Coupled CFD-DEM modelling to predict how EPS affects bacterial biofilm deformation, recovery and detachment under flow conditions

The deformation and detachment of bacterial biofilm are related to the structural and mechanical properties of the biofilm itself. Extracellular polymeric substances (EPS) play an important role on keeping the mechanical stability of biofilms. The understanding of biofilm mechanics and detachment can help to reveal biofilm survival mechanisms under fluid shear and provide insight about what flows might be needed to remove biofilm in a cleaning cycle or for a ship to remove biofilms. However, how the EPS may affect biofilm mechanics and its deformation in flow conditions remains elusive. To address this, a coupled computational fluid dynamic – discrete element method (CFD-DEM) model was developed. The mechanisms of biofilm detachment, such as erosion and sloughing have been revealed by imposing hydrodynamic fluid flow at different velocities and loading rates. The model, which also allows adjustment of the proportion of different functional group of microorganisms in the biofilm, enables the study of the contribution of EPS towards biofilm resistance to fluid shear stress. Furthermore, the stress-strain curves during biofilm deformation have been captured by loading and unloading fluid shear stress to study the viscoelastic properties of the biofilm.

Optimal Cleaning Cycle Scheduling under Uncertain Conditions: A Flexibility Analysis on Heat Exchanger Fouling

Fouling is a substantial economic, energy, and safety issue for all the process industry applications, heat transfer units in particular. Although this phenomenon can be mitigated, it cannot be avoided and proper cleaning cycle scheduling is the best way to deal with it. After thorough literature research about the most reliable fouling model description, cleaning procedures have been optimized by minimizing the Time Average Losses (TAL) under nominal operating conditions according to the well-established procedure. For this purpose, different cleaning actions, namely chemical and mechanical, have been accounted for. However, this procedure is strictly related to nominal operating conditions therefore perturbations, when present, could considerably compromise the process profitability due to unexpected shutdown or extraordinary maintenance operations. After a preliminary sensitivity analysis, the uncertain variables and the corresponding disturbance likelihood were estimated. Hence, cleaning cycles were rescheduled on the basis of a stochastic flexibility index for different probability distributions to show how the uncertainty characterization affects the optimal time and economic losses. A decisional algorithm was finally conceived in order to assess the best number of chemical cleaning cycles included in a cleaning supercycle. In conclusion, this study highlights how optimal scheduling is affected by external perturbations and provides an important tool to the decision-maker in order to make a more conscious design choice based on a robust multi-criteria optimization.

Cleaning cycle optimisation in non-tracking ground mounted solar PV systems using Particle Swarm Optimisation

The effect of installation azimuth angle in the optimization of the cleaning cycle of a solar photovoltaic plant was experimentally investigated in this study. The optimum cleaning cycle was determined using Particle Swarm Optimization algorithm cognizance of the fact that different orientations have different soiling rates. Soiling rates on three different azimuth configurations were experimentally investigated and an exponential soiling loss model was developed for each configuration for use in the optimization problem. Azimuth angle differences of ±12.5% were found to have a significant influence on soiling of as much as 28.29% for the selected location. The North of North West configuration was found to be optimal as opposed to the generally accepted North configuration for maximum energy generation at a minimum cost of energy. This configuration generated 0.87% more energy at unit energy cost of $0.093 compared to the North configuration which had a minimum cost of $0.113. The optimized cleaning cycles were 35 days for the optimal configuration while the North configuration had an optimized cleaning cycle of 28 days. A 17.7% difference in the cost of energy was recorded due the influence of soiling. The study revealed that for minimizing the unit energy cost, it is necessary to take into effect the influence of soiling.

Effect of Venturi Structures on the Cleaning Performance of a Pulse Jet Baghouse

In order to study the effect of venturi structures on the cleaning performance of a pulse jet baghouse, several cleaning tests are carried out applying the pulse jet test bench. Experiments are performed using different filter bags with venturi tubes of varying throat diameters and lengths. Peak pressure and peak pressure arrival time are selected to evaluate the cleaning efficiency under different conditions. On this basis, a series of comparative experiments are conducted between a typical venturi-type and a new venturi tube design on a semi-industrial pulse jet baghouse. The effects of the new venturi-type on baghouse cleaning performance and dust emission are studied. The results show that pulsing air pressure has a significant effect on bag-cleaning performance; the larger the throat diameter and the shorter the venturi length, the higher the peak pressure and the shorter the peak pressure arrival time. This improves the cleaning efficiency of the filter bags. The new venturi design changes the distribution of the cleaning pressure in the vicinity of the bag mouth, effectively extending the cleaning cycle and reducing the energy consumption as well as reducing the dust emission concentration of the baghouse.

The role of ferrous clays in the interpretation of wettability – a case study

Clean sandstone, with minimal clay content, is expected to be strongly water wet once the rock has been through an effective cleaning process. Even samples containing significant clay minerals are usually expected to be water wet after appropriate cleaning. However, tests carried out on core samples from Fields in three different global locations show mixed indices, even for clean state samples where no aging with crude oil has taken place. A few hypotheses for this behaviour considered herein are: whether the cleaning method was adequate, whether wettability was altered by an external factor, or if wettability was due to mineral composition. This paper presents the results obtained from wettability studies on fresh, clean and restored state core plug samples from three different Fields. Wettability indices were obtained by using the combined Amott-USBM method. Petrography was performed on sample end-trims to investigate the possible presence of halite or barite in the clean state samples, thought to be from drilling fluid infiltration, which should have been removed by the methanol cleaning cycle. This showed no organic material or salt (halite), negating wetting change from inefficient cleaning. From a reactive clays [1] model perspective, these rock samples are considered clean-sand (i.e. illite/ smectites- as total clay content), determined by XRD analysis, are lower than 10%. SEM and XRD results showed the presence of grain-coating chlorite in one sample set and glauconite grains in the others. Only once the unusual wettability indices were obtained was the grain-coating chlorite identified as chamosite by SEM/EDX, which is an iron-rich form of chlorite. The presence of chamosite or glauconite appears to influence the wetting tendency. In summary, USBM vs Amott wettability indices of the analysed samples are consistent between both methods, showing a mixed to oil-wet tendency for all samples where chamosite was identified, regardless of the initial test condition. Samples with glauconite appeared to be more mixed wet after wettability restoration. The results suggest that iron rich clay/mineral content is the main contributor to the oil wet tendency of the evaluated rocks.