AGEING: LOOKING BACK AND LOOKING FORWARD

Ageing is the process of transformation of a fouling layer following its initial formation, whereby the deposit converts to another, usually more cohesive, form. Its importance, and the lack of understanding in this area, was identified by Epstein in a paper published in 1983. A review of subsequent work indicates that the topic has attracted significant attention in some fields since, particularly in the deposition of waxes in crude oil pipelines. A quantitative model of ageing in chemical reaction fouling, linking heat transfer with the evolution of deposit thermal conductivity, is presented here. This model, which is based on simple physical principles, is both plausible and computable, and highlights the importance of timescales, temperature sensitivities and mode of testing on the results obtained from thermal fouling experiments. Support for the modeling approach is given by laboratory measurements of thermal conductivity and data published by Fan and Watkinson (2008). Salient lessons are identified, particularly in relating experimental studies to operating units. The lessons are illustrated with a discussion of the impact of ageing in a heat exchanger on a crude oil preheat train. The need for further quantitative work in this area is highlighted, as well as the scope for exploiting current knowledge. INTRODUCTION Ageing (or aging) refers to the transformation of a fouling deposit after it has been formed on a heat transfer surface. In 1981 Epstein (published in 1983) identified ageing as one of the principal mechanistic stages in the process of fouling. In the same paper he described ageing as one of the fields in which the fundamental understanding was most poorly developed. It has attracted significant attention in some applications since, but in others it remains just another unknown factor in heat transfer. Ageing plays an important role in most of the fouling mechanisms identified by Epstein: corrosion, biofouling, crystallization and chemical reaction. It could be argued that ageing plays a smaller role in particulate fouling, except when this occurs in parallel with another mechanism, or when the presence of a particulate deposit facilitates another mechanism such as the formation of biofouling in the pores of such a layer. Ageing arises because the fouling layer is, by definition, a static layer of material on the heat transfer surface and is therefore exposed to process conditions over an extended timescale, which promote transformations which would not arise in the moving fluid during its passage through the unit. Ageing is important because the transformations determine the physical and chemical nature of the deposit, and consequently (i) exchanger thermal and hydraulic behaviour; (ii) the dynamics of the fouling process; (iii) the nature of the material recovered after heat transfer is interrupted: ageing can mask the identity of the material originally deposited; (iv) the ease of removal, particularly in cleaning: the symbiotic relationship between fouling, ageing and cleaning was discussed by Wilson (2005). Figure 1 depicts how wall conditions can differ from those in the bulk fluid in terms of temperature, thereby driving kinetic and thermodynamic changes. Figure 1 Schematic of temperature distribution in a heat transfer unit subject to fouling, with wall temperature Tw, bulk temperature Tb and depositfluid interface temperature Ts. Other distributed factors include concentration (e.g. of oxygen), giving rise to different reaction mechanisms such as anaerobic digestion in biofilms and electrochemical potential, enhancing corrosion; and shear imposed by the fluid. Within the stationary deposit there may occur wall bulk fluid