Scale-up of Gas-liquid Mass Transfer in Oscillatory Multi-orifice Baffled

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Kinetics, Catalysis, and Reaction Engineering

Scale-up of Gas-liquid Mass Transfer in Oscillatory Multi-orifice Baffled Reactors (OMBRs) Safaa M.R. Ahmed, Anh N. Phan, and Adam P. Harvey Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Industrial & Engineering Chemistry Research

Scale-up of Gas-liquid Mass Transfer in Oscillatory Multi-orifice Baffled Reactors (OMBRs)

Safaa M.R. Ahmed a, b, Anh N. Phan a, Adam P. Harvey a, * a

School of Engineering, Newcastle University, Merz Court, Claremont Road, Newcastle Upon Tyne NE1 7RU, UK

b

Department of Chemical Engineering, College of Engineering, Tikrit University, Tikrit, Salah ad Din, IQ 34001, Iraq. *

Corresponding Author (Email: [email protected] )

__________________________________________________________________________________ ________ Abstract The effect of scale-up on air-water mass transfer in “oscillatory multi-orifice baffled reactors” (OMBRs) was investigated. Reactors of 10, 50 and 100 mm diameter were studied. The reactors had the same aspect ratio, dimensionless baffle spacing, open cross-sectional area, and orifice diameter. Air-water flow patterns were visualized using a high-speed camera, and the kLa was determined based on dissolved gas measurements. On the basis of these results, a dimensionless scale-up correlation, based on Sh, Reo (oscillatory flow Reynolds number), ReG (superficial gas velocity Reynolds number) and the diameter ratio, was developed and successfully validated against this data and literature values.

Keywords: two phase flow, mass transfer, oscillatory baffled reactor, scale-up

1. Introduction Although the conventional bubble column (BC) has been widely used in industry to enhance mass transfer of gas-liquid systems, scale-up in up flow bubble columns has often proved unpredictable due to non-uniform mixing and uncontrolled bubble size at larger scales1,2,3. In addition, it has been reported4 that in order to operate in the bubbly flow regime, which is usually the desired regime due to the associated high rates of mass transfer, the gas flow must be below 0.05 m/s (Figure 1), which constrains the scale-up of BCs.

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Figure 1. Air-water flow regimes at different bubble column scale4 (re-drawn); where DT is the tube diameter, and UG is the superficial gas velocity. The oscillatory baffled reactor is a continuous tubular reactor containing periodically spaced baffles, with an imposed oscillatory flow via use of diagrams, pistons or bellows. It has been proven to increase kLa by 4- to 10-fold compared to BCs 5, 6, 7, 8, and 75% compared to a stirred tank fermenter (STF)9. The kLa is enhanced by periodic formation and dissipation of vortices in the baffle cavities, which reduce bubble size and increase gas holdup10. Ni and Gao (1996a)5 studied the effect of the orifice baffle spacing, aeration rate, and the oscillation intensity on mass transfer enhancement for gas-liquid system in two OBRs of 50 mm and 100 mm diameter with the same aspect ratio (L/D=10.5). They proposed correlations for individual cases as shown in Equations (11) and (12): 𝑃

𝑘𝐿 𝑎 = 0.0186 (𝑉)0.4 𝑈𝐺 0.32

(50 mm ID)

(1) 5

𝑃

𝑘𝐿 𝑎 = 0.0256 (𝑉)0.425 𝑈𝐺 0.37

(100 mm ID)

(2)

Where kLa is the overall mass transfer coefficient (s-1), P/V is power density per unit volume (W m-3), and UG is the superficial gas velocity (m s-1). The oscillation amplitude was shown to have a stronger effect on kLa than the oscillation frequency5, 8. Multi-orifice baffles have been shown to cause a high degree of plug flow11 and display a wider operating window for bubbly flow and higher rates of mass transfer than other designs, including helical, single-orifice, and integral baffle design10. In addition, the multi-orifice design has been suggested to be the most appropriate for gas-liquid systems as it allows good control of bubble size12, 13, 14. Although parameters such as oscillation conditions, gas velocity, and baffle design have been studied, there have


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not yet been any studies dealing with the effect of OBR scale up. Therefore, this study was performed to investigate mass transfer enhancement in an air-water system over a range of OBR scales, as an important step toward industrial application of OBRs for gas-liquid reactions and separations.

Although conventional OBRs (single orifice plate baffle design) have been shown to exhibit significant gas-liquid mass transfer enhancement (up to 10-fold), all the studies were single scale5, 6, 8. Despite a number of studies on scale-up of OBRs, based on residence time distributions11, 15, no correlation for scale-up of OBRs based on mass transfer has yet been developed. Ni et al. (2006) developed an empirical equation for determining mass transfer coefficients, but the correlations were size-dependent and the choice of large baffle-spacing (1.8 times tube diameter) was unsuitable for gas-liquid systems10, 11, 14, 16

. Previous work in this area10 has demonstrated that the multi-orifice design significantly

outperforms other designs (helical, single-orifice), and should be the design of choice for gas-liquid mass transfer applications. Hence, the main aim of this work was to determine kLas in OMBRs over a wide range of oscillation conditions and column diameters, such that a usable correlation could be developed.

2. Experimental apparatus and procedure 2.1. Experimental setup and kLa quantification

Figure 2 represents the equipment used for kLa determination:



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Figure 2. Schematic diagram for the scale-up study Three scales of oscillatory multi-orifice baffled reactors, OMBRs, were used with dimensions as detailed in Table 1. Baffle spacing (l = 0.5 D), aspect ratio (L/D = 22.5), baffle thickness (Tb = 2 mm) and open cross-sectional area (α’ = 0.37) were kept the same in all designs. The multi-hole design was used because it was shown in a previous study10 that they allow enhanced control of the bubble flow regime, if the orifice diameters were of fixed diameter. Here, the orifice diameters were fixed at 3.5 mm. This maintains the same air-water flow pattern at all scales14 and maintains the bubble size. This should lead to more predictable scale-up between scales. This choice also, of course, dictates that the number of holes must vary between scales (if the open cross-sectional area is to be kept constant).

Table 1. OMBR Specifications

OMBR diameter, mm

10

50

100

0.018

2.21

18

225

1125

2250

Baffle spacing, l, mm

5

25

50

Number of orifice (n)

3

75

300

Diameter of supporting rods, mm

1.75

2

2

Diameter ratio (n do/D)

1.05

5.25

10.5

Operational capacity, L Column height, mm

The OMBR columns were made of transparent Perspex tubing. The baffles were 3D-printed (uPrint SEplus) using ABS material (acrylonitrile butadiene styrene). Each rig setup consisted of a tube mounted vertically on an oscillator to provide sinusoidal oscillation conditions. Two oscillation units were used: the patented magnetic design of Servo Tube was used for a 10 mm diameter tube to provide a frequency of 1–15 Hz and centre-to-peak amplitudes (xo) of 1-15 mm. At the larger scales, a stainless steel bellow assembly was used to provide frequencies from 1.25-10Hz, and xo=1.85-8 mm. The OMBRs were operated in semi-batch mode (continuous gas flow; zero liquid flow) at atmospheric pressure and room temperature (~20 oC). The temperature and pressure were continuously monitored and logged via

a Mettler Toledo data transmitter. The liquid (distilled water) height (H) were fixed at a certain value, H=22.5 x D mm from the gas inlet stream, for all the scales. Prior to each experiment, the system was de-oxygenated by injecting nitrogen gas (oxygen-free, BOC Ltd) until the dissolved oxygen (DO) reached zero. Then the gas supply was switched from nitrogen to oxygen and the DO concentration was


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monitored by a DO probe until saturation was achieved. The air was injected to the rigs using 1-2 mm internal diameter tubing. No sparger was used, as the premise of the work was to investigate mass transfer enhancements based on oscillation alone. A Cole-Parmer air flow meter was used to measure the air velocity at the column bottom.

2.2. Method of kLa calculation A Mettler Toledo DO probe, 12 mm O.D., (response time: 33 s) was located at the top of the column (at a position of H=19 x D mm from the gas inlet stream). DO values were recorded using PicoLog data (PicoLog ADC 20) at fixed intervals (1s). The effects of the gas and liquid dynamics on the probe response were accounted for by compensating for the kLa value of each run using a first order model (Eq. 3)7, 9, 14. The probe constant, Kp, was determined by a step change in concentration technique and checked during the run to eliminate the potential for errors due to drift7. For example, Kp was found to be 0.0071±0.0005s-1 for the 50 mm scale. However, this Kp was checked during each run, to take into

account changes in the membrane during the run that could alter Kp14. kLa was determined for each experiment from the O2 dissolution plots by best-fitting the experimental O2 dissolution profiles data to Equation 4: 𝐶𝐿∗ −𝐶𝐿 (𝑡) 𝐶𝐿∗ −𝐶𝐿,0

= exp (−𝐾𝑝 𝑡)

(3)

𝐶 ∗ −𝐶

𝐶𝐿 (𝑡) = 𝐾𝐿 −𝐾𝐿,0𝑎 {𝐾𝑝 𝑒𝑥𝑝[−𝑘𝐿 𝑎(𝑡 − 𝑡0 ] − 𝑘𝐿 𝑎 𝑒𝑥𝑝[−𝐾𝑝 (𝑡 − 𝑡0 ]} 𝑝

𝐿

(4)

… where CL and CL* are the oxygen solubility and the saturated oxygen concentration, respectively, in the liquid. Note, the maximum value of CL was 14500 mg l-1 while CL* was about 95-97% of CL max. CL,o is the oxygen concentration at time t = 0, and t0 is the time delay. Table 2 shows the experimental operating conditions, oscillation conditions (frequency, f and amplitude xo) and aeration rate, the volumetric flow rate of air per volume of liquid per minute (vvm).

Table 2. Operating conditions Condition

Range

f (Hz)

1-12

xo (mm)

1.85-10



vvm

0-1.0

Reo′

0-5000

St ′

0.05, 0.07, 0.12, 0.25

2.3. Modification of dimensionless groups The oscillatory motion of the fluid within OBRs is described by the oscillatory Reynolds number (Reo) and the Strouhal number (St). As mixing and flow behavior in multi-orifice OBRs are different from that in a single-orifice OBR, the effect of the orifice number, n, orifice diameter, do, and cross sectional area, α, on the performance of OMBR must be taken into account10, 14. Therefore, the modified mixing conditions (Eq. 5 and 6), proposed by Pereira et al14, were applied to describe the effect of the parameters, n, do, and α, on the flow regimes and mass transfer coefficient12. In addition, α was also modified (Eq. 7) to incorporate n:

2𝜋𝑓𝑥𝑜 𝜌 𝐷 1−𝛼 ) ( 𝑛) √ 𝛼 2 𝜇 √

𝑅𝑒𝑜′ = ( 𝑆𝑡 ′ =

𝐷 1 4𝜋𝑥𝑜 √𝑛

𝛼 ′ = (𝑛

𝑑𝑜2 )= 𝐷2

=

1 𝑆𝑡 √𝑛

𝛼 ∗𝑛

1

1−𝛼

= 𝑅𝑒𝑜 ( 𝑛) √ 𝛼2

(5)

√

(6) (7)

…where ρ is the fluid density (kg.m-3), μ is the fluid viscosity (Pa.s), f is the oscillation frequency (Hz), xo is the oscillation amplitude (m), and D is the OMBR diameter (m). Reo′ and St ′ are the modified oscillatory Reynolds number and Strouhal number respectively. n is the number of orifices, do is the orifice diameter (m), α is the open cross sectional area, and α′ is the modified open cross sectional area.

2.4. Flow regime identification A previous study10 showed that the OMBR achieves bubbly flow over a wider range of operating conditions than other OBR designs (helical-baffle OBR, integral-baffle OBR, and single orifice-baffle OBR). In addition, bubble size was shown to be lower in the OMBR, indicating higher interfacial contacting area. Here, the bubble size was investigated as a function of scale for the OMBR design, using a high-speed CCD camera (Photron FastCam) with a frame rate of 4000 frames per second (fps) to follow any slight change in the ﬂow pattern. A Perspex optical box filled with glycerol was also fitted mid-way up each OMBR (Figure 2). A sequence of image snapshots were taken to track bubbles over a wide range of aeration rates, in the range vvm = 0–1, and oscillation condition, Reo′ = 0–5000. The flow regime map for individual scales was established based on the collected images and videos. In addition, the vvm value at the transition regime was quantified using gas hold-up (εG) plots1, 4, 10, as shown in Equation 8 where a sudden change in the gas hold-up slope was observed1. The gas-hold up


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was measured using the visual observation method as well as the images and videos recorded by the high speed camera with average standard deviation of ±1.8% of the experimental duplicates.

𝜀𝐺 = 𝐻−𝑁

𝐻 ∗ −𝐻 𝑏

(8)

𝑇𝑏 (1−𝛼′ )

…where εG is the gas holdup, H is the liquid height in the absence of gas (m), H* is the liquid height with the presence of gas (m), Nb is the total number of baffles in the column, Tb is the baffle thickness (m), and α′ is the modified open cross-sectional area.

3. Results and Discussion 3.1. Flow regime maps Figure 3 shows images of air-water flow patterns and bubble tracking inside OMBRs at different aeration rates and oscillation conditions. It can be observed that slug flow and churn flow appeared only at the 10 mm scale, as shown in Figure 3(a). At the 50-100 mm scales, only bubbly flow was observed. It should also be noted that, for those reactors in the bubbly flow regime, the bubble size decreased with increasing oscillation conditions (Figure 3(b) and (c)).



Figure 3. Air-water flow visualization in the OMBRs. Scale bar corresponds to 3.5 mm In conventional BCs, slug flow is expected at D ≤ 500 mm, churn flow at D > 500 mm, and bubbly flow at all scales when UG< 0.05 m s-1 (see Figure 1)1, 5. Hence, there is clearly a larger “operating window” for bubbly flow for the OMBR. This wide range of bubbly flow operating conditions in OMBRs could be exploited at industrial scale. These observations correlate with the gas-hold up data shown in Figure 4. The change in the slope of the 10 mm scale at vvmtr of 0.53 correlates with the transition from bubbly to churn flow. At larger scales, however, the kLa increased monotonically with increasing vvm.


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Figure 4: Gas Hold-up vs vvm at three different OBR diameters Based on the visual observation and the quantitative data (the gas hold-up data), the flow regime maps were established over a wide range of mixing conditions: Reo′ = 0-5000 and vvm=0-1. At scales ≥50 mm, only the bubbly flow regime was observed at all tested conditions whereas there were three distinctive flow regimes observed at 10 mm scale (Figure 5). This is perhaps due to the difference in the hole number, n=300 for 100 mm i.d., and 75 for 50 mm i.d. compared with n=3 for 10 mm i.d. This means that there is more “orifice edge per unit diameter” at the larger scales, which will increase the interaction between bubbles and edge, leading to increased “bubble chopping”. Also, as observed, the small baffle spacing, l=0.5D, in the 50 mm and 100 mm designs acts against slug and churn bubble development in the inter-baffle zones. Note that these features are not observed in conventional bubble columns, where bubble sizes increase with height, as they coalesce. It should also be noted that the premise of this research was to investigate scale-up by maintaining orifice size and scaling up by orifice number, in the belief that this would lead to more predictable scale-up.



Figure 5. Flow regime maps for vertical OMBR, D=10 mm 3.2. kLa in the OMBRs 3.2.1. Effect of aeration rate (gas velocity) and OMBR scale Figures 6 (a) to (d) show the impact of the superficial gas velocity, vvm, on kLa at all scales over a wide range of mixing conditions, Reo′ and St′.


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Figure 6. Effect of aeration rate (vvm) and the OMBR scale on the mass transfer coefficient at (a) no oscillatory flow, Reo′ =0, (b) Reo′=410 and St'=0.25, (c) Reo′ =1231 and St'=0.12, and (d) Reo′ =2461 and St'=0.07 Clearly, the kLa significantly increases with aeration rate at all diameters, and increases with tube diameter. The increase with flow rate was expected, as increased flow rate increases the number of bubbles, resulting in increased air-water interfacial area, even without applying oscillation. For example, at Reo′=0, the kLa of the 100 mm scale increased from 0.009 s-1 to 0.04 s-1 as vvm increased by a factor of 10 (Figure 6 (a)), which agreed well with previous gas−liquid mass transfer studies using singleorifice and multi-orifice oscillatory columns9, 14. kLa also increased with oscillation amplitude (xo), due to an increase in the number of strong eddies generated because of the interaction with the small orifice resulting in reduction in bubble size6-8, 10. For example, at vvm=0.5, kLa at the 100 mm scale increased from 0.046 s -1 to 0.082 s-1, as Reo′ increased from 410 to 2461 (Figure 6, b) and d)). kLa also increased with the OMBR diameter. This is because an increase in the column diameter leads to an increased number of orifices, n, therefore, the population of small bubbles increases. It was observed that the 100 mm scale exhibited the highest kLa values, 0.0086 s-1- 0.082 s-1 compared to 0.0046-0.0272 s-1 at the 10 mm scale.

The enhancement increased to a maximum at vvm=0.1 at the 100 mm scale when the oscillation increased from 410 to 2461 (see Figure 7, b)), with approximately 5-fold, 4-fold and 3.5-fold increases for the 100 mm, 50 mm, and 10 mm scales respectively.



Figure 7. Effect of vvm on the enhancement of kLa in all the OMBR scales at a) Reo′=410 and St'=0.25, and b) Reo′=2461 and St'=0.07. 3.2.2. Effect of Fluid Oscillation Figures 8 (a) to (c) show the effect of oscillation for the three scales at three aeration rates: vvm=0.1 (Figure 8 (a)), vvm=0.3 (Figure 8 (b)), and vvm=0.5 (Figure 8 (c)):


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Figure 8. Effect of oscillation condition and the OMBR diameter on the mass transfer coefficient at (a) vvm=0.1, (b) vvm=0.3, and (c) vvm=0.5 Three specific patterns can be observed from Figure 8:  At Reo′ ≤ 1000 and St' ≤ 0.12, kLa variation was negligible for the 10 mm scale. This is because weak eddies are generated, therefore, little bubble breakage was minimal and bubbles residence times were not increased7, 14.  At Reo′ >1000 and St' = 0.07, the 50 mm and 100 mm scales showed a significant increase in kLa due to the strong eddies generated leading to increased bubble residence time and enhanced mass transfer.  At very high oscillations, Reo′ >4000 and St' = 0.05, (Figure 8 (a) - (c)) kLa decreased with Reo′ for all tested scales. For the 10 mm scale, there was a transition regime from the homogeneous, bubbly, regime to the heterogeneous, churn, regime10. The gas-liquid contacting area in churn flow is lower, as large bubbles form, surrounded by a thin film of liquid5, 8, 10. For the 50-100 mm scales, kLa also decreased due to a sudden increase in foam generation.

3.3. Scale-up correlation A correlation for predicting mass transfer coefficient, kLa, in OMBRs was established based on the experimental results of kLa as shown in Eq. 9a when 0< Reo′ ≤5000, and Eq. 9b when Reo′ =0. The correlation represents the relationship between kLa or Sh as a function of operating conditions, Sh=f (

n do D

, Reo′, ReG).

𝑆ℎ =

𝑘𝐿 𝑎 𝐷 2 𝐷𝑒𝑓𝑓

= 0.003 (

𝑛 𝑑𝑜 𝐷

)

0.2

𝑅𝑒𝑜′

0.38

𝑅𝑒𝐺 0.32


… 0< Reo′ ≤ 5000

(9a)


𝑆ℎ𝑜 =

𝑘𝐿 𝑎 𝐷 2 𝐷𝑒𝑓𝑓

= 0.01 (

𝑛 𝑑𝑜 𝐷

)

0.2

𝑅𝑒𝐺 0.32

… Reo′ = 0

Page 14 of 19

(9b)

Where Sh is Sherwood Number, Sho is Sherwood Number at Reo′ = 0, Deff is diffusivity (m2/s) the Deff values used in this work were imported from a standard table of diffusivity of air-water at T=20 oC and atmospheric pressure. The Deff was included in the left side of the correlation (Eq. 9) to help us to develop the main correlation (the right side of Eq. 9), Reo′ is the modified oscillatory Reynolds number, ReG is Reynolds number based on superficial gas velocity (Eq. 10), and (n do/D) is the diameter ratio (dimensionless). 𝑅𝑒𝐺 =

𝜌 𝑈𝐺 𝐷 𝜇

(10)

The correlations (Eq. 9 a and b) were validated using experimental data obtained from this study and data available in the literature5, 7, 10, 14, as shown in Figure 9, below. The predicted data agreed well with experimental data with a high correlation coefficient, i.e. R2 >0.95. Therefore, the developed correlation can be applied to predict kLa for OMBR across these scales.

Figure 9. Sherwood number Predicted vs Sherwood number experimental for all the OMBRs scales

used here and in the literature 5, 7 ,10 ,14. The correlation is a “step forward” in this area, as the first applicable over a range of scales. However, it should be noted that it was developed based on data for fixed orifice diameter, i.e. assuming that OMBR scale-up would be via increasing the number of orifices, rather than their diameter. Furthermore, the data here was based upon a fixed baffle separation of 0.5D. It may be that the baffle spacing has


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some effect here in that the baffle spacing decreased with tube diameter, perhaps leading to greater coalescence at the larger tube diameters for instance, but this has not been apparent in this set of experiments. Further validation (or invalidation) of this correlation should be performed by investigating the extent of its accuracy for multi-hole designs of varying diameter, designs of varying baffle spacing and designs of varying open cross-sectional area, and combinations of these factors.

4. Conclusions The effect of scale-up on the mass transfer characteristics of oscillatory multi-orifice baffled reactors (OMBRs) of fixed orifice diameter, open cross-sectional area and relative baffle spacing (0.5D) was investigated over a wide range of gas velocities and oscillation conditions, at three different scales: diameters of 10, 50 and 100 mm. The bubbly flow regime was only observed at the large scales, 50 mm and 100 mm diameter. This could be exploited in the gas-liquid systems at any scale ≤100 mm i.d., as bubbly flow is typically the desired regime, and here occurs over a wide range of operating conditions. The oscillation could significantly enhance mass transfer, by as much as a factor of 4. A simple scale-up correlation was developed to render scale-up of mass transfer in these reactors more predictable. The correlation relates Sh to Reo', the oscillatory Reynolds number, Re G, the gas flow Reynolds number, and a diameter ratio. The correlation was validated by plotting predicted data against the experimental data collected from this study and from literature (at various OBR designs and scales). 96% of the data set was within 30% confidence limits, confirming the validity of the correlation.



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5. Acknowledgment Financial support from the Higher Committee for Education Development in Iraq (HCED) to author Safaa M.R. Ahmed is gratefully acknowledged.

Abbreviations

BC C C* CD Co CT D Deff do

bubble column the dissolved oxygen concentration (g l-1) the saturated dissolved oxygen concentration (g l-1) the orifice discharge coefficient, dimensionless initial oxygen concentration at time t = 0 s, (g l-1) computerised tomography OBR diameter (m) diffusivity, m2 s-1 orifice diameter (m)

F H H* kLa kLao

oscillation frequency (Hz) liquid height with the presence of gas (m) liquid height with the absence of gas (m) volumetric mass transfer coefficient (s-1) volumetric mass transfer coefficient with the absence of oscillation (s-1) baffle spacing/helical pitch (m) Column length (m) the number of baffles the diameter ratio, dimensionless number of baffles in the column (-) the number of baffles per unit length (m -1) oscillatory baffled column oscillatory single-orifice baffled reactor particle image velocimetry, net flow Reynolds number for gas, dimensionless oscillatory flow Reynolds number, dimensionless the modified oscillatory flow Reynolds number, dimensionless Sherwood number, dimensionless Sherwood number at Reo′=0, dimensionless Strouhal number, dimensionless the modified Strouhal number, dimensionless

L L n n do/D Nb Nb OBC OMBR PIV ReG Reo Reo′ Sh Sho St St' Symbols t Tb UG vvm vvmtr xo

time, sec the baffle thickness (m) superficial gas velocity (m s-1) aeration rate aeration rate of transition regime oscillation amplitude (m) 16 ACS Paragon Plus Environment

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Greek Letters α α′ ɛ µ µG ρ ρG ω

open cross-sectional area, dimensionless the modified open cross-sectional area, dimensionless gas hold up, dimensionless water viscosity (Pa s) gas viscosity (Pa s) water density (kg/m3) gas density (kg/m3) angular frequency (2πf)

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17 ACS Paragon Plus Environment


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