PTQ Q4 2023 Issue

Optimising fouled distillation units

Case studies discuss optimisation strategies and enhanced distillation unit performance against fouled conditions

Soun Ho Lee Valero Energy Corporation

F ouling is a critical issue in distillation unit opera- tions. Fouled conditions not only downgrade unit performance but also reduce unit run length. Online equipment cleaning techniques can be applied while a unit is running. However, the effectiveness of online cleaning is limited, and fouling remains in most cases. Thorough distillation equipment cleaning and/or upgrad- ing can resolve or mitigate fouling. However, these activi- ties require a distillation unit outage, resulting in a loss of production. What if a unit outage is not feasible and a distil- lation unit continues operating while handicapped? Is there a way to reinstate distillation unit performance without a unit outage? Case study 1: Background The crude distillation unit (CDU) under discussion belongs to a refinery that processes blended feedstocks among light, medium, and heavy crude slates. Charged crudes are heated through two parallel preheat trains and furnaces and then introduced into the crude atmospheric tower. This tower separates crudes into intermediate products: u Unstabilised light naphtha v Heavy naphtha w Kerosene x Diesel y Atmospheric tower bottoms (ATB). The existing flow scheme was designed in the early 1990s to process medium-gravity Middle East crude slates. Current crude slates consist of light domestic crude and heavy imported crude slates. Heavy naphtha streams were withdrawn from two different locations and introduced to a single heavy naphtha side stripper. It was believed that a single nozzle was not big enough to achieve targeted heavy naphtha draw rate, and two side- draw nozzles could help relieve hydraulic limitations. Seven fractionating trays were arranged between the top naphtha pumparound and the heavy naphtha lower draw nozzle, and eight trays were also positioned between the heavy naphtha lower draw and kerosene pumparound sec - tion. The kerosene pumparound and product had different draw locations to maximise kerosene-diesel fractionation. The crude atmospheric tower configuration is shown in Figure 1 . Case study 1: Problem description This bespoke CDU faced multiple challenges in recent

Top reux

O - gas

Light naphtha

Top pumparound

7 trays

8 trays

Heavy naphtha

Kerosene pumparound

Kerosene

Wash

Diesel pumparound

Feed

Diesel

Stripping steam

ATB

Figure 1 Crude atmospheric tower

PTQ Q4 2023