Most refiners were concerned only with the main kerosene fraction of their distillate and with enlarging this middle cut as much as possible. To accomplish this they usually extended ordinary distillation into destructive distillation of the heavier (higher-boiling) fractions above the kerosene range. However they proceeded, they inevitably obtained in ordinary distillation a large, lighter fraction before reaching the kerosene range. With the exception of those who cracked excessively down to coke, they also could not avoid producing a second rough fraction much heavier than kerosene. It was these two fractions that comprised by-product stock--that is, material from which useful by-products could be obtained by further distillation or treating.
The first light fraction of ordinary distillation was crude naphtha. Since it always contained some hydrocarbons boiling in the kerosene range and higher, this fraction required further distillation and treating. Though variable, its volume might equal 20 per cent or more of a low-gravity