Hydrodynamics of organic and ionic liquids in a slurry bubble column reactor operated at elevated temperatures

Accepted Manuscript Hydrodynamics of organic and ionic liquids in a slurry bubble column reactor operated at elevated temperatures Manuel Götz, Jonathan Lefebvre, Friedemann Mörs, Rainer Reimert, Frank Graf, Thomas Kolb PII: DOI: Reference:

S1385-8947(15)01455-2 http://dx.doi.org/10.1016/j.cej.2015.10.044 CEJ 14320

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

8 August 2015 17 October 2015 19 October 2015

Please cite this article as: M. Götz, J. Lefebvre, F. Mörs, R. Reimert, F. Graf, T. Kolb, Hydrodynamics of organic and ionic liquids in a slurry bubble column reactor operated at elevated temperatures, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.10.044

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Hydrodynamics of organic and ionic liquids in a slurry bubble column reactor operated at elevated temperatures Manuel Götz*1, Jonathan Lefebvre2, Friedemann Mörs1, Rainer Reimert2, Frank Graf1, and Thomas Kolb2

1: DVGW-Forschungsstelle at the Engler-Bunte-Institute of the Karlsruhe Institute of Technology (KIT), Engler-Bunte-Ring 1, 76131 Karlsruhe, Germany

2: Karlsruhe Institute of Technology, Engler-Bunte-Institute, Fuel Technology, Engler-BunteRing 1, 76131 Karlsruhe, Germany *Corresponding author: Phone: +49 721 608 4 2568, Fax: +49 721 606-172, Email: [email protected] Highlights •

A specially designed gas sparger formed small bubbles and large interfacial areas

•

Hydrodynamics of ionic liquids operated above 373 K has been investigated for the first time

•

At elevated temperature, the behavior of ILs is dominated by their surface tension

•

The influence of particle size, solid concentration, and particle density has been investigated

•

An increase in particle density has been observed to reduce the gas holdup significantly

Abstract Hydrodynamics (e. g. gas holdup, bubble size, and flow regime) significantly influences the performance of slurry bubble reactors. In this work, the influence of gas, liquid, and solid phase properties on the hydrodynamics was investigated at temperatures up to 573 K and pressures up to 0.5 MPa. Amongst the investigated liquids, the hydrodynamic behavior of 1

ionic liquids at elevated temperatures is ruled by their high surface tension leading to small gas holdups and formation of large bubbles compared with heat transfer oils. The solids strongly influence the hydrodynamics but their effects are still not fully understood. Therefore, the behavior of solids was investigated in this work. The results indicate that small concentrations of fine particles reduce bubble coalescence and subsequently stabilize the homogeneous regime. However, the primary bubble size increases with the addition of solids. Furthermore, it has been shown that the gas holdup decreases with an increasing difference between particle and liquid density. Keywords Hydrodynamics; Gas Holdup; Slurry Bubble Column Reactor; Three-Phase Reactor; Elevated Temperature; Ionic Liquid

2

1

Introduction

Slurry bubble column reactors (SBCR) are widely used in the chemical industry [1-3]. Usually, the gas and liquid phases are reactants and/or products, whereas the solid phase is a catalyst. Typical reactions (industrial and research) performed in such reactors are FischerTropsch synthesis, hydrodesulphurization and hydrogenation of fatty oils, and other hydrogenations and oxidations [1, 2, 4-10]. Recent experiments on SBCR have been done in which liquids have been used as an inert medium for three-phase methanation [11-16] and for three-phase methanol synthesis [17-19]. Sometimes, the solid phase is a reactant (e. g. coal liquefaction [20]). In SBCRs, the volumetric mass transfer coefficient kLa [21, 22] and thereby the overall conversion rate [1, 13, 23] is strongly influenced by the hydrodynamics of the system. The hydrodynamics in a SBCR is rather complex. The hydrodynamics include gas holdup (εG), bubble size (dB), backmixing, and flow regime. According to Deckwer [1], three flow patterns can be found in SBCRs, the homogeneous regime (narrow bubble size distribution), the heterogeneous regime (small and large bubbles exist in parallel), and the undesired slug flow regime. The transition between the regimes is not sharp. According to Kazakis et al. [24], the term pseudo-homogeneous is used to describe the regime in which the gas holdup increases linearly with increasing uG, but without uniformity in the bubble size distribution. Several parameters affect the gas holdup and the bubble size. These parameters include the physical properties of the gas, liquid, and solid phases, the operating variables (gas velocity and solid concentration), the dimensions of the reactor, and the design of the gas distributor [8, 21, 23, 25-28]. Furthermore, a parameter can affect the gas holdup in different ways depending on the flow regime (e. g. gas density [29] and sparger design [21, 30]). Most studies regarding hydrodynamics of bubble columns were performed in aqueous solutions or alcohols [28]. However, ionic liquids which are salts (usually organic) with a melting point below 373 K have gained attention in the past years [31-34]. Few studies were found that investigated the hydrodynamics [35-37] and the gas-liquid mass transfer [38] of these liquids. According to these studies, the behavior of ionic liquids is ruled by their high viscosity. The high viscosity leads to bubble coalescence and to relatively small gas holdups. However, all the studies with ionic liquids were performed at temperatures between 288 – 308 K. At higher temperatures (e. g. more than 373 K), ionic liquids show lower viscosities resulting in a different hydrodynamic behavior. For the first time, this work investigates the hydrodynamics of ionic liquids at elevated temperatures (up to 343 K). Besides the gas and liquid properties, addition of solids can have a strong effect on the hydrodynamic behavior of bubble columns. In most studies, enhancement of bubble coalescence is observed if solids are present [25, 39, 40]. This not only leads to larger bubbles and destabilization of the homogenous regime but also to a reduction in both gas hold up as well as volumetric mass transfer coefficient [10, 21, 41-47]. However, contradictory results have also been reported in literature. According to Rabha et al. [44], small concentrations (< 3 %) of very small particles (< 100 µm) do not influence the hydrodynamics. Luo et al. [48] reported that the effect of the particle size on εG can be neglected in the range of 44 - 254 µm. In a few studies, even a positive effect on εG was 3

reported if small solid concentrations are used [49-51]. In section 3.2, an attempt has been made to clarify these different observations. The findings described above are only valid for wettable particles. In contrast, small nonwettable particles are reported to accumulate at the gas-liquid interface reducing bubble coalescence thereby increasing εG and kLa [25, 52]. Throughout this work, wettable particles were used. Often, three-phase reactors are operated at large gas velocities in the heterogeneous regime [1]. The large gas velocity results in high kLa values [2, 8]. In reactions such as FischerTropsch synthesis and hydrogenation, the unconverted gaseous educts can be recycled, since the product is a liquid. However, this does not apply to reactions such as the abovementioned three-phase methanation. In three-phase methanation, both educts (H2, CO, CO2) and the desired product (CH4) are gaseous. Furthermore, the produced CH4 is to have a high purity of at least 95 %. [23]. Since separation of methane from the unconverted educts would be associated with a large expenditure, a high conversion rate is to be reached in one pass. Consequently, small gas velocities are required. This article focuses on the hydrodynamics of SBCRs operated at low gas velocities (below 0.04 m/s). The aim of the article is to investigate and evaluate the principle behavior of an SBCR operated at elevated temperature and pressure.

4

2

Experimental

The experiments were carried out using a bubble column of
 = 0.0246 m. For several

2.1 Experimental setup

reasons, such a small reactor diameter had to be used. First, the costs of ionic liquids are in the order of several 1000 €/kg. Another reason is that the same reactor diameter was used to

perform methanation experiments as described in [12, 13]. Based on such experiments, the conversion rates measured in an SBCR can be linked directly to the underlying hydrodynamics [23]. At our laboratories, the methanation reaction cannot be carried out in a reactor with a diameter of 0.15 m or larger, since several hundred m³/h of H2 and CO would be required (gas recycling is not possible, since H2 and CO are converted into CH4). As shown in the supplementary material (Table S.7), we carefully proved that the qualitative findings described in this work are independent of the reactor diameter (except otherwise stated). In the homogeneous regime, for instance, a larger reactor diameter would only lead to lower gas holdup values [53-57], but its behavior would be similar. The reactor is equipped with a glass sector of 0.18 m in length. The glass sector was pressed between two steel flanges. Plate springs were used to make up for the reactor expansion at high temperatures (see Figure 1). For the gas holdup experiments demonstrated in this article, the glass cylinder was installed 0.28 m above the gas sparger. Further experiments were performed with different positions of the glass cylinder, i.e. with different slurry heights. The variation of the height/diameter ratio had no effect on the measured gas holdup (see supplementary material, Figure S.1). Perforated plates were used as gas spargers. Unless otherwise specified, a perforated plate consisting of 435 holes with a hole diameter of 100 µm (resulting in a free area of 0.72 %) produced by laser drilling has been used (named G-25-1). The gas sparger was specially designed to reach high gas holdups and to show defined pitches of the holes (Phole = 10 dhole). Besides that, experiments with a plate with 132 holes (dhole = 100 µm, Phole = 2 10-3 m) were performed (free area of 0.22 %, named G-25-2).

5

Figure 1:

Bubble column with glass sector used in this work

The gases (N2, H2, CO, CO2 and Ar) were supplied by means of a set of mass-flow controllers (Bronkhorst). These gases were chosen to vary the gas density and because of the envisaged application (three-phase methanation) described in [13, 58]. The feed gas was preheated to the desired temperature before entering the column. The liquid suspension temperature was measured near the gas distributor region. Three heat transfer oils and four ionic liquids have been studied. These liquids have been chosen because their properties would fit to the operating conditions of a three-phase methanation reactor [13, 23]. A list of the tested liquids and their relevant properties can be found in Table 1. The purity of the ILs was at least 99%, and the water content was below 300 ppm. More information regarding the property data can be found in [23, 58] and in the supplementary material (Table S.6). The thermal stability of the ionic liquids in the presence of H2 and N2 is discussed in [59]. As solid phase, a commercially available porous Ni/Al2O3 (ρP = 1585 kg/m³) catalyst was selected because of its good methanation performance [12, 13]. Furthermore, pure non-porous Ni particles have been used (ρP = 8900 kg/m³).

6

Table 1:

Ionic Liquid (IL)

Oil

Type of liquid

List of the tested liquids with some property data; extrapolated data are printed in italics [23, 58, 60-68] Temperature in K

Density in kg/m³

Viscosity in 10-3 Pa s

Surface tension in 10-3 N/m

Dibenzyltoluene (DBT) M = 272 g/mol

473 573

911.1 836.5

0.71 0.38

24.12 15.43

Polydimethylsiloxane 1 (X-MT) M unknown

473 573

790.1 700.7

2.60 1.43

See X-BF

Polydimethylsiloxane 2 (X-BF) M ≈ 3200 g/mol

473 573

820.1 740.1

5.08 2.58

10.26 5.07

[BMMIM][Tf2N] M = 433 g/mol

473 573

1259.0 1167.2

1.43 0.45

25.34 21.06

[N1114][Tf2N]* M = 396 g/mol

473 573

1237.7 1149.5

1.61 0.51

31.66 27.98

[PMPip][Tf2N] M = 422 g/mol

473 573

1259.3 1173.4

1.56 0.48

28.9 25.26

[P(14)666][Tf2N]* M = 764 g/mol

473 573

940.0 867.7

2.48 0.75

19.26 11.41

Liquid

* Pictures of the bubble column operated with these liquids can be found in the supplementary material (Figure S.2)

2.2 Experimental methods The range of investigated experimental conditions is given in Table 2. With respect to the envisaged application (see section 1), low gas velocities of 0.004 – 0.04 m/s had been investigated. Table 2:

Investigated range of experimental conditions Parameter

Unit

Range

T

K

293 – 573 ILs up to 270

p

MPa

0.1 – 0.5

uG

m/s

0.004 – 0.04

ρG

kg/m³

0.0423 – 3.221

ρP

kg/m³

Ni/Al2O3: 1585 Ni: 8900

CSL

-

0 – 4.6 %

dp

µm

50 – 400

The gas holdup  was calculated from the gas-flushed (GLS) and the gas-free (LS) slurry

sector. Experiments were repeated at least two times, each time with a ratio ℎ /
 > 12. heights (see Eq. 1). The heights were measured with a ruler and the abovementioned glass

For  > 5 %, the relative error was usually below 1 %. When foaming occurred, the slurry

7

height was measured just under the foam layer. When foaming was too stark, the gas holdup measurement was not taken into account.  =

ℎ − ℎ ℎ

(1)

Speed Camera Casio EX-FH2. The bubble diameter
was then identified using a ruler For measurements of the bubble size, several pictures of the SBCR were taken using a High IMAGEJ was used. At least 30 different bubble diameters were used to calculate


present on the pictures (see Figures S.2 and S.3). Thereby, the image processing software

according to the Sauter mean diameter. For spherical bubbles, which was the usual case, the Sauter mean diameter of the bubbles can be calculated with Eq. 2.
=

∑
  ∑
 

(2)

The flow regime was identified using the bubble size distribution (occurrence of large bubbles as shown in the supplementary material (Figure S.2 and Figure S.3)), the fluctuation and

deformation of the liquid level, and the graph of  vs. . In this publication, the heterogeneous regime is defined when the first large bubble appears and slug flow is defined by the occurrence of bubbles with
> 0.5∙
 .

8

3

Results and discussion

3.1 2-phase system 3.1.1 Influence of gas velocity and gas density Gas velocity In the (pseudo-)homogeneous regime, the gas holdup increases almost linearly with increasing gas velocity [1]. For the X-BF / N2 system and for the gas sparger used in this work, the homogeneous regime can be observed up to a gas velocity of 0.01 – 0.015 m/s (Figure 2). The occurrence of large bubbles (heterogeneous regime) at uG = 0.015 m/s is regime transition is typical for perforated plates [1]. For  > 0.03 m/s, slug flow is observed. shown in the supplementary material (Figure S.3). This early homogeneous / heterogeneous

This early transition is typical for small reactor diameters and liquid viscosities higher than 3 10-3 Pa∙s [54]. Deckwer [1] and Shah et al. [55] also report the occurrence of slug flow at gas velocities above 0.025 m/s.

Figure 2:

Effect of gas velocity on the gas holdup; at 0.1 MPa (black) and 0.5 MPa (white) (X-BF / -3

N2, T = 523 K (ηL = 3.5 10 Pa s)); similar data for DBT / N2 can be found in the supplementary material (Figure S.1)

One aim of this work was to achieve high gas holdups since a good gas-liquid mass transfer is advantageous for the envisaged application described in [13, 58]. Compared to other systems described in literature, relatively high gas holdups (Figure 2) and low bubble rise velocities of  < 0.1 m/s (Figure 3) were achieved in the homogeneous regime (both caused

by small dB and large NB). According to Deckwer [1],  = 0.18 – 0.24 m/s are typical values.

Krishna [42] measured  > 0.20 m/s for a water / air system with a reactor diameter
 = 0.05 m. In this work, the small orifices of 100 µm and the large number of orifices per area lead to small bubbles of dB = 1 – 2 10-3 m and a large specific interfacial area aGL up to 700 m-1 (see supplementary data, Figure S.4). The influence of the gas sparger is stronger in non9

coalescing media [69]. As described in section 3.1.2, the heat transfer oils at temperatures above 473 K can be assumed as non-coalescing media leading to stabilization of small gas bubbles. The small bubble diameter leads to reduced buoyancy and subsequently to smaller bubble rise velocities. Another aspect to explain the observed low bubble rise velocities is the relatively small reactor diameter (
 = 0.0246 m) used in this study. Small reactor diameters

result in greater wall effects which slow down the bubble rise in the column. Furthermore, an increase in εG can be found with decreasing dR [53-57].

Figure 3:

Effect of superficial gas velocity on the bubble rise velocity (white) and the bubble diameter (black) (X-BF / N2, p = 0.1 MPa, T = 523 K)

In the literature, there is obviously no information about perforated plates with very small holes as used in this work. Typically and possibly due to manufacturing reasons, perforated plates have relatively large holes diameters in the range of millimeters (free plate area afree usually < 1 %). On the contrary, porous plates usually have a pore size below 100 µm and their specific free plate area afree is much higher because of the small pitch of holes (afree often > 10 %). In order to assess the effect of the pitch of holes, two perforated plate gas spargers with different pitches of holes were investigated. Figure 4 shows the influence of the pitch of holes and the number of holes on the gas holdup. For the gas sparger G-25-1, dB/2 is larger than Phole. Therefore, bubble coalescence could be assumed. However, this assumption has not been confirmed. The gas sparger with the smaller pitch (G-25-1) leads to a significantly higher gas holdup (Figure 4 left) and to a larger number of bubbles per reactor volume (Figure 4 right). The larger number of holes for sparger G-25-1 causes smaller gas velocities per hole and a higher bubble production rate, whereas the non-coalescing media helps to avoid bubble coalescence. Furthermore, the gas sparger G-25-2 with the lower number of holes causes an earlier regime transition (Figure 4 right). Since the G-25-1 gas sparger leads to higher gas holdups and smaller bubbles, this gas sparger was used for all the experiments described below.

10

Figure 4:

Comparison of two perforated plates; G-25-1 (black): 100 µm holes, 1 10-3 m pitch, 435 -3

holes; G-25-2 (white): 100 µm holes, 2 10 m pitch, 132 holes (DBT / N2, p = 0.1 MPa, T = 533 K); for bubble size (Table S.1) and for video (0.02 m/s, G-25-2) see supplementary material

Gas density The gas density was varied by using three different gases (H2, N2, and CO2) and additionally by applying N2 at 0.1 MPa, 0.3 MPa, and 0.5 MPa (see supplementary material, Table S.2). In the investigated range, the gas density (and therefore the molecular weight) has a small effect on the gas holdup (Figure 5). A small effect of the gas density in the homogeneous regime was also described in literature [29, 53]. An increasing gas density reduces bubble coalescence. According to Wilkinson and van Dierendonck [70], this is explained by the Kelvin–Helmholtz instability of larger bubbles. Consequently, the gas density effect is rather small in the homogeneous regime because there is no bubble coalescence. The dependence of εG on ρG is shown in Eq. 3. As shown in Table 3, the constant K is small in the homogeneous regime.  ~

Table 3:

(3) Literature data for the dependence of εG from ρG in the homogeneous regime Author Reilly et al. [29] Wilkinson et al. [53] Hikita et al. [71]

K (Eq. 3) 0.04 0.03 0.06

In the heterogeneous regime, the influence of gas density via reduced bubble coalescence is not negligible [27, 29, 53, 72-74]. However, in this work, the heterogeneous regime was only observed in a small uG range (e. g. Figure 2: heterogeneous regime from 0.015 – 0.02 m/s) because of the small reactor diameter. According to our own data and to literature data [1, 75, 76], the small bubble gas holdup is much higher than the large bubble gas holdup even in the heterogeneous and slug flow regimes as long as small gas velocities < 0.05 m/s are used (i. e. NSB >> NLB as shown Figure 8 left). Accordingly, the gas density does not have an influence on the gas holdup for the experimental setup used in this work. 11

Figure 5:

Effect of gas density on the gas holdup (X-BF / H2, N2 or CO2, T = 523 K); see Table S.2 for gases and pressures used

3.1.2 Influence of liquid properties Viscosity, density, and surface tension are the most important properties of pure liquids with respect to hydrodynamics. According to a correlation developed by Behkish et al. [26], the gas holdup increases with increasing liquid density and decreases with increasing surface tension and viscosity (Eq. 4, SI-units).  ~

. .  ∙ .

(4)

The effect of liquid density has not been investigated systematically in this work. The liquid density influences buoyancy and drag force of a gas bubble. Buoyancy and drag force have contradictory effects. On the one hand, increasing ρL increase the bubble rise velocity (increased buoyancy). On the other hand, according to Geary and Rice [77] and to Eq. 5 ([78], from [79]), an increase in ρL leads to smaller bubbles formed at the gas sparger (see supplementary material, Figure S.5). Besides the correlation of Behkish et al., other correlations also show an increasing gas holdup with increasing liquid density [21, 71, 80]. 1 
#$%& 3
#$%&  3
#$%&   ,- ./,#$%& + +
=! + )* ' ' '

(5)

Liquid viscosity At temperatures above 473 K, all the investigated liquids have moderate viscosities (Table 1). According to the literature, an increasing liquid viscosity ηL promotes bubble coalescence. Thereby, the heterogeneous regime is observed earlier (i.e. at a smaller uG,trans ). Furthermore, the bubble size increases and a decrease is observed in both εG and kLa [21, 12

24, 27, 45, 53, 76, 81-85]. However, as for gas density, the effect of liquid viscosity depends on the flow regime. To investigate the viscosity effect, two silicon oils (X-BF and X-MT) with different chain lengths are compared. Figure 6 shows a slightly higher gas holdup for the more viscous XBF, as long as the homogeneous regime is compared. This seems to be in disagreement with Eq. 4, but Stegeman et al. [81] also found an increase in gas holdup with increasing viscosity for small gas velocities. Assuming the same bubble size formed at the gas sparger in both silicon oils, an increased viscosity leads to higher drag forces reducing the bubble rise velocity and subsequently increases the gas holdup. For gas velocities > 0.02 m/s, the gas holdup in X-BF is lower because of a regime transition to the slug flow regime. Consequently, the higher viscosity of X-BF leads to an earlier regime transition. This is in agreement with findings of other authors ([30, 76, 86] and Eq. 4).

Figure 6: Comparison of gas holdup for different liquids (N2, p = 0.1 MPa, T = 473 K)

Ionic liquids (ILs) usually have a high viscosity at low temperatures. However with increasing temperature, the viscosity of ionic liquids decreases strongly, whereas the surface tension decreases slowly. Consequently, these liquids are well suited for the investigation of the effects of viscosity. For example, an increase in temperature from 293 K to 473 K decreases the viscosity of [BMMIM][Tf2N] by 99 %, whereas the surface tension drops only by 23 % (from 33 mN/m to 25.3 mN/m). Kaji et al. [35] and Zhang et al. [36] investigated the hydrodynamics of ILs at moderate temperatures (288 – 308 K). According to these studies, the behavior of ionic liquids is ruled by their high viscosity. The high viscosity leads to bubble coalescence and to relatively small gas holdups. This is in agreement with our own measurements, shown in Figure 7 (abscissa in logarithmic scale). At 293 K (ηL = 133 mPa s) slug flow prevails at the lowest investigated gas velocity of 0.005 m/s. It is well-known that the homogeneous regime can no longer exist if the liquid viscosity is too high [1, 30, 42, 76, 82]. By increasing the temperature, the viscosity drops and consequently the regime transition shifts to higher gas velocities.

13

Figure 7:

Influence of viscosity on the flow regime for [BMMIM][Tf2N] (black points: measured gas velocities, N2, p = 0.1 MPa); the measured bubble size for this IL can be found in the supplementary material (Table S.3); the viscosity was changed by varying the temperature between 293 – 473 K

In Figure 6, [BMMIM][Tf2N] can be compared with X-BF at 473 K. At this temperature, X-BF has a 3.6 times higher viscosity but, nevertheless, the gas holdup in X-BF is much higher compared to [BMMIM][Tf2N]. This finding must be explained by a 2.5 times lower surface tension. Consequently, the behavior of ionic liquids at elevated temperature is ruled not only by their viscosity but also by surface tension. This result extends the findings of Kaji et al. [35] and Zhang et al. [36] who had shown that viscosity is the predominant factor of ILs at low temperatures (288 – 308 K).

Liquid surface tension An increased surface tension results in increased bubble diameters dB which in turn reduces εG (Eq. 4) and kLa [21, 30, 42, 45, 53, 69, 85]. Also, the regime transition is shifted to smaller uG. A higher surface tension stabilizes large bubbles avoiding bubble breakup. The reduced bubble deformation due to a high surface tension can also reduce the mass transfer coefficient [87]. In Figure 8, the bubble population is shown for DBT, X-BF, and an IL. Small bubbles can be observed for the oils X-BF and DBT. In contrast, the use of the IL leads to large bubbles and a smaller number of bubbles per reactor volume. Consequently, the small gas holdup measured in [BMMIM][Tf2N] (Figure 6) is in agreement with the large bubble size as depicted in Figure 8.

14

Figure 8:

Pictures of the bubble column operated at uG = 0.02 m/s for three liquids (N2, p = 0.1 MPa, T = 473 K); the circle marks a large bubble; DBT: homogeneous regime, X-BF and [BMMIM][Tf2N]: heterogeneous regime

X-BF vs. DBT Comparing the gas holdup data in the homogenous regime when DBT and X-BF are used, it is observed that in the case of DBT, the gas hold up is lower than in X-BF (Figure 6). This behavior can be attributed to the higher surface tension of DBT leading to larger bubbles formed at the gas sparger (Eq. 5). To verify this assumption, the correlation of Geary and Rice [77], which is based on force balances during different stages of bubble formation, was used to calculate the bubble diameter formed at the gas sparger: •

X-BF: dB,calc = 1 10-3 m

• DBT: dB,calc = 1.3 10-3 m The calculated bubble diameters are smaller than the measured: 1.6 and 1.9 10-3 m, respectively. However, both methods (calculation and measurement) result in larger bubbles for DBT. DBT vs. ILs Surprisingly, the oil DBT with nearly the same surface tension as the ILs leads to smaller gas bubbles than [BMMIM][Tf2N] (Figure 8), and in DBT the regime transition is shifted to higher gas velocities (Figure 6 and Figure 8). In order to verify this discrepancy, the gas holdup in DBT at 423 K was compared with that in [PMPip][Tf2N] at 473 K (the effect of the changed gas density is negligible as shown in section 3.1.1). At these – different – temperatures, the two liquids have almost identical surface tensions and viscosities: • •

[PMPip][Tf2N], 473 K: ηL = 1.6 10-3 Pa s; DBT, 423 K:

-3

ηL = 1.3 10 Pa s;

σL = 28.9 10-3 N/m; -3

σL = 28.5 10 N/m;

ρL = 1259 kg/m³ ρL = 948 kg/m³

The results shown in Figure 9 seem to confirm the discrepancy. Furthermore, gas holdups for two more ionic liquids are given. It can be seen that all the three ionic liquids show the same gas holdup. This is not surprising since they have similar physical properties. In the 15

homogeneous regime (uG < 0.01 m/s), εG measured in DBT is also similar but at higher gas velocities significantly higher gas holdups are found. Consequently, the difference in the behavior of the oils and the ionic liquids cannot be attributed to differences in viscosity and in surface tension alone. The high densities of ILs should result in even higher gas holdups (see Eq. 4).

Figure 9:

Comparison of DBT at 423 K with three ionic liquids at 473 K; at these temperatures, all four liquids have similar ηL and σ L values

Surfactants To resolve the discrepancy, the presence of surfactants needs to be considered. Surfactants such as tensides and electrolytes drastically reduce bubble coalescence [88] resulting in a stabilization of the homogeneous regime and in increased gas holdup values. According to Ruzicka et al. [89], the use of low concentrations of CaCl2 in water (e. g. 0.06 mol/l) stabilized the homogeneous regime by ≈ 100 % and the gas holdup in the heterogeneous regime (at uG = 0.1 m/s) was more than doubled. However, the liquid properties ρL, ηL, and σL only changed by 0.5 %, 1.8 %, and 0.3 %, respectively, after surfactants addition. This leads to the presumption that surfactants are present in DBT and cause the difference between the ILs and DBT. The foaming tendency of DBT (see Figure 8) confirms this presumption, because surfactants are known to cause foaming [69, 90, 91]. Both foaming and bubble coalescence inhibition are based on the same mechanism, the Marangoni effect (description see below): Bubble coalescence is believed to occur in three steps [92, 93]. 1. Approach of two bubbles forming a thin film between them with an initial thickness of typically 1-10 µm. 2. Thinning of the film by drainage of the liquid: When the thickness of the film is reduced to ≈ 100 nm, van der Waals attraction increases the draining rate, while the electrostatic double layer repulsive force decreases it. Step 2 is believed to be the rate controlling step. 3. As the film thickness reduces below 10 nm, it ruptures, leading to coalescence. In the thin liquid film between two bubbles, the concentration of positive surfactants is lower than in the liquid bulk. The lower concentration causes an increase in the surface tension of 16

the film. The Marangoni effect results from this surface tension gradient causing flow in the direction of the higher surface tension area (healing of the film) [89, 90, 93]. For the gas sparger used in this work, surfactants should not influence the gas holdup in the homogeneous regime. But they stabilize the homogenous regime resulting in a shift of the transition velocity to higher values. This is clearly visible in Figure 9.

17

3.2 3-phase system It is well known that increasing solid concentrations reduce the gas holdup [8, 21, 94]. This negative effect of solid concentration is an important aspect for the design of catalytic slurry bubble column reactors. In such reactors, the catalyst concentration has a dual effect on the reactor performance. For low catalyst concentrations, the intrinsic reaction is rate limiting (see Eq. 6) and an addition of catalyst at constant GHSV (Gas Hourly Space Velocity) increases the effective (overall) reaction rate (e. g. [13]). 2,344 = 2-, ∙

5678 9:

(6)

reduces the mass transfer coefficient , ;. In the mass transfer limited regime (Eq. 7), the However, an increase in catalyst concentration decreases εG (see section 3.2.1), and thus effective reaction rate decreases with decreasing , ;. ∗ 2,344 = <, ;= ∙ <> − > =

(7)

This predicament reiterates the necessity of understanding the system hydrodynamics to enable a high reactor performance.

3.2.1 Solid concentration As expected, an increasing volume concentration of solid CSL has a negative effect on the gas holdup (Figure 10). In the literature, the decrease in gas holdup with solid concentration is often attributed to the increase in the slurry viscosity, thereby theoretically enhancing bubbles coalescence [27, 48, 95]. However, the change of slurry viscosity cannot be the only explanation [44, 49, 96]. Our own calculations for a X-BF / solid system at 523 K show that a solid concentration of 4.6 % increases the slurry viscosity by only 20 % (see supplementary data, Table S.4), which cannot account alone for the observed gas holdup decrease (small influence of viscosity as shown Eq. 4).

18

Figure 10:

Effect of volume concentration of solid on the gas holdup (X-BF / N2 / Ni/Al2O3, p = 0.1 MPa, T = 523 K, dP = 100 - 160 µm)

An increase in solid particle diameter
@ has a negative impact on the gas holdup (Figure

3.2.2 Particle size

11). This is in agreement with most literature findings [44, 50, 51]. Furthermore, some authors report no effect or even a positive effect (as described in section 1) of small particles on the gas holdup compared to solid-free systems.

Figure 11:

Effect of particle diameter and gas velocity on the gas holdup (X-BF / N2 / Ni/Al2O3, p = 0.1 MPa, T = 523 K, CSL = 2.4 %, the dots for dP < 100 µm were set to 75 µm based on a sieve analysis)

To investigate the dual effect of particles, three ranges of particle size were compared: Small particles: dP < 100 µm As shown in Table 4, the homogeneous regime is present at higher gas velocities uG compared with the solid-free system if a small solid concentration CSL of 2.4 % is used. 19

Consequently, small particles can stabilize the homogeneous regime. The regime transition from heterogeneous to slug flow regime is also shifted to higher uG. Such an effect was already reported by Sada et al. [51]. They showed that wettable particles with dP < 3 µm impede bubble coalescence due to particle presence in bubble film. Although bubble coalescence is reduced in our work, addition of fine particles causes a decrease in gas holdup in the homogeneous regime (see Figure 11). The findings of Sada et al. and the results presented here can be explained by an increase in primary bubble size formed at the gas sparger if solids are present (see supplementary data, Figure S.6). According to Luo et al. [97], solids increase the primary bubble size formed at the sparger compared with solid-free systems due to suspension inertial force. The suppression of bubble coalescence is not relevant in the homogeneous regime since there is no considerable bubble coalescence. However, suppression of bubble coalescence could explain the extended uG range of the homogenous regime compared with the solid-free system. In our experiments, for a further increase in particle content from 2.4 to 4.6 % (Table 4), the regime transition to the heterogeneous regime was observed earlier. As a consequence, the reduction of bubble coalescence by small particles is only valid for small solid concentrations. Based on the observed effects and on literature data, as a rule of thumb it can be said: small concentrations of fine particles reduce bubble coalescence and subsequently stabilize the homogeneous regime. Furthermore, according to [41, 49], small contents (i. e. a few percent) of fine particles (i. e. < 100 µm) tend to increase the gas holdup (suppressing coalescence). In contrary Figure 11 shows that this suppression does not necessarily cause an increase in gas holdup since primary bubble size formed at the sparger can be increased by particles. Nevertheless, the suppression of coalescence can be confirmed. Table 4:

Flow regime for particles with dP < 100 µm (N2 / X-BF, p = 0.1 MPa, T = 523 K, similar results were obtained at 473 K and 573 K)

uG in m/s

0

Solid concentration CSL in % 2.4 3.4

4.6

0.006

homogeneous

homogeneous

homogeneous

homogeneous

0.01

homogeneous

homogeneous

homogeneous

heterogeneous

0.015

heterogeneous

homogeneous

heterogeneous

heterogeneous

0.02

heterogeneous

heterogeneous

0.025

slug flow

slug flow

unknown

slug flow slug flow

Medium size particles: dP = 100 – 160 µm The results for this particle size are already shown in Figure 10. Suppression of coalescence is not observed anymore. The gas holdup decreases if solids are added as described in literature (see above).

20

Large particles: dP = 200 – 400 µm Large particles drastically influence the hydrodynamics of bubble columns. The homogeneous regime is not observed at all, and the regime transition to slug flow happens at a low gas velocity of uG = 0.01 m/s (Figure 11). The effect that the homogeneous regime is not formed anymore has already been described by Krishna et al. [94], though under very different conditions (dR = 0.38 m, CSL = 36 % und dP ≈ 40 µm). The negative effect of large particles could be explained by insufficient particle fluidization. In particular for low gas velocities as well as for larger particles, the volume fraction of the solid most likely exhibits a gradient along the column height [98]. To evaluate this possibility, the critical gas velocity required for complete suspension of solid this correlation, uG > 0.027 m/s would be required to fluidize particles with
@ = 400 µm (the used parameters and the equation (Eq. S.2) can be found in the supplementary material, Table S.5). Hence, the observed early regime transition can be explained by inefficient solid

particles was calculated applying a correlation developed by Koide et al. [99]. According to

fluidization: solid particles lay on the gas sparger and deform emerging bubbles leading to bubble coalescence [100].

3.2.3 Particle density Besides concentration and diameter, the particle density ρP should also have an impact on the hydrodynamics. However, studies concerning this parameter are rare. In [101], a decrease in εG with increasing ρP is observed. This is in agreement with the correlation of Behkish et al. [26] (Eq. 8).  ~exp <−2.231FG − 0.157
K K − 0.242=

(for liquids without surfactants)

(8)

The results shown in Figure 12 prove a negative influence of solid particle density on the gas holdup. A higher particle density leads to an apparent higher slurry density. An increase in slurry density leads to greater buoyancy decreasing the gas holdup. The addition of nickel particles (ρP = 8900 kg/m³) to pure X-BF with a concentration CSL of 3.4 % results in a slurry density (ρSL) increase of 34 %. Furthermore, Figure 12 shows that regime transition to the heterogeneous and slug flow regime is shifted to lower gas velocities due to an increasing particle density.

21

Figure 12:

Effect of particle density on the gas holdup (X-BF / Ar); black: Ni/Al2O3 (ρSL = 848 kg/m³); white: Ni (ρSL = 1080 kg/m³); p = 0.1 MPa, T = 493 K, CSL = 3.4 %, dP = 20 – 50 µm; (see supplementary data, Figure S.7, for comparison on a mass basis)

Another effect of the solid density could be related to the particle settling velocity (Eq. 9; Stokes' law; terminal velocity) and to the particle Reynolds number (Eq. 10). In this case, the difference between particle and liquid density would be of interest. Of course, it needs to be considered that the swarm settling velocity is different to the terminal settling velocity. Usually, the swarm settling velocity is smaller [102], except when particle clusters are formed. @,M&NO = QR@,O =

' ∙
@ ∙ <@ − = 18 ∙ 

(9)

@,M&NO ∙
@ ∙  

(10)

Koide et al. [21] compared glass spheres (ρP = 2500 kg/m³) with bronze spheres (ρP = 8770 kg/m³), where the glass spheres exhibited higher εG and kLa values. They developed a correlation considering the effect of the particle density as given in Eq. 11. According to this equation, the gas holdup depends on the difference between particle and liquid density. <@ −  =  U ~ S1 + T ∙ <1 −  = 

V

(11)

To prove the influence of ReP,∞ and (ρP - ρL), further experiments were performed whose results are shown in the following sections. Temperature variation In gas-liquid systems, the gas holdup usually increases with increasing temperature because surface tension and viscosity decrease with increasing temperature and their influence on εG 22

is much stronger than that of the decreasing gas density (e. g. [28, 43], Eq. 3 and 4 and own experimental data). However, in three-phase systems another effect has been observed. According to Eq. 9, an increase in temperature leads to an increase in uP,set∞, since ηL decreases (and (ρP – ρL) increases slightly). Furthermore, an increasing temperature increases the particle Reynolds number (Eq. 10). As shown in Figure 13, an increase in ReP,∞ leads to lower gas holdups. The temperature effect (decreasing gas holdup with increasing temperature) observed here was also reported in the literature [43, 103].

Figure 13:

Effect of particle Reynolds number and gas velocity on the gas holdup in a three-phase system (X-BF / Ar / Ni/Al2O3, p = 0.1 MPa, CSL = 3.4 %, dP = 20 – 50 µm)

Comparison of liquids with different ηL in three-phase systems In comparison to DBT, X-BF should lead to higher gas holdups in a three-phase system because of its higher viscosity (Table 1). The smaller viscosity of DBT would cause higher settling velocities and higher ReP,∞ (X-BF: ReP,∞ = 0.11; DBT: ReP,∞ = 5.1). The smaller gas holdup of DBT in three-phase systems is shown in Figure 14: the gas holdup in the system N2 / DBT / Ni/Al2O3 is much lower than in the system N2 / X-BF / Ni/Al 2O3. In particular for the higher catalyst loading of 4.4 vol%, the difference between the two liquids is significant: in XBF, the homogeneous regime can be observed up to a gas velocity of 1 cm/s. In contrast, the homogeneous regime disappears if DBT is used and slug flow already prevails at uG =1 cm/s. Obviously, solids, possibly due to their high specific surface, neutralize the effect of surfactants. The results confirm that the influence of solids is related to the particle settling velocity and the particle Reynolds number, respectively. However, further investigations are needed to improve the understanding and to take the swarm settling velocity into account.

23

Figure 14:

Comparison of X-BF (- -) and DBT (··) without solids (white), with CSL = 2.3 % (grey) and CSL = 4.4 % (black) (N2 / Ni/Al 2O3, T = 573 K, p = 0.1 MPa, dP = 100 – 160 µm and ReP,∞ = 0.11 for X-BF and 100 – 200 µm and ReP,∞ = 5.1 for DBT)

Finally, the gas holdup in ionic liquids for three-phase systems is shown in Figure 15. A temperature of 533 K was selected because this temperature was assumed as the maximum possible operation temperature of the IL [N1114][Tf2N]. The addition of solids caused a decrease in gas holdup. The suppression of coalescence by small amounts of fine particles was not observed during these experiments. Further data for gas holdup and bubble size can be found in the supplementary material (Figure S.6).

Figure 15:

Gas holdup for DBT and the IL [N1114][Tf2N] in a three-phase system (Ar / Ni/Al2O3, T = 533 K, p = 0.1 MPa, dP < 100 µm, CSL = 0 and 2.5 %, respectively)

24

4

Summary

The aim of the work was to investigate the parameters influencing the hydrodynamics of a slurry bubble column reactor operated at elevated temperatures up to 573 K with heat transfer oils and with ionic liquids. First, a gas sparger was developed which produced a large gas-liquid interfacial area. As expected, gas spargers with small gas velocities per hole cause smaller bubbles and higher gas holdups. In particular, the use of a perforated plate with small holes (100 µm) and a defined pitch of holes of 1 10-3 m in combination with noncoalescing media enables the formation of small bubbles (1 – 2 10-3 m) and a large gas-liquid interfacial area (up to 700 m-1). The gas density was found to have a small effect on the gas holdup in the investigated range (mostly homogeneous regime). The influence of the liquid properties is different for different flow regimes. The following conclusions can be drawn: •

In the homogeneous regime, an increasing viscosity has no effect or even a positive effect on the gas holdup

•

In the homogeneous regime, the surface tension is the most important liquid property

•

An increase in viscosity and surface tension destabilizes the homogeneous regime

•

The hydrodynamics of the investigated heat transfer oils is significantly influenced by surfactants

The behavior of ionic liquids was extensively studied and can be summarized as follows: •

The use of ionic liquids leads to bubble coalescence and consequently to low gas holdups and to a destabilization of the homogeneous regime

•

The hydrodynamic behavior of ionic liquids is ruled by their viscosity at low temperatures (e. g. small uG,trans ) and by the surface tension at elevated temperatures (> 373 K)

•

All the investigated ionic liquids show a similar hydrodynamic behavior

For the influence of solids it was found that: •

Particles increase the primary bubble size formed at the gas sparger

•

Small concentrations of small particles (<< 100 µm) can reduce bubble coalescence, otherwise particles enhance bubble coalescence

•

An increase in density difference between solid and liquid density (e. g. by increasing the particle density) reduces the gas holdup

25

5

Nomenclature

5.1 Symbols and abbreviations Symbol

Unit

Name

[BMMIM][Tf2N]

-

1-Butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

[N1114][Tf 2N]

-

Butyltrimethylammonium bis(trifluoromethylsulfonyl)imide

[PMPip][Tf 2N]

-

1-Methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide

[P(14)666][Tf 2N]

-

Trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide

A, B, C

-

Constants

afree

-

Free area of the gas sparger

aGL

m

CSL

-

Gas-free volume concentration of catalyst

c

mol/m³

Concentration

DBT

-

Dibenzyltoluene

d

m

Diameter

dhole

m

Diameter of the gas sparger holes

FV

m³/s

Volumetric flow rate

-1

-1

Gas-liquid interfacial area per unit volume of reactor

GHSV

h

Gas Hourly Space Velocity

g

m/s²

Standard gravity

hGLS

m

Height of the gas-flushed column (Gas-Liquid-Solid)

hLS

m

Height of the gas-free column (Liquid-Solid)

IL

-

Ionic liquid

K

-

Constant

km

-

Constant in Eq. 5 related to the drag coefficient

kLa

1/s

Volumetric mass transfer coefficient

M

g/mol

Molecular weight

mcat

kg

Catalyst mass

N

-

Number

Phole

m

Pitch of holes

p

MPa

Pressure

SBCR

-

Slurry Bubble Column Reactor

T

K

Temperature

uB

m/s

Bubble rise velocity

uG

m/s

Superficial gas velocity

uG,crit

m/s

Critical gas velocity required for complete suspension of solid particles

uG,trans

m/s

Transition gas velocity homogeneous/heterogeneous

uP,set∞

m/s

Terminal settling velocity

ReP,∞

-

Particle Reynolds number (based on uP,set∞)

ri,eff

m-³ mol

Effective reaction rate W

 YZ[

/X Y8

\

26

-1

s

-1

rm,i

kg mol -1 s

Mass-based intrinsic reaction rate

V

m³

Volume

X-BF

-

Trade name of an oil based on Polydimethylsiloxane

X-MT

-

Trade name of an oil based on Polydimethylsiloxane (shorter chains than X-BF)



Pa s

Dynamic viscosity

-

Gas holdup

ρ

kg/m³

Density

σ

N/m

Surface tension

.

η

5.2 Subscripts * = equilibrium

B = bubble

calc = calculated

G = gas phase

het = heterogeneous Regime

i = serial number i = gas “i”

L = liquid

LB = Large Bubbles

P = particle

R = reactor

S = solid

SB = Small Bubbles

SL = Slurry

6

Acknowledgment

This publication is based on a work funded by the Federal Ministry of Education and Research, Germany.

27

7

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Highlights Hydrodynamics of organic and ionic liquids in a slurry bubble column reactor operated at elevated temperatures

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A specially designed gas sparger formed small bubbles and large interfacial areas

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Hydrodynamics of ionic liquids operated above 373 K has been investigated for the first time

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At elevated temperature, the behavior of ILs is dominated by their surface tension

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The influence of particle size, solid concentration, and particle density has been investigated

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An increase in particle density has been observed to reduce the gas holdup significantly

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