An unknown problem
Apart from people working in a refinery, few people have heard about fouling, or the accumulation of dirt on heat exchangers. At first glance, it seems like a complex, technical issue, something very far from people’s reality. However this is a top-tier “energy thief,” which complicates the sector’s efforts to reduce its CO2 emissions and has a direct impact on the world economy.
At refineries, crude is preheated in heat exchangers before entering the furnaces. However the constant flow of different types of fluids causes progressive accumulation in this equipment, which bit by bit experiences a reduction in its capacity to heat crude. The result is that, when the crude goes through the furnaces, more energy must be used to achieve the required temperature and continue the distillation process.
This accumulation of dirt and heat exchangers is a common phenomenon in refineries throughout the world, which requires these facilities to use, according to various sources, between 2% and 3% more energy, which equals 0.25% of industrialized countries’ GDP.
The problem is so significant that the United Kingdom carried out a three-year program with an estimated investment of €15 million to find formulas that reduced the impact of fouling. At the same time, universities and research centers work throughout the world to determine why it is produced and how it can be measured and prevented.
RepHEN, an innocative solution
Repsol has also been looking for solutions to this global problem for years. In 2010, it developed an internal tool known as rePHEN, Repsol Heat Exchanger Network. This model simulates networks to analyze their maintenance condition, compare their performance in their current condition and “clean” condition, and calculate the extra energy expense associated with the accumulation of dirt that is needed to heat the crude.
Information generated helps specialist design a more appropriate cleaning strategy for the exchangers as they are able to determine what exchanger is the dirtiest at each moment in time and what type of cleaning can have the most impact on the network without taking time for a general shutdown.
And that is not this tool’s only benefit. Periodic analysis of the exchangers condition is helping Repsol technicians to adopt new measures to prevent them from becoming dirty, such as redesigning equipment, installing bypass is out of service, and while operating, as well as adding new additives to the crew before putting it into the network.
In order to properly understand the problem of dirt accumulation, we must be familiar with the role that heat exchangers have at a refinery.
When crude reaches a refinery, the first step consists in heeding it in furnaces normally powered with natural gas. In this way the required temperature is reached to then separate the products in distillation columns. To avoid excessive energy use in the furnaces, the crude is preheated in what are known as heat exchangers, taking advantage of the heat available from other currents.
How does this heat exchange system work? In this equipment, the crude’s cold current comes into contact (without mixing)with the hot current of the products that just came out of the distillation column. In this way a double benefit is achieved: the crude’s temperature increases before entering the furnace and product school to be sent to storage tanks and other units. As a result, the furnace uses less fuel, which also reduces CO2 emissions to the atmosphere.
The problem is that, as the months go by, the exchangers become progressively dirtier. Deposits of organic and nonorganic material act as an insulating layer that hinders heat exchange and increases the resistance to the flow of liquid. The result is that the crude is preheated less, meaning that this temperature deficit must be made up for in the furnace by burning more fuel.