October 2018

Heat Transfer

Methodology for model-based, real-time prediction of corrosion in heat exchangers

In the chemical and process industries, gases or vapor mixtures must be cooled to temperatures near the water dewpoint in a variety of thermal unit operations.

Holzer, G., Jeitler, E., Prozess Optimal CAP GmbH

In the chemical and process industries, gases or vapor mixtures must be cooled to temperatures near the water dewpoint in a variety of thermal unit operations. Such mixtures often contain substances that develop corrosive characteristics if the temperature falls below the water dewpoint. In that case, depending on the gas composition, acids can be produced and may cause severe damage to a heat exchanger. This kind of corrosion, called pitting, may remain undetected for a long time and ultimately lead to sudden equipment failure.

FIG. 1. Example of a rigorous, CFD-based heat exchanger calculation.
FIG. 1. Example of a rigorous, CFD-based heat exchanger calculation.

One strategy to avoid pitting is to use corrosion inhibitors; however, such inhibitors require a significant increase in cost and can be difficult to dispense under varying process conditions. Consequently, most processes operate with thermal safety “gaps” of pressure and temperature to the water dewpoint. A considerable drawback of this approach is that the capacity of the heat exchanger equipment is not fully utilized.

Calculation approach.

To overcome such limitations, this article suggests a process prediction method for the real-time estimation of the lowest possible heat exchanger surface temperature. This can help optimize the potential of the process.

The key features of the new approach include the application of rigorous thermodynamics, consideration of all relevant facility components needed for a complete mass and energy balance, and a rigorous heat exchanger calculation that considers surface temperatures and dead zone temperatures for the thermodynamic state. The process includes several steps:

  1. Simulation of the crude distillation unit (CDU)
  2. Connection with the distributed control system (DCS)
  3. Determination of significant factors influencing the dewpoint
  4. Optimization of the simulation
  5. Determination of the correlation between process data and lowest pipe wall temperature
  6. Influence of the gas flow on the pipe wall temperature.
FIG. 2. Velocity contours resulting from a CFD simulation interacting with a simulation model.
FIG. 2. Velocity contours resulting from a CFD simulation interacting with a simulation model.

The thermodynamic calculation provides the theoretical water dewpoint as a function of the process parameters. Considering that sensitive variables, such as the composition of the multicomponent process stream, have a significant influence on the water dewpoint, the accurate thermodynamic description of the complete system is one challenge of the method.

The customized computational fluid dynamics (CFD) simulation, which interacts with the simulation model, provides the complete spatial distribution of the heat exchanger surface temperatures and dead zone temperatures (Fig. 1). The velocity contours resulting from such a simulation are illustrated in Fig. 2.

Model results.

FIG. 3. Integration of the proposed approach in the process control system.
FIG. 3. Integration of the proposed approach in the process control system.

Both components are combined into a process prediction model, where an interface between the process prediction model and the process control system (PCS) is used for real-time transmissions of the process parameters to the model and the recommended process parameters generated by the model back to the PCS. These recommendations can be used as guidelines for the operator or directly implemented as PCS command variables in view of an automated optimal operation mode. Such an implementation is illustrated in Fig. 3.

Takeaway.

The proposed methodology combines rigorous thermodynamics with CFD simulation into a novel process prediction model that significantly enhances the overall availability of the process, ensures the definite prevention of pitting corrosion and increases the efficiency of the process. HP

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