Combustion and emission characteristics of biofuels in diesel engines

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This study was concerned with the performance of biofuels in diesel engines. Generally, the basic combustion and emission characteristics of Rapeseed Oil (RSO) and Soya Oil (SO) result in a lower in-cylinder pressure peak than diesel. This led to the reduction of Nitrogen Oxides (NOx) emissions and to relatively high soot emissions. Further measurements of RSO were done in order to investigate the influence of injection pressure, injection timing and Exhaust Gas Recirculation (EGR) on combustion and emission characteristics. A high soot emission from RSO was reduced by increased injection pressure. Moreover, injection timing also had to be varied in order to reduce the soot emissions from RSO. The retarded injection timing (3 deg bTDC) and increased injection pressure (1200 bar) for the blend of 30% RSO resulted in a reduction of soot emission to the same level as from diesel fuel. Further investigation regarding the soot emissions was done for Rapeseed Methyl Ester (RME) under turbocharged engine operation. The application of the boost pressure resulted in stable engine operation at a late injection timing of 5 deg aTDC. A simultaneous reduction of soot and NOx emissions has been achieved for RME at an injection timing of TDC and high EGR percentage (40 – 50 %). The soot particles size distribution under different engine operating conditions for RME and diesel has also been investigated. Moreover, the characteristic of Electrostatic Mobility Spectrometer (EMS) and the design of primary dilution system have been provided in order to understand the influence of the dilution process and to obtain more real results. Generally, RME showed less particles concentration in the nucleation mode when compared to diesel. Moreover, high EGR caused a shift of the particles from the nucleation mode by agglomeration into the accumulation mode for both fuels. The effect of injection pressure could only be seen in the accumulation mode, where high injection pressure slightly reduced the concentration number. The soot emission was effectively reduced by the usage of the diesel particulate filter (DPF). For this purpose, the soot particles size distributions before and after the DPF have been measured at different engine speeds and loads. At low engine torque, the soot was effectively filtered while the operation under high engine loads resulted in low soot particle concentration especially in the nucleation mode, after the DPF

Experimental and theoretical studies of protein transport in hollow-fibre bioreactors for mammalian cell culture

Cultivation of marnrnalian cells in the extracapillary space (ECS) of hollow-fibre bioreactors (HFBRs) is increasingly used for the production of useful proteins such as monoclonal antibodies. One of the greatest challenges associated with the operation of HFBRs is the maintenance of a uniform cell growth environment. In particular, the distributions of growth-factor proteins can be highly heterogeneous, leading to poor performance or even failure of the culture. Another important aspect of HFBR operation is the harvesting of product proteins from the ECS. Considering the high costs of the product and media proteins, there is a strong motivation for carrying out studies that will provide a better understanding of protein behaviour in HFBRs. The main focus of this thesis was the development of mathematical models describing different aspects of protein transport in HFBRs. The models were validated using protein concentration data collected during cell-free HFBR experiments. A one-dimensional Krogh cylinder model was employed to analyse hindered transmembrane transport relevant to the leakage of smaller proteins from the ECS. A two-dimensional porous medium model (PMM) was used to simulate open-shell operations such as harvesting. An advanced, three-dimensional PMM formulation permitted an extensive analysis of gravity-influenced free-convective ECS protein transport at different HFBR orientations. The dynamics of protein leakage in HFBRs was found to depend on many factors, including initial protein placement, perfusion flow rate, and the addition of a nonleaking protein to the ECS. An open-shell ECS shunt was predicted to be a viable alternative to the traditional closed-shell HFBR configuration, while cocurrent harvesting was found more efficient than countercurrent harvesting. Most studies revealed that the protein transfer between the fibre bundle and the manifolds played a significant role in ECS protein redistribution. Numerous model simulations confirmed the occurrence of forced-convective downstream polarisation of ECS proteins under typical operating conditions. The heterogeneity of protein distribution was greatly reduced by directing the perfusion flow upward, in which case strong free-convective flows mixed the contents of the ECS. These effects were most pronounced in cartridges oriented vertical-up, which might be a nonstandard but nonetheless promising configuration for use in future HFBR cell culture.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

Modelling of fluid flow and protein transport in hollow-fibre bioreactors

A mathematical model (the Porous Medium Model, PMM) was developed to predict the fluid flow and solute transport in hollow-fibre devices, with a particular emphasis on hollowfibre bioreactors (HFBRs). In the PMM, both the extracapillary space (ECS) and the lumen side are treated as interpenetrating porous regions with a continuous source or sink of fluid. The hydrodynamic equations of the PMM are based on Darcy's law and continuity considerations while the transport of the ECS protein is described by the time-dependent convective-diffusion equation. Compared to the earlier Krogh Cylinder Model (KCM), in which the fluid flow and protein transport are assumed to be the same for each fibre, the PMM represents an improved approach in which the spatial domain corresponds to the real dimensions of the hollow-fibre module. Thus, it can be applied to operating conditions where macroscopic radial pressure and concentration gradients exist, such as in open-shell operations. It was demonstrated that, in the absence of radial gradients, the PMM becomes mathematically equivalent to the one-dimensional KCM. The PMM also takes into account the osmotic pressure dependence on the ECS protein concentration, which causes a coupling of the hydrodynamic and protein transport equations. The Porous Medium Model was tested by applying it to one- and two-dimensional closed-shell operations. Both confirmed that a significant polarization of the ECS protein occurs in the direction of the existing pressure gradients under dominant convective transport conditions. The downstream polarization of protein affects HFBR hydrodynamics by virtually shutting down the flow in a significant portion of the ECS due to locally high osmotic pressures. It can also facilitate harvesting of the product protein by increasing its concentration near the downstream ECS port. Modelling studies of the hydrodynamics of hollow-fibre devices in the partial and full filtration modes of operation were carried out for a wide range of membrane permeabilities (10"14<LP< 10-7 m). It was demonstrated using the PMM that, for membranes with permeabilities below about 10"13 m, practically all of the pressure drop between the inlet lumen and outlet ECS ports is due to the hydraulic resistance of the membrane. If the Lp value is increased above approximately 10'12 m, this assumption, commonly made in order to experimentally determine membrane permeabilities, begins to break down. Also, for membrane permeabilities exceeding this value, the ECS and lumen flow rates predicted by the PMM and KCM for the partial filtration mode become significantly different. Modelling of the inoculation phase of HFBR operation is used as another example application of the Porous Medium Model. PMM simulations of the inoculation phase showed that, in the case of a Gambro HFBR with a membrane permeability of the order of 10"15 m, the protein concentration distribution at the end of the inoculation period is very non-uniform and most of the shell side remains free of protein. Using a lower-concentration inoculum solution partially alleviates this problem. Alternatively, a relaxation phase with all ports closed can be applied after inoculation to help homogenize the contents of the ECS by diffusion and osmotically-driven convection. However, this process may be fairly time-consuming and may pose the risk of cell starvation due to oxygen limitations. It is suggested that introduction of the inoculum through both ECS ports simultaneously or periodic changes of the flow direction may be more efficient ways of carrying out the inoculation process. The cell-packed conditions, which exist in the ECS during the production and harvesting phases of HFBR operation, can significantly decrease the ECS hydraulic conductivity and, to a lesser extent, the effective protein diffusivity due to a decrease in the ECS porosity. The ECS permeability value affects the magnitude of convective transport in the shell side and hence the rate of protein removal from the ECS and the product concentration in the harvested solution, thereby influencing the overall efficiency of the process. High-cell-density conditions in the ECS might not allow achievement of high product removal rates and product harvest concentrations. Two modes of harvesting, the closed-lumen mode (with only the two ECS ports open) and the standard mode (with only the downstream ECS port and both lumen ports open), were compared and showed no significant differences in their efficiencies. It was found that the downstream polarization of the ECS protein prior to harvesting can considerably improve the efficiency of this process.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat