Peter J. Heggs

CFD modelling of a pilot-scale counter-current spray drying tower for the manufacture of detergent powder

ABSTRACT A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.

Transient Response of Heat Exchangers with One Infinite Capacitance Fluid

Abstract Information on the transient response of heat exchangers is essential to produce the optimum heat exchanger operation for the required function. A transient response analysis is presented for a heat exchanger with an infinite capacitance fluid on the shell side. The inlet temperature of the finite capacity fluid is stepped, and the heat exchanger response is represented by the change in the temperature of this fluid at the outlet of the tubes. The results cover all practical combinations of the dependent parameters and are presented as a set of charts. Results of previous workers in this field are discussed. Their charts cover a very restricted range of heat transfer conditions and are therefore of limited use. In addition, assumptions have been incorporated into their analyses which result in major discrepancies.

Predictions of Heat Transfer and Flow Circulations in Differentially Heated Liquid Columns With Applications to Low-Pressure Evaporators

Numerical computations are presented for the temperature and velocity distributions of two differentially heated liquid columns with liquor depths of 0.1 m and 2.215 m, respectively. The temperatures in the liquid columns vary considerably with respect to position for pure conduction, free convection, and nucleate boiling cases using one-dimensional (1D) thermal resistance networks. In the thermal resistance networks the solutions are not sensitive to the type of condensing and boiling heat transfer coefficients used. However, these networks are limited and give no indication of velocity distributions occurring within the liquor. To alleviate this issue, two-dimensional (2D) axisymmetric and three-dimensional (3D) computational fluid dynamics (CFD) simulations of the test rigs have been performed. The axisymmetric conditions of the 2D simulations produce unphysical solutions; however, the full 3D simulations do not exhibit these behaviors. There is reasonable agreement for the predicted temperatures, heat fluxes, and heat transfer coefficients when comparing the boiling case of the 1D thermal resistance networks and the CFD simulations.

Process Modelling of Entrained Flow Gasification

Abstract This paper presents the development of a mathematical model in gPROMS of a process for the generation of high quality synthetic fuels from alternative fuels. The first stage is the model of an entrained flow gasifier for the generation of syngas thermodynamically to ensure the feasibility of the gasifier. This is done by minimising the Gibbs free energy of the system and applying the system constraint of the Lagrange multipliers that represent the chemical potential of each species in the system. The best syngas ratio (H 2 /CO) of around 1 is predicted at temperatures greater than 1400 K with the least amount of carbon dioxide produced. The model was subsequently put through a sensitivity analysis study to determine the effect of property estimation methods. The simulations are shown to be insensitive to the different Equation of State (EOS) methods. Hence considerable time can be saved by using the ideal gas assumption instead of the other more complicated EOS methods.

Vacuum Operation of a Thermosyphon Reboiler

The research facility at the University of Manchester in the Morton Laboratory is a full scale replica of an industrial sized natural circulation thermosyphon reboiler, which comprises 50 tubes of 3 m length and 25.4 mm OD. The facility is operated under vacuum. Water is used as the process fluid and condensing steam is the heating source. Experimental datasets were obtained for the reboiler and have been presented in the form of profile plots of feed rate, fluid recirculation, recirculation ratio and vapour quality. The data elucidate the effect of pressure [0.1 to 1.0 bar] and heat duties [78 to 930 kW] on the performance of the reboiler. Three distinct modes of operation have been observed. Mode one is defined as a flow-induced instability or geysering (low heat duty) and exists below a definite transitional point that is independent of process pressure. Mode two is a region of stable operation that occurs above the threshold of the flow-induced instability, while mode three, which is defined as the heat-induced instability (density-wave instability), is pressure dependent obtained at high duties and is characterised by violent oscillations. These instability thresholds represent the lower and upper limits of operation of the reboiler. The region of stable operation is enveloped between the two limits and is very dependent on process pressure as it progressively becomes smaller as the vacuum becomes lower. These studies led to unique experimental observations, which revealed the existence of intermittent reversed flow in the entire loop. The use of throttling in the heat-induced unstable region to return to stable operation tends to be over a narrow range, outside of which the sole way to regain stability is to lower the heat load or increase the process pressure. In the region of flow-induced instability, throttling the fluid at the inlet is useless and actually makes the situation worse. These instabilities are alleviated by increasing the heat load.Copyright

A two-dimensional, finite-difference model of the oxidation of a uranium carbide fuel pellet

The oxidation of spent uranium carbide fuel, a candidate fuel for Generation IV nuclear reactors, is an important process in its potential reprocessing cycle. However, the oxidation of uranium carbide in air is highly exothermic. A model has therefore been developed to predict the temperature rise, as well as other useful information such as reaction completion times, under different reaction conditions in order to help in deriving safe oxidation conditions. Finite difference-methods are used to model the heat and mass transfer processes occurring during the reaction in two dimensions and are coupled to kinetics found in the literature.

Modelling the Oxidation of Spent Uranium Carbide Fuel

Uranium/mixed carbide fuels are a candidate fuel for future nuclear reactors. However, in order to be implemented, a clear outline for their reprocessing must be formed so as to reduce the volume of nuclear waste produced as much as possible. One proposed method is to oxidise the uranium carbide into uranium oxide which can then be reprocessed using current infrastructure. A mathematical model has been constructed to simulate such an oxidation from a combination of finite-difference approximations of the relevant equations describing the heat and mass transfer processes involved. Available literature was consulted for reaction coefficients and information on reaction products, however the behaviour of the produced oxide is uncertain. The model was built accounting for this uncertainty and the resultant predictions will assist in characterising the proposed reprocessing method for carbide fuels.

A CFD-process model of steam generation in a power plant by a thermosyphon system

Abstract A novel combined CFD-process model of steam generation in a coal-fired natural circulation boiler is driven by heat flux data from a detailed CFD simulation of the furnace. The circulation mass flow at steady-state is calculated and the fluid-side heat transfer coefficient as a function of height is post-processed and can be linked back to the flux boundary condition in the CFD model. It is demonstrated that the circulation rate is approximately 8 times the feed flow, which is significantly higher than the “rule of thumb” values of 3-5 times the feed flow. The resulting model can provide vital information on the operation of steam generation systems in coal-fired power plants and lead to the development of improved control strategies with increased opportunity for flexibility of operation and reduction of thermal stresses in the tube walls.

Modelling a Vertical Thermosyphon Reboiler Operating Under Vacuum Using Data from Rigorous Experimental Studies

Thermosyphon reboilers represent effectively a pumpless system, in which natural, gravity-assisted circulation takes place. Although these units are the most commonly used in the chemical industry, Arneth and Stichlmair (2001), their operation has only been considered within the context of wider experimental programmes with few studies conducted below atmospheric pressure. In addition, previous research carried out to determine the operating characteristics of thermosyphon reboilers decoupled the problems related to heat transfer into a tube side and a shell side, usually by means of an electrically heated single tube (uniform heating). Thus, it is of paramount importance to look at the coupled problem to obtain better estimates of the heat transfer coefficients for the condensing steam in the shell and the heated process fluid in the tubes. The work described in the present article is carried out in this context and provides a detailed description of a mathematical model developed to predict the steady-state performance of a vertical thermosyphon reboiler. A number of operating variables have been predicted. Analysis of these predictions and comparisons with the generated experimental data, reported by Alane and Heggs (2007), resulted in good agreement. The resultant model could be used for optimisation studies on existing thermosyphon reboilers and the design of new ones.