N.F. Kirkby

Kinetics of HCl reactions with calcium and sodium sorbents for IGCC fuel gas cleaning

Abstract Experiments for HCl sorption by calcium and sodium sorbents have been carried out with simulated gases of compositions typical for “air-blown” and “oxygen-blown” IGCC fuel gases. The concentration of CO2 affects the behaviour of HCl sorption by Ca-sorbents but does not influence the performance of Na-sorbents in the temperature range 300–600°C. The sorbent particle size has a slight effect on HCl sorption by Na2CO3 in the range 90–250 μm. The optimum temperature for the reaction was found to be between 400 and 500°C. This has been attributed to a combination of reaction kinetics and sintering. A mathematical model has been developed to simulate gas solid reactions in fixed beds. The coefficient of gas diffusion through the product layer (NaCl) was obtained at 300–600°C.

Formation of product layers in solid-gas reactions for removal of acid gases

Gas-solid reactions occurring in the dry scrubbing of acidic emissions (e.g. SO 2 , H 2 S, and HCl) from flue gases cease at certain maximum conversions, X m (often <100%). Based on free energy-work analysis, a crystallisation and fracture model is presented to describe the mechanism for the formation of a product layer (PL). As a special feature of the model, the chemical potential of a reaction is shown to be dependent on mechanical work. A population model is proposed which suggests that the reaction rate depends on the population of nuclei of the critical size (PNC) at the reaction interface and the reaction stops when the PNC approaches zero. The existence of a maximum thickness of PL, h m , has been shown mathematically; h m is demonstrated to be influenced by various parameters. It is proposed that the structure of a PL is characterised by the initial critical radius (r ki ) of the nuclei. The larger the value of r ki , the more porous is the PL with a larger coefficient of PL diffusion and less mechanical work. A low conversion of sorbent results from either a small or excessively large r ki . A high conversion can only be achieved around the optimum value of r ki . Experimental data available in the literature are interpreted successfully by the present model for reactions of Ca-sorbents with SO 2 , H 2 S and HCl

Evaluation of poly (ADP-ribose) polymerase inhibitor ABT-888 combined with radiotherapy and temozolomide in glioblastoma

BackgroundThe cytotoxicity of radiotherapy and chemotherapy can be enhanced by modulating DNA repair. PARP is a family of enzymes required for an efficient base-excision repair of DNA single-strand breaks and inhibition of PARP can prevent the repair of these lesions. The current study investigates the trimodal combination of ABT-888, a potent inhibitor of PARP1-2, ionizing radiation and temozolomide(TMZ)-based chemotherapy in glioblastoma (GBM) cells.MethodsFour human GBM cell lines were treated for 5 h with 5 μM ABT-888 before being exposed to X-rays concurrently with TMZ at doses of 5 or 10 μM for 2 h. ABT-888′s PARP inhibition was measured using immunodetection of poly(ADP-ribose) (pADPr). Cell survival and the different cell death pathways were examined via clonogenic assay and morphological characterization of the cell and cell nucleus.ResultsCombining ABT-888 with radiation yielded enhanced cell killing in all four cell lines, as demonstrated by a sensitizer enhancement ratio at 50% survival (SER50) ranging between 1.12 and 1.37. Radio- and chemo-sensitization was further enhanced when ABT-888 was combined with both X-rays and TMZ in the O6-methylguanine-DNA-methyltransferase (MGMT)-methylated cell lines with a SER50 up to 1.44. This effect was also measured in one of the MGMT-unmethylated cell lines with a SER50 value of 1.30. Apoptosis induction by ABT-888, TMZ and X-rays was also considered and the effect of ABT-888 on the number of apoptotic cells was noticeable at later time points. In addition, this work showed that ABT-888 mediated sensitization is replication dependent, thus demonstrating that this effect might be more pronounced in tumour cells in which endogenous replication lesions are present in a larger proportion than in normal cells.ConclusionsThis study suggests that ABT-888 has the clinical potential to enhance the current standard treatment for GBM, in combination with conventional chemo-radiotherapy. Interestingly, our results suggest that the use of PARP inhibitors might be clinically significant in those patients whose tumour is MGMT-unmethylated and currently derive less benefit from TMZ.

Alteration with reaction progress of the rate limiting step for solid-gas reactions of Ca-compounds with HCl

Abstract Solid-gas reactions of calcium compounds with hydrogen chloride (HCl) are used in dry scrubbing of HCl from hot gases. The experimental data in the literature were analysed using the shrinking unreacted core model. It has been shown that the rate limiting step for the reactions alters with the extent of solid conversion in this time sequence: chemical reaction control, combined control, product layer diffusion control, and chemical reaction control again. The extent of solid conversion with time could not be predicted by the classical correlations obtained by assuming a single rate limiting step. A modified correlation was presented to obtain product layer diffusion coefficients. A new model, the crystallisation and fracture model, was proposed to describe the mechanism of the formation of the solid product layer. Based on free energy-mechanical work analysis, the model can fundamentally explain the reasons for the change of the rate limiting step, the existence of the maximum conversions of sorbent, the variation of the structure of the solid product layer with temperature, and the effect of gaseous products. The model may also be applicable to other solid-gas reaction systems.

A mathematical model of brain tumour response to radiotherapy and chemotherapy considering radiobiological aspects

Glioblastoma is the most frequent and malignant brain tumour. For many years, the conventional treatment has been maximal surgical resection followed by radiotherapy (RT), with a median survival time of less than 10 months. Previously, the use of adjuvant chemotherapy (given after RT) has failed to demonstrate a statistically significant survival advantage. Recently, a randomized phase III trial has confirmed the benefit of temozolomide (TMZ) and has defined a new standard of care for the treatment of patients with high-grade brain tumours. The results showed an increase of 2.5 months in median survival, and 16.1% in 2 year survival, for patients receiving RT with TMZ compared with RT alone. It is not clear whether the major benefit of TMZ comes from either concomitant administration of TMZ with RT, or from six cycles of adjuvant TMZ, or both. The objectives were to develop our original model, which addressed survival after RT, to construct a new module to assess the potential role of TMZ from clinical data, and to explore its synergistic contribution in addition to radiation. The model has been extended to include radiobiological parameters. The addition of the linear quadratic equation to describe cellular response to treatment has enabled us to quantify the effects of radiation and TMZ in radiobiological terms. The results indicate that the model achieves an excellent fit to the clinical data, with the assumption that TMZ given concomitantly with RT synergistically increases radiosensitivity. The alternative, that the effect of TMZ is due only to direct cell killing, does not fit the clinical data so well. The addition of concomitant TMZ appears to change the radiobiological parameters. This aspect of our results suggests possible treatment developments. Our observations need further evaluations in real clinical trials, may suggest treatment strategies for new trials, and inform their design.

Patchy cleaning of rigid gas filters—II. Experiments and model validation

Abstract Rigid ceramic filters have emerged as the most promising technology for cleaning of hot gases due to their resistance to attack by aggressive gases and high temperatures. Their potential may be further extended by using them in combination with injection of a dry sorbent to remove acid gases and other chemical contaminants. A crucial factor in their successful utilisation is the ability of the cleaning pulse to remove the deposited cake effectively from the filter surface. In this study, laboratory experiments have been carried out using a single ceramic candle filter. The non-steady-state behaviour of the filter in the initial period of filtration was followed. ‘Patchy’ cleaning of the filter was observed and is consequently identified as the major cause of the inefficiency of filter cleaning indicated by the measured residual pressure drops. A simple correlation has been proposed to determine the cleaned fraction from pressure differences as a measure of the efficiency of cleaning. The conditioning curves have been successfully simulated using a recently developed probabilistic model and it has been shown that the model can also be applied to simulate the experimental results obtained in a pilot plant operating at high temperatures. The modelling results are consistent with the experimental observation that patchy cleaning with a thin residual dust layer in the cleaned areas can explain the conditioning behaviour.

Patchy cleaning of rigid gas filters-I. A probabilistic model

Abstract Filtration with ceramic filters is a major technology for removal of particulate emissions at high temperatures. In combination with injection of a dry sorbent (‘dry scrubbing’), ceramic filters are also used to remove gaseous contaminants. Such filters are regenerated by application of a reverse pulse of cleaning gas; this frequently leads to partial or ‘patchy’ cleaning, rather than complete simultaneous cake detachment. This paper presents a probabilistic model to describe, as a result of patchy cleaning, the area distribution of cake patches with different ages, the thickness distribution, the velocity distribution, and the pressure drop history. A generalised probability distribution has been derived. Different modes of patchy cleaning have been proposed and can be simulated by changing the value of a weighting factor contained in the generalised distribution. Incorporated into a fixed-bed reaction model, this model makes it possible to examine the effect of patchy cleaning on dry scrubbing of acid gases in filter cakes.

Combined reaction and separation in pressure swing processes

Abstract Theoretical studies of a novel reactor which combines pressure swing adsorption (PSA) and chemical reaction are presented; such a reactor is referred to as a pressure swing reactor, or PSR. The design is based on a conventional two-bed PSA process, in which many of the usual cycle configurations for operation are possible, e.g. simple and purge cycles. Each bed contains a mixture of an active catalyst for reaction, and a selective adsorbent for the adsorption of one or more of the reaction species. Theoretical calculations predict that such a process may lead to greater conversions than conventional steady flow reactors, and thus allow a lower temperature of operation for a desired conversion. Furthermore, for equilibrium reactions of the general formaA ⇌bB +cC, and especially when an adsorbent can be chosen such that the adsorption equilibrium constants are in the sequenceHB > [HA,HC], possible improvements over equilibrium conversions are indicated. In this work, theoretical investigations have concentrated on three types of reversible reaction schemes; isomerisation, dissociation/disproportionation, and dehydrogenation; potential advantages of a PSR have been illustrated for these. As a test case, experimental investigations have concentrated on the dehydrogenation reaction of methylcyclohexane to toluene, for which a Pt Al2O3 catalyst was found to have suitable activity at temperatures as low as 450 K. Pulse chromatography experiments have been carried out to scan the high temperature (400 K to 700 K) adsorption properties of methylcyclohexane, toluene and hydrogen on some commercial adsorbents; clay-based adsorbents were found to be particularly suitable for this case, yielding the desired sequence of adsorption strengths.

Enhancement of catalytic reaction by pressure swing adsorption

Abstract A theoretical study of an adsorptive reactor which combines multibed pressure swing adsorption and chemical reaction is presented; such a reactor is referred to as a pressure swing reactor, or PSR. Studies have concentrated on an asymptotic case in which there is the ideal propagation of concentration waves within the reactor beds; the method of characteristics was employed in the solution of the governing PSR equations. The studies assessed the effects of operating conditions, and cycle configurations, on the PSR performance. Calculations indicate enhanced reactant conversion when compared to conventional steady state plug flow operation. In particular, for some reversible reactions, substantial improvements over equilibrium yields have been calculated. For example, for the dissociation reaction 2A ⇔ B + C, and where B is the only adsorbing component, approximately two-fold improvements over the equilibrium yield of product B have been predicted. Such reaction enhancement can be attributed to the limitation of the backward reaction, which results from the separation of the product species B and C. In addition to the method of characteristics, a cells-in-series method for the asymptotic case has been developed, and found to yield calculations consistent with the method of characteristics solutions. In a third numerical approach, the spatial discretisation technique of orthogonal collocation on finite elements was applied to the governing PSR equations, and the resulting system of ordinary differential equations solved by a standard integration algorithm. In this case, many of the simplifying model assumptions were relaxed, allowing, for example, the simulation of a non-isothermal PSR with finite mass transfer rates. One practical significance of reaction enhancement by pressure swing adsorption is a lower temperature of operation than in a conventional reactor. This would lead to savings in the energy requirements of the reactor, and limit the rate and degree of catalyst deactivation due to coke deposition or sintering.

Optimal dosing of cancer chemotherapy using model predictive control and moving horizon state/parameter estimation

Model predictive control (MPC), originally developed in the community of industrial process control, is a potentially effective approach to optimal scheduling of cancer therapy. The basis of MPC is usually a state-space model (a system of ordinary differential equations), whereby existing studies usually assume that the entire states can be directly measured. This paper aims to demonstrate that when the system states are not fully measurable, in conjunction with model parameter discrepancy, MPC is still a useful method for cancer treatment. This aim is achieved through the application of moving horizon estimation (MHE), an optimisation-based method to jointly estimate the system states and parameters. The effectiveness of the MPC-MHE scheme is illustrated through scheduling the dose of tamoxifen for simulated tumour-bearing patients, and the impact of estimation horizon and magnitude of parameter discrepancy is also investigated.