OXYGEN TRANSFER IN HORIZONTAL PIPELINE AERATORS WITH NOZZLES

Horizontal pipeline and tubular loop aerators are of interest for fermentation and waste water treatment and are ideally suited for continuous processing. A major drawback is that these pipeline contactors invariably operate in the “elongated bubble and plug” regime in which the mass transfer rate is low. This article evaluates the performance of a horizontal pipeline aerator fitted with nozzles equispaced along its length to enhance mass transfer rates by promoting turbulence and augmenting effective interfacial area. Such devices can also be advantageously used in long pipe lines as in the case of treating waste while it is being transported. Pressure drop and overall liquid-side mass transfer coefficient data are reported as functions of liquid (water) and gas flow rates and nozzle size and spacing. It is shown that for all the conditions studied, k_La = 0.026(ΔP/L)^1.036 and that the pressure gradient is given by a simple correlation, provided an empirical parameter which characterises a nozzle is known. Preliminary investigations on the effect of surfactant ad the presence of suspended solids (size 75 μm) on mass transfer coefficient are also reported. Very high values of power dissipation can be achieved in such aerators without mechanically moving parts and high values of mass transfer coefficient can be realized.     

导流管型鼓泡塔中牛顿型和非牛顿型液体的氧传递系数

在塔径分别为φ0.188m和φ0.112m各三组不同直径导流管的中心区充气塔中，测定水和CMC溶液的氧传递系数并提出关联算式.讨论中心区充气和环隙区充气塔的不同管径比Di／D0对氧传递系数HLa和液体循环的影响.无论在空气－水或空气－CMC溶液体系，结构参数Di／D0值在0.6左右，液体循环时间和KLa值为最小     

MICROBUBBLES FOR ENHANCEMENT OF OXYGEN TRANSFER IN XANTHAN GUM FERMENTATION

This study investigates the use of microbubble sparging for enhancing oxygen transfer and product yield during xanthan gum fermentation. Fermentation runs with full air sparging and partially substituted microbubble sparging were compared for the parameters biomass, xanthan, oxygen uptake, and energy levels. Microorganisms were found compatible at numerous Tween-20 levels and further showed considerable resistance to shear conditions in the microbubble generator. Microbubbles with a size of about 145 μm, gas hold up of 65%, and foam stability of about 3 min. resulted at standardized microbubble generator operating conditions (8000 rpm, 2 min., 300 ppm surfactant). A comparison of both methods showed that partially substituted microbubble sparging increased oxygen uptake by 50%, final biomass levels by 17%, and final xanthan gum yield by 30%. Results indicate that X. campestris is amenable to conditions encountered within a microbubble generator and that microbubble sparging improved oxygen transfer and xanthan gum yield without affecting gum quality.     

MICROBUBBLES FOR ENHANCEMENT OF OXYGEN TRANSFER IN XANTHAN GUM FERMENTATION

This study investigates the use of microbubble sparging for enhancing oxygen transfer and product yield during xanthan gum fermentation. Fermentation runs with full air sparging and partially substituted microbubble sparging were compared for the parameters biomass, xanthan, oxygen uptake, and energy levels. Microorganisms were found compatible at numerous Tween-20 levels and further showed considerable resistance to shear conditions in the microbubble generator. Microbubbles with a size of about 145 μm, gas hold up of 65%, and foam stability of about 3 min. resulted at standardized microbubble generator operating conditions (8000 rpm, 2 min., 300 ppm surfactant). A comparison of both methods showed that partially substituted microbubble sparging increased oxygen uptake by 50%, final biomass levels by 17%, and final xanthan gum yield by 30%. Results indicate that X. campestris is amenable to conditions encountered within a microbubble generator and that microbubble sparging improved oxygen transfer and xanthan gum yield without affecting gum quality.     

OXYGEN TRANSFER IN THE WATER-JET VESSEL

This is an investigation of the gas holdup and the volumetric mass transfer coefficient of a plunging water jet in an air-water system. We sound k_La to be directly proportional to gas holdup in two regions. For the first time, this has been clarified in the plunging liquid jet system. The volumetric mass transfer coefficient and the gas holdup have been well correlated in terms of the Froude number, liquid jet length, nozzle diameter and vessel diameter.     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

LIQUID-SOLID MASS TRANSFER AT HORIZONTAL WOVEN SCREENS WITH UPWARD COCURRENT GAS-LIQUID FLOW

Rates of liquid-solid mass transfer at horizontal single screens and an array of horizontal parallel separated screens were studied under upward cocurrent gas (N₂)-liquid bubbly flow using the electrochemical technique. Variables studied were gas and liquid flow rates, and screen characteristics (e.g., mesh number and wire diameter)

Under the present conditions where relatively low solution flow rates were used the rate of mass transfer was found to be mainly determined by the gas flow rate. For a given gas flow rate, the mass transfer coefficient decreased with increasing solution flow rate. The data for single screen were correlated with a dimensionless equation. Rates of mass transfer at an array of separated horizontal screens were lower than those at the single screen by an amount ranging from 3 to 45% depending on screen mesh number and flow conditions. The importance of the present study for building continuous high space time yield catalytic, and electrochemical reactors suitable for electrochemical air pollution control is highlighted.     

FREE CONVECTION MASS TRANSFER AT HORIZONTAL CYLINDERS

Natural convection mass transfer at horizontal cylinders was studied using an electrochemical technique involving the deposition of copper from acidified copper sulphate solution. Cylinder diameter and copper sulphate concentrations were varied to provide a range of Sc. Gr from 1.55 × 10¹⁰ to 2 × 10¹². Under these conditions, the data were correlated by the equation:

Measurement of mass transfer distribution revealed the fact that mass is transferred at the lower hemicylinder by a laminar flow mechanism and at the upper hemicylinder by a turbulent flow mechanism.     

MOBILIZATION OF SLUDGE IN HORIZONTAL CYLINDRICAL TANKS WITH SUBMERGED NOZZLES

Over 378,000 m³ (100 million gal) of radioactive waste is being stored in hundreds of tanks at several U.S. Department of Energy (DOE) sites. The environment within the tanks is highly radioactive and chemically harsh. The waste typically consists of a heterogeneous sludge layer covered by a layer of supernatant. A few of the tanks have leaked to the environment, while others are corroding. Removing the waste from the tanks and processing it to a stable final form are desired to prevent any additional contamination of the environment. The tanks vary in size and geometric shape and do not have systems for removing the sludge waste.

Mobilization and mixing studies were conducted with a surrogate sludge (e.g., kaolin clay) using submerged jets in two sizes of horizontal tanks with nominal capacities of 0.87 m³ (230 gal) and 95 m³ (25,000 gal). Mobilization efficiencies and mixing times were determined for single and bidirectional jets in both tanks with the discharge nozzles positioned at various locations in the tanks. Approximately 80% of the surrogate sludge was mobilized in the 95-m³ tank using a fixed bidirectional jet (inside diameter = 0.035 m) and a jet velocity of 6.4m/s (21 ft/s). The effective cleaning length, which is defined as the distance between the jet and the edge of the remaining sludge bank, was approximately 3.2 m (10 ft) on each side of the bidirectional jet.     

环流反应器中肌苷发酵液的氧传递研究

根据肌苷发酵过程主要是溶氧传质控制的结论,本文研究了操作变量和装置结构变量对环流发酵反应器中氧传递的影响,定量地得到气含率,液体循环速度和结构因素对氧传递的关系。对发酵反应器中溶氧浓度分布进行了摸拟计算,依据计算的结果提出了较佳的 D_E/D 和 L_E/D 的结构。     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.     

MASS AND MOMENTUM TRANSFER WITH NON-NEWTONIAN FLUIDS IN FLUIDIZED BEDS

A new model for non-Newtonian fluid flows through fluidized beds is developed by extending the Richardson-Zaki concept. In the proposed model, the existing correlations for an isolated single sphere are simply extended to those for a fluidized bed by incorporating a voidage function. The model is found to be in a good agreement with other correlations for flows of a non-Newtonian fluid through multi-particle systems. The model is also applied to interpret mass transfer in fluidized beds.     

CONVECTIVE MASS TRANSFER WITH PULSATILE FLOW IN STRAIGHT RIGID TUBES

This work is a numerical study of mass transport with laminar pulsatile flow through a straight rigid tube with no backflow. Convective oxygen transport is analyzed for a reactive fluid (blood) and nonreactive fluids. The literature contains several experimental and theoretical studies of this problem; however, the conclusions drawn from these investigations are contradictory. The results of this study can be used to explain many of these contradictions.     

ON THE METHODS FOR THE DETERMINATION OF THE OXYGEN TRANSFER COEFFICIENT IN MECHANICALLY AGITATED VESSELS

This paper reports on the determination of the overall oxygen transfer coefficient in mechanically agitated vessels. Many variants of the sulfite oxidation method are compared with the dynamic method. A new variant of the sulfite oxidation method, called the reaction time method, is proposed. Overall oxygen mass transfer coefficients were obtained with two probes having significantly different time constants and for various agitation levels. The advantages and disadvantages of the main methods used for the determination of the oxygen transfer coefficient are reviewed and discussed.     

NATURAL CONVECTION MASS TRANSFER INSIDE HORIZONTAL SQUARE DUCTS

Rates of free convection mass transfer inside horizontal square ducts were measured by electrochemical technique. Physical properties of the solution and duct side length were changed to provide Sc.Gr range of 7.38 × 10⁸ - 1.4 × 10¹¹. Under these conditions the data were correlated by the equation Sh= 0.382(Sc.Cr)^0.278 A comparison between the measured overall rate of mass transfer with that calculated by summing the rates of mass transfer at the separate sides of the duct showed that the measured value is higher than the calculated value owing to convective interaction between adjacent surfaces.     

MASS TRANSFER IN RECIRCULATING BLOOD FLOW

An experimental investigation was done on oxygen transfer into laminar flows of whole blood and saline downstream of an abrupt pipe expansion. This was used as the experimental model for regions of separated flow in the arterial system. The flow was fully-developed prior to the tube expansion with Reynolds numbers between 160 and 850. The results showed high transfer rates over most of the region downstream of the step, with maximum transfer occurring near the reattachment point. In addition, oscillations of the transfer rate were measured at the larger Reynolds numbers of 450 and 850.     

环流反应器和搅拌槽传质特性的分析

本文测定了提升管为单段和提升管分为三段的气升式环流反应器中的液相体积氧传质系数，在螺带式搅拌桨区应器中测定了液相体积氧传质系数和功率消耗，实验体系为模拟生物发酵液非牛顿流体特性的核甲基纤维素水溶液，本文还从液相体积氧传质系数及单位液相体积的功耗所产生的氧传质效果方面对提升管不分段和提升管分为三段的气升式环流反应器与螺带式搅拌反应器进行了比较。     

MASS TRANSFER IN GAS-SPARGED POROUS ELECTRODES

The electrochemical limiting current method was used to measure mass transfer coefficients with cocurrent upward gas-liquid flow in packed bed electrodes. Liquid-solid and overall gas-solid coefficients were determined from limiting current data in the presence of inert or reactive gas flows with the use of a plug flow model. The presence of inert gas flow increased mass transfer coefficients over those in single-phase flow by a factor of up to 1.7. As a result of boundary layer penetration by gas bubbles, sparging with reactive gas increased mass transfer rates by a factor of up to 3.5.