Nov-2022

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

Article Summary

Crude 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 influence 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 operations based on overall heat exchanger performance (overall 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 overcome 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 outlet 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 atmospheric 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 column, 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 corrosion. 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 hydraulic 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 new exchanger in the flashed crude train, establishing the performance of the entire crude preheat train under the fouled conditions and benchmarking the unit are essential. This required evaluating exchanger performance under the design fouling and actual fouling during the unit run length.

Exchanger fouling
The fouling factor applied during the design phase to the shell and tube side of an exchanger is a measure of additional resistance to heat transfer caused by deposits, which may include fouling and a corrosion layer on the tube surface. This resistance depends on the type of fluid, presence of particulates/contaminants, heat transfer surface material, temperature, flow velocities, and the operating period between successive cleaning actions. The design fouling factor provides excess area to enable the heat exchanger to perform the required duty. In fouling analysis, the fouling condition is expressed by the following formula:

Actual Fouling % = (1 - U Actual /U Clean) x 100
Design Fouling % = (1 - U Design/U Clean) x 100

Where:
U Actual = Actual overall heat transfer coefficient based on the actual fouling
U Clean = Clean overall heat transfer coefficient based on the program rating
U Design = Design overall heat transfer coefficient
The U Actual is obtained from rigorous modelling of the unit and the crude preheat train exchangers.

Rigorous exchanger modelling for test run
A unit performance test run for a blended crude slate was generated, which included crude feed and product rates from the atmospheric and vacuum column, column operating conditions, product distillations and properties, PA temperatures and rates, and the temperature and pressure profile for the exchangers in the crude preheat train. These parameters were used to develop a simulation model with the crude feed produced by blending the products from the two columns.

The off-gas produced from the cracking of heavy ends in the atmospheric and vacuum heaters was also included as feed to the two columns. The model matched product rates, properties, distillation, and fractionation (gap/overlap) between adjacent streams and PA rates and supply and return temperatures.

In the atmospheric column simulation, the naphtha PA return temperature from E-11 A-D is controlled to achieve the mix temperature of the naphtha PA return and the overhead reflux above the water dew point to prevent free water condensation and minimise corrosion in the atmospheric column overhead.