In many refineries most of theC4’s and lighter are removed from the atmospheric column overhead distillate in the first column of the light end unit. This is the unit’s debutanizer column. Some refineries however chose to separate the light naphtha and lighter from the heavy naphtha first. There is no specific reason one can assume its really a question of the specific refinery’s economic criteria. Having separated the C4’s and lighter as a distillate from the naphtha the distillate enters a depropanizer where C3 and lighter are separated as a distillate from the C4’s. This distillate is further fractionated in a deethanizer column where the C3 is removed as the bottom product. There is no distillate product from this unit but all the gas lighter than C3 leaves the tower as a vapor usually routed to the refinery fuel gas system.
The C4 portion of the overhead distillate is fractionated in the debutanizer so that it meets finished product specification with respect to itsC5 content. The fractionation in the depropanizer will be such that the C3 content of the bottom product—C4 LPG will meet the butane LPG specification with respect to RVP (Reid Vapor Pressure). The fractionation in this tower will also be such that theC4 content in the overhead distillate will meet the propane LPG specification which leaves as the bottom product from the deethanizer with respect to its C4 content. The fractionation in the de-ethanizer will be such as to ensure that the C2 and lighter content of the propane LPG will be such as to meet that LPG’s Reid Vapor Pressure specification.