Liquid-liquid extraction, also referred to as solvent extraction, processes are widely used to purify products in which the product contaminants are preferentially soluble in one of the liquid phases and the desired product is preferentially soluble in the other phase which is immiscible with the first liquid phase. The commercial extractors employing solvent extraction include both unagitated and agitated columns such as mixer-settlers all of which rely on the transfer of the component to be separated (the consolute component) from one liquid phase through the interface with the other (second) liquid phase and into the second phase. It is common for gelation and or emulsification between the first and second liquid phases to develop when so influenced by the consolute component.
Illustrative of industrial applications of solvent extraction are separation of aromatic and aliphatic hydrocarbons, desulfurization of hydrocarbons, butadiene separation, caprolactam extraction, manufacture of acetic acid, pharmaceutical processes such as for antibiotics and vitamins, phenol production, manufacture of synthetic elastomers and inorganic metal extraction applications such as is used in conversion of ilmenite to titantium dioxide. Oftentimes these extraction processes utilized a hydrocarbon such as hexane as one liquid component and water as the other liquid component relying on the solubility of the consolute component in water for its removal from and resultant purification of the desired product which is soluble in the hexane.
Such a liquid-liquid extraction procedure is commonly used in the production of saturated or unsaturated, amorphous, elastomeric olefin copolymers. These include copolymers of ethylene with propylene, and terpolymers of ethylene with propylene and with a cyclic or acyclic nonconjugated diolefin, such as 1,5-cyclooctadiene, 1,4-hexadiene, 6-methyltetrahydroindene, methylene-norbornene, ethylidene-norbornene, etc. These copolymers and terpolymers are usually produced by solution processes, in the presence of catalytic systems consisting of compounds of the transition metals of Group V of the Periodic Table of Elements, preferably vanadium compounds, such as vanadium triacetylacetonate, vanadium oxychloride, and of organometallic compounds of metals of Groups I to III of the Periodic Table of Elements, preferably aluminum compounds.
When the process provides the polymers of appropriate molecular weight, it is useful to quench the polymerization by introducing a quenching agent such as water.
In such processes, it is necessary to purify the polymeric compounds by removing the catalyst residues therefrom because, besides contributing to the formation of ash content in the polymeric product, the presence in the polymer of these residues promotes the occurrence of oxidative degradation processes with consequent loss of quality of the polymeric product.
It is known from the prior art to purify elastomeric products in solution in suitable organic solvents, by a solvent extraction process involving washing the polymer solutions with aqueous solutions of suitable extracting agents which are capable of forming water soluble compounds with the catalyst residues and subsequently separating the aqueous phase from the organic phase. Unfortunately, quenching with water and/or washing with aqueous solutions of extracting agents, complexing agents, pH of the system, viscosity of one or both phases and the efficacy of mixing can result in emulsification and/or gelation of one or more of the components and/or poor contact between the extracting agent and the catalyst residues being extracted.
Various specific approaches to removing catalyst residue from the polymers are hereafter set forth: U.S. Pat. No. 3,337,514, to Knabeachuh et al, requires contacting a solution of an alpha-olefin copolymer with steam, then with aqueous mineral acid, then with water under turbulent conditions, and separating the copolymer solution from the aqueous phases; U.S. Pat. No. 3,740,381 approaches purification by washing with an aqueous solution of extracting agent in the presence of a propylene monomer and a solvent for the polymer product; U.S. Pat. No. 3,804,815 treats the polymer product with aqueous caustic, followed by filtration and removal of the aqueous phase; and, U.S. Pat. No. 4,016,349 utilizes polar liquid extractants (such as water) supported on a finely divided, solid material such as diatomaceous silica, silica gel, alumina or molecular sieves which material is then separated from the polymer solution.
There is also a need to prevent gel/emulsion formation in liquid-liquid extraction processes, particularly those in which water is one of the immiscible phases, and a need for a simple and efficient process for removing metal catalyst residues from alpha-olefin hydrocarbon polymer solutions, and especially a process suitable for continuous catalyst residue removal.