Off-gas Data Research Articles

Potential of off-gas analyses for sequentially operated reactors demonstrated on full-scale aerobic granular sludge technology

This work shows how more variables can be monitored with a single off-gas sampler on sequentially operated than on continuously fed and aerated reactors and applies the methods to data from a full-scale aerobic granular sludge reactor as a demonstration and to obtain insight in this technology. First, liquid-gas transfer rates were calculated. Oxygen (O2) absorption and carbon dioxide (CO2) emission rates showed comparable cyclic trends due to the coupling of O2 consumption and CO2 production. Methane (CH4) emissions showed a stripping profile and nitrous oxide (N2O) emissions showed two peaks each cycle, which were attributed to different production pathways. Secondly, aeration characteristics were calculated, of which the gradual improvement within cycles was explained by surfactants degradation. Thirdly, liquid phase concentrations were estimated from off-gas measurements via a novel calculation procedure. As such, an average influent CH4 concentration of 0.7 g·m−3 was found. Fourthly, reaction rates could be estimated from off-gas data because no feeding or discharge occurred during reaction phases. The O2 consumption rate increased with increasing dissolved oxygen and decreased once nitrification was complete. Fifthly, greenhouse gas emissions could be derived, indicating a 0.06% N2O emission factor. Sixthly, off-gas gave an indication of influent characteristics. The CO2 emitted per kg COD catabolized corresponded with the TOC/COD ratio of typical wastewater organics in cycles with balanced nitrification and denitrification. High nitrogen removal efficiencies were associated with high catabolized COD/N ratios as estimated from the O2 absorption. Finally, mass balances could be closed using off-gas O2 data. As such, an observed yield of 0.27 g COD/g COD was found. All these variables could be estimated with a single sampler because aeration without feeding creates a more homogeneous off-gas composition and simplifies liquid-phase mass balances. Therefore, off-gas analyzers may have a broader application potential for sequentially operated reactors than currently acknowledged.

Mathematical Modeling of Postcombustion in an Electric Arc Furnace (EAF)

Numerical modeling was used to study the capability of postcombustion in an electric arc furnace (EAF) equipped with virtual lance burners. The CO flow rate at the molten bath surface was estimated using the off-gas data obtained close to the outlet of an EAF. Then, the effect of the secondary oxygen flow rate on postcombustion was studied. The results show a CO flow rate of 0.6 kg·s−1 and 0.8 kg·s−1 for operation modes of burner and burner + lancing. Increase of the secondary oxygen flow rates of 60% and 70% result in 17% and 7% increase in the postcombustion ratio (PCR) for the burner and burner lancing modes, respectively.

Application of genetic algorithm to improve an electric arc furnace freeboard model based on practical data

Electric arc furnaces are applied to produce steel by melting scrap in a batch process. Despite of the significant role of the off-gas in the electric arc furnace environmental and energy feature, limited research has been carried out specifically on the modelling of the electric arc furnace freeboard. In this research, a reactor base model for electric arc furnace with burner and without burner is extended by considering more inputs to approach real conditions. There are some uncertainties in the measurement or calculation of electric arc furnace inputs due to different constraints. Regarding the multi objective problem and the conflicting nature of objectives, a genetic algorithm is applied to improve the model accuracy. The genetic algorithm found the optimum input parameters in the validated range based on the best compatibility with actual off-gas data. The comparison among different results illustrates the effective performance of genetic algorithm for this problem. This accurate model can be applied in the study of recycling, storage condition and control of the electric arc furnace off-gas for final purpose of energy saving and environmental protection.

Reduction Behavior of Carbon Composite Pellets Including Alumina and Silica at 1273 K and 1373 K

The effect of alumina and silica on the reaction behavior of carbon composite pellet was investigated by thermogravimetric analysis at 1273 K (1000°C) and 1373 K (1100°C). X-ray diffraction was used for the phase analysis of iron oxide changing from hematite to reduced iron. The overall reaction was divided into three stages according to phase transformation of iron oxide. CO and CO2 from off-gas data was used for kinetic analysis and for understanding reaction mechanism. The Boudouard reaction was largely influenced by alumina and silica that changed CO gas concentration resulting in different reduction behavior of the pellets. Alumina increased the reaction rate of carbon composite pellet while silica decreased the overall reaction rate.

Evaluation of oxygen transfer rates in stirred‐tank bioreactors for clinical manufacturing

Several methods are available for determining the volumetric oxygen transfer coefficient in bioreactors, though their application in industrial bioprocess has been limited. To be practically useful, mass transfer measurements made in nonfermenting systems must be consistent with observed microbial respiration rates. This report details a procedure for quantifying the relationship between agitation frequency and oxygen transfer rate that was applied in stirred-tank bioreactors used for clinical biologics manufacturing. The intrinsic delay in dissolved oxygen (DO) measurement was evaluated by shifting the bioreactor pressure and fitting a first-order mathematical model to the DO response. The dynamic method was coupled with the DO lag results to determine the oxygen transfer rate in Water for Injection (WFI) and a complete culture medium. A range of agitation frequencies was investigated at a fixed air sparge flow rate, replicating operating conditions used in Pichia pastoris fermentation. Oxygen transfer rates determined by this method were in excellent agreement with off-gas calculations from cultivation of the organism (P = 0.1). Fermentation of Escherichia coli at different operating parameters also produced respiration rates that agreed with the corresponding dynamic method results in WFI (P = 0.02). The consistency of the dynamic method results with the off-gas data suggests that compensation for the delay in DO measurement can be combined with dynamic gassing to provide a practical, viable model of bioreactor oxygen transfer under conditions of microbial fermentation.

Monitoring off-gas O2/CO2 to predict nitrification performance in activated sludge processes

Nitrification/denitrification (NDN) processes are the most widely used technique to remove nitrogenous pollutants from municipal wastewater. The performance of nitrogen removal in the NDN process depends on the metabolism of nitrifying bacteria, and is dependent on adequate oxygen supply. Off-gas testing is a convenient and popular method for measuring oxygen transfer efficiency (OTE) under process conditions and can be performed in real-time. Since carbon dioxide is produced by carbonaceous oxidizing organism and not by nitrifiers, it should be possible to use the off-gas carbon dioxide mole fraction to estimate nitrification performance independently of the oxygen uptake rate (OUR) or OTE. This paper used off-gas data with a dynamic model to estimate nitrifying efficiency for various activated sludge process conditions. The relationship among nitrification, oxygen transfer, carbon dioxide production, and pH change was investigated. Experimental results of an online off-gas monitoring for a full-scale treatment plant were used to validate the model. The results showed measurable differences in OUR and carbon dioxide transfer rate (CTR) and the simulations successfully predicted the effluent ammonia by using the measured CO(2) and O(2) contents in off-gas as input signal. Carbon dioxide in the off-gas could be a useful technique to control aeration and to monitor nitrification rate.

Simple adaptive pH control in bioreactors using gain-scheduling methods

A simple well-performing adaptive control technique for pH control in fermentations of recombinant protein production processes is described and its design procedure is explained. First, the entire control algorithm was simulated and parameterized. Afterwards it was tested in real cultivation processes. The results show that this simple technique leads to significant reductions in the fluctuations of the pH values in microbial cultures at a minimum of expenditures. The signal-to-noise ratio and thus the information captured by the pH signal were increased by about an order of magnitude. This leads to a substantial improvement in the noise of many other process signals that are used to monitor and control the process. For instance, respiratory off-gas data of CO(2) and its derived carbon dioxide production rate signals from the cultures carry much less noise as compared to those values obtained with conventional pH control. Detailed process analysis revealed that even very small pH jumps of 0.03 values during the fermentation were shown to result in pronounced deflections in CO(2)-volume fraction of 8% (peak to peak). The proposed controller, maintaining the pH within the interval of 0.01 around the setpoint, reduces the noise considerably.

Determination of activated sludge biological activity using model corrected CO 2 off-gas data

Carbon dioxide (CO 2) online off-gas monitoring is useful to detect changes in biological activity for activated sludge systems especially under limited oxygen conditions like under simultaneous nitrification–denitrification (SND) where respirometric measurements are not applicable. So far, the influence of the bicarbonate system on the liquid–gas transfer of CO 2 prevented the wider use of off-gas CO 2 for monitoring purposes in wastewater treatment. The objective of the paper is to demonstrate a practical method to correct measured off-gas CO 2 as an indicator of biological activity by taking into account pH shifts (resulting in CO 2 release or retention) and changes in influent alkalinity. The simple model is based on the physicochemical system of the bicarbonate/CO 2 equilibrium and the liquid–gas mass transfer for aerated systems. Standard on-line measurements (pH, temperature, flow rates) and periodical alkalinity titration serve as input data to estimate the influence of the carbonate system on the CO 2 off-gas concentrations measured on-line. For a particular plant the CO 2 mass transfer coefficients are derived from measurements compared to the theoretical calculation from oxygen mass transfer. The model estimates the biological carbon dioxide production rate (CPR; heterotrophic activity) by the correction of the measured carbon dioxide transfer rate (CTR; C-flux by the off-gas) with the calculated inorganic carbon dioxide transfer rate ( r F) considering bicarbonate consumption (autotrophic activity).

Off-Gas Source Estimation for Boiling Water Reactors under Various Operation Conditions

Radioactive fission gases (krypton and xenon) are observed in boiling water reactor (BWR) plants without defective fuel under various operation conditions. The off-gas in a BWR plant without defective fuel arises from fissile impurities within the cladding materials and/or fissile material deposited on the cladding surface. To estimate the off-gas source in operating plants, the source estimation method considering the off-gas transport time from production to measurement was applied to the data collected under various plant operation conditions. This method was verified by the adaptation of three sets of off-gas data groups to the source of cladding impurity and/or deposition actually observed in the plants. The judgment as to whether off-gas derives from a cladding impurity or a deposition was made by analyzing the data of 11 BWRs with different off-gas levels.

NO x emission from electric arc furnace in steel industry: contribution from electric arc and co-combustion reactions

Off-gas measurements were conducted at industrial electric arc furnaces (EAF) in Germany in order to investigate the interrelation of NOx emission with installed plant equipment (e.g. gas burner) and process data (e.g. carbon input). Off-gas data monitor rapid changes in off-gas composition, temperature, and volume flow rates of air into the furnace indicating the transient state batch process of scrap melting. From the measurements two distinct sources of NOx emission are clearly distinguished: (1) NOx formation in the electric arc plasma during the start-up period of the melting process in an oxidizing furnace atmosphere after the charging of the furnace. (2) NOx formation from post-combustion of CO/CO2/H2 gas with air inside and outside the furnace. Whereas the contribution from arc ignition is similar for different types of EAFs, other contributions depend on furnace equipment and operation, e.g. gas burners, use of air as carrier medium for carbon or dust injection, air-tightness of furnace, and parameters of off-gas extraction by EAF dedusting system. The positive effect of the minimum volume flow rate of air into the furnace by controlled off-gas extraction to total NOx emission is shown.

Oxidative pyrolysis of organic ion exchange resins in the presence of metal oxide catalysts

Pretreatment of the organic ion exchange resins by oxidative pyrolysis is effective for volume reduction before vitrification. In this work, pyrolysis of two nuclear-grade resins, Purolite NRW100 (cationic) and NRW400 (anionic), was examined using a laboratory-scale theromgravimetric analyzer (TGA) in air. It was shown that the cationic resin was harder to degrade and was less volatile compared to anionic resin. Off-gas data revealed the presence of SO 2, CO 2, CO, and H 2O during oxidative pyrolysis of cationic resin from 30 to 800 °C. Trimethylamine, CO 2, CO, and ethyl formate were found in the case of anionic resin. In addition, oxidative pyrolysis of the mixed resins (50/50 wt.%) showed the existence of the gases nearly at the temperatures where the gases would evolve if the results of two different resins were superimposed. Several metal salts including CuSO 4·5H 2O, CuO, and FeSO 4·7H 2O, as well as the ions Cu 2+ pre-loaded on the resins had a catalytic effect on the oxidative pyrolysis of cationic resins, in which the decomposition of functional groups and polymer matrices was enhanced. Such catalytic effect was highlighted by a large decrease in activation energy calculated according to a degradation mechanism involving four consecutive reactions.

Demonstration of the defense waste processing facility vitrification process for tank 42 radioactive sludge – melter feed preparation

AbstractThe Defense Waste Processing Facility (DWPF), at the Savannah River Site (SRS), is processing and immobilizing the radioactive high level waste sludge at SRS into a durable borosilicate glass for final geological disposal. The DWPF is currently processing the second, million gallon batch of radioactive sludge. This second batch is primarily from Tank 42. Each time a new batch of radioactive sludge is to be processed by the DWPF, the process flowsheet is to be tested and demonstrated to ensure an acceptable melter feed and glass can be made. This demonstration was completed in the Shielded Cells Facility in the Savannah River Technology Center at SRS.This paper presents the processing and offgas data, and compositional analyses obtained during the preparation of a melter feed for this demonstration. A second paper in this conference describes the properties of the glass produced from this feed. The demonstration used Tank 42 sludge slurry and the DWPF process control strategy for blending the sludge slurry with Frit 200 to make an acceptable melter feed. To prepare feed for the melter, the flowsheet requires that the radioactive sludge slurry be treated with nitric and formic acid to adjust rheology and remove mercury. During this step, hydrogen is formed from the decomposition of the formic acid. The acidified sludge slurry is then mixed with the prescribed amount of glass forming frit and evaporated to the proper weight percent solids to prepare feed to the melter. During this step hydrogen is also formed. Results indicate that the H2 generation rate is below the DWPF safety limits and an acceptable melter feed was produced.

Waste minimization pretreatment via pyrolysis and oxidative pyroylsis of organic ion exchange resin

Pyrolysis and/or oxidative pyrolysis of organic ion exchange resins and other combustible waste may be effective pretreatment processes before vitrification. To further examine these processes, organic ion exchange resins were pyrolyzed or oxidatively pyrolyzed with the use of a small-scale, commercial TGA. Volatilization of the anionic and cationic resins was observed separately for each resin as a function of temperature for pyrolysis and oxidative pyrolysis conditions. The quantity of remains or residue was found to be dependent on the method employed. Three different methods were examined with the TGA to pretreat the resins: pyrolysis; oxidative pyrolysis; and oxidative pyrolysis of ash remaining after the pyrolysis of resin. The latter two methods were found to provide better volume reduction than the pyrolysis-only process. Between the two types of resins, cationic and anionic, the cationic exchange resin was less volatile. Pyrolysis and oxidative pyrolysis of mixed resin (50% cation and 50% anion by wt.) showed volatilization at the temperatures where volatilization was observed for each of the separate resins. Because of certain limitations of the commercial TGA, tube furnace experiments were performed, generally, to examine the pyrolysis of larger quantities of cationic, anionic, and mixed resin, and to examine off-gas characteristics. The cationic resin-only and anionic resin-only gravimetric results showed good agreement with the smaller-scale TGA results. SEM pictures of the different variants of the resin (cationic, anionic, and mixed) show a different morphology for each. Off-gas data showed the presence of H 2S, SO 2, CO, and NO during the pyrolysis of cationic resin. CO was observed during the pyrolysis of anionic resin. The mixed resin trials showed the presence of the gases approximately at the temperatures where the gases would evolve if the results of the two different resins (cationic and anionic) were superimposed. However, the amount of hydrogen sulfide relative to the sulfur dioxide was found to increase significantly compared to the results of the cationic resin-only trials.

Application of Pneumatic Fracturing to Enhance In Situ Bioremediation

A field pilot demonstration integrating pneumatic fracturing and in situ bioremediation was carried out in a gasoline-contaminated, low permeability soil formation. A pneumatic fracturing system was used to enhance subsurface air flow and transport rates, as well as to deliver soil amendments directly to the indigenous microbial populations. An in situ bioremediation zone was established and operated for a period of 50 weeks, which included periodic subsurface injections of phosphate, nitrate, and ammonium salts. Off-gas data indicated the formation of a series of aerobic, denitrifying, and methanogenic microbial degradation zones. Based on soil samples recovered from the site, 79% of soil-phase benzene, toluene, and xylenes (BTX) was removed by the integrated technology. From mass balance calculations, accounting for all physical losses, it was estimated that 85% of the total mass of BTX removed (based on mean concentration levels) was attributable to biodegradation.

Fast inferential adaptive optimization of a continuous yeast culture based on carbon dioxide evolution rate

A fast inferential, multivariable adaptive optimization algorithm based on a fast responding off-gas data, the carbon dioxide evolution rate (CER), has been developed and applied to a continuous baker's yeast culture to maximize the cellular productivity in simulation and experimental studies. In the simulation study the process was optimized based on CER measurements using readily available steady-state data on the ratio between the cellular productivity and the CER. It was shown that the algorithm is two to three times faster than the algorithm based on cell mass concentration measurements. In the experimental study the CER was maximized without any information on the relationship between the cellular productivity and the CER. It took about 40 h for the process to converge, while about 80 h was required when the optimization was based on cell mass measurements. The attained steady state was found to be different but fairly close to that obtained with cell measurements. Briefly discussed is a switching to the cell-mass-based algorithm at the final stage of the optimization to overcome a potential inaccuracy.