Control of scaling in heat exchangers through the use of physical treatments

Summary :

In order to study the impact of the heterodyne effect on the formation of mineral deposits from domestic water networks in a heat exchanger, an experimental pilot was set up to test this technology on two types of heat exchangers, plate and joint exchanger and platelet exchanger. Several operating parameters such as flow rate, time, and temperature were experimentally tested in order to identify their influence on calcium carbonate precipitation and scale formation in the exchanger. In order to approve the efficiency of the heterodyne effect, a comparison was made with other physical treatments that were tested on the same pilot, namely: ultrasonic treatment at two different frequencies, low frequency, and low-intensity electromagnetic treatment, and the ultrasonic-electromagnetic coupling. The first part of this work focused on numerical modeling, two software were used, Phreeqc interactive and COMSOL Multiphysics 6.0. On the one hand identify the nature of crystallized polymorphism, whether at the pilot-scale or at the industrial scale. And on the other hand to identify the sensitive areas for the formation of the deposit in order to know the location of the ultrasonic system, and to model the dispersion of the ultrasonic waves at different frequencies and power, either in the water or in the metallic surfaces of the exchanger. The results of the numerical analysis obtained by these models were then compared to the experimental measurements. A series of laboratory-scale experiments, with or without the physical treatment, is performed on an exchanger that has been insulated from the outside with an insulating cover and equipped with thermocouples in order to evaluate the impact of the physical treatment tested on the heat transfer. 100 Liters of water with a scaling capacity (30°F) was heated to 50°C within the exchanger for 72 hours in a closed circuit. Due to the high content of dissolved calcium carbonate and the decrease of its solubility when the temperature increases, a precipitate of calcium carbonate forms inside the heat exchanger, but also in the water, as well as on the walls of the exchanger. At the end of each experiment, the deposit is dissolved in 5% (vol.) nitric acid and analyzed by ICP-AES spectroscopy to perform a complete material balance and determine the amount of scale precipitated on the exchanger plates. A dismantling of the exchanger was carried out in each type of test in order to analyze the distribution of the deposit in the whole of the plates by 3D digital microscopy and confocal microscopy, to analyze the precipitated polymorphism by RAMAN spectroscopy and scanning electron microscopy, and the size distribution of the precipitated particles by laser granulometry. The results obtained show that the heterodyne-guided waves allow reducing 65,46% of the deposit. On the other hand, the treatment that allows better reduction is the electromagnetic treatment with a percentage of reduction of 76,83%, without changing the nature of polymorphism. At present, all the physical treatments tested allow a reduction in the amount deposited, but do not prevent the total formation of the deposit. These methods aim to minimize the occurrence of fouling. They are preventive rather than corrective methods.