1. Field of the Invention
This invention relates to the manufacture of phenol by the acid cleavage of cumene hydroperoxide. In particular, this invention relates to controlling acidity levels in phenol-acetone streams in order to produce good quality phenol and to maximize the amount of AMS by-product isolated.
2. Description of Related Art
In general, phenol is prepared by the oxidation of cumene by air, followed by an acid catalyzed cleavage of the resulting cumene hydroperoxide to phenol and acetone. The acid cleavage is typically effected by means of strong inorganic acid, usually sulfuric acid, or by addition of SO.sub.2 which may be oxidized in-situ to sulfuric acid. Other suitable inorganic acids include aqueous hydrochloric or perchloric acid. The acid cleavage reaction mixture typically includes primarily phenol, acetone, and cumene. In addition to the principal products, the acid cleavage reaction mixture will further contain varying amounts of minor by-products, principally alpha-methylstyrene, dimethylphenylcarbinol, water, acetophenone, p-cumylphenol and various alpha-methylstyrene dimers. Of these components alpha-methylstyrene is a useful by-product and it is desirable to recover it.
Dimethylphenylcarbinol is relatively unstable and decomposes to alpha-methylstyrene and condensation products under the influence of acidity and/or temperature. The decomposition of dimethylphenylcarbinol produces water and is a reversible reaction. Consequently, it cannot proceed to completion in the presence of water.
In the process of recovering phenol from the acid cleavage reaction mixture, the reaction mixture is initially neutralized, either directly by the addition of a base or indirectly by contact with an ion exchange resin. The neutralized reaction mixture is fed to a distillation train where alpha-methylstyrene, phenol and acetone are recovered. In the early stages of the distillation, water is removed and desirably dimethylphenylcarbinol decomposes to the aforementioned by-products which are removed in subsequent distillation stages.
In this process, a key element is the control of acidity in the effluent stream fed to the distillation columns. If more than 1 or 2 ppm residual inorganic acid is present, a large portion of alpha-methylstyrene is decomposed to alpha-methylstyrene dimers or reacts with phenol to form cumylphenol. On the other hand, if the acidity is too low, dimethylphenylcarbinol doesn't decompose until the phenol still bottoms leading to alpha-methylstyrene contamination of the phenol. The present invention is particularly directed towards controlling the acidity of the neutralized cleavage mixture in order to produce good quality phenol and maximize the isolated yield of by-product alpha-methylstyrene by minimizing its decomposition to side-products.
Present methods for monitoring acidity in phenol-acetone involve diluting the product with water and measuring the resultant pH in the aqueous phase. Because of the necessity of adding water, this approach is complicated and usually run as a batch operation. Such an operation suffers from unduly large time lags between the taking of the sample and reporting the results.
A significant improvement in process control and thus yield of alpha-methylstyrene would involve the direct monitoring of acidity in the phenol-acetone stream. Extensive studies of indicator electrodes and potentiometric titrations in hydrocarbon and related solvents have been made in order to directly monitor the degree of acidity in fuel oils and lubricating oils. The results have been summarized by G. J. Hill in Reference Electrodes p433-463 (1961) and references therein. In general, the same electrodes used in aqueous systems are used in nonaqueous systems. Most of the work has been based on a combination of indicator electrode and aqueous reference electrode and contains many instances where the addition of a protic solvent, such as alcohol to the solution to be titrated, was necessary before satisfactory results could be obtained. The necessity of adding an external solvent to a process stream to measure acidity complicates continuous monitoring. Thus, a test side-stream with metering pumps and related control equipment is normally required. Moreover, addition of such solvent would introduce impurities to the process were the test stream to be returned to the process stream. As a result, the test stream is usually disposed of, adding to the cost of the process. Our work also confirmed that standard electrode systems in a phenol-acetone-cumene system in the absence of added water are unsuitable due to instability over time in the voltages observed.