Recycle Ratio Research Articles

Hypersaline produced water clarification by dissolved air flotation and sedimentation with ultrashort residence times

Treatment and reuse of some produced waters is made difficult due to their hypersalinity, high concentrations of myriad other dissolved and suspended components, specialized technology requirements (modularity, portability, and short residence times), and lack of existing information on their processing. In this work, produced water containing ∼100,000 mg/L total dissolved solids from the Permian Basin was coagulated with aluminum chlorohydrate (ACH) and flocculated with an anionic high molecular weight organic polymer prior to dissolved air flotation (DAF) and sedimentation to reduce turbidity to < 4 NTU and iron < 0.8 mg/L (>95% removal in both cases) with a total coagulation-flocculation-sedimentation/flotation residence time of only 5 min. Two advantages of DAF over sedimentation were noted: (i) DAF required only half the dosage of the pre-hydrolyzed ACH coagulant to remove ∼90% of turbidity and iron even without the organic polymeric flocculant and (ii) DAF even operated successfully without ACH coagulation (i.e., using only the organic polymeric flocculant) evidencing its lower chemical dosing needs. Further, DAF attained all water quality and operational goals at a recycle ratio of only 12% demonstrating that it outperformed sedimentation to generate clean brine at relatively reduced excess energies necessary for air saturation. Higher DAF recycle ratios reduced turbidity and iron removal possibly due to floc breakage. Colloids were effectively destabilized by double layer compression (due to high water salinity), charge neutralization (via adsorption of Al13 polycations), and enmeshment (precipitation of amorphous aluminum). They were flocculated via interparticle bridging (by the anionic organic polymeric flocculant) to create large, compact flocs facilitating ultrashort flotation/sedimentation times. Direct evidence for these individual coagulation and flocculation mechanisms were obtained using electrophoretic mobility measurements, thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, optical microscopy, computational image and video analysis, and scanning electron microscopy – energy dispersive X-ray spectroscopy.

Plant-wide modeling of a metropolitan wastewater treatment plant to reduce energy consumption and carbon footprint.

A real metropolitan wastewater treatment plant (RWWTP) serving a population equivalent of 1.55 million was modeled to reduce energy consumption and carbon footprint (CFP). An approach was proposed to handle the dilution factor and partial aeration due to discontinuous air diffuser locations in the Bardenpho-5 configuration. Various operational, structural, and configurational modifications were evaluated. Results indicated that management scenarios might provide conflicting outcomes for different targets. Reduced energy consumption may not result in lower CFP at the same time. Moreover, operational changes that would impact total nitrogen (TN) concentrations and N2O release may significantly impact CFP. A policy of using a modified Bardenpho-5 process with reduced internal recycle (IR) ratio, waste activated sludge (WAS), and return activated sludge (RAS) flow rates provided the lowest CPF. Modified Bardenpho-5 process and replacing belt thickeners with gravity thickeners supplied the highest savings in energy consumption. Overall, up to 14% and 20% reductions were possible in the energy consumption and CFP of the plant, respectively. The RWWTP may save up to 10% in energy expenses annually by operational modifications.

Behaviour of eccentrically loaded circular recycled aggregate concrete-filled stainless steel tube stub columns

This paper presents an experimental and numerical study of eccentrically loaded circular recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns. The testing programme was conducted on twelve specimens designed with different stainless steel tube section sizes and recycled coarse aggregate replacement ratios, and included tensile coupon tests, concrete cylinder tests and eccentric compression tests. The test results, including the failure loads and modes, load–mid-height lateral deflection curves and evolution of neutral axis, were reported and discussed. Following the testing programme, a numerical modelling programme was conducted, with finite element models developed and validated against the test results and then employed to perform parametric studies to generate further numerical data. The confinement effects were analysed based on the numerical data. The relevant design interaction curves, as set out in the European code, American specification and Australian/New Zealand standard, were assessed for their applicability to circular RACFSST stub columns under combined compression and bending. On the basis of the quantitative and graphical assessments, it has been found that (i) each set of codified design interaction curve leads to similar assessment results for eccentrically loaded circular RACFSST stub columns with different recycled coarse aggregate replacement ratios and (ii) the failure load predictions from the codified design interaction curves are conservative.

Thermodynamic limitations to direct CO2 utilisation within a small-scale integrated biomass power cycle

Partially recycling CO2-rich exhaust gases from a syngas fuelled internal combustion engine to a biomass gasifier has the capability to realise a new method for direct carbon dioxide utilisation (CDU) within a bioenergy system. Simulation of an integrated, air-blown biomass gasification power cycle was used to study thermodynamic aspects of this emerging CDU technology. Analysis of the system model at varying gasifier air ratios and exhaust recycling ratios revealed the potential for modest system improvements under limited recycling ratios. Compared to a representative base thermodynamic case with overall system efficiency of 28.14 %, employing exhaust gas recycling (EGR) enhanced gasification system efficiency to 29.24 % and reduced the specific emissions by 46.2 g-CO2/kWh. Further investigation of the EGR-enhanced gasification system revealed the important coupling between gasification equilibrium temperature and exhaust gas temperature through the syngas lower heating value (LHV). Major limitations to the thermodynamic conditions of EGR-enhanced gasification as a CDU strategy result from the increased dilution of the syngas fuel by N2 and CO2 at high recycling ratios, restricting equilibrium temperatures and reducing gasification efficiency. N2 dilution in the system reduces the efficiency by up to 2.5 % depending on the gasifier air ratio, causing a corresponding increase to specific CO2 emissions. Thermodynamic modelling indicates pre-combustion N2 removal from an EGR-gasification system could decrease specific CO2 emissions by 9.73 %, emitting 118.5 g/kWh less CO2 than the basic system.

Hydrodynamic Properties Investigation of Ebullated Bed Reactor Using Non-Newtonian Liquid

The importance of using EBR has been renewed recently due to the sharp increase in heavy feedstocks sent to refineries and the hydrocracking process. Most of these feedstocks have a non-Newtonian behavior. The performance of this type of reactor using non-Newtonian liquid is complicated and has not been covered well yet. Hence, the present work is devoted to elucidating the effect of the non-Newtonian behavior of fluid on the hydrodynamic properties of a three-phase (gas-liquid-solid) reactor under operating conditions of different values ​​of gas velocity (2, 4, 6) cm/sec, liquid velocity (0.9, 1.39, 1.8, 2.3) cm/sec, and recycle ratio (1.5, 2, 2.5). The study observed the effect of non-Newtonian behavior using polymethyl Cellulose (PMC) at different concentrations (0.1, 0.2, 0.3, and 0.4) wt%. The pressure gradient method was used to elucidate the minimum liquid fluidization velocity and to estimate hold up, while the imaging method was used to measure the bubble's size. The results showed that the higher the gas velocity, the lower the minimum liquid fluidization velocity. As the intensity of the non-Newtonian behavior increased, gas velocity showed the opposite effect. The results also showed that increasing the velocity of liquid and gas and the intensity of the non-Newtonian increase the gas hold-up. The bubbles characteristics, represented by bubble size results, show that small bubbles appear at low gas velocities, and these bubbles collapse as gas and liquid velocities increase as well as liquid viscosity.

Design guideline for CO2 to methanol conversion process supported by generic model of various bed reactors

This study developed a novel design guideline for CO2 to methanol (CTM) conversion process supported by a generic model for various types of bed reactors (BRs) as the process economics depends on BR selection. Using commercially feasible BRs, such as fixed bed (FxB), bubbling fluidized bed (BFB), turbulent fluidized bed (TFB), and fast fluidized bed (FFB), we analyzed the CTM process to determine the optimum recycle ratio, gas velocity over the minimum fluidization velocity (GV/MFV) ratio, inlet reactor pressure, and inlet reactor temperature. At full recycle of unconverted gas, the optimum GV/MFV ratio was 0.1 for the process using an FxB reactor, while conversion using a TFB reactor was a better option than those using BFB and FFB reactors at the GV/MFV ratio of 7.03. Meanwhile, the CTM process using various types of BRs performed the minimum cost at 55 bar for reactor pressure and 498 K for reactor temperature. Owing to the competitiveness of the FxB and TFB reactors at small and large CO2 source rates, respectively, further analysis of the optimal conditions was conducted for these reactors, and a design guideline for the CTM process using these reactors was developed. The effect of the uncertainty parameters on the process design was analyzed to determine the CO2 source rate, green H2 cost, and CO2 cost. The design guideline was suggested for proper CTM process at different uncertainty values of CO2 source rate and material costs under a vision up to 2050 (for net-zero emissions). Finally, the design guideline integrated with methanol purifiers highlighted its reliability for high-purity methanol production. The production cost (∼340 $/tMeOH at 2050) for green methanol (mitigated −0.78 to −0.83 kgCO2/kgMeOH) was highly competitive with the cost (310 to 520 $/tMeOH) of traditional methanol production emitted 1.6 to 2.971 kgCO2/kgMeOH. The developed guidelines are useful for the design, operation, and decision-making of the CTM process. In addition, the developed models for BRs can contribute to other CO2 utilization processes.

New hybrid membrane vacuum swing adsorption process for CO2 removal from N2/CO2 mixture: modeling and optimization by genetic algorithm.

In this study, a new innovative hybrid membrane/vacuum swing adsorption (VSA) process is developed, modeled, and optimized for removal of CO2 from flue gases. The process benefits from the advantages of membrane simplicity and the high product quality of the adsorption system. The main advantage of this new process is the simultaneous increases of both CO2 purity and its recovery. To achieve this objective, in the first step, a membrane system using PEBAX nano-composite membrane was modeled. In the second step, a VSA system using zeolite 13X was modeled. The adsorption equilibrium was predicted by the Toth isotherm. To increase the modeling accuracy, the mass transfer rate was calculated based on the quasi-second-order model. At the final step, the hybrid membrane/VSA process was modeled. Comparison of the new hybrid membrane/VSA with the stand-alone VSA process shows that the CO2 product concentration was increased by 39% and the recovery was improved by 8%. To study the process limitations and increase the product quality, a sensitivity analysis was performed on vacuum pressure, membrane stage cut, and recycle ratio. Based on the results, decreasing the membrane stage cut to 15% and applying a recycle ratio equal to 2 will increase the product quality with the cost of increasing the equipment size. Finally, to achieve the required purity and recovery specification in industrial applications, the process was optimized using the genetic algorithm. Based on these results, it is possible to produce CO2 with 94.7% purity and 99% recovery and N2 with 99.9% purity and 97.3% recovery by regenerating the adsorbents at 0.01 bar, setting the membrane stage cut equal to 11%, keeping the recycle ratio at 1.89, and adjusting the purge-to-feed ratio to 2%.

Preliminary feasibility study for hydrogen storage using several promising liquid organic hydrogen carriers: Technical, economic, and environmental perspectives

Liquid organic hydrogen carriers (LOHC) are promising alternatives to conventional H2 media owing to their novelty in the storage and transportation of H2. Herein, a comprehensive feasibility study is reported for hydrogenation processes using several promising LOHC systems: N-ethylcarbazole (NEC)–perhydro-NEC (12H-NEC), dibenzyltoluene (DBT), perhydro-dibenzyltoluene (18H-DBT), toluene (TOL)–methylcyclohexane (MCH), and CO2–methanol (MeOH). Detailed process simulation using Aspen Plus® reveals a considerable amount of stored H2 for DBT–18H-DBT (7.20 kmolH2 h−1) and the lowest unit cost of H2 storage (7.00 $ kgH2-1) by itemized cost estimation. In addition, the effects of operating temperature, recycle ratio of unreacted H2, and scale of H2 storage on economic feasibility are investigated, and the expected cost reductions to 2.83, 4.21, 3.84, and 2.37 $ kgH2-1 are evaluated for NEC–12H-NEC, DBT–18H-DBT, TOL–MCH, and CO2–MeOH, respectively. The sensitivity analysis quantifies the variation in the total unit cost of H2 storage and identifies the costs of the H2 reactant and labor as key economic parameters in LOHC hydrogenation. Furthermore, 0.16–0.43, 0.78–1.72, 1.25–1.73, and 1.12–5.52 kgCO2 kgH2-1 of unit CO2 emissions are evaluated by integrative carbon footprint analysis.

Experimental and numerical investigations of recycled aggregate concrete-filled stainless steel tube stub columns under combined compression and bending

This paper reports experimental and numerical investigations into the behaviour and resistance of recycled aggregate concrete-filled stainless steel tube stub columns under combined compression and bending. Eccentric compression tests were conducted on twelve stub column specimens fabricated from austenitic stainless steel tubes with two cross-section sizes and concretes with three recycled coarse aggregate replacement ratios (0%, 35% and 70%). The test results, including the failure loads, load–mid-height lateral deflection curves, evolution of neutral axis and confinement effect, were discussed in detail. The testing programme was followed by a numerical modelling programme, where finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data over a wide range of cross-section dimensions and loading combinations. Given that there are no existing design codes for recycled aggregate concrete–stainless steel composite structures, the relevant codified design rules for natural aggregate concrete filled-carbon steel tube stub columns under combined compression and bending, as set out in the European code, American specification and Australian/New Zealand standard, were evaluated for their applicability to recycled aggregate concrete filled-stainless steel tube stub columns, based on the test and numerical data. The evaluation results revealed that the European code and Australian/New Zealand standard generally provide an acceptable level of design accuracy, while the American specification yields unduly conservative failure load predictions.

An advanced coal-based zero-emission polygeneration system using water-gas shift reaction and syngas recycle to achieve different methanol and electricity distributions

To achieve different products distributions and improve energy conversion performance for the coal utilization with zero carbon emission, an advanced polygeneration system incorporating the methanol synthesis and Allam power cycle (an oxy-combustion, direct-fired supercritical carbon dioxide Brayton cycle) is proposed. The water-gas shift reaction is combined with the syngas recycle to obtain different products (i.e., methanol and electric power) distributions. Moreover, different from the conventional coal-to-methanol process, the carbon dioxide generated by the shift reaction is not separated from the shifted syngas but enters the Allam cycle, which can be simply split from the cooled exhaust without additional energy penalty. In other words, the carbon dioxide separation energy penalty of the methanol production is reduced to zero by connecting the coal-to-methanol process and the Allam cycle. In this study, the material flow and energy conversion characteristics of the proposed polygeneration system is revealed. The influences of the carbon monoxide shift ratio and syngas recycle ratio on the methanol productivity, net electric power throughput and the corresponding fuel saving ratio are comprehensively analyzed. The optimum strategy of the combination of the water-gas shift reaction and syngas recycle is obtained with the methanol/electricity energy ratio ranging from 0.75 to 2.71; and the highest fuel saving ratio soars to 5.79% as the methanol/electricity ratio is 1.55 at the carbon monoxide shift ratio of 0.17 and the syngas recycle ratio of 0.8.

Performance and bacterial community analysis of a two-stage A/O-MBBR system with multiple chambers for biological nitrogen removal

A two-stage anoxic/oxic (A/O)-moving bed biofilm reactor (MBBR) system with multiple chambers was established for municipal wastewater treatment. At the total hydraulic retention time (HRT) of 11.2 h and nitrate recycling ratio of 1, the removal efficiencies reached 83.8%, 82.5%, and 77.8% for soluble chemical oxygen demand (SCOD), 98.0%, 97.5%, and 94.9% for ammonia nitrogen (NH4+-N), and 91.8%, 92.0%, and 87.7% for total inorganic nitrogen (TIN) in summer, autumn and winter, respectively. Biofilms with functional bacterial populations were formed in the pre-anoxic reactors, the pre-oxic reactors, the post-anoxic reactors and the post-oxic reactors of the two-stage A/O-MBBR system. The highest nitrification potential was found in the last oxic reactor of the first A/O-MBBR subsystem with the highest relative abundances of the functional genes including [EC:1.14.99.39] and [EC:1.7.2.6]). The highest denitrification potential was found in the post-anoxic reactors with the highest relative abundances of the functional genes including [EC:1.7.2.1], [EC:1.7.2.5] and [EC:1.7.2.4]. This work constructed an efficient municipal biological nitrogen removal technology to achieve high effluent nitrogen standards in winter, and investigated its working mechanism to provide a basis for its design and optimization.

Two-stage membrane cascades for post-combustion CO2 capture using facilitated transport membranes: Importance on sequence of membrane types

The use of membrane module performance data obtained in industrially-relevant environment as the basis in process simulation can lead to a more realistic prediction of a CO2 capture system. In this work, we report the use of two classes of industrially validated membranes, i.e., hybrid facilitated transport membranes (HFTMs), which are characterized by higher permeances and lower selectivity, and the fixed site carrier (FSC) polyvinylamine (PVAm) membrane, which is characterized by lower permeance and higher selectivity relative to each other, to study the potential of these membranes in two-stage configurations for post-combustion CO2 capture applications. Two-stage cascades with and without recycle streams were simulated for a target CO2 recovery of >80% and purity of 80–99.5%. Recycle systems were found to contribute in reaching high purity targets of CO2 >90% at the fixed recovery of 90%. The positioning of membranes with different properties in different stages was found to influence the performance of the system significantly. Processes employing HFTMs in the first stage coupled with a PVAm membrane in the second stage performed best with the lowest total energy/membrane area requirement and recycle ratio for a target of 90% recovery and >90% purity of CO2. The process employing HFTMs in both stages outperformed all other cases in terms of membrane area required. The case employing PVAm membranes in both stages performs at its optimum only at a lower purity requirement (<90%). This study reveals the importance of using an optimized combination of membranes with different separation capabilities at different stages.

A Study of Recycling Process of Cobalt-based Conversion Coating Formed on Aluminium Alloys

In this study, the influence of different cyclic treatment and reuse process on the performance of the conversion film was systematically studied by electrochemical test, surface characterization and determination of cobalt ions in the conversion solution, and new technology of chemical conversion of cobalt salts in aluminum alloys were determined. Mode A indicated the practice of cyclical processing of the test object in the original conversion solution, in which components of the circulation liquid were consumed continuously. After the first cycle, the Ecorr and Rp were – 691.8 V and 63.5 kΩ. In the later time, Rp was just about 13.0 kΩ, and the corrosion resistance of conversion film degraded progressively. Mode B represented the practice of adding to the original solution before cyclical processing of the test object, so that volume of the solution remains unchanged, and components of the circulation liquid kept increasing. RP increased from 9.3 kΩ to the 69.9 kΩ at the beginning of the cycle, followed by a slow descent, and content of cobalt in the film was stable, also higher than that under model A. This demonstrates that mode A has high utilization of the cobalt in the conversion solution in the early recycling, whereas mode B can prolong the cyclic service life of the conversion solution in circulation. After adding oxidants, as NaClO3, NaBrO3, H2O2, NaClO, and KMnO4, to the circulation liquid before recycling, the reuse and recycle ratio of cobalt reaches up to 41.3 %, and Rp can be promoted to123.2 kΩ at most. The life of the recycling was prolonged, while the costs of chemical conversion of cobalt were reduced.

Effects of effluent recycle on treatment performance in a vertical flow constructed wetland

As part of the effort to improve design and operation schemes of decentralized wastewater systems, the Danish EPA funded a research initiative, to re-evaluate the Danish guidelines for VFCW with an aim to improve its design. Therefore, the aim of this study is to assess the effect of recycling water from the effluent of the VFCW to the pump chamber at several recycle ratios and to compare the effects of recycle to the sedimentation tank in lieu of the pump chamber. In addition, water quality changes within the depth of the bed were assessed in order to make recommendations on an appropriate depth for effective treatment. The results showed overall the system performed very well regardless of the recycle ratio with removal rates of TSS, TOC, BOD5, NH4-N and TP around 99% and thus easily exceeding the national discharge requirements. Despite that, the better results for TN effluent concentrations, below 45 mg/L, were achieved for recycling water from the effluent of VFCW to pump chamber at recirculation rates of 50% to 100%. The changes in concentrations of measured parameters with depth suggest that almost all transformations occurred with the top 20 cm of the 1.0 m thick effective bed depth, thus, the results show that the VFCW dimensions given in the Danish guidelines may be overdesigned from the perspective of meeting current discharge limits.

Biobased plastic: A plausible solution toward carbon neutrality in plastic industry?

Biobased plastic exhibits unique benefits for achieving carbon neutrality, a key step toward reducing atmospheric greenhouse gases, due to its stability, high carbon content, and origin of carbon by photosynthesis. Herein we evaluate the role and potential of biobased plastic as an alternative carbon reservoir which is completely artificial, since most plastic polymers are synthetic and massively produced after the 1950 s. Model simulation indicates that plastic, under usage, burial, and littering, forms a growing carbon reservoir, sinking 6.82 gigatons of carbon (GtC) in 2020. Plastic-formed carbon is estimated to stack up to 19.4–23.2 GtC in 2060 under various production scenarios. However, only 18–40% of carbon stored in plastic is biobased carbon, equivalent to approximately 31–48 gigatons of carbon dioxide. Without any low carbon energy upgrade, carbon neutrality is difficult to achieve even with 90% biobased plastic substitution and 50% recycling ratio. Because extra GHG emissions are generated as a result of increasingly using incineration as a post-treatment strategy in response to increasing waste generation, the annual net GHG emission continues to rebound after the bio-based plastic substitution and plastic recycling approach their upper limits. Additional strategies are therefore needed to achieve complete carbon neutrality.

Efficient nitrogen removal from onsite wastewater by a novel continuous flow biofilter

Soil-based passive biofiltration system is an economically feasible technology for nitrogen removal from onsite wastewater. However, the conventional design requires a large system footprint with limited treatment capacity. In this study, a novel continuous flow biofilter (CFB) with adjustable recirculation and continuous flow pattern was developed for onsite wastewater treatment with a small footprint. Efficient total nitrogen removal (80.1–97.5%) was observed at various hydraulic loadings (0.03–0.12 m3 m−2 d−1), nitrogen loadings (1.1–8.6 g N m−2 d−1) and recycle ratios (2–3) when treating septic tank effluent (STE), with low effluent TN (0.7–13.6 mg N L−1). Nitrous oxide was observed in the denitrification effluent indicating incomplete denitrification at elevated dissolved oxygen levels (3.3–5.8 mg L−1). Nitrogen removal rate (2.9–7.0 g N m−2 d−1) and ammonium removal rate (2.4–7.2 g N m−2 d−1) were positively correlated with nitrogen loadings increase (1.1–8.6 g N m−2 d−1) but were not significantly impacted by the hydraulic loading rate change (0.08–0.12 m3 m−2 d−1). The total biomass abundance and nitrifying microorganisms decreased significantly as the nitrification columns depth increased, while homogeneous microbial distribution was observed in the denitrification columns. The abundance of ammonium oxidizing archaea (AOA) increased significantly at increased hydraulic and nitrogen loading rate, while the ammonium oxidizing bacteria (AOB) abundance remained steady. The abundance of functional genes involved in denitrification process (nirS, nirK and nosZ) responded differently when hydraulic and nitrogen loading rate changes. Collectively, this study suggested the CFB could efficiently remove nitrogen from onsite wastewater with fluctuating influent compositions and various hydraulic loadings.

Experimental Analysis of Recycled Aggregate Concrete Beams and Correction Formulas for the Crack Resistance Calculation

This paper studies the similarities and the differences between natural aggregate concrete (NAC) beams and recycled aggregate concrete (RAC) beams in terms of concrete material properties, bearing capacity, and crack resistance and further proposes correction formulas for cracking moment calculation of RAC beams that consider different recycled aggregate substitution ratios. First, the basic mechanical properties (e.g., cube compressive strength, axial compressive strength, and elastic modulus) of RAC blocks were tested; second, comparative bending tests of RAC beams and natural aggregate concrete beams were conducted, in order to obtain the cracking moments, yield bending moments, ultimate bending moments, mid-span deflections and crack development forms; finally, numerical simulations were performed to investigate the cracking performances of RAC beams. Through analyzing the experimental results, it is found that the crack development pattern of RAC beams resembles that of natural aggregate concrete beams, while the cracking performance of RAC beams significantly deviates from that of NAC beams. Finally, the correction formulas for the cracking moment calculation of RAC beams are proposed based on the numerical simulation results and verified by data from other researchers, which can be regarded as a correction for the cracking moment calculation formulas in the current code for RAC beams with respect to varied recycled concrete aggregate substitution ratios.

Anaerobic Co-Digestion of Food Waste with Sewage Sludge: Simulation and Optimization for Maximum Biogas Production

Anaerobic co-digestion (ACD), where two or more substrates are digested simultaneously, is able to prevent the problems associated with mono-digestion. The aim of this study is to develop a simulation model of ACD of food waste (FW) with sewage sludge (SS) for biogas production coupled with pre-treatment, sludge handling and biogas upgrading using SuperPro Designer v9.0. The Design Expert v13 is employed to perform optimization and evaluate the effect of hydraulic retention time (HRT), sludge recycle ratio, water to feed ratio (kg/kg) and SS to FW ratio (kg/kg) on the methane flow, chemical oxygen demand (COD) and volatile solids (VS). The results show that the methane yield of 0.29 L CH4/g COD removed, COD removal efficiency of 81.5% and VS removal efficiency of 69.2% are obtained with a HRT of 38.8 days, water to feed ratio (kg/kg) of 0.048, sludge recycle ratio of 0.438 and SS to FW ratio (kg/kg) of 0.044. Economic analysis has shown this study is feasible with a payback time of 6.2 years, net present value (NPV) of $5,283,000 and internal return rate (IRR) of 10.2%. This indicates that the ACD of FW and SS is economically feasible in a larger scale.

Analytical and experimental investigations of recyclic double pass photovoltaic thermal system

ABSTRACT Hybrid photovoltaic-thermal (PV/T) system consisting of a double-pass solar air heater under external recycling has been proposed for the combined generation of electrical and thermal power from the same area. One of the main concerns with PV modules is their poor efficiency due to high temperatures caused by extreme sun radiation. One of the most prevalent approaches for reducing the temperature of PV modules and improving performance is to use hybrid photovoltaic thermal (PV/T) systems. The heat generated by the PV panel is conveyed via working fluids such as air which is further utilized for other applications such as space heating. The present research explored the theoretical and experimental investigation of the proposed system for its thermal, electrical, and combined efficiency. The investigated double pass photovoltaic thermal system (PV/T) under recycling operation mainly consists of a glass cover, a PV module that is located below the glass, a back plate, and insulating material that is placed below the back plate. The theoretical model describes heat transfer characteristics and the divergence in the temperatures of the anticipated photovoltaic thermal system. Furthermore, under identical geometric and operational flow conditions, analytical results are compared with experimental results and showed that good agreement is achieved in the acceptable range. The performances of the proposed system that includes the thermal, electrical, and combined efficiency was measured for the mass flow rate ranging from 0.03 to 0.15 kg/sec, recycle ratio 0.3–1.8, irradiation 300–1000 W/m2, packing factors 0.4–0.95, and varying channel depth ratio ranging from 1.0 to 6.0. The maximum growth in net electrical power of the current design is 43.32% more than the previous design with external recycling at a mass flow rate of 0.15 kg/sec, recycle ratio of 0.9, and at depth ratio of 3.

Heat integration and waste minimization of biomass steam gasification in a bubbling fluidized bed reactor

Steam gasification of biomass generates a hydrogen-rich product gas that is diluted in unreacted steam and contaminated with tar. In this study, a novel heat integration alternative is conceptualized through syngas cooling to near-ambient temperature via an indirect water condenser downstream of a bubbling fluidized bed gasifier. The steam generated from heat recovery is used for biomass gasification, whereas the condensed bio-oil (unreacted tar and steam) is collected and recycled to the gasifier for further conversion. A one-dimensional kinetic model is developed and tested against experimental data from the literature on steam gasification of bio-oil and steam gasification of biomass in two separate bubbling fluidized bed gasifiers. The average absolute deviation of model predictions for CO2, CO, CH4 and H2 mole fractions are 60.85%, 14.25%, 30.17% and 10.75% for bio-oil steam gasification and 80.63%, 27.70%, 67.28% and 4.71% for biomass steam gasification, respectively. Based on the agreement observed between the model predictions and experimental data over substantial ranges of operating conditions, the same approach is adopted for modeling integrated biomass and recycled bio-oil gasification. Increasing bio-oil recycle ratio increases water conversion and chemical efficiency at the cost of decreasing carbon conversion and thermal efficiency of biomass gasification process. In addition to reactor temperature and steam-to-biomass ratio, tar recycle ratio can be utilized as an optimization variable to compromise between process chemical and thermal efficiencies, as well as wastewater production. Finally, an economic analysis shows that an incremental return on investment of >33% is achievable for the integrated process.