Investigation of Pyridine Synthesis in a Fast Fluidized Bed Reactor

Subscriber access provided by Gothenburg University Library

Article

Investigation of pyridine synthesis in a fast fluidized bed reactor Shuaishuai Zhou, Zelong Liu, Di Qin, Xiao Yan, mengxi liu, Chunxi Lu, and Guangzhou Jin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04456 • Publication Date (Web): 07 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 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

Industrial & Engineering Chemistry Research

Investigation of pyridine synthesis in a fast fluidized bed reactor Shuaishuai Zhou1, Zelong Liu2, Di Qin1, Xiao Yan1, Mengxi Liu1, Chunxi Lu1*, Guangzhou Jin2 1. College of Chemical Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China 2. Department of Chemical Engineering, Beijing Institute of Petro-Chemical Technology, Beijing 102617, China *Corresponding author. {e-mail: [email protected].} Abstract Pyridine has been generally synthesized by aldehydes and ammonia in a turbulent fluidized bed reactor. In this paper, a fast fluidized bed reactor was proposed for pyridine synthesis. Experiment result shows that the yields of pyridine and 3-picoline decrease while the selectivity of pyridine over 3-picoline is increased. A model was proposed to predict the performance of the fast fluidized bed reactor, the average prediction deviation is 6%. The influence of mass transfer, heat transfer and backmixing of gas phase is represented by a modification factor, and the mean value of this modification factor is 0.75 within the experiment operating conditions. By model prediction, the reaction should be terminated when the critical point of R2 is reached to avoid over reaction. To optimize the pyridine and 3-picoline product yield and minimize coke product yield, the reaction temperature should be kept around 723 K. Keywords: pyridine synthesis, fast fluidized bed reactor, experiment, modeling, modification factor 1 INTRODUCTION Pyridine and 3-picoline have been widely used for synthesis of pharmaceuticals and agrochemicals, benefiting from high chemical reactivity as well as biological activity 1. Traditionally, pyridine bases were obtained from coal tar and suffered from high sulfur content. In 1920s, large demand of pyridine and pyridine bases in the market encouraged development of new technologies for pyridine synthesis. Now, most of the pyridine bases are produced based on Chichibabin condensation process 1. Work associated with pyridine synthesis process has been rare and mainly focused on: catalysts, reaction mechanisms, reaction kinetics and reactors. Reddy 1 proposed that the development of catalyst can be divided into two stages, with respect to silica-alumina catalysts and zeolite catalysis respectively. Until now the most widely used zeolite catalysts are HZSM-5 catalysts, benefiting from the design of an appropriate shape selective system. Because of multiple side reactions produced in pyridine synthesis reaction process, the reaction mechanism of the process has been vague and unclear. Up to now, it has been confirmed that during reaction process, the imide intermediates is produced, which is the key component to form pyridine bases 2-6. Reports regarding the reaction kinetics of pyridine synthesis reaction has been few. Reddy 7 carried out experiment in a 20 mm ID Pyrex reactor dealing with HZSM-5 catalysts. Results suggests that the reaction rate is only determined by reaction operating conditions and acetaldehydes concentration. As for reactor types, Goe 8 suggested that reactor should be chosen from the basic categories of fixed-bed and fluid-bed forms, and it was usually determined by the catalysts properties and the reactions involved. Rao 9 synthesized pyridine bases by ammonia and aldehydes as well as methanol. A W-ZSM-5 catalyst was loaded in a fixed-bed Pyrex reactor with 20 mm internal diameter. The reaction temperature is 693 K, it was found that the pyridine and 3-picoline yields reached 61.5% and 16.8%, respectively. Reddy 7 synthesized pyridine bases by ammonia and aldehydes over HZSM-5 catalysts. Under a reaction temperature of 673 K, the yields by weight for pyridine and 3-picoline were 44.4% and

*Corresponding author. Tel./fax: +861089733803. E-mail address: [email protected]. 1 Environment ACS Paragon Plus

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

13.0%. Rao’s and Reddy’s work indicated that the fixed-bed reactor had high selectivity of pyridine over 3-picoline. However, there still exist two problems for scaling up the fixed bed pyridine synthesis reactor. Firstly, the pyridine synthesis reactions are highly exothermic, leading to a big problem of removing massive heat from the fixed bed reactor. Secondly, pyridine bases synthesis reaction produces coke which gradually deactivates the catalyst. Therefore, periodic in-situ regeneration is necessary and the problem of massive heat removal still restricts the scale up of the fixed bed reactor. It leaves the fluidized bed reactor to be a favorite choice, due to the advantages of quick and massive heat and mass transfer, excellent gas and catalyst contact and continuous operation. Francis 10 synthesized pyridine and 3-picoline by aldehydes, methanol and ammonia. A silica-alumina catalyst was loaded in a fluidized-bed reactor which was operated under a temperature of 773 K. The yields by weight of pyridine and 3-picoline were 35% and 27% respectively. Feitler 11 produced pyridine bases in both fixed bed and fluidized bed reactors. Within a large range of reaction temperature from 723 K to 773 K, the fluidized bed reactor offers higher production yield than the fixed bed reactor does. In a fluidized bed reactor, Goe 8 synthesized pyridine over metal ions modified shape-selective zeolite catalyst. They found the alkyl-pyridine derivatives involving aldehydes, methanol and ammonia. The highest yield by weight of pyridine and β-picoline were 34% and 16%, respectively. Since the obvious advantages of fluidized bed reactor, some commercial scale pyridine base synthesis units incorporated fluidized catalyst beds. In industrial reactors, a problem that results in frequent shut down is the excessively coking on the gas distributor. According to previous studies 1,7, the main reactions are typical parallel series reactions and suppressing gas and solid back-mixing reduce the coke formation. Previous studies 12-17 indicate that by increasing the superficial gas velocity the flow pattern transfers from turbulent fluidized bed to fast fluidized bed. Li 18 investigated the gas backmixing in fluidized beds spanning the entire range from the bubbling regime to the dilute transport regime using continuous injection of a helium tracer. By analyze the experiment data, they found that the Peclet Number of fast flow is nearly twice as that in turbulent flow, which indicates a less backmixing in fast flow. In this paper, the fast fluidized bed is adopted to reduce the back-mixing. Bai 19 proposed a model to predict the performance of a fast fluidized bed reactor in FCC riser regeneration process. In Bai’s model, the gas flow is taken as plug flow and the radial difference of particle concentration was also omitted, but the prediction result is acceptable. With the development of computer hardware and computational techniques, numerical simulation has become one of the most significant means of exploring the complex flow and reaction characteristics in fast fluidized bed reactor and a lot of work has been carried out in recent years. Thelogs 20-21 and Gao 22 incorporated lumped kinetic models into three dimensional (3-D) computational fluid dynamics to predict the performance of FCC riser reactors. Li 23 simulated the gas-solid flow and catalytic cracking reactions in a FCC riser. However, lack of experiment data, no report on the reactor modeling of pyridine synthesis process is found. In this paper, a simplified model was proposed to modeling the performance of fast fluidized bed reactor in pyridine synthesis.

2 EXPERIMENT Figure1 shows the schematic diagram of the experiment apparatus (Beijing Huiersanji Green Chem-Tech co., Ltd), which consisted of a fast fluidized bed reactor of 14 mm ID and 3500 mm in height and also a regenerator of 500 mm ID and 1000 mm height was placed side by side with the reactor. A ceramic filter 2

ACS Paragon Plus Environment

Page 2 of 23

Page 3 of 23 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


was used to separate catalysts from gas products. Formaldehydes and acetaldehydes feedstock flow were pumped and measured by a electronic balance (EP20K, Changshu Yiou Instrument and Meter Plant, China) and were preheated to 373 K. Then they were mixed with ammonia, which was regulated by a glass rotameter (LZB-4F, Tianjin FDS DODD Instrument Co., Ltd., China). The mixture of feedstock was introduced into the bottom of the fast fluidized bed reactor by a nozzle, mixing with hot regenerated catalysts from the regenerator and vaporizing. The vapor and catalysts flowed upwards in the fast fluidized bed reactor and were separated by a filter. Then the gas product was liquefied in two condensers and collected by a flask. The spent catalysts was stripped by steam and then introduced into the regenerator. After regeneration, spent catalyst was conveyed to the fast fluidized bed reactor through regeneration pipe. The products were analyzed by a gas phase chromatograph (GC7890II, Shanghai Tianmei Keji Instrument Co., Ltd., China). The initial acetaldehydes concentration CA0 was calculated by eq (1) to eq (4).

CA0 

nA0 V

(1)

where nA0 is the mole flow rate of acetaldehydes, which was obtained by multiplying the mass flow rate of aldehydes flow and its corresponding composition. V is the volumetric flow rate of both aldehydes flow and ammonia flow at reaction conditions at the bottom of the riser, which was derived by Ideal gas law.

PV  nRT

(2)

where n is the mole flow rates of both aldehydes and ammonia and can be calculated by the following equation.

n  nA0 / xA0  nm

(3)

where xA0 is the mole fraction of acetaldehydes in aldehydes flow, nm is the mole flow rate of ammonia and was calculated by

nm  (1/ af  1)nA0 / am

(4)

where ηaf is acetaldehydes to formaldehydes ratio by mole (ATFR), ηam is ammonia to aldehydes ratio by mole (ATAR), which were obtained by the composition of both aldehydes and ammonia before experiment. Reddy 7 suggested that the volume change in a micro reactor can be neglected, therefore the gas velocity was assumed to be constant in the fast fluidized bed reactor. The superficial gas velocity in the entrance of

3



Page 4 of 23

the fast fluidized bed reactor is calculated by eq (5).

ug 

4V  D2

(5)

where D is the internal diameter of the fast fluidized bed reactor. The reaction temperature was measured by five thermocouples which located along the riser, as shown in Figure 2. The reaction temperature was the average of the temperatures registered at points 2, 3 and 4. The operation pressure at the entrance is registered by the pressure gauge located at the bottom of the riser, which is taken as the reaction pressure. For such a small experiment apparatus, it was difficult to measure the catalysts circulation rates directly by instruments. Alternatively, the heat balance method was used to calculate the catalysts circulation rates. Figure 3 shows the flows for catalysts circulation rate calculation. According to heat balance of the computation node, eq (6) is obtained.

m H i

i



 D 2Gs 4

LT   mo H o

(6)

Pyridine yield yp and 3-picoline y3p yield were calculated by eq (7).

y p (%) 

5N P 100% 2 N ar  N fr

(7)

6 N3P y3 p (%)  100% 2 N ar  N fr

where yp and y3p were pyridine yield and 3-picoline yield respectively. Np and N3p are the mole number of pyridine and 3-picoline in the products. Nar and Nfr are the mole number of acetaldehydes and formaldehydes in feedstock.

3. MODELING In the present work, a flow-reaction model is proposed to investigate the influence of the operation parameters other than Gs on production yield. 3.1 Reaction Kinetics Up to now, the work regarding pyridine synthesis reaction kinetics has been rare. Reddy

7

proposed a

reaction mechanism based on typical parallel series reactions. As showed in Figure 4, the feedstocks (mixture of acetaldehyde, formaldehyde and ammonia) first produce pyridine and 3-picoline, then a part of 4


Page 5 of 23 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


pyridine and 3-picoline forms coke at different rate. The main reactions are given by eq (8) and eq (9). The byproducts in pyridine synthesis process are 2-picoline, 4-picoline, methylamine, dimethylamine and trimethylamine, which can be minimized by adjusting the composition of the feed-stock and the ratios of mixture.

2CH3CHO+HCHO+NH3   C5H5 N+3H2O+H2

(8)

2CH3CHO+2HCHO+NH3   C6H7 N+4H2O

(9)

Based on the above mechanism and experiment results, Reddy

7

proposed the following reaction rate

expressions, and the reaction constants and activation energy are listed in Table S1.

rA  (k1  k2 )CA 2a

(10)

rD  (k1CA 2  k3CD )a

(11)

rE  (k2CA 2  k4CE )a

(12)

In this paper, the reactions and the catalysts are same as in Reddy’s experiments, and the above reaction kinetics is adopted. 3.3 Reactor Model The reactor model in this paper is obtained based on the following assumptions. I. For fast fluidized bed reactor, the gas phase could be taken as plug flow 19, 24-26. II. The temperature radial and axial differences is omitted, which means the heat transfer rate inside the fast fluidized bed reactor is great enough to guarantee uniform temperature distribution. III. The superficial gas velocities is unchanged along the riser height 7. Based on mass balance, the following equations could be obtained.

dC A0 (k1  k2 )C A2  (h)  a dh ug ( h )

(13)

dCD 0 (k1C A2  k3CD )  (h )  a dh ug ( h )

(14)

dCE 0 (k2C A2  k4CE )  (h )  a dh ug ( h )

(15)

The catalysts activity “a” is determined by the catalysts residence time. Reddy relationship between catalysts activity and residence time can be described by eq (16).

5


7

proposed that the


a  a0e kd tc

Page 6 of 23

(16)

where a0 represents the activity of fresh catalysts and equals to 1. kd is the deactivation reaction constant and could be calculated by eq (17) 7.

kd  kd 0e



Ed RT

(17)

For the above reactor model, the influence of gas phase axial back-mixing, mass transfer and heat transfer on production yield is simplified. To make up for this simplification, a modification factor k is proposed. The modified reactor model is as follows:

dC A0 (k  k )C 2  (h) k 1 2 A a dh u g ( h)

(18)

dCD 0 (k1CA2  k3CD )  (h) k a dh u g ( h)

(19)

dCE 0 ( k C 2  k 4CE )  ( h) k 2 A a dh u g ( h)

(20)

The modification factor k is closely related to operating conditions

27-29.

For lack of experiment data with

pyridine synthesis reaction system, eq (21) is proposed to describe the relation between the modification factor and the operating conditions.

k  b1Gs b2 u g b3 T b4

(21)

By fitting the experiment data, the parameters in Eq (21) is obtained, which are b1=4.04，b2=0.272， b3=0.692，b4=-0.549. It seems that the catalysts circulation rate has greater influence on modification factor than superficial gas velocity does, and the reaction temperature has a negative influence on modification factor. The average value of the modification factor during experiment is 0.75. 3.3 Catalysts Concentration Axial Distribution Previous works indicated that, for a fast fluidized bed reactor, the particle concentration axial distribution is in “S” shape 30. At the bottom of the fast fluidized bed, a dense bed is formed due to limited gas carry over ability 31. Above the dense bed, a dilute phase is formed and usually exists in core-annulus structure 19,32.

Based on large number of industrial experiment data, Lu

33

proposed a correlation to predict the

particle concentration axial distribution in fast fluidized bed as shown in eq (22).

6


Page 7 of 23 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


  1.923ug 0.915h 0.347Gs1.362

(22)

Since Lu’s model is based on analysis of industrial experiment, and Lu’s experiment operating conditions cover our, this paper adopts Lu’s model.

4 EXPERIMENT RESULTS The experiment operation conditions are listed in Table 1. Table 1. Variation range of operating parameters Parameters

Variation range

CA0 (mol/L) ug (m/s) Tr (K) Pr (kpa) Gs (kg/m2/s)

0.0053~0.0056 0.7~1.8 723~798 180~190 11.9~22.1

Gs: catalysts circulation rate based on the reactor cross sectional area

During the experiments, the values of ATFR and ATAR were unity, ratios widely used in pyridine bases industries. Table 2 and Table S2 showed the experimental production yield. Table 2. Production yield under different operating conditions

1 2 3

CA0 (mol/L)

V (L/min)

Tr (K)

Gs (kg/m2/s)

yp

y3p

yp+y3p

yc

0.0056 0.0055 0.0052

6.5 6.6 6.8

723 748 798

12.8 12.8 20.6

0.27 0.22 0.43

0.08 0.07 0.10

0.35 0.29 0.53

0.11 0.11 0.17

4.1 Selectivity From Table 2 and Table S2, it is seen that the selectivity of pyridine over 3-picoline is about 3, which nearly one and a half times as much as industrial unit’s selectivity of 2.2. For fast fluidized bed reactor, the catalysts residence time is less than turbulent fluidized bed reactor. Previous works 34-35 pointed out that in a fast fluidized bed reactor the catalysts concentration follows a typical core-annulus distribution. Catalysts in the annulus zone tend to flow downward, which make the flow pattern deviate from plug flow. Besides, the slip velocity between gas phase and catalysts

36-37

increase the catalysts residence time. Three types of

models are proposed to predict the catalysts residence time distribution in circulating fluidized bed reactors 38,

which are the dispersion model, core-annulus/incremental tracer balance model and stochastic model. In

this paper, the model proposed by Wei

39, 40

is employed to predict the catalysts particle residence time

distribution. Wei 39 used the phosphorescent particle tracer technique to measure the particle residence time distribution in a riser with a diameter of 140 mm and height of 7200 mm. The superficial gas velocity and 7



Page 8 of 23

solids circulation rate are in the range of 2.6 to 8.0 m/s and 8 to 80 kg/m2/s respectively. The experiment results showed that the RTD curve of the riser has two peaks. Wei 39 suggested that the first narrow peak is the particles with small extent of dispersion and the second wider but low peak stands for the particles with much longer residence time and much wider residence time distribution. By assuming the dispersion coefficients are independent on axial position, Wei

39

proposed the following equations to give the

variation of tracer concentration c with time t and distance x.

 2 cs c c Das 2  U ss s  s x x t

(23)

 2 cc c c  U sc c  c 2 x x t

(24)

Dac

c  cs  cc (1   )

(25)

where γ is the fraction of dispersed particles. For open-open boundary conditions: x=±∞, cc=0, cs=0 and the initial condition: x=0, cc= cc0δ(t), cs= cs0δ(t) where cs, cc, Uss and Usc are the tracer concentration and superficial velocities of the dispersed particles in the riser. Das, Dac are the axial diffusion coefficient of dispersed particles and clusters respectively. The analytical solutions for the above equations are as follows:

cs 

(  t ) Pes 1 Pes s exp( s ) 2 t 4t s

(26)

cc 

(  t ) Pec 1 Pec c exp( c ) 2 t 4t c

(27)

where

Pes 

U ss h0 U h h h , Pec  sc 0 , s  0 , c  0 Das Dac U ss U sc

To obtain the mean residence time, the value of Peclet number should be determined firstly. Wei

39

calculated the Peclet number of dispersed particles and clusters from experiment data, and found that the 8


Page 9 of 23 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


Peclet numbers are around 100 and 3 for the dispersed particles and clusters respectively. By using the above model, the mean residence time in this experiment is ranged from 10.7 s to 24.5 s. Compared to the average residence time in the turbulent fluidized bed reactor, which is around 1 hour, the mean residence time of the fast fluidized bed reactor is dramatically reduced. The particle residence time is closely related to the catalysts activity. Figure 5 is the catalysts activity variation with residence time based on eq (16) and eq (17). By eq (17), the catalysts activity could be calculated, which is large than 0.99 for the fast fluidized bed reactor and is 0.95 for turbulent fluidized bed reactor. Since the pyridine synthesis reactions with ammonia and aldehydes are parallel reactions, and the reaction rate of R1 is larger than R2, an increase of catalysts activity will raise the selectivity of pyridine over 3-picoline. The experiment results in this paper also justified this phenomenon. 4.2 Production Yield The highest pyridine bases production yield is 65% in this experiment, and the corresponding reaction conditions are reaction temperature of 773 K, superficial gas velocity of 0.9 m/s, and the catalysts circulation rate of 20.6 kg/m2/s. The pyridine bases production yield is about 10% lower than that in a commercial turbulent fluidized bed reactor, probably because the contact of feed stocks and the catalysts in the riser was not as intensive as that in turbulent fluidized bed reactor. This phenomenon suggests that the catalyst circulation flow rate may have a strong influence on the production yield in the fast fluidized bed reactor. 4.3 Influence of Catalysts Circulation Rate on Production Yield Figure 6 illustrates the influence of catalysts circulation rate on the production yield. The increase of catalysts circulation flow rate brought an increase in mass flow rate ratio of catalysts to feedstock, leading to an intensive gas-catalyst contact. Moreover, the mean catalyst activity in the fast fluidized bed reactor becomes higher, increasing the product yields and selectivity. As shown in Figure 6, with the increase of the catalyst circulation flow rate from 15 to 22 kg/m2/s, the production yield increases from 28% to 52%. Similar phenomenon was also observed by Reddy 7. Furthermore, the increase in pyridine yield seems greater than in the increase in 3-picoline yield, resulting in high selectivity of pyridine over 3-picoline.

5 MODELING RESULTS Eq (18) to eq (21) formed the reactor model for this fast fluidized bed reaction system. Figure 7 is the comparison between experiment data and model prediction, the average prediction deviation is 6%. The

9



Page 10 of 23

model application range is listed in Table 1. The influence of different operating parameters on production yield is as follows.

5.1 Reaction Process in a fast fluidized bed reactor Figure 8 is reactants and products concentration distribution along riser axial direction. The reaction temperature is 773 K, and the superficial gas velocity is 1 m/s. Catalysts circulation rate is 25 kg/m2/s and initial acetaldehyde concentration is 0.0055 mol/L. It is seen that with the increase in reactor height, the acetaldehyde concentration decreases, while the pyridine and 3-picoline concentrations only increase before 1.5 m. Meanwhile, the coke concentration increases continually. Eq (14) and eq (15) indicate that pyridine reaction (R1) rate will become zero when the aldehyde concentration and the pyridine concentration satisfy eq (28). And 3-picoline reaction rate (R2) will become zero as the aldehyde concentration and the 3-picoline concentration satisfy eq (29).

k1CA2  k3CD

(28)

k2CA2  k4CE

(29)

As the reactions carry out, acetaldehyde concentration decreases, then the reaction rate of R1 and R2 becomes less than zero, which is named as over-reaction section. The reaction period with positive reaction rate of R1 and R2 is named as normal reaction section, and the corresponding reactor height for the reaction rate of zero is named as critical point. According to Figure 8, the critical point of R1 is around 1.5 m, and the critical point of R2 is around 0.8 m, which indicates the critical point of R1 is higher than that of R2. To minimize the coke produce, the reaction should be terminated when the critical point of R2 is reached, and the quantified criterion is expressed by eq (30).

CA 

k 4C E k2

(30)

5.2 Influence of Superficial gas velocity on predicted Production yield An increase of superficial gas velocity may affect production yield by enhancing mass transfer between gas phase and catalysts, decreasing catalysts concentration and reducing reactants residence time. Eq (27) shows that an increase in superficial gas velocity increases the modification factor, which in turn results in an increase of reaction rate. On the other hand, eq (22) shows that higher superficial gas velocity leads to lower catalysts concentration, and thereby a decrease in reaction rate. Besides, higher superficial gas velocity means shorter residence time of reactants, improving selectivity at a loss of conversion. Figure 9 10


Page 11 of 23 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


is the influence of superficial gas velocity on production yield. With the increase in superficial gas velocity, the pyridine and coke yields decrease, while 3-picoline yield firstly increases and then decreases.

5.3 Influence of Reaction Temperature on predicted Production yield The influence of reaction temperature on production yield is reflected by reaction constants. For the reactions involved in pyridine synthesis process, an increase in reaction temperature raises the reaction rate. Figure 10 illustrates the influence of reaction temperature on production yield. It seems that the reaction temperature has small influence on production yield in the experiment. Because the reactions involved is exothermic, an increase of reaction temperature pushes the reaction progress toward the reactant direction. As the result, limited by reaction equilibrium, the production yield slightly changes with temperature.

5.4 Influence of Initial Acetaldehyde concentration on predicted Production yield An increase in initial acetaldehyde concentration may raise the reaction rate effectively, and promote the conversion of reactants. Figure 11 gives the influence of initial acetaldehydes concentration on production yield. It is seen that with increase in initial acetaldehyde concentration, the production yield increases. The coke yield has the smallest increase magnitude with the increase of initial acetaldehyde concentration, which means increase of initial acetaldehyde concentration favors the produce of pyridine and 3-picolinie.

6 CONCLUSIONS A fast fluidized bed reactor is proposed for gas phase pyridine synthesis. A hot mode experiment apparatus was established, and the production yield under different operating conditions was measured. A model is proposed to investigate the influence of operating conditions on production yield. Experiment result indicates that the production yield in the fast fluidized bed reactor is slightly smaller than in an industrial turbulent fluidized bed reactor, while the selectivity of pyridine over 3-picoline is 1.5 times as that in the industrial turbulent fluidized bed reactor. In the modeling process, a modification factor is proposed to present the influence of mas transfer, heat transfer and backmixing on reaction rate. The mean value of the modification factor is 0.75 within this experiment operating conditions. The model accurately predicts the production yield with an average deviation of 6%. By model prediction, the reaction should be terminated when the critical point of R2 is reached to avoid over reaction. To optimize the pyridine and 3-picoline production yield and minimize coke production yield, the reaction temperature should be kept around 723 K.

11



SUPPORTING INFORMATION Tables with the production yield under different operating conditions and the value of reaction kinetic constants. ACKNOWLEDGMENTS This work was supported by NKBRDP (the National Key Basic Research Development Program ) of China (973, 2012CB215000).

12


Page 12 of 23

Page 13 of 23 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


a CA0 cs cc Das Dac Gs h h0 Hi Ho L k1 k2 k3 k4 mi mo nA0 Nar Nfr Np N3p Pr Pec Pes R rA rD rE Tr t ug Us Usc Uss V Vr xA xD xE yp

NOMENCLATURE activity of catalysts, for fresh catalysts the value is unity concentration of acetaldehydes in feedstock (mol/L) dispersed particle concentration (kg/m3) clustering particle concentration (kg/m3) axial solid dispersion coefficient of dispersed particles (m2/s) axial solid dispersion coefficient of cluster (m2/s) catalysts circulation rate based on reactor cross sectional area (kg/m2/s) the height of the reaction zone (m) the height of the reactor (m) the enthalpy of the feed stocks in the inlet of the computation node, (kJ/kg) the enthalpy of the feed stocks in the outlet of the computation node, (kJ/kg) the hear latent of the catalysts, (kJ/kg/K) reaction rate constant for reaction A to D (L/(mol.s)) reaction rate constant for reaction A to E (L/(mol.s)) reaction rate constant for reaction D to F (s-1) reaction rate constant for reaction E to F (s-1) the mass flow rate of the feed stocks in the inlet of the computation node, (kg/s) the mass flow rate of the feed stocks in the outlet of the computation node, (kg/s) number of mol of acetaldehydes in feedstock (mol) mole number of acetaldehydes in feedstocks (mol) mole number of formaldehydes in feedstocks (mol) mole number of pyridine in the products (mol) mole number of 3-picoline in the products (mol) pressure of feedstock (Pa) axial Peclet number of cluster axial Peclet number of dispersed particles ideal gas constant 8.314 (J/(mol.K)) reaction rate for acetaldehydes (mol/(L.s)) reaction rate for pyridine (mol/(L.s)) reaction rate for 3-picoline (mol/(L.s)) reaction temperature (K) residence time (s) superficial gas velocity (m/s) average solids velocity (m/s) average solids velocity of cluster (m/s) average solids velocity of dispersed particles (m/s) volumetric flow rate (m3/h) volume of riser (m3) yield of acetaldehydes yield of pyridine yield of 3-picoline pyridine production yield 13



y3p 3-picoline production yield yc coke production yield Greek symbols ηaf acetaldehydes to formaldehydes ratio ηam ammonia to aldehydes ratio △ difference τc residence time of cluster (s) τs residence time of dispersed particles (s)

14


Page 14 of 23

Page 15 of 23 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


REFERENCES (1) Reddy, K. S. K.; Srinivasakannan, C.; Raghavan, K. V. Catalytic vapor phase pyridine synthesis: a process review. Catal. Surv. Asia. 2015, 16, 28-35. (2) Jin, F.; Cui, Y.; Rui, Z.; Li, Y. Effect of sequential desilication and dealumination on catalytic performance on ZSM-5 catalyst for pyridine and 3-picoline synthesis. J. Mater. Res.. 2010, 25, 272-282. (3) Calvin, J. R.; Davis, R. D.; McAteer C. H. Mechanistic investigation of the catalyzed vapor-phase formation pyridine and quinoline bases using 13CH2O, 13CH3OH, and deuterium-labled aldehydes. Appl. Catal., A. 2005, 285, 1-23. (4) Baldev, S.; Sisir, K. R.; Krishnadeo, P. S.; Tarun, K. G. Role of acidity of pillared ineter-layered clay (PILC) for the synthesis of pyridine bases. J. Chem. Technol. Biotechnol. 1998, 71, 246-252. (5) Kalevaru, V. N.; Madaan, N.; Martin, A. Synthesis, characterization and catalytic performance of titania supported VPO catalysts for the ammoxidation of 3-picoline. Appl. Catal., A. 2011, 391, 52-62. (6) Sisir, K. R.; Banikar, G.; Shyam, K. R. Studies on the synthesis of 2 & 4-picoline –correlation of acidity with the catalytic activity. Stud. Surf. Sci. Catal. 1998, 113, 713-719. (7) Reddy, K. S. K.; Screedhar, I.; Raghavan, K. V. Kinetic studies on vapour phase pyridine synthesis and catalyst regeneration studies. Can. J. Chem. Eng. 2011, 89, 854-863. (8) Goe, G. I.; Davis, R. D. Pyridine base synthesis process and catalyst for same. U. S. Patent 5,218,122, 1993. (9) Rao, R. R.; Kulkarni, S. J.; Subramanyam, M.; Rama Rao, A. V. Synthesis of pyridine and picolines over modified silica-alumina and ZSM-5 catalysts. Reac. Kinet. Catal. Lett. 1995, 56, 301-309. (10) Francis, E. Cislak; William, R. Wheeler. Synthesis of pyridine and 3-picoline. U. S. Patent, 2,807,618, 1957. (11) Feitler, D.; Wolfgang, S.; Henry, W. Process for the production of pyridine or alkyl substituted pyridines. U. S. Patent 4,675,410, 1987. (12) Bi, H. T.; Ellis, N.; Abba, I. A.; Grace, J. R. A state-of-the-art review of gas-solid turbulent fluidization. Chem. Eng. Sci. 2000, 55, 4789-4825. (13) Jaiboon, O.; Chalermsinsuwan, B.; Mekasut, L.; Piumsomboon, P. Effect of flow patterns/regimes on CO2 capture using K2CO3 solid sorbent in fluidized bed/circulating fluidized bed. Chem. Eng. J. 2013, 219, 262-272. (14) Zhang, K.; Pei, P.; Brandani, S.; Chen, H; Yang, Y. CFD simulation of flow pattern and jet penetration depth in gas-fluidized beds with single and double jets. Chem. Eng. J. 2012, 68, 108-119. (15) Richtberg, M.; Richter, R.; Wirth, K. -E. Characterization of flow patterns in a pressurized circulating fluidized bed. Powder Technol. 2005, 155, 145-152. (16) Kim, S. W.; Kirbas, G.; Bi, H.; Lim, C. J.; Grace, J. R. Flow behavior and regime transition in a high-density circulating fluidized bed riser. Chem. Eng. Sci. 2004, 59, 3955-3963. (17) Wang, W.; Lu, B.; Li, J. Choking and flow regime transitions: Simulation by a multi-scale CFD approach. Chem. Eng. Sci. 2007, 62, 814-819. (18) Li, J.; Weinstein, H. An experimental comparison of gas backmixing in fluidized beds across the regime spectrum. Chem. Eng. Sci. 1989, 44, 1697-1705. (19) Bai, D; Zhu J.-X.; Jin, Y.; Yu, Z. Novel Designs and Simulations of FCC Riser regeneration. Ind. Eng. Chem. Res. 1997, 36, 4543-4548. (20) Theologos, K. N.; Markatos, N. C. Advanced modeling of fluid catalytic cracking riser-type reactors. AIChE J. 1993, 39, 1007–1014. (21) Theologos, K. N.; Nikou I. D.; Lygeros, A. I.; Markatos, N. C. Simulation and design of fluid-catalytic cracking riser-type reactors. AIChE J. 1997, 43, 486–498. (22) Gao, J.; Xu, C.; Lin, S.; Yang, G.; Guo, Y. Simulations of gas-liquid-solid 3-phase flow and reaction in FCC riser reactors. AIChE J. 2001, 47, 677–692. (23) Li, J.; Fan, Y. -P.; Lu, C. -X.; Luo Z. -H. Numerical simulation of influence of feed injection on hydrodynamic behavior and catalytic cracking reactions in a FCC riser under reactive conditions. Ind. Eng. Chem. Res. 2013, 52, 11084–11098. (24) Gayán, P.; de Diego, L. F.; Adánez, J. Radial gas mixing in a fast fluidized bed. Powder Technol. 1997, 94, 163-171. (25) Werther, J.; Hartge, E. U.; Kruse, M. Radial gas mixing in the upper dilute core of a circulating fluidized bed. Powder Technol. 1992, 70, 293-301. (26) Dry, R. J.; White, C. C. Gas residence-residence time in a high-velocity circulating fluidized bed of FCC catalyst. Powder Technol. 1989, 58, 17-23. (27) Chen, X.-Z.; Luo, Z.-H.; Yan, W.-C.; Lu, Y.-H.; Ng, I.-S. Three-dimensional CFD-PBM coupled 15



model of the temperature fields in fluidized bed polymerization reactors. AIChE J. 2011, 57, 3351-3366. (28) Guan, Y.; Chang, J.; Zhang, K.; Wang, B.; Sun, Q. Three dimensional CFD simulation of hydrodynamics in an interconnected fluidized bed for chemical looping combustion. Powder Technol. 2014, 268, 316-328. (29) Lu, B.; Zhang, J.; Luo, H.; Wang, W.; Li, H.; Ye, M.; Liu, Z.; Li, J. Numerical simulation of scale-up effects of methanol-to-olefins fluidized bed reactors. Chem. Eng. Sci. 2017, 171, 244-255. (30) Smolders, K.; Baeyens, J. Gas fluidized beds operating at high velocities: a critical review of occurring regimes. Powder Technol. 2001, 119, 269-291. (31) Chan, C. W.; Seville, J., Yang, Z.; Baeyens, J. Particle motion in the CFB riser with special emphasis on PEPT-imaging of the bottom section. Powder Technol. 2009, 196, 318-325 (32) Bi, H. Some issues on core-annulus and cluster models of circulating fluidized bed reactors. Can. J. Chem. Eng. 2010, 80, 809-817. (33) Lu, C. X.; Wang Z. Research of industrial fast fluidized bed reactor. Petro. Technol. 1991, 20, 695-699 (in Chinese). (34) Bai, D.; Zhu, J.-X.; Jin, Y.; Yu, Z. Internal recirculation flow structure in vertical upward flowing gas-solids suspensions, Part I. A core/annular model. Powder Technol. 1995, 85, 171-178. (35) Tallon, S.; Davies, C. E.; Barry, B. Slip velocity and axial dispersion measurements in a gas-solid pipeline using particle tracer analysis. Powder Technol. 1998, 89, 125-131. (36) Wang, J. Length scale dependence of effective inter-phase slip velocity and heterogeneity in a gas-solid suspensions. Chem. Eng. Sci. 2008, 63, 2294-2298. (37) Li, D.; Ray, A. K.; Ray, M. B.; Zhu, J. Catalytic reaction in a circulating fluidized bed riser: Ozone decomposition. Powder. Technol. 2013, 242, 65-73. (38) Harris, A. T.; Davidson, J. F.; Thorpe, R. B. Particle residence time distributions in circulating fluidized beds. Chem. Eng. Sci. 2003, 58, 2181-2202. (39) Wei, F.; Zhu, J.-X. Effect of flow direction on axial solid dispersion in gas-solid cocurrent upflow and downflow system. Chem. Eng. J. 1996, 64, 345-352. (40) Wei, F.; Jin, Y.; Yu, Z.; Chen, W.; Mori, S. Lateral and axial mixing of the dispersed particles in CFB, J. Chem. Eng. Jpn. 1995, 28, 506-510.

16


Page 16 of 23

Page 17 of 23 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


Figures

1. carbon dioxide detector; 2,8,11. condenser; 3. regenerator; 4,5. valve; 6. stripper; 7. separator; 9. riser; 10. bottom of riser; 12. pipe condenser; 13. gas-liquid separator; 14. liquid separation tower; 15-1~2. piston pump; 16-1~2. heater; 17. glass rotameter; 18. electronic balance; 19. product collector; 20. refrigerator; Figure 1. Process of experiment apparatus

17



Figure 2. Axial position of thermometers

Figure 3. Computation node for Gs calculation

18


Page 18 of 23

Page 19 of 23 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


A: acetaldehyde B: ammonia C: formaldehyde D: pyridine E: 3-picoline F: coke Figure 4. Reaction mechanism (7)

Figure 5. The catalysts activity variation with residence time

19



Figure 6. The influence of catalysts circulation rates on production yield

Figure 7. Comparison between model prediction and experiment result

20


Page 20 of 23

Page 21 of 23 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


Figure 8. Concentration of reactants and products distribution along riser axial direction

Figure 9. The influence of superficial gas velocity on production yield by model prediction

21



Figure 10. The influence of reaction temperature on production yield by model prediction

Figure 11. The influence of initial acetaldehydes concentration on production yield by model prediction

22


Page 22 of 23

Page 23 of 23 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


For Table of Contents Only

23


Investigation of Pyridine Synthesis in a Fast Fluidized Bed Reactor

Recommend Documents