SO 2 Removal in a Pilot Scale Rotating Packed Bed

Abstract

Absorption of SO₂ from a gas mixture with an ammonia-based solution was investigated using a counter current rotating packed bed (RPB) with SiC structured packing and plastic packing. Effects of operating parameters, such as the rotational speed, liquid flow rate, gas flow rate, liquid–gas ratio, inlet concentration of SO₂, and the alkalinity of absorbent on the removal efficiency of SO₂, were examined. Experimental results showed that the plastic packing had a better desulfurization performance under the same operation conditions. Moreover, correlations of gas-side volumetric overall mass transfer coefficients of the RPB with the two types of packing were obtained and the predicted data were in good agreement with experimental results.

Introduction

The SO₂ emitted from the combustion of coal and other fossil fuels leads to serious environmental pollution and does harm to human health. To control the SO₂ emission, a variety of flue gas desulfurization (FGD) schemes have been developed and applied worldwide, including wet, semidry, and dry processes. In industrial practice, SO₂ emission reduction processes use three main methods: wet, dry, and semidry methods, as shown in Table 1. Wet technologies are the most often used methods for SO₂ emission reduction in chemical plants. They have the advantage of low cost. The semidry method was also a popular method and has some advantages of wet and dry technologies. Less corrosion attacks the device for the dry technology since no liquid is present in the process of SO₂ absorption. However, it also has the shortcomings of low SO₂ removal efficiency and large equipment (Kaminski, 2003). Among them, the wet FGD process is the most popular technology used in industrial practice, since it has the advantages of high desulfurization efficiency and high utilization rate of desulfurization reagents (Scala et al., 2004; Chu and Hwang, 2005). The ammonia-based wet FGD has drawn increasing attention in China for a long time (De et al., 2001; Jiang et al., 2011). Due to its lower investment cost, higher desulfurization efficiency, and useful by-products, the ammonia-based wet FGD has been recognized as the most probable process to upgrade the existing desulfurization systems to meet the new and more strict emission standard enacted in China. For instance, for coal-fired power plants, the Chinese government enforced the new national regulation on emission in early 2012. For the sake of comparison, the regulations in some other countries were also listed and can be found in Li et al. (2013) and Chu et al. (2014). In the ammonia-based wet FGD process, the absorbent solution [containing (NH₄)₂SO₃ and NH₄HSO₃] reacts with the SO₂ from the flue gas. The following Reactions (1)–(3) mainly occur during the absorption of SO₂ into the desulfurization solution (Gao et al., 2010; Jia et al., 2010), and Reactions (4)–(5) generate by-products of (NH₄)₂SO₄, which is a useful fertilizer (Jia et al., 2010). \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm SO}_2 ( { \rm g} ) \leftrightarrow { \rm SO}_2 ( { \rm aq} ) \tag{1}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ \rm SO}_2 ( { \rm aq} ) + { \rm H}_2{ \rm O} \leftrightarrow { \rm H}_2 { \rm SO}_3 \tag{2}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}( { \rm NH}_4 ) _2 { \rm SO}_3 + { \rm H}_2{ \rm SO}_3 \leftrightarrow 2{ \rm NH}_4{ \rm HSO}_3 \tag{3}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}2 ( { \rm NH}_4 ) _2{ \rm SO}_3 + { \rm O}_2 \leftrightarrow 2 ( { \rm NH}_4 ) _2 { \rm SO}_4 \tag{4}\end{align*} \end{document} \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}2{ \rm NH}_4{ \rm HSO}_3 + { \rm O}_2 \leftrightarrow 2{ \rm NH}_4{ \rm HSO}_4 \tag{5}\end{align*} \end{document}

Table 1.

Comparison of Wet, Dry, and Semidry Flue Gas Desulfurization

Methods	Advantages	Drawbacks	References
Wet-FGD
Lime/limestone scrubbers method	Mature technology, applied widely, low cost, by-product of gypsum	High operating costs, corrosion, acidic wastewater, scaling, blocking	Kaminski (2003)
Double alkali method	Low cost, no scaling and blocking, by-product of gypsum	High operating costs, corrosion, acidic wastewater	Keeth et al. (1991)
Ammonia-based FGD	High utilization rate of desulfurizing agent, high efficiency, useful by-product	High operating costs, corrosion, ammonia loss	Gao et al. (2010)
Seawater desulfurization method	Low cost, low operating costs, no scaling and blocking	Low sulfur content, high power consumption, heat pollution, geographical restrictions	Kaminski (2003)
Semidry-FGD
Spray dry scrubber using lime as sorbent	Low cost, low operating costs	Instability and lack of reliability	Kaminski (2003)
Furnace sorbent injection	Low cost, low operating costs		Keeth et al. (1991)
Duct sorbent injection	Low cost, low operating costs		Kaminski (2003)
Dry-FGD
Tampella LIFAC	No wastewater, short process flow	High operating costs, large equipment	Kaminski (2003)
CFD-FGD	No wastewater, short process flow	High operating costs, large equipment	Kaminski (2003)

FGD, flue gas desulfurization.

The wet FGD was usually carried out in columns packed with various packings (Kiil et al., 1998) or a spray tower (Dou et al., 2009), a rotating stream tray scrubber (Sun et al., 2002), and so on. However, there are difficulties to implement the upgrade of the existing desulfurization system by employing and building a new column due to the limitation of the area and space in some chemical factories. It is, therefore, a promising pathway to adopt process intensification technology and equipment (Wu et al., 2007; Akyalçın and Kaytakoğlu, 2010) to solve the above problem. Higee technology originally invented by Ramshaw and Mallinson (1979, 1981) is one of the typical process intensification technologies carried out in a rotating packed bed (RPB) that is widely applied in various gas–liquid contacting processes, such as distillation (Chu et al., 2013), desulfurization (Wang et al., 2009; Jiang et al., 2011), deaeration (Zhao et al., 2010), chemosynthesis (Sun et al., 2011), H₂S removal (Guo et al., 2014a), CO₂ capture (Luo et al., 2012; Guo et al., 2014b), and nanoparticle preparation (Chen et al., 2000). In an RPB, the efficiency of mass transfer and micromixing can be up to 1–3 orders of magnitude larger than that in a conventional packed column (Jiang et al., 2011). The type of packing loaded in the rotor of an RPB is significant for the performance of mass transfer and mixing efficiency. Table 2 gives a summary of various packings used in RPBs and offers the progress of correlation development of RPBs. The size, shape, and material of packing loaded in RPBs have great influence on the mass transfer. Modified correlations of mass transfer coefficient with dimensionless parameters (Re, Ga, Fr, Sc, We) based on different systems (CO₂-NaOH, SO₂-NaOH, O₂-H₂O, methanol-ethanol, and so on) were also proposed. The exponents of the centrifugal acceleration in these correlations were different due to the different RPB structures and systems.

Table 2.

Summary of Packing Performance Loaded in Rotating Packed Beds

Type	a (m²/m³)	ɛ	ID (mm)	OD (mm)	h(mm)	System	Results	References
Porous plate packing	671.8	0.827	40	150	80	CO₂-NaOH	k_La_e=3.689×10⁻³ Re_G^0.5449 We_L^0.5476 Ga^0.3428	Jiao et al. (2010)
	574	0.911	40	150	80	CO₂-NaOH	k_La_e=3.968×10⁻³ Re_G^0.1399 We_L^0.4336 Ga^0.2724
Structured packing	803	0.956	38	80	20	O₂-H₂O	K_Ga: 0.30–0.66 (1/s)	Lin and Jian (2007)
	855	0.95	24	44	120	O₂-H₂O	K_Ga: 0.74–1.72 (1/s)
Stainless steel wire mesh	125	0.97	210	380	100	H₂S-MEDA	—	Guo et al. (2014a)
	—	—	78	158	50	CO₂-NaOH	α_t/α_p= 66,510 Re^−1.41 Fr^−0.12 We^1.21 φ^−0.74	Luo et al. (2012)
	688	0.427	10	60	20	O₂-H₂O	k_Lαd_p/Dd_t(1−0.95 V₀/V_t – 1.13 V_i/V_t)=0.35 Re^0.17 Gr^0.3 Sc^0.5 We^0.3 (α_t/α_p)^−0.5 (σ_t/σ_p)^0.14	Chen et al. (2006)
	825	0.950	10	60	20	O₂-H₂O
	280	—	60–290	100–310	—	Methanol–ethanol	k_Gα=1.06 (F-factor)^2.14 (ω²r_av)^0.51	Mondal et al. (2012)
Blade packing	299	0.99	195	625	295	Methanol-1-butanol	K_Ga: 1.8–2.53 (1/s)	Lin and Jian (2007)
	93	0.99	195	625	295	Methanol-1-butanol	K_Ga: 2.9–5.8 (1/s)	Hsu and Lin (2011)
	163	0.99	18	78	20	Methanol-1-butanol	K_Ga: 1.13–1.34 (1/s)	Sung and Chen (2012)
Block packing	1,700	0.9	40.5	—	28	SO₂-NaOH	k_Gα_e=5.97 Re^0.74 Gr^0.14 Sc^0.33[(r₀² – r_i²)α_p/d_p]^−0.52 (α_pD_G/d_p)	Shivhare et al. (2013)
Split packing	—	—	140	272	15, 45	Ethanol–water	k_Gα=5.818×10⁻³ Re^0.9976 Gr^0.09167 Sc^−0.33 (D_G/d_p²)	Li et al. (2014)
	—	—	117	284	53	Ethanol–water	k_Gα=5.757×10⁻³ Re^1.0989 Gr^0.05156 Sc^−0.33 (D_G/d_p²)
Metal foam (Ni-Cr)	1,700	—	40.5	123	28	CO₂-NaOH	Counterrotation:k_Lα_e=0.0395 Re_L^0.4971 Fr_L^0.3840 Sc_L^0.5 (α_pD_L/d_p)	Rajan et al. (2010)
	2,500	—	40.5	123	28	CO₂-NaOH	Corotation:k_Lα_e=0.0589 Re_L^0.7168 Fr_L^0.3133 Sc_L^0.5 (α_pD_L/d_p)
	1,700	0.9	40.5	—	28	SO₂-NaOH	Counterrotation:k_Gα_e=0.23 Re^0.8 Gr^0.18 Sc^0.33[(r₀²−r_i²)α_p/d_p]^−0.17 (α_pD_G/d_p)	Shivhare et al. (2013)
							Corotation:k_Gα_e=0.98 Re^0.63 Gr^0.21 Sc^0.33[(r₀²−r_i²)α_p/d_p]^−0.28 (α_pD_G/d_p)
	1,700	0.9	—	150	30	SO₂-NaOH	Counterrotation:k_Gα_e=7.62×10⁻³ Re^0.976 Gr^0.132 Sc^0.333 (α_pD_G/d_p)	Reddy et al. (2006)
							Corotation:k_Gα_e=7.62×10⁻³ Re^0.852 Gr^0.194 Sc^0.33 (α_pD_G/d_p)
SiC packing	125	0.97	100	300	100	SO₂-ammonia	K_ya=2.6×10⁻⁴ Re_G^0.60 Fr_G^0.34 Ga^0.47	This work
Plastic packing	150	0.65	100	300	100	SO₂-ammonia	K_ya=6.7×10⁻⁵ Re_G^0.77 Fr_G^0.21 Ga^0.26

Since the media of the ammonia-based wet FGD are strongly corrosive, the corrosion resistance of the packing loaded in the rotor plays an important role for the long-time running of the RPB. In this work, the SiC structured packing and plastic packing were chosen in view of their excellent corrosion resistance. The SO₂ removal efficiency of a pilot scale RPB with these two types of packing was investigated using the ammonia-based solution as absorbent. The dependence of the SO₂ removal efficiency on the factors such as the rotational speed, liquid flow rate, gas flow rate, liquid–gas ratio, inlet concentration of SO₂, and the alkalinity of absorbent was evaluated. Correlations for mass transfer coefficient were obtained based on the experimental data.

Experimental

Experimental apparatus

Figure 1 illustrates the main structure of the RPB. The rotor had an inner diameter of 100 mm, outer diameter of 300 mm, and axial height of 100 mm. Photos of the two packings, which were loaded in the rotor of the RPB, are shown in Figure 2. The SiC structured packing (Fig. 2a) has a void fraction of 0.95, density of 3,210 kg/m³, and specific surface area of 125 m²/m³. The plastic packing (Fig. 2b), made of polyethylene mesh, has a specific surface area of 150 m²/m³, void fraction of 0.65, strand diameter of 1 mm, and pore size of 3 mm.

FIG. 1.

Schematic diagram of rotating packed bed (RPB). (1) Gas inlet; (2) rotor; (3) shell of RPB; (4) liquid inlet; (5) gas outlet; (6) liquid outlet; (7) rotation axis.

FIG. 2.

Photos of packing. (a) SiC structured packing; (b) plastic packing.

Experimental procedure

Figure 3 schematically depicts the experimental setup for SO₂ removal. The simulated flue gas was a mixture of air from a blower and SO₂ (>99.9%; Beijing Huayuan Gas Chemical CO., Ltd.) from a gas cylinder. The gas flow rate was measured by a pitot tube and was then introduced into the RPB through its gas inlet (as shown in Fig. 1). The gas stream flowed inward from the outer periphery of the rotor because of a pressure difference. The ammonia-based solution with a total concentration of 129 g/L was prepared by dissolving (NH₄)₂SO₃ and NH₄HSO₃ (Industrial purity; Shandong Tiantaigangsu CO., Ltd.) into deionized water. The absorbent was pumped from the storage tank into the liquid inlet, and the liquid flow rate was measured by a rotameter. The solution was sprayed onto the inner periphery of packing through liquid distributors installed in the eye of the rotor. The absorbent flowed outward by the centrifugal force and made full contact with the gas stream in a countercurrent mode in the packing of the RPB. The SO₂ in the gas stream dissolved into the liquid stream and reacted with the absorbent. The cleaned gas was discharged from the gas outlet of the RPB, while the SO₂-rich aqueous solution was discharged from the liquid outlet of the RPB into the liquid tank.

FIG. 3.

Schematic diagram of experimental setup. (1) SO₂ analyzer; (2) waste liquid tank; (3) stock tank; (4) liquid pump; (5) flow meter; (6) RPB; (7) three-phase asynchronous motor; (8) transducer; (9) SO₂ cylinder; (10) centrifugal blower; (11) U-tube; (12) pitot tube.

The concentration of the absorbent was analyzed using the titration method (HG/T2784-1996 and HG/T2785-1996). The concentration of SO₂ in the inlet and outlet gas streams was measured by an automatic analysis apparatus (KM940; Kane Quintox Co.). All the data were obtained under steady-state operation.

In this experiment, the SO₂ removal efficiency in the RPB is defined as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} \eta = (C_{SO2 , in} - C_{SO2 , out} ) / C_{SO2 , in} \times 100 . \tag{6}\end{align*} \end{document}

where η is the SO₂ removal efficiency (%). C_SO2, in and C_SO2, out are the SO₂ concentrations (mg/m³) at the inlet and outlet of flue gas streams, respectively.

Results and Discussion

Unless otherwise specified, the operational parameters were set as follows: rotating speed (N) 400–1,000 rpm, gas volumetric flow rate (Q_V) 75–160 m³/h, liquid volumetric flow rate (Q_L) 300–400 L/h, SO₂ inlet concentration (C_i) 1,000–5,500 mg/m³, total salt concentration 190±2 g/L, alkalinity 13–18, and room temperature (20°C).

Effect of rotational speed on SO₂ removal efficiency

Figure 4 shows the effect of rotational speed of the RPB on the SO₂ removal efficiency. The SO₂ removal efficiency increased with the increasing of rotational speed of the RPB for both types of packings. As an important parameter of an RPB, the rotational speed has great influence on the liquid flow pattern. The calculated high-gravity factor in this work is about 20–240g, corresponding to 400 and 1,400 rpm, respectively. At higher rotational speed, vigorous impingement and uniform dispersion of the liquid will occur frequently, which lead to a higher mass transfer rate and results in a higher absorption rate. Moreover, the removal efficiency of the SiC structured packing starts at a lower value compared with the plastic packing. When the rotational speed was less than 1,000 rpm, the efficiency for SiC structured packing increases more rapidly than for the plastic packing. When the rotational speed exceeded 1,000 rpm, the increasing trend gradually slowed down for both packings. The possible reason for the above trend is that the two packings have different structures of the internal pores or flow channels. The plastic packing consisted of uniform PE fibers and layers. Liquid hydrodynamics was speculated to be different inside the packing. Low rotational speed for the SiC packing results in a small centrifugal force and this can cause liquid maldistribution, which affects the removal efficiency in a negative way. The plastic packing has more uniform flow channels and even a lower rotational speed works well for that case. A higher rotational speed also means higher power consumption, which must be considered in industrial applications. In these above experiments, the SO₂ removal efficiency was above 97.5% at a rotational speed of 1,000 rpm, and this rotational speed was chosen as the appropriate one by taking both power consumption and removal efficiency into consideration.

FIG. 4.

Effect of rotating speed on SO₂ removal efficiency. C_SO2, in=2,860 mg/m³, TA=16, Q_L=340 L/h, Q_G=114 m³/h.

Effect of liquid flow rate on SO₂ removal efficiency

The effect of liquid flow rate on SO₂ removal efficiency is shown in Figure 5. With increasing liquid flow rate, the SO₂ removal efficiency of the RPB with two types of packing increased significantly as long as the Q_L was less than 360 L/h and then remained almost unchanged for values greater than 360 L/h. For the gas–liquid reaction, there are two steps: (1) SO₂ gas transfers to liquid and (2) SO₂ in the liquid reacts with the absorbent in the liquid. For step (1), the RPB can enhance the mass transfer process, but it cannot slow down or speed up the chemical reaction process (2). The reason for the increasing curves in Figure 5 is that when the liquid flow rate was increased, it could intensify the gas–liquid contacting and, thus, increase the surface renewal frequency to improve the mass transfer process and the removal efficiency. Similar as above, the RPB with plastic packing removed SO₂ more efficiently than that with SiC structured packing.

FIG. 5.

Effect of liquid flow rate on SO₂ removal efficiency. C_SO2, in=3,300 mg/m³, TA=16, Q_G=114 m³/h, N=1,000 rpm.

Effect of gas flow rate on SO₂ removal efficiency

Figure 6 shows the effect of gas flow rate on SO₂ removal efficiency and it decreased with the increasing of gas flow rate for the two types of packing. The gas flow rate has a contradictory influence on the absorption process. The increase of the inlet gas flow rate will increase the gas velocity, which leads to an increasing of the relative velocity between the liquid and gas, which is favorable to the absorption. Simultaneously, however, the gas–liquid contact time was shortened with the increasing of the gas flow rate, which is unfavorable to the absorption. It can be inferred that the latter influence is dominant based on the data in Figure 6. For the same situation, the plastic packing performs better than SiC structured packing, and the SO₂ removal efficiency exceeds 97%.

FIG. 6.

Effect of gas flow rate on SO₂ removal efficiency. TA=16, Q_L=340 L/h, N=1,000 rpm, C_SO2, in=2,900 mg/m³.

Effect of liquid–gas ratio on SO₂ removal efficiency

Figure 7 displays the effect of liquid–gas ratio (Q_L/Q_G) on SO₂ removal efficiency. It is well known that the liquid–gas ratio is one of the most important parameters for the FGD technology evaluation. A higher liquid–gas ratio can enhance the mass transfer rate, but increases power consumption and operation cost. Therefore, an appropriate ratio is determined by the removal efficiency and economic consideration. In Figure 7, the SO₂ removal efficiency increases considerably as the liquid–gas ratio increases and as long as Q_L/Q_G<3.55×10⁻³, then the increasing slows down for Q_L/Q_G>3.55×10⁻³. Compared with SiC structured packing, the plastic packing is more efficient for SO₂ removal. At a fixed Q_G, an increase in the liquid flow rate would increase the effective wetting area of the packing. Consequently, the effective surface area for mass transfer was increased, which is favorable to SO₂ absorption. The range of 2.5 ∼ 4×10⁻³ for Q_L/Q_G can be considered to be appropriate and can guarantee the desired level of SO₂ removal efficiency.

FIG. 7.

Effect of liquid–gas ratio on SO₂ removal efficiency. C_SO2, in=3,420 mg/m³, TA=16, Q_G=114 m³/h, N=1,000 rpm.

Effect of inlet SO₂ concentration on SO₂ removal efficiency

Figure 8 illustrates the effect of inlet SO₂ concentration on SO₂ removal efficiency. The SO₂ removal efficiency decreases with the increasing of inlet SO₂ concentration. In comparison with SiC structured packing, the SO₂ removal efficiency of the RPB with the plastic packing was higher. When the inlet SO₂ concentration increased, the mass transfer driving force is enhanced based on the double film theory, which is beneficial for the absorption. However, the absorbent desulfurization capacity is constant and it gradually becomes a limiting factor when the inlet concentration becomes even higher, which results usually in a slow decrease of the SO₂ removal efficiency. The SO₂ removal efficiency is still above 95% even when the inlet SO₂ concentration reaches 5,000 mg/m³ for the SiC structured packing and it is above 97% for the plastic packing.

FIG. 8.

Effect of inlet SO₂ concentration on SO₂ removal efficiency. TA=16, Q_L=340 L/h, Q_G=114 m³/h, N=1,000 rpm.

Effect of alkalinity on absorption on SO₂ removal efficiency

Alkalinity is an important indicator of the absorption capacity of the absorbent and also one criterion of the by-products in the industry. The quantity of (NH₄)₂SO₃ in the desulfurization solution can be indicated by the alkalinity. If the alkalinity is equal to 1, then 5.8 g (NH₄)₂SO₃ is present in 1 L of desulfurization solution. The alkalinity has great impact on SO₂ removal efficiency, as shown in Figure 9. The SO₂ removal efficiency increased with the increasing of the alkalinity. Sulfur dioxide is an acidic gas; therefore, the increase of alkalinity of the absorbent is favorable to the absorption. The range of alkalinity adopted usually is 16–18 to avoid the excessive loss of ammonia.

FIG. 9.

Effect of alkalinity of absorption on SO₂ removal efficiency. C_SO2, in=2,960 mg/m³, Q_L=340 L/h, Q_G=114 m³/h, N=1,000 rpm.

Mass transfer coefficient

Gao et al. (2010) reported that the absorption of SO₂ was controlled by the gas film when the (NH₄)₂SO₃ concentration exceeds 0.05 mol/L. In our experiment, the absorbent concentration was always greater than 0.05 mol/L, and thus, the absorption of SO₂ was regarded as being controlled by the gas film. The gas-side volumetric overall mass transfer coefficient can be expressed as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}N_A = K_y ( y_A - y_e ). \tag{7}\end{align*} \end{document}

Based on a mass balance of a volume element of packing as shown in Figure 10, it can be described as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}- Gdy_A = N_A \times a \times 2 \pi r \times dr \times h. \tag{8}\end{align*} \end{document}

FIG. 10.

Schematic of packing.

Combining Equations (7) and (8) gives the following: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}- Gdy_A = K_y \cdot ( y_A - y_e ) \times a \times 2 \pi r \times dr \times h. \tag{9}\end{align*} \end{document}

For ammonia-based absorption of SO₂, the reaction is an instantaneous reaction in the liquid film, so that y_e can be approximately considered as 0. Equation (9) then becomes the following: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}- Gdy_A = K_y \times y_A \times a \times 2 \pi r \times dr \times h. \tag{10}\end{align*} \end{document}

Integrating Equation (10) gets the following: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}K_ya = Gln ( y_{in} / y_{out} ) / ( \pi h ( R_0^{\ 2} - R_i^{\ 2} ) ). \tag{11}\end{align*} \end{document}

This way, K_ya values can be calculated based on experimental results using Equation (11). Moreover, based on dimensional analysis, the main variables involved in the SO₂ absorption by an RPB can be referred to as the following dimensionless numbers: Re_G, Fr_G, and Ga. Thus, the correlations of K_ya for the different packings can be expressed as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}K_y a = ARe_G^{ \ \ \rm K1} Fr_G^{ \ \ \rm K2} Ga^{ \rm K3}. \tag{12}\end{align*} \end{document}

The parameters A, K₁, K₂, K₃ can be determined by a multiple linear regression, so that the correlation for K_ya can be obtained. For the SiC structured packing: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}K_y a = 2.6 \times 10^{ - 4} Re_G^{ \ \ 0.60} Fr_G^{ \ \ 0.34} Ga^{0.47}. \tag{13}\end{align*} \end{document}

For the plastic packing: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}K_y a = 6.7 \times 10^{ - 5} Re_G^{ \ \ 0.77} Fr_G^{ \ \ 0.21} Ga^{0.26}. \tag{14}\end{align*} \end{document}

Figure 11 shows the K_ya values obtained according to experimental results versus the values predicted by the correlations. The range of K_ya is within 3–7 1/s. The deviations between the calculated and experimental data are within ±10% and ±5% for SiC and plastic packing, respectively. The calculation formula of Ga includes the high-gravity factor (β), which is directly related to the rotational speed. The exponents of β are 0.47 and 0.26 for SiC and plastic packing, respectively. Comparing these two exponents, the rotational speed, thus, has a smaller influence on the SO₂ removal efficiency for the plastic packing.

FIG. 11.

Comparison of the predicted and experimental K_ya values. (a) SiC structured packing; (b) plastic packing.

Comparison

Table 3 shows the comparison of SO₂ removal by RPBs equipped with different types of packing. It can be found that the RPB with packing is an effective way for removal of SO₂ from flue gas. SiC structured packing and plastic packing are a feasible choice for industry application, especially the plastic packing due to its excellent corrosion resistance as well as its high cost efficiency.

Table 3.

Comparison of SO₂ Removal by Rotating Packed Beds Equipped with Different Packings

Packing	a (m²/m³)	ɛ	ID (mm)	OD (mm)	h (mm)	C_SO2, in (mg/m³)	Absorbent	Efficiency (%)	References
SiC structured packing	125	0.95	100	300	100	5,000	Ammonia-based solution	>95	This work
Plastic packing	150	0.65	100	300	100	5,000	Ammonia-based solution	>97
Stainless wire mesh packing	—	—	70	206	80	<5,000	Dual alkali solution	99	Wang et al. (2009)
	650	0.925	59	132	40	5,085	Sodium citrate buffer solution	97	Jiang et al. (2011)
Metal foam (Ni-Cr)	1,700	0.9	—	150	30	2,500–4,000	Sodium hydroxide solution	—	Rajan et al. (2010)
	1,700	0.9	40.5	—	28			—	Rajan et al. (2010)

Conclusions

In this work, the desulfurization performance of a countercurrent RPB with SiC structured packing and plastic packing using an ammonia-based solution was investigated. Sulfur dioxide removal efficiency increased with the increasing of rotational speed, liquid flow rate, liquid–gas ratio, and alkalinity, while it decreased with the increase of gas flow rate and inlet SO₂ concentration. The two types of packing loaded in the RPB for the SO₂ removal have a desulfurization efficiency above 95%. The RPB with plastic packing had better performance than SiC structured packing. In addition, the correlations of mass transfer coefficients of RPB for SiC structured packing and plastic packing were obtained, the predicted data were in good agreement with the experimental results with deviation of ±10% and ±5%, respectively. Compared with the traditional desulfurization technology, the RPB occupied only 20% of the normal space needed to achieve a better desulfurization effect.

Footnotes

Acknowledgments

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

Author Disclosure Statement

No competing financial interests exist.

Nomenclature

Greek Symbols

Dimensionless Groups

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SO 2 Removal in a Pilot Scale Rotating Packed Bed

Abstract

Abstract

Introduction

Experimental

Experimental apparatus

Experimental procedure

Results and Discussion

Effect of rotational speed on SO2 removal efficiency

Effect of liquid flow rate on SO2 removal efficiency

Effect of gas flow rate on SO2 removal efficiency

Effect of liquid–gas ratio on SO2 removal efficiency

Effect of inlet SO2 concentration on SO2 removal efficiency

Effect of alkalinity on absorption on SO2 removal efficiency

Mass transfer coefficient

Comparison

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

Nomenclature

Greek Symbols

Dimensionless Groups

References

Effect of rotational speed on SO₂ removal efficiency

Effect of liquid flow rate on SO₂ removal efficiency

Effect of gas flow rate on SO₂ removal efficiency

Effect of liquid–gas ratio on SO₂ removal efficiency

Effect of inlet SO₂ concentration on SO₂ removal efficiency

Effect of alkalinity on absorption on SO₂ removal efficiency