Mass Transfer Study of Dehydration by Triethylene Glycol in Rotating

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Thermodynamics, Transport, and Fluid Mechanics

Mass Transfer Study of Dehydration by Triethylene Glycol in Rotating Packed Bed for Nature Gas Processing Shao-Bo Cao, Ping Liu, Liangliang Zhang, Bao-Chang Sun, Haikui Zou, Guang-Wen Chu, Yong Luo, and Jian-Feng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04813 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Industrial & Engineering Chemistry Research

Mass Transfer Study of Dehydration by Triethylene Glycol in Rotating Packed Bed for Nature Gas Processing Shaobo Cao1, Ping Liu1, Liangliang Zhang1,*, Baochang Sun 1, Haikui Zou1, Guangwen Chu 1,2, Yong Luo1, Jianfeng Chen 1,2 1

Research Center of the Ministry of Education for High Gravity Engineering and

Technology, Beijing University of Chemical Technology, Beijing, 100029, PR China 2

StateKey Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China

Abstract: To ensure steady run of gas transmission lines and meet the utilization specification of downstream processes, gas dehydration operation is employed in offshore natural gas production, where has an urgent demand for space and size intensivism. In this work, a rotating packed bed (RPB) was used to intensify the triethylene glycol (TEG) dehydration process and the effect of operating variables on the overall volumetric mass transfer coefficient (Kya) and height of mass transfer unit (HTU) was investigated. The value of HTU in RPB was measured to be 4.3-7.9 mm, which was at least an order of magnitude lower than that in conventional columns, indicating the significantly reduction in the size of the dehydration apparatus. The predicted Kya matches well with the experimental data and the mean relative error (MRE) is only 6.68%. Keywords: Dehydration, TEG, Mass transfer, Intensification, RPB

*

Corresponding author. Tel: +86-10-64443134; fax: +86-10-64434784. E-mail address: [email protected]

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1. INTRODUCTION In addition to being an important source of primary industrial energy source, natural gas (NG) is also widely used in chemical industry and daily life1. Because of the cleaner nature, it is believed that the demand for NG will continuously grow in the future. Therefore, besides the exploitation of onshore natural gas, there has recently been an increase in exploitation of offshore natural gas. By far, the production of offshore nature gas are still facing many challenges, such as harsh environments, extreme climates, and limited space 2-3. NG usually contains a large amount of water vapor and is fully saturated when it is mined from oil and gas reservoirs4. Water vapor in NG can make several operational problems during the production and transmission process. For example, the formation of gas hydrates and free water (liquid) by water vapor can lead to plugging of valves and pipelines, reduction of pipeline transmission capacity and increase of power consumption. Besides, in wet natrual gas, CO2 and H2S can form acid, which corrodes the pipelines and equipments5-8. To ensure smooth operation of gas transmission lines and meet the utilization specification of downstream processes, gas dehydration operation is necessary in NG industry. Common methods for NG dehydration include absorption with liquid solvents, adsorption with solid desiccants, condensation, membrane separation, and supersonic separation9-15. At present, absorption with liquid solvents is the most widely used method for NG dehydration industry. Glycols are the most widely used desiccants in the NG dehydration process because they satisfy most of the industrial criteria2, 4, 16 . TEG is by far used in about 95% of the glycol dehydration units for NG streams as it exhibits most of the desirable criteria of commercial suitability including high water capacity, high thermal stability and low vaporization loss17.

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Traditional gas-liquid contactor has been used in NG-TEG dehydration system for a period of time. The main types of contactor used in previous studies are packed tower, and spray tower18-20. Packed tower configurations arouse lots of interest because they can achieve mass transfer intensification by providing larger gas-liquid contact area21. Although these contactors have been improved by structure and internals modification to enhance the gas-liquid mass transfer, the size and weight of these contactors are still relatively large, which not only makes the high cost of the equipment itself, but also brings additional construction cost of offshore platform due to the increase in space demand and load-bearing capacity of the platform structure. Consequently, it is urgent to develop spatially intensive and high effective alternative dehydration technology to meet the applicative demand of offshore platform. Meanwhile, the technique should be robust enough to adapt to harsh environments and extreme climates to ensure reliability and availability in offshore location3. Process intensification can help address some of these issues by using technologies which are energy efficient and use of minimal space3, 22. Higee (high gravity) has been deemed as an advanced process intensification technique23. Rotating packed bed (RPB), which is the typical device of Higee, can generate centrifugal force by high-speed rotating rotor to simulate high gravity environment23-25. It has been reported that the mass transfer is greatly intensified in PRB and the total investment and size of the equipment will be consequently minimized26-28. For example, Rajan et al.33 studied the mass transfer characteristics of the CO2 absorption process in a novel RPB, and found that the mass transfer coefficient for a RPB can be 2-20 times that in a conventional packed column. Works by Reddy et al.34 indicated that a volume reduction of distillation and absorption units by 2 orders of magnitude is feasible for a Higee system. These studies demonstrate the process intensification potential of RPB. Until now, RPB has

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been widely applied in many field of chemical industry, including absorption of acid gas, distillation, polymer devolatilization and deoxidation29-35. In addition, RPBs used for seawater deoxygenation and NG desulfurization have successfully achieved continuous running for several thousands of hours on the offshore platform, which well demonstrates that RPB is robust enough to adapt to the offshore environment and ensure reliability for industrial operation36. Based on these previous studies and successful applications, it is believed that RPB will be an attractive alternative for offshore natural gas dehydration operation. Actually, the concept of NG dehydration using Higee has been reported by Agarwal et al.37, who developed a design procedure for Higee absorption/distillation systems. A desigh study in their report for NG dehydration case shows that significant volume reduction can be achieved using Higee over conventional packed beds. Their design was based on a general mass transfer correlation because there still lacked a specific correlation for NG-TEG dehydration system, which was actually the foundation of an accurate RPB design for dehydration process. This work therefore is aimed at investigating the mass transfer performance of the dehydration process in RPB. The volumetric overall mass transfer coefficient (Kya) and height of mass transfer unit (HTU) were used to characterize the mass transfer characteristics of the TEG dehydration process in RPB. The effects of different operating conditions, such as high gravity level (β), gas flow rate (G), liquid flow rate (L), water content in the inlet gas stream (yin), TEG concentration (wTEG ) and absorption temperature (T), on Kya and HTU were studied based on single-factor experiment. To illustrate the advantage of RPB in the mass transfer of the dehydration process, similar wet gas dehydration experiments were carried out in a packed column. In addition, dimensionless numbers were used to correlate Kya for the dehydration process in RPB contactor over wide ranges of operating conditions.

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2. EXPERIMENTAL SECTION 2.1. Materials. In this work, the TEG (purity ≥ 99.27%) was purchased from Shanghai Aladdin Biological Reagent Co. Ltd., N2 (purity ≥ 99.9%) was purchased from Beijing Ruyuanruquan Technology Co. Ltd. Before starting the experiment, water was added to TEG in order to get different TEG concentrations. 2.2. Experimental Procedure. Figure 1 shows a flowchart of experimental apparatus for wet gas dehydration process and nitrogen is used to simulate NG in this process. It is believed that the effect of this substitution on the experimental results can be neglected since our experiment is focused on investigating the mass transfer characteristics of the dehydration process in RPB.

1,8-nitrogen cylinder; 2,7,9,10-valve; 3,6,18-flowmeter; 4-water storage tank; 5-gas mixer; 11-circulating water tank; 12,15-pump; 13,17-gas moisture; 14-RPB; 16-TEG storage tank Figure 1. The experimental set-up for gas dehydration in the RPB

The wet gas dehydration experiment was performed as follows. Firstly, TEG at a certain volume flow rate was pumped from a TEG storage tank into RPB through a liquid inlet. Liquid distributor of RPB sprayed TEG into the inner edge of the packing, which was made of wire mesh. The TEG absorbent moved outward and left from the outer edge of the RPB through a centrifugal force. Meanwhile, the N2 feed gas 5



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outflowed from a nitrogen cylinder was saturated with water after flowing into a water storage tank. The N2 gas with saturated water was then mixed with another N2 feed gas in a gas mixer forming a wet feed gas stream, which contained certain content of water. Wet gas feed stream flew inward from the outer edge of RPB by a pressure-driving force. Gas and liquid streams had a countercurrent contact in the RPB and the water in the wet gas feed stream was absorbed into TEG in this process. Finally, water-loaded TEG exited from RPB’s liquid outlet and the dehydrated gas exited from RPB’s gas outlet. The water content in the wet feed gas stream and dehydrated gas was measured by two gas moisture meters (type RHD-601, rang 0-30000 ppm, accuracy 30 ppm), respectively. The water content in TEG was measured by the Karl Fischer titrator (type KLS701, accuracy 5%). The water content of inlet and outlet gas stream were not recorded until the gas moisture meters arrived at stabilization. The water content in the TEG could be converted into TEG concentration according to Eq.(1) 𝑤𝑇𝐸𝐺 = 𝑤 − 𝑤𝐻2 𝑂

(1)

where w is TEG initial concentration and it’s value is 99.27% in this work, wH2O is water content in TEG. Table 1. Specifications of the RPB Used in This Work Item

Units

Value

Inner radius of the packing, r1

cm

1.72

Outer radius of the packing, r2

cm

4.10

Axial height of the packing, H

cm

1.64

Packing volume, Vp

cm3

71.33

Porosity of packing

0.97

Surface area per unit volume of the packing

m2·m−3

500

Table 1 gives the specifications of RPB mentioned above. The operation conditions were as follows: liquid flow rate (L) of 6-14 L·h-1, gas flow rate (G) of 2506


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360 L·h-1, high gravity level (β) of 12-168, absorption temperature (T) of 293-313 K, TEG concentration in mass fraction (wTEG ) of 97.81-99.27%, and the water content in the inlet gas stream (yin ) of 9000-18500 ppm. This work is aimed at investigating the mass transfer performance of the dehydration process in RPB, which was shown in terms of Kya and HTU. The Kya and HTU are derived based on two-film theory. The detailed derivation process from the experimental parameters and measured results are given in the Supporting Information. For a comparison, similar dehydration experiments were also performed in a conventional packed column. The inner diameter and height of the packed column were 5cm and 140 cm, respectively. Stainless steel θ-ring packing with height and diameter of 5 mm was used to filled the column. And the height of packing layer is 110 cm.

3. RESULTS AND DISCUSSION 3.1. Effect of Gas Flow Rate on Kya and HTU.

Kya (kmol·m-3s-1)

0.14

0.68

T=25℃, β=34, yin=15000 ppm, wTEG=98.95%，L=6 L·h-1

0.64

0.13

0.60 0.56

0.12

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.52 0.11

Kya HTU

0.10 240

270

300

330

0.48

360

-1

G (L·h )

Figure 2. Effect of gas flow rate on Kya and HTU

Figure 2 displays the effect of gas volumetric flow rate on Kya and HTU in RPB. It was clear that an increase in gas flow rate led to a higher Kya and HTU. Turbulence of the system increased with an increase in gas flow rate, which led to the decrease of 7



mass transfer resistance and thus an increase of Kya. Meanwhile, increasing the gas flow rate meant less TEG solution per unit of wet gas and consequently lowered the water removal efficiency, which resulted in a decrease in NTU and subsequently an increase in HTU. These results are in accordance with the conclusions reported by Chuang et al 38. 3.2. Effect of Liquid Flow Rate on Kya and HTU.

0.68

Kya (kmol·m-3s-1)

0.14

T=26℃, β=34, yin=15000 ppm G=250 L·h-1, wTEG=99.02%

0.64

0.13

0.60 0.56

0.12

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.52 0.11 Kya

0.48

HTU

0.10 6

8

10 L (L·h-1)

12

14

Figure 3. Effect of liquid volumetric flow rate on Kya and HTU

Figure 3 shows the effect of liquid volumetric flow rate on Kya and HTU. Clearly, Kya increased while HTU decreased with the increase of the liquid flow rate. On the one hand, an increase of liquid flow rate could enhance the turbulence on the liquid side. Hence, increasing the liquid flow rate leaded to a thinner liquid boundarry conditions which reduced the mass ransfer resistance. On the other hand, increasing the liquid flow rate led to an increase of liquid holdup in RPB, which would generate more droplets, threads, and films forming larger gas-liquid contact area in the RPB. Both of the two effect enhanced the gas-liquid mass transfer. Thus, there was an obvious increase in Kya and decrease in HTU with increasing the liquid flow rate. 3.3. Effect of High Gravity Level on Kya and HTU. 8


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The high gravity level (β) reflects the centrifuge force within RPB, which is equal to acceleration of gravity. It could be calculated by the following equation: 𝛽=

𝜔2 𝑟

(2)

𝑔

where ω is the angular speed of the RPB, r is the geometric average radius of the packing and g is the acceleration of gravity (9.8 m·s-2).

0.11

0.80 -1

T=25℃, yin=9000 ppm, G=250 L·h , -1

L=10 L·h , wTEG=99.22%

-3 -1

Kya (kmol·m s )

0.75 0.10 0.70

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.09 0.65

Kya HTU 0.08

0.60 0

40

80

β

120

160

Figure 4. Effect of RPB’s high gravity level on Kya and HTU

Figure 4 plots the effect of high gravity level on Kya and HTU in the RPB. It showed that Kya increased and HTU decreased obviously with an increase in high gravity level of RPB ranging from 12 to 67. However, only a small effect on Kya was observed when the high gravity level was further increased. It has been proved that the high gravity level has great influence on the liquid flow pattern upon the surface of the packing and inside the interspace of the packing. The liquid flow pattern in the packing of RPB varied from liquid films (β100g)23, and the latter flow pattern would provide more gas-liquid contact area, which was beneficial to the mass transfer process. Simultaneously, higher rotating speed caused faster gas-liquid interface renewal, thereby reducing the gas-liquid mass transfer resistance and resulting in a better gas-liquid mass transfer effect. However, an 9



increase in high gravity level could also slightly reduce the liquid holdup in the packing, and thus might decrease the gas-liquid contact area. This partly offset the extent of reduction in mass transfer resistance, which prompted Kya to be not obviously improved with the increase of the high gravity level when β was higher than 67. 3.4. Effect of Water Content in the Inlet Gas Stream on Kya and HTU.

0.14

0.68

T=25℃, G=250 L·h-1, L=10 L·h-1, wTEG=99.22%, β=168

Kya (kmol·m-3s-1)

0.64 0.13 0.60 0.12

0.56

0.11

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.52 Kya

0.10 8000

HTU 12000

16000

0.48 20000

yin (ppm) Figure 5. Effect of water mole fraction of the inlet gas stream on Kya and HTU

Figure 5 presents the effect of water content in the inlet gas stream on Kya and HTU in RPB. It was shown that Kya increased and HTU decreased when the water content of the inlet gas stream raised. However, the change was not significant. Based on the two-film theory, the increase of water content in the inlet gas stream meant increasing the mass transfer driving force of gas phase, which was beneficial for allowing more H2O molecules to travel from gas bulk to the gas-liquid interface. On the other hand, the increase of gas content meant more water was absorbed into the TEG solution, leading to the increase of dilution speed of liquid phase and cosequently the decrease of liquid phase mass transfer driving force. As a result, only a slight increase in Kya was observed in this work. 3.5. Effect of TEG Concentration on Kya and HTU.

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0.13

0.84

G=250 L·h-1, yin=15000 ppm, L=10 L·h-1, T=25℃, β=34.38

0.78

Kya (kmol·m-3·s-1)

0.12

0.72 0.11 0.66 0.10 0.60 0.09 Kya 0.08 97.5

HTU 98.0

98.5 wTEG (%)

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.54 0.48

99.0

Figure 6. Effect of TEG concentration on Kya and HTU

Figure 6 displays the effect of inlet TEG concentration on Kya and HTU. It can be seen that Kya obviously rose and HTU reduced when the TEG concentration increased. But Kya and HTU didn't vary linearly with a change of the TEG concentration. The increase of TEG concentration led to the enhancement of driving force in liquid side, which was beneficial to mass transfer process. When the concentration of TEG increased, the water content in the outlet gas stream decreased and the value of the equilibrium water content in gas phase should also decrease. Because these two variations were simultaneous, the change of the Kya and HTU didn't vary linearly with the change of TEG concentration. 3.6. Effect of Absorption Temperature on Kya and HTU. Figure 7 presents the effect of temperature on the measured Kya and HTU. With increasing temperature from 20 to 40 ℃, Kya had an obvious increase and HTU decreased. This was because increasing the temperature would decrease the viscosity of the TEG. Meanwhile, increasing the temperature would also increase the diffusion coefficient since it was proportional to the temperature and inversely proportional to viscosity. Hence, increasing the temperature leaded to better flow and gas-liquid mixing

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in the packing that subsequently caused easier diffusion and mass transfer of water vapor in TEG. Although high temperature gave rise to the decrease of Henry’s law constant for water-TEG system, which led to a decrease in logarithmic mean driving force ( △ym ), this unfavorable effect was not dominating factor of the water-TEG absorption process in our investigated temperature range. Excessive temperature (higher than 40 ℃) easily caused the foaming of the absorbent, which led to abnormal operation of the absorption process in the contactor, so excessive temperature range was not investigated in this study.

G=250 L·h-1, wTEG=99.22%, L=6 L·h-1,

0.15

0.7

β=34.38, yin=19000 ppm

Kya (kmol·m-3s-1)

0.14 0.6

0.13 0.12

0.5

HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.11 0.10

Kya

0.4

HTU

0.09 20

25

30 T (℃)

35

40

Figure 7. Effect of absorption temperature on Kya and HTU

3.7. Comparison. Dehydration experiments in a traditional packed column was conducted under the similar operating conditions as a comparision. Figure 8 shows the effect of the gas flow rate on Kya and HTU in the packed column. The experimental conditions were listed in Table 2. It was evident that Kya was nearly proportional to the gas flow rate, which was similar to the experimental results in RPB.

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0.004

44

Kya (kmol·m-3s-1)

0.003 40 HTU (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.002 36 0.001

Kya HTU

0.000

32 80

120

160

200

240

G (L·h-1) Figure 8. Effect of gas flow rate on Kya and HTU in packed column

The mass transfer performance of wet gas dehydration process in different contactor including packed column, spray column and RPB was compared in Table 2. It was reported that HTU was about 120-180 mm for the packed column and 680-950 mm for spray column in the previous report. In this study, HTU was measured to be 4.3-7.9 mm for RPB and 345-401 mm for packed column. Relatively speaking, the packed column has better mass transfer performance than spray column in the TEG dehydration process. Meanwhile, it could be seen that the value of HTU for RPB is at least one order of magnitude lower than that for the packed column. This well indicates that RPB possesses excellent gas-liquid mass transfer intensification performance for dehydration process using TEG. The size of the dehydration contactor is expected to be reduced to less than 1/10 compared to the traditional packed column by using Higee technology. Thus it could be envisioned that RPB will exhibit a great potential for wet gas dehydration process, especially for space confined situations, such as offshore platform.

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Table 2. Comparison of Dehydration Performance Between the Packed Column and RPB Packed column

Spray column18

Packed column38

RPB

(this work)

(with fin coil)

Temperature, K

298

297-303

293-295

293-313

Gas flux, m3·m-2·s-1

0.0113~0.0340

No report

1.2-1.66

0.0160~0.0230

Gas-liquid ratio

20-60

510

120-160

18-60

Inlet water content, ppm

15000

21160-27680

17400

9000-18500

TEG concentration,%

99.12

93.6-94.3

95

97.81-99.27

HTU, mm

345-401

680-950

120-180

4.3-7.9

3.8. Empirical Correlation of Kya Establishing an accurate correlation related to Kya is very important for the design of RPB and predicting the effect of operation parameters on the dehydration process. Based on the single-factor experiment, the variables found to influence the value of Kya are the gas flow rate, the liquid flow rate, the diffusion coefficient of water in the gas, the TEG concertation, the high gravity level, water content in the inlet gas stream, and the packing character of RPB, physical properties of both air and the liquid. The Kya for gas dehydration using TEG solution in a RPB can be expressed as a function of the following parameters: (3)

𝐾𝑦 𝑎 = 𝑓(𝑀𝑡 , 𝑑𝑝 , 𝐺, 𝐿, 𝑦𝑖𝑛 , 𝑤𝑇𝐸𝐺 , 𝐷𝑣 , 𝜇𝑣 , 𝛽)

Usually, the above variables are arranged into pertinent dimensionless groups and mass transfer correlation in terms of the dimensionless groups can be obtained from the dimensional analysis and linear regression. Chung et.al38 developed two mass transfer correlations for random packing and structured packing in packed tower as below: Mass transfer correlation for random packing: 2 𝐾𝑦 𝑎𝑀𝑡 𝑑𝑝

𝐷𝑣 𝜌𝑣

𝐿

= 6.33 × 10−5 (1 − 𝑋)−0.09 (𝐺)0.27 𝑆𝑐𝑣 0.333 𝑅𝑒𝑣 1.38

Mass transfer correlation for structured packing: 14


(4)

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2 𝐾𝑦 𝑎𝑀𝑡 𝑑𝑝

𝐷𝑣 𝜌𝑣

𝐿

= 9.03 × 10−6 (1 − 𝑋)−0.05 (𝐺)0.26 𝑆𝑐𝑣 0.333 𝑅𝑒𝑣 1.34

(5)

However, these correlations are not applicable to predicting the dehydration performance in RPB because they lack the effect of high gravity level on Kya. Besides, the (1-X) in the above correlations, which represents the mass transfer driving force, is defective. Because it can only represent the effect of TEG purity on mass transfer driving force and misses the effect of water content in inlet gas on mass transfer driving force, which is proved notable in this study. A new correlation, which takes the high gravity level and the complete mass transfer driving force into account, is given as below: 2 𝐾𝑦 𝑎𝑀𝑡 𝑑𝑝

𝑦

where

y 1-wTEG

𝐺

= 𝛼(1−𝑤

𝐷𝑣 𝜌𝑣

𝑇𝐸𝐺

)𝛾 ( 𝐿 )𝛿 𝑆𝑐𝑣 𝜀 𝑅𝑒𝑣 𝜂 𝛽 𝜃

(6)

represents the mass transfer driving force between the TEG solution and

gas phase. According to penetration theory39 and surface-renewal theory40-41, the mass transfer coefficient is theoretically proportional to Dv0.5. However, many experimental studies have shown that Kya is actually proportional to Dv0.67. Thus, the exponent of the Schmidt number ε is fixed at 0.333 in this study. The value of α, γ, δ, η, and θ are constant and obtained by linear regression of the experimental data. The final correlation can be given as follow: 2 𝐾𝑦 𝑎𝑀𝑡 𝑑𝑝

𝐷𝑣 𝜌𝑣

𝑦

= 0.1697(1−𝑤

𝐺

𝑇𝐸𝐺

)0.231 ( 𝐿 )−0.236 𝑆𝑐𝑣 0.333 𝑅𝑒𝑣 1.614 𝛽0.125

(7)

The mean relative error (MRE) of the calculated values (Kya, cal) from the experimental data (Kya, exp) can be calculated by Eq.(8) 1

𝑀𝑅𝐸 = 𝑁 ∑𝑁 𝑖=1 |

(𝐾𝑦 𝑎𝑖,𝑐𝑎𝑙)−(𝐾𝑦 𝑎𝑖,𝑒𝑥𝑝) (𝐾𝑦 𝑎𝑖,𝑒𝑥𝑝)

| × 100%

(8)

where N represent the number of the experimental data. As is shown in Figure 9, the calculated Kya values matches well with the experimental data and the MRE is only

15



6.68%. And the deviation of all the calculated values is within ±20%. In Eq.(7), 0.231, -0.236, 1.614 and 0.125 represent the inlet gas and absorbent characteristic, the RPB operation characteristic, the gas flow characteristic and high gravity field characteristic. The exponent of β is positive indicating that Higee technology can effectively intensify the mass transfer of the dehydration process.

+20%

0.20 -3

-1

Predicted Kya (kmol·m ·s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.16

-20%

0.12

0.08

0.04 0.04

0.08

0.12

0.16 -3

0.20 -1

Experimental Kya (kmol·m ·s ) Figure 9. Comparison of the predicted and experimental Kya values

3.9. Case Study To demonstrate the advantage of using Higee technique, a RPB contactor for NGTEG dehydration process is designed based on the empirical correlation of Kya, which is given in section 3.8. The specifications of the NG-TEG dehydration process, which is taken as typical of an offshore platform (Example 20-11 of the literature42), is listed in Table 3. The composition of the NG stream is simplified as a mixture of methane and water vapor. Table 3. NG Dehydration Design Problem Specification Item

Value

Gas flow rate

1490 kmol·h-1

Inlet gas mole fraction

0.9982 CH4; 0.0018 H2O

16


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Pressure, atm

41.4

Operation temperature, ℃

40

Mole fraction of water in outlet gas

0.00014

Liquid flow rate

11.66 kmol·h-1 (99 wt % TEG)

For a comparison, a RPB is designed to get the same separation efficiency with the packed column provided in the litrature42. And the design is based on the design procedure developed by Agarwal et al.37 To get the whole size of the contactor and the volumn of the packing, the inner radius, outer radius and the height of the RPB were calculated, respectively. The related result is shown in Table 4. As it shown in Table 4, the packing volume of packed column and RPB are 1.28 m-3 and 0.0079 m-3 respectively. This result indicates that using RPB can reduce volume of dehydration contactor by 2 orders of magnitude, which is in accordance with the conclusions reported by Reddy et al 34. Table 4. Comparison of Dimension Between RPB and Packed Column RPB design

Packed column

rotational speed

1500 rpm

inner radius of the packing, r1

0.087 m

diameter

0.73 m

outer radius of the packing, r2

0.14 m

height

3.05 m

axial height of the packing, H

0.21 m

packing volume, Vp

0.0079 m-3

packing volume

1.28 m-3

RPB volume, VRPB

0.013 m-3

Dimensions

Volume

17



CONCLUSIONS This work studied the mass transfer characteristics of dehydration process using TEG in RPB systematically. It was found that the mass transfer performance of TEG dehydration process was effected by many process parameters. Kya increased with gas flow rate, liquid flow rate, high gravity level and TEG concentration in this work. A mass transfer correlation for Kya that could display the effect of various operating conditions was established for the TEG dehydration process in RPB. The predicted Kya matches well with the experimental results and the MRE is only 6.68%. A comparison of the mass transfer characteristics of dehydration process between RPB, packed column and spray column revealed that the value of HTU in RPB was more than 1 order of magnitude lower than that in a conventional column, which indicated significantly reduction in the size of the dehydration apparatus. In summary, RPB exhibited great potential for industrial wet gas dehydration process, especially for space confined situations, such as offshore platform.

ASSOCIATED CONTENT Supporting Information Derivation process of Kya in a rotating packed bed.

AUTHOR INFORMATION Corresponding Authors *Tel: +86-10-64443134; fax: +86-10-64434784. E-mail address: [email protected].

ACKNOWLEDGEMENTS This work was financially supported by National Key R&D Program of China (No. 2017YFB0603300) and the fundamental research funds for the central universities (JD1706). 18


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NOTATION yin =inlet mole fraction of water in gas phase ,% yout =outlet mole fraction of water in gas phase ,% y=mole fraction of water in gas phase, % y*=equilibrium mole fraction of water in gas phase, %

△ym =logarithmic average of the absorption driving force r1 =inner radii of the packing, m r2 =outer radii of the packing, m r=geometric average radius of the packing, m dr=radial thickness of the volume element, m w=TEG initial concentration, % wH2O =water content in TEG, % wTEG =TEG concentration, % g=acceleration of gravity, m·s-2 G=gas flow rate, L·h-1 L=liquid flow rate, L·h-1 ω=angular speed, rad·s-1 Kya=gas phase volumetric mass transfer coefficient, kmol·m-3·s-1 H=axial length of the packing, m N= number of the experimental data NH2O =absorption rate of H2O, kmol·m-3·s-1 NTU=number of mass transfer units HTU=height of mass transfer unit ,cm Dv=diffusion coefficient, m2·s-1 Mt=molecular weight of the solute, kg·kmol-1 19



Vp = packing volume, cm3 ρv=density of gas, kg·m-3 μv=viscosity of the gas phase, kg ·m·s-1 3

dp=spherical equivalent diameter of packing ( √

6𝑉𝑝 𝜋

), m

T=absorption temperature, K α, γ, δ, ε, η, θ=coefficients in correlation β=high gravity level Dimensionless Groups Sc𝑣 = Schmidt number =

μ𝑣 𝜌𝑣 𝐷𝑣

Re𝑣 = Reynolds number =

𝜌𝑣 𝑑𝑝 𝑢𝑣 𝜇𝑣

20


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Mass Transfer Study of Dehydration by Triethylene Glycol in Rotating

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