Fouling Mitigation by design

John M. Nesta and Christopher A. Bennett

Fluor Canada Ltd., Calgary, Alberta, Canada and

Heat Transfer Research, Inc., College Station, Texas, USA

Crude oil, which fouls via multiple mechanisms, is one of the most problematic of all process fluids. However, crude oil preheat train exchanger fouling can be mitigated with the no-foul design strategy¹. The key points of this no-foul design method are to maximize shear stress and minimize wall temperature. Furthermore, replace fouling factors with 20% excess area when both fluids are within the scope of medium-through-high boiling point hydrocarbon mixtures with API gravity less than 45. If one fluid is outside of this scope, use a fouling factor determined from experience (if necessary) and multiply the in-scope fluid heat transfer coefficient by 0.83. Application of this field-proven design methodology will significantly lower capital costs and substantially increase run time between cleanings (Table 1).

Table 1. No-foul versus standard heat exchanger designs^a

Parameter	No – foul design	Standard design	Standard design with 10% coefficient margin
Surface area, m^2	832	1,564	1,875
Estimated cost, US$	996,000	1,527,000	1,775,000
Clean overall coefficient, W/m^2 K	361	231	204
Total fouling resistance, m^2 K/W	0.000634	0.00199	0.00268
Fouling margin, % excess surface	22	46	55
Shell side
Pressure drop, kPa	175	66.9	63.4
Velocity, m/s	0.61	0.34	0.30
Shear stress^b, Pa	14.2	4.8	4.1
Tube side
Pressure drop, kPa	185	66.2	55.2
Velocity, m/s	2.2	1.1	0.91
Shear stress, Pa	15.6	4.5	3.3

^aService is residue stripper bottoms/preflash bottoms exchanger

^bShell side shear stress is weighted for window and cross flow

¹J. Nesta and C. A. Bennett, Reduce fouling in shell-and-tube heat exchangers, Hydrocarbon Processing 83(7), 77 – 82 (2004).