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INTERNATIONAL JOURNAL OF C HEMICALR EACTOR E NGINEERING

Volume 6 2008 Article A29

Optimization of Vapor Phase PyridineSynthesis Hindered by Rapid Catalyst

DeactivationSuresh Kumar Reddy Kuppi Reddy Inkollu Sreedhar

Kondapuram Vijaya Raghavan Shivanand Janardan Kulkarni

Machiraju Ramakrishna

Indian Institute of Chemical Technology, sureshreddy [email protected] Institute of Technology & Science, [email protected] Research and Development Organisation, kvraghavan [email protected] Institute of Chemical Technology, [email protected] Indian Institute of Chemical Technology, [email protected]

ISSN 1542-6580Copyright c 2008 The Berkeley Electronic Press. All rights reserved.


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Optimization of Vapor Phase Pyridine Synthesis

Hindered by Rapid Catalyst Deactivation

Suresh Kumar Reddy Kuppi Reddy, Inkollu Sreedhar, Kondapuram VijayaRaghavan, Shivanand Janardan Kulkarni, and Machiraju Ramakrishna

Abstract

The synthesis of pyridine bases from acetaldehyde, formaldehyde and ammo-

nia through aminocyclization continues to provide the best prospect for meetinggrowing demand. A proper selection of catalyst and standardization of processparameters are vital to achieve a market friendly product distribution and reactoroperation. In this work, the major responsible factors for enhancing the activityand selectivity of HZS-5 catalysts have been identied and their individual andcombined effects on aldehyde conversion, coke formation and selectivity to pyri-dine formation have been assessed. A priori assessment of catalyst time on streambehavior has been achieved by modeling the catalyst deactivation process.

KEYWORDS: aminocyclization, pyridine synthesis, zeolite catalysis, processstandardization, modeling catalyst decay

K. Suresh Kumar Reddy is thankful to CSIR, New Delhi for the award of SRF-GATE fellowship.The authors acknowledge the support and encouragement provided by the Director, Indian Instituteof Chemical Technology, Hyderabad (India).


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1. INTRODUCTION

Pyridine is an industrially important chemical in organic reactions and itssynthetic production from easily available feedstocks like acetaldehyde,formaldehyde and ammonia through aminocyclization provides the best prospectfor meeting its growing market demand. Vapor phase aminocyclization requires

proper selection of catalyst and standardization of process parameters to achieve amarket friendly product distribution and reactor operation.

Vapor phase pyridine synthesis from aldehydes and ammonia was firstreported by (Chichibabin, 1924). Since then, several industrially relevant

processes employing a range of cost competitive raw materials were patented. Thefirst commercial production of pyridine began in 1953 in USA and subsequentlyICI, Rutgerwerk, Nepara, Koei Chemicals and others followed ( Baylis et. al.,1976, Chang et. al., 1980, Feitler et. al., 1987, cheng et. al., 1989, Shimizu et.al.,1993). A range of carbonyl containing precursors including acrolein and acetonewere tried in fixed and fluid bed reactors.

Solid acid catalysed vapor phase pyridine synthesis has enhanced the prospects of cumulative benefits of high yield, selectivity and environmentalcleanliness. However very few breakthroughs have been achieved in catalyst lifeextension. The present work focusses on catalyst and process standardization invapor phase pyridine synthesis to attain high selectivity to pyridine formation andminimum coke deposition. The role of catalyst acidity and Al +3 content, aldehydeand ammonia inputs, contact time and other process parameters on conversion of limiting reactant and selectivity towards pyridine formation received attention.Critical factors contributing to the catalyst deactivation have been investigated toevolve a model to simulate its time on stream performance.

2. CATALYST CHOICE

The pyridine synthesis proceeds in two or more reaction pathways involvingcondensation, cyclization and hydrogen transfer. Substituted solid acid catalystshave been reported (Calvir, 2005, Golunski et.al.,1986) to provide attractiveyields of pyridine due to their shape selectivity and ability to reduce the formationof higher alkylated derivatives. Pentasil type of zeolites and ion exchangedsilicalites with medium SiO 2/Al 2O3 ratios (30-120) have more pronounced effect

on pyridine formation since their pore diameters are almost similar to that of pyridine and are thermally stable (Sato et.al., 1994a). In the present work, HZSM-5 is accordingly, employed as the catalyst.

Figure 1 Highlights the major products obtained from aminocyclizationwhen various combinations of aldehydes and ketones are employed. Reaction 2 is

Kuppi Reddy et al.: Optimization of Vapor Phase Pyridine Synthesis

Published by The Berkeley Electronic Press, 2008


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commercially favoured for pyridine synthesis. The following main and secondaryreactions are important for the present studies as shown in scheme 1(Sato et.al.,1994b).

Above reactions are characterized by noticeable increase in the number of moles of product molecules as compared to the reactants indicating nofavorable effect of pressure increase on reaction rates. In the present work theyield is defined as mole % of acetaldehyde converted into pyridine whereas theselectivity refers to the ratio of moles of pyridine to the moles of 2 and 3 picolinesformed.

Figure 1. Reported principal products from ammonia reaction with variouscombinations of aldehydes and ketones

1.acetaldehyde2.acetaldehyde and

formaldehyde3.acrolein4.acrolein and

acetaldehyde5.acrolein and

propionaldehyde6.acrolein and

acetone

HZSM-5 (240)T = 350 - 400 0cP= 1 atm

1. 2 and 4- picolines

2. pyridineand 3-

picolines3. 3-picolines4. pyridine5. 3-picolines6. 2-picolines

2 International Journal of Chemical Reactor Engineering Vol. 6 [2008], Article A29

http://www.bepress.com/ijcre/vol6/A29


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Scheme 1. Reaction pathways in aminocyclization

3. EXPERIMENTAL

3.1 Packed bed downflow reactor

The vapor phase pyridine synthesis is carried out at atmospheric pressure in acontinuous tubular downflow Pyrex reactor (20 mm i.d) with the lengths of

preheater and reaction sections being 15 and 4 cm respectively. The catalyst (4gms) is loaded into the reaction section followed by glass beads (5 mm dia) in the

preheater section. The catalyst is activated by calcination with airflow at 723 K for 6h. The catalyst bed temperature is measured with a thermocouple placed atthe center of reactor cross section. The product is cooled and condensed using ice-cold water and collected at the bottom. Sufficient number of ice-cooled traps areused to collect lower boilings. Details of laboratory reactor set up are given in

Figure 2 .For process standardization studies, the aldehyde mixture consisting of

99.97 wt% acetaldehyde and 35% W/V of aqueous formaldehyde is cooled (283

K) and fed into reactor preheating zone in liquid form. Their mole ratio is variedfrom 0.5 to 3.0 and also their flow rates from 1 to 8 ml/hr. The reactiontemperature is varied from 523 to 673 K. The other parameters viz., Si/Al ratio of the catalyst is varied from 40 to 240 and anhydrous ammonia (gas) to aldehyderatio from 1 to 10. To study the effect of H 2O on the catalyst activity,35% W/V formaldehyde is diluted with the predetermined quantity of water.

2C H 3 C H O + H C H O + N H 3 + 3H 2 O + H 2

NPyridine

3C H 3 C H O + N H 3N

2-PicolineC H 3

+ 3H 2 O + H 2

2C H 3 C H O + 2 H C H O + N H 3 + 4H 2 O

3-PicolineN

C H 3




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Figure 2. Experimental setup for pyridine synthesis

The reaction products are analyzed by GC (shimadzu-14B) using a

chromosorb-102 column (6' length, =1/8, 80/100 mesh). Nitrogen is used as thecarrier gas and the column, injector and detector are maintained at 473,533 and553 K respectively. The retention times of the products are compared withauthentic compounds.

3.2 TPD of ammonia

The temperature programmed desorption of ammonia (TPDA) is used to assessthe catalyst acidity by employing Autochem 2910(Micromeritics, USA) system.250 mg of oven-dried catalyst sample (at 383 oK for overnight) is taken in a U-shaped quartz sample tube and is pretreated at 473 oK for 1 hr by passing 99.999%

pure helium (50 ml/min). It is then adsorbed with anhydrous ammonia (AU: 17)from a 10% NH 3-He mixture at 353K flowing at 75 ml/min. It is subsequentlyflushed at 378 K for 2h to remove physically adsorbed ammonia. The TPDAstudies are carried out from ambient temperature to 1173 K at 10 oC/min heatingrate. The amount of NH 3 desorbed is calculated using GRAMS-32 software.

TI 201 a, b, c

FI 101

VENTTI 202

TWOSTAGE R CW-INREGUL TI 203ATOR 1 TIC 101 SV 101

NH 3 0 E101 CWR

ELECTRIC TI 204C SV 101 1 HEATER Y

SV 102LINDE P 101

R

SV 101 STORAGE VESSELS P101: METERING PUMP R101: REACTOR CW-IN: COOLANT IN CW R: COOLANT RETURNSV 101: SEPARATOR FI 101: WET GAS FLOW METER E101: CONDENSOR TIC 101: TEMP INDICATING CONTROLER TI 201 a, b, c; TI 202,203,204: TEMP INDICATOR




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3.3 CHNS analysis

The carbon, hydrogen, nitrogen, and sulfur content of the catalyst samples have been evaluated employing ELEMETAR-VARIO-EL analyser. The sample (0.4mgminimum) is taken in a tin foil and placed in the carousel. It falls into thecombustion tube where the sample is oxidized at 1420 K. The oxides are thenreduced to their respective elements.

3.4 XRD

The X-ray Diffraction (XRD) studies are conducted with Siemens d-5000Diffractometer using nickel filtered Cu K radiation, to characterize thecrystalline coke structure. The deposition of coke inside the catalyst pores is alsoverified by means of changes in XRD spectra of coked and freshly preparedcatalysts. In this method, the deviation of Xray beam of a particular wavelength

by a crystalline particle is related to the interplaner distance of the diffracting planes. The angle of incidence is estimated by the well known Bragg equation.

3.5 Thermal analysis (TGA/DTA)

Thermogarvimetric and differential thermal analysis have been conductedemploying TGA/SDTA Mettler Toledo 851 e instrument to study the cokeoxidation characteristics which is relevant for properly designing catalystregeneration process. The weight loss and temperature increment above areference sample are closely monitored. It also allows to determine the amount of coke released from the catalyst during the thermal treatment. 10mg of catalystsample is placed in an electro balance and heated at a rate of 10 0C/min by

passing nitrogen gas. The weight loss is recorded. The first differential of TGAcurve provides DTA profile.

3.6 BET surface area

BET surface area measurements has been done by employing multi-point Gemini2360 instrument (Micromeritics, USA), employing single point method using a30% N 2 and 70% He mixture. The sample is pre-treated to remove all moistureunder flow of pure He (99.99%) at 473 K for 2 hours. The gas mixture is then

passed through the sample tube placed in a liquid N 2 bath (77 K). The peak areasof adsorption and desorption are used to measure the specific surface area of thesample.




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3.7 IR

FT-IR spectra of various samples reported in this study are recorded on a Nicolet740 FTIR spectrometer and some of them in Bio-Rad 175c instrument at ambientconditions using KBr as the diluent to study the coke deposits and their possiblelocation. Approximately 1mg of catalyst sample is mixed with 100mg of IR gradeKBr and placed in a clean KBr dye. About 6 tons of pressure is employed toobtain a transparent disc of sample embedded in KBr. It is loaded into the sampleholder and scanned in the mid IR region viz., 100 to 4000cm -1 to obtain FT-IR spectrum.

4. RESULTS AND DISCUSSION

4.1 The role of catalyst acidity

Pyridine bases are formed through series-parallel reactions in vapor phaseaminocyclization and the catalyst acidity has been reported to have significantinfluence on product distribution (Kulkarni et.al., 1994) with the followingobservations:

1. Presence of both Lewis and Bronsted acid sites on catalyst in athermodynamic equilibrium.

2. Bronsted sites promote carbonium ion formation whereas Lewis sites preferentially adsorb ammonium ions. The former is vital for aminocyclization.

3. High catalyst acidity retards pyridine synthesis due to predominance of unreactive adsorbed ammonium ions and enhanced catalyst coking ability.

4. The optimum level of catalyst acidity for pyridine synthesis is around150mol/gm.

5. Pentasil structure in HZSM-5 and silicalite promotes selective formationof pyridine under medium level of acidity.

The concentration of Al 3+ ions in catalyst as represented by silica-alumina ratio(hereafter refered to as Si/Al) provides a strong basis for catalyst acidityoptimization. In the present work, Si/Al is varied from 40 to 240 for a better understanding of the catalyst acidity effect on yield and selectivity in pyridinesynthesis. The results are shown in Figure 3 . The present authors found (SureshKumar Reddy et.al., 2008) that enhancement of Si/Al from 40 to 90 contributes to

high conversion and high coke formation. However, a different trend is observedat higher Si/Al viz., higher pyridine selectivity and lower conversion levels. Thetotal ammonia uptake at S/A=240 was estimated as 200 mol/gm, a value verynear to the reported (Snape et.al., 1997, Darragh et.al., 1978) optimum value. Thesame catalyst had shown ( Figure 4 ), a significant increase in pyridine selectivity




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and minimum coke formation though at lower pyridine yield. Table 1 providessurface area of the fresh and coked catalyst samples, which shows the negativeeffect of coking and insensitivity of Si/Al ratio to the surface area. The surfacearea loss is minimal in case of coked catalyst with Si/Al = 240.

4.2 The role of aldehyde ratio

The studies of Chichibabin and others (Ramachandra Rao et.al., 1995) establishedthe critical role of acetaldehyde for achieving a favorable reaction pathway to the

pyridine base formation. In the present work, formaldehyde/acetaldehyde moleratio (hereinafter referred to as aldehyde ratio) is varied between 0.5 to 3 tostandardize the conversion and selectivity levels in pyridine synthesis. Figure 5 shows the experimental results. An aldehyde ratio of 1.0 is found to provide highselectivity to pyridine formation and attractive yield

Figure 3. Ammonia TPD patterns with Freshly prepared ZSM-5 (a, b, c, d)with Si/Al ratio 40, 90, 150, 240 respectively.




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Table 1. Surface area of fresh and coked catalystSurface area (m 2/g)S.No Catalyst

Fresh Catalyst Coked catalyst1 HZSM-5 (40) 382.6 72.2

2 HZSM-5 (90) 349.1 32.8

3 HZSM-5 (150) 358.0 86.5

4 HZSM-5 (240) 382.2 273.7(T=375 OC, F=2 ml/hr, W=4 gm Catalyst, CH 3CHO /HCHO/NH 3 mole ratio 1:1:4)

Figure 4. Effect of Si / AI ratio on pyridine yield and selectivity(T = 375 0C; F = 2ml/hr; W = 4gm catalyst; aceteldehyde/formaldehyde/ammonia

ratio 1:1:4)

0

10

20

30

40

50

60

0 40 80 120 160 200 240 280

Si/Al ratio

P y r

i d i n e

Y i e l d %

0

0.5

1

1.5

2

2.5

3

3.54

4.5

5

Yield %

Selectivity




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Figure 5. Aldehyde ratio effect on yield and selectivity(T=375 OC, F=2 ml/hr, W=4 gm Catalyst, acetaldehyde/formaldehyde/ammonia mole ratio1:1:4)

4.3 The role of ammonia

Aldehydes react with ammonia even at 80-100 oC in the presence of alumino-silicates to form compounds containing both hydroxyl and amino groups.However at 350-400 oC, further reactions occur to form pyridine bases dependingupon the composition of aldehyde mixtures. Golunski et.al., (1986) reported thataluminosilicates preferentially adsorb ammonia to promote surface carbocations.Farberov et.al., (1975) proposed a mechanism for pyridine base synthesisconsisting of adsorption of reactants at the acidic sites of the catalyst to formcarbonium ions. Nucleophilic addition then occurs on the catalyst surface to formadsorbed imines, which are subsequently cyclized. The mole ratio of ammonia toaldehydes is therefore, an important process parameter for optimizing the pyridineyields. The patent literature shows the variation of this ratio in 0.5-1.0 range under different process conditions. In the present work, the ammonia to aldehyde ratio isvaried in a much broader range of 1-10. Figure 6 presents the experimental resultsshowing an increase in the yield of pyridine bases with an increase in ammoniatill the ammonia to aldehyde ratio attains 1:4 and a declining trend thereafter.

0

5

10

15

20

25

30

35

4045

50

0.5 0.75 1 2 3Aldehyde mole ratio

% y

i e l d

0

1

2

3

4

5

6

7

89

10

s e

l e c

t i v

i t y

pyridine yield

Selectivity towards pyridine




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Figure 6. Effect of aldehyde/ammonia ratio on yield and selectivity

(T=375 OC, F=2 ml/hr, W=4 gm Catalyst viz., HZSM-5 (240), acetaldehyde/formaldehyde moleratio 1: 1)

4.4 The role of water

Van Der Gaag et.al., (1986) reported that catalyst deactivation due to cokeformation can be more rapid when no water was present in the feed. Their studieshave also shown that water assists in reducing coke and polymeric materialformation by keeping the catalyst surface clean. Golunski et.al., (1986) reported

positive effect of water on the yield of pyridine bases and stability of the catalyst.In the present work, the water/ total aldehydes molar ratio is varied from 1.42 to6.13 to identify its effect on yield, selectivity and coke formation. Details

presented in Table 2 demonstrated the importance of water content in thealdehyde feed not only in standardization of aminocyclization process but also incontaining coke and polymeric deposit formations.

0

20

40

60

80

100

120

1:01 1:02 1:04 1:06 1:10

Mole ratio (aldehydes/NH 3)

T o t a

l Y i e l d %

0

1

2

3

4

5

6

7

8

S e

l c t i v i

t y

Total Yield

Slectivity




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Figure 7. Effect of temperature on yield and selectivity(F=2 ml/hr, W=4 gm Catalyst, acetaldehyde/formaldehyde/ammonia mole ratio 1:1:4,space velocity = 0.275gm/hr/gm catalyst)

Table 2. The effect of water formation on cokeWater/aldehydes(moleratio)

Acetaldehyde(ml/hr)

Pyridineyield %

Selectivity Coke a Polymeric b

1.42 0.840 67.5 3.63 0.048 0.1092.47 0.653 70.4 3.87 0.030 0.0693.77 0.514 57.0 3.98 0.019 0.0474.33 0.470 53.0 4.02 0.017 0.0255.07 0.423 51.0 4.10 0.015 0.0176.13 0.371 50.0 4.17 0.012 0.015

T=375 oC, W=4 gm Catalyst HZSM-5 (240), Feed rate = 2 ml/hr, acetaldehyde/ammonia 1:4 moleratios of reactants, aldehyde ratio 1:1, a: gms of coke/gms of aldehyde fed, b: gms of polymeric/gms of aldehyde fed.

4.5 The role of temperature

The condensation of aldehyde and ammonia leading to the formation of pyridine bases is reported by Golunski et.al., (1986) to proceed through carbonium ionformation catalysed by Bronsted acid sites. At high temperatures, the catalyst

0

10

20

30

40

50

60

7080

90

200 250 300 350 400 450

Temperature ( oC)

Y i e l d %

0

0.5

1

1.5

2

2.5

S l e c t

i v i t y

Yield

Selectivity




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surface becomes dehydroxylated and Bronsted centers are reported to betransformed into their Lewis counterparts resulting in a drop in overall conversionand selectivity. The profiles shown in Figure 7 shows the influence of processtemperature on total conversion of aldehydes with high conversion recorded at648 K and best selectivity at 623 K though with relatively higher coke formation.

4.6 The role of space velocity

Space velocity (W/F) of feed, defined as the number of reactor volumes of feed atspecified conditions that can be treated in unit time, is varied in the range of 0.137to 1.1 gm/hr/gm catalyst to study its effect on pyridine synthesis. Results are

presented in Table 3 . A space velocity of 0.275-gm/hr/gm.catalyst is found to provide best results in terms of high conversion and low coke formationtendencies.

Table 3. Space velocity variationsYield, %Space

velocity,hr -1 Pyridin

e2-

Picoline3-

Picoline

TotalConversion,%

Selectivitya

Coke b

0.137 55.9 15.1 25.9 96.9 3.0 0.0624

0.275 56.3 14.7 24.0 95.0 1.7 0.0519

0.550 33.4 5.1 19.7 58.2 1.6 0.1476

1.100 29.9 0.7 9.2 39.8 3.6 0.1822

T=375 oC, W=4 gm wt of Catalyst HZSM-5 (40), Acetaldehyde/formaldehyde/ammonia 1:1:4mole ratio, a: moles of pyridine/moles of picolines, b: gms of coke/gms of aldehyde fed,

4.7 Combined effect of selected process parameters

Employing a combination of selected process parameters Viz., T=375 oC, W/F =2,aldehyde to ammonia ratio =1:4 and aldehyde ratio = 1:1 for HZSM-5 (240)catalyst, a selectivity of 4.9 could be achieved at 55% conversion level and cokedeposition has been limited to 0.03625 gm / gm catalyst. This result demonstratesthe importance of multiparameter process standardization in enhancing selectivityand containing coke deposition in vapor phase pyridine synthesis.




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5. CATALYST DEACTIVATION

Carbon deposition on the catalyst in the vapor phase pyridine synthesis (Feitler et.al., 1987, Goe et.al., 1993, McAteer et.al., 1998) occurs fairly rapidlynecessitating regeneration and recycle of catalyst. Fixed, fluid and moving bedreactors have been tried by researchers to achieve minimum interruption inoperation for reasonably long spells. The fluid bed seems to have a distinct edgeover other reactor configurations since catalyst regeneration can be carried out ina closed cycle. Golunski et.al., (1986) inferred that the coke deposition is a shapeselective process involving the formation of large polycyclic aromatic molecules,which are unlikely to be formed in the narrow channels or intersections of thecatalyst particles. The results presented in previous sections show that the Si/Alratio of catalyst below 150, 350-375 oC temperature range, space velocity above0.275gm/hr/gm (catalyst), aldehyde ratio around 1 and aldehyde/ammonia ratio

below 1:4 are conducive to coke formation.

5.1 Characterization of coked catalyst

In the present work catalyst samples are subjected to IR, XRD, CHNS and BETsurface area measurements. The IR profile ( Figure 8 ) of the deactivated catalystshows an increase of 1585cm -1 band and a decrease in 3400cm -1 band. Thus theoptimization of OH groups on HZSM-5 catalyst surface will be a key factor for minimizing of coke formation during pyridine synthesis.

The XRD patterns of coked catalyst ( Figure 9) displayed similar reflections at 2 = 7.93,8.90,23.19,23.84 and 45.33 which are suggestive of highstability of HZSM-5 catalyst under reaction conditions. No reflections could beattributed to coke formation indicating its likely amorphous nature. However, aslight shift in its intensity has been noted indicating a probable catalyst pore

blockage due to coke formation. Thermogravimetric analysis of coke oxidation isundertaken to study the weight loss patterns of catalyst at different S/A ratios. Thecoke moiety is lost between 723-973 oC with no subsequent loss. It is found to bemaximum when S/A is around 90 and minimum when S/A is 240.




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Figure 8. IR patterns of (a) Fresh HZSM-5; (b), (c), (d), (e) patterns of cokedHZSM-5 With Si/Al ratio 240,150,90,40 respectively




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Figure 9. XRD patterns of fresh and coked catalysts




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The composition of coke is evaluated by CHNS analyzer. It varies withSi/Al ratio of catalyst (Table 4 ). An enrichment of hydrogen content occurs asSi/Al is enhanced. At Si/Al = 240, the hydrogen enrichment is found to berelatively high and nitrogen content is low by making this composition somewhatunique. Incidently the selectivity towards pyridine is also found to be high at thisratio. The increasing hydrogen content suggests that the coke formation is due totrapped hydrocarbons and it greatly depends on aldehyde conversion.

Table 4. Coke weight and compositionCatalyst

HZSM-5 (40)

HZSM-5 (90)

HZSM-5 (150)

HZSM-5 (240)

% Wt lossDue to oxidation

5.43

9.16

3.44

3.03

Cokecomposition

C H 1.23 N0.11

C H1.28

N0.09

C H 2.39 N0.11

C H 2.62 N0.04

T=375 OC, F=2 ml/hr, W=4 gm wt of Catalyst, acetaldehyde/formaldehyde/ammonia mole ratio1:1:4

5.2 Deactivation function

The modeling of time variant catalyst activity will facilitate preassesment of time

dependent reaction rate and product distribution for evolving appropriativerecycle policies. Voorhies (1945) reported a simple correlation (reciprocal power)for modeling catalyst deactivation with the presumption that coke formationoccurs through the diffusion of its precursors through a carbon layer of a specificthickness and is proportional to the amount of deposited coke. The correlationconstants are specific to a particular feed, catalyst and a set of operatingconditions. Weekman et.al., (1968) developed a model based on exponentialdecay of catalyst. Dumez et.al., (1978) employed a catalyst deactivation functionwhich is multiplied with intrinsic reaction rate.

The kinetics of complex reactions like catalytic cracking is evaluated based on lumped parameter models (Lee et.al., 1989) to link coke formation with

the conversion and yield of main products. The present authors reported (SureshKumar Reddy et.al., 2008) following rate equation for coke formation in pyridinesynthesis.




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Figure 10. Best-fit catalyst decay model results compared with experimental timeon stream coke formation

(F = 2ml/hr; W = 4 gm catalyst (HZSM-5 (240)); acetaldehyde/formaldehyde/ammonia mole ratio1:1:4; T = 375 0C)

dc c/dt = r C0*C

Where r C0 is the initial coking rate, which is a function of partial pressureand temperature of the reaction system and is a constant for a given set of processconditions. C c is the weight of coke formed per unit weight of catalyst and C isthe deactivation function. Employing least square analysis with SAA software, theexperimental C c Vs t data was processed to evaluate the best fit based on theminimum arithmetic average error. The exponential catalyst decay model wasfound to provide the best fit. Figure 10 provides the experimental data vis a visthe best fit model prediction with C = exp (-24.1157*Cc), Cc = (24.1157) -1 * ln(1+1.078*t) as reported by the present authors.

6. SUMMARY

The vapor phase synthesis of pyridine has been studied in a packed bed downflowreactor by employing a HZSM-5 catalyst by varying its acidity levels and process

parameters viz., temperature, aldehyde ratio, ammonia, water content, temperatureand space velocity to achieve high selectivity towards pyridine formation at

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0 50 100 150TOS, (hrs)

C c ( g m . o

f . c o

k e / g m . o

f . a l d e

h y d e )

Cc(experimental)

Cc(exponential decay)




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attractive yield levels. The HZSM-5 catalyst with silica to alumina ratio of 240 isfound to provide the best product distribution with least amount of cokeformation. The water content of the aldehyde feed mixture has a significantinfluence on pyridine yield and coke formation. The multiprocess parameter standardization is very essential to achieve high conversion efficiency andminimum coke formation in aminocyclization for pyridine synthesis. The time -on- stream behavior of the catalyst is predicted very accurately in comparisonwith the experimental data by employing an experimental decay model proposedearlier by the present authors.

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Darragh J. I., Process for producing pyridine and methyl pyridines, U.S. patent

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Dumez F. J, Froment G. F., Dehydrogenation of 1-Butene into ButadieneKinetics, Catalyst Coking, and Reactor Design, Ind. Eng. Chem. Process Des.Develop., Vol. 15 291-301(1976).

Farberov M. I., Antonova V. V., Ustavschikov B. F., Titova. N. A., Synthesis of pyridine bases from aldehydes and ammonia, Zh.Prikl.Khim., Vol. 46, 1587-1592 (1973).

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alkyl substituted pyridines U.S. Patent 4,675,410 (1987).

Goe G. I., Davis R. D., Pyridine base synthesis process and catalyst for same, U.S. Patent 5,218,122 (1993).




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