Polytetrafluoroethylene Wire Mesh Packing in a Rotating Packed Bed

Oct 14, 2016 - was used as wire mesh packing in a rotating packed bed. (RPB). The effective interfacial area (ae) and the volumetric liquid-side mass-...
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Polytetrafluoroethylene Wire Mesh Packing in a Rotating Packed Bed: Mass-Transfer Studies Qiu-Yun Chen,†,‡ Guang-Wen Chu,*,†,‡ Yong Luo,*,†,‡ Le Sang,†,‡ Li−Li Zhang,‡ Hai-Kui Zou,‡ and Jian-Feng Chen†,‡ †

State Key Laboratory of Organic−Inorganic Composites and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China ABSTRACT: Polytetrafluoroethylene (PTFE) material, which is well-known for its excellent anticorrosion properties, was used as wire mesh packing in a rotating packed bed (RPB). The effective interfacial area (ae) and the volumetric liquid-side mass-transfer coefficient (kLae) of the RPB with PTFE packing was studied experimentally by a NaOH−CO2 chemical absorption system and an oxygen−water physical desorption system, respectively. Experimental results showed that both ae and kLae increased with decreasing fiber diameter and pore size. As for material, mass-transfer performance of the PTFE packing was lower than that of the stainless steel wire mesh packing but is applicable in some high-corrosion and -viscosity environments. Moreover, correlations for ae and kLae were proposed.

1. INTRODUCTION A rotating packed bed (RPB), which uses centrifugal force instead of gravity, is regarded as an outstanding contactor or reactor for the process intensification of gas−liquid, liquid− liquid, and gas−liquid−solid systems.1,2 A high centrifugal field produces a powerful shearing force on the liquid in the RPB, which leads to the formation of thin liquid films, ligaments, and small droplets, resulting in an enhanced mass transfer.3 Researchers reported that the mass-transfer coefficient in the RPB could be 1−3 orders of magnitude higher than that in the conventional packed bed.2,4 Packing can enlarge the gas−liquid contacting area; thus, it is a vital component for the mass-transfer devices. Studies of structure and material of packing have become a research hotspot of the RPB.5 Chen et al.6 compared the influence of various shapes and materials of packing on the volumetric liquid-side mass-transfer coefficient (kLae) in a RPB. They indicated that the wire mesh packing gave a kLae that was much higher than that of the bead packing. Because of low cost and high mass-transfer performance, stainless steel wire mesh packing is widely used in the RPB for laboratory and industrial uses at present. However, chemical processes sometimes involve multiple acid gases like H2S, HF, and HCl, which even corrode stainless steel after being dissolved in water or vapor. Moreover, pitting corrosion is a general but serious form of local corrosion on metal surfaces by seawater and some other acid solutions, which leads to stress corrosion cracks or perforation of stainless steel.7−9 When the corrosive solution is introduced to a RPB, the rotor’s dynamic balance would be damaged if the packing is corroded, resulting in severe © 2016 American Chemical Society

vibrations and consequently influencing the long-time running of the RPB. Therefore, it is important to improve the anticorrosion ability of packing in a RPB. Polytetrafluoroethylene (PTFE) is an excellent inert and electrical insulting polymer material which does not suffer from electrochemical corrosion and pitting corrosion; therefore, it has a potential use as packing. Besides the property of anticorrosion, the weight and friction coefficient of PTFE are lower than those of the same volume of stainless steel. It is beneficial to save the motor’s energy and maintain the cleanness of the packing surface. Additionally, PTFE has the widest operating temperature range among most polymer materials.10−12 To favor the RPB design with PTFE packing, the first step is to research the mass-transfer performance of the RPB with PTFE packing. Required by the RPB design, the effective interfacial area (ae) and kLae are two key parameters to assess the mass-transfer performance of a RPB with PTFE packing. Yang et al.13 have proposed the well-known chemical absorption method of CO2 absorption into NaOH solution to measure ae of a RPB with stainless steel wire mesh packing. Their experimental results confirmed the existence of the end effect zone. Luo et al.14 measured ae in a RPB with plain-woven stainless steel wire mesh packing of various sizes of fiber diameter and opening. Their data showed that RPB with thinner fiber diameter and smaller opening size of plain-woven stainless steel packing Received: Revised: Accepted: Published: 11606

July 10, 2016 September 22, 2016 October 14, 2016 October 14, 2016 DOI: 10.1021/acs.iecr.6b02630 Ind. Eng. Chem. Res. 2016, 55, 11606−11613

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Industrial & Engineering Chemistry Research resulted in a higher ae. Furthermore, Luo et al. proposed the following correlation for ae of wire mesh packing (eq 1): ae = 6.651 × 104ReL−1.41We1.21Fr −0.12φ−0.74 at (1) The oxygen−water system is usually employed to measure kLae in gas−liquid contactors. Ramshaw and Mallinson15 conducted experiments in oxygen−water system, and the results indicated that the mass-transfer efficiency of RPB was much higher than that of the traditional packed column. Tung and Mah16 modeled and correlated the Ramshaw and Millinson data to prove that the film penetration theory is applicable to Higee processes. The correlations of interfacial area and liquidside mass-transfer coefficient for the RPB were simply proposed by replacing the gravitational acceleration of the correlations for the traditional packed column with the centrifugal acceleration. Kumar and Rao17 further confirmed this viewpoint. To evaluate the mass-transfer efficiency of a RPB, both ae and kLae were considered in some literature by NaOH−CO2 and oxygen−water systems, respectively. Rajan et al.18 evaluated both ae and kLae for two types of novel split RPBs. Results showed that the mass-transfer performance of the counter-rotation RPB was better than that of the corotation device. Chen and Tsai19 examined the ae and kLae in the RPB equipped with baffles. Their experimental data showed that the RPB with baffles could increase ae, but kLae was lower in the RPB with baffles. To the best of our knowledge, to date there is no study on mass transfer in a RPB with PTFE packing. Therefore, this work aims to measure the ae and kLae in a RPB with different types of PTFE wire mesh packings under different operation conditions, such as rotational speed, liquid flow rate, and gas flow rate. Moreover, the values of ae and kLae of RPB with PTFE and stainless steel wire mesh packing were compared. Based on all the experimental data, correlations of ae and kLae were proposed.

Figure 1. PTFE wire mesh woven patterns: (a) photograph of the rotor loaded with PWP, (b) schematic structure of PWP, (c) photograph of the rotor loaded with KWP, (d) schematic structure of KWP, and (e) structure of one strand.

Table 1. Packing Parameters

2. EXPERIMENTAL SECTION 2.1. Packing. There are two weaving modes of wire mesh packing used in this study: plain-woven packing (PWP) and knit-woven packing (KWP). Figure 1a shows a photo of the rotor loaded with PWP, and Figure 1b displays the detail structure of the PWP. The PWP is made of wire mesh, woven through interweaving warp and weft threads up and down. The densely interlacing points make the PWP rigid and difficult to deform. Figure 1c shows a photo of the rotor loaded with KWP, and Figure 1d displays the detail structure of KWP. The KWP is made by bending the yarn into circles and then intermeshing these circles into wire mesh. With this special loop structure, the KWP is equipped with the characteristics of high extensibility and flexibility. Figure 1e shows the strand made from fibers (mono filament). A strand made up with several fibers can be regarded as increasing the fiber diameter of the wire mesh. Detailed dimensions of four PTFE packings (named KWP-1, KWP-2, PWP-1, and PWP-2) are given in Table 1. Additional experiments were conducted by using stainless steel wire mesh packing (named PWP-ss) which has the same fiber diameter and opening size with PWP-1, and the details are also shown in Table 1. 2.2. RPB. Figure 2 displays the structure of experimental RPB that can be loaded with the above packings in the rotor. The height, inner radius, and outer radius of the rotor are 15,

name

number of fiber in one strand

single fiber diameter (mm)

opening size (mm)

specific surface area (m2/m3)

KWP-1 KWP-2 PWP-1 PWP-2 PWP-ss

1 3 1 1 1

0.3 0.3 0.2 0.2 0.2

3−4 3−4 0.9 0.6 0.9

1300 1780 4690 4270 1860

material PTFE PTFE PTFE PTFE stainless steel

Figure 2. Main structure of RPB. 1, shell; 2, liquid distributor; 3, gas outlet; 4, gas inlet; 5, packing; 6, rotor; 7, motor; 8, liquid outlet.

40, and 82 mm, respectively. The shaft of the RPB is driven by a motor, and the rotational speed ranges from 0 to 2800 r/min 11607

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meters (Visiferm DO 120, Hamilton). The kLae values could be calculated by the descriptions below. Considering a differential element of annular ring in the section of the packing with radius r, height z, and thickness dr, the mass balance equation of oxygen in the liquid can be expressed as6

(rpm). Gas enters the rotor from the stationary housing and then flows through the packing from the outer edge to the inner edge of the rotor. Gas leaves the RPB from the center tube after contacting with the liquid. Liquid is sprayed to the inside packing from the liquid distributor, which is set with four 1 mm diameter nozzles aligned along the length direction. After counter-current contacting with gas inside the RPB, the liquid leaves the RPB at the liquid outlet. 2.3. Experimental Procedures and Calculations. Figure 3 shows the schematic of the experimental setup. For the CO2−

L dC L = kLa(C L* − C L)2πrz dr

(4)

where CL is the oxygen concentration in water and CL* is the equilibrium concentration associated with the gas concentration. For the overall packing area, the mass balance equation of oxygen in the liquid can be expressed as L(x − x0) = G(y − y0 ) = G(mx* − 0)

(5)

The stripping factor (S) can be defined as mG (6) L The equilibrium mole fraction of the liquid x* can be calculated by combining eqs 5 and 6. x − x0 x* = (7) S S=

Equilibrium concentration can be calculated as

C L* = Figure 3. Experimental setup to measure ae and kLae. 1, stock tank; 2, online dissolved oxygen meters; 3, RPB; 4, flowmeter; 5, nitrogen cylinder; 6, carbon dioxide cylinder; 7, drain tank; 8, pump; A1, sample at gas outlet; A2, sample at gas inlet; B1, sample at liquid inlet; B2, sample at liquid outlet.

NaOH system to measure ae, a NaOH solution of ∼1 mol/L was pumped from a stock tank and sprayed onto the inner edge of the rotor with the liquid flow rate range of 18−30 L/h. The CO2 and N2 mixed gas, at ∼10 vol % CO2, was supplied from pressure cylinders. The gas with gas flow rate ranging from 800 L/h to 1600 L/h counter-currently contacted with NaOH solution inside the RPB. CO2 concentration at both the gas inlet and outlet were detected by two infrared gas analyzers, respectively. Liquid samples were taken at the liquid inlet and outlet. Composition concentrations of liquid samples were measured by an automatic potentiometric titrator. If the experimental conditions of the NaOH−CO2 system can meet the requirement of DCO2k2COH−/k2L ≥ 9,20 the gas−liquid masstransfer interface area can be written according to Danckwerts21 as

L πz(r0 2 − ri 2)

⎡ ln⎢ 1 − ⎣

(

1 S

C L,i

)C

1−

L,0

1 S

1⎤ + S⎥ ⎦

(9)

3. RESULTS AND DISCUSSION 3.1. Effective Interfacial Area (ae). 3.1.1. ae of RPB with KWPs. Figure 4 shows the effect of rotational speed, liquid flow rate, and gas flow rate on ae in the RPB with KWP-1 and KWP2. As expected, ae increases with the increase of the rotational speed and liquid flow rate but is independent with the gas flow rate. Comparing the influence of fiber diameter of the packing in the RPB, we found that ae of KWP-1 is about 20% higher than that of KWP-2, even though the specific surface area of the KWP-1 is smaller than that of KWP-2. It is indicated that there is no direct relationship between the specific surface area and the effective interfacial area. Wire mesh packing with the thinner fiber diameter has a higher mass-transfer performance probably because of the stronger cutting capacity on fluid and lower liquid flow resistance. 3.1.2. ae of RPB with PWPs. The effective interfacial area for different opening size and materials of PWP-1, PWP-2, and PWP-ss are shown in Figure 5. When the results of PWP-1 with PWP-2 are compared, ae was enhanced as the rotational speed and liquid flow rate increased, but hardly any influence with the gas flow rate was observed. Additionally, the smaller opening size wire mesh packing provides a higher mass-transfer efficiency. The possible reason is that the smaller opening size wire mesh takes advantage of higher powerful shearing force to tear the big liquid droplets into smaller droplets, which results in a bigger gas−liquid contacting area. A comparison of the stainless steel packing and PTFE packing shows that ae values of PWP-ss are about 20% higher than those of PWP-1.

NCO2 HPCO2 DCO2k1

(8)

S

Integrating the equation from inner radius (r0) to outer radius (ri), kLae can be calculated as follows: kLae =

A=

C L − C L,0

(2) 14

The effective interfacial area is given by A ae = (3) V where V represents the overall volume of the packing. Figure 3 also illustrates the diagram of the experimental setup of the oxygen−water system. The water at room temperature was pumped into RPB by the liquid distributor. The nitrogen entered the RPB and contacted with water at the gas flow rate of 50−250 L/h. The inlet and outlet liquid oxygen concentrations were measured by two online dissolved oxygen 11608

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Figure 5. Effects of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on ae in the RPB with PWP-1, PWP-2, and PWP-ss of different opening size and materials.

Figure 4. Effects of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on ae in the RPB with KWP-1 and KWP-2 of different fiber diameters.

Part of the reason is that PTFE material is more flexible than stainless steel packing; thus, droplets are not easy to be sheared off when they are deposited on the PTFE wire mesh under the centrifugal field. 3.1.3. Correlations of ae. Correlations for ae in the RPB with PTFE packings have been rarely studied to date. Luo et al.14 fitted a correlation for ae in a RPB with a variety of plain-woven stainless steel wire mesh packings (as shown in eq 1), but the correlation is probably limited with the experimental packing material. To improve the scope of application of eq 1, a parameter defined as the critical surface tension (σc) was introduced in the correlation of eq 1 to reflect the influence of different packings’ surface properties on mass-transfer efficiency. The surface tension (σ) of ∼1 mol/L NaOH solution is

approximate to the surface tension of water of 72 mN/m at 25 °C,6 while the σc values of PTFE and stainless steel are 18.5 and 75 mN/m, respectively.22,23 The dimensionless group (φ), which could be calculated by the ratio of the surface area of one cell of the actual wire mesh to the surface area of one equivalent sphere, was defined to stand for the shape parameter of wire mesh as shown in eq 10. φ=

2πd ·(c + d) π ·d p2

(10)

In the above equation, c and d were assumed to be the opening size and fiber diameter of the wire mesh, respectively. dp = 6(1 − ε)/at was defined as the effective diameter of the 11609

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Industrial & Engineering Chemistry Research Table 2. Effective Interfacial Area Correlation Regression Results

a

parameter

α

β

γ

δ

κ

ζ

R2a

PWP KWP

11.057 21.558

0.359 0.283

0.448 0.317

−0.224 −0.218

−0.202 −0.748

1.099 1.099

0.96 0.96

Goodness of fit R2 = 1 − (∑((ae)calculated − (ae)experimental)2)/(∑((ae)average − (ae)experimental)2)

equivalent sphere. Therefore, the empirical correlation was proposed in the form of eq 11 to predict ae in the RPB with different packings. ⎛ ⎞ζ ae β γ δ κ σ = αReL We Fr φ ⎜ ⎟ at ⎝ σc ⎠

(11)

The values of regressed parameters are reported in Table 2. The parity plot between the experimental values and correlation of the fitted values are displayed in Figure 6. It

Figure 6. Comparisons of experimental and calculated data of ae.

can be seen that the fitting data in this work is within ±20% of the experimental data. To investigate the applicability of this correlation, the comparisons between the calculated results by eq 11 and experimental data from Luo et al.14 who researched ae for PWPs and Yang et al.13 for KWPs of stainless steel material are presented in Figure 6. The graph indicates that the maximum deviation is ∼30%, which suggests that the correlation proposed in this work has good prediction ability. 3.2. Volumetric Liquid-Side Mass-Transfer Coefficient (kLae). 3.2.1. kLae of RPB with KWPs. Figure 7 displays the influence of different rotational speed, liquid flow rate, and gas flow rate on the kLae values of KWPs. It can be seen that kLae increases with the increase of the rotational speed and liquid flow rate. On one hand, this result is mainly ascribed to a higher ae generated just as mentioned above; on the other hand, higher rotational speed and liquid flow rate make the liquid leave from the packing surface more quickly, which is beneficial to accelerate the liquid surface renewal rate. However, gas flow rate plays an insignificant role in kLae because oxygen desorption is a mass-transfer process by liquid film control.19 Comparison of the two KWPs aims to explore the impact of fiber diameter on kLae. The results showed that kLae of the RPB with KWP-1 was about 10% higher than that with KWP-2. 3.2.2. kLae of RPB with PWPs. As shown in Figure 8, kLae increases with the increase of the rotational speed and liquid flow rate, but gas flow rate has a limited effect on the liquid-side mass-transfer coefficient. In detail, the values of kLae increase

Figure 7. Effects of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on kLae in the RPB with KWP-1 and KWP-2 of different fiber diameters.

from 0.18 to 0.35 s−1 when the rotational speed increases from 800 to 2400 rpm. The values of kLae increase from 0.23 to 0.32 s−1 when the liquid flow rate increases from 18 to 30 L/h. Comparing the kLae of PWP-1 with PWP-2, we found that the opening size does not show an obvious effect on kLae in the 11610

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the packing, which probably reduces surface renewal rate and decreases the kL. Stainless steel packing shows a kLae that is higher than that of PTFE packing, not only because of a larger ae the stainless steel wire mesh provided but also probably because of the lower liquid flow resistance in the stainless steel wire mesh packing. 3.2.3. Correlations of kLae. A correlation of kLae for the RPB with different types and materials of packings used in this work can be obtained based on the experimental data. It can be seen from the experimental results that kLae in the RPB has similar influence factors with ae, so the correlation shown in eq 12 could be improved from eq 11. ⎛ σ ⎞ζ = αReL We Fr φ ⎜ ⎟ DO2 ·a t ⎝ σc ⎠

kLa ·d p

β

γ

δ κ

(12)

The values of regressed parameters are reported in Table 3. To view the trend of kLae with these parameters, both PWPs and KWPs are positively correlated to the centrifugal acceleration (ac) and negatively correlated to the shape factor (φ) and critical surface tension (σ). As shown in Figure 9, all of the experimental values lie within ±20% of the calculated data. Equation 12 and Table 3 provide a reasonable estimation of kLae in the RPB with various packings.

Figure 9. Comparisons of experimental and calculated data of kLae.

3.3. Comparisons of Different Packings. Four types of PTFE wire mesh packings are studied to explore the masstransfer performance of the anticorrosive PTFE material used as packing in the RPB. One stainless wire mesh packing was employed for the comparisons with PTFE packing. Besides the influence of the packing’s fiber diameter and opening size, the woven pattern is also a significant factor in mass-transfer efficiency. Table 4 shows the average experimental results under the experimental conditions. Results show that the stainless steel packing provides the highest ae and kLae among all packings because of its small opening size and powerful liquid cutting ability. Although the mass-transfer efficiency of

Figure 8. Effect of (a) rotational speed, (b) liquid flow rate, and (c) gas flow rate on kLae in the RPB with PWP-1, PWP-2, and PWP-ss of different opening size and materials.

RPB. PWP-2 has a kLae that is slightly higher than that of PWP1 except for rotational speeds less than 1200 rpm. Smaller opening size wire mesh provides a larger gas−liquid interfacial area. Dense wire mesh may result in a high flow resistance in

Table 3. Volumetric Liquid-Side Mass-Transfer Coefficient Correlation Regression Results

a

parameter

α

β

γ

δ

κ

ζ

R2a

PWP KWP

25.237 80.681

0.815 1.195

0.214 0.105

−0.270 −0.253

−0.154 −0.643

0.936 0.936

0.99 0.99

Goodness of fit R2 = 1 − (∑((kLae)calculated − (kLae)experimental)2)/(∑((kLae)average − (kLae)experimental)2) 11611

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Industrial & Engineering Chemistry Research Table 4. Average ae and kLae of Each Packing packing type

ae (m2/m3)

kLae (1/s)

KWP-1 KWP-2 PWP-1 PWP-2 PWP-ss

980 835 1053 1083 1262

0.31 0.28 0.27 0.28 0.32

at = surface area of the packing per unit volume of the bed (m2/m3) ac = centrifugal acceleration (m/s2) Ci = concentration of CO2 at the gas−liquid interface (mol/ L) COH− = concentration of OH− in NaOH solution (mol/L) CL = oxygen concentration in liquid phase (mol/L) CL* = equilibrium concentration associated with the gas concentration (mol/L) CL,0 = concentration of solute in inlet liquid stream (mol/L) CL,i = concentration of solute in outlet liquid stream (mol/L) D = diffusion coefficient (m2/s) DCO2 = diffusivity of dissolved CO2 gas (m2/s) DO2 = diffusivity of dissolved O2 gas (m2/s) g = gravitational force (m/s2) k1 = reaction rate constant of first-order reaction (1/s) k2 = reaction rate constant of second-order reaction (m3/ (kmol·s)) kL = liquid-side mass-transfer coefficient (m/s) kLae = volumetric liquid-side mass-transfer coefficient (1/s) G = gas flow rate (m3/s) L = liquid flow rate (m3/s) m = phase equilibrium constant N = rotational speed (r/min) ri = inner radius of rotor (m) ro = outer radius of rotor (m) S = stripping factor V = total volume of the packing (m3) x = mole fraction of solute in liquid stream x* = equilibrium mole fraction of solute in liquid stream associated with the mole fraction of solute in gas stream x0 = mole fraction of solute in the inlet liquid stream y = mole fraction of solute in gas stream y0 = mole fraction of solute in the inlet gas stream

PTFE is lower than stainless steel wire mesh, PTFE is also a great choice to be a long-running packing loaded in industrial RPBs when applied to some special chemical processes of highcorrosion and -viscosity environments. As for the dependence on woven pattern, KWP packings present a higher kLae but a lower ae than PWP packings. For design purposes, the conservative estimate of ae and kLae of the RPB with PTFE packing could be 20% and 10% loss, respectively, if using the PTFE packing to replace the stainless steel wire mesh packing in the packing size design of the RPB. Considering the mechanical strength of the packing, KWP-2 can be suitable for the strong centrifugal field because it was woven by strands with big fiber diameter.

4. CONCLUSIONS Because of the advantages of anticorrosive materials such as PTFE, mass-transfer efficiency of four different PTFE wire mesh packings were investigated by a NaOH−CO2 chemical absorption system and a water−oxygen physical desorption system in a RPB. As a result, ae and kLae were enhanced with the increasing liquid flow rate and rotational speed, while the gas flow rate has no significant influence on them. Smaller opening size and thinner fiber diameter result in higher masstransfer performance. The comparisons of PTFE and stainless steel wire mesh packings show that PTFE packing has a lower ae and kLae. Correlations of ae and kLae for the RPB with KWPs and PWPs are proposed. The correlation of ae significantly fits well to the experimental data of previous literature. Results indicate that the correlation of ae and kLae can be applicable to various materials of wire mesh packings in the RPB. In short, the RPB with PTFE packing, which has an anticorrosive ability and acceptable mass-transfer efficiency, is a new choice for industrial application in corrosive environments.

■

Greek Letters

ε = porosity of the packing ρ = density of liquid (kg/m3) σc = critical surface tension of material (mN/m) σ = surface tension of water (mN/m)

Dimensionless Groups

Fr = Froude number =

Q L2 rω2(2πrz)2 d p ρL Q Ld p

Re = Reynolds number =

AUTHOR INFORMATION

■

Corresponding Authors

*Tel: +86 10 64446466. Fax: +86 10 64434784. E-mail: [email protected]. *Tel: +86 10 64446466. Fax: +86 10 64434784. E-mail: [email protected].

We = Weber number =

(2πrz)μL

ρL Q L 2d p (2πrz)2 σ

REFERENCES

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Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21406008, U1462127, and 21436001) and the Fundamental Research Funds for the Central Universities (YS1401).

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NOMENCLATURE A = gas−liquid mass-transfer interface area (m2) ae = gas−liquid effective interfacial area (m2/m3) 11612

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DOI: 10.1021/acs.iecr.6b02630 Ind. Eng. Chem. Res. 2016, 55, 11606−11613