Dryer Absorber Research Articles

The long-term engineering properties and sustainability indices of dewatering hydrated lime mortars through Jacaranda seed pods

The progressive increase in the industrialisation, consequently the energy consumption and carbon footprint reacquaint the sustainable and environmental credentials of hydrated lime mortars and re-congregate the implications of such binders in masonry construction. The low water retaining characteristics of hydrated lime mortars that often results in insufficient bond development with the subsequent substrate, were manipulated through the incorporation of powdered Jacaranda seed pods and silica fume to establish mortar-masonry optimisation in practice. Although there are numerous studies addressing the agricultural waste incorporation in cement-based materials, the incorporation of Jacaranda seed pods on construction materials have not been reported in the literature. Characterisation study comprising the elemental composition and physical properties of Jacaranda seed pods were essential in assessing the compatibility of pods utilisation in hydrated lime mortars. The paper reports authentic experimental research results on the utilisation of Jacaranda seed pods as a binder substitute in hydrated lime mortars concerning the effect of dewatering on the long-term engineering properties and sustainability indices. The utilisation of powdered Jacaranda seed pods and silica fume demonstrated the eligibility of manipulating the strong water retaining characteristics of hydrated lime mortars to attain improved mortar-substrate optimisation in masonry construction. It is essentially shown in the paper that the degree of dewatering, governed by the incorporation of powdered Jacaranda seed pods and silica fume herein, could not be underestimated as this phenomenon yields approximately 18% increase in the compressive strength of hydrated lime mortars comprising both at short- and long-terms. Although the incorporation of both the powdered Jacaranda seed pods and silica fume decreased the water penetration depth and porosity of mortars by ~11% and 17% respectively, dewatering, an inevitable incident that occurs as soon as the freshly mixed mortars are placed on dry absorbent substrates, further yields a greater decrease in these physical measurements at 1 year. Incorporation of powdered Jacaranda seed pods and silica fume generated a crucial improvement on cost efficiency and eco-strength efficiency of mortars both at 91 days and 1 year. The results have shown that more than 30% reduction on the carbon footprint could be established through this practice. It is essentially established in the paper that the degree of dewatering, experienced at freshly-mixed stage, determines the authentic performance of hydrated lime mortars in masonry construction and governs the veritable sustainability analysis to be performed. The results reported in this paper does not only encourage the re-introduction of the hydrated lime binder in construction practice but also assert a cleaner alternative waste management route for the agricultural wastes.

Enhanced Steam Temperature Enabled by a Simple Three-Tier Solar Evaporation Device.

Interfacial water evaporation technology by using solar energy provides one of the promising pathways for freshwater shortage management. However, current research mainly focuses on the improvement of evaporation efficiency by macro or microregulations, ignoring the steam temperature, which is a manifestation of the quality of water. Herein not only is a high‐rate solar evaporation achieved but also steam temperature is enhanced by a simple three‐tier (wet absorber–air gap–dry absorber) device. In a routine interfacial evaporation test, the evaporator achieves a stable evaporation rate up to 2.15 kg m−2 h−1 under one sun, demonstrating a competitive evaporation rate compared with other reports. With the three‐tier device, the steam temperature can increase 33.7%, 41.13%, and 47% without dry absorber under one sun, two sun, and three sun illumination, respectively. At the same time, the steam temperature can be as high as 95.5 °C under three sun intensities. This work provides the possibility of using a simple three‐tier device for high‐temperature steam generation without extra energy input, which contributes to an idea for future research on the production high‐quality water.

Selective sulfur removal from semi-dry flue gas desulfurization coal fly ash for concrete and carbon dioxide capture applications

High-sulfur mixed fly ash residues from semi-dry flue gas desulfurization units in coal-fired power plants are unsuitable for use as supplementary cementitious material (SCM) for concrete production or carbon dioxide utilization. In this work, we explore the potential for upcycling a representative spray dry absorber ash (10.44 wt% SO3) into concrete-SCM by selective sulfur removal via weak acid dissolution while simultaneously exploring the possibility for CO2 capture. Towards this effort, parametric studies varying liquid-to-solid ratio, acidity, and CO2 pressure were conducted in a batch reactor to establish the sulfur removal characteristics in de-ionized water, nitric acid, and carbonic acid, respectively. The dissolution studies show that the leaching of sulfur from calcium sulfite hemihydrate, which is the predominant S phase, is rapid and achieves a concentration plateau within 5 min, and subsequently, appears to be controlled by the primary mineral solubility. Preferential S removal was sufficient to meet SCM standards (e.g., 5.0 wt% as per ASTM C618) using all three washing solutions with 0.62–0.72 selectivity (S^), defined as the molar ratio of S to Ca in the leachate, for a raw fly ash with bulk S^ = 0.3. Acid dissolution with 1.43 meq/g of ash or under 5 atm CO2 retained > 18 wt% CaO and other Si-, Al-rich phases in the fly ash. Based on the experimental findings, two sulfur removal schemes were suggested for either integration with CO2 capture and utilization processes using flue gas or to produce fly ash for use as a SCM.

Fermentative Quality and Animal Acceptability of Ensiled Persimmon Skin with Absorbents for Practical Use in Ruminant Feed.

Simple SummaryAlthough persimmon skin (PS) has a high nutrition content, it loses much of its nutrient value through effluent, which hampers its use as a feed. To reduce effluent production during ensiling, we tested three kinds of moisture absorbents (kraft pulp, wheat bran, and beet pulp). We evaluated five types of table scale silages: persimmon skin only, persimmon skin plus Lactobacillus buchneri inoculum, and persimmon skin plus L. buchneri plus each absorbent in turn. We observed low effluent and gas loss in absorbent-treated groups, implying a positive contribution of these absorbents to fermentation quality. Further, we performed an in vitro culture test of PS silage supplemented with an absorbent mixture. The proportion of methane to total gas decreased in the ensiled group compared with that in the other groups. Finally, we conducted a preliminary feeding experiment on dry ewes to assess the palatability of PS with a partial substitution of up to 20% dry matter, and found no adverse effect on feed intake and acceptability. Our results highlight the potential use of PS as a part of ruminant feed.Persimmon skin (PS), while representing an attractive feed source, requires an appropriate preservation procedure to increase its shelf life. We assessed the fermentation quality, in vitro ruminal incubation, and intake of persimmon skin silage ensiled with different dry absorbents. We prepared the silage on a table scale (Experiment 1) and evaluated five different mixtures: PS without an additive, PS plus Lactobacillus buchneri inoculum (LB), and PS plus LB plus each of the absorbents kraft pulp, wheat bran, or beet pulp. We opened the laboratory bags, kept at 25 °C, at 0, 14, 28, and 60 days for fermentation quality and chemical analysis (n = 3 for each measurement). Further, with an in vitro rumen simulated cultivation study (Experiment 2), we evaluated the fermentation pattern of PS with a mixture of two absorbents (kraft pulp and wheat bran) either raw (no fermentation) or ensiled (n = 4 for each treatment). Finally, we conducted an in vivo experiment using six dry ewes assigned based on their body weight to two experimental groups in a crossover design of two periods (Experiment 3). We fed a control group a 100% basal diet (tall fescue hay and concentrate mixture) and ensiled PS (PSS) group, a 20% dry matter substitution of tall fescue with PS silage mixed with kraft pulp as the sole absorbent. The results of Experiment 1 show, regardless of the absorbents used, the effluent volume of the lab bags was lower in absorbent-treated groups (p < 0.001). In Experiment 2, the condition of the PS with absorbents (raw or ensiled) did not affect the total gas production (p > 0.05), but we observed an increased propionate proportion in PSS with absorbents compared to basal diet (p = 0.019). The proportion of methane to the total gas in PSS group was considerably reduced compared with that in the other groups (p < 0.001). As we did this incubation study with a single run, a more detailed evaluation in the future would verify these observations. In the animal trial (Experiment 3), dry matter intake was similar between groups (p > 0.05), but ewes spent a shorter time eating in the PSS-fed group (p = 0.011). Here we present the practical use of PSS as part of ruminant feed in which dry absorbents prevented dry matter loss.

Total Integrated Operation of Gasifier, Hot Gas Desulfurizer, and SEWGS Process for Pre-combustion CO2 Capture Using Real Syngas from Coal

SEWGS (Sorption Enhanced Water Gas Shift) process is one of options for pre-combustion CO2 capture using dry absorbent and water gas shift catalyst. In this process, the thermodynamic equilibrium in the shift reaction can be enhanced to give more hydrogen yield by adding a CO2 absorbent into the shift reactor. Total integrated system consists of coal gasifier, hot gas clean-up process (two interconnected fluidized bed desulfurizer), and one loop SEWGS process. In this presentation, continuous operation results of the hot gas clean-up process and one loop SEWGS process using dry absorbents at high temperature and pressure conditions were discussed. Dry absorbents (for desulfurization, CO2 absorption) were produced by KEPCO RI (Korea Electric Power Corporation Research Institute) by means of spray-drying method. Continuous operation was performed at high pressure (21bar for gasifier, 20bar for hot gas clean-up process, and 18bar for SEWGS process) and reaction temperature was 550oC for desulfurizer and 236oC for SEWGS reactor. Totally integrated operation was maintained up to 50hours. We could get high sulfur capture efficiency (99.6%) in the desulfurizer, high CO conversion (98.9%) and CO2 capture efficiency (98.0%) in the SEWGS reactor.

Evaluation of absorbent materials for use as ad hoc dry decontaminants during mass casualty incidents as part of the UK's Initial Operational Response (IOR).

The UK’s Initial Operational Response (IOR) is a revised process for the medical management of mass casualties potentially contaminated with hazardous materials. A critical element of the IOR is the introduction of immediate, on-scene disrobing and decontamination of casualties to limit the adverse health effects of exposure. Ad hoc cleansing of the skin with dry absorbent materials has previously been identified as a potential means of facilitating emergency decontamination. The purpose of this study was to evaluate the in vitro oil and water absorbency of a range of materials commonly found in the domestic and clinical environments and to determine the effectiveness of a small, but representative selection of such materials in skin decontamination, using an established ex vivo model. Five contaminants were used in the study: methyl salicylate, parathion, diethyl malonate, phorate and potassium cyanide. In vitro measurements of water and oil absorbency did not correlate with ex vivo measurements of skin decontamination. When measured ex vivo, dry decontamination was consistently more effective than a standard wet decontamination method (“rinse-wipe-rinse”) for removing liquid contaminants. However, dry decontamination was ineffective against particulate contamination. Collectively, these data confirm that absorbent materials such as wound dressings and tissue paper provide an effective, generic capability for emergency removal of liquid contaminants from the skin surface, but that wet decontamination should be used for non-liquid contaminants.

Influence of substrate and sand characteristics on Roman cement mortar performance

When formulating repair mortars standard test specimens should be used with caution as these cannot be considered representative of samples of mortars collected on site. This work reports an approach to repair mortar formulation which takes into account the influence of porous substrates, sand characteristics and mortar thickness on the properties of both fresh and hardened Roman cement mortars. It is shown that mortars cast on a dry absorbent substrate show modified properties such as increased strength and decreased water absorption coefficient, the degree of which is a function of sand grading and surface characteristics, sample thickness and substrate sorptivity.

알칼리 활성화된 고로슬래그 페이스트의 물리화학적 특성 및 이산화탄소 흡수능 평가

PURPOSES: In this study, alkali-activated blast-furnace slag (AABFS) was investigated to determine its capacity to absorb carbon dioxide and to demonstrate the feasibility of its use as an alternative to ordinary Portland cement (OPC). In addition, this study was performed to evaluate the influence of the alkali-activator concentration on the absorption capacity and physicochemical characteristics. METHODS: To determine the characteristics of the AABFS as a function of the activator concentration, blast-furnace slag was activated by using calcium hydroxide at mass ratios ranging from 6 to 24%. The AABFS pastes were used to evaluate the carbon dioxide absorption capacity and rate, while the OPC paste was tested under the same conditions for comparison. The changes in the surface morphology and chemical composition before and after the carbon dioxide absorption were analyzed by using SEM and XRF. RESULTS: At an activator concentration of 24%, the AABFS absorbed approximately 42g of carbon dioxide per mass of paste. Meanwhile, the amount of carbon dioxide absorbed onto the OPC was minimal at the same activator concentration, indicating that the AABFS actively absorbed carbon dioxide as a result of the carbonation reaction on its surface. However, the carbon dioxide absorption capacity and rate decreased as the activator concentration increased, because a high concentration of the activator promoted a hydration reaction and formed a dense internal structure, which was confirmed by SEM analysis. The results of the XRF analyses showed that the CaO ratio increased after the carbon dioxide absorption. CONCLUSIONS : The experimental results confirmed that the AABFS was capable of absorbing large amounts of carbon dioxide, suggesting that it can be used as a dry absorbent for carbon capture and sequestration and as a feasible alternative to OPC. In the formation of AABFS, the activator concentration affected the hydration reaction and changed the surface and internal structure, resulting in changes to the carbon dioxide absorption capacity and rate. Accordingly, the activator ratio should be carefully selected to enhance not only the carbon capture capacity but also the physicochemical characteristics of the geopolymer.

The CO2 Absorber Based on LiOH

Abstract Carbon dioxide absorbers have been used in anesthesiology for many years. However, this process is not limited to this field of medicine. Removing carbon dioxide from human environment is used in other areas as well: mining industry, submarines, scuba diving, space travel and many others. The rationale to remove carbon dioxide from confined spaces is that cannot be eliminated otherwise. Anesthesia practitioners are well aware of this component of the circle system, the carbon dioxide absorber. In daily practice the clinician is less concerned with what kind of substance fills the dedicated canister, as this is usually in the care of the maintenance personnel. The appearance of Sevoflurane and Desflurane, with their own chemical characteristics, prompted the clinician to dedicate new attention to these absorbents. The classical substances used for this purpose are different combinations of limes. The practical concern of the anesthesiologist is to notice when the absorbent is consumed and call for its replacement. Still, many other aspects remain: compound A formation with Sevoflurane, carbon monoxide formation with Desflurane and dry absorbent for instance. The latest member of these products in the medical field is the LiOH carbon dioxide absorbent. Although used for many years in the space exploration, its way into the operating room is a rather recent achievement. Special chemical properties and high absorptive capacity make this new type of absorbent an attractive option for modern anesthesia practice. The article below invites the reader through a short journey on the history of the CO2 absorbents and anesthesia circuits, Lithiumas a chemical element and, finally, to this new type of absorbent.

Experimental Study on Corrosion Characteristics of CO2 Pipeline for Offshore Transport and Geological Storage

Among the technologies, CCS (Carbon dioxide capture and storage) technology is assumed and arising as crucial technology for reducing anthropogenic carbon dioxide and bridging to hydrogen economy. CCS technology includes carbon dioxide capture on the large point source, transport of captured carbon dioxide stream and storage of carbon into safe geological formations. Among the CCS processes, capture side is developing with many demonstration projects, focusing on the efficiency improvement of wet and dry absorbents, material improvement for membrane, construction cost down, etc. But in transport side, the technology is in the initial stage. In Korea, site surveys are mainly focused and operated on the offshore saline aquifers in the east sea of Korea and rather few surveys are processed in the on land side. For the offshore carbon dioxide geological storage, there are two options for transportation, using pipeline or ship. To utilize the offshore pipeline it is possible to use high chrome steel alloy to response to seawater corrosion, though the cost for construction might take huge budget for installation mainly cause of material cost. Otherwise, we considered it might be possible to apply the anti-corrosion coating outside of the carbon steel pipeline to mitigate seawater corrosion and to reduce the cost, so to secure steady and robust operation of offshore carbon dioxide transportation with little concern of corrosion issue. In this study, we investigated the possibility of using carbon steel pipeline with internal phosphate coating and external thermal spray coating as candidate for the offshore carbon dioxide transportation pipeline material. The base material for this study is carbon steel. The pipe internal corrosion test was conducted with fluids of CO2 mixture with additives of water and SO2 gas, and the external corrosion was carried out with 3.5%w NaCl solution. In the case of internal corrosion, all the specimens resulted in corroded surface with rust covered when SO2 was injected over 175 ppmv. In the case of external corrosion, zinc thermal spray with epoxy coating showed that more epoxy sealer is remained than aluminum thermal spray coating. The test result will be used to figure out economically efficient and safe candidate pipeline material.

Abatement of mercury emissions in the coal combustion process equipped with a Fabric Filter Baghouse

The purpose of this study was to investigate the dependence of mercury emissions on coal ranks and electric utility boilers equipped with Fabric Filter Baghouses (FF). A comparison of mercury emission rates and fly ash properties was made between a circulating Fluidized Bed Combustor (CFBC) with FF and a Pulverized Coal (PC) combustor with FF during the burning of all three ranks of American coals. The data were collected from the Environmental Protection Agency Information Collection Request (EPA ICR) and WKU ICSET’s mercury testing program. A statistical stepwise regression procedure was used to determine significant factors such as coal rank and types of boilers equipped with FF on mercury emissions during coal combustion. The higher mercury emission rates were generally found in both CFB and PC units when lignite was burned. The lower mercury emission rates were generally found in both CFB equipped with FF and PC units equipped with FF when bituminous coal was burned. There was a statistically significant lower mercury emission in the CFBC equipped with FF than that in the PC units when sub-bituminous coal was burned. Lower mercury emission rates in electric utility boilers equipped with FF are due to the active fly ash generated with a larger specific surface area and pore volume. Higher mercury emission rates observed during lignite-fired boilers may be due to their lower specific area of fly ash, which results from lower LOI, as well as the pore blockage by selenium (Se) for Texas lignite; and sodium (Na) and potassium (K) for North Dakota lignite. There is no significant mutual benefit for the mercury captured by the addition of Spray Dry Absorber (SDA) or selective non-catalytic reduction (SNCR) in the CFBC system.

Expansion of high flow mixtures of gypsum-bonded investments in contact with absorbent liners

To evaluate the expansion of high flow mixtures of gypsum-bonded investments in contact with two dry absorbent liners since early setting. Cristobalite-Dentsply and Cristobalite-Polidental gypsum-bonded investments were tested under 10 setting conditions (n=4): Normal and Hygroscopic (controls); four conditions of 'Dry-Lining' (set in contact with 3 different amounts of dry cellulose liner: 0.2, 0.4 or 0.8 g and with 0.1 g of a granulated Super Absorbent polymer liner) and four conditions of 'Dry-Lining with hygroscopic expansion' (initially setting in contact with dry absorbents, as 'Dry-Lining' conditions, followed by water immersion immediately after loss of gloss). The percent of expansion at 2h was analyzed. The 'Dry-Lining' condition, with 0.4 and 0.8 g of cellulose, for brand Dentsply (1.41+/-0.13 and 1.42+/-0.16) and Polidental (1.14+/-0.17 and 1.01+/-0.22) allowed a significant higher expansion than the controls Normal (Dentsply=0.30+/-0.04, Polidental=0.31+/-0.05) (P<0.05). The 'Dry-Lining with hygroscopic expansion' condition with 0.8 g of cellulose expanded significantly more (3.57+/-0.55) than the control Hygroscopic (2.63+/-0.31) in brand Dentsply (P<0.05). Super Absorbent did not produce different expansion (Dentsply=0.86+/-0.02, Polidental=0.79+/-0.18) than the controls Normal and Hygroscopic (Dentsply=2.93+/-0.17, Polidental=2.12+/-0.40). The water removal with dry cellulose absorbent liner before loss of gloss increased the potential of setting expansion for Normal and Hygroscopic conditions. Thus, when using high fluid mixtures, which are more desirable for investment pouring, it will be possible to obtain enough setting expansion potential in order to adequately compensate for wax and alloy shrinkage in the casting process.

Carbon monoxide production from sevoflurane breakdown: modeling of exposures under clinical conditions.

Isoflurane, enflurane, sevoflurane, and especially desflurane produce carbon monoxide (CO) during reaction with desiccated absorbents. Of these, sevoflurane is the least studied. We investigated the dependence of CO production from sevoflurane on absorbent temperature, minute ventilation (VE), and fresh gas flow rates. We measured absorbent temperature and in vitro CO concentrations when desiccated Baralyme reacted with 1 minimum alveolar anesthetic concentration of (2.1%) sevoflurane at 2.3-, 5.0-, and 10.0-L VE. Mathematical modeling of carboxyhemoglobin concentrations was performed using an existing iterative method. Rapid breakdown of sevoflurane prevented the attainment of 1 minimum alveolar anesthetic concentration with low fresh gas flow rates. CO concentrations increased with VE and with absorbent temperatures exceeding 80 degrees C, but concentrations decreased with higher fresh gas flow rates. Average CO concentrations were 150 and 600 ppm at 2.3- and 5.0-L VE; however, at 10 L, over 11,000 ppm of CO were produced followed by an explosion and fire. Methanol and formaldehyde were present and may have contributed to the flammable mixture but were not quantitated. Mathematical modeling of exposures indicates that in average cases, only patients < or =25 kg, or severely anemic patients, are at risk of carboxyhemoglobin concentrations >10% during the first 60 min of anesthesia. Sevoflurane breakdown in desiccated absorbents is expected to result in only mild carbon monoxide (CO) exposure. Completely dry absorbent and high minute ventilation rates may degrade sevoflurane to extremely large CO concentrations. Serious CO poisoning or spontaneous ignition of flammable gases within the breathing circuit are possible in extreme circumstances.

Low-flow and minimal-flow anesthesia with sevoflurane

Due to its low solubility and the high maximum concentration delivered by the vaporizer sevoflurane is especially suitable for the performance of low flow anaesthetic techniques. High flow phases for wash-in or wash-out of anaesthetic gases can be kept short, the difference between the volatile's concentration in the fresh gas and within the breathing system is comparatively small, and the time constants are short even during low flow anaesthesia. The monitoring, required to sufficiently ensure the safety of the patients, corresponds to the current obliging technical safety standards. As compound A may accumulate in the breathing system, sevoflurane should not be administered with fresh gas flows lower than 1.0 l/min, until the scientific discussion on nephrotoxicity of this substance in humans is solved. Low flow anaesthesia guarantees a sufficient and continuous wash-out of trace gases. Thoroughly the use of sevoflurane with dry soda lime must be avoided, as this volatile in an extreme exothermic reaction is absorbed nearly totally and degraded to a considerable degree by dry carbon dioxide absorbent. The gaseous degradation products are pungent and possibly may be harmful to the patients. Only by low flow anaesthesia the use of sevoflurane will gain an economically and ecologically acceptable range of efficiency.

Mechanistic aspects of carbon monoxide formation from volatile anesthetics.

Desflurane, enflurane and isoflurane can be degraded to carbon monoxide (CO) by carbon dioxide absorbents, whereas sevoflurane and halothane form negligible amounts of CO. Carbon monoxide formation is greater with drier absorbent, and with barium hydroxide, than with soda lime. The mechanism, role of absorbent composition and water, and anesthetic structures determining CO formation are unknown. This investigation examined sequential steps in anesthetic degradation to CO. Carbon monoxide formation from anesthetics and desiccated barium hydroxide lime or soda lime was determined at equimole and equiMAC concentrations. Carbon monoxide formation from deuterium-substituted anesthetics was also quantified. Proton abstraction from anesthetics by strong base was determined by deuterium isotope exchange. A reactive chemical intermediate was trapped and identified by gas chromatography-mass spectrometry. The source of the oxygen in CO was identified by 18O incorporation. Desflurane,enflurane,andisoflurane(difluoromethylethyl ethers), but not sevoflurane (monofluoromethyl ether), methoxyflurane (methy-ethyl ether), or halothane (alkane) were degraded to CO. The amount of CO formed was desflurane > or = enflurane > isoflurane at equiMAC and enflurane > desflurane > isoflurane at equimole concentrations. Proton abstraction from the difluoromethoxy carbon was greater with potassium than with sodium hydroxide, but unmeasurable with barium hydroxide. Carbon monoxide formation was correlated (r = 0.95-1.00) with difluoromethoxy (enflurane > desflurane > isoflurane > or = methoxyflurane = sevoflurane = 0) but not ethyl carbon proton abstraction. Deuterium substitution on enflurane and desflurane diminished CO formation. Chemical trapping showed formation of a difluorocarbene intermediate from enflurane and desflurane. Incorporation of H2(18)O in barium hydroxide lime resulted in C18O formation from unlabeled enflurane and desflurane. A difluoromethoxy group is a structural requirement for haloether degradation to CO. Results are consistent with initial base-catalyzed difluoromethoxy proton abstraction (potassium > sodium hydroxide, thus greater CO formation with barium hydroxide lime vs. soda lime) forming a carbanion (reprotonated by water to regenerate the anesthetic, hence requirements for relatively dry absorbent), carbanion decomposition to a difluorocarbene, and subsequent difluorocarbene reaction to form CO.

Low and minimal flow inhalational anaesthesia.

To describe the pharmacokinetic behaviour and practical aspects of low (0.5-1 l.min-1) and minimal (0.25-0.5 l.min-1) flow anaesthesia. A Medline search located articles on low flow anaesthesia, and computer simulated anaesthetic uptake models are used. Most, 85-90%, of anaesthetists use high fresh gas flow rates during inhalational anaesthesia. Low/minimal flow anaesthesia with a circle circuit may avoid the need for in-circuit humidifiers, raise the temperature of inspired gases by up to 6 degrees C, reduce cost by about 25% by reduction of fresh gas flows to 1.5 l.min-1, and reduce environmental pollution with scavenged gas. Knowledge of volatile anaesthetic pharmacokinetic behaviour facilitates the use of minimal/low flow rates. Small amounts of nitrogen or minute amounts of methane, acetone, carbon monoxide, and inert gases in the circuit are of no concern, but the degradation of desflurane (to carbon monoxide by dry absorbent) and sevoflurane (to compound A by using a fresh gas flow of > 2 l.min-1) must be avoided. With modern gas monitoring technology, safety should be no more of a concern than with high flow techniques. The use of fresh gas flow rates of < 1 l.min-1 for maintenance of anaesthesia has many advantages, and should be encouraged for inhalational anaesthesia with most modern volatile anaesthetics.

Rehydration of desiccated Baralyme prevents carbon monoxide formation from desflurane in an anesthesia machine.

Desiccated carbon dioxide absorbents degrade desflurane, enflurane, and isoflurane to carbon monoxide (CO) in vitro and in anesthesia machines, which can result in significant clinical CO exposure. Carbon monoxide formation is highest from desflurane, and greater with Baralyme than with soda lime. Degradation is inversely related to absorbent water content, and thus the greatest CO concentrations occur with desflurane and fully desiccated Baralyme. This investigation tested the hypothesis that rehydrating desiccated absorbent can diminish CO formation. Baralyme was dried to constant weight. Carbon monoxide formation from desflurane and desiccated Baralyme was determined in sealed 20.7-ml vials without adding water, after adding 10% of the normal water content (1.3% water), and after adding 100% of the normal water content (13% water) to the dry absorbent. Similar measurements were made using an anesthesia machine and circle system. Carbon monoxide was measured by gas chromatography-mass spectrometry. Carbon monoxide formation from desflurane in vitro was decreased from 10,700 ppm with desiccated Baralyme to 715 ppm and less than 100 ppm, respectively, when 1.3% and 13% water were added. Complete rehydration also decreased CO formation from enflurane and isoflurane to undetectable concentrations. Desflurane degradation in an anesthesia machine produced 2,500 ppm CO in the circuit, which was reduced to less than 180 ppm when the full complement of water (13%) was added to the dried absorbent. Desflurane is degraded by desiccated Baralyme in an anesthesia machine, resulting in CO formation. Adding water to dried Baralyme is an effective means of reducing CO formation and the risk of intraoperative CO poisoning. Although demonstrated specifically for desflurane and Baralyme, rehydration is also applicable to enflurane and isoflurane, and to soda lime.

The response of anesthetic agent monitors to trifluoromethane warns of the presence of carbon monoxide from anesthetic breakdown.

Trifluoromethane and CO are produced simultaneously during the breakdown of isoflurane and desflurane by dry CO2 absorbents. Trifluoromethane interferes with anesthetic agent monitoring, and the interference can be used as a marker to indicate anesthetic breakdown with CO production. This study tests representative types of gas monitors to determine their ability to provide a clinically useful warning of CO production in circle breathing systems. Isoflurane and desflurane were reacted with dry Baralyme at 45 degrees C. Standardized samples of breakdown products were created from mixtures of reacted and unreacted gases to simulate the partial degrees of reaction which might result during clinical episodes of anesthetic breakdown using 1% or 2% isoflurane and 6% or 12% desflurane. These mixtures were measured by the monitors tested, and the indication of the wrong agent or a mixture of agents due to the presence of trifluoromethane was recorded and related to the CO concentration in the gas mixtures. When presented with trifluoromethane from anesthetic breakdown, monochromatic infrared monitors displayed inappropriately large amounts of isoflurane or desflurane. Agent identifying infrared and Raman scattering monitors varied in their sensitivity to trifluoromethane. Mass spectrometers measuring enflurane at mass to charge = 69 were most sensitive to trifluoromethane. Monochromatic infrared monitors were unable to indicate anesthetic breakdown via interference by trifluoromethane, but did indicate falsely elevated anesthetic concentrations. Agent identifying infrared and Raman monitors provided warning of desflurane breakdown via the interference of trifluoromethane by displaying the wrong agent or mixed agents, but may not be sensitive enough to warn of isoflurane breakdown Some mass spectrometers provided the most sensitive warnings to anesthetic breakdown via trifluoromethane, but additional data processing by some patients monitor units reduced their overall effectiveness.

Degradation of halothane, enflurane, and isoflurane by dry soda lime to give carbon monoxide

In the presence of completely dry soda lime volatile anaesthetics will decompose to carbon monoxide (CO). In an in vitro study, the absorbent (soda lime, ICI) was dried with a constant gas flow of 11/min oxygen for 120 h. The weight loss during the drying was 17.1%. Two vol% of halothane, enflurane or isoflurane in oxygen was administered with a constant flow of 0.51/min oxygen through the completely dry absorbent. Concentrations of gases were measured before and after the absorbent using mass spectrometry (MGA 1100, Perkin-Elmer) and an electrochemical NO monitor (Mini PAC CO, Dräger). The temperature inside the soda lime was monitored continuously. Shortly after adding the anaesthetic to the oxygen passing through the absorbent, carbon monoxide appeared in the outlet of the soda lime container. The measured peak concentrations varied around 450 ppm (halothane), 3500 ppm (enflurane) and 3800 ppm (isoflurane). The temperature inside the absorbent rose from the ambient temperature (19.8 degrees C) to a maximum of 52.1 degrees C during CO production and decreased when the CO production lowered after approximately 1 h (all anaesthetics). During CO production no measurable concentration of halothane left the absorber. After passing through the absorbent the concentrations of isoflurane and enflurane were slightly lower than the corresponding concentrations in the fresh gas measured before absorption.

New inhalation anesthetics

Recently, two new halogenated volatile anaesthetics, sevoflurane and desflurane, have been approved for clinical use in Germany. Their low solubility in blood is the most important common property, and this represents the most obvious difference from the inhalational anaesthetics currently used. Extensive clinical and experimental evaluations have confirmed the superior pharmacokinetic properties predicted. Both sevoflurane and desflurane provide more rapid emergence from anaesthesia, permit easier titration of the anaesthetic dose during maintenance and offer more rapid recovery from anaesthesia. For sevoflurane, there are additional advantages: a pleasant odor, negligible airway irritation, and excellent pharmacodynamic characteristics that even provide cardiovascular stability comparable to isoflurane. A certain disadvantage and source of potential nephrotoxicity result from the metabolism of sevoflurane (2-5%) to anorganic fluoride and degradation to compound A in carbon dioxide absorbents. The extensive clinical data reported to date have revealed no evidence that sevoflurane has adverse renal effects. New insight into the pathomechanism of nephrotoxicity associated with either production of fluoride or compound A may well support clinical experience. Desflurane strongly resists in vivo metabolism and because of this it appears to be devoid of toxicity. Nevertheless, potential side-effects may result from degradation in dry absorbents and subsequent release of CO, from its extreme pungency and irritating airway effects. Thus, desflurane is not recommended for induction of anaesthesia, especially in children. The tendency for desflurane transiently to stimulate sympathetic activity, especially at concentrations above 1.0 MAC, limits its application in patients with cardiac disease.