Diffusive Airflow Research Articles

Estimating air diffusion contribution to pressure decay during membrane integrity tests

A set of bench-scale experiments were conducted to estimate the contribution of air diffusion to pressure decay in an intact immersed membranes. The apparatus consisted of a pressurized vessel, an air compressor, a pressure regulator and an inverted water-filled burette for measuring the volume of diffused air. The results obtained from air diffusion measurements allowed for consistent determination of membrane’s true bubble-point pressure and the size of the largest pore. The results also indicated that air flowrate due to diffusion through an intact wetted membrane is relatively constant for a specific membrane with the same surface area and water temperature. Furthermore, it was demonstrated that the pressure decay in an intact membrane was primarily due to air diffusion through water-filled pores. Therefore, the dilution effect observed during pressure decay tests on membranes with large surface area can be mainly attributed to air diffusion through intact pores. Based on the experimental results a mathematical model was developed that estimates the amount of air diffusion through an intact wetted membrane as a function of applied pressure. The model also estimates the contribution of the diffusive airflow to pressure decay test. It is proposed that subtracting this contribution from the measured pressure decay tests can lead to more sensitive tests and better estimates of true log reduction values for microorganisms. This paper presents the development and verification of the mathematical model for estimating diffusion air flowrate through an intact ZeeWeed®500 membrane.

Estimating air diffusion contribution to pressure decay during membrane integrity tests

A set of bench-scale experiments were conducted to estimate the contribution of air diffusion to pressure decay in an intact immersed membranes. The apparatus consisted of a pressurized vessel, an air compressor, a pressure regulator and an inverted water-filled burette for measuring the volume of diffused air. The results obtained from air diffusion measurements allowed for consistent determination of membrane’s true bubble-point pressure and the size of the largest pore. The results also indicated that air flowrate due to diffusion through an intact wetted membrane is relatively constant for a specific membrane with the same surface area and water temperature. Furthermore, it was demonstrated that the pressure decay in an intact membrane was primarily due to air diffusion through water-filled pores. Therefore, the dilution effect observed during pressure decay tests on membranes with large surface area can be mainly attributed to air diffusion through intact pores. Based on the experimental results a mathematical model was developed that estimates the amount of air diffusion through an intact wetted membrane as a function of applied pressure. The model also estimates the contribution of the diffusive airflow to pressure decay test. It is proposed that subtracting this contribution from the measured pressure decay tests can lead to more sensitive tests and better estimates of true log reduction values for microorganisms. This paper presents the development and verification of the mathematical model for estimating diffusion air flowrate through an intact ZeeWeed ®500 membrane.

Development of a new integrity testing system

As membrane filtration systems are more commonly used in the water treatment industry, the call for a reliable, fast and on-line integrity testing system becomes more urgent. Especially where membrane filtration is used in potable water production for the removal of pathogenic microorganisms, the integrity of such a system is of the utmost importance. Membrane integrity testing can be performed in a number of ways, where the pressure-hold or vacuum-hold test and the diffusive airflow test are well known. Although relatively widely applied in membrane plants worldwide, all of these methods exhibit major drawbacks like off-line testing and the lack of a direct relationship between the measured data and the removal efficiency (log-removal). To overcome the above mentioned drawbacks of conventional integrity testing systems, a new kind of integrity test, the Spiked Integrity Monitoring System or SIM®-System was developed by NORIT Membrane Technology in close co-operation with Water Supply Company North Holland and IWW Rhenish-Westphalian Institute for Water Research, which combines the accuracy of a challenge test with the speed of a pressure test, while keeping the system under test in operation.

The issue of dynamic hyperinflation in acute respiratory distress syndrome patients.

Dynamic hyperinflation is produced by a diffuse expiratory flow limitation impairing exhalation under mechanical ventilation. It constitutes a serious clinical problem in patients exhibiting bronchial asthma or chronic obstructive pulmonary disease, when mechanical ventilation is required. But this phenomenon may also complicate respiratory support in acute respiratory distress syndrome (ARDS) patients. The presence of diffuse airflow limitation in ARDS patients has sometimes been noticed in the past, but its consequences have only recently been emphasised in a dynamic study, using bedside recording of flow/volume loops during respiratory support. More recently, by recording systematically a prolonged expiration, the current authors have observed a localised airflow limitation in a majority of ARDS patients, constituting another potential factor of dynamic hyperinflation under respiratory support. In the same report, the current authors' have emphasised the impact of this limitation on the shape of the pressure/volume loop. At a time when increasing respiratory support is proposed to improve carbon dioxide clearance in acute respiratory distress syndrome submitted to protective ventilation, dynamic hyperinflation may become a major clinical problem in this setting.

Effects of snow and firn ventilation on sublimation rates

AbstractVapor transport in snow and firn plays a key role in post-depositional changes of many reactive species, in mass and energy balance of large snow-covered areas, and in metamorphic changes of the crystal structure of the snow itself. While conventional estimates of vapor transport and sublimation rate are based on diffusion of water vapor from the snow surface to the atmosphere, ventilation (airflow through interstitial pore spaces) in snow and firn can affect the top several meters of firn, yet the effects of ventilation on sublimation rates in firn have not been previously investigated. In this paper we present the first calculations of sublimation rates due to advective/ diffusive airflow and vapor transport in the snow and firn. Airflow velocities, vapor transport and sublimation rates are calculated using a two-dimensional finite-element model. the airflow patterns induce regions of sublimation and regions of condensation within the snow and firn. Because small surface sastrugi move in time as the snow surface profile changes due to wind redeposition, the subsurface flow and regions of condensation and sublimation will also likely change on a time-scale of days to weeks. However, if the roughness features are such that they move very little over time, it is likely that the regions of condensation and sublimation will have a noticeable effect on the microstructure of the firn. While the highest mass-transfer rates occur near the surface, the depth of vapor transport and phase change depends on firn properties, wavelength of the snow surface roughness, and temperature.

Assessing pathogen removal efficiency of microfiltration by monitoring membrane integrity

This study was conducted to investigate the ability of various methods of monitoring membrane integrity to respond to changes in actual membrane integrity imposed by the compromised fibers within the microfiltration unit. In addition, the pilot-scale MF unit was challenged with high concentrations of coliform, Cryptosporidium, and spore, in order to assess the pathogen removal capability of microfiltration. A correlation between the integrity tests and microbial challenge data was also made. The integrity tests investigated in this study were pressure decay and diffusive air flow tests (direct integrity tests), and turbidity and particle counting (indirect integrity tests). Both pressure decay (PDT) and diffusive air flow (DAF) tests were sensitive enough to detect one damaged fiber out of 66,000. The extent of fouling did not affect the sensitivity of the PDT and DAF, showing that PDT and DAF tests are a simple, reliable means to monitor membrane integrity under field conditions. Indirect integrity monitoring using turbidity and particle counting, however, responded poorly to changes in membrane integrity. Microbial challenge study demonstrated that microfiltration was capable of removing various pathogens including Cryptosporidium, at the level required by drinking water regulations, under even adverse operating conditions. Finally, PDT and DAF tests showed a better correlation with actual microbial removal efficiency of microfiltration than turbidity and particle counting. The turbidity and particle counting grossly underestimated the removal of pathogen larger than MF membrane pore size due to poor sensitivity.

Demonstrating the integrity of a large scale microfiltration plant using a Bacillus spore challenge test

For protection against Cryptosporidium and Giardia New Zealand drinking water regulations established a goal of removal of all particles of 1μm and larger. The City of Tauranga, New Zealand, commissioned a 36 ML/d (9.5 mgd) microfiltration (MF) plant, the Joyce Road Water Processing Plant (JRWPP), to meet this stringent reduction goal. The plant consists of ten USF Memcor 90M10C units. This paper details a bacillus spore challenge test conducted on the JRWPP. The objectives were to (1) confirm that the plant could achieve the required 5 log reduction of particles, and (2) evaluate direct and indirect integrity testing methods as means to confirm log-5 particle reduction performance over the life of the plant. The test used Bacillus megaterium spores dosed into the feed water with the plant operated to maximise the challenge on the MF hollow fibre membranes. The results showed (1) the ability of an integral MF plant to reject Bacillus megaterium spores and achieve at least log 5.9 reduction in organisms; (2) Bacillus spores are a good medium for testing as they are robust, easy to handle, cultured in large quantities, and easily identified; (3) the spore challenge test cannot be economically used to monitor full-scale plant performance; (4) particle counting results correlated poorly with bacillus counts in feed or filtrate; and (5) the USF Memcor integrity test methods, which included a pressure hold test and a diffusive airflow test, underestimated spore rejection.

Estimation of cleanup time in layered soils by vapor extraction

Soil vapor extraction (SVE) is a standard remediation technique for cleaning up soils contaminated by volatile organic compounds (VOCs). A key parameter for planning SVE operations is the time required to reach the desired cleanup standard. In this paper, an approximate analytical solution is developed, which allows the fast estimation of cleanup times in layered unsaturated zones. The contaminants are assumed to be dissolved in the pore water, sorbed on the soil matrix and mixed with the soil air. Liquid organic phase is absent. For partitioning between gas and water and water and solid, local-equilibrium is assumed. The analytical solution is based on the well mixed reservoir model and on the plug flow model. It is shown that, for a number of scenario cases, the results of the analytical solution are, for practical purposes, in reasonable agreement with a numerical solution of the partial differential equations for the local-equilibrium advection–dispersion model of mass transport in porous media by Fickian diffusion and Darcian air flow. The results are displayed in terms of Peclet number of molecular diffusion (PNMD). In the analytical solution three different approximations are used. The PNMD range is divided into three intervals, representing different transport regimes. At low PNMD, molecular diffusion dominates transport in both layers. In this interval cleanup time is estimated by the average of the plug flow time for one pore volume through the layer of higher permeability, and the cleanup time estimated by the mixed reservoir model. At intermediate PNMD values, advective transport dominates in the more permeable layer and molecular diffusion in the less permeable. Consequently, cleanup time is limited by diffusive mass transfer from the less to the more permeable layer. In this interval, the estimation of cleanup time is entirely based on the mixed reservoir model. At high PNMD values, transport is governed in both layers by advection. Here, cleanup time is estimated by applying the plug flow model to the layer of lower permeability. The range of soil permeability values, for which such high PNMD values can be achieved, as well as the range of the actual air flow rates, are identified. The limitations of the validity of local-equilibrium assumption at high PNMD values are discussed using the relationship between desorption rates and gas advection fluxes. Further, the sensitivity of cleanup time estimates with respect to the independent dimensionless parameters characterizing the SVE configuration considered, is studied.

Predicting log removal performance of membrane systems using in-situ integrity testing

Two of the most sensitive methods currently available for monitoring membrane integrity are the pressure decay test (PDT) and diffusive air flow (DAF) tests. Both these tests are capable of monitoring small changes in membrane integrity, even in very large membrane arrays. The DAF test, for example, is capable of detecting a single broken fibre in a membrane array containing over one million fibres. This enables operators to control membrane integrity at levels well above 4 log removal of Giardia or Cryptosporidium, without relying on filtered water quality monitors such as turbidity or particle counters. The tests form part of an overall membrane control strategy known as the Memsure™ process, involving process integrity monitoring, and identification and isolation of faulty modules for later repair.

Automatic monitoring of membrane integrity in microfiltration systems

Two in-situ testing methods developed by Memcor have been shown to provide a sensitive measure of system integrity for microfiltration membranes. These are the pressure decay test (PDT) and diffusive air flow test (DAF). The tests enable operators to measure the log removal capability of membrane systems directly, rather than relying solely on filtered water quality monitors such as turbidity and particle counters. The DAF test, for example, is capable of detecting a single broken fibre in a membrane array containing over one million fibres. This paper discusses the principles and application of the PDT and DAF methods to the monitoring and control of membrane system integrity.

Factors Affecting the Performance of Diffused-Air Aeration Systems for Aquaculture

A diffused-air aeration system which operated at low gauge pressure (1.1 to 4.3 m H20) was operated with six or twelve diffusers at five depths between 0.91 and 3.35 m and six air flow rates between 28 and 255 l/minute. The standard oxygen transfer rate (SOTR) or amount of oxygen transferred per hour increased markedly with increasing diffuser depth and air flow rate, but the power required to deliver air to the diffusers also increased as air flow rates and diffuser depths increased. Best standard aeration efficiency (SAE) values (2.0 to 2.5 kg O2/kW·hr) were obtained at air flow rates of 28 and 57 1/minute. With six diffusers and 28 and 57 air/minute, peak SAE occurred at 2.13 and 2.74 m, respectively. With twelve diffusers, peak SAE was achieved at 0.91 m for 28 1 air/minute and at 2.74 m for 57 1 air/minute. In order to efficiently deliver air into water at a high rate, a large number of diffusers is needed.

Diffuser spacing simplified

An approach is suggested for simplifying the choice of correct spacing for ceiling diffusers. The method depends on satisfying an air change rate condition and a throw condition simultaneously. The air change rate condition is defined by the diffuser layout and ceiling height, expressed as a graph of diffuser spacing versus air flow per diffuser. The throw condition is the criterion for satisfactory air movement in the occupied space expressed as a relationship between throw and the distance to the next diffuser. It is envisaged that air change rate graphs could be drawn up for common diffuser layouts and ceiling heights. Diffuser throw data could then be made available as an overlay on transparent film so that the consequences of differing diffuser options would easily be ascertained by superposition of the throw condition onto graphs of constant air change rate.

Viscous slipstream flow downstream of a centerline Mach reflection

Viscous slipstream flow downstream of triple shock wave intersection in supersonic diffuser air flow, using Pitot and static pressure probe measurements