Aromatic extraction

Also known as aromatic extraction, it is a liquid-liquid extraction process in which an extractant is used to separate aromatics from hydrocarbon mixtures. It is mainly used to recover light aromatics (benzene, toluene, and various xylene) from catalytic reforming and hydrocarbon cracking gasoline; it is sometimes also used to recover naphthalene from catalytic cracking diesel. The non-aromatic residues remaining after the extraction of aromatics are known as raffinate oil. The boiling points of light aromatic hydrocarbons and non-aromatic hydrocarbons with a similar number of carbon atoms differ very little (for example, benzene at 80.1°C, cyclohexane at 80.74°C, 2,2,3-trimethylbutane at 80.88°C); sometimes azeotropes are formed as well, so they cannot be separated by distillation. Taking advantage of the fact that aromatics have a higher solubility in certain solvents than non-aromatics, liquid-liquid extraction can be used to recover aromatics with very high purity. Common extractants include diethylene glycol ether (diglycol), triethylene glycol ether (triglycol), tetraethylene glycol ether (tetraglycol), and sulfolane; dimethyl sulfoxide, N-methylpyrrolidone, and N-formylmorpholine are also used (see table). In 1952, the American company Universal Oil Products developed the Udex process for extracting aromatics using diethylene glycol ether as a solvent; after its implementation in Houston, this method was widely used. 　　Aromatics are present in high amounts in reformed gasoline; it contains no impurities such as olefins or sulfides, making it easier to process. Pygase contains a high amount of dienes, olefins, and styrene, as well as small amounts of organic compounds containing sulfur, nitrogen, and oxygen. Dienes tend to polymerize easily, and sulfides are difficult to remove from aromatic compounds. Therefore, before extracting aromatics from pyrolysis gasoline, a two-stage hydrogenation treatment must be carried out to remove the aforementioned impurities. 　　The process flow is illustrated using the treatment of catalytically reformed gasoline with diethylene glycol ether (see figure). The raw material is extracted in an extraction tower by counter-current contact with a solvent at a temperature of 125–140°C, with a solvent-to-raw material ratio of about 15:1. The bottom product of the extraction tower contains aromatics dissolved in the solvent; this solvent is sent to the stripping tower (absorption) to be separated from the aromatics. The solvent at the bottom of the tower is recycled back to the extraction tower, while the product at the top is sent to an aromatic washing tower where residual solvents are removed, resulting in a pure aromatic mixture. The non-aromatics from the top of the extraction tower are sent to a washing tower to remove residual solvents. The bottoms of both wash towers contain water and solvent; in the solvent removal and recovery tower, after the water is distilled off, the solvent at the bottom of the tower is sent to the extraction tower for reuse.

Got it; it seems my trip to Kaikawa wasn’t in vain after all