Process Thermal Efficiency Research Articles

Optimising thermal efficiency of direct contact membrane distillation by brine recycling for small-scale seawater desalination

A technique to optimise thermal efficiency using brine recycling during direct contact membrane distillation (DCMD) of seawater was investigated. By returning the hot brine to the feed tank, the system water recovery could be increased and the sensible heat of the hot brine was recovered to improve thermal efficiency. The results show that in the optimal water recovery range of 20 to 60% facilitated by brine recycling, the specific thermal energy consumption of the process could be reduced by more than half. It is also noteworthy that within this optimal water recovery range, the risk of membrane scaling is negligible — DCMD of seawater at a constant water recovery of 70% was achieved for over 24h without any scale formation on the membrane surface. In contrast, severe membrane scaling was observed when water recovery reached 80%. In addition to water recovery, other operating conditions such as feed temperature and water circulation rates could influence the process thermal efficiency. Increasing the feed temperature and reducing the circulation flow rates increased thermal efficiency. Increasing the feed temperature could also mitigate the negative effect of elevated feed concentration on the distillate flux, particularly at a high water recovery.

Theoretical and experimental studies of immediate assisted solar air gap membrane distillation systems

A new design of the air gap membrane distillation (AGMD) system equipped with a solar absorber was investigated theoretically and experimentally for saline water desalination, which integrated the immediate assisted solar (IAS) absorber as an additional heat supply source and the AGMD process to produce high purity water, say the IAS–AGMD system. The theoretical formulations were developed and the resultant equations were solved by the Newton–Raphson method. The theoretical predictions show that the IAS–AGMD system accomplishes a better device performance in pure water productivity than that of the conventional AGMD system. The good agreement was achieved between the theoretical predictions and the experimental runs in the present study. The effects of the fluid inlet temperature, volumetric flow rate, air gap thickness, and incident solar radiation on the heat transfer efficiency and pure water productivity were also delineated. The theoretical results indicate that the pure water productivity increases with increasing the inlet temperature of hot fluid, volumetric flow rate, and incident solar radiation but with decreasing the air gap thickness and inlet cold fluid temperature. Moreover, the theoretical prediction of the optimal process thermal efficiency in the IAS–AGMD system was obtained as the air gap thickness is 2.5 mm.

Influence of S/C ratio and temperature on steam reforming of model biogas over a metal-foam-coated Pd–Rh/(CeZrO2–Al2O3) catalyst

A model biogas consisting of 60% CH4 and 40% CO2 was reformed by steam at 101.3kPa pressure over a metal-foam-coated 1.31wt% [Pd(7)–Rh(1)]/(CeZrO2–Al2O3) catalyst in a tubular reactor at GHSV 10,000h−1. Reactant conversion, product selectivity, process thermal efficiency and coke formation were evaluated by varying S/C ratio as 1.00–2.00 in the temperature range of 923–1123K. CO2 in the biogas as well as steam reacted to reform CH4, whose conversion was significantly contributed by dry reforming at 973K for S/C ratio of 1.25 or 1.50. Dry reforming contribution was attenuated by steam reforming as S/C ratio or reaction temperature increased. H2/CO selectivity increased and CO/(CO+CO2) selectivity decreased as S/C ratio increased; the former decreased and the latter increased as reaction temperature increased. CO2 conversion was not so high due to the competition of dry reforming with WGS/RWGS. Process thermal efficiency and coke formation were dependent upon S/C ratio and reaction temperature in a rather complicated manner. According to the performance guidelines defined in this study, S/C=1.50 and 1023K were selected as optimum steam-biogas reforming condition, at which the catalytic stability was maintained for 200h onstream period at the increased GHSV of 20,000h−1.

Process flow sheet evaluation of a nuclear hydrogen steelmaking plant applying very high temperature reactors for efficient steel production with less CO2 emissions

Recently, CO2 reduction is an important problem for steelmaking. Substitution of coal, presently used as a reducing agent of iron ore in blast furnaces, to hydrogen produced by non-fossil energy is a way to reduce CO2 emissions. In this study, the idea of nuclear hydrogen steelmaking (NHS) system was investigated using very high temperature reactor (VHTR) and thermochemical hydrogen production iodine–sulfur (IS) process. Heat input and CO2 emissions including material production, material transportation, and electricity generation were evaluation criteria. Results of the NHS system were compared with those of a conventional blast furnace steelmaking (BFS) system. Influence of heat input options to the steelmaking process and hydrogen production thermal efficiency of IS process were investigated for the NHS system. Though heat input to the NHS system was 130–142% of that to the BFS system, CO2 emissions of the system were 13–21%. Pre-heating of hydrogen by coal combustion before blowing to a shaft furnace was effective to decrease heat input, although CO2 emissions increased. Direct pre-heating by nuclear heat was also effective without increase of CO2 emissions if close location of the nuclear reactor to the steelmaking plant was publicly accepted. Hydrogen production thermal efficiency had a significant influence on the heat input. Conceptual design of a plant unit of the NHS system producing steel of 1.47×106t/y with 2 VHTRs of 600MW heat and a shaft furnace and an electric arc furnace was proposed. The steelmaking scale was a little smaller than those of the recent largest Midrex® plants.

Modeling of air-gap membrane distillation process: A theoretical and experimental study

A one dimensional (1-D) air gap membrane distillation (AGMD) model for flat sheet type modules has been developed. This model is based on mathematical equations that describe the heat and mass transfer mechanisms of a single-stage AGMD process. It can simulate AGMD modules in both co-current and counter-current flow regimes. The theoretical model was validated using AGMD experimental data obtained under different operating conditions and parameters. The predicted water vapor flux was compared to the flux measured at five different feed water temperatures, two different feed water salinities, three different air gap widths and two MD membranes with different average pore sizes. This comparison showed that the model flux predictions are strongly correlated with the experimental data, with model predictions being within +10% of the experimentally determined values. The model was then used to study and analyze the parameters that have significant effect on scaling-up the AGMD process such as the effect of increasing the membrane length, and feed and coolant flow rates. The model was also used to analyze the maximum thermal efficiency of the AGMD process by tracing changes in water production rate and the heat input to the process along the membrane length. This was used to understand the gain in both process production and thermal efficiency for different membrane surface areas and the resultant increases in process capital and water unit cost.

Process synthesis and economics of combined biomethanol and CHP energy production derived from biomass wastes

AbstractBACKGROUND: This paper reports on process synthesis and economics of combined methanol and CHP (combined heat and power) energy production from crude biooil, waste glycerol produced in biodiesel factories and biomass wastes using integrated reactor design for hydrogen rich syngas. This new process consists of three process steps: (a) pyrolysis of organic waste material to produce biooil, char and pyrogas; (b) steam assisted hydrogasification of the crude glycerol wastes, biooil mixed with pyrogas for hydrogen rich gas; and (c) a low temperature methanol synthesis process. The H2‐rich gas remaining after methanol synthesis is recycled back to the pyrolysis reactor, the catalytic hydro‐gasification process and the heat recovery steam generator (HRSG).RESULTS: The breakeven price of the Hbiomethanol process yields positive net financial NPV and IRR above 600 USD per tonne. The total capital cost for a small‐scale methanol plant of capacity 2 tonne h−1 combined with a cogeneration plant of capacity 2 MWe power is estimated to be 170.5 million USD.CONCLUSION: Recycling gas allows the methanol synthesis reactor to perform at a relatively lower pressure than conventionally while the plant still maintains a high methanol yield. The integrated hydrogasification reactor and energy recovery design process minimizes heat loss and increases the process thermal efficiency. The Hbiomethanol process can convert any condensed carbonaceous material and liquid wastes, to produce methanol and CHP. Copyright © 2012 Society of Chemical Industry

Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H2O to I2 in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H2, with a Section energy requirement of 367 kJ/mol H2, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.

MODELING TEMPERATURE AND SALT CONCENTRATION DISTRIBUTIONS IN DIRECT CONTACT MEMBRANE DISTILLATION

Modern desalination technologies, applied to seawater and brackish water, offer effective alternatives in a variety of circumstances. Because of its low energy consumption, accessible running conditions and simple maintenance, membrane distillation (MD) has become one of the cheapest technologies for seawater desalination. The membrane distillation driving force is the transmembrane vapor pressure difference that may be maintained with an aqueous solution colder than the feed solution in direct contact with the permeate side of the membrane giving rise to the configuration known as direct contact membrane distillation (DCMD). The main objective of the present study is to provide a detailed numerical analysis of the heat and mass transfer in DCMD and to offer useful basic detailed information about the nature of the process that is needed for process improvement and optimization. Moreover, the present study is carried out to investigate the effect of parameters such as the inlet temperatures of the hot and cold solutions, the concentration of the feed (hot) solution, the inlet velocity of the hot and cold solutions on the process characteristics of DCMD desalination evaluated in terms of the permeate flux and the process thermal efficiency. The direct contact membrane distillation process has been modeled as a two-dimensional coupled problem in which a simultaneous numerical solution of the momentum, energy and diffusion equations of the feed and cold solutions have been carried out with permeation taken into account. Velocity and temperature distributions inside the membrane feed and cold solution channels are obtained. Some of the principal conclusions drawn from the present study are: (1) increasing the inlet temperature of the hot solution has a major effect on the permeate flux, (2) increasing the feed water salt concentration decreases the permeate flux, (3) the inlet velocities of the hot and cold solutions have a relatively strong effect on the permeate flux.

Numerical evaluation of the energetic performances of structured and random packed beds in regenerative thermal oxidizers

Regenerative thermal oxidizers (RTO) can be conveniently used to control volatile organic components (VOC) emissions, because of their thermal efficiency and cost effectiveness. In the RTO technology, beds of inert material are used in order to heat the polluted air by cooling burnt gases, through a sequence of cyclic operations which cut the fuel requirements. A computational 1D unsteady model, able to account for both structured and random packed bed regenerators, is developed and applied to realistic plant conditions. Process thermal efficiency and gas pressure drop are calculated as functions of the system geometry and operating parameters. The code can be usefully employed in the analysis and design of RTO systems and in order to choose the more suitable type of regenerator, structured or random packed bed (even considering various particle shapes). Energetic performances of both random and structured regenerators were compared, showing that the first ones exhibit a little higher thermal efficiency but also an elevated pressure drop, at a same value of exchange surface per unit volume of the bed. Random packed bed regenerators resulted less attractive from the energetic point of view and their usage is advisable if their lower cost satisfy economical requirements.

Modeling Process Characteristics and Performance of Fixed and Fluidized Bed Regenerative Thermal Oxidizer

The abatement of volatile organic components (VOC) in emissions is an extremely stringent environmental problem. One of the most frequently used technologies is the regenerative thermal oxidation (RTO), because of its thermal efficiency and cost-effectiveness. In the present paper, a computational one-dimensional unsteady model, able to account for fixed as well as fluidized beds of particles, is developed, validated, and applied to realistic plant conditions. Process thermal efficiency and gas pressure drop are calculated as functions of the system geometry and operating parameters. Results of a validation against experimental data are presented first, showing very good agreement. The overall thermal efficiency is obtained as the performance index for packed beds of spheres and Rashig rings and fluidized beds of spheres, allowing a direct comparison of the two systems. The simulation results clearly show that, despite the high efficiency of gas−solid contact in fluidized beds, their thermal performances are unacceptably poor, because of the cyclic nature of the process.

Technical aspects of the production of dried extract of Maytenus ilicifolia leaves by jet spouted bed drying

This work presents an evaluation of the performance of jet spouted bed with inert particles for production of dried extracts of Maytenus ilicifolia leaves. The development of the extraction procedure was carried-out with the aid of three factors and three levels Box–Behnken design. The effects of the extraction variables, temperature ( T ext); stirring time ( θ); and the ratio of the plant to solvent mass ( m p/ m s) on the extraction yield were investigated. The drying performance and product properties were evaluated through the measurement of the product size distribution, loss on drying ( U p), flavonoid degradation ( D) and, process thermal efficiency ( η). These parameters were measured as a function of the inlet temperature of the spouting gas ( T gi), the feed mass flow rate of the concentrated extract relative to mass flow rate of the spouting gas ( W s/ W g), the ratio between the feed flow rate of spouting gas relative to feed flow rate at a minimum spouting condition ( Q/ Q ms) and the static bed height ( H 0). A powder product with a low degradation of active substances and good physical properties were obtained for selected operating conditions. These results indicate the feasibility of this drying equipment for the production of dried extracts of M. ilicifolia Martius ex Reiss leaves.

Optimization with Genetic Algorithms of a Gas Turbine Cycle with H2-Separating Membrane Reactor for CO2 Capture

A gas turbine power process with CO2 capture through precombustion decarbonization, which employs a H2-separating membrane reactor is presented. Optimization with the process thermal efficiency as objective function is made through the use of genetic algorithms. The use of genetic algorithms enabled a division of the optimization parameters into two groups; one group where the values are at their optimum at the limit of the investigated parameter range, and one group where there actually is an optimum within the investigated range. It was found that the process has a severe efficiency penalty caused by the use of heat from hydrogen combustion for the reforming process. The process is a zero CO2 emission power process and also NOx emissions should be low, due to the inherent mixing of hydrogen with steam.

Thermal analysis in fluidized bed drying of moist particles

This paper deals with thermal modeling of the fluidized bed drying of wet particles to study heat and mass transfer aspects and drying thermal efficiencies. The model is then validated with the literature experimental data obtained for corn. A parametric investigation is undertaken to study the effects of the inlet air temperature, the air velocity and the initial moisture content of the material (i.e. corn) on the process thermal efficiency. The results show that the thermal efficiencies of the fluidized bed drying decrease sharply with decreasing moisture content of corn and hence increasing drying time, and apparently become the lowest at the end of the drying process. This clearly indicates that the moisture transfer from the material depends strongly on the air temperature, air velocity and the moisture content of material. A good agreement is obtained between the model predictions and the available experimental results.

PH—Postharvest Technology: Monitoring and Performance of a Commercial Grain Dryer

This paper presents detailed in situ experimental results for the performance of a commercial batch grain dryer under real operating conditions while drying barley. The distribution of key drying parameters of air temperature, relative humidity, airflow, grain moisture content, drying time and energy use are measured. For the two trials conducted, the overall cycle-averaged process thermal efficiency is 48·4 and 41·5%, respectively. This is achieved when the average initial moisture contents of the grain are, respectively, 15·7 and 17·2% and the highest plenum temperatures 41·4 and 34·0°C. During drying, unlike the stack-on temperatures, the exhaust air temperatures after the grain stack are fairly stable and only mildly affected by the variation of ambient conditions. After the initial heating period, the air enthalpy remains practically constant through the grain stack section. Close to the dryer centre, because of the presence of the auger, the air velocity is considerably lower than other parts of the dryer. The drying appears to be dominated by a drying front advancing through the grain stack. The reduction of grain moisture content is much faster during the last few hours of the drying process. The experiments highlight the importance of accurate measurement and representation of product moisture contents and airflow.

The curing of powder coatings using gaseous infrared heaters: An analytical model to assess the process thermal efficiency

This experimental investigation is structured to determine the efficiency of energy transfer between a gaseous infrared burner and a powder coated surface being cured. The results from the experimental burner tests are interpreted using a Monte Carlo based numerical simulation. This simulation incorporates both an Edwards' exponential wide band model for the spectrally selective energy absorption and emission in the flue gas, and test measurements for the powders' spectral absorption characteristics. The results from the simulation show that efficiency of the energy transfer from the natural gas stream to the powdered surface ranges between 25 % and 30 %, when the burner is operating at stoichiometric conditions. The highest efficiency was obtained for the darkest coloured powder. The energy efficiency was also found to increase with burner temperature. However, the quality of the cured surface tended to decrease if the temperature rise was significant.

Hynol—An economical process for methanol production from biomass and natural gas with reduced CO 2 emission

The Hynol process is proposed to meet the demand for an economical process for methanol production with reduced CO 2 emission. This new process consists of three reaction steps: (a) hydrogasification of biomass, (b) steam reforming of the produced gas with additional natural gas feedstock, and (c) methanol synthesis of the hydrogen and carbon monoxide produced during the previous two steps. The H 2-rich gas remaining after methanol synthesis is recycled to gasify the biomass in an energy neutral reactor so that there is no need for an expensive oxygen plant as required by commercial steam gasifiers. Recycling gas allows the methanol synthesis reactor to perform at a relatively lower pressure than conventional while the plant still maintains high methanol yield. Energy recovery designed into the process minimizes heat loss and increases the process thermal efficiency. If the Hynol methanol is used as an alternative and more efficient automotive fuel, an overall 41% reduction in CO 2 emission can be achieved compared to the use of conventional gasoline fuel. A preliminary economic estimate shows that the total capital investment for a Hynol plant is 40% lower than that for a conventional biomass gasification plant. The methanol production cost is $0.43/gal for a 1085 million gal/yr Hynol plant which is competitive with current U.S. methanol and equivalent gasoline prices. Process flowsheet and simulation data using biomass and natural gas as cofeedstocks are presented. The Hynol process can convert any condensed carbonaceous material, especially municipal solid waste (MSW), to produce methanol.

Technical evaluation of UT-3 thermochemical hydrogen production process for an industrial scale plant

The continuous production of UT-3 thermochemical hydrogen has been successfully achieved through the operation of the bench-scale model plant. In parallel with this operation, a commercial size UT-3 hydrogen generating plant was conceptually designed to assess the process thermal efficiency. The applicability of the membrane gas separator has been evaluated for the case of applying a membrane separator to an Fe reactor in the UT-3 process. The encouraging results showed that the UT-3 process with the membrane separator has the potential to improve the process thermal efficiency and economical efficiency.

Development of Liquefaction Process for Natural Gas.

A new liquefaction process for natural gas is developed to improve process thermal efficiency while using industrial standard equipment. The new process consists of a pre-cooling section which uses mixed refrigerant with brazed aluminum plate-fin exchangers or a spool wound heat exchanger, and a liquefaction section which uses an iso-entropic expander. Thermal efficiency of the new liquefaction process is confirmed to be of the highest level compared with other liquefaction processes. Also, since the new liquefaction process is constructed with commonly available industrial equipment, it can be readily adapted to liquefaction plants of any capacity without requiring expensive and specially designed equipment.

Thermal Upgrading of Low-Grade, Low-Rank South Australia Coal

Abstract Experimental data are reported on the extents and rates of removal of impurities from a low-grade, low-rank South Australia lignite by thermal treatment of coal-water slurries in the temperature range 250-3S0“C, Removal of up to 80% of the water, 50% of the oxygen, 40% of the organic sulfur. 60% of the sodium chloride, and 30% of the “ash” constituents has been achieved, with a coal product yield of about 80% (dry, mineral-matter-free basis), containing over 98% of the original coal energy. Process thermal efficiency is estimated to be ∼95%. A distributed activation energy model provides a satisfactory fit to experimental data for the evolution of volatile species.

A method for the techno-economic evaluation of chemical processes—improvements to the “OPTIMO” code

This paper describes the improvement of a chemical engineering code [1] developed specifically for the evaluation and analysis of multi-step processes. The capabilities of the original code have been retained in that the mass and energy balances are the basis of a heat exchange network synthesis. However, rather than complete the evaluation with the process thermal efficiency as before, the program has been extended in order to accommodate a more detailed nuclear reactor-chemical plant coupling and new routines were written so that the hydrogen production cost can be estimated. The code was applied to two sulphur-based cycles and the results are presented later.