Importance of side strippers in crude distillation unit operations
Role of side stripping, types of side strippers, equipment selection, optimisation strategies, and start-up guidelines in performance targets and unit reliability
Soun Ho Lee Valero Energy Corporation
T he crude atmospheric column is the core of achiev- ing performance targets in the crude distillation unit (CDU). Optimisation and improvement of the column has been cited in numerous industry publications. Meanwhile, side strippers, which are coupled to the col- umn, are often overlooked. Poor side stripper operations downgrade unit performance and economics. Unit performance enhancements using side stripper configuration and operation changes are presented with actual case studies. The first case study covers optimisa - tion strategies for the side stripper under hybrid operation. Repurposing and reconfiguring of multiple side strippers for product yield and quality enhancement is discussed in the second case study. CDU side strippers In the typical CDU design, crude oil is heated and intro- duced to the crude atmospheric column flash zone. The multiple fractionation sections positioned above the flash zone mainly function as rectification services. The back-end compositions of the multiple products are controlled through the sections. Meanwhile, the front-end compositions of the products cannot be controlled precisely in these sections. The streams withdrawn from the crude atmospheric column are at ‘half-fractionated’ conditions. Managing the front-end compositions of the products is necessary to meet target product specifications, including flash point. Therefore, most CDUs are equipped with multiple side strippers. Side stripping actions are driven by either a reboiler or stripping medium. Hybrid configurations, which use reboiler and stripping steam simultaneously, are also observed. A side stripper with a reboiler and a side stripper with strip- ping medium have their own benefits and weaknesses. Details of these side strippers are discussed as follows. Side stripper with reboiler The vapour stream generated by the side stripper reboiler contacts the descending liquid stream in continuous unit operation. As the liquid stream contains a relatively higher amount of the light boiling range materials, this disequilib- rium drives mass transfer of the light boiling range materi- als from the liquid to the vapour at each stripping device of the column.¹ The light boiling range materials are stripped
out of the liquid stream. The transferred light boiling range materials are eventually returned to the crude atmospheric column as vapour, while the stripped liquid is run down as the stripper bottom product. In this stripping mechanism, without the presence of stripping medium, only stripped light boiling range materials are recycled back the crude atmospheric column. The side stripper with reboiler usually retains a high vapour-to-liquid traffic ratio and achieves better stripping performance compared to the side stripper with stripping medium. This performance benefit is more pronounced in the kerosene side stripper, which requires stringent flash point control. Other high-temperature process streams, These unique differences in process conditions between side stripper types should not be ignored for side stripping equipment sizing and operations. Otherwise, inferior side stripping performance can be experienced such as the pumparound draws, are often used as side stripper reboiler heating media to improve the unit energy efficiency. Reboiler fouling is one of the major weaknesses in this side stripper configuration. This fouling often down - grades side stripping performance and unit reliability. Side stripper with stripping medium In the side stripper with stripping medium configuration, the selected stripping medium is introduced to the stripper bottom at gas state. Introduced stripping medium contacts the descending liquid stream and reduces partial pressures of the light boiling range materials in the liquid stream. Reduced partial pressure conditions liberate and vapourise the light boiling range materials. Vapourised light boiling range materials and the stripping medium are transferred back to the crude atmospheric column together. This par- ticular stripping mechanism increases traffic across the
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PTQ Q3 2025
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