Scholarly article on topic 'Gas Hold-up in Three Phase Co-current Bubble Columns'

Gas Hold-up in Three Phase Co-current Bubble Columns Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — S. Kumar, R.A. Kumar, P. Munshi, A. Khanna

Abstract Bubble columns are used in a large number of applications in chemical engineering. The important variables that affect the gas holdup, bubble dynamics and flow regime in a bubble column are gas and liquid velocities, liquid viscosity, liquid surface tension, design of the gas distributor, solid concentration and column diameter. Experiments have been performed in a 15cm diameter co-current slurry bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35μm have been used as solid phase. Solid loading up to 9% has been used. The superficial gas velocity varies from 1.0 to 16.28cm/s and superficial liquid velocity varies from 0 to 12.26cm/s. Effects of liquid height, liquid velocity, gas velocity and solid concentration over gas holdup for both two and three phase co-current flows have been studied. For batch case the liquid height didn’t affect the gas holdup. The gas holdup increases with increase in gas velocity for both two and three phase co-current columns. For two phase and three phase flow up to 1% solid loading; at low superficial gas velocity i.e. in the homogeneous regime, the increase in liquid velocity doesn’t show any change in the gas holdup. For higher gas velocities i.e. in the heterogeneous regime, increase in liquid velocity decreases the gas holdup rapidly. Above 1% solid loading, liquid velocity effect over gas hold-up is negligible. With increase in solid concentration for co-current bubble column the gas holdup slightly increases or remains constant up to 5% loading; beyond this loading there is a significant decrease in gas holdup

Academic research paper on topic "Gas Hold-up in Three Phase Co-current Bubble Columns"

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Engineering

Procedía

ELSEVIER

Procedía Engineering 42 (2012) 851 - 863

www.elsevier.com/locate/procedia

20th International Congress of Chemical and Process Engineering CHISA 2012 25 - 29 August 2012, Prague, Czech Republic

Gas hold-up in three phase co-current bubble columns

S. Kumara, R. A. Kumara, P. Munshib, A. Khannaa a*

aDepartment of Chemical Engineering, IIT Kanpur, UP-208016, India bDepartmentofMechanicalEngineering, IIT Kanpur, UP-208016, India

Abstract

Bubble columns are used in a large number of applications in chemical engineering. The important variables that affect the gas holdup, bubble dynamics and flow regime in a bubble column are gas and liquid velocities, liquid viscosity, liquid surface tension, design of the gas distributor, solid concentration and column diameter. Experiments have been performed in a 15 cm diameter co-current slurry bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35 ^m have been used as solid phase. Solid loading up to 9% has been used. The superficial gas velocity varies from 1.0 to 16.28 cm/s and superficial liquid velocity varies from 0 to 12.26 cm/s. Effects of liquid height, liquid velocity, gas velocity and solid concentration over gas holdup for both two and three phase co-current flows have been studied. For batch case the liquid height didn't affect the gas holdup. The gas holdup increases with increase in gas velocity for both two and three phase co-current columns. For two phase and three phase flow up to 1% solid loading; at low superficial gas velocity i.e. in the homogeneous regime, the increase in liquid velocity doesn't show any change in the gas holdup. For higher gas velocities i.e. in the heterogeneous regime, increase in liquid velocity decreases the gas holdup rapidly. Above 1% solid loading, liquid velocity effect over gas hold-up is negligible. With increase in solid concentration for co-current bubble column the gas holdup slightly increases or remains constant up to 5% loading; beyond this loading there is a significant decrease in gas holdup

© 2012 Published by Elsevier Ltd. Selection under responsibility of the Congress Scientific Committee (Petr Kluson)

Keywords: Bubble column; three phase flow; solid effect; co-current flows; gas hold-up

a* Corresponding author. Tel.: +91512-2597117; fax: +91512-2597104. E-mail address: akhanna@iitk.ac.in.

1877-7058 © 2012 Published by Elsevier Ltd. doi: 10.1016/j.proeng.2012.07.470

1. Introduction

Bubble columns are mostly used as multiphase reactors in chemical, petrochemical, biochemical and metallurgical industries [1]. As compared to other reactors both in design and operation, the major advantages bubble columns provide are excellent heat and mass transfer characteristics, less maintenance and low operating costs due to lack of moving parts and can easily handle solids. The important variables affecting the gas hold up in a bubble column are gas and liquid velocities, liquid viscosity, surface tension, design of the gas distributor, solid concentration and column diameter [2-5].

Generally, at low superficial gas velocities bubbles are small and uniform in size [6]. Their size and uniformity depends on the properties of the liquid. It also depends on the design of the gas distributor and the column diameter. Here, bubble coalescence rate along the column is insignificant [7]. Hence, if the gas is distributed uniformly at the column inlet, a homogeneous bubble column is obtained.

At high superficial gas velocities, the bubble coalescence rate increases significantly, the gas-liquid flow becomes heterogeneous and the bubble column contains a mixture of large and small bubbles [7]. The size of large bubbles depends on the design of the gas distributor, column diameter and physical properties of the liquid. The hydrodynamics, mixing and transport properties of a heterogeneous bubble column are considerably different from that of a homogeneous column.

Numerous invasive and non-invasive techniques have been used to estimate the average gas hold up [89]. Measuring the gas holdup using DPT is non-invasive, hence does not interrupt bubble column operation. This method has been used in semi-batch bubble columns [10-12], and cocurrent bubble columns [13-14].

Shah et al. [15] studied the effect of liquid velocity in a downward flow bubble column. Tang and Heindel [16] studied the effect of sparger orientation in cocurrent and batch flow and observed a decrease in holdup with increasing liquid velocity. In cocurrent flow, the liquid velocity reduces the relative velocity between the liquid and gas and hence, the bubble- induced turbulence intensity. Hills has measured gas holdup in a 15cm diameter bubble column at gas superficial velocities of 0.07-3.5 m/s and liquid superficial velocities of 0-2.7 m/s. Hills [17]. He reported a decrease in gas holdup with an increase in liquid velocity. Fujie et al. [18] and Friedel et al. [19] also reported a decrease in gas holdup with an increase in liquid velocity in down flow bubble columns of 45 cm and 15 cm internal diameter, respectively.

Most of the published literature report that the gas holdup decreases with increasing solid concentration [13, 20-23]. The presence of solid increases the bubble size, which results in bigger and faster bubbles [13, 23-25]. In the presence of solid the increment in bubble population from small to large bubbles has been observed by Swart et al. [21]. Further, the reduction of bubble breakup [25-26] and increase of mixture viscosity [13, 22, 27-28] also contribute for the reduction of holdup.

Some other researchers have also observed a dual effect of solids on gas holdup [13, 29-32]. It indicates the presence of two counteracting physical mechanisms. Khare and Joshi [31] have shown that this dual effect leads to a maxima at about c = 0.6% of fine alumina particles. Banisi et al. [33] suggested that a small amount of fine particles (suppressing coalescence) and large amount of big particles (break up oflarge bubbles) tend to increase the holdup (reduce meanbubble speed).

Li and Prakash [34] have shown that gas holdups due to the small bubble fraction decreases while their rise velocities increases almost linearly with increasing slurry concentration up to a slurry concentration of about 20% v/v. The observed decrease in the small bubble rise velocity at higher slurry concentrations has been explained in terms of accumulation of fine bubbles in the system. The fine bubble fraction, which is included in the smaller bubble fraction, can account for the decrease in smaller bubble rise velocity and a corresponding increase in gas holdup at high slurry concentrations. The holdup due to large bubble slightly decreases up to solid concentration of 20% v/v. afterwards it is practically independent of concentration. This decrease has been attributed to the increment in the rise velocity of large bubbles for increasing solid concentration.

Swart et al [21] studied the gas holdup of air/paraffin oil in the presence of porous silica particles having 38 m mean diameter at atmospheric conditions in batch operation. They concluded that increasing slurry concentration reduces the total gas holdup, because of the destruction of the small bubble population. Gandhi et al [26] studied the effect of glass beads (35 m average size) concentration in water up to 40 vol% in discrete step of 10% on the gas holdup in a batch column. They also found that the average gas holdups decreased with increasing slurry concentration but the rate of decrease was less for higher slurry concentrations. But the literature that considers the effect of solid concentration from 1 % to 10 % is scarce. Mena et al [32] studied the effect of solid concentrations in the range of 1-30% in a batch column for average particle size = 2.1 mm. They reported a significant increase in gas holdup as solid concentration increases from 1-5%. In the range of 5-30%, sudden reduction in gas holdup occurs as the solid concentration increases.

Our focus of work is to find the effect of gas & liquid velocities and solid concentration over three phase co-current flows with glass beads of 35 m average size as the solid phase.

2. Experimental Details and Analysis

All The material used for the construction of the column is plexi-glass. For ease of installation and dismantling for cleaning purpose, the column is divided into three sections, each of 64 cm mutually attached through flanges. The inner diameter of the column is 15 cm and thickness of the column is 5mm. The total height of the column is 2.72 m. Gas phase enters from the bottom of the column through a gas distributor. Liquid phase enters through the conical section of the column at the bottom of the column. A distributor plate is provided for uniform distribution of the liquid phase. Five taps are provided at axial locations to measure the pressure drop in the column using Differential Pressure Transducers (DPTs). Liquid flows through 3.75 cm pipe line, which can sustain up to the pressure 15 bars. An outlet of 5 cm pipe line is provided at top of the column for disengagement of gas and liquid/slurry phase. Compressed air flows through 1.25 cm Stainless Steel (SS) pipe line. Compressed air has been used as gas phase and water has been used as liquid phase. Glass beads of density 2500 kg/m3 have been used as solid particles. Solid loading up to 9% wt/vol has been used. The superficial gas velocity varies from 1.13 to 16.28 cm/s and superficial liquid velocity varies from 0 to 16.04 cm/s. Details of operating parameters have been given in Table 1. A 1000 liter capacity tank is used to store the water for recirculation. An agitator with two impellors has been installed in the tank to properly mix the slurry. The schematic diagram of experimental setup has been shown in Figure 1.

Fig. 1. Schematic diagram of the experimental setup

For a three phase system, the pressure gradient is defined as

The sum of volume fraction of individual phases must be equal to unity. Hence,

The volume fractions of solid and liquid phases in liquid-solid slurry phase can be defined as

Here / and * are volume fraction of liquid and solid in the slurry phase. The volume fractions in three-phase suspension can be defined in terms of volume fractions in liquid-solid slurry as

= i _ ('_±_) , MM

vt+^sG^- Pi)/

Tang and Heindel [14] have developed an alternate method to calculate the average gas hold-up as shown in equation (8). They concluded that this always produces less error compared to the equation (7).

where P — is the pressure difference between the lower and higher ends of column section and Po,ui is the corresponding static pressure difference when no gas is flowing in the column, keeping all other conditions as same. Eq. (7) and Eq. (8) both neglect the effect of wall shear stress and liquid acceleration due to void changes that may influence gas holdup in cocurrent bubble columns [15, 16]. In our case the effect of wall shear stress is negligible, since the wall shear stress is significant only after Ui> 40 cm/s for air water system [35].

We have applied equation (8) for calculating the gas holdup values. Pressure fluctuations have been measured using the four DPTs as a function of the superficial gas velocity. The distances of the DPTs are 25.3 cm, 81.3 cm, 106.6 cm and 147.3 cm from the sparger. These are piezoelectric sensors supplied by the Honeywell International, USA (ST 3000 Smart Pressure Transmitter). Dynamic pressure measurements have been carried out at a frequency of 50 Hz with a total acquisition length of 10000 points (200 s) for each measurement. For measuring liquid/slurry flow, electromagnetic flow meter (Model No: AQUAMAG by Krohne Marshall) is used. Air rotameter (PG-1,2 and 8 by Eureka Equipments) has been used to measure the gas flow rate.

3. Effect Of Various Parameters

3.1. Effect ofstatic height of liquid

All Experiments were performed to measure the effect of static height of liquid on overall gas holdup in a batch column. Pressure drop was measured to calculate the overall gas holdup. Experiments were performed for 1 and 1.8 m of static liquid height. From Figure 2, it can be observed that static height of liquid has no significant effect on gas holdup. Literature has shown that for H/D> 5 and diameter greater than 10-15 cm; the effect of liquid height over gas hold-up is negligible [2, 5]. Our bubble column satisfies both the criteria.

Fig. 2.

Effect of liquid height on gas holdup

3.2. Effect ofGas Velocity

Most published studies have shown that increasing the superficial gas velocity leads to increase in the gas holdup [6, 34, 36-38]. The dependence of the gas holdup on the superficial gas velocity has been defined by the following power-law expression [39]:

where n is dependent on the flow regime. Initially, the gas holdup seems to increase sharply and almost linearly with the superficial gas velocity in the homogeneous flow regime where the exponent n in Equation (9) is generally reported to be in the range of 0.8-1.2 [2,39]. In the heterogeneous regime the value of n decreases and it lies in the range 0.4-0.8.

Figure 3 is a sample graph showing the effect of superficial gas velocity over gas holdup. The gas holdup increases sharply first, and the slope becomes relatively flat at a later stage. These two different slopes correspond to the homogeneous and heterogeneous regime respectively.

Superficial gas velocity (Cm/s)

Fig. 3. Effect of Superficial gas velocity over gas holdup

3.3. Effect ofLiquid 'Velocity

Most published studies have shown that increasing the superficial gas velocity leads to increase in the gas holdup [6, 34, 36-38]. The dependence of the gas holdup on the superficial gas velocity has been defined by the following power-law expression [39]:

Experiments were performed for liquid velocity up to Ui = 16.04 cm/s. In the present study, for two phase flows overall gas holdup was observed to decrease with increasing liquid velocity as shown in Figure 4. For, low superficial gas velocity the effect of liquid velocity on hold up is less. As we increase the superficial gas velocity, the hold-up decreases at a faster rate for change in the liquid velocity. Cocurrent flow of both gas and liquid, leads to an increase of bubble rise velocity. Bubbles with higher rise velocity leave the column faster, i.e., residence time of gas phase decreases which leads to lower holdup.

For three phase co-current flows, experiments were performed with different solid concentrations up to 9% wt/vol. Figure 5 & 6 shows that below 1% solid loading the trend is the same as in two phase flow. But for higher solid loading, the effect of liquid velocity over hold-up is insignificant as shown in Figure 7. High concentration of solid particles promotes the bubble coalescence, and thus decreases the hold-up. In the presence of high solid concentration, the liquid velocity hasn't much impact over the gas hold up.

Fig. 4. Effect of liquid velocity over gas holdup for two phase co-current flow

Fig. 5. Effect of liquid velocity over gas holdup for three phase co-current flow

Fig. 6. Effect of Slurry velocity over Gas holdup for 0.5% solid concentration

Fig. 7. Effect of liquid velocity over gas holdup for 9.0% solid concentration

3.4. Effect ofSolid Concentration

Mena et al. [32] have explained the effect of solid concentration in for particle size 2.1 mm and density 1023 kg/m3 in a batch bubble column. The addition of solid changes the density and viscosity of the liquid. Each particle surface creates a no-slip condition for the liquid, where the liquid velocity must be zero. Hence, extra velocity gradients arise and the viscous dissipation increases. This leads to apparent increase in the viscosity. The rise velocity of a bubble is reduced because of increased slurry viscosity. The hydrodynamic forces and mutual collision of bubbles and particles also reduces the speed of bubbles [28]. This reduction in bubble rise velocity increases the gas holdup. The bubble coalescence is also promoted in viscous media that allows the formation of bigger bubbles. The suspension in the slurry in general promotes the coalescence. This reduces the overall hold up. This effect increases with increase in slurry concentration. The effective viscosity of slurry is given by ^eff = ^*(l+2.5®s), where ®s is the slurry concentration. These two competing mechanisms lead to maxima in the gas holdup vs solid loading curve around 3-5% solid loading.

In the present case glass beads with 35 m average size were used as solid particles in the three phase co-current air-water system. The liquid velocity has been varied from 0 to 16.04 cm/s. Solid concentration was varied in two different ranges from 0% to 1% and from 1% to 9%. For every liquid velocity, it was observed that the gas holdup is either constant or slightly increases up to 5% solid loading, but there was a considerable decrease in the holdup for solid concentrations above 5 %. Figure 8 and 9 show the phenomena clearly. Figure 9 shows that the maximum gas holdup occurs at 3% solid loading.

0.20-,

Us = 12.26 cm/s

Solid loading (wt%)

■ 0.1% 0.3%

0.5% 0.7% 1.0%

0 2 4 6 8 10 12 14 16 18

Superficial Gas Velocity(U ), cm/s

Fig. 8. Effect of Solid Loading over Gas holdup for (0-1%) solid loading

0.16 -

Solid loading (wt%)

■m— 1.0% • - 3.0% -a— 5.0% 7.0%

0 2 4 6 8 10 12 14 16 18

Superficial Gas Velocity(Ua), cm/s

Fig. 9. Effect of Solid Loading over Gas holdup for (1-9%) solid loading

4. Conclusions

Experiments have been performed for two and three phase bubble columns in batch and co-current mode of operation. The solid loading has been varied in two ranges (0-1) % and (1-9) %. There is no effect of liquid height over gas holdup in batch column. The gas holdup increases with increase in gas velocity for both two and three phase flow. The slope of this curve is more for homogeneous regime and less for heterogeneous regime. For co-current flows, at low superficial gas velocity the increase in liquid/slurry velocity doesn't show any change in the gas holdup. For higher gas velocities with increase in liquid/slurry velocity the gas holdup decreases rapidly. For co-current flows, with increase in solid concentration the gas holdup is nearly same or slightly increases up to 5% solid loading. There is a significant decrease in gas holdup for solid concentration above 5%. This shows that the presence of solid has a dual effect over the gas holdup. Specifically for higher solid concentrations (>5%) the gas holdup decreases sharply

Acknowledgements

The authors acknowledge the research grant support provided by Chevron, USA; Advanced Refining Technologies (ART), USA and Hindustan Petroleum Corporation Limited (HPCL), India.

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