Effect of liquid viscosity on gas holdups in a reversed flow jet loop reactor

The effect of draft tube diameter and liquid viscosity on overall and annulus gas holdups were studied in a reversed flow jet loop reactor. The draft tube diameter to reader diameter ratio (D_d / D) and liquid viscosity were varied in the ranges 0.34-0.67 and 1.5-43 mPa. s, respectively. The maximum gas holdup was obtained when the D_d / D value ranged btween 0.47 and 0.61. The gas holdup decreased with increasing viscosity. Empirical correlations are presented to predict the gas holdups.     

Gas holdup in a two‐phase reversed flow jet loop reactor

Experimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (E_gd) and gas holdup with circulation loop (E_g). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.     

Studies on the hydrodynamic behavior and mass transfer in a down‐flow jet loop reactor with a coaxial draft tube

The effects of certain pertinent parameters such as gas and liquid flow rates and nozzle position on the behavior of a down‐flow jet loop reactor (DJR) have been studied. The mean residence times of gas and liquid phases and the gas holdup within the reactor have been measured. In addition, the overall volumetric mass transfer coefficient, and the influence of the gas flow rate and the position of the nozzle inside the draft tube on the latter has been determined. Correlations have been presented for the gas holdup and k_La which take into account the length of the draft tube and the nozzle immersion height. The k_La values obtained at different power per unit volume (P/V) values in the DJR used in the present study compare favorably with data presented for stirred tanks and bubble columns in the literature. The liquid residence time distribution (RTD) within the reactor has been studied by tracer analysis for various operating conditions and nozzle immersion height and the results are indicative of the high mixing intensities that can be obtained in such reactions. © 2001 Society of Chemical Industry     

Local bubble size distribution,gas–liquid interfacial areas and gas holdups in an up-flow ejector

Particle image velocimetry (PIV) was used to measure local bubble size distributions (BSD), gas–liquid interfacial areas and gas holdups in an up-flow ejector, based on the water–air system with different liquid and gas flow rates under the presence/absence of the swirl body. The results show that the bubble flow patterns are different whether to add the swirl body into the nozzle, especially at low gas flow rate because the bubbles formed “bubble chain” in the ejector with swirl. The mean bubble sizes D₃₂ of the two are both related to the pressure drop between import and export, gas ratio and liquid flow. The interfacial area and D₃₂ are both mainly dependent on the local gas holdups. The mean bubble sizes in the absence of swirl body are smaller than that in the presence of swirl under different operating conditions. The gas holdups and interfacial area are larger with swirl than those without swirl. With the increase of the gas fraction, the differences of D₃₂, a_t and e_G become smaller.     

The effect of geometrical parameters on the flow field of an opposed jet rim mix head: Equal flow and matched fluids

The effect of geometrical parameters on the flow field present in the mix head used in reaction injection molding (RIM) are presented for Re_d (Re_d = 4Q/πdv, nozzle diameter d, fluid kinematic viscosity v and the volumetric flow rate Q through the nozzle) representative of commercial practice. Quantitative velocity measurements of the flow field in a mix head have not been reported in the literature despite the extensive use of the mix head for reaction injection molding. Flow visualization and velocity measurements using a laser Doppler anemometer have been obtained for different values of geometrical parameters such as mix chamber diameter (D), distance from the nozzle inlet to the closed end of the chamber (H), and nozzle needle position (N). The ratio of the mix chamber to nozzle diameter (d) (D* = D/d) was a significant parameter affecting the flow field. The distance from the impingement point to the closed end of the chamber was found to have little effect on the observed flow field beyond the impingement area. A nozzle needle position that partially constricted the nozzle opening was found to decrease the axial distance to unidirectional flow within the mix chamber.     

Axial mixing of liquid in a turbulent-bed contactor

The axial dispersion of liquid in a 12-in. turbulent-bed contactor has been investigated for three packing sizes: ½-in., 1-in. and 1½-in. The gas and liquid flow rates were varied from 500 to 2700 lb./(hr.)(sq. ft.) and from 1500 to 11,000 lb./(hr.)(sq. ft.) respectively. The transient response technique using KCl solution as the tracer was employed for this purpose. The experimentally determined residence-time distribution curves were interpreted by means of a one-dimensional dispersion model. The axial dispersion coefficient, D_L, was found to increase with increasing gas flow rate, liquid flow rate, or packing size. In terms of Peclet number (N_Pe = ū d_p/D_L), the present data showed that N_Pe was dependent on Reynolds number (N, = d_p ū ρ/μ), Gallileo number (N_Ga = d_p³ ρ³ g/μ²), and reduced gas mass velocity (Δ = (G-G_mf)/G_mf), but the ratio of the Peclet number for a turbulent contactor to the Peclet number for a fixed-bed contactor, N_Pe/N_Peo, depended only on Δ, and the diameter ratio d_p/d_t. A correlation of N_Pe/N_Peθo with Δ and d_p/d_t is presented.     

固体装载率对局部相含率与液体速率的影响

在高径比(H/D=22.2)的气升式内环流反应器中,以空气-水-石英砂为物系,研究了固体装载率和表观气速对导流筒内局部固含率、下降区固含率、下降区液体速率的影响,以及导流筒内局部气含率、固含率随轴向高度的分布规律。结果表明,固体体积分数小于或等于1.0%时,导流筒内局部固含率和下降区固含率与表观气速无关;下降区液体速率随表观气速和固体装载率的增加而下降;固定表观气速,在固体体积分数小于或等于2.0%时,导流筒内气含率在轴向是均匀分布的。     

Experimental Studies on Suspension of Solid Particles in a Low‐Shear Stirred Vessel

A low‐shear stirred vessel was explored. Experimental studies on the suspension of solid particles in solid‐liquid and gas‐solid‐liquid systems were conducted to examine the performance of this new reactor. The method based on the power number curve was modified to determine the critical impeller speeds required for just complete off‐bottom suspension of solids under non‐gassed (N_js) and gassed conditions (N_jsg) in this reactor, and a PC‐6A fiber‐optic probe for the measurement of solid distribution was used to complementarily validate this method. A more homogeneous flow field was gained with a draft tube installed, so that the standard deviations of average shear rate and maximal shear rate are reduced. The modified power consumption method can determine N_js and N_jsg, and the values of N_js with a draft tube are much lower than those without it. N_jsg increases slightly with increasing gas flow rate, and N_jsg with a higher solid weight fraction is larger in this lower‐shear reactor.     

RTD studies in trickle bed reactors packed with porous particles

The residence time distribution (RTD) for liquid phase in a trickle bed reactor (TBR) has been experimentally studied for air-water system. Experiments were performed in a 15.2 cm diameter column using commerical alumina extrudates with D/d_p ratio equal to 75 to eliminate the radial flow differences. The range of liquid and gas flow rates covered was 3.76 < Re_L < 9.3 and 0 < Re_G < 2.92. The axial dispersion model was used to compute axial dispersion coefficient. The effect of liquid and gas flow rates on total liquid holdup and axial dispersion was investigated. The total liquid holdup has been correlated to liquid and gas flow rates.     

Effect of geometrical parameters of draft tubes and clear liquid height on gas holdup in a bubble column for gas dispersion into tubes

The effects of the geometrical parameters of draft tubes and the clear liquid height on the average gas holdup E_G in a 0.16 m I.D. bubble column for gas dispersion into the tubes were experimentally studied in an airtap water system. The gas holdup depended on the superficial gas velocity U(ING), the kinds of gas spargers, the diameter and length of the draft tubes, clearance C_b between the lower end of the draft tube and the bottom of the bubble column, and the clear liquid height H_L. E_G increased with decreasing hole diameter of the gas sparger at a small gas velocity U_G, but did not depend on the kinds of gas spargers at a large U_G. E_G decreased with increasing clear liquid height H_L. The effect of H_L on E_G was well expressed by the modified three-region model. The experimental data of E_G were correlated.     

Hydrodynamics and mass transfer behavior in multiple draft tube airlift contactors

The draft tube configuration significantly affected the performance of an airlift contactor. The multiple draft tube configuration was demonstrated to give a better gas-liquid mass transfer when compared with a conventional one-draft-tube system. The airlift with a larger number of draft tubes allowed a higher level of bubble entrainment, which rendered a high downcomer gas holdup. This resulted in a higher overall gas holdup in the contactor. Liquid velocity was also enhanced by increasing the number of draft tubes. The ratio between downcomer and riser cross sectional areas, A _d /A _r , had a great effect on the system performance, where a larger A _d /A _r led to a lower downcomer liquid velocity and smaller quantity of gas bubbles being dragged into the downcomer. This resulted in low gas holdup, and consequently, low gas-liquid interfacial mass transfer area, which led to a reduction in the overall volumetric mass transfer coefficient. The presence of salinity in the system drastically reduced the bubble size and subsequently led to an enhancement of gas entrainment within the system. As a result, higher gas holdups and gas-liquid interfacial area were observed, and hence, a higher rate of gas-liquid mass transfer was obtained.     

Hydrodynamic and mass transfer characteristics of external-loop airlift reactors without an extension tube above the downcomer

The effects of the horizontal connection length (0.1≤L_c≤0.5 m), the downcomer-to-riser cross-sectional area ratio (0.11≤A_d/A_r≤0.53) and the superficial gas velocity (0.02≤U_G≤0.18 ms^-1) on gas holdups in the riser and downcomer, the circulation liquid velocity, the mixing time, and the overall volumetric mass transfer coefficient were determined in external-loop airlift reactors without an extension tube above the downcomer [configuration (a)]. For otherwise fixed conditions, the absence of the extension tube strongly affected the hydrodynamic and mass transfer characteristics of external-loop airlift reactors. In contrast with the external-loop airlift reactor with the extension tube [configuration (b)], a large air pocket formed in the top horizontal connection and the surface aeration took place in the external-loop airlift reactor without the extension tube [configuration (a)]. As a result, the riser circulation liquid velocity in configuration (a) was slower than that in configuration (b). The riser and downcomer gas holdups, the mixing time and the overall volumetric mass transfer coefficient in configuration (a) were larger than those in configuration (b), respectively.     

Hydrodynamics in down flow jet loop reactor

Gas holdup and liquid circulation time were measured in a down flow jet loop reactor for air–water system. It was observed that the circulation time decreases with an increase in nozzle diameter, draft tube to column diameter ratio and liquid velocity. Th gas holdup increases with an increase in gas and liquid velocities. The optimum draft tube to column diameter ratio was found to be 0.438. Correlations for gas holdup and circulation time involving operational and geometrical variables are presented.     

三相下喷式环流反应器的传质性能

在三相非牛顿型流体体系中，对下喷式环流反应器传质特性进行了实验研究。讨论了表观气速、能量耗散速率、导流筒直径与反应器直径比、喷嘴直径、导流筒下端距反应器底部的距离、固体装填量、羧甲基纤维素钠（ＣＭＣ）溶液浓度及其流变特性对它的影响。实验结果表明，容积传质系数随表观气速和能量耗散速率的增加有所增加，在实验条件下，发现最优的导流筒直径与反应器直径比在０．４～０．４５这一范围、固体装填量大约为３％（体积百分比）、导流筒下端距反应器底部的距离为０．０８ｍ左右。同时提出了容积传质系数的经验关联式。     

CFD Modeling of a Bubble Column Reactor Carrying out a Consecutive A → B → C Reaction

In this paper, we develop a CFD model for describing a bubble column reactor for carrying out a consecutive first‐order reaction sequence A → B → C. Three reactor configurations, all operating in the homogeneous bubbly regime, were investigated: (I) column diameter D_T = 0.1 m, column height H_T = 1.1 m, (II) D_T = 0.1 m, H_T = 2 m, and (III) D_T = 1 m, H_T = 5 m. Eulerian simulations were carried out for superficial gas velocities U_G in the range of 0.005–0.04 m/s, assuming cylindrical axisymmetry. Additionally, for configurations I and III fully three‐dimensional transient simulations were carried out for checking the assumption of cylindrical axisymmetry. For the 0.1 m diameter column (configuration I), 2‐D axisymmetric and 3‐D transient simulations yield nearly the same results for gas holdup ?_G, centerline liquid velocity V_L(0), conversion of A, χ_A, and selectivity to B, S_B. In sharp contrast, for the 1 m diameter column (configuration III), there are significant differences in the CFD predictions of ?_G, V_L(0), χ_A, and S_B using 2‐D and 3‐D simulations; the 2‐D strategies tend to exaggerate V_L(0), and underpredict ?_G, χ_A, and S_B. The transient 3‐D simulation results appear to be more realistic. The CFD simulation results for χ_A and S_B are also compared with a simple analytic model, often employed in practice, in which the gas phase is assumed to be in plug flow and the liquid phase is well mixed. For the smaller diameter columns (configurations I and II) the CFD simulation results for χ_A are in excellent agreement with the analytic model, but for the larger diameter column the analytic model is somewhat optimistic. There are two reasons for this deviation. Firstly, the gas phase is not in perfect plug flow and secondly, the liquid phase is not perfectly mixed. The computational results obtained in this paper demonstrate the power of CFD for predicting the performance of bubble column reactors. Of particular use is the ability of CFD to describe scale effects.     

Radial gas mixing characteristics in a downer reactor

In a downer reactor (0.1 m-I.D.x3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0–40kg/m²s) and particle size (84, 164 Μm) on the gas mixing coefficient have been determined. The radial dispersion coefficient(D _r ) decreases and the radial Peclet number (Pe_r) increases as gas velocity increases. At lower gas velocities, D_r in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, D_r increases with increasing solids circulation rate (G_s), however, D_r decreases with increasing G_s at higher gas velocities. Based on the obtained D_r values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.     

Gas holdup and overall volumetric mass transfer coefficient in a modified reversed flow jet loop reactor

Experiments were conducted in a modified reversed flow jet loop reactor having the liquid outlet at the top of the reactor to determine the gas holdup and overall volumetric mass transfer coefficient in the air-water system. The influence of gas and liquid flow rates, and the draft tube to reactor diameter ratio were studied. It was observed that both gas holdup and volumetric mass transfer coefficient increased with increased gas and liquid flow rates and were found to be significantly higher in the modified reactor compared to the conventional one. The optimum draft tube to reactor diameter ratio was found to be in the range of 0.4 to 0.5. Empirical correlations are presented to predict gas holdup and overall volumetric mass transfer coefficient in terms of operational and geometrical variables.     

Gas holdup and gas entrainment of a plunging water jet with a constant entrainment guide

The gas holdup and gas entrainment of a plunging liquid jet with a gas entrainment guide in an air-water system was investigated. The measurement of the gas holdup was performed using an over-flow method. The turbulent jet velocity calculated on an inside nozzle diameter in the range from 4.4-26.5 m/s for this system has been used in our correlations. The gas holdup has been well correlated in terms of 1/H(v₀² + 2gH₁), H₁ d₀ and the gas entrainment in terms of 1/H_w(v₀² + 2gH₁), H₁, d₀. The jet power requirement was also obtained from experimental data.     

Three‐phase airlift internal loop reactor: correlations for predicting the main fluid dynamic parameters

Three‐phase airlift loop reactors have many industrial applications. Examples include applications in the chemical, metallurgical, biochemical and minerals‐processing industries; fluid dynamics in such systems is a critical factor affecting efficiency. The experimental work was carried out at a pilot‐plant scale in a tank with a cylindrical shape and conical bottom (height 1.25 m; diameter 0.42 m), water and air were used as liquid and gas phases, and for different solid phases, different loads of glass spheres (diameters: 0.25 and 1 mm; density 2.6 g cm^?3), and polystyrene cylinders (diameter and length 3 mm; density 1.0 g cm^?3) were introduced. Air was injected through the bottom of the tank by means of 12 nozzles (diameter 1 mm each). An internal draft tube riser was tested on different configurations as its diameter was varied (44, 82, 125 and 240 mm) as was its height (1050 and 630 mm). Corresponding liquid velocities in the adjacent annular downcomer were determined by a thermal tracer technique, solid holdups were determined by conductivity methods, riser overall gas holdups were deduced from the liquid level, and riser gas holdup from manometer readings. Several adjusted correlations were considered in a method to predict the main fluid dynamic parameters (solids and gas holdups, and superficial liquid velocities). Copyright © 2005 Society of Chemical Industry