Revamps 2022 Issue

Crude preheat train fouling and fix-up

Rigorous modelling of crude preheat train exchangers allows for fouling progression of the heat exchangers to be monitored throughout the crude unit run length

S D Radia Fluor Canada Ltd

C rude preheat train fouling is a serious operating problem in a refinery crude and vacuum unit. It reduces exchanger performance, increases furnace duty and associate fuel cost, reduces cooling capacity, and increases train pressure drop. The resulting increase in fuel costs and loss in throughput can significantly influ - ence refinery profitability. Crude oil is generally contaminated with water, salt, wax, sand, and mud, containing metal oxides and other particulates. These particulates deposit on heat exchanger surfaces, promoting the build-up of organic and inorganic fouling. Common techniques to prevent fouling include filtration, desalting, fouling inhibition, crude compatibility analysis, and preheat train revamp. Cleaning of exchangers is also carried out during unit operation to maintain throughput, maximise run length, and maximise operating performance. Fouling trends are often monitored by refinery opera - tions based on overall heat exchanger performance (over - all heat transfer coefficient x area or UA) estimations in the preheat train. These estimations cannot distinguish the performance penalty caused by fouling or that caused by flow reduction. A rigorous simulation model with a detailed rating of the preheat train exchangers can over - come this problem. Rigorous modelling of crude preheat train exchangers is described using a detailed heat exchanger rating program within a simulation model of an atmospheric and vacuum unit at an existing refinery. The exchangers are modelled with shell and tube side temperature and flow profiles that match the actual performance of various feed, product, and pumparound (PA) streams. The modelling establishes the fouling state of the shell and tubes to obtain the actual performance (duty and out - let temperature) of the crude preheat train exchangers. The modelling allows the fouling progression of the exchangers to be monitored through the crude unit run length. Crude preheat train In this case study, the crude preheat train consists of raw crude, desalted crude, and flashed crude preheat in an existing refinery, as shown in Figures 1 and 2 . The atmo - spheric and vacuum columns with feed, product, and PA streams are shown in Figures 3 and 4 . The raw crude is mixed with wash water and heated in the raw crude preheat train upstream of the desalters with

naphtha and jet PA and products from the atmospheric col - umn, and medium vacuum gas oil (MVGO) PA and products from the vacuum column. Temperature bypasses of hot streams are used to control the temperature of the return streams to the two columns for adequate PA heat removal. The raw crude temperature to the desalters is controlled to reduce brine, sediment, and water and water-soluble salts in the two desalters to minimise column overhead corro - sion. The desalter pressure is maintained high enough to prevent light crude oil fractions and water vaporising. The desalted crude is further preheated using MVGO PA and the product streams from the atmospheric and vacuum columns in a desalted crude preheat train and routed to a preflash drum. Light ends from the crude and entrained water from the desalters are flashed off and routed to the flash zone of the atmospheric column. This configuration prevents the vaporisation of these components upstream of the heater pass valves and potential pass flow imbalances. The flashed crude from the preflash drum is further heated using MVGO product and PA, medium diesel PA, heavy atmospheric gas oil PA, and product streams from the atmospheric and vacuum column in a flashed crude preheat train. The preheated flashed crude is further heated in the atmospheric heater and sent to the atmospheric column. The majority of atmospheric tower bottoms (ATB) from the atmospheric column is sent to the vacuum column via the vacuum heater, where it is fractionated into light, medium, and heavy vacuum gas oils. The slop oil from the vacuum seal drum in the column overhead is recycled to raw crude at the crude preheat train inlet, while the sour water from the same drum is sent to the stage 2 desalter as make-up water. Atmospheric tower bottoms bypass circuit Some of the ATB is bypassed around the vacuum column and sent to another unit for further processing. The ATB bypass is cooled with the flashed crude for heat recovery and finally cooled in an air cooler (E-30) prior to export to another unit. The ATB to this air cooler can run very hot due to fouling in the upstream exchanger, which causes numerous leaks. The air cooler also badly fouls up, and the high-pressure drop across this exchanger causes a hydrau - lic limit on the ATB circuit. A new exchanger to reduce the inlet temperature to E-30 by recovering heat from the ATB bypass was proposed to debottleneck the ATB bypass circuit. Before integrating the

Revamps 2022