(186b) Consider Thermal and Hydraulic Impacts of Fouling in Crude Preheat Exchanger Design

Krishnan S. Chunangad, Robert Chang, Lou Curcio, Richard Casebolt

ExxonMobil Research and Engineering, 22777 Springwoods Village Parkway, Spring, TX 77389, USA

Abstract

The overall energy efficiency of an oil refinery is highly affected by the performance of its crude preheat train which is often severely hindered by fouling, the deposition of unwanted material on thermal surfaces. Crude oils are complex fluids and many species such as salts, particles, wax, asphaltenes, coke and colloids can deposit on the tube surfaces thus impeding heat transfer and increasing pressure drops. As a result, the energy recovered in the preheat train is reduced, more fuel is burned at the furnace downstream of the pre-heat train, which in turn increases CO₂ emissions, throughput may be reduced, and units need to be periodically taken offline for cleaning. Annual losses associated to fouling can be quantified in the order of millions of dollars each year in individual refineries.

Traditional design methodologies deal with fouling in a very simplistic way by using fixed fouling factors. This neglects entirely any dynamics of the deposition process, as fixed fouling factors are not capable of capturing variations in fouling behavior as a function of process conditions. Moreover, information on the hydraulic impact of fouling is typically neglected. As a result, it is not possible to quantify the effects that a proposed design may have on the heat transfer and pressure drop performance through the exchanger run-length. To improve this aspect, a methodology that accounts for both thermal and hydraulic effects of fouling is needed.

This paper reports on the work where thermal data and hydraulic data as available, on crude preheat exchangers in seven different preheat trains, over several cleaning cycles, have been used to estimate the key fouling parameters in the Ebert-Panchal (1999) model using Hexxcell Studio™, a commercial software package for the thermo-hydraulic analysis, estimation and prediction of fouling in refinery heat exchangers. The data covers many crude blends with tubeside shear stresses ranging from 2 to 40 Pa at the hot end of the crude preheat train with multiple metallurgies. The simultaneous consideration of both temperatures and pressures allowed estimating the growth of fouling thermal resistance and hydraulic resistance (fouling layer thickness) with time and its effect on the exchangers’ thermal and hydraulic performance. This information can be used to improve the design methodology for crude preheat exchangers to minimize fouling and maximize energy performance and train throughput.  

In general, a good fit of the thermal-hydraulic plant data was achieved with the fouling model. The work also highlighted some known limitations of the Ebert-Panchal (1999) model. Both aspects will be briefly discussed in the presentation.

Keywords: heat exchange; crude oil; fouling; thermo-hydraulic performance; Ebert-Panchal; resistance;