alkylation of isobutane and 2-butene by concentrated sulfuric acid in a

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Kinetics, Catalysis, and Reaction Engineering

ALKYLATION OF ISOBUTANE AND 2-BUTENE BY CONCENTRATED SULFURIC ACID IN A ROTATING PACKED BED REACTOR Yuntao Tian, Zhenxing Li, Sijing Mei, Miaopeng Sheng, Jian-Feng Chen, Guang-Wen Chu, Liangliang Zhang, Adrian C. Fisher, and Haikui Zou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02570 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Industrial & Engineering Chemistry Research

ALKYLATION OF ISOBUTANE AND 2-BUTENE BY CONCENTRATED SULFURIC ACID IN A ROTATING PACKED BED REACTOR Yuntao Tian†,‡, Zhenxing Li‡,§, Sijing Mei‡,§, Miaopeng Sheng‡, Jian-feng Chen†,‡,§, Guangwen Chu‡,§, Liangliang Zhang§*, Adrian C. Fisher†, Haikui Zou‡*

†

Beijing Advanced Innovation Center for Soft Matter Science and Engineering,

Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡

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

Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China §

State Key Laboratory of Organic-Inorganic Composites, Beijing University of

Chemical Technology, Beijing, 100029, People’s Republic of China

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Abstract

A novel Rotating Packed Bed (RPB) reactor is firstly adopted to intensify the reaction of isobutane alkylation with 2-butene catalyzed by H2SO4. This work investigated reaction performance, and reaction conditions were optimized. Under the optimal conditions, the research octane number (RON) reached 98.85. Meanwhile, the yields of C8 and trimethylpentanes were 90.65% and 85.47%, respectively. The reaction efficiency was tremendously improved by using RPB due to its high efficiencies of mass transfer and micro-mixing. More importantly, the inner mole ratio of isobutane to 2-butene was dramatically decreased in RPB, which means the energy cost of material cycling in the alkylation process could be extremely reduced. Moreover, an empirical correlation model was proposed to predict the multi-product yields and RON with a deviation within ±10%. In conclusion, RPB reactor is a highly promising industrial platform for the process of H2SO4 alkylation.

Keywords: Rotating packed bed; Alkylation; Isobutane; 2-Butene; Sulfuric acid.


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1. INTRODUCTION Automobiles and boilers emission, which are mainly composed of nitride oxides (NOx) and sulfur oxide (SOx), are the major source of air pollution. Meanwhile, the amount of automobiles in the world has reached a high volume and the number will continue climbing along with world’s growing population and economic demand, especially in developing countries like China and India.1 It will inevitably aggravate the environmental pollution caused by the combustion of olefins, aromatics and impurities such as sulfur and nitrogen compounds derived from motor gasoline.2, 3 Environmental protection standard is becoming increasingly stringent due to the environmental issues.4-6 Therefore, it is highly necessary to improve the quality of gasoline and make it more environmentally friendly. Alkylate oil is one of the ideal additives of blending component for gasoline as it contains no impurities such as sulfur and nitrogen compounds, aromatics and olefins. Alkylate oil helps in reducing the pollutants emission from the combustion process. Thus, there is a huge demand of alkylate in the future to appeal the criteria of the automobile exhaust gas emission standard. Refineries usually produce alkylate by alkylation of isobutane with C4 olefins to produce alkylate, which can easily get from fluid catalytic cracking (FCC) operation.7, 8 Currently, many types of catalysts have been applied to alkylation reaction, containing concentrated sulfuric acid (H2SO4), hydrofluoric acid (HF), ionic liquid and solid acid. 9-17 Ionic liquid has high viscosity and it is unstable in wet and air environment. Superacid sites would be dropped easily from solid acid, resulting in deactivation. HF possesses a high volatility and it is 3 ACS Paragon Plus Environment


extremely dangerous due to its toxicity. Once leaked, HF will cause severe casualties and environmental harm.18 Compared to these catalysts, the H2SO4 process is relatively safe, stable and most of the new alkylation factories prefer to choose sulfuric acid as catalyst. The H2SO4 process still occupies a massive market percentage, and the improvements of this process have attracted highly academic interests.19-21 The alkylation reaction is a rapid reaction with complicated products and it has mutual immiscibility phases between acid and hydrocarbon (isobutane and butene). Alkylation reaction actually occurs at interface between acid and hydrocarbon phases, which producing mixture of C8 components.3,

22, 23

Owing to the significant

differences in densities and viscosities between the H2SO4 and hydrocarbon phases, the efficiencies of mass transfer and micro-mixing in alkylation reactor have become critical issues and may even determine the distribution of alkylation products directly. On the one hand, the mass transfer of isobutane through interface of hydrocarbon and acid is the rate-determining step for alkylation reaction due to the low solubility of isobutane and the rapid reaction rate of butene.3, 16, 20, 23-25 On the other hand, besides the main alkylation reaction, butene can react by themselves to produce oligomers, which will crash into fragments, forming hydrocarbons contain more than 5 carbon atoms. These hydrocarbons with lots of molecular structures have low research octane number (RON).24 Because the solubility of butene in acid is higher than it of isobutane, butene could transfer to the interface of the two phases easier than isobutane. This triggers the aggregation of butene at the interface and consequent 4 ACS Paragon Plus Environment

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occurrence of the polymerization of butene. As the result of alkylation reaction, the product is made up of C5~7, C8 and C9+. Besides, the main components of C8 are trimethylpentanes (TMPs) and dimethylhexanes (DMHs), and the RONs of C5~7, TMPs, DMHs and C9 are 40~90, 100~109, 50~70 and 20~90, respectively. Therefore, the TMPs are the key and optimal components. Hence, intensifying the mixing between acid and hydrocarbon phases will be beneficial to restrain the polymerization. Based on the previous discussion, the improvement of H2SO4 process mainly focuses on strengthening micro-mixing and enhancing the mass transfer process. The eductor reactor was introduced by Orgral International Technologies Company, Mobil Oil Company and Amarjit Company in lieu of mechanical propel mixer in order to meet high mixing requirements of sulfuric acid alkylation.26 Dupont developed Stratco horizontal stirred reactor to obtain good mixing and dispersing efficiency by high-speed circulation of material flows. CDAlky process27 was carried out by Lummus using vertical reactor with a self-owned patent packing, instead of mechanical mixing devices to create efficient mass transfer surface between two phases. In previous years, these three modified technologies were applied to H2SO4 alkylation process successfully. Moreover, these technologies have verified that enhanced mixing and mass transfer process between acid and hydrocarbon phases is beneficial for improving alkylate quality. In these industrial processes, the external mole ratio of isobutane to 2-butene are usually at the range of 5:l to 8:1. However, to enhance the mass transfer of isobutane to the reaction interface and restrain the 5 ACS Paragon Plus Environment


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polymerization of butene, isobutane is largely recycled within the reaction zone, which increases the internal mole ratio of isobutane to 2-butene dramatically. The internal mole ratio of isobutane to 2-butene may be as high as 1000:l at the interface between the two phases, leading to massive energy cost of these processes.3 Therefore, high efficient mass-transfer equipment and technology is urgently needed to not only promote the quality of product but also decrease the mole ratio of isobutane to 2-butene (I/O) of reaction zone. Rotating packed bed (RPB) reactor is an excellent multiphase device because of its high efficiency of micro-mixing and mass transfer.28, 29

In RPB, high-speed rotating packing generates strong shear force, followed by

packing tearing liquid into tiny liquid elements with a size of 10-5~10-4 m,

30-32

including droplets, threads, and films. This results in a significant enhancement on micro-mixing and mass transfer between liquid elements.33,

34

Besides, liquid

elements will be renewed and remixed when passing through each packing layer. Moreover, previous studies have demonstrated that RPB is eligible to intensify the viscous-convective deformation and micro-mixing between liquid elements with a viscosity above 1 Pa·s.35-40 Therefore, RPB is considered as a promising reactor for H2SO4 alkylation process. The objective of this work is to provide a potential candidate for developing new H2SO4 alkylation process or upgrading the existing H2SO4 alkylation process. In this work, RPB is adopted as a new reactor to enhance the alkylation reaction performance of isobutane with 2-butene using sulfuric acid as catalyst. The effects of operating conditions, including reaction time (t), the volume ratio of sulfuric acid to 6 ACS Paragon Plus Environment

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hydrocarbon (A/HC), reaction temperature (T), RPB rotational speed (NRPB), and pressure (P), on reaction performance in the RPB are investigated to provide elementary data that is necessary to optimally design this process.

2. EXPERIMENTAL SECTION 2.1 Feed and catalyst Isobutane and 2-butene were purchased from Beijing Jumingcheng Gas Equipment Technology Development Co. Ltd. and the detailed compositions were shown in Tables 1 and 2, respectively. Concentrated sulfuric acid (purity of 95-98%) was obtained from Beijing Chemical Works.

Table 1. The composition of the butane Components

Content, %

i-Butane

98.59

n-Butane

1.20

Others

0.21

Table 2. The composition of the 2-butene Components

Content, %

t-Butene

53.18

n-Butene

1.07

i-Butene

0.66

c-Butene

44.78

Others

0.31 7

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2.2 Apparatus and process Figure 1 showed the schematic diagram of experimental alkylation process. The hydrocarbon with a required mole ratio of isobutane to 2-butene (I/O) was stored in the mixed hydrocarbon tank. Two cooling tanks, loaded with anti-freezing liquid, were used to control the temperatures of RPB and materials. Prior to the experiment, nitrogen (N2) was used to purge all the pipelines and RPB to maintain an inert atmosphere and ensure the entire assembly was leak-proof under a certain pressure. The pressure in the system was maintained by N2. Meanwhile, there was a cooling system in the RPB reactor to control the temperature. Cooling water was supplied from cooling bath to the shell surrounding the RPB and the spiral tube inside the RPB. Then, the cooling water returned to the bath. When temperature and pressure reached the desired values, the feed was forcedly flowed into RPB by N2 via a liquid flowmeter, and sulfuric acid was pumped into RPB simultaneously. In addition to that, the flow rates of hydrocarbon and H2SO4 were both 10 L/h. Two streams, were mixed and reacted in RPB by passing through the packing, and then the liquid mixture entered into a stirred tank through the liquid outlet of RPB to continue the reaction. When the operation was completed, the liquid mixture was separated into two phases automatically, and products were stored in product tank. At last, the liquid samples


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were separately collected from the liquid outlet of RPB and product tank, and the gas sample was obtained from the gas sampling point.

Figure 1. Schematic diagram of the experimental alkylation process

(1-mixed hydrocarbon tank, 2-cut-off valve, 3-filter, 4-thermostatic tank, 5-one-way valve, 6-rotating packed bed with spiral tube and shell, 7-storage tank of sulfuric acid, 8-pump,9-stirred tank,10-product tank, CWS-1/2-cooling liquid supply, CWR1/2-cooling liquid return)

2.3 Analysis methods of samples The gas phase was analyzed by a SHIMADZU GC-2014C gas chromatograph, which was equipped a flame ionization detector (FID) and a chromatographic column of GsBP-Plot Al2O3 (50 m × 0.53 mm × 15 µm). The temperatures of the injector and FID detector were 70 °C and 250 °C, respectively. The feed gas analysis result was



shown in figure 2. In the figure, the six peaks are isobutane, n-butane, t-2-butene, n-butene, isobutene, c-2-butene, successively. The liquid phase alkylates components in the product were analyzed by a Dongxi GC-4000A gas chromatograph, which was equipped an FID detector and a chromatographic column of GsBP-PONA (100 m × 0.25 mm × 0.25 µm). The temperatures of injector and FID were both 280 °C. The chromatogram of the liquid as shown in figure 3.

A

4000000

B 6

3

Intensity

1

3000000

Intensity

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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4

5

2000000 11.0

11.5

12.0

12.5

13.0

R.Time (min)

1000000

2 0 0

5

10

15

20

25

R.Time (min)

Figure 2. Hydrocarbon Analysis of Gas Chromatography A: Full figure of GC; B: Enlarged figure: 1. isobutane, 2.n-butane, 3. t-2-butene, 4. n-butene, 5.isobutene, 6. c-2-butene.


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8000000

6000000

Intensity

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


4000000

2000000

0

0

10

20

30

40

50

60

RetentionTime (min)

Figure 3. Liquid Sample Analysis of Gas Chromatography Qualitative analysis of the liquid sample was obtained by Agilent 7890B-5977A GC-MS. Area normalization method, which was corrected by factors of all components with the standard substance benzene24, 41, was adopted to quantify various components. The quantification of various components was based on the peak area and calculated by the area normalization method. The correction factors of all components were close to 1.0 with the standard substance benzene24,

41

. The

conversion of butene was calculated by x=

xin − xout × 100% xin

(1)

where x was the conversion of butene; xin was the mass fraction of the butene in the feed tank; xout was the mass fraction of the butene in the gas phase. The RON of alkylate oil was calculated by41 n

RON= ∑ vi ⋅ RON i i =1


(2)


where i was a component in alkylate oil; RONi was the RON of pure substance i; and vi was the relative volume fraction of component i in the alkylate, which was calculated on the basis of the mass fraction and the relative density of component i. The RON and relative density of the components were adapted from the report date by Liu et al.41.

3. RESULTS AND DISCUSSION 3.1 Effect of I/O mole ratio The effect of I/O mole ratio on product distribution and alkylate RON showed in figures 4 and 5. The yields of C8, TMPs, TMPs/DMHs ratio and alkylate RON increased distinctly with increasing I/O mole ratio. The yields of C5~7, DMHs, and C9+ showed an evident drop in increasing I/O mole ratio.

100

80

100

C5~C7 C8 C9+ RON

90

60 80

RON

Yield (%)

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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40

70 20

0

60 10:1

15:1

30:1

I/O mole ratio

Figures 4. Effect of I/O mole ratio on alkylate composition and RON (t=10 min, P=0.5 MPa, NRPB=1200 r/min, T=8 °C, and A/HC=1:1 v:v).


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90

75

15

TMPs DMHs TMPs/DMHs

12

9 45 6

TMPs/DMHs

60

Yield (%)

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


30 3

15

0

0 10:1

15:1

30:1

I/O mole ratio

Figure 5. Effect of I/O mole ratio on C8 distributions and TMPs/DMHs ratio (t=10 min, P=0.5 MPa, NRPB=1200 r/min, T=8 °C, and A/HC=1:1 v:v). Shi Yi

[48]

had investigated the effect of I/O mole ratio in the agitator reactor with

values of 15:1, 30:1 and 100:1. It showed that increasing I/O mole ratio would significantly promote the quality of alkylate oil. In addition to that, with the I/O mole ratio increasing, the content of 2-butene reduced obviously. The 2-butene was surrounded by isobutane so that the opportunities of 2-butene contacting isobutane were rarely few. Hence, 2-butene was possible to alkylate with isobutane rather than oligomerization. According to the carbocation mechanism, the isoalkyl cations were more possible to react with isobutane. Moreover, the alkylate oil was too few to be collected due to high I/O mole ratio. The little liquid sample is hard to be analyzed by GC. In addition, the I/O mole ratio of 30:1 was an optimal option to choose in the following experiments.

3.2 Effect of reaction time 13 ACS Paragon Plus Environment


In this study, reaction was actually carried out in two reactor, a RPB and a stirred tank following the RPB. Residence time was counted the reaction time in these two reactors. Owing to the alkylation process had a series of subsequently multi-complex reactions, such as chain rearrangement, the optimal product was usually obtained after these following reactions. Hence, the initial reaction product would be inserted into the stirred tank for subsequent reaction to obtain the final product. The effect of reaction time on product distribution and alkylate RON were shown in figures 6 and 7. The yields of C8, TMPs and alkylate RON increased with increasing reaction time before the 6 min, after which the values increased slightly. The yields of C5~7, DMHs and C9+ showed a slight drop with increasing reaction time. Although TMPs/DMHs ratio showed an increasing tendency, the alkylate RON barely changed after the 6th min.

100

100

C5~C7 C8 C9+ RON

80

98

60

94 92

40

RON

96

Yield (%)

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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90 20 88 0

86 2

4

6

8

10

Reaction time (min)

Figures 6. Effect of reaction time on alkylate composition and RON (P=0.5 MPa, NRPB=1200 r/min, T=8 °C, A/HC=1:1 v:v, and I/O=30:1 mol:mol). 14 ACS Paragon Plus Environment

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100

16

TMPs DMHs TMPs/DMHs

80

14

60 10 40

TMPs/DMHs

12

Yield (%)

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


8 20

6

0

4 2

4

6

8

10

Reaction time (min)

Figure 7. Effect of reaction time on C8 distributions and TMPs/DMHs ratio (P=0.5 MPa, NRPB=1200 r/min, T=8 °C, A/HC=1:1 v:v, and I/O=30:1 mol:mol). Isobutane reacted with 2-butene in the presence of H2SO4 and then generated a series of alkylation components in a very short time span. Because of the highly reactive carbonium intermediate in the generally accepted mechanism, the intrinsic reaction rate was extremely fast. Combined with the mechanism of carbonium ion24 and two-steps reaction42, 43, isobutene was generated through fast isomerization of 2-butene and 1-butene and then converted into tert-butyl cation (iC4+). Considerable amounts of sec-butyl sulfates were obtained from tert-butyl cation and acid. At the first step, these sec-butyl sulfates predominantly formed trimers, tetramers, and pentamers, such as mono-sec-butyl sulfate (MBS) and di-sec-butyl sulfate (DBS), then reacted rapidly to form low polymers in the C8-C20 range mainly, as heavy ends (HEs). Therefore, MBS and DBS were able to react with isobutane to produce C8 and light ends (LEs), whereas the sec-butyl sulfates could react to form sulfuric acid. Meanwhile, the rearrangement process of acyclic hydrocarbon was almost completed 15 ACS Paragon Plus Environment


within 6 minutes in this work, and the alkylate RON remained at a high value. In addition to that, the reaction time was fixed at 6 minutes in the further research.

3.3 Effect of the rotational speed of RPB

100

100

C5~C7 C8 C9+ RON

80

98

60

94 92

40

RON

96

Yield (%)

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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90 20 88 0

86 0

200

400

600

800

1000

1200

Rotational speed (r/min)

Figure 8. Effect of RPB rotational speed on alkylate composition and RON

(P=0.5 MPa, T=8 °C, t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol).


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16

100

TMPs DMHs TMPs/DMHs

80

14

60 10 40

TMPs/DMHs

12

Yield (%)

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


8 20

6

4

0 0

200

400

600

800

1000

1200

Rotational speed (r/min)

Figure 9. Effect of the rotational speed of RPB on C8 distributions and TMPs/DMHs ratio

(P=0.5 MPa, T=8 °C, t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol).

The effect of RPB rotational speed on product distribution and alkylate RON were shown in figures 8 and 9. With increasing rotational speed, the yields of C8, TMPs, TMPs/DMHs and alkylate RON increased while the yield of C9+ decrease. When the RPB rotational speed exceeded 900 r/min, no obvious changes in yields were observed. In addition, the yields of C5~7, and DMHs showed a slight drop when rotational speed increased. Alkylation was a rapid reaction which had complicated products and mutually immiscible interphase, and it was taking place at the interface between acid and 17 ACS Paragon Plus Environment


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hydrocarbon. Furthermore, the high solubility of isobutane in the acid phase would decrease side reactions such as polymerization.3 Li et al.21 had demonstrated that high stirring speed in stirred tank reactor helped generate small-sized hydrocarbon droplets in acid and thus provided large interfacial area and high mixing efficiency for the transport of isobutane, which improved the selectivity of C8. The results were consistent with their conclusions. With the rotational speed increasing, the higee level was increasing, and the mean droplet size was decreasing that was calculated from Sang et al.[49] The comparison table was shown in table 3. It was well known that the size of the droplet would affect the mass transfer and micro-mixing performance between the hydrocarbon and catalyst. The smaller the droplet size, the better the mass transfer and micro-mixing performance between the two phases. At the low speed, the mean droplet size was reducing fast with the rotational speed increased. Based on the results, we found the size of the droplet was sharply decreased when the higee level was increased below 100 g. But when the higee level is above 100 g, the increasing of the RPB higee level had very little effect on the size of the liquid size. Thus, the mass transfer and micro-mixing intensification of the RPB maybe reach the limited value and the yield remains unchanged.

Table 3. The comparison of droplet size and rotational speed

Rotational Speed

HIGEE level

Mean Droplet Size of Concentrated Sulfuric Acid

Mean Droplet Size of Hydrocarbon

r/min

/

mm

mm

150

2.32

8.00

8.99

300

9.26

5.38

6.04


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600

37.05

3.61

4.06

750

57.90

3.18

3.57

900

83.37

2.86

3.21

1200

148.21

2.43

2.73

1400

201.74

2.22

2.49

To sum up, both phases of acid and hydrocarbon were torn by high-speed rotating packing into tiny liquid elements, and these liquid elements provided a huge interfacial area, which was beneficial to mass transfer process between acid and hydrocarbon phases. The collisions between liquid elements intensified the mass transfer process. Additionally, liquid elements would be remixed and renewed when passing through each packing layer. All these factors enhanced the mass transfer and accelerated the mixing between two phases, and the effects became significant with increasing rotational speed of RPB. For isobutane alkylation reaction, the micro-mixing efficiency directly affected the distribution of alkylation products. Thus, high rotational speed was beneficial for the achievement of desired components with high alkylate RON.



3.4 Effect of A/HC

100

100

C5~C7 C8 C9+ RON

98 96

60

94 92

40

RON

Yield (%)

80

90 20 88 86

0 0.6

0.8

1.0

1.2

A/HC

Figure 10. Effect of A/HC on alkylate composition and RON

(NRPB=1200 r/ min, P=0.5 MPa, T=8 °C, t=6 min, and I/O=30:1 mol:mol).

100

80

16

TMPs DMHs TMPs/DMHs

14

60 10 40 8 20

6

0

4 0.6

0.8

1.0

1.2

A/HC


TMPs/DMHs

12

Yield (%)

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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Figure 11. Effect of A/HC on C8 distributions and TMPs/DMHs ratio

(NRPB=1200 r/min, P=0.5 MPa, T=8 °C, t=6 min, and I/O=30:1 mol:mol). The effect of A/HC on product distribution and alkylate RON were shown in figures 10 and 11. With increasing A/HC, the yields of C8, TMPs, TMPs/DMHs and alkylate RON increased while the yield of C9+ decreases. When A/HC exceeded to 0.85, no obvious changes in the yields were observed. Additionally, the yields of C5~7 and DMHs showed a slight decrease while increasing A/HC. According to the view proposed by Ende et al.,44 the interfacial area between two phases was highly dependent on A/HC, and the maximum interfacial area of 640 cm2/cm3 was obtained at an acid volumetric ratio of around 75% in their study. They recommended that an acid volume fraction of 60-65% was high enough to obtain alkylate in good quality and quantity. In this work, an A/HC ratio of 0.85 (an acid volumetric fraction of 46%) was high enough to obtain good product distributions and RON, furthermore, this value was lower than that recommended by Ende et al., which benefited from the enhancements of mass transfer and micro-mixing between acid and hydrocarbon phases as discussed in section 3.2. And Li et al.21 used sulfuric acid with trace caprolactam as the catalyst and found the optimal A/HC to get the result of an 88% selectivity of C8 was 1.50. Results showed good reaction performance could be obtained under relatively low A/HC in RPB, and this represents that the dosage of acid could be reduced when RPB was adopted as the reactor.



3.5 Effect of reaction temperature The reaction heat was mainly generated in the butene conversion reactions. Almost all the butene (99%) was converted in the RPB. The adiabatic temperature rise was calculated as about 5 K. [48] Owing to the rotational packed in the RPB, it was so difficult to measure the temperature in the reaction area directly, so the temperatures of inlet and outlet were monitored by thermocouple detectors. Based on our measurement, the temperature rise of the reactant is less than 2 K by using this cooling method. Therefore, the spiral tube was positive to cool the materials, and the inlet temperature of the RPB was set to the measure point of the temperature. Hence, the reaction was regarded as isothermal process.

100

100

C5~C7 C8 98 C9+ RON

80

96 60 94

RON

Yield (%)

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

Page 22 of 38

40 92 20

90

0 2

4

6

8

10

12

14

88 16

Reaction temperature (°C)

Figure 12. Effect of reaction temperature on alkylate composition and RON

(NRPB=1200 r/min, P=0.5 MPa, t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol).


Page 23 of 38

100

18

80

TMPs DMHs 16 TMPs/DMHs

60

12 10

40

TMPs/DMHs

14

Yield (%)

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


8 20 6 0

4 2

4

6

8

10

12

Reaction temperature (°C)

Figure 13. Effect of reaction temperature on C8 distributions and TMPs/DMHs ratio

(NRPB=1200 r/min, P=0.5 MPa, t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol). Figures 12 and 13 showed the effect of reaction temperature on product distribution and alkylate RON. With increasing reaction temperature, the yields of C8, TMPs, TMPs/DMHs and alkylate RON decreased while the yields of C9+ increased. No obvious changes in yields of C5~7 and DMHs were observed. Under the circumstance of the reaction temperature was below 8 °C, the yields of C8 and alkylate RON were higher than 88.6% and 97.7%. While the reaction temperature was 2 °C, the highest RON yield of 98.85 was acquired. Some side reactions like oligomerization45, cracking and disproportionation24 could be simultaneously occurred. Both rate constants of 1-butene oligomerization and alkylation increased, due to the increasing temperature, and oligomerization would generate more components of lower alkylate RON, such as C9+. In addition, cracking 23 ACS Paragon Plus Environment


and disproportionation would generate DMHs with low RON. Moreover, isobutane alkylation reaction was an exothermic reaction, but a high temperature would expedite both alkylation and side reactions. The reaction rate constants of side reactions were even 1~2 orders of magnitude larger than it of the alkylation reaction24, which meant that low temperature favored the formation of the desired components (like TMPs). However, the high viscosity of sulfuric acid and low reaction rate at low temperature would hinder mass transfer process and reaction performance. Therefore, relatively low temperature was incompatible. In this work, the optimal reaction temperature was 8 °C and the alkylate RON which was higher than 97 could be acquired.

3.6 Effect of pressure

100

100

60

94 92

40

RON

C5~C7 98 C8 C9+ RON 96

80

Yield (%)

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

Page 24 of 38

90 20 88 86

0 0.3

0.4

0.5

0.6

0.7

Pressure (MPa)

Figure 14. Effect of pressure on alkylate composition and RON

(NRPB=900 r/min, T=8 °C,t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol). 24 ACS Paragon Plus Environment

Page 25 of 38

100

80

15

TMPs DMHs TMPs/DMHs

60 9 40

TMPs/DMHs

12

Yield (%)

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


6 20

0 0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

3 0.65

Pressure (MPa)

Figure 15. Effect of pressure on C8 distributions and TMPs/DMHs ratio

(NRPB=900 r/min, T=8 °C, t=6 min, A/HC=1:1 v:v, and I/O=30:1 mol:mol). The effect of pressure on product distribution and alkylate RON were shown in figures 14 and 15. The yields of C8, TMPs, TMPs/DMHs and alkylate RON increased directly with increasing pressure up to 0.4 MPa while the yields of C9+ decreased. No obvious changes in yields of C5, C6, C7, and DMHs were observed. Under the circumstance of the reaction pressure was over 0.4 MPa, the yields of C8 and the alkylate RON were higher than 87.3% and 97.4. The by-products generation and low RON generated from hydration reaction in gas phase, increasing pressure was beneficial to liquid-phase reaction and improved the quality of alkylate. The saturated vapor pressure of the feed was about 0.2 MPa ( from NIST Database ) at a reaction temperature of 8 °C, so the system pressure was necessarily to be remained higher than 0.2 MPa to avoid the volatility of 25 ACS Paragon Plus Environment


Page 26 of 38

hydrocarbon.46 Because the reaction system was kept at a specified pressure, the partial pressure of gaseous hydrocarbon was enclosed in the reaction system, it was difficult to flow through the RPB. However, owing to the existing of the hydrocabon in gas phase, there were still gas phases reactions in the system. But it was difficult to distinguish whether the product was formed from gas reaction or not, and it was hard to quantify the contribution of gas reaction. Based on the previous study, the gas phase reaction had negative contributions in the alkylation process.47 According to Figure 14, a high RON of 98.0 was obtained when pressure increased from 0.3 to 0.6 MPa, but there was no obvious difference in RON between 0.5 and 0.6 MPa. Considering the operating cost, 0.5 MPa was chosen as the optimal reaction pressure.

3.7 Correlation model According to the experimental results, alkylate product distributions were connected to many factors including reaction time, reaction temperature, A/HC and rotational speed of RPB. Liu et al.41 investigated the isobutane alkylation performance catalyzed by composite ionic liquid, and an empirical equation associating reaction performance with operation conditions was established as 2

 IL   IL  y = a + b ⋅ T + c ⋅T + d ⋅  + e ⋅  +f  HC   HC  2

2

 I  I ⋅   + g ⋅   +h ⋅ t 2 + i ⋅ t O O

(3)

where y referred to the yield of components in alkylate (wt %), including C5-7, TMPs, DMHs, C9+, and the alkylate RON; T was the reaction temperature; t was the contact time; IL/HC was the volume ratio of ionic liquid to hydrocarbon; I/O was the


Page 27 of 38 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


mole ratio of isobutane to olefin; a, b, c, d, e, f, g, h, and i were the coefficients of the respective parameters in Eq. 3. Based on previous studies41,

43

and the analysis of our experimental results, an

empirical correlation was developed to predict product distributions and alkylate RON as 2

 A   A  y=m1 +m 2 t + m3 t+m 4 T +m5 T+m6   +m7    HC   HC  2

2

2

2

N  N   I I +m8  RPB  +m9  RPB  +m10 P 2 +m11P+m12   +m13   O O  150   150 

(4)

where y referred to the yield of components in alkylate (wt%) and the alkylate RON; m1-m13 were the coefficients of the respective parameters in Eq. 4. Through curve fitting, the optimum coefficients of m1-m11 were obtained and listed in Table 4. Figure 16 showed the comparison between calculated values and experimental results. It could be seen that the calculated values agreed well with experimental results with a deviation of ±10%.

Table 4. Coefficients of the correlation model Correlation C8

TMPs

DMHs

C9+

RON

m1

-128.73

185.24

29.57

-144.42

14.69

m2

-0.24

0.15

-0.07

-0.28

0.03

m3

5.13

-3.30

1.56

5.89

-0.70

m4

-0.01

0.00

0.00

-0.01

0.00

m5

-0.51

0.69

-0.27

-0.66

0.13

m6

-54.17

34.25

-14.98

-60.29

5.00

coefficients



m7

136.31

-88.34

38.06

150.88

-12.31

m8

-0.44

0.43

-0.15

-0.42

0.00

m9

5.86

-5.34

2.03

5.98

-0.21

m10

-76.48

69.17

-37.18

-79.40

1.99

m11

90.76

-86.31

41.65

93.56

-2.55

m12

0.06

-0.02

0.03

0.08

-0.01

m13

0.47

-1.79

-0.02

-0.18

0.53

100

Experimental C8 value (wt, %)

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

Page 28 of 38

80

+10% 60

-10% 40

C8 C9+ RON TMPs DMHs

20

0 0

20

40

60

80

100

Calculated C8 value (wt, %)

Figure 16. Relevancy of calculated values and experimental results

4. CONCLUSIONS In this work, a novel rotating packed bed reactor is adopted for the first time to investigate the reaction performance of isobutane alkylation catalyzed by concentrated sulfuric acid. The optimal reaction time, A/HC, reaction temperature, rotational speed of RPB, and reaction pressure were 6 min, 1:1, 2 °C, 1200 r/min and 28 ACS Paragon Plus Environment

Page 29 of 38 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


0.5 MPa, respectively. Under these optimal operating conditions, the yields of C8 and TMPs were 90.65% and 85.47%, respectively, the ratio of TMPs to DMHs was 16.48, and the alkylate RON was 98.85. Furthermore, a correlation model is applied to predict product distribution and alkylate RON, which showed reliable relevance towards experimental results. In addition to the utilization of energy, the stirring paddle shall stir all the materials in the tank to make the processes of mixing and mass transfer between the materials effective. In comparison, the RPB only deal with material which stays on the packed motor. Hence, the energy efficiency of the RPB is much greater than that of the stirred tank. Moreover the high RON alkylate oil is obtained at an I/O ratio of 30:1 in RPB, in which internal I/O is also fairly low. The results showed that high quality alkylate oil can be acquired with a low inner I/O in the RPB due to its high efficiencies of mass transfer and micro-mixing. It could also decrease the massive energy that the compressors and pumps are used to cycle the unreacted isobutane and mix with fresh feed to increase the inner I/O in the reactor. In short, alkylation of isobutane and 2-butene by H2SO4 in RPB has great prospects as it can observably decrease energy consumption of streams recycling and acquire high RON alkylate oil.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]



*E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT

We would like to thank Salman Sheikh and Nkounkou Mapela Marvin Feriel for the language modification. This work was supported by the National Natural Science Foundation of China (Nos. U1662123, 21436001), and Key Technologies R&D Program (No. 2014BAE13B01).

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57x68mm (300 x 300 DPI)

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alkylation of isobutane and 2-butene by concentrated sulfuric acid in a

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