Utilization of various projectiles to mitigate fouling in tubular heat exchangers

Abstract

Heat exchangers are the workhorse of most chemical, petrochemical, food processing and power generating processes. Of the many types of heat exchangers, approximately 60% of the market is still dominated by shell and tube heat exchangers. One major problem of heat exchangers and particularly the shell and tube type is directly related to the deposition of unwanted materials on the heat transfer surfaces. Fouling may cause one or more of several major operating problems: i) reduction of heat transfer, ii) under-deposit corrosion, iii) increased pressure loss and iv) flow mal-distribution. There are many different mitigation techniques available in the market to maintain the surface of heat exchangers clean to some extent. Among them are projectiles of various shapes, materials and hardnesses which circulate via a separate loop through the exchanger. The advantages of this method include effective fouling mitigation and stable operating conditions. Having said that, there are nevertheless numerous unanswered questions such as optimum injection interval, minimum required shear force to remove fouling layers, applicability of projectiles at elevated temperatures, minimum required velocity of projectile propulsion, and the criterion for the selection of projectiles for any specific fouling process. The present study, as part of a European Project entitled Clean-Ex, endeavors to address some of these questions. A test rig was designed and constructed to simulate conditions under which fouling occurs in water service processes. The rig includes an online cleaning device which enables introduction of projectiles for various operating scenarios including i) continuous or ii) at different time intervals. A comprehensive set of experimental runs was carried out for crystallization fouling of CaSO4 solutions with and without projectiles. Due to laboratory restrictions, fouling runs were performed at accelerated conditions to rigorously characterize the impact of projectile cleaning in terms of injection intervals and various types of projectiles. The experimental results showed that the projectiles are capable of removing parts of the fouling layer at the early stage of the fouling process. The cleaning efficiency decreases as the fouling layer builds up such that the projectile is not effective when the asymptotic fouling is approached. In addition, shorter injection intervals of the projectiles decrease the asymptotic fouling resistance. Sintering of the fouling layer which hinders the cleaning action of projectiles should be accounted for this phenomenon. Furthermore, all projectiles decreased the induction time of the fouling process. The asymptotic fouling resistance was also approached much quicker compared to the case of no injection. The performance of any projectile lies in a trade-off between its size, texture and stiffness. Stiffness produces a shear force required to dislodge the deposit and size is required to maintain the contact area between projectile and the surface. Accordingly, a criterion was developed to determine the optimum projectile size and stiffness for best cleaning performance. The criterion shows that bigger and softer projectiles cannot last for a long time injection processes. Given the importance of size and stiffness, the projectiles were subsequently divided into two groups of hard and soft due to the required stiffness and velocity to move the projectile within the tube. To discriminate between these two groups, a new term called contact stability factor or Z factor is proposed which is a function of stiffness and size. A mechanistic model has also been developed to predict the asymptotic fouling resistance when projectiles are in operation, based on injection rate, fouling rate and removal rates. The model predicts the asymptotic fouling resistance with an accuracy of 69% based on CaSO4 concentration, saturation concentration, injection interval, shear force and contact stability of the tube with the projectile.