Apr-2012

Crude preheat fouling control helps reduce CO2 emissions

Modelling key performance indicators quantifies the impact of fouling in the hot preheat train and helps to identify the benefits of a planned treatment programme

Andre Vanhove and Alain Pothuaud
GE Water & Process Technologies

Article Summary

Optimising energy savings and minimising the CO2 footprint are two key objectives for refineries in today’s difficult economic environment. GE Water & Process Technologies (GE W&PT) has developed a programme that helps meet both objectives, while maintaining or even improving overall profitability. A comprehensive chemical treatment and monitoring programme can reduce the fouling rate in the crude unit (CDU) hot preheat exchanger network and subsequently minimise CO2 emissions.

A 100 000 bbl/d Western European refinery was experiencing fouling in the hot preheat exchangers of the CDU, resulting in a loss of heat duty of around 25% per year. We audited the system during two consecutive runs, and quantified the heat duty loss by regression analysis and normalisation. As the heat duty loss is directly proportional to the furnace inlet temperature decline, the audit quantified the slope of the normalised furnace inlet temperature as the baseline. Compensating for the decline in furnace inlet temperature requires increased fuel in the furnace, and this consequently contributes to an increase in CO2 emissions. By tracking the CDU furnace fuel demand, CO2 emissions can also be quantified.

Fouling in the preheat exchanger network is frequently caused by the precipitation of asphaltenes, often as a result of incompatibility of crude blends or instability of the asphaltenes at elevated temperatures. GE has developed a new patented Field Fouling Monitor (FFM) tool to identify and predict potential instabilities of crudes and crude blends.

Our chemical treatment programmes are typically designed to reduce the fouling rate by at least 50%, but are primarily focused on the optimal economic return. The FFM is used to adjust chemical injection rates in response to changing crude quality and the related fouling tendencies.

After one year of treatment, a 56% reduction in fouling rate was achieved, representing a 56% reduction in fuel consumption increase, as well as a significant reduction in CO2 emissions. Representing about 5% of the total refinery CO2 emissions, this reduction provided the refinery with improved operating flexibility, while still operating within CO2 emissions limits.

General description of fouling
Heat exchanger network systems are used to recover as much sensible heat as possible from process streams around the CDU by preheating the feedstock prior to entering the furnace. The more heat transferred to the feed in the exchangers, the less energy/fuel is required to heat the crude to the required distillation temperature range. The hottest exchangers have a direct impact on the furnace inlet temperature. Exchangers with the highest heat flux or low velocity usually show the highest fouling rates. Fouling is caused by the precipitation of solids materials present in the feed or formed in the heating process of the crude preheat system. Two major types of material can cause the deposit formation in the equipment: inorganic and organic.

Inorganic
Solid inorganic contaminants in crude oils or reprocessed streams, such as corrosion products, sand, silt or catalyst fines, can deposit in low-velocity sections of an exchanger and cause hydraulic and/or thermal obstructions. Most of the salts in the crude should be removed via the desalter. Some of the inorganic solids can be removed in the desalter, if the appropriate wetting chemistry is applied. Maximum removal of salt and solids helps reduce the fouling potential in the crude preheat system. A caustic solution is sometimes injected in the desalted crude to reduce corrosion in the crude distillation overhead section, but this can also enhance the fouling potential if not applied correctly.

Organic
The predominant mechanism of organic fouling is the precipitation of organic components, which become insoluble in the system, such as high 
molecular weight hydrocarbons, asphaltenes or coke.

Unsaturated compounds, usually resulting from reprocessing of cracked material, can polymerise via free radical mechanisms and are catalysed by the presence of metals and/or low levels of oxygen.

Polycondensation reactions can occur in the presence of components such as carboxylic acid and certain nitrogen components, and the mechanism is defined as a non-free radical polymerisation reaction.

Asphaltenes precipitation is often a key contributor to fouling in crude preheat systems. The asphaltenes are kept in the crude as a colloidal mixture and are stabilised by the presence of natural dispersants. Crude oils are mostly processed as blends of several crudes and, depending on their interaction with asphaltene stability, the peptisation of these asphaltenes in the process can be insufficient. As a result, they can become insoluble and deposit onto heat transfer equipment. Typically, the stability of asphaltenes is inversely proportional to the temperature, and asphaltenes are more likely to drop out of solution in the hottest area of the preheat train (see Figure 1).

Crude unit fouling control and monitoring
The hot preheat trains typically consist of several exchangers arranged in both parallel and series configurations. Some designs allow for bypassing of individual or banks of exchangers in the event rapid fouling necessitates offline cleaning during the normal run length.

In addition to basic feedstock analysis, several other parameters need to be considered when designing an appropriate chemical treatment programme. Operating conditions, such as exchanger configuration and design (velocity and turbulent or laminar conditions), desalter performance, solids removal, caustic injection, crude oil blends, storage, handling and crude oil compatibility, should all be reviewed.

Antifoulants can help minimise equipment fouling in various ways. Some are designed to reduce the particle size of fouling material, others keep particles dispersed, and still others create a film to prevent deposits from adhering to metal surfaces. Although not a complete list of antifoulant functionalities, most effective chemical treatment programmes offer a combination of functions, selected to meet the specific needs of the individual system. It is also usually beneficial to pretreat the system to minimise the initial high fouling rate immediately after startup (see Figure 2).