(378a) Detailed Thermo-Hydraulic Modelling of Crude Oil Heat Exchangers: Abnormal Fouling Behaviour Due to Inorganic Deposits

Crude oil fouling in heat exchangers in oil refineries leads to energy losses, operating difficulties and economic penalties. Heat exchangers at the hot end of preheat trains, where thermal fouling of organic matter is the main fouling mechanism, are traditionally considered to be more important from the point of view of energy efficiency and recovery. As a result, studies generally focus on the thermal impact of deposition of organic matter. Inorganic matter, however, is often found in analysis of deposits, although typically in heat exchangers located upstream of the desalter (cold end). Such inorganics have an important effect on the heat transfer properties of the deposit layer due to their higher conductivity with respect to that of organic deposits.

Here, a case study is presented based on recently published plant data from the Esfahan Refinery (Iran)¹. According to the reference paper, severe fouling in the key heat exchanger, i.e. the most affected by fouling, led to inadmissible pressure drops and to the decision of shutdown, cleaning and overhaul. The analysis of the deposit inside the tubes revealed a composition with 80_wt% inorganic matter (including SiO₂, CaSO₄, and iron oxides). This deposit composition is certainly unexpected, since the heat exchanger is located downstream of the desalter. This is a real example of hydraulic performance, rather than thermal, being the limiting factor for a cleaning and mitigation decision, and of abnormal fouling behaviour due to unexpected deposition of inorganics. The overhaul work, planned before the shutdown (and therefore before knowing the analysis results), was a thermal based approach aimed at reducing the high temperature of the heating fluid by introducing a new heat exchanger. Fouling mitigation was expected but, after restart, the fouling behaviour did not improve, and the pressure drop raised quickly reaching values after a year of operation of similar magnitude to those before the overhaul. Posterior analysis led to the conclusion of this high content of inorganic salts being due to desalter inefficiency.

A detailed dynamic, distributed model for heat exchangers undergoing fouling², previously shown to capture the complex, interacting thermo-hydraulic, fouling and ageing phenomena for thermal organic fouling, is here adapted to account for the effect of presence of inorganics. This is, to the author’s knowledge, the first time this effect is included in the modelling of crude oil fouling. Pressure drop and temperature measurements, before and after a network overhaul and cleaning, and information on deposit composition are available for the tube-side of this exchanger. The detailed thermo-hydraulic model of heat exchangers, together with the available data and reasonable assumptions, is applied to simulate the unit most adversely affected by fouling. A novel modification to previous parameter estimation methodologies is introduced to fit key fouling parameters based on pressure drop measurements, instead of temperature. Information on the composition of the deposits from experimental analysis is used as an indicator of the percentage of inorganic and organic material. Then, information about the conductivity of the inorganic portion is extracted by considering possible values, corresponding to different inorganic species, and evaluating the temperature predictions against measured temperature data.

The results presented demonstrate the need of considering the enhanced conductivity of the fouling deposits due to inorganic material in order to capture main trends in reduction of thermal and hydraulic performance. They show a good match between experimental and calculated data both before and after the overhaul, despite the considerable number of assumptions, and seem to indicate that the same fouling behaviour was still present after the retrofit work. This indicates that the primary cause of fouling was not solved by the overhaul work and reinforces the hypothesis of a malfunctioning of the desalter as a possible cause.

The results also demonstrate the importance of considering both temperature and pressure drop measurements in the interpretation of plant data and fitting of model parameters, and that consideration of thermal effects in isolation would not permit to extract sufficient information about the nature of the deposit.

1. Mozdianfard MR, Behranvand E. A field study of fouling in CDU preheaters at Esfahan refinery. Appl Therm Eng. 2013;50(1):908–917. 

2. Coletti F, Macchietto S. A Dynamic, Distributed Model of Shell-and-Tube Heat Exchangers Undergoing Crude Oil Fouling. Ind Eng Chem Res. 2011;50(8):4515–4533.

Acknowledgments

This research was performed under the UNIHEAT project. The authors wish to acknowledge the Skolkovo Foundation and BP for financial support, and Hexcell Ltd. for providing the modelling framework used in this work.