ARTICLE pubs.acs.org/IECR
Sparger Type Influence on the Hydrodynamics of the Draft Tube Airlift Reactor with Diluted Alcohol Solutions Ivana M. Sijacki,*,† Milenko S. Tokic,† Predrag S. Kojic,† Dragan Lj. Petrovic,† Miodrag N. Tekic,† Mirjana S. Djuric,† and Slobodan S. Milovancev‡ † ‡
Department of Chemical Engineering, Faculty of Technology, University of Novi Sad, 403063 Novi Sad, Serbia Department of Energy, Electronics and Telecommunication Engineering, Faculty of Technical Sciences, University of Novi Sad, 403065 Novi Sad, Serbia
bS Supporting Information ABSTRACT: The objective of this work was to investigate the influence of normal aliphatic alcohols (from methanol to n-octanol) and the gas sparger type (single orifice, perforated plate, and sinter plate) on the hydrodynamics of a draft tube airlift reactor. The results showed that the presence of alcohols led to both an increase in the gas holdup and reduction of the downcomer liquid velocity, in comparison to the values obtained in water. The type of gas sparger also changed the hydrodynamic behavior—the gas holdup increased with dispersion effectiveness, from single orifice to sinter plate, while the effect on the downcomer liquid velocity was in conjunction with the liquid phase properties. Simple correlations were developed to predict the gas holdup and the downcomer liquid velocity. Beside the gas superficial velocity, the proposed correlations include the surface tension gradient, as a unique parameter which represents the liquid phase properties, and sparger related variables: the ratio of the sparger’s total openings area and the draft tube cross-sectional area and also orifice (pore) size of the sparger.
1. INTRODUCTION Although airlift reactors (ALRs) are suitable for many different processes (biomass or metabolites production, wastewater treatment, gas-liquid or gas-liquid-solid chemical reactions, etc.), there are still many issues concerning the behavior, design, and scale-up of these reactors for specific industrial applications. The investigations conducted already included the use of different liquids and additives in order to simulate the complex real industrial systems. Among others, dilute nonviscous alcohol solutions were used as the liquid phase in ALRs. The addition of alcohols, known surfactants, inhibits the coalescence of bubbles and, therefore, has a strong influence on the hydrodynamics and mass transfer in these contactors. The effect of alcohols on coalescence suppression is related with the chemical structure of the molecule, for it is composed of a hydrophobic part (nonpolar carbon chain) and a hydrophilic part (polar -OH group). Such molecule composition causes the accumulation of alcohol molecules on the gas-liquid interface with the carbon chain oriented toward the center of the bubble. Therefore, the monolayer around the gas bubble is formed, contributing to the bubble rigidity. When two bubbles approach, a thin liquid film forms between them, with a tendency to flow outward as the bubbles get closer. Normally, the gas-liquid interface on both sides of the film would stretch. However, the film has lower alcohol concentration in comparison to the one on the bubble surface. This causes the increase in the surface tension of the film, resulting in surface tension gradient forces. The stretching of the bubble surface is retarded, the drainage of the liquid film between the bubbles is slower, and consequently, the coalescence of bubbles is hindered.1-3 When a bubble moves through the liquid, the adsorbed surfactants are pushed to the r 2011 American Chemical Society
back of the bubble, which results in the surface tension gradient opposed to the tangential shear stress. The drag on the bubble is increased, and the rise velocity is reduced.3-5 Having in mind the above-mentioned factors, many researchers reported that the addition of small quantities of aliphatic alcohols increased the gas holdup, in comparison to pure water, in bubble columns (BCs),2,4,6 continuous BCs,4 draft tube airlift reactors (DT-ALRs),7-13 external loop airlift reactors (ELALRs)5,14 and split-rectangular airlift reactors (SR-ALRs).15 Also, the addition of alcohols increased the gas holdup in highly viscous saccharose solutions; with the possibility of full coalescence repression.16 Along with the changes in the gas holdup, the induced liquid velocity in ALRs is also affected by alcohol addition. Through formation of smaller bubbles, which are easily dragged in the downcomer, the resulting difference between the riser and the downcomer gas holdup, proportional to the driving force for liquid circulation, in the presence of alcohols becomes lower. Hence, the liquid velocity in alcohol solutions is decreased, in comparison to that obtained in water.9,11-13,15,17,18 However, depending on reactor geometry, it has been reported10,19 that the liquid phase velocity can be increased in the presence of alcohols. Table 1 presents a review of studies conducted in DT-ALRs with diluted alcohol solutions. The impact of alcohol, both in BCs and ALRs, is more pronounced as the number of C-atoms in the molecule chain increases,2,4,6,11-16 but also as the alcohol concentration Received: September 29, 2010 Accepted: January 20, 2011 Revised: December 20, 2010 Published: February 08, 2011 3580
dx.doi.org/10.1021/ie101989r | Ind. Eng. Chem. Res. 2011, 50, 3580–3591
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Review of Studies of Hydrodynamics and Mass Transfer in Dilute Alcohol Solutions in Draft Tube Airlift Reactors distributor type and ref Fields
and
D (m)
DR/D (%)
0.152
63
investigated
characteristics
liquid
parameters
porous plate
1 wt % ethanol
εG, tC
Slater7 Weiland8
0.2
sinter plate, do = 150-200 μm
0.22 wt % iso-propanol
εG, tC
Kennard and
0.22
45
sinter plate, do = 150 μm
10 g/L ethanol
εG
0.1
50
perforated plate, do = 1 mm
0.5 wt % n-butanol
εG, WLD, tC,
200, 400, 600 ppm and 0.25 wt % n-propanol
kLa εG, εGR, εGD
59; 74; 88
Janekeh9 Petrovic et al.10 Muthukumar
0.19
49
perforated plate, do = 1 mm
0.106
51
single orifice, do = 4 mm
and Velan11 Albijanic et al.12 Sijacki et al.13
0.106
51
single orifice, do = 4 mm
1 wt % methanol, ethanol, n-propanol, iso-propanol, n-
εG, WLD, tC,
butanol
kLa
3.2 wt % methanol, 0.46 wt % ethanol, 0.032 wt % n-
εG, WLD
propanol, 0.032 wt % iso-propanol, 0.011 wt % n-butanol, 0.0057 wt % n-pentanol, 0.0051 wt % n-hexanol, 0.002 wt this paper
0.106
51
% n-heptanol and 0.002 wt % n-octanol the same as in Sijacki et al.13
single orifice, do = 4 mm;
εG, WLD
perforated plate, do = 1 mm; sinter plate, do = 100-160 μm
increases.2,3,6,11,15,16 Nevertheless, the research by Keitel20 showed that a minimum and upper limiting concentrations exist, in which range the alcohol addition has a marked influence on the global hydrodynamics. Increasing the alcohol concentration above the upper limiting concentration value only enhances the liquid phase frothing and bubble coalescence.2,17 Assuredly, the design of the gas sparger must have a certain influence on the primary gas dispersion. Many researchers reported that the sparger configuration has impact on the bubble size and, hence, on the hydrodynamics and mass transfer, in BCs21-23 and ALRs,24-26 only at low gas velocities. In uniform bubbly flow, at low flow rates, it has been shown22-26 that bubble size strongly depends on the orifice size. As the gas input increases, in the heterogeneous regime, the size of the bubbles in dispersion is generally independent of the size at birth and it is controlled by the equilibrium between the dynamic pressure force, which tends to break the bubble, and the surface tension force, which attempts to preserve the shape and size. Therefore, the sparger influence on the global hydrodynamics and mass transfer at high gas flow rates diminishes.21-26 The extensive research of Merchuk et al.,25 who applied seven different spargers in a DT-ALR, showed that sparger design greatly affects the gas holdup and hence liquid circulation velocity, in uniform bubbly flow and transition flow. A sparger with a smaller pore size created smaller bubbles and led to a significant increase in the riser gas holdup. Besides that, completely different changes in the riser gas holdup, with an increase of the gas throughput for different sparger geometry, could be observed: with perforated spargers, the changes were more gradual and the transition between the regimes was not sharp, in comparison to the sinter spargers. Smaller bubbles were easily dragged in the downcomer leading to the decrease in driving force for liquid circulation and, thus, hampering the liquid circulation velocity.25 Similar observations were reported by Cao et al.27 for the EL-ALR with four spargers of different geometry and pore size. On the other hand, contrary conclusions were published.24,28,29 The discrepancies
about the influence of the gas sparger could be ascribed to the different reactor scale, reactor configuration, and the gas flow rate applied in the research.30 But, the most pronounced impact of the gas distributor is observed for the noncoalescing systems.2,16,24,31 Camarasa et al.2 conducted experiments in a BC with three different distributors: single orifice, multiple orifice, and porous plate. It was concluded2 that distributor type affects both the changes in the gas holdup curves and the transition between regimes in the BC. The influence of the distributor was more enhanced in the presence of n-butanol and n-pentanol. With the porous plate as gas sparger and in alcohol solutions, a narrow distribution of small bubbles is observed, almost the same size within the column as of those formed at the distributor.2 The research of Zahradnik et al.16 showed that the alcohol addition (from ethanol to n-octanol) to viscous saccharose solutions led to a significant difference in the bubble bed behavior and the gas holdup in a BC, even in the case of two perforated plate distributors with the same free plate area, but different hole diameters. The favorable effect of alcohols increased with the increasing length of their carbon chains.16 Snape et al.31 reported a similar synergistic effect of sparger design and liquid phase properties on values of gas holdup and liquid circulation velocity in an EL-ALR with aqueous solutions of inorganic salts as coalescence inhibitors. They31 also indicated that a suppression of the gas distributor effect in ALRs can be ascribed to the influence of superposed liquid flow on the two phase flow pattern and bubble rise velocity in the riser. Correlations for prediction of the gas holdup, downcomer liquid velocity, liquid circulation time, mixing time, and volumetric mass transfer coefficient in ALRs have been proposed in various forms, based on investigations in different systems.32 Knowing that the only property of diluted alcohol solutions that differs considerably from water is their surface tension, several correlations were developed for this specific liquid phase. Posarac and Tekic6 and Albijanic33 improved Hughmark’s correlation by introducing the number of C-atoms in the alcohol molecule 3581
dx.doi.org/10.1021/ie101989r |Ind. Eng. Chem. Res. 2011, 50, 3580–3591
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Experimental setup.
chain (CN) as the variable, beside the surface tension of the corresponding solution. However, a better choice was made by Albijanic et al.12 who introduced the surface tension gradient as a representing variable, instead of mutually dependent variables: the CN value and the surface tension. The validation of the mentioned correlations12 for the prediction of the hydrodynamics in the DT-ALR with diluted alcohol solutions was confirmed in our previous research13 with an expanded range of alcohols (from methanol to n-octanol). All of these correlations12,13,33 were developed for a DT-ALR with diluted alcohol solutions but only with single orifice as the gas sparger. None of the studies published so far have gone into details concerning the possible synergistic influence of alcohol addition and the gas sparging effectiveness on the hydrodynamics of the DT-ALR. The aim of this paper is to investigate this influence.
2. EXPERIMENTAL SETUP The experiments were conducted at 20 ( 1 °C and atmospheric pressure in a glass DT-ALR, with geometrical details presented in Figure 1. The air, sparged into the draft tube, was used as the gas phase. The gas flow rates were controlled and measured by a rotameter. Three different most commonly used
sparger types were applied: a single orifice (4 mm i.d.), perforated plate (7 holes of 1 mm i.d., triangular pitch), and sinter plate (100-160 μm pores, average pore size 115 μm, porosity 8%). Porosity and average pore size of the sinter plate were obtained by porosimeter (Porosimeter 2000 with Macropore Unit 120). This data was used to estimate a sum of cross-sectional areas of all pores available for gas flow through the sinter plate distributor. Tap water and dilute alcohol solutions from methanol to noctanol were used as the liquid phase. The concentrations of alcohols used in this experimental work, and the main physical properties of the liquid phase at 20 °C are summarized in Table 2. Surface tensions of liquid phases were obtained by tensiometer (Torsion Balance, Model OS) with (0.0001 N/m accuracy. The surface tension gradient (-dσ/dCA) was estimated from the slope of the experimental σ versus CA curve (Figure 2). Such a choice of alcohols and their concentrations resulted in achieving an extensive range of surface tension gradient values. Table 2 presents, also, the coefficient of determination (R2) as a measure of the goodness of fit of the surface tension gradient. The overall gas holdup was determined by the volume expansion technique with an error
