My MEng and PhD degrees were obtained at the University of Cambridge. I worked for Brown and Root (UK) for one year before joining the lecturing staff in chemical engineering at the University of Cambridge. I was elected a member of the IChemE and became a chartered engineer three years later. I moved to the University of Surrey in 2001 and was elected FIChemE in 2006.
University roles and responsibilities
- Undergraduate Admissions Tutor
16 FEB 2021
Surrey joins £1.2m industry project to improve efficiency of chemical recycling technology
- Multiphase flows including oil production flows (2, 3 and 4 phase), in circulating fermenters, jet engines and dissolved gas floatation tanks.
- Energy optimisation of sewage treatment processes
- Capture and recycling or destruction of odours including VOCs
- Treatment of process water from Hydro-Thermal Carbonisation
- Chemical recycling of waste plastics.
- Start date: July 2020
- End date: September 2022
Collaborating with Recycling Technologies alongside the University of Birmingham and Cranfield University, we will contribute to the development of an online monitoring system which will predict and control Plaxx® (a recycled feedstock which can be used to manufacture new plastic) quality based on input composition and process conditions.
Completed postgraduate research projects I have supervised
I have supervised students who have obtained their doctorates with the following titles:
- The properties and abrasive behaviour of sand in hoppers
- Cavitation in the flow through an orifice
- The discharge of solid/liquid mixtures from storage vessels
- The measurement of diffusion coefficients in non-Newtonian liquids
- Large bubbles in circulating fermenters
- Circulating fluidised beds
- Near horizontal conveying of sand by two-phase gas-liquid flows in pipelines
- Residence time distributions in circulating fluidised beds
- Forces on pipe bends due to two-phase flow
- Bubble break-up and mass transfer in down flow
- Vertical dense-phase conveying of large particles
- Air-current segregation in alumina silos
- Oil-water separation on a production platform
- Simulation of oil sprays formed by leaks in pipes
- Transient flows in gas turbines
- Using a screw extruder as a granulator
- Optimising energy use in the water industry
- Regeneration of VOCs from from GAC adsorbent
- Dissolved gas floatation of oils
- Direct contact condensation
I teach on the following modules:
- Energy and Industrial Systems
- Process and Equipment Design
- Chemical Product Design.
Courses I teach on
Gear M, J Sadhukhan, R Thorpe, R Clift, J Seville, M Keast (2018) A life cycle assessment data analysis toolkit for the design of novel processes - a case study for a thermal cracking process for mixed plastic waste, Journal of Cleaner Production, 180, 735-747. https://doi.org/10.1016/j.jclepro.2018.01.015
Mauro Lafratta, Rex B. Thorpe, Sabeha K. Ouki, Achame Shana, Eve Germain, Mark Willcocks, Jacquetta Lee (2020)Dynamic biogas production from anaerobic digestion of sewage sludge for on-demand electricity generation, In: Bioresource Technology310123415 Elsevier
Oliver Crossley, Rex Thorpe, Dominik Peus, Judy Lee (2020)Phosphorus recovery from process waste water made by the hydrothermal carbonisation of spent coffee grounds, In: Bioresource Technology301122664 Elsevier
Dissolved air flotation (DAF) is often used after a primary gravity separator to enhance the quality of wastewater, so it can be released to streams, rivers or the sea. The main aim of the DAF experiments reported here was to measure the oil droplet removal efficiency, (η) mostly in the range 15-80 μm from oil-in-water mixtures. The DAF tank used in this investigation was a scale model of real DAF unit. Two kinds of oil, vegetable and mineral and two types of water, fresh and salty were used, and four other operating parameters were varied. A droplet counting and oil-in-water measuring methods were used to estimate the η. Dimensional analysis concluded that the η in this experiment is a function of eight other dimensionless groups and the experimental data has been subjected to multivariable linear regression. The resulting correlation was found to have a root mean square error of 6.0%, but predict η outside the range zero and one. An alternative mathematical formulation was devised that cannot predict η outside the range. Regression of the data by this formulation, which had the same number of adjustable parameters as the linear regression, was successful with a lower root mean square error of 5.5%.
In this paper, data is published on the removal of H2S and VOCs by a Biotrickling Filter (BTF) demonstration plant, namely a SULPHUS™, which was installed by Thames Water in late 2015. These data, along with some data already published by Sempere et al. (2018), were compared to the predictions of a number of existing and novel models for the removal of a single pollutant by a biofilm. The two widely used models of Ottengraf and van den Over (1983) were found to be inadequate with sum of squares of errors of 11 and 41 mg2m-6 respectively. These models are based on zero-order kinetics in the biofilm which according to the M-M kinetic model, are likely to be inaccurate at low pollutant concentration. The odour control unit was designed to produce low emission levels of less than 1 ppmv of H2S, rendering the zero-order assumption unlikely to be accurate. A model based on first-order kinetics, which also has some support in the literature, was found to be a better, but not a good, fit to the data with a sum of squares of errors of 4.7 mg2m-6. A novel model for the BTF based on M-M kinetics was found to be a good fit to the shape of the data with the lowest sum of squares of errors of 2.5 mg2m-6. This novel M-M model was also identified as the best fit for VOC data from the same unit. Other publications support the M-M approach with a product of saturation constant and Henry’s Law constant of about 50 mg m-3, which is equivalent to an H2S level in the gas phase of about 40 ppmv. Broad agreement was found between the SULPHUS™, experiments and data in the literature for other BTFs destroying H2S under the zero-order regime, at V_max value of about 0.3 g/m3/s. This paper represents an attempt to harmonise a literature that was previously disparate, which has not previously been attempted.
The transient temperature distribution and volumetric heat transfer coefficient during the 16 inception of flooding in a three-phase bubble type direct contact condenser have been 17 experimentally investigated. The flooding mechanism and the factors affecting the onset of 18 flooding of the three-phase direct contact column are not considered. A short Perspex column 19 of 70 cm total height and 4 cm internal diameter utilising two immiscible fluids was studied. 20 Pentane vapour with initial temperatures of 40°C, 43.5°C and 47.5℃ was the dispersed phase 21 and tap water at a constant temperature (19℃) was the continuous phase. Only 48 cm of the 22 column was used as the active height and different mass flow rates of both phases were used. 23 The experimental results showed that the instantaneous temperature distribution along the 24 direct contact column tends to be uniform when the direct contact column is working under 25 flooding conditions. Furthermore, the volumetric heat transfer coefficient increases as the 26 dispersed mass flow rate is increased towards the flooding limit and remains constant along 27 the column height. In addition, the dispersed phase mass flow rate that leads to flooding 28 increased with increasing mass flow rate of the continuous phase. The initial temperature of 29 the dispersed phase did not have a considerable effect on the flooding inception limit under 30 the present experimental conditions
An experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapour at three different initial temperatures (40°C, 43.5°C and 47.5°C), with differing mass flow rates, and tap water at a constant initial temperature (19°C) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapour (dispersed) phase mass flow rate. Furthermore, it was observed that the initial driving temperature difference had no effect on the volumetric heat transfer coefficient. While, the temperature gained by the continuous phase has a considerable effect.
A SULPHUS™ biotrickling filter (BTF) and an ACTUS™ polishing activated carbon filter (ACF) were used at a wastewater treatment plant to treat 2,432 m3·h–1 of air extracted from sewage sludge processes. The project is part of Thames Water’s strategy to reduce customer odour impact and, in this case, is designed to achieve a maximum discharge concentration of 1,000 ouE·m–3. The odour and hydrogen sulphide concentration in the input air was more influenced by the operation of the sludge holding tank mixers than by ambient temperature. Phosphorous was found to be limiting the performance of the BTF during peak conditions, hence requiring additional nutrient supply. Olfactometry and pollutant measurements demonstrated that during the high rate of change of intermittent odour concentrations the ACF was required to reach compliant stack values. The two stage unit outperformed design criteria, with 139 ouE·m–3 measured after 11 months of operation. At peak conditions and even at very low temperatures, the nutrient addition increased considerably the performance of the BTF extending the time before activated carbon replacement over the one year design time. During baseline operation the BTF achieved values between 266-1,647 ouE·m–3 even during a 6 days irrigation failure of the biofilm.
The earlier in the development of a process a design change is made, the lower the cost and the higher the impact on the final performance. This applies equally to environmental and technical performance, but in practice the environmental aspects often receive less attention. To maximise sustainability, it is important to review all of these aspects through each stage, not just after the design. Tools that integrate environmental goals into the design process would enable the design of more environmentally friendly processes at a lower cost. This paper brings together approaches based on Life Cycle Assessment (LCA) including comparisons of design changes, hotspot analysis, identification of key impact categories, environmental break-even analysis, and decision analysis using ternary diagrams that give detailed guidance for design while not requiring high quality data. The tools include hotspot analysis to reveal which unit operations dominate the impacts and therefore should be the focus of further detailed process development. This approach enables the best variants to be identified so that the basic design can be improved to reduce all significant environmental impacts. The tools are illustrated by a case study on the development of a novel process with several variants: thermal cracking of mixed plastic waste to produce a heavy hydrocarbon product that can displace crude oil, naphtha, or refinery wax or be used as a fuel. The results justified continuing with the development by confirming that the novel process is likely to be a better environmental option than landfill or incineration. The general approach embodied in the toolkit should be applicable in the development of any new process, particularly one producing multiple products.
This study investigates the recovery of phosphorus from the process water obtained through hydrothermal carbonisation (HTC) of a ‘wet’ biomass waste, namely spent coffee grounds. HTC was shown to liberate more than 82% of the total phosphorus in the grounds in the form of dissolved ortho-phosphate. Nanofiltration was used to concentrate the inorganic nutrients of the HTC process water, achieving a mass concentration factor of 3.9 times. The natural stoichiometry of phosphorus, magnesium and ammoniacal nitrogen in the nanofiltration retentate was favourable for struvite precipitation. 92.8% of aqueous phosphorus was recovered as struvite through simple pH adjustment, yielding a total phosphorus recovery of 75% from the feedstock spent coffee grounds.
Computational fluid dynamics (CFD) is a simulation technique widely used in chemical and process engineering applications. However, computation has become a bottleneck when calibration of CFD models with experimental data (also known as model parameter estimation) is needed. In this research, the kriging meta-modelling approach (also termed Gaussian process) was coupled with expected improvement (EI) to address this challenge. A new EI measure was developed for the sum of squared errors (SSE) which conforms to a generalised chi-square distribution and hence existing normal distribution-based EI measures are not applicable. The new EI measure is to suggest the CFD model parameter to simulate with, hence minimising SSE and improving match between simulation and experiments. The usefulness of the developed method was demonstrated through a case study of a single-phase flow in both a straight-type and a convergent-divergent-type annular jet pump, where a single model parameter was calibrated with experimental data.
The UK Water Industry currently generates approximately 800GWh pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 - conventional AD with CHP, 2 - Thermal Hydrolysis Process (THP) AD with CHP, 3 - THP AD with bio-methane grid injection, 4 - THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 - THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP. The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.
Low-grade energy cycles for power generation require efficient heat transfer equipment. Using a three-phase direct contact heat exchanger instead of a surface type exchanger, such as a shell and tube heat exchanger, potentially makes the process more efficient and economic. This is because of its ability to work with a very low temperature driving force, as well as its low cost of construction. In this study, an experimental investigation of the heat transfer efficiency, and hence cost, of a three-phase direct contact condenser has been carried out utilising a short Perspex tube of 70 cm total height and 4 cm internal diameter. Only 48 cm was used for the direct contact condensation. Pentane vapour with three different initial temperatures (40℃, 43.5℃ and 47.5℃) was contacted with water with an inlet temperature of 19℃. In line with previous studies, the ratio of the fluid flow rates was shown to have a controlling effect on the exchanger. Specifically, the heat transfer efficiency increased virtually linearly with this ratio, with higher efficiencies also being observed with higher flow 2 rates of the continuous phase. The effect of the initial temperature of the dispersed phase was shown to have a lower order impact than flow rate ratio. The capital cost of the direct contact condenser was estimated and it was found to be less than the corresponding surface condenser (shell and tube condenser) by 30 times. An optimum value of the continuous phase flow rate was observed at which the cost of the condenser is at a minimum. Keywords: Three-phase direct contact condenser, heat transfer efficiency, costing
The performance of a particular type of horizontal three phase separator (bucket & weir) was evaluated on the Alba Field, situated offshore of the coast of Equatorial Guinea, by coding a set of equations for the design of the separators. Output parameters such as the oil and water residence times, liquid droplet settling/rising times, minimum lengths for gas-liquid disengagement, and holdup and surge times for the oil bucket and the water compartment, were checked regularly against max/min values for good operation. Thus it was possible to assess the likely behavior of the four Alba field separators to changes in operating variables while also comparing the results obtained from the equations with real field data for performance. The performance of the separators was fairly consistent, even though oils of varying viscosity and temperature were processed. The values of parameters at which performance deteriorated was somewhat different from those usually quoted in the literature for good operation. © BHR Group 2007 Multiphase Production Technology 13.
The aim of this work was to study the potentials and benefits of dynamic biogas production from Anaerobic Digestion (AD) of sewage sludge. The biogas production rate was aimed to match the flexible demand for electricity generation and so appropriate feeding regimes were calculated and tested in both pilot and demonstration scale. The results demonstrate that flexibilization capability exists for both conventional AD and advanced AD using Thermal Hydrolysis Process (THP) as pre-treatment. Whilst the former provides lower capability, flexible biogas production was achieved by the latter, as it provides a quick response. In all scenarios, the value of the biogas converted into electricity is higher than with a steady operational regime, increasing by 3.6% on average (up to 5.0%) in conventional and by 4.8% on average (up to 7.1%) with THP. The process has proven scalable up to 18m3 digester capacity in operational conditions like those in full scale.
Trials performed by Thames Water on a Sludge Powered Generator (SPG) have used sludge from a Thermal Hydrolysis Process (THP) as feed. Data from the trials with THP product sludge at Thames Water's Crossness SPGs was subject to data analysis by converting the trial data into flows of operating cost. Sludge is a mixture of many chemicals and these would be very time consuming to analyse for combustion performance in full detail. Therefore sludge has been simplified to a mixture of water and a single combustible chemical component (coniferyl alcohol) with the same heat of combustion as water-free sludge and roughly the right elemental analysis. This simplification enables the thermal behaviour of the combustion, including its tendency to extinguish without support fuel, to be captured. Both the simplified model and the data analysis from the trial show the THP product sludge is a viable fuel which produces a net financial benefit to the SPG’s operation.
The power system needs flexible electricity generators. Whilst electricity generation from anaerobic digestion (AD) of sewage sludge has traditionally been baseload, transforming the generation capacity into a modern flexible operator is an opportunity to further valorise the resource. This work aims to demonstrate that AD of sewage sludge can support flexible generation and be operated dynamically in a relevant operational environment, to promote full scale implementation. A demonstration scale plant (20 m3 conventional AD reactors) was used to test several feeding regimes designed to return a biogas production rate that matches the demand. Two demand profiles are defined, either by common corporate power purchase agreements or by the main balancing mechanism used by the grid operator in UK. Demand-driven biogas production is demonstrated in this relevant operational environment, and the flexibilisation performance is positive in all scenarios. The value of the biogas increases by up to 2%, which outperforms the results obtained at pilot scale. Additionally, an increase in biogas yield is observed. Whilst transitional imbalances are recorded, they last for few hours and the overall stability is not affected. In conclusion, these trials demonstrate demand-driven biogas production is a feasible operational solution and full-scale implementation is possible.
A semi-analytical model for the drag coefficient of a swarm of two-phase bubbles, condensing in direct contact with an immiscible sub-cooled liquid, has been developed. The analysis used a cellular model configuration, assuming potential (but not inviscid) flow around the reference two-phase bubble in the cell. The effect of the condensation ratio within the two-phase bubbles was included using an approximate relation. The drag coefficient for a wide range of Reynolds numbers (0.1. ≤. Re. ≤. 1000) has been found using the viscous dissipation integral method, and the effect of the liquid content within the two-phase bubble or the half opening angle (β), and the system void fraction (α) were examined. The drag coefficient has been found to increase with the condensation ratio and with the void fraction of the system. The present model agrees well with previously available experimental data and theoretical predictions for single bubbles or particles.
Experiments are described on the pneumatic conveying of 2.7mm alumina particles up a vertical riser of internal diameter 46.4mm or 71.4mm. The particles entered the riser from a fluidised bed, via a short horizontal pipe and a bend of radius 75mm. Measured variables included solids flow rates, air flow rates, inlet and outlet air pressures P 1 and P 2, and the pressure profile in the riser. The solids flow rate was consistent with some earlier models of similar systems, in which the plugs of packed solids move up at a velocity of about U-U mf, where U=superficial air velocity and U mf=incipient fluidising velocity. Solids-wall friction is significant and suppresses fluidisation. To model the system approximately, a conveying efficiency=(power for air compression)/(rate of gain of potential energy of solids) is defined and correlated against solids flux. It was found that the conveying efficiency tended to an asymptote just above 20%. The correlation led to a tentative design formula, Eq. (6), for predicting P 1-P 2 at a given solids flow rate. P 1-P 2 is typically between 50% and 100% of the pressure drop needed to support a column of solids of height equal to that of the riser.It was concluded that plug flow pneumatic conveying is a satisfactory technology for transporting coarse particles which cannot be conveyed in leaner regimes due to the possibility of pipeline erosion or solids attrition. © 2012 Elsevier B.V.
This paper outlines the industrial problem of air current segregation in alumina storage silos which occurs with the handling of the feedstock alumina in aluminium plants. One significant parameter, the air extraction rate, was studied in an experimental silo which was manufactured for this purpose. The experiments conducted in small scale devices displayed the interaction between the particle flow and air current segregation. Results from these experiments show that the increase of the silo air extraction rate reduces air current segregation. The dimensional analysis method has been applied to form dimensionless groups out of the significant parameters. Five dimensionless groups were obtained which is unwieldy. To reduce the number of dimensionless groups the physical properties were lumped into the terminal velocity. This simplified approach gives three dimensionless groups. Initial experiments justify further research to establish weather the simplified approach can scale the dynamic of the flow and the degree of segregation from a small scale silo to industrial equipment.
An experimental investigation of heat exchange in a three-phase direct contact condenser was carried out using a 70-cm-high Perspex tube with a 4-cm inner diameter. The active direct contact condenser comprised 48 cm. Pentane vapour at three initial temperatures (40℃,43.5℃, and 47.5℃) and water at a constant temperature (19℃) were used as the dispersed and continuous phases, respectively, with different mass flow rate ratios. The results showed that the continuous phase outlet temperature increased with increasing mass flow rate ratio. On the contrary, the continuous phase temperature decreased with increases in the continuous mass flow rate. The initial temperature of the dispersed phase slightly affected the direct contact condenser output, which confirms a latent phase effect in this type of heat exchanger.
In this paper, the resultant hydrodynamic force ( FR , where 2 2 FR Fx Fy ) acting on pipe bends will be discussed. A hypothesis that the peak (resultant) forces, FR, peak acting on pipe bends can be described by the normal distribution function will be tested, with the purpose of predicting the mean of the FR, peak ( FR, mean ) and the standard deviations of the FR, peak ( FR, standard deviation ) generated. This in turn allows prediction of the probability of the largest forces that occasionally occur at various flow rates. This information is vital in designing an appropriate support for the piping system, to cater the maximum force over a long period of operation. Besides, this information is also important in selecting a pipe material or material for connections suitable to withstand fatigue failure, by reference to the S-N curves of materials. In many cases, large numbers of response cycles may accumulate over the life of the structure. By knowing the force distribution, ‘cumulative damage’ can also be determined; ‘cumulative damage’ is another phenomenon that can cause fatigue, apart from the reversal maximum force.
Calibration and sensitivity studies in the computational fluid dynamics (CFD) simulation of process equipment such as the annular jet pump are useful for design, analysis and optimisation. The use of CFD for such purposes is computationally intensive. Hence, in this study, an alternative approach using kriging-based meta-models was utilised. Calibration via the adjustment of two turbulent model parameters, C_μ and C_2ε, and likewise two parameters in the simulation correlation for C_μ was considered; while sensitivity studies were based on C_μ as input. The meta-model based calibration aids exploration of different parameter combinations. Computational time was also reduced with kriging-assisted sensitivity studies which explored effect of different C_μ values on pressure distribution.
Energy usage is increasing around the world due to the continued development of technology, and population growth. Solar energy is a promising low-grade energy resource that can be harvested and utilised in different applications, such solar heater systems, which are used in both domestic and industrial settings. However, the implementation of an efficient energy conversion system or heat exchanger would enhance such low-grade energy processes. The direct contact heat exchanger could be the right choice due to its ability to efficiently transfer significant amounts of heat, simple design, and low cost. In this work, the heat transfer associated with the direct contact condensation of pentane vapour bubbles in a three-phase direct contact condenser is investigated experimentally. Such a condenser could be used in a cycle with a solar water heater and heat recovery systems. The experiments on the steady state operation of the three-phase direct contact condenser were carried out using a short Perspex tube of 70 cm in total height and an internal diameter of 4 cm. Only a height of 48 cm was active as the direct contact condenser. Pentane vapour, (the dispersed phase) with three different initial temperatures (40℃,43.5℃ and 47.5℃) was directly contacted with water (the continuous phase) at 19℃. The experimental results showed that the total heat transfer rate per unit volume along the direct contact condenser gradually decreased upon moving higher up the condenser. Additionally, the heat transfer rate increases with increasing mass flow rate ratio, but no significant effect on the heat transfer rate of varying the initial temperature of the dispersed phase was seen. Furthermore, both the outlet temperature of the continuous phase and the void fraction were positively correlated with the total heat transfer rate per unit volume, with no considerable effect of the initial temperature difference between the dispersed and continuous phases.