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Oct-2012

Value of monitoring exchanger networks

A rigorous exchanger simulation model can be used to calculate the true cost of fouling in crude preheat networks

LAURA COPELAND
Nalco Company

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Article Summary

Heat exchanger fouling has a direct impact on profitability. Over time, fouling leads to higher energy consumption, higher maintenance costs, reduced feed rates and shorter intervals between turnarounds. The relationship between fouling and energy becomes more significant when you consider the link between additional fuel gas consumption, higher CO2 emissions and the detrimental impact on a refinery’s energy intensity index (EII). The environment and the total cost of operation (TCO) are negatively impacted.

Proven energy savings can be realised when the fouling of a crude unit preheat network exchanger can be effectively monitored. Monitoring will determine how fouling in a network changes with time. Crude units see the highest charge rates and the largest temperature increase of any refinery unit,1 so the benefits of a successful monitoring and fouling control programme can be significant.

This article will include a brief review of crude unit heat exchanger fouling mechanisms, how fouling affects energy management costs, and potential solutions.
 
Fouling mechanisms
What is fouling? It is the formation of deposits in process equipment that impedes the transfer of heat and increases the resistance to fluid flow. Several physical, operational and chemical factors can combine to form these deposits. Most crude preheat deposits have low thermal conductivity and reduce heat transfer. Fouling can have a substantial economic impact upon a refiner’s profitability when it causes throughput reductions due to hydraulic limits or furnace tube temperature limits. Fouling always leads to energy losses when fuel has to be increased to the furnace to make up for lower crude temperatures coming from the fouled crude preheat network.

What causes fouling?
There are three main operational factors that lead to fouling: blending crudes, the velocity through the process and crude quality. Changes to any of these factors can lead to a change in fouling throughout the process.

When crudes are blended together, there is the potential for instability that can lead to fouling. If the crudes are processed individually, the fouling potential can be different than if two or more crudes are blended together. For example, a refiner could be processing a heavy, low API crude and have very little fouling, but when they blend a light, high API crude with it they see increased fouling in their crude preheat train. The introduction of another type of crude has caused instability in what is normally a stable crude. Preheat train monitoring can be used to support the refiner’s decision-making process when implementing a strategy to prevent fouling deposition due to the incompatibility of crudes.2 Figure 1 shows the results of testing done on the Nalco Fouling Potential Analyzer (FPA), where each crude individually has a lower fouling potential than when they are blended together. The FPA value is the inflection point of each trend, and a lower FPA value equals less stability. In this example, when crude A is blended with crude B, the stability decreases and, therefore, there is a higher potential for fouling.

Another operational factor that can cause fouling is a change in the velocity through the process. Initially, most heat exchangers are designed and sized to achieve a maximum heat transfer at design throughput conditions. Often these design conditions achieve very low fouling rates due to the high velocity and proper baffle design and spacing. As throughput changes or heat exchangers are added or redesigned, velocity changes and the rate of fouling can also change.

Particulates travelling along with the crude have a greater potential to fall out and cause fouling at lower velocities. A proper monitoring programme should take into account the velocities of the various streams, how they are changing with time, and the impact on fouling and temperatures throughout the system. Just as many exchanger networks may encounter a “cleaning” effect from a sudden increase in throughput, they can also experience a “fouling event” with a sudden lowering of throughput. A well-monitored system can locate which exchangers tend to foul the most as a result of decreased throughput (velocity).

The third operational factor that can lead to fouling issues throughout the refinery is the quality of the incoming crude. Crude oil can contain asphaltenes and inorganic materials that can contribute to fouling in the system. Asphaltenes are the most common fouling material found in the hot preheat train. They are naturally stabilised by resins that prevent them from agglomerating, but asphaltenes can easily agglomerate when destabilised and cause fouling. Another type of foulant is existent debris such as sand or sediment carried in the crude oil that may be deposited when stressed by heat. The deposition of inorganic salts can result in fouling if the refinery has no desalting capability or if the desalters are not working properly. Finally, one more potential type of fouling material is polymeric gums that can form if a reactive stream is added to the crude oil. Figure 2 shows an example plot of normalised furnace inlet temperature (NFIT). This shows how a change in incoming crude to a refinery could have a significant impact on fouling. More discussion of NFIT can be found in the next section of this article. 

Cost of fouling + cost of fouling control = total cost of operation
It is possible to operate a crude preheat to achieve the lowest TCO by calculating the total cost of fouling and the total cost of fouling control. The costs of fouling are all related to the extra fuel burned in the furnace due to the fouling layer inhibiting heat to be transferred to the crude. As the fouling increases in the different exchangers, the crude exiting each exchanger leaves at colder and colder temperatures. That lower temperature crude must be heated up to the fixed furnace exit temperature in order for the refiner to meet their target cut points. This additional fuel due to fouling is difficult to calculate without a proper heat exchanger simulator being run on a regular basis.

Refiners will also change the pumparound rates to manipulate the cuts in the atmospheric tower for maximum profitability. This will also add or delete heat from the preheat, but this is not due to fouling. A proper monitoring programme will be able to distinguish the difference between temperature losses due to operational changes from temperature losses due to fouling. This can be achieved by calculating a NFIT using a base set of operating conditions. The NFIT will be equal to the actual furnace inlet temperature (FIT) as long as the operating conditions remain the same. When pumparound flow rates or temperatures change, the heat load to the preheat will change and affect the FIT, causing the FIT and the calculated NFIT to be different. The difference will be the result due solely to operating changes between the base case and the current case.

Figure 3 is an example of the differences that can be seen between FIT and NFIT. The NFIT will show the temperature decline due to fouling, while the FIT will show the temperature decline due to both fouling and operational changes. The NFIT trend is useful to show the impact of changing any variable that has an effect on the fouling rate. 

The decline in temperature due to fouling can be converted into lost BTUs (energy) that must be made up in the furnace by burning extra fuel. Incorporating furnace efficiency and cost of fuel, the NFIT reflects a cost of fouling.


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