Alkylation waste acid components and their variations

Thread Content

Note: Due to my limited knowledge, the views presented below may have many shortcomings; I kindly ask for your understanding and guidance. Regarding the component analysis of alkylated waste acid: The alkylation reaction requires an acid concentration of over 88.5%; when this concentration drops, the waste acid is discharged from the acid sedimentation tank in the reaction system into the waste acid tank. The typical composition of the waste sulfuric acid from alkylation units is as follows: 88.5–90.0% titratable acid, 3–5% organic hydrocarbons, 1.5–2% liquefied gas, 1.0–1.5% water, 0.5–1.0% organic sulfates, along with small amounts of sulfonic acids and other components ； The organic hydrocarbons in question are primarily isooctane components; these useful components have not been effectively recovered to date, which represents a significant flaw in the facility and results in substantial economic losses. For an alkylation plant with a capacity of 200,000 tons, the annual loss due to waste acid carrying along liquefied gas and isooctane amounts to around 3 million in terms of economic value. Scientific and reasonable methods for recovery should be given priority by enterprises. The isooctane entrained in the waste acid is not constant; as it moves through the discharge system of the waste acid treatment plant, its composition continues to change, which hinders the stable operation of the waste acid cracking unit and increases its energy consumption. During the storage of waste acid, as the temperature rises to ambient levels, the components mentioned above in the waste acid will continue to react, and these reactions will proceed more rapidly. If the aforementioned organic hydrocarbon—isooctane—is not recovered, “organic hydrocarbons and organic sulfates” will react in the waste acid storage tank to form high-carbon-chain polymers; these polymers are generally known as acid-soluble oils or “red oils,” and they contain large amounts of double-bonded conjugated dienes, C5 rings, and branched compounds. And these polymers can react with sulfuric acid to produce sulfur dioxide and water. Therefore, as time passes and the temperature rises (to around 30°C), the levels of sulfur dioxide and water in the waste acid increase, which further leads to a decrease in the H/C ratio of the polymer. This results in the formation of high-carbon-chain polymers and solid deposits, causing blockages in the boiler tubes of the cracking unit. Waste acid contains a high amount of organic hydrocarbons, and as it is stored for longer periods, this leads to an increase in high-carbon solid deposits, carbonization of some organic substances, and an abundance of iron ions. In the design of the cracking furnace, the residence time for the cracking of waste acid is kept within a certain range; however, impurities such as high-carbon solid deposits require a longer time to be processed, which leads to incomplete combustion and the formation of impurities like biochemical sulfur, thereby causing blockages in the tubes of the boiler section ； At the same time, high levels of iron ions can also form iron oxide at high temperatures, which, along with the aforementioned sulfides and other impurities, can clog the furnace tubes as well as the tubes of the air-flue gas heat exchanger. The aforementioned impurities are hard in texture and difficult to remove; as production continues, the pores become increasingly small, the pressure difference across the furnace tube increases, and ultimately production can no longer be sustained. The decomposition of the high-carbon chain structures formed from organic substances in spent acid requires fuel, which in turn leads to an increase in fuel gas consumption and oxygen consumption, as well as a series of other abnormal conditions.