Few people beyond those who work in refineries have heard of fouling, or dirtying of the heat exchangers. At first, it may sound like a complex, technical issue, and something that is very far from most people’s reality. However, this powerful “energy thief” undermining sector efforts to reduce its CO2 emissions has a direct impact both on the world economy and on the fight against climate change.
At refineries, crude oil is preheated in the heat exchangers before entering the furnaces. However, the constant flow of different types of fluids causes the progressive dirtying of these machines, and their capacity to heat crude oil is gradually reduced. The result is that, when the crude oil enters the furnaces, more energy must be consumed to reach the required temperature and continue the distillation process.
This dirtying of the heat exchangers is a common phenomenon in refineries around the world, which obliges these facilities to consume, according to different sources, between 2% and 3% more energy, equal to 0.25% of the GDP of industrialized countries.
The magnitude of the problem is such that the United Kingdom has implemented a three-year program with an initial investment of €15 million to find formulas that reduce the impact of fouling. At the same time, universities and research centers all over the world are working to find out why it happens and how it can be measured and prevented.
Repsol has also been working on this global problem for years. In 2010 we launched an internal tool called repHEN (Repsol HEAT Exchanger Network). It is a model that simulates exchanger networks to analyze their state of conservation, compare their performance in their current state and in a "clean" state, and calculate the additional energy costs associated with fouling required to heat the crude oil.
The information generated allows specialists to design the most appropriate cleaning strategy for the exchangers, since it allows them to know which exchanger is dirtier at any given time or what cleaning can have more impact on the network without having to wait for a general shutdown.
That's not the only benefit of this tool. The regular analysis of the state of the exchangers is helping Repsol's specialists to adopt new measures for preventing fouling, such as the redesign of equipment, the installation of bypasses to take them out of service and clean them while the unit is operating, or the addition of new additives to the crude oil before it enters the network.
In order to correctly understand the problem of fouling, we need to understand the role that heat exchangers play in a refinery.
When crude oil arrives at a refinery, the first step consists of heating it in furnaces, which usually run on natural gas. In this manner, the required temperature is reached in order to then separate the different products in the distillation column. To prevent excessive energy use in the furnaces, the crude is preheated in so-called heat exchangers, leveraging the heat available in other currents.
How does this heat exchange system work? In this equipment, the cold current of the crude comes into contact (without mixing) with the hot current of the already distilled products that just came out of the distillation column. This system has a dual benefit: the crude is heated before it enters the furnace, and the products cool off so they can be sent to storage tanks or other units. As a result, the furnace consumes less fuel, thereby also reducing CO2 emissions into the atmosphere.
The problem is that the heat exchangers are progressively dirtied with each passing month. Deposits of organic and inorganic matter act as an insulating layer that makes it more difficult to exchange heat and increases resistance to the flow of fluids. The result is that the crude oil is preheated less, which means this temperature deficit must be offset inside the furnace by burning more fuel.
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