A reaction engineering approach to modelling of crude oil fouling deposits : analysis, monitoring and cleaning

• Main Author: Diaz Bejarano, Emilio
• Other Authors: Macchietto, Sandro
• Published: Imperial College London 2016
• Subjects: 660
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.754663

Refineries are amongst the largest industrial energy users. The preheat trains of their crude distillation units, extensive heat exchanger networks, are key facilities for energy efficiency. Fouling deposition on heat transfer surfaces leads to substantial energy losses, fuel consumption, operating difficulties, CO2 emissions and even throughput losses. Modelling of crude oil fouling deposits has been traditionally limited to thermal resistances. However, consideration of the local change in deposit thickness and the evolution of its properties due to ageing or changes in composition is important to capture the thermal and hydraulic impact of fouling. A more fundamental approach is needed to account for deposition mechanisms, assess the effect on operations, relate cleaning effectiveness to deposit state, and assist in monitoring, prediction, and design. In this thesis, a reaction engineering approach to describe the crude oil fouling deposit is proposed. The deposit is modelled as a spatially distributed multi-component solid of time-varying thickness. It has the ability to track the composition of each point as the deposit builds up and evolves. This determines the local physical properties of the layer, such as thermal-conductivity. The deposit model is implemented within a distributed, thermo-hydraulic model for a shell-and-tube heat exchanger developed in past works. This allows evaluating the heat duty and pressure drop for given operating conditions and deposit characteristics. Relevant applications of the approach include: 1) simulation of organic deposition, ageing and different deposition-offsetting mechanisms; 2) seamless simulation of full cleaning, partial cleaning and fouling resumption after cleaning; and 3) the ability to describe mixed organic-inorganic fouling, the significant effects of inorganics on the performance of the exchanger and the potential use of such effects to detect changes in fouling behaviour. Taking this modelling framework as basis, a thermo-hydraulic fouling analysis method is presented. By using various simplified modes of the deposit model, it is possible to: first, interrogate plant data to evaluate the fouling state over time; and second, fit suitable deposition models to enable prediction of fouling behaviour as a function of operating conditions. The analysis method includes the evaluation of cleaning effectiveness. The combined fouling and cleaning information can be used to find economically advantageous cleaning schedules for heat exchanger networks. Finally, a novel heat exchanger and fouling monitoring approach, the TH-λ method, is presented that permits simultaneous visualization of the time variation in thermal and hydraulic performance, comparison with operational limits, evaluation of the impact and state of fouling, and detection of changes in fouling behaviour. Tube pressure drop is either measured or calculated via soft-sensors. The above models and methods are demonstrated through a number of theoretical and industrial case studies that highlight the importance of considering thermal, hydraulic and deposit compositional aspects in the study of crude oil fouling. Overall, an integrated framework is set-up that has immediate application for monitoring refinery heat exchangers, data interpretation, development of predictive models, cleaning scheduling and detection of abnormal situations, and therefore can potentially lead to improved energy efficiency, significant economic savings, and safer operations.