Caprolactam as a New Additive To Enhance Alkylation of Isobutane

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Caprolactam as a New Additive to Enhance Alkylation of Isobutane and Butene in H2SO4 Liantang Li, Jisong Zhang, Kai Wang, and Guangsheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04557 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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Industrial & Engineering Chemistry Research

Caprolactam as a New Additive to Enhance Alkylation of Isobutane and Butene in H2SO4 Liantang Li, Jisong Zhang, Kai Wang, Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, China Abstract Searching for new additives to enhance alkylation of isobutane and butene in H2SO4 has attracted interests of academic as well as industrial since the process was introduced to produce high quality oil product in large scale. This research focused on alkylation of isobutane and butene with additive of caprolactam (CPL) in H2SO4. With the new additive of caprolactam, selectivity of C8 was obviously improved due to the improved solubility of isobutane in H2SO4 and the decreased acidity. Different additive amounts of caprolactam were test, showing that 1.0 wt% is optimal and selectivity of alkane C8 can be improved from nearly 80% to 88%. And the effects of stirring speed, reaction temperature, acid to hydrocarbon volume ratio (H/C), isobutane to butene molar ratio (I/O), reaction time and variety of olefin were also studied carefully and respectively. Extended duration runs were practiced which has demonstrated that the system was stable. This new additive is potential to be applied in large scale industrial processes to improve the quality of alkylation. Key words: Alkylation; caprolactam;

H2SO4;

butene

1. Introduction Isobutane alkylation is an important process in industry to produce alkylates, which is considered to be a quite valuable blending component for gasoline pool owing to such advantages as no olefins or aromatics, low vapor pressure as well as high octane number.1, 2 Hence, much attention has been devoted to alkylation. Alkylation of light olefins with isobutane can occur in the presence of strong acid catalysts like sulfuric Corresponding author. Email: [email protected]. Tel.: +86-10-62783870.

ACS Paragon Plus Environment


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acid （H2SO4），hydrofluoric acid (HF), solid catalyst and ionic liquids.3-6 +

H C4 H10 + C4 H 8 → C8 H18

(1)

Zeolites were firstly introduced in alkylation in the 1960s by Sun and Mobil.7, 8 Since then, many studies have been carried on in this field.5,

9, 10

And it is well

acknowledged that solid catalysts show good initial catalyst activity and selectivity. Peter et al. investigated the reaction with HFAU zeolite with a framework of Si/Al (the ratio of 4.5) as the catalyst.11 Zeolite BEA has also been examined as catalyst in alkylation by Gautam et al.12 They found that BEA showed better catalyst stability and higher alkylate selectivity. Hamzehlouyan et al. also explored the process of alkylation using zeolite as catalyst, which showed that the pore of zeolite was important and had exerted effect on the reaction.9 Although solid catalysts show great reaction activity, this kind of technology has not been commercialized yet. Two major problems prevent it wide application in industry. One is the fast catalyst deactivation because of the decrease of density of active acid sites. The other is the regeneration of catalysts. In recent years, ionic liquids used as catalysts in alkylation has been extensively reported

4, 5, 13-18

. Liu et al reported the process of alkylation of isobutane with

2-butene using [AlCl4CuCl]- as catalyst.13 Under their experimental conditions, the main product obtained was trimethylpentane whose selectivity is higher than 85%, and the research octane number is about 98-101. The ionic liquid composition had large effect on the alkylation.19, 20 Liu et al. indicated that the composite ionic liquid can improve the quality of alkylates, with the yields of C8 as high as 88.86%.21 Aschauer et al. also explored the intrinsic kinetics of the alkylation using isohexane/2-hexene and isopentane/2-pentene with chloroaluminate ionic as catalysts.22 Though ionic liquid may be a promising catalyst in alkylation, more work needs to be done before it can be applied in industry for the high price of ionic liquid as well as the corrosion of Cl- in ionic liquid. So far, sulfuric acid （H2SO4）and hydrofluoric acid (HF) have been commonly used as the catalysts in the industrial alkylation since last century and sulfuric acid is


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adopted more than HF considering safety and environmental issues.1 The alkylation process with H2SO4 as catalyst is more complex than that with HF.23-31 Sun et al. studied the alkylation kinetics of isobutane with butene using sulfuric acid as catalyst considering

three

important

alkylates

including

trimethylpentane

(TMPs),

dimethylhexanes (DMHs) and heavy ends (HEs).32 Their results indicated that the addition of H+ to olefin led to a strong exothermic reaction. According to the studies of Lee et al., kinetic constants for alkylation of isobutane with 1-butene and 1-butene oligomerization are 2.25×108 and 9.5 ×108 cm3/mol/s at 298K, respectively.30 Reaction pathways network is shown in scheme 1. Olefin would be protonated by H2SO4 first and then react with olefin to obtain TMP+ or DMH+, which would react with isobutane and obtain TMP or DMH as the main products. Heavy ends (HE) are considered to be the products of TMP+ or DMH+ reacting with olefin. Light ends (LE) come from the fragmentation of large isoalkyl cations. Besides, olefin would be polymerized and produce some side products. The characteristics of alkylation in H2SO4 are the low solubility of isobutane in H2SO4 and the fast oligomerization of olefin. Li et al. pointed out that the stirring rate, reaction time and the feed ratio have effect on the dissolved amount of feed or the ratio of butane and alkenes in sulfuric acid and thus affect the alkylate quality.23 They also investigated the variables that affect the alkylation process, including the mass transfer steps and the solubility of isobutane in the acid phase. Moreover, Li et al. found that under any experimental conditions, the increased agitation, isobutane to olefin ratio and the reaction time could improve the alkylate quality.24 In addition, Mosby et al. investigated the effect of temperature, agitation, acid strength, isobutane to 1-butene ratio, acid to hydrocarbon ratio on the alkylation of 1-butene and isobutane using sulfuric acid as the catalyst.33 According to the results of Mosby, agitation and residence time in the reactor have a significant effect on alkylation. Besides, Sprow et al. noted the importance of interfacial area in the reaction system. Improving the interfacial area in the reaction zone is beneficial to improving the quality of alkylates,34 which provides guidance for the further study of alkylation using H2SO4 as catalyst.



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Scheme 1. Reaction pathways network of alkylation with H2SO4 as catalyst

According to the researches above, one promising method to improve the quality of alkylates is to improve the interfacial area of two phases and the solubility of isobutane in H2SO4. As shown in literatures and patents35-39, some additives were added to improve the solubility of isobutane in H2SO4 and further improve the quality of alkylates, such as dodecylbenzene sulfonic acid40, trifluoromethane sulfonic acid41, p-phenylenediamine42, sulfonamide43. According to the results of Chen et al., adding cationic or ampholytic surfactants can change the solubility of isobutane/alkenes, and then improve the yield.44 Tang et al. studied the alkylation in acidic ionic liquids and mixtures of ionic and acid liquid (H2SO4 or CF3SO3H).45 The results show that the conversion is higher than 95%, and selectivity of C8 is above 70%, and the value of TMP/DMH is higher than 7 with the mixture as catalyst. Although the mentioned materials can improve the quality of alkylates, the price and the stability need to be taken into consideration. Caprolactam is an important chemical product with the acceptable price produced from Beckmann rearrangement of cyclohexanone oxime in oleum. It can combine with H2SO4 and form a stable sulfate which may improve the solubility of isobutane in H2SO4.46 Here the objective of this work is to study alkylation in H2SO4 with additive of caprolactam. The amount of caprolactam in H2SO4 was studied and the suitable amount was selected for further research. The important factors which affect the alkylation such as stirring rate, temperature, reaction time, ratio of isobutane to butene, ratio of acid to hydrocarbon were investigated systematically. Besides, extended duration experiments were carried out to test the stability of the reaction system. The work may provide a new reference to improve the quality of alkylates. 2. Experimental


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2.1 Materials and chemicals H2SO4 (A.R 95-99%) was purchased from Beijing Chemical Plant. Isobutane, 1-butene, 2-butene and iso-butene were purchased from ZhaoGe gas Plant. Caprolactam is purchased from J&K. All chemicals were used without further purification. A drop interfacial tension meter (OCAH200, DataPhysics Instruments of Germany) was used to measure the interfacial tension. An Ubbelohde viscometer was used to measure the viscosities of the solution of H2SO4 and caprolactam.

2.2 solubility measurement The isobutane solubility in H2SO4 was determined in an autoclave. A certain amount of H2SO4 was added in an autoclave and then the autoclave was flushed with N2 to remove the air in autoclave. Autoclave temperature was controlled by water bath and when temperature was at the set value, isobutane was introduced within a short time. After that, the agitator was turned on to dissolve isobutane in H2SO4. The pressure was recorded in different times till the pressure maintained in a certain value. The pressure drop was observed as a result of the dissolving isobutane. According to ideal gas equation of state, we could obtain the amount of dissolved isobutane, hence, we could calculate the solubility.

2.3 Alkylation apparatus and process Figure 1 shows the alkylation apparatus. The reaction was performed in an autoclave. A certain volume of H2SO4 was added into a 500 mL autoclave. And then nitrogen (N2) was introduced to purge the autoclave. Later, the pressure of the autoclave was set to 0.3MPa with N2 to keep the hydrocarbon in liquid. Next, the agitator was kept at a certain rate (250-1000 r/min). Then the temperature of autoclave (7 °C to 19 °C) was controlled by a water bath. When the temperature of autoclave reached the set value, the hydrocarbon feed was pumped into the reaction autoclave by a metering pump at a set flow rate from a piston storage tank. The hydrocarbon of a certain I/O molar ratio was stored in a feed storage tank. In all experiments, the



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reaction time was recorded as long as the hydrocarbon was pumped into the autoclave. After the reaction, certain amount of dodecane was added into the autoclave and sufficient time (20 min) was given to allow alkylates extracted into dodecane. After that, the product was analyzed. The experiments were conducted with sulfuric acid to hydrocarbon volumetric ratio from 1.13 to 3, isobutane to butene (I/O) from 10 to 150, and reaction time from 2.1 min to 10.5 min.

Figure 1. Schematic of alkylation apparatus (1-Nitrogen, 2-reducing valve, 3-water tank, 4-metering pump, 5-piston storage tank, 6-one way valve, 7-autoclave, 8-atomopheric valve)

2.4 Analysis The

alkylate

components

in

the

product

were

identified

with

gas-chromatography-mass spectrometry (GC-MS). The amounts of alkylate components were determined by Agilent 7890 Gas Chromatograph which was equipped with a hydrogen flame ionization detector. The chromatographic column is AB-5MS capillary column (30 m ×0.25 mm×0.25 µm). And the quantification of different components is based on the peak area for correction factors of all components are almost close to 1.0 with the standard substance benzene.21,32


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ܵ௜ =

ௐ೔

(2)

ௐ೟೚೟ೌ೗

where i presents the component of the alkylates. In our experimental conditions, we divided these components into three groups, C5-C7, C8 and C9+.

2.5 Reliability of experiments In order to ensure the reliability of experiments, a group of experiments were conducted under the same conditions (reaction temperature 12 °C, stirring speed 1000 r/min, H/C ratio 1.5, isobutane/2-butene ratio 100:1, reaction time 10.5 s, and catalyst H2SO4), and results are list in Table 1. From the results, standard deviation of C5-C7, C8 and C9+ are 1.72%, 1.71% and 0.82%, respectively. And the relative standard deviations are all below 10%, indicating that the reliability of the experiments is acceptable.

Table 1. Repeatability of Three Experiments at the Same Conditions

item

1

2

3

Average

Standard

Relative

value

deviation

standard

(%)

deviation (%)

Component Selectivity(%) C5-C7

13.5

12.5

10.7

12.2

1.2

9.6

C8

76.2

79.3

80.2

78.5

1.7

2.2

C9+

10.3

8.2

9.1

9.2

0.8

8.9

3. Results and discussion 3.1 Effect of caprolactam on H2SO4 physical properties Physical properties would be changed when caprolactam is added into H2SO4. Interfacial tension of acid and hydrocarbon with different amounts of caprolactam is shown in Figures 2. The data of Figures 2 demonstrates that the interfacial tension



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does not change much when additive amount is under 1.5%. The effect of caprolactam on viscosity of H2SO4 and the solubility of isobutane in H2SO4 were also explored and the results are presented in Figure 3. These data imply that adding caprolactam in H2SO4 has little effect on H2SO4 viscosity while increases the solubility of isobutane. Under our experimental conditions, the solubility of isobutane in H2SO4 is 0.038 wt% at 12 °C, while the solubility improved to 0.099 wt% when additive amount of caprolactam in H2SO4 increases to 4wt%. According to the literature47, caprolactam in H2SO4 is protonated to form caprolactamium hydrogen sulfate as shown in scheme 2. Caprolactamium hydrogen sulfate increases the solubility of isobutane in H2SO4 while it has little influence on physical properties of H2SO4.

Figure 2. Interfacial tension of H2SO4 and hydrocarbon with different additive amount of caprolactam

Figure 3. Viscosity of H2SO4 and solubility of isobutane in H2SO4 with different additive amount


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of caprolactam

Scheme 2. Reaction of caprolactam in H2SO4

3.2 Effect of amount of caprolactam in H2SO4 Different additive amounts of caprolactam in H2SO4 were examined, e.g 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt% and 3 wt%. Figure 4 shows the results of alkylation with different amounts of additive in H2SO4. From the results, it can be observed that adding caprolactam in H2SO4 can improve the selectivity of C8 effectively. When using pure H2SO4 without any caprolactam as isobutane/2-butene is 100:1, the selectivity of C8 is about 80%. As the additive amount of caprolactam is 0.5 wt% and 1.0 wt%, the selectivity of C8 can be as high as 85% and 88%, respectively. When isobutane/2-butene is 8:1, the selectivity of C8 increases from 36.80% to 62.44% as the addition amount of caprolactam is 1 wt%. Based on Figure 3, the improved solubility of isobutane in H2SO4 is thought to be the main reason. According to the previous literature48, the mass transfer of isobutane to acid is an important step in alkylation reaction and high concentration of isobutane in H2SO4 is required to promote main reaction. The additive of caprolactam improves the isobutane solubility in H2SO4 hence increases the amount of carbocation ionic, thus, improves the selectivity of C8. However, the results in Figure 4 indicate that with the increase of the additive amount of caprolactam, the selectivity of C8 decreases as the additive amount is more than 1.0 wt%. A possible explanation for this phenomenon is that additive amount of caprolactam also affect the acidity strength of H2SO4. Acidity strength of H2SO4 is an important factor to control the ionization level. According to the previous results, high acid strength has adverse effect on the quality of alkylates and lower acid strength is also bad for the quality of alkylates.48 49 In the results of Mosby et. al. 90%-93% acid



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strength is the most suitable concentration to reduce the side reaction and improve the selectivity of C8.33 Adding large amount of caprolactam could reduce the acid strength. Based on these findings and previous results, we choose the additive amount of 1.0 wt% in the further studies. Table 2 shows the effect of addition of caprolactam at different I/O molar ratio. From the results, it could be found that the addition of caprolactam could effectively improve the selectivity of C8, especially at lower I/O molar ratio.

Figure 4. Selectivity of alkylates using different caprolactam amounts in H2SO4 as catalysts (T=12 °C, stirring speed=1000 r/min, H/C=1.5, t=10.5 min) (a) isobutane/2-butene=100:1 (b) isobutane/2-butene=8:1 Table 2.Comparisons of with and without addition caprolactam at different I/O molar ratio I/O molar ratio

Selectivity of C8 (%)

8:1 100:1 150:1

0% caprolactam

1wt% caprolactam

36.8 78.5 90.4

62.5 88.1 91.4

3.3 Effect of stirring speed According to the previous studies, the mixing performance of two phases is important for alkylation and efficient agitation is required to ensure the good contact of two phases to produce high quality alkylates.33 The effect of stirring speed on alkylation using H2SO4 with additive of caprolactam is exhibited in Figure 5. Stirring speed of 250 r/min, 500 r/min, 750 r/min and 1000 r/min in alkylation were investigated. These data indicate that high stirring speed leads to high selectivity of


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C8. When stirring speed is 250 r/min, selectivity of C8 is about 50 %. As the stirring speed is 1000 r/min, selectivity of C8 reaches 88%. The results are consistent with founding that high agitation promotes high quality of alkylates.25,

29, 50

In the

alkylation system, high stirring speed decreases the droplet sizes of hydrocarbon in sulfuric acid, increasing the interfacial surface area for the transport of isobutane, which improves the selectivity of C8. Base on the results, a speed of 1000r/min is chosen for further studies.

Figure 5. Effect of stirring speed on the alkylate composition (T=12 °C, caprolactam=1.0 wt%, H/C=1.5, isobutane/2-butene =100:1, t=10.5min)

3.4 Effect of reaction temperature Reaction temperature could not be ignored for alkylation.3,

48

The effect of

temperature on alkylation in H2SO4 with additive of caprolactam were studied and the results are shown in Figure 6. According to the results, temperature is an significant factor for the reaction.44 When reaction temperature increases from 7 °C to 12 °C, the selectivity of C8 is changed from 91% to 88%. When temperature is increased further, the selectivity of C8 decreases from 88% to 50% as the temperature is changed from 12 °C to 19 °C. One reason maybe that temperature could change the physical properties of reactants, such as viscosity, density and interfacial tension. Another reason is that the main reaction of alkylation is a strong exothermic reaction, and lower temperature restrains side reaction, e.g. polymerization, disproportionation and cracking. But too low temperature causes high viscosity of H2SO4 and need more



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energy to obtain lower temperature. Hence, according to the results, we chose 12 °C to further studies.

Figure 6. Effect of reaction temperature on the alkylate composition (caprolactam=1.0 wt%, stirring speed=1000 r/min, H/C=1.5, isobutane/2-butene =100:1, t=10.5 min)

3.5 Effect of H/C volumetric ratio The volumetric ratio of sulfuric acid to hydrocarbon is an important factor for the reaction because the ratio has influence on the interfacial area between two phases.51 As the reference mentioned, when the volumetric of H/C is higher than 1, sulfuric acid in the system is continuous phase, otherwise, hydrocarbon is continuous phase. It will causes poor quality of alkylates if hydrocarbon is continuous phase.33 In sulfuric acid-continuous condition, the dispersion size becomes smaller with the increasing of volumetric ratio, resulting in the better selectivity of C8. Base on the previous studies, we investigated the effect of H/C volumetric ratio in our reaction system. As is shown in Figure 7, the selectivity of C8 changed a little with the H/C changing from 1.13 to 3. When the ratio is about 1.5, the selectivity of C8 is 88% and the value is the highest among the different experimental conditions. This results agree well with the results of Ende et al.51 In their results, they found that when the H/C is 1.50-1.86, the dispersion of hydrocarbon in H2SO4 is the best, which improved the interface area of two phases. In our further experiments, H/C volumetric ratio is chosen as 1.5.


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Figure 7. Effect of H/C volumetric ratio on the alkylate composition (T=12 °C, caprolactam=1.0 wt%, stirring speed=1000 r/min, isobutane/2-butene =100:1, t=10.5 min)

3.6 Effect of I/O molar ratio I/O molar ratio has significant influence on quality of alkylates. The effect of I/O molar ratio is shown in Figure 8. The data reported here suggest that with the increase of I/O molar ratio from 10 to 150, selectivity of C8 increases from 39% to 91%. The tendency of results is consistent with the results of industry and previous studies33. The internal I/O can be as high as 300-1000 for the recycle of H2SO4 in industry. As reported by Lee et al.30, the solubility of butene in H2SO4 is about 2.5 times than isobutane, hence, High I/O means improving the dissolved isobutane and promoting formation of carbocation, which also means reducing the side reaction, such as polymerization of butene. This result also provides guidance for improving the quality of alkylates.



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Figure 8. Effect of isobutane to butene molar ratio on the alkylate composition (T=12 °C, caprolactam=1.0 wt%, stirring speed=1000 r/min, H/C=1.5, t=10.5 min)

3.7 Effect of reaction time Reaction time is also an important factor for a reaction. Different reaction time was investigated and the results are shown in Figure 9. The results confirm that the selectivity of C8 increases with the increase of reaction time. The reason for this phenomenon is that I/O is related to the reaction time under our experimental conditions. I/O increased as the reaction was prolonged for the initial I/O molar ratio was as high as 100. With result like this, we take the attitude a new technology is needed to enhance internal I/O molar ratio to reduce side reaction.

Figure 9. Effect of reaction time on the alkylate composition (T=12 °C, caprolactam=1.0 wt%, stirring speed=1000 r/min, H/C=1.5, isobutane/2-butene


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=100:1)

3.8 Effect of variety of olefin Different kinds of olefin were studied and the results are shown in Figure 10. As is shown in Figure 10, the olefin has effect on the selectivity of alkylates. In our experimental conditions, 2-butene as feed has the highest selectivity of C8, followed by 1-butene, and iso-butene has the lowest selectivity of C8. The results are in substantial agreement with the results of Emmanuel et al.52 They found that slightly better quality of alkylates when using 2-butene or 1-butene, and the iso-butene has the worse quality. With 2-butene and 1-butene as feeds, the RON is about 96-98 and 95.7-97.7, respectively. While with 1-butene as feed, the RON is 91-92. In the study of mechanism of alkylation using H2SO4 as catalyst Researches show that 1-butene isomerizes into 2-butene before reaction.50 As for iso-butene, polymerization occurs, easily forming high carbon materials.53 Besides, solubilities of three kinds of olefin are different and the dissolved ratio of isobutane to butene is smallest when using iso-butene as feed 25, 48, which leads to the poorest quality alkylates with iso-butene as feed. These results are also important to the kinetic study of alkylation.

Figure 10. Effect of category of butene on the alkylate composition (T=12 °C, caprolactam=1.0 wt%, stirring speed=1000 r/min, H/C=1.5, I/O=100:1, t=10.5 min)

3.9 Stability of catalyst Extended duration runs experience was performed due to stable properties of



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catalyst are required in industry. Selectivity of different components shows that additive of caproclatam in H2SO4 is quite stable. Selectivity of C8 could be kept at nearly 83.53% as reaction time is as long as 52.5 min (reaction temperature 12 °C, stirring speed 1000 r/min, H/C ratio 1.5, isobutane/2-butene ratio 100:1, and caproclatam amount 1.0 wt%), indicating that the stability of this kind of catalyst is remarkable and potential to be applied in industry to improve the quality of alkylates. According to the above experimental results, caprolactam as a new additive could enhance alkylation effectively with H2SO4 as catalyst. Comparison of our results and others is shown in Table 3. As can be seen in Table 3, additive of caprolactam can improve the selectivity of C8 from 29.0% to 39.8%. As shown in Table 3, selectivity of C8 (I/O is 10) is nearly equal to the value with HFAU Zeolite as catalyst (I/O is 40). Besides, selectivity of C8 can be as high as 88.1% when I/O is 100 in our experimental condition, which is higher than that with C32H68P-2AlCl3 as catalyst. On the whole, adding this new additive is a promising method to improve the quality of alkylation and may be applied in large scale industrial process.

Table 3. Comparison between our work and other researches Catalyst

I/O

H/C

Temperature (°C)

H2SO433 HFAU Zeolite11 C32H68P-2AlCl316 H2SO4+caprolactam H2SO4+caprolactam

5 40 110 10 100

1

1.5 1.5

10 50 80 12 12

Stirring speed (r/min)

Reaction time (min)

Selectivity of C8 (wt%)

2000

1 96 90 10.5 10.5

29.0 44.0 68.9 39.8 88.1

1000 1000

4. Conclusion

Alkylation of isobutane and butene with different amounts of caprolactam added in H2SO4 were investigated. Appropriate amount of caprolactam is required to adjust the acidity strength of H2SO4 and improve the solubility of isobutane in the H2SO4. When


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Additive amount of caprolactam is 1.0 wt%, the highest quality of alkylates is achieved under our experimental conditions. The higher stirring speed is good for improving the dispersion of hydrocarbon in H2SO4 and improving the selectivity of C8. Lower temperature is also beneficial for the selectivity of C8. H/C volume ratio results show that the best H/C volume ratio is 1.5. I/O also has important effect on alkylates, and higher I/O is good for the selectivity of C8. The selectivity of C8 increases with the increase of reaction time. The variety of olefin has effect on the selectivity of C8. 2-Butene has the highest selectivity of C8 and iso-butene has the lowest value. On the whole, adding caprolactam in H2SO4 is a promising method to improve the quality of alkylates. In order to understand the reaction deeply, more work should be done, such as dispersion, mass transfer and the reaction kinetics.

Acknowledgements

We gratefully acknowledge the support from the National Natural Science Foundation of China (Nos. 91334201, U1463208 and 21506110).

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Caprolactam as a New Additive To Enhance Alkylation of Isobutane

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