Heat Exchanger Fouling and Cleaning : Fundamentals and Applications Engineering Conferences International Year 2003 Challenges in Cleaning : Recent Developments and Future Prospects

There is considerable scope for optimisation of processes subject to fouling by effective management of cleaning. The cleaning cycle starts with the (oftencomplex) material generated during the fouling cycle. The nature of the deposit determines the most appropriate cleaning method, which can often be optimised significantly via knowledge of the key mechanisms involved in deposit removal. Links between deposit ageing and cleanability need to be established and quantified. There is a wide range of cleaning methods available, and attention is focused here on cleaning-inplace (CIP) techniques. Modern instrumentation allows cleaning (and deposit materials behaviour) to be probed to greater degree than ever before, but the removal technology is only part of the cleaning process. Monitoring and validation of cleaning are equally important, particularly for process plant used in flexible manufacture or subject to batch assurance requirements. Individual sensors are unlikely to meet all monitoring criteria, so future approaches will require reconciliation and interpretation of on-line data from multiple devices. Many industries stand to learn from practice and approaches the food, pharmaceuticals and electronics sectors, where these concepts are well established. The definition of 'cleanliness' will vary from sector to sector, but the needs of minimising environmental impact, accurate monitoring, assurance and suitable training for operators are common to all. INTRODUCTION 'CLEANLINESS' Cleaning, the removal of deposit layers from equipment surfaces, is an essential operation for many industrial processes subject to fouling. Exceptions arise where the surface is replaced rather than renewed for subsequent processing, either because (i) the risks posed by re-use are too great, (ii) the down-time required for cleaning too large, or (iii) the cost of achieving the required state of cleanliness is greater than a replacement strategy. Replacement strategies are seldom employed in the process industries, which indicates that the aim of the cleaning process may simply be to restore process efficiency in terms of heat transfer efficiency or pressure drop. In other sectors, such as the food, biotech and high value chemicals manufacture, issues such as crosscontamination of products, separation of products in flexible manufacturing operations, microbial impacts on hygiene, batch integrity and traceability are extremely important, so that 'cleanliness' includes chemical and biological aspects, usually at the micro-scale. Nano-scale criteria for cleanliness are found in the medical and micro-electronic sectors, where avoidance of molecular contamination (e.g. allergen carryover) is the principal aim. These examples are quoted in order to illustrate that any discussion of cleaning must incorporate a definition of the associated degree of cleanliness desired, as this will dictate the choice of technologies and determine the extent to which cleaning can be verified. The latter point raises the issue of assurance, which a major part of validated cleaning processes defined as A procedure whose effectiveness has been proved by a documented programme providing a high degree of assurance that a specific cleaning procedure, when performed appropriately, will consistently clean a particular piece of equipment to a pre-determined level of cleanliness. (Crockford, 2003) In sectors where the aim of cleaning is to restore process performance, assurance may not be ranked strongly: in others, it is essential for compliance with quality criteria. Assurance therefore requires the use of monitors or testing, which need to be planned as a cleaning protocol is developed, as quantitative data will be required in order to prove compliance. Likewise, operators will need to be trained to interpret the data generated both qualitative and quantitative in order to identify problems and/or solutions. Figure 1 illustrates some of the factors to be considered during development of validated cleaning processes in many food and biotech operations. It assumes that the nature of the foulant to be cleaned can be identified and is generated consistently, which is not always the case. In the food industry, for instance, seasonal variations create deposit inconsistency, while product changeovers in multi-product plant may require different cleaning strategies for the same equipment. Furthermore, the technologies employed and emphasis given to particular factors will be determined by the application, the stated 'cleanliness' aims, reproducibility and reliability. A key feature of the Figure is that cleaning, possibly even more than fouling, involves more than technology: effective cleaning is about effective management. 1 Wilson : Challenges in Cleaning Published by ECI Digital Archives, 2003 This paper does not seek to present a definitive account of existing cleaning technologies, as these are covered in detail elsewhere (e.g. Müller-Steinhagen, 2000; Graβhoff, 1997). Rather, the intention is to identify where challenges and opportunities exist in implementing effective cleaning management, and suggest areas where progress is being made, or attention needs to be directed. Moreover, cleaning is understood to mean the removal of existing fouling layers in dedicated stages, as opposed to those mitigation or antifouling techniques whereby foulant is removed continuously during processing, without interrupting manufacturing. Particular attention is directed towards cleaning-inplace technologies, which are highly favoured in the food and biotech sectors owing to their suitability for verification as they avoid contamination arising from opening equipment. Several of the references are drawn from a recent conference on this topic in the food industry (Wilson et al., 2002). The emphasis lies on applications to heat transfer equipment. It must be noted that related cleaning problems occur in non-heated systems and deposition need not always be heat transfer induced: biofilms, for example, which are not considered in detail, give rise to cleaning (and sterilisation) problems in both. Figure 1 Factors to be considered in developing cleaning methods (after Crockford, 2003) FOULING – SETTING INITIAL CONDITIONS The view of cleaning as a necessary chore, when fouling mitigation methods prove unable to prevent deposit build-up, ignores the inherent symbiosis between fouling and cleaning. The first aspect is of initial states: the surface layers generated during the fouling process obviously form the starting material for any cleaning protocol, and likewise the state of the surface remaining after any cleaning stage is of crucial importance in later operation, as it will dictate the ability of foulants to attach. The extent of fouling is rarely uniform in heat exchangers, either due to surface temperature and shear stress distributions arising from the flow pattern in the unit, or variations in chemical reaction with bulk or surface temperature and composition. As fouling proceeds, this variation in distribution will cause a variation in fouling level (e.g. high fouling coverage giving rise to lower local flow rates and therefore higher fouling rates) and in fouling properties – particularly those linked to ageing. The second aspect concerns process operating conditions, as a design selected to give optimal mass transfer, heat transfer, temperatures or reactions during a cleaning stage may not be able to operate at the conditions needed to mitigate fouling most effectively. The nature of the foulant can is also likely to be affected by operating conditions, apart from temperature (and ageing): biofilm characteristics are well known to vary with flow conditions and oxygen concentration, while at a finer scale, Lelièvre et al. (2002a) reported that the removal rate of Bacillus spores was affected by the flow conditions applied during both the soiling (attachment) procedure and the cleaning stage. This aspect is particularly important when equipment is used for different products, where it will have not been optimized for either stage. Ageing Ageing has long been recognized as an important stage in fouling – indeed, discussed for crude oil II. Determine and establish worst case(s) I Define and characterise what is to be cleaned Qualify and calibrate equipment Define specification for clean acceptance criterion Identify and characterise all process inputs and sources of variability Develop standard operating procedures and train operators Select cleaning technique, agents and disposal Identify suitable measurement, testing and monitoring systems III. Establish cleaning protocol: devise validation plan, test criteria, and validation protocols 2 Heat Exchanger Fouling and Cleaning: Fundamentals and Applications, Art. 21 [2003] http://dc.engconfintl. org/heatexchanger/21 exchangers by Atkins in 1962 – is an essential feature in dairy operations (Visser et al., 1997). Ageing was classified as one of the fundamental steps in Epstein’s 5×5 fouling mechanism matrix (1981) and was described by Müller-Steinhagen (2000) as the most poorly understood aspect of fouling, yet is a key factor in linking fouling and cleaning processes as one seeks to optimize potential mitigation routes. Ageing arises in many fouling mechanisms, and almost always yields a stronger, more resistant deposit, e.g. reaction to bind heterogeneous food matrices together (Liu et al. 2002): ‘consolidation’ of particulate deposits (Turner and Klimas, 2001). Several mathematical models of fouling now feature ageing considerations (scaling – Bohnet et al., 1997; waxes – Singh et al., 2000), which have been made possible by the availability of measurement techniques to quantify ageing effects and the recognition of the importance of this factor. There is considerable scope for further work in this field, particularly in linking the effectiveness of cleaning stages to the extent and nature of fouling. The rate and type of cleaning wi