduced into a flow of a hydrocarbon liquid, dissolves quickly and causes a decrease in hydrodynamic resistance.”
The prototype disadvantages are:                Poor quality of the polymer component        Necessity of regenerating a big volume of solvents        Pollution of the environment.        