Sulfur Dioxide Desorption Characteristics of Basic Alkali Aluminum

Mar 7, 2018 - Compared with the smooth tube, the spring tube was found to be much ... The results showed that the flow rate of BASS rich in SO2, heati...
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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Sulfur Dioxide Desorption Characteristics of Basic Alkali Aluminum Sulfate Desulfurization Rich Liquid by Falling Film Evaporation with a Spring Tube Yanjun Zhang, Xianhe Deng, and Shuangfeng Wang* Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China ABSTRACT: This paper presented a novel technology for the desorption of SO2 in the basic aluminum sulfate solution (BASS) by falling film evaporator with spring tube. The liquid film falling along the internal surface of the evaporation tube was heated by the steam outside. The desorption performances of the vertical single falling film evaporators with spring tube and smooth tube were comparatively investigated. The film distribution on the inner surface of the two tubes were also analyzed through visualization experiments. Compared with the smooth tube, the spring tube was found to be much better in heat transfer and mass transfer. Thus, analysis on the operating parameters for the desorption of SO2 was performed only in the spring tube. The results showed that the flow rate of BASS rich in SO2, heating steam temperature, vacuum of the system, and initial concentration of SO32− played important roles in the desorption performance.

Wang10 comparatively investigated the desorption efficiency of BASS-SO2 in the packing tower and the supergravityrotating-bed. It was found that the supergravity-rotating-bed had better performance in desorption efficiency than the packing power. Under the same conditions, the desorption rate in the supergravity-rotating-bed was 1.7 times larger than that in the packing power. It was mainly due to the better gas− liquid contact in the supergravity-rotating-bed. The effect of ultrasound on SO2 desorption from sodiumalkali-desulfurization-rich solution was investigated by Xu et al.11 They compared the ratio of H2SO3 and NaHSO3 in the solution. They found that the ratio reached the same values respectively in the conditions of ultrasound and without ultrasound, but the decomposition time in the ultrasound is less than that without ultrasound. Thus, they concluded the ultrasound only accelerated the decomposition of H2SO3 but had little influence on the decomposition of NaHSO3. In addition, Xue et al.12 also applied the ultrasound technique into the desorption of citrate SO2 rich solution. The result showed that, under the same conditions, using ultrasound could enhance the desorption efficiency by 25%. Meanwhile, the microwave method was also employed by some researchers into the desorption of SO2 from the adsorbent or adsorbate. Zhang et al.13 applied the microwave method for the regeneration of activated carbon in desulfurization. They found, under the same conditions, that greater microwave power would desorb larger concentration of SO2.

1. INTRODUCTION SO2 pollution and acid rain which are caused by flue gas emission not only cause huge property loss but also endanger human health, and thus severely restrict the development of society.1−3 Choosing a high quality desulfurizer and promising desulfurization process is pretty crucial. Wet flue gas desulfurization (FGD) technology is one of the domain methods to solve the flue gas emission pollution.4−7 One of its significant benefits is that the absorbent could be regenerated. However, the research of wet FGD were mostly focused on the absorbing capability, attention was seldom paid to the desorption characteristics, and much less to its industrial applications. Basic aluminum sulfate solution (BASS) desorption regeneration process is a method in which BASS absorbs SO2 from the flue gas and then is refreshed by heater and release of SO2.8 It is considered as one of the most important wet FGD technologies. Chemical equation of the process can be expressed as follows: Al 2(SO4 )3 ·Al 2O3(aq) + 3SO2 (g) ↔ Al 2(SO4 )3 ·Al 2(SO3)3 (aq)

(1)

This approach has significant economic benefits, since the absorbent could be regenerated and prepared with cheap raw materials of Al2(SO4)3 and CaCO3.9 As mentioned earlier, the research of BASS in desorption characteristics needs to be paid much more attention, especially in its industrial applications. Usually, the traditional tank reactors used for SO2 desorption from BASS rich in SO2 (BASS-SO2) suffered from a low desorption rate and a poor desorption efficiency. It is mainly due to the serious back mixing process and inefficient heating process. © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December 5, 2017 March 5, 2018 March 7, 2018 March 7, 2018 DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Schematic of the visualization experiment.

Figure 2. Schematic diagram of the experimental setup.

The falling film evaporator serves as a high efficient heat and mass exchanger. Considering in the industrial process such as the thermal power generation there is usually much waste steam generated which belongs to the low-grade heat source, it seems that the falling film evaporation is a perfect method that could utilize this waste steam. Thus, it is promising that the falling film evaporation is employed into the desorption of BASS-SO2 to realize the refreshment of the absorbent (BASS). Huang et al.17,18 experimentally investigated the desorption of BASS-SO2 in the falling film evaporator respectively with smooth tube and converging-diverging tube. Under the conditions of heating temperature of 381.15 K, liquid flow rate of 0.005 kg/s, sulfur concentrations of 0.06 mol/L, aluminum concentration of 20 g/L, and basity of 20%, the converging-diverging tube (desorption efficiency 94.2%) performed better in the desorption efficiency than the smooth tube (desorption efficiency 83.7%). In consideration of the difficulty in the fabrication of converging-diverging tube, we

Nowadays, vacuum technology was also extensively applied into the gas−liquid separation process.14,15 Chen et al.16 investigated the effects of operating parameters on SO2 desorption performance in a lab-scale reactor. The experiment was conducted under the vacuum conditions with stirrer in the liquid. The experimental results showed that the SO 2 desorption efficiency could reach above 95% at the pressure below 20 kPa. Thus, Chen et al.16 concluded that the desorption performance of BASS-SO2 could be significantly improved with great decrease of pressure. They also found that the heating temperature and the stirring speed could influence the desorption performance. Although the methods mentioned above could strengthen the SO2 desorption, they are confronted with problems such as low efficiency, high energy consumption (especially the electric power), and lack of industrial application. Therefore, a new desorption method is needed for the desorption of BASS-SO2. B

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

the vacuum pump (13). Meanwhile, the second vapor was condensed in the cooling equipment (9), and the SO2 was absorbed by the NaOH solution (12). 2.1.3. Distributor. A distributor in the falling film evaporation experiment was needed to achieve a uniform film flow on the inner surface of the evaporation tube. In this work, a kind of plug-type hollow-tube spiral groove distributor (Figure 3) made of stainless steel was employed. The detail

applied a spring tube as the evaporation tube for the desorption of BASS-SO2. Besides, seldom research was found on the desorption reaction in the falling film evaporator with the spring tube. This paper investigated the desorption properties of SO2 from the BASS-SO2 in a single vertical falling film evaporator with spring tube. The spring was inserted into the smooth stainless tube to form a spring tube. The BASS-SO2 was heated up by the steam outside. The desorption reaction took place on the inner surface of the spring tube under the vacuum condition. The high desorption rate and high desorption efficiency was obtained. Furthermore, compared with the former mentioned desorption methods, this technique has much superiority in energy conservation and economic benefits, for it could sufficiently utilize the waste steam generated in the industry.

2. EXPERIMENT SECTION 2.1. Experimental Setup. 2.1.1. Visualization Experiment. The flow conditions of the falling film in the evaporation tubes were simulated and investigated by a visualization experiment, shown in Figure 1. The inner diameter of the glass tube is 17 mm, which is the same as the stainless tube applied in the desorption. The red ink was fed into the distribution room (4), and its flow rate was controlled by the rotameter (1). The liquid level height in the distribution room could be measured by the dividing rule (3), and it reflected the flow resistance of the distributor (2). The liquid in the distribution room flowed into the glass tube (6) through the annulus (5) between the distributor and the tube. The flow conditions of the falling film on the inner surface of glass tube could be observed directly. 2.1.2. Falling Film Evaporation Experiment. The desorption experimental system was illustrated in Figure 2. It was similar to the system reported by Huang.17 A stainless tube was served as the evaporation tube with the inner diameter of 17 mm, the heating length of 2.3 m, and the wall thickness of 1 mm. Before the heating process was conducted, the nocondensable gas, in the annulus between evaporation tube and outer tube, must be cleared by shutting down K3 and opening K1 and K4. Then, K1 and K4 were closed and K3 was opened, and the steam flowed into the annulus between evaporation tube and outer tube. The pressure of the steam was monitored by pressure gauge (5), and its temperature was measured by digit thermometer (6). Both of them could be controlled by adjusting K3. The condensate water which flows through the steam trap K5 was collected and measured by the counting cup (7). The vacuum of the system was measured by the vacuum meter (4), and it could be adjusted by K2. The K2 connected the distribution room (14) and the outer circumstance. The BASS-SO2 was pumped from a tank (1) into the distribution room (14). It flowed into the falling film evaporation tower (15) through annulus between the distributor and the evaporation tube. The BASS-SO2 ran down along the inner surface of the evaporation tube and was heated by the steam outside. The desorption reaction took place on the inner surface of the tube, with SO2 and second vapor released from the falling film. After desorption, the two phase fluid [mainly BASS(l), SO2 (g), and second vapor (g)] ran into the tank (8) at the bottom of the evaporation tower. The temperature of the liquid phase was measured by the digit thermometer (16). Afterward, the gas phase was taken out by

Figure 3. Distributor used in the experiment.

Table 1. Parameters of the Distributor parameters

size

length (mm) inner diameter (mm) smooth length (mm) smooth outer diameter (mm) spiral slot length (mm) length in the evaporation tube (mm) outer diameter in the evaporation tube (mm) spiral slot number spiral groove depth (mm) spiral groove width (mm) thread pitch (mm) thread lead (mm) thread inclination angle

250 12 190 18 60 50 16.5 5 1 3 5 25 30°

parameters of the distributor were given in Table 1. Its effects were specified as follows. (1) When the BASS-SO2 flowed through the annulus between the distributor and the evaporation tube, the liquid made the axially rotational and downward movement. Thus, the liquid obtained the tangential velocity along the circumferential direction. It is beneficial for distributing the liquid film evenly on the inner surface of the evaporation tube. (2) The liquid level could be kept at a certain height above the annulus due to the flow resistance. It is essential in the case when many evaporation tubes are installed in parallel forms, as it could ensure the liquid in the distribution room flowing into each tube uniformly. (3) The channel throughout the distributor could balance the system pressure between the distribution room and the evaporation tube. 2.2. Preparation of the BASS-SO2. The BASS was prepared according to a reported method by Chen et al.9 The BASS-SO2 had to be prepared not long before the desorption experiment was conducted. As described in Figure 4, the pure SO2 was injected into the BASS (volume of 19 L, aluminum concentration of 20 g/L, and basicity of 30%). The SO2 was absorbed by the BASS and in the form of SO32− in the C

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

where d = 17 mm is the inner diameter of the evaporation tube, l = 2.3 m is the heating length of the evaporation tube, and the heat exchange area A = 0.123 m2. The energy utilization efficiency y of the falling film evaporation could be calculated as y=

liquid. The concentration of the SO32− could be measured by the standard iodometric titration method within a small variation of ±1%.19 The flow rate of SO2 and the time injected were controlled to make the SO32− in the BASS reach a required concentration. 2.3. Data Analysis. The desorption rate v can be calculated as (C2 + C3)V2 (2) t 2− where C2 is the concentration of SO3 absorbed by NaOH, mol/L (Figure 2, label number 12); C3 is the concentration of SO42− absorbed by NaOH, mol/L (Figure 2 label number 12); V2 = 10 L is the NaOH solution volume; and t is the desorption time, min. It should be noted that, under the heated condition, the SO32− is easily oxidized to SO42−, which cannot be detected by the standard iodometric method. So it is inaccurate to calculate the desorption efficiency by immediately measuring the residual concentration of SO32− in the BASS after desorption with this method. Therefore, the ion chromatograph equipment (792 Basic IC manufactured by the Metrohm Company, Switzerland) was employed to measure the concentration of SO32− and concentration of SO42− in the NaOH solution within a small variation of ±1%. The concentration of residual SO2 in the gas after absorption by NaOH (Figure 2, label number 12) was detected by the following methods: as shown in the following picture, the gas passing through the vacuum pump (Figure 2, label number 13) was injected into a test tube which contained the NaOH solution. The concentration of the SO32− and SO42− in the test tube was determined by the ion chromatograph equipment. We found that the amount of residual SO2 in the gas was actually very little. Therefore, we concluded that the SO2 was nearly all absorbed by the NaOH solution in the container (Figure 2, label number 12). The desorption efficiency η can be calculated as v=

(C2 + C3)V2 × 100% C1V1

q=

H2 + H3 tA

(6)

where t is the desorption time, min; and A is the heat exchange area A = 0.123 m2. The heat transfer coefficient h could be calculated as q h= (7) Δt where ΔT is the temperature difference between the steam outside and the film inside.

3. RESULTS AND DISCUSSION 3.1. Visualization Experiment Results Discussion. 3.1.1. Smooth Tube Experiment. First, the falling film distribution in the smooth tube was investigated at different flow rates. As shown in Figure 5, the flow rates in four tubes, arranged from left to right, are 20 L/h, 30 L/h, 40 L/h, and 50 L/h, respectively. The liquid ran down along the inner surface of the glass tube in the bundle form rather than film, demonstrating that the film could not be formed on the inner surface of the smooth tube. 3.1.2. Spring Tube Experiment. The falling film distribution in the spring tube was also comparatively explored. The spring tube was made by inserting the spring (screw pitch = 4 mm, external diameter = 16.5 mm, and wire diameter = 1 mm) into the former smooth glass tube. As described in Figure 6, the flow rates in four tubes, arranged from left to right, are 15L/h, 20L/h, 25L/h, and 30L/ h, respectively. The falling film distribution in the spring tube

(3)

SO32−

where C1 is the concentration of in BASS-SO2, mol/L; C2 is the concentration of SO32− absorbed by NaOH solution, mol/L; C3 is the concentration of SO42− absorbed by NaOH solution, mol/L; V1 = 19 L is the volume of BASS-SO2; and V2 = 10 L is the NaOH solution volume. The heat exchange area A of the evaporation tube could be calculated as A = πdl

(5)

H1 is the total energy inputted into the falling film evaporation. It can be calculated as H1 = h1ρV3, where h1 is the phase change enthalpy of the water at steam pressure in the annular tube; ρ = 998.2 kg/m3 is the density of the condensed water at temperature of 293 K; and V3 is the volume of the condensed water in the counting cup (Figure 2, label number 7). H2 is the energy absorbed by the rise of temperature of BASS-SO2, which can be calculated as H2 = ρ1V1(T2 − T1)Cp, where Cp = 4.187 kJ kg−1 K−1 is the specific heat of the BASSSO2, ρ1=1062.41 kg/m3 is the density of the BASS-SO2, V1 = 19 L is the total volume of the BASS-SO2, T2 is the boiling temperature of the BASS in the evaporation tube, and T1 is the environment temperature. H3 is the energy absorbed by the phase change of the solvent in the BASS-SO2 (i.e., water) in the evaporation tube, which can be calculated as H3 = h3ρ2V4, where h3 is the phase change enthalpy of the water at the pressure in evaporation tube, ρ2 = 998.2 kg/m3 is the density of the condense water at temperature of 293 K, and V4 is the volume of the condense water in the container (Figure 2, label number 9). By calculation, the energy utilization efficiency of the total system could be achieved up to 85%. The heat flux q could be calculated by the following equation:

Figure 4. Preparation of the BASS-SO2.

η=

H2 + H3 × 100% H1

(4) D

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

entirely wet by the liquid. Therefore, in our next works, the desorption experiment was conducted at the liquid flow rates no less than 30 L/h. 3.2. Desorption Experiment Results Discussion. 3.2.1. Comparison between Spring Tube and Smooth Tube. First, the desorption characteristics of the BASS-SO2 in the falling film evaporation were comparatively investigated in the spring tube and smooth tube. The initial concentration of SO32− in the BASS-SO2 was maintained at 0.19 mol/L. The BASS-SO2 was desorbed respectively in the smooth tube and the spring tube. The BASS-SO2 flow rates varied from 30 L/h to 60 L/h. The system pressure in the evaporation tube was defined as 20 kPa (absolute pressure), and the heating steam temperature was set at 373 K. The heat transfer coefficients of the two tubes were exhibited in Figure 7. The heat transfer rates of the two tubes both

Figure 5. Falling film distribution on the inner surface of smooth tube.

Figure 7. Heat transfer coefficients in smooth tube and spring tube varying with BASS-SO2 flow rates. The initial concentration of SO32− was 0.19 mol/L, system pressure was 20 kPa (absolute pressure), and heating steam temperature was 373 K. Figure 6. Falling film distribution on the inner surface of the spring tube.

increased with the rise of BASS-SO2 flow rate. It is mainly due to two reasons. One reason is the laminar boundary layer which is near the evaporation tube wall in the falling film getting thinner as the flow rate is rising. Another reason is the fluid flow in the film becoming more turbulent at a larger flow rate. Thus, the convection heat transfer was intensified along the radial direction of the evaporation tube in the falling film. Besides, it is evident that the heat transfer rate in the spring tube is much more than that in the smooth tube. The heat transfer coefficient in the spring tube is averaging larger by about 38% than that in the smooth tube. As mentioned in the former section 3.1, it is mainly due to the much more uniform and steady film distribution in the spring tube. The distribution of film in the spring tube was also clarified in Figure 8. The desorption rate varying with BASS-SO2 flow rates in the two tubes was depicted in Figure 9. As the initial concentration of SO32− in the BASS-SO2 was certain (0.19 mol/L), so the inflow rate of SO2 flowing into the evaporation tube increased linearly with the BASS-SO2 flow rate. The desorption rates in the two tubes both increase with the rise of BASS-SO2 flow rate. It mainly ascribes to the rising turbulence intensity of the falling film. Meanwhile, the transfer frequency of the fluid micelles between the gas−liquid interface and the film body became more active. Consequently, the update rate of the SO2

was much more uniform than that in the smooth tube. When the film ran down along the inner surface of the spring tube, it encountered the spring and redistributed on the inner surface of the tube at the obstacle effect of the spiral coil. Meanwhile, the liquid also made the rotation movement along the spring coil, which was not only beneficial for the stability of the film but also enhanced its interior turbulence. Furthermore, the spring oscillated regularly along the tube under the impact force of the falling film,20 which conversely made the falling film get thinner and more steady. It should be noted that there was some local dry spots in the falling film at the low flow rates (15 L/h, 20 L/h, and 25 L/h). It is a frustrating phenomenon as the dry spot could deteriorate the recycle of the BASS in desorption reaction. At these regions, the BASS will precipitate and scale on the inner surface of the evaporation tube due to the excessive water evaporation. Besides, the scaling problem is also an unpleasant factor for the heat and mass transfer process in the falling film evaporation. Thus, larger liquid flow rate is needed to avoid these local dry spots. It could also be seen from Figure 6, at the last flow rate (30 L/h), all dry spots disappeared and the tube wall was E

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

The desorption efficiency varying with the BASS-SO2 flow rate in the two tubes was present in Figure 10. Despite the

Figure 8. Distribution of falling film in the spring tube.

Figure 10. Desorption efficiency in smooth tube and spring tube varying with BASS-SO2 flow rates. The initial concentration of SO32− was 0.19 mol/L, system pressure was 20 kPa (absolute pressure), and heating steam temperature was 373 K.

desorption rate increase, the desorption efficiency in the two tubes both decreased. It was mainly because the thickness of the falling film gradually got thicker as the BASS-SO2 flow rate increased. It played a negative part in the heat and mass transfer process. As shown in Figure 9, although the desorption rate increased at the higher BASS flow rate, the increase range of the desorption rate could not catch up with that of the SO2 inflow rate. Therefore, it resulted in a decline in the desorption efficiency as the BASS-SO2 flow rate increased. In the industrial process, the total regeneration amount of BASS is related to both the flow rate of BASS-SO2 and the desorption efficiency. The high flow rate of BASS-SO2 will result in poor desorption efficiency. So, considering the practical application and the economic factor, the optimal flow rate of BASS-SO2 is 40 L/h. 3.2.2. Effects of Parameters on Desorption in Spring Tube. It was investigated in the previous section that the spring tube exhibited much better performance in the desorption reaction, as compared with the smooth tube. And the influence of the BASS-SO2 flow rate on the desorption was also discussed. Thus, the exploration on other operating parameters such as the system pressure, the initial concentration of SO32−, and the heating steam temperature were done only in the spring tube in the next works (sections 3.3−3.5). 3.3. Effect of System Pressure. The initial concentration of SO32− in the BASS was maintained at 0.19 mol/L. The desorption experiment was conducted with the initial pressure of the system varying from 20 to 100 kPa (absolute pressure). The BASS-SO2 flow rate was set as 40 L/h and the heating steam temperature was 373 K. As shown in Figure 11, both the desorption rate and the desorption efficiency dropped down with the increase of system pressure. As the system pressure increased, the partial pressure of the SO2 in the evaporation tube also rose. It resulted in a decline in the driving force for the SO2 released from the falling film into gas phase. Meanwhile, the bubble point of the BASSSO2 also rose up due to the increase of system pressure in the

Figure 9. Desorption rates in smooth tube and spring tube varying with BASS-SO2 flow rates. The initial concentration of SO32− was 0.19 mol/L, system pressure was 20 kPa (absolute pressure), and heating steam temperature was 373 K.

in the interface was accelerated. According to the two-film theory based on gas−liquid mass transfer, the process of desorption is driven by the concentration difference between the liquid phase and the gas phase.21 Thus, the mass transfer rate in the falling film was intensified. Besides, the desorption action in the spring tube is much more active than that in the smooth tube. The desorption rate in the spring tube is averaging an improvement by about 19%, as compared with that in the smooth tube. Compared with the smooth tube, the spring tube exhibited much more superiority in the heat transfer and mass transfer process. It is mainly because the falling film in spring tube could flow in a more uniform and stable movement, as compared with the smooth tube. Besides, the obstacle and the guide effect of the spiral coil in the spring tube could also enhance the falling film turbulence. It stimulated the convection effect in the radial direction of the evaporation tube. Furthermore, the oscillation of the spring coil along the axial direction of the tube made the laminar sublayer of the film get thinner. It decreased the thermal resistance and made the convection along the radial direction in the film stronger. F

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 11. Effect of the system pressure on the desorption rate and the desorption efficiency of BASS-SO2. The initial concentration of SO32− was 0.19 mol/L, BASS-SO2 flow rate was 40 L/h, and heating steam temperature was 373 K.

Figure 12. Effect of initial SO32− concentration on the desorption rate and the desorption efficiency of BASS-SO2. BASS-SO2 flow rate was 40 L/h, system pressure was 20 kPa (absolute pressure), and heating steam temperature was 373 K.

evaporation tube. Thus, the temperature difference between the heating steam outside and the falling film inside decreased, which led to the decline in the convection heat transfer effect. It also means the transfer frequency of the fluid micelles between the gas−liquid interface and the film body became less active and the update rate of the SO2 in the interface dropped down. In addition, the secondary steam in the evaporation tube became weak due to the rise of the system pressure. So its stretching and disturbing effect on the falling film was also weakened which led to an increase in the mass transfer resistance. Although the solubility of the SO2 in the BASS liquid declined as the film temperature rose, it took a relatively minor part compared with the former factors. Therefore, both the desorption rate and the desorption efficiency dropped down with the increase of system pressure. In the practical application, the regeneration of BASS must take both the desorption efficiency and the vacuum pump power into consideration. Although lower system pressure contributes to the desorption of BASS-SO2, it would consume more vacuum pump power. So it is uneconomical to maintain the system pressure at too low a level. Therefore, the system pressure P = 40 kPa (absolute pressure, desorption efficiency 90%) would be a proper value in the industrial process. 3.4. Effect of SO32− Concentration. The initial concentration of SO32− in the BASS-SO2 was regulated by controlling the flow rate of pure SO2 and the time injected into the BASS. The initial system pressure was set as 20 kPa, and the BASSSO2 flow rate was 40 L/h. The heating steam temperature was maintained at 373 K. As exhibited in Figure 12, the desorption rate rose with the augment of the concentration of SO32−. When the initial concentration of SO32− in the liquid film rose, the driving force in the mass transfer process was enhanced. Thus, the desorption reaction in the falling film was accelerated, while the amount of SO32− flowing into the system also increased as the concentration of SO32− rose. The increase range in desorption rate could not catch up with that of the SO32− inflow. Therefore, the desorption efficiency dropped down with the increase of initial SO32− concentration in the BASS-SO2. In the industrial process, the absorption efficiency and the desorption efficiency should be taken into account simulta-

neously. In order to ensure a high desorption efficiency of BASS-SO2 and keep the BASS recycled, the amount of SO2 that BASS absorbed should not be too high. Thus, the concentration of SO2 in the BASS-SO2 must be kept below 0.3 mol/L. 3.5. Effect of Heating Steam Temperature. The initial concentration of SO32− in the BASS-SO2 was maintained at 0.19 mol/L. The system pressure was set as 20 kPa, and the BASS-SO2 flow rate was 40 L/h. The heating steam temperature outside the evaporation tube varied from 373 to 393 K. As described in Figure 13, both the desorption rate and the desorption efficiency increased with the rise of heating steam

Figure 13. Effect of heating steam temperature on the desorption rate and the desorption efficiency of BASS-SO2. The initial concentration of SO32− was 0.19 mol/L, BASS-SO2 flow rate was 40 L/h, and system pressure was 20 kPa (absolute pressure).

temperature. As the steam temperature increased, the bubble point of the BASS rose slowly due to the low system pressure in the evaporation tube. Thus, the temperature difference between the heating steam outside and the falling film inside was magnified, which stimulated the convection effect along the G

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research radial direction of the evaporation tube. It also means the transfer frequency of the fluid micelles between the gas−liquid interface and the film body became more active, and the update rate of the SO2 in the interface increased. Besides, the solubility of SO2 in the BASS-SO2 also declined due to the rise of film temperature. It also contributed to the acceleration of desorption reaction. Therefore, the desorption rate and the desorption efficiency both increased with the rise of heating steam temperature. In the practical applications, the falling film is heated by the waste steam generated from other industrial processes. The heating steam temperature will not be too high due to low steam pressure. Besides, the economic factor should also be taken into consideration. Lower steam pressure means a lower grade heat source could be applied. Therefore, the heating steam temperature T = 373 K (desorption efficiency 92.4%) was proper in the practical application.

4. CONCLUSION This paper presents a novel technology to the desorption of BASS-SO2 by falling film evaporation with a spring tube. Compared with the traditional desorption methods, this technique has much superiority in energy conservation and economic benefits in practical application, as it could sufficiently utilize the waste steam in the industrial production. Besides, the desorption performances of the vertical single falling film evaporators with spring tube and smooth tube were comparatively investigated. Key findings of the study are shown as follows. (1) The spring tube had better performance on the uniformity and stability of the falling film compared with the smooth tube. But there were still local dry spots in the film for the spring tube at the low flow rate range. (2) Compared with the smooth tube, the heat transfer coefficient in the spring tube was averaging an increase of about 38%, and the desorption rates in the spring tube were averaging improvements by 19%. (3) The influence of operating parameters such as BASS-SO2 flow rates, system pressure, heating steam temperature, as well as the concentration of the SO32− were sufficiently explored. Both the decline of the system pressure and the rise of the steam temperature could promote the desorption rate and the desorption efficiency. Although the increase of BASS-SO2 flow rate and the rise of initial concentration of SO32− could stimulate the desorption rate, the efficiency of desorption declined because the increase of desorption rate could not catch up with the increase of SO2 inflow rate.

■

■

BASS = basic aluminum sulfate solution BASS-SO2 = basic aluminum sulfate solution rich in SO2 v = the desorption rate C1 = the concentration of SO32− in BASS-SO2 C2 = the concentration of SO32− absorbed by NaOH solution, mol/L C3 = the concentration of SO42− absorbed by NaOH solution, mol/L V1 = the volume of BASS-SO2 V2 = the NaOH solution volume V3 = the volume of the condensed water in the counting cup V4 = the volume of the condensed water t = the desorption time, min η = the desorption efficiency d = the inner diameter of the evaporation tube l = the heating length of the evaporation tube A = the heat exchange area, m2 y = the energy utilization efficiency H1 = the total energy input into the falling film evaporation H2 = the energy absorbed by the rise of temperature of BASS-SO2 H3 = the energy absorbed by the phase change of the solvent h1 = the phase change enthalpy of the water at steam pressure h3 = the phase change enthalpy of the water at the pressure in the evaporation tube ρ = the density of the condensed water Cp = the specific heat of the BASS-SO2 T1 = the environment temperature T2 = the boiling temperature of the BASS in the evaporation tube q = the heat flux h = the heat transfer coefficient ΔT = the temperature difference between the steam outside and the film inside

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-20-22236929. E-mail: [email protected]. ORCID

Shuangfeng Wang: 0000-0001-7779-1750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Program of International Science and Technology Cooperation of China (Grant 2016YFE0118100) and Dongguan Innovative Research Team Program (Grant 2014607119).

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NOMENCLATURE FGD = flue gas desulfurization H

DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.iecr.7b05036 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX