• How to prevent fouling in feed / effluent exchanger of hydrotreatement unit in refinery?



  • Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

    This is a common operating issue in hydroprocessing units. Several factors can lead to the fouling issues in feed/effluent heat exchangers of hydroprocessing units, the main factors are described below:

    1: High concentration of particulate material in the feed: An adequate filtering of the feed stream is essential to prevent fouling in heat exchanger of hydroprocessing units. This is a special concern when the hydroprocessing units process cracked feeds like coker gasoils, it's important to evaluate the performance of the feed filtering system as strategy to solve fouling issues;

    2: Dissolved oxygen in the feed stream: This is a well known concern in hydroprocessing operation, especially for hydrotreating units which process chemical unstable feeds like coker gasoils. It's fundamental that the processing unit rely on oxygen stripper column or operates receiving the feed stream from a tank with nitrogen blanket aiming to avoid the reaction between oxygen and hydrocarbon which will result in polymerization (gumm) which will produce fouling in the heat exchangers;

    3: High salt concentration in the feed: This is a normal concern for straight run feeds, it's important to check the performance of the crude oil desalting system in order to minimize this effect;

    4: Operating Conditions of the Heat Exchangers: Higher skin temperatures can increase fouling formation and this is worsened by low velocities in the equipement. To prevent this, the design should be verified to analyse the best operating point and define adequate operating conditions;

    5: Asphaltenes Precipitation: A blend of heavy and paraffinic feeds can produce precipitation of asphaltenes due to chemically unstable blends, this is a concern for residue hydroprocessing units. For high severity hydroprocessing units, mainly hydrocrackers, the presence of polynuclear aromatics (PNA) can be another factor which can result in fouling in heat exchangers.

    To solve this issue, it's important to carry out a deep study to identify the main cause of fouling and define a strategy to deal with this, in some cases process adjustments are enough to solve the problem. If necessary, the antifouling dosage can be considered.