A NOVEL FOULING CELL TO STUDY INFLUENCE OF SOLUTION ON CRYSTALLISATION FOULING

A novel fouling cell has been developed to assess the effect of process solution chemistry on crystallisation fouling. This device consists of a jacketed vessel with two heated vertical copper fingers suspended in the solution from above. The substrates are heated by a recirculating flow of liquid and operate under conditions of constant temperature driving force. The fouling cell was designed to emphasise simplicity yet still allow the key surface material and coating chemistry effects to be captured. The thermal performance of the fouling cell was characterised using thermochromic liquid crystals. The cell was used to study crystallisation fouling from aqueous solutions containing calcium carbonate. One advantage of the device is that when identical test sections are used, separate destructive analytical techniques can be used to determine the composition and microstructure of the fouling layer(s). The results are compared with those obtained from a simple co-current heat exchanger operating under forced flow conditions. Good agreement in deposit structure and composition was found. The rate of fouling was monitored from heat transfer measurements and from changes in solution composition. Mass balances confirmed that the change in solution chemistry matched the growth in deposit. Kinetic analyses indicated that deposition was controlled by mass transfer to the surface. INTRODUCTION Heat exchanger fouling is the build-up of unwanted material on heat transfer surfaces, and the deleterious effects of fouling on heat transfer are well documented for many industrial and domestic systems. Often it is not practicable to use real heat exchangers from industry or from domestic devices to study characteristics of a fouling process. Reasons for this include, but are not necessarily limited to; expense (some heat exchanger configurations can have complex geometries and are therefore expensive to produce), time (on some systems it can take an unreasonably long time to deposit sufficient fouling material for an effective study) and practicality (heat exchangers can be parts of very complicated systems and decommissioning and/or recommissioning can be technically challenging, time consuming and expensive). Therefore, it is convenient to build laboratory analogues for real heat exchanger systems in order to reproduce the characteristics of a fouling system to be studied. This paper presents details of the design and development, as well preliminary experiment results for, a test cell designed to study the influence of solution chemistry on the prevalence of crystallisation fouling. Crystallisation fouling is one of the five principal fouling mechanisms identified by Epstein (1983). It occurs in heat exchangers when the concentration of a solute exceeds its solubility limit at the heat transfer surface. This paper focuses on inverse solubility solutions (containing solutes whose solubility decreases with increasing temperature – often termed ‘retrograde’ behaviour) and heated surfaces. The surface provides pathways for heterogeneous nucleation and a crystalline deposit may nucleate and grow there. Examples of inverse solubility salts include calcium carbonate, calcium sulphate and magnesium silicate (Bott, 1995; Brooke, 1984). Inverse solubility salts can also participate in particulate fouling, when precipitation of crystalline material in the bulk solution leads to deposition at the heat transfer surface (see Hawthorn and Wilson, this conference). Crystallisation fouling can also arise with normal solubility salts when the solution is subject to evaporation and on cooled surfaces. The test cell presented here is similar in concept to the ‘cold finger’ devices used to study wax deposition (Jennings and Weispfennig, 2005). In heating, ventilation and air conditioning (HVAC) systems, the deposition of calcium carbonate and other hardness salts can be problematic since these systems are designed to run for long periods of time with little maintenance. Water softening units can be used but would require continual renewal. The composition of the feed water is largely determined by the geology of the installation location. It is useful to know the susceptibility of a device or system to fouling, and the composition of the feed water is a key determining factor. Certain waters may preclude the sale of an HVAC device in an area or necessitate re-engineering of a system to counteract fouling (Kawaley, 2015). It is therefore important to use a systematic approach to study the effect of solution composition on fouling. This paper reports results from an ongoing study of hard water scaling on copper surfaces. Heat Exchanger Fouling and Cleaning – 2017 ISBN: 978-0-9984188-0-3; Published online www.heatexchanger-fouling.com 49 We present a fouling rig which has several desirable and unusual features that aid the study of the effect of solution composition on fouling. Firstly, the design of the apparatus emphasises simplicity and most of the parts are standard fittings. The test cell shares similarities with the heated probe system employed by Geddert et al. (2011), where electrically heated test strips were suspended in a reservoir of the solution of interest. Their unit operated under constant heat flux while our device employs constant temperature driving force. The solution of interest only contacts the heat transfer surfaces and the glass walls of the reservoir. This means that fouling upstream of the test section, which can occur in more complex apparatuses, cannot occur. Furthermore, the system is easy to clean between experiments. The apparatus features two identical test sections. This allows separate, destructive analyses to be undertaken using deposit generated from a single batch of process solution. In this study, one sample was subject to quantitative compositional analysis using inductively coupled mass spectrometry (ICP-MS) and the second was used for microstructure and composition analysis by scanning electron microscopy (SEM) and related methods. The deposit is formed on the external surface of a vertical cylindrical annulus. Surface temperature is a key parameter and the clean heat transfer performance of the test cell was characterised using thermochromic liquid crystals (TLCs). TLCs change in colour at a particular temperature and are used in medical devices, thermometers and novelty mugs. The colour change and transition temperature is determined by the liquid crystal material. The temperature transition width for the TLCs used here was 0.1 K. Their use in heat transfer studies, for determining local heat transfer coefficients, is well established in aerospace applications (Ireland & Jones, 2000, Manzhao et al. 2008) where the heated fluid is usually air. The use of TLCs to study liquid phase heat transfer is much less common because most TLCs are not very stable in water. This was mitigated in this study by the use of microencapsulated TLCs. The ability to determine the local surface temperature, and thus estimate the local heat transfer coefficient, was particularly important given the complex and non-standard fluid flow configuration in the fouling cell reservoir: extensive computational fluid mechanics simulations similar to those reported by Huang et al. (2012) would otherwise have been required. EXPERIMENTAL METHODS Design and optimisation of fouling rig Figure 1 shows schematics and photographs of the fouling rig. A 5 litre, 150 mm inner diameter cylindrical jacketed glass vessel serves as the reservoir. The charge of liquid in the reservoir was stirred by a 70 mm magnetic stirrer bar rotating at 150 rpm. Its temperature was maintained by circulation of water from a heater/chiller unit through the jacket. The glass walls allow the solution to be monitored for precipitation; insulation can be applied if needed. The Reynolds number at the tip of the stirrer is approximately 22 000 (at a bulk temperature of roughly 50°C). The stirring speed was set at the fastest rate that the stirrer could be operated stably. The purpose of the stirrer bar was primarily to ensure that the fluid in the vessel was well mixed and there little variation in temperature in the bulk liquid. Separate studies (not reported here) established a weak dependency of ho on stirrer speed. The system contains two test devices which are mounted in the reservoir lid on a diameter, equidistant from the centre. The test device consists of a flow connection (Figure 2(c)) and a test section (Figure 2(a)). These studies used copper test sections 160 mm long, wall thickness 0.65 mm, fabricated from standard 15 mm o.d. domestic copper piping (alloy C12200). In these experiments the bottom 120 mm of the finger is in contact with the process solution when the rig is assembled. This sets the area for heat transfer. Figure 1. Fouling cell: (a) schematic; (b) photograph. The bottom end of each test section was sealed by crimping closed a 5 mm length using a vice, with a small amount of glue located inside the crimped region to ensure that the section remained watertight. The crimped design proved superior to soldering a cap on to the end of the tube as crimping maintained uniform surface chemistry. Tests with a soldered cap gave preferential nucleation at the joint, caused by the tin and other metals present in the solder alloy. Heating flow inlet