Coating and hydrodynamics of random column packing Dixon rings

Abdelraouf, Mohamed and Rennie, Allan and Aiouache, Farid and Najdanovic, Vesna (2023) Coating and hydrodynamics of random column packing Dixon rings. PhD thesis, Lancaster University.

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Abstract

In this thesis, three main objectives were covered using experiments and modelling studies. Firstly, the coating of Dixon rings cheaply and repeatably using alumina sol-gel. Secondly, the new understanding of fluid flow over curved wire meshes. Thirdly, the effect of Dixon rings on process parameters such as pressure drop, liquid holdup and mass transfer coefficient were investigated using experiments and model. Packed columns are filled with solid structures to improve the heat and mixing of multiphase processes. The packing usually interacts with the fluid, increasing the mixing but creating a pressure drop. Packing are typically evaluated based on their hydrodynamic behavior, such as pressure drop, liquid holdup, and mass transfer coefficient. Packing with low-pressure drop, high mass transfer, and high flooding velocities are the preferred choice for users. Wire mesh packing are an example of packing that offer low-pressure drop and increased mass transfer. The pore openings in the mesh provide a path for the gas to flow, resulting in good mixing and low-pressure drop. Dixon rings are an example of wire mesh packings made of stainless steel and rolled into a cylindrical shape with a bisecting section in the middle. The stainless-steel structure of Dixon rings makes them resistant to high temperature and chemical constraints. Packing with such properties has great potential in water treatment, hydrocracking, and more. However, the hydrodynamic behavior of Dixon rings has not been investigated deeply in the literature. Secondly, a new understanding of fluid flow on a microscale and macroscale over curved surface of the wire mesh was achieved. The macroscale experiments were conducted in a semi-pilot plant with gas and liquid flow in counter-current for the uncoated Dixon rings. The effect of Dixon rings on pressure drop, liquid holdup, and mass transfer were measured and compared to commercial packing of similar size and material. The microscale study was focused on the liquid flow distribution, which is critical in the design and operation of packed columns. The liquid flow distribution changes the packing's wetting efficiency, affecting several hydrodynamic parameters, such as the liquid holdup and the effective surface area. The liquid wetting efficiency was evaluated using imaging experiments and CFD simulations for the coated and uncoated Dixon rings. The flow regime and several hydrodynamic parameters such as liquid holdup, effective surface area, and pressure drop were anticipated to affect the mass transfer capabilities highly. Finding smaller unit operations is required to cut down the global emission. It is not a surprise that process intensification is needed in packed columns. Packing with a tunable surface can achieve both hydrophobic and hydrophilic properties that are highly desired due to their tendency to control the liquid dispersion and wetting properties. Changing the surface properties using uniform coating is a common approach to produce tunable contact angle and surface wetting. Coating metals have been fraught with difficulties, and Dixon rings have a complex geometry which hinders their use in many applications, specifically when a coating is required. This research considered the application of a uniform adhesive coating via the sol-gel deposition method of alumina on Dixon rings. The coated film was investigated for the first time on stainless steel wire mesh for use as Dixon rings. The kinetics of deposition of the sol-gel was followed for a range of initial compositions of the coating, such as the ratios of alumina to water, acid content, polyethyleneimine binder content and the number of deposition cycles. The coated Dixon rings were characterised by surface optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Brunauer, Emmett and Teller (BET). Adhesive tests included assessing coating behaviour against shear forces, ultrasound, and temperature constraints. Well-adhered alumina with a thickness of up to 20 µm was successfully deposited. Imaging experiments and 3D volume-of-fluid modelling investigated the microscale behaviour of liquid flow over coated and uncoated Dixon rings. An alumina coating modified the surface of the wire mesh ring to reach both hydrophilic and hydrophobic characteristics. The contact angle was varied due to the coating and resulted in different mass and heat exchanges of the multiphase chemical systems. The cycle of capillary droplet flow over the uncoated ring exhibited penetration of the hydrophilic mesh openings, adherence to the surface of the ring, and accumulation as drips at the bottom region of the rings. However, over the hydrophobic ring, the droplet exhibited low adherence to the ring surface, accumulation at the top surface of the ring, no penetration of the openings, slipping by the gravitational forces over the vertical curvature and accumulation as drips at the bottom region. In agreement with the classical observations at the macroscale, the observations at the pore-scale confirmed the increase of the wetting efficiency, liquid holdup and effective surface area at increased liquid flowrate and reduced contact angle. The 3D model had a relative deviation of 7.21 % with Stichlmair’s model for the liquid holdup, particularly in the hydrophilic zone of the contact angle and low flow as well as a relative deviation of 14.24% with Linek’s model for effective area, particularly in the hydrophobic range of the contact angle. Considering the results at the pore scale, the packing material was then investigated at a larger scale using the non-coated Dixon rings exclusively. The hydrodynamic and mass transfer properties for Dixon rings packing are not currently available within the literature. A semi-pilot plant was used to evaluate the pressure drop, the liquid holdup, and the mass transfer coefficient for Dixon rings packing 5/8 and 1/4 in. The study was extended to various operational conditions for gas and liquid counter-current flow. The results were compared to commercial random packing and validated, showing that Dixon rings offer low-pressure drop, high liquid holdup, and mass transfer coefficient. This study successfully investigated a method to coat metallic wire mesh packing using sol-gel. The thickness and distribution of the coating were investigated by a parametric analysis for several ratios such as the acid, water, and Al content. Microscopic and SEM tests showed that the coating has a uniform thickness, and adhesive tests were conducted to ensure the coating is well attached to the Dixon rings’ surface. This coating method can be used for any type of metallic wire mesh substrate to produce alumina coating with tunable thickness. A microscopic study was implemented to investigate the liquid flow behaviour for coated and uncoated Dixon rings using a 3D VOF model and imaging experiments. The liquid holdup and the effective surface area from the 3D model were compared to results from the literature. The coated Dixon rings have a hydrophobic nature, while the uncoated rings have hydrophilic properties. The pressure drop, liquid holdup and mass transfer value were reported for Dixon rings for the first time on a macroscopic level. Based on the experimental data, models were developed to predict the dry pressure drop and the loading point. The model showed good reliability with a SE less than 5%