Fouling by Asphaltenes After and During Conversion
During resid conversion processes, such as visbreaking or hydroconversion, more saturated pendant groups are cracked off asphaltenes, making them less soluble. Meanwhile, the more saturated volatile liquids formed during the conversion are often nonsolvents for asphaltenes. As a result, these converted asphaltenes often become insoluble when the reaction product is cooled to form sediments and foul heat exchangers. Therefore, distilling out the volatile liquids from the heavy product reduces the hot filtration sediment even though the concentration of converted asphaltenes is increased. The removal of nonsolvent allows more converted asphaltenes to dissolve. On the other hand, if the asphaltenes become insoluble during conversion, they form a liquid crystalline form of coke, the carbonaceous mesophase, inside the reactor. Usually, the most effective method to mitigate this form of coke formation is also to remove the nonsolvent by stripping out the volatile liquids from the conversion reactor.
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