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Butadiene production from bioethanol and acetaldehyde over tantalum

oxide-supported spherical silica catalysts for circulating fluidized bed

Tae-Wan Kim ⇑,  Joo-Wan Kim, Sang-Yun Kim, Ho-Jeong Chae, Jeong-Rang Kim, Soon-Yong Jeong,Chul-Ung Kim ⇑

Research Center for Green Catalysis, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 305-600, Republic of Korea

h i g h l i g h t s

The effect of size and structural properties of Ta2O5/silica sphere was studied.

 The various ETB reaction conditions for Ta2O5/silica sphere were studied.

 Ta2O5/Q-6 catalyst showed good mechanical strength and catalytic performance.

 Ta2O5/Q-6 catalyst tested for hydrodynamic properties in the cold-bed CFB.

a r t i c l e i n f o

 Article history:

Available online 16 October 2014

Keywords:

Ethanol to butadiene

Spherical silica

Tantalum oxide

Circulating fluidized bed

a b s t r a c t

The chemical 1,3-butadiene (BD) is usually produced from ethanol and acetaldehyde (ethanol to BD, ETB)

over Ta2O5/SiO2 catalyst in fixed-bed system. However, the ETB process has a short catalyst-regeneration

cycle due to rapid deactivation of the catalyst by coke. To overcome this problem, a circulating fluidized-

bed (CFB) reactor could be used in the ETB reaction for continuous regeneration of deactivated catalyst.

The catalyst supplied to a CFB reactor must be spherical, show good catalytic performance, and be

mechanically strong. In this study, a series of samples of Ta 2O5   catalyst on silica spheres with different

pore and particle sizes, were prepared. The catalysts were examined for mechanical strength by anattrition test and in ETB reaction in the fixed-bed reactor. The Ta 2O5/Q-6 catalyst, with particles size of 

75–150 lm, showed good mechanical strength and the best catalytic performance. This optimum spher-

ical catalyst was tested to examine its hydrodynamic properties in a cold-bed CFB reactor.

 2014 Elsevier B.V. All rights reserved.

1. Introduction

The chemical 1,3-butadiene (BD) is an important chemical

intermediate in the petroleum industry because BD is a base mate-

rial used for producing commercially important synthetic rubbers

and polymers [1]. The main application of BD is in the production

of acrylonitrile butadiene styrene and styrene butadiene rubber,

which is used for manufacturing automotive tires. Due to the con-

tinuous growth of the global economy, especially in China and

India, the global automotive tire market is also expected to glow

consistently and to reach about USD 180 billion by 2017. This

potential attracts many investors and researchers to the produc-

tion of BD   [2]. At present, the increasing rate of demand for BD

has created a serious deficit in BD production by steam cracking

of naphtha due to the depletion of petroleum, high price of oil,

and environmental issues. These conditions have motivated a push

to develop alternative technologies for BD production from renew-

able, and non-petroleum resources   [1]. In addition, long-term

shortages of C4  chemicals are expected to be the outcome of the

availability of huge quantities of natural gas caused by recent

American shale-gas revolution. This has driven a shift to lighter,

gas-based feed-stocks; away from heavier, oil-based feed-stocks,

in the petrochemical industry [3]. This shift to lighter feed-stocks

has resulted in a significant reduction of BD production because

BD is now primarily produced as a byproduct of ethylene cracking

of the oil-based feed-stocks [4]. Ethanol is the most abundant bio-

based, sustainable resource because industrial ethanol is mainly

produced via fermentation of biological-feed-stocks such as sugar

and corn [1,5]. The production of ethanol as a biofuel was greater

than 100 billion liters in 2011 and expected to increase   3–7%

annually in the years from 2012 to 2015   [6]. As the bioethanol

market grows rapidly, the manufacture of bio-ethanol-based

http://dx.doi.org/10.1016/j.cej.2014.09.110

1385-8947/  2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +82 42 860 7257; fax: +82 42 860 7508

(T.-W. Kim). Tel.: +82 42 860 7504; fax: +82 42 860 7508 (C.-U. Kim).

E-mail addresses: twkim@krict.re.kr (T.-W. Kim), cukim@krict.re.kr (C.-U. Kim).

Chemical Engineering Journal 278 (2015) 217–223

Contents lists available at   ScienceDirect

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building blocks for the chemical industry has attracted a lot of 

attention. Therefore, if technological advances can be achieved in

the near future, BD production using bioethanol would be the most

promising, sustainable and renewable technology among various

on-purpose BD-production technologies [7].

The catalytic conversion of ethanol into BD (ETB) is a well-

known industrial process. The ETB process has been used from

the 1920s, but was scrapped due to lower oil prices enabled byincreasing oil production in early 1960s [1,5,8–14]. However, the

ETB process is now again becoming attractive as a potential alter-

native due to high oil prices and the generous supply of bioethanol

[1,5,7,15,16]. ETB processes are mainly divided into two kinds.

Lebedev’s process has only one step, and Ostromyslensky’s process

has two [1,5,8–15]. The latter was commercialized by the Carbide

and Carbon Chemicals Corporation in the United States   [17–22],

and involves two series of reactions (ethanol (EtOH) dehydro-

genation to acetaldehyde (AA), and then EtOH-AA condensation

to BD) in a fixed-bed system. Silica supported tantalum oxide

(Ta2O5/SiO2) is the best-known catalyst for the second step of the

ETB process [17–22].

However, both ETB processes have short catalyst-regeneration

cycles (12 h for the one-step process, and 120 h for the two-step

process) because of the rapid deactivation of the catalyst by coke

formation [17–22]. In order to continuously remove the coke from

the catalyst, a circulating fluidized bed (CFB) reactor could be

applied to the ETB reaction. The CFB catalytic reactors were first

tried to develop fluid-catalytic-cracking (FCC) technology for the

production of gasoline from the oil to replace old bubbling-bed

reactors [23]. Since the 1960s, CFB reactors have been applied to

various chemical processes such as gasification and combustion

of methane, biomass and coal  [24,25], FCC, Fischer–Tropsch syn-

thesis, and methanol-to-olefin (MTO). The CFB reactor may easily

control the heat generated by exothermic reactions, enhance mass

transfer in a gas–solid reaction system, and provide continuous

regeneration of the coke-choked catalyst  [26]. However, the cata-

lyst used in the CFB reactor need be resistant both to severe heat

and to mechanical stresses due to operating conditions thatinclude high temperature and high velocity flow. A catalyst with

low attrition resistance could cause many problems in the CFB sys-

tem and could eventually shut down the overall process. Therefore,

it is essential that a spherical Ta2O5/SiO2  catalyst with both good

catalytic performance and significant mechanical strength be pro-

vided as the ETB catalyst in CFB reactors.

In this study, we report for the first time spherical-shaped tan-

talum–silica based catalysts in order to apply CFB reactor for the

continuous remove of coke in ETB reaction. The catalytic perfor-mance and mechanical strength of the Ta2O5/SiO2  spherical cata-

lysts were examined using a fixed-bed reactor and an attrition

test, respectively. After selection of the optimum spherical catalyst,

having both high catalytic activity and attrition resistance, we

evaluated the hydrodynamic properties of the optimum catalyst

using a cold-bed, circulating fluidized bed reactor.

2. Materials and methods

 2.1. Preparation of Ta 2O5  supported spherical silica catalysts

Commercial silica spheres (CARiACT Q-n, where   n   stands for

the pore size of the silica) were obtained from Fuji Silysia. All

Fig. 1.   CFB reactor apparatus for 1,3-butadiene production from ethanol and

acetaldehyde (1: riser, 2: stripper, 3: regenerator, 4: stripper slide valve, 5:

regenerator slide valve, 6: stripper transfer line, 7: regenerator transfer line, 8: flue

gas control valve, 9: product gas control valve, 10: regenerator air inlet, 11: feed

inlet, 12: riser nitrogen inlet, 13: stripper nitrogen inlet, 14: flue gas outlet, 15:product gas outlet).   Fig. 2.   Nitrogen sorption isotherms and pore size distributions of the catalysts.

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of the spherical silica-supported 2 wt% tantalum oxide (Ta2O5)

catalysts were prepared by an impregnation method using etha-

nol as a solvent. A measure (100 g) of spherical silica support

was added to 1000 ml of ethanol, which also contained 32 g of 

tantalum pentachloride (TaCl5, Aldrich). After stirring for 2 h,

the ethanol was removed using a rotary evaporator and the sam-

ples were dried at 120  C for 10 h. The CARiACT Q-n-supported

Ta2O5   samples (Ta2O5/Q-n) were obtained by calcination at500 C for 5 h.

 2.2. Characterization

Nitrogen adsorption isotherms were measured at  196  C on a

Micromeritics Tristar 3000 volumetric adsorption analyzer. Before

the adsorption measurements, all samples were outgassed at

300 C in a degassing station. The Brunauer–Emmett–Teller (BET)

equation was used to calculate the apparent surface area from

the adsorption data obtained at   P /P 0   between 0.05 and 0.2. The

total volume of micro- and mesopores was calculated from the

amount of nitrogen adsorbed at  P /P 0 = 0.95, assuming that adsorp-

tion on the external surface was negligible compared to adsorption

in the pores. The pore size distributions were calculated by analyz-

ing the desorption branch of the N2  sorption isotherm using the

Barret–Joyner–Halenda (BJH) method. Scanning electron micro-

scope (SEM) images were obtained with a Philips XL-30S fieldemission guns (FEG) SEM operated at 10 kV. The samples were

coated with gold before SEM measurement. The attrition test of 

the spherical catalyst was determined using the ASTM D5757-95

method. After 50 g of the catalyst was fluidized by air at 10 l/min

for 5 h, the amount of entrained catalyst collected in the thimble

filter was measured. The attrition rate is defined as the ratio of 

the amount of entrained catalyst to the total amount of catalyst

(50 g) used [26]. The bulk density of the catalyst was determined

by measuring the mass and volume of a catalyst sample using a

balance and a graduated cylinder, respectively.

 2.3. Catalytic test in a fixed-bed reactor 

The production of 1,3-butadiene (BD) from ethanol and acetal-

dehyde was performed in a fixed bed reactor system with a 3/8

inch stainless steel (SUS) tube reactor  [7]. The reaction tempera-

ture was controlled by a type-K thermocouple (Omega) and a PID

controller. A mixture of ethanol (99.9 wt%, Samchun) and acetalde-

hyde (85 wt%, Aldrich) with ethanol, to provide an acetaldehyde

molar ratio of 2.5   [17,21], was fed into the catalytic reactor at

0.66 ml/h by a high-performance liquid chromatography (HPLC)

pump. The catalyst (0.25 g) was loaded in the middle of the SUS

tube. Before the reaction, the catalyst was warmed up to thereaction temperature (350 C, heating rate = 5.0 C/min) with a

5 ml/min flow of the carrier gas (N 2). The reaction was then per-

formed with a liquid hourly space velocity (LHSV) of 1.0 h1 at

350  C. The effluent gas products were measured using a gas chro-

matograph (6100GC, Young Lin Instrument Co.) equipped with a

flame ionization detector (FID). Products were detected by the

FID using a capillary HP Plot Q column (0.53 mm id 40 micron

thickness 30 m length). Total conversions and BD selectivities

were calculated using the following equations.

 2.4. Catalyst circulation test in the CFB reactor 

A schematic diagram of the pilot CFB plant (riser length: 4 m,

riser inner diameter: 14.6 mm) used in the experiments appears

in Fig. 1. Hydrodynamic properties such as the catalyst circulation

rate were examined by means of a cold-bed CFB test (without

heating). First, 2.6 kg of the optimum catalyst was prepared as in

Section   2.1   and loaded into the CFB reactor. The cold-bed test

was performed under 25 psig of N2 gas pressure in a CFB pilot plant

with a mechanical slide valve.

3. Results and discussion

 3.1. Properties of the Ta 2O5 /Q-n catalysts

Fig. 2 shows the nitrogen physisorption isotherms and pore size

distributions for the three different Ta2O5/Q-n samples. The capil-

lary condensation step increased and the amount of N2 adsorption

below P /P 0 = 0.2 decreased with an increase in the pore diameter.

This indicates that the number of mesopores in the Ta 2O5/Q-n sam-ple increased, while the number of micropores decreased, with

increasing of pore size. The pore size distribution curves for the

 Table 1

Physical properties of catalysts.

Samples Ta (wt%)a S BET  (m2 g1)b V t  (cm

3 g1)c wavg   (nm)d wBJH  (nm)

e Dbulk (g/ml)f  Attrition test (%)

Ta2O5/Q-3 1.84 688 0.37 2.2 2.0 0.77 0.30

Ta2O5/Q-6 1.90 530 0.58 5.9 7.3 0.53 0.21

Ta2O5/Q-10 1.91 214 1.15 13.3 16.6 0.38 2.93

a Ta, Ta loading measured by ICP-AES.b S BET, apparent BET specific surface area.c V t, total pore volume.d wavg, the average pore size.e

wBJH, the pore size calculated using the BJH method and deduced from the highest point of pore size distribution curve.f  Dbulk, bulk density.

Total conversion ¼ ðTotal C moles ðC moleunreacted EtOH þ C moleunreacted AAÞÞ

Total C moles  100

BD selectivity ¼

  C moleBD in productsTotal C moles in products except for EtOH and AA 100

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Ta2O5/Q-n samples also show an increase in the mesopore volume

with increasing pore diameter. The detailed structural properties of 

the Ta2O5/Q-n  samples are summarized in   Table 1. The average

pore sizes of the Ta2O5/Q-n  catalysts are shown to range from 2.2

to 13.3 nm. The pore sizes of Ta 2O5/Q-3 and Ta2O5/Q-6 calculated

by the desorption branch of the isotherm using the BJH method,

were larger than the average pore sizes due to overestimation by

the BJH method of the range of large mesopores  [27]. The specificsurface area gradually decreased with an increase in pore diameter,

while the pore volumes significantly increased from 0.37 to

1.15 cm3/g. This enlargement of pore volume was mostly caused

by the increase in the mesopore volume of the Ta2O5/Q-n catalysts.

The bulk density and mechanical strength of the Ta2O5/Q-n cat-

alysts examined by attrition test are shown in  Table 1. The Ta2O5/

Q-6 catalyst exhibited the highest attrition resistance (0.21%),

while Ta2O5/Q-10 showed the lowest attrition (2.93%). With an

increase of pore size, the surface area and bulk density decreased.This indicates that larger pore size might lead to a mechanically

Fig. 3.   SEM images of catalysts: (a) Ta2O5/Q-3, (b) Ta2O5/Q-5, and (c) Ta2O5/Q-10.

Fig. 4.   EtOH/AA total conversion (open symbol) and BD selectivity (solid symbol) of Ta 2O5/Q-n  catalysts (circle: Ta 2O5/Q-3, square: Ta 2O5/Q-6, triangle: Ta 2O5/Q-10).

 Table 2

Catalytic performance of Ta2O5  supported Q-n catalysts in a fixed bed reactor at 350  C, LHSV of 1 h1 at 30 h.

Sample EtOH/AA conv. (%) Carbon selectivity (C mol%)

Ethylene Propylene Butene isomers 1,3-BD Ethoxy ethane Ethyl acetate Crotonaldehyde Acetic acid Others*

Ta2O5/Q-3 10.9 4.6 1.1 0.7 47.8 4.9 3.4 18.0 0.6 19.0

Ta2O5/Q-6 31.5 2.7 1.6 1.6 72.0 0.2 0.6 0.3 6.6 14.4

Ta2O5/Q-10 30.6 1.8 1.2 1.6 70.3 2.0 2.0 6.4 3.7 11.0

* Unidentified compounds mainly consisting of heavier compounds than acetic acid in GC chromatography.

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weak catalyst, resulting from a less dense silica framework given

similar-sized silica-support. Thus, the catalyst with the largest

pore size (Ta2O5/Q-10) showed the lowest attrition resistance.

SEM images of the tantalum-oxide-loaded spherical silica cata-

lysts are displayed in Fig. 3. All SEM images of the Ta2O5/Q-n  cat-

alysts show a spherical morphology with a particle size around

200–300 lm.

 3.2. Catalytic test for ETB in the fixed-bed reactor 

To investigate the effects of the mesopore size, and to deter-mine the optimum mesopore size, the catalytic activities of the cat-

alysts (particle size 200–300 lm) with different pore diameters

were examined for ETB reaction in the fixed-bed reactor.   Fig. 4

shows the total conversion of ethanol (EtOH) and acetaldehyde

(AA), and the 1,3-butadiene (BD) selectivity of three spherical sil-

ica-supported tantalum oxide catalysts with different pore diame-

ters. The conversions for all samples gradually decreased with

time-on-stream due to the formation of coke in the catalysts, while

the selectivities of BD were almost constant over the entire reac-

tion time. This clearly reveals that the CFB reactor needs to contin-

uously regenerate catalyst deactivated by accumulation of coke.

Among the series of Ta2O5/Q-n  catalysts, the total conversion and

the BD selectivity of the Ta2O5/Q-3 catalyst showed the lowest val-

ues among all catalysts due to its smaller pore size. The catalystswith pore size above 6 nm showed good conversion and BD selec-

tivity. The Ta2O5/Q-6 catalyst with the middle mesopore size (aver-

age pore size 5.9 nm) showed the highest catalytic performance

(total conversion = 31.5% and BD selectivity = 72.0%), which was

slightly higher than that of the Ta2O5/Q-10 catalyst (total conver-

sion = 30.6% and BD selectivity = 70.3%). As shown in  Table 1, the

mesopore size of the Ta2O5/Q-10 catalyst (13.3 nm) was over two

times larger than that of the Ta2O5/Q-6 catalyst (5.9 nm), whereas

the BET-specific surface area of the Ta2O5/Q-10 catalyst (214 m2/g)

was over two times less than that of the Ta2O5/Q-6 catalyst

(530 m2/g). From structural analyses of the catalysts and results

from the catalytic tests, the Ta2O5/Q-6 catalyst appears to be the

best candidate for ETB reaction because it has both a moderately

large mesopore size and a high surface area, which facilitate goodaccessibility of the reactants and products, as well as good disper-

sion of tantalum oxide within the silica  [7]. The detailed product

selectivities are shown in   Table 2, including unidentified com-

pounds. These were heavy compounds and would mainly cause

deactivation of the catalyst by creation of coke. The formation of 

heavy compounds was gradually reduced with increasing meso-

pore size of the catalysts. This indicates that a catalyst with large

mesopores forms lesser amounts of heavy compounds compared

with catalysts with small mesopores, due to better accessibility

and diffusion of the reactants and the products [7].

The optimum spherical silica-supported tantalum oxide cata-

lyst is Ta2O5/Q-6 because Ta2O5/Q-6 showed both good catalytic

performance and good mechanical strength, which are essentialfor the catalyst in a CFB reactor. To investigate the effect of the par-

ticle size of the Ta2O5/Q-6 catalyst for the ETB reaction, we pre-

pared four Ta2O5/Q-6 samples with different particle sizes

(Fig. 5): 75–150, 75–200, 200–300, and 300–500 lm. As shown

in   Fig. 6   and   Table 3, the conversions of particles less than

300 lm were 30.1–33.4% at a reaction time of 30 h, and the BD

selectivity increased with decreasing particle size of the catalyst.

Fig. 5.  SEM images of Ta2O5/Q-6 catalysts of different particle size: (a) 75–150 lm, (b) 75–200 lm, (c) 200–300 lm, and (d) 300–500 lm.

Fig. 6.  EtOH/AA total conversion (open symbol) and BD selectivity (solid symbol) of 

Ta 2O5/Q-6 catalysts of different particle size (circle: 75–150 lm, square: 75–200 lm, triangle: 200–300 lm, diamond: 300–500 lm).

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The smallest particle size, 75–150 lm, for the Ta2O5/Q-6 catalyst,

shows the best catalytic performance, while the largest particle

size, 300–500 lm, displayed the lowest catalytic activity. Small

particle sizes should facilitate the diffusion of reactants and prod-

ucts during the catalytic reaction, which is an effect similar to that

from large mesopores as discussed above [7]. In addition, the selec-

tivity of heavy compounds (others in  Table 3) in the particle size

range 300–500 lm was 20.0%, which is the largest amount among

the catalysts due to relatively hard diffusion of reactants and prod-

ucts from the catalyst with large particle size, compared with that

of those with small particle size.

 3.3. Cold-bed CFB test 

As discussed Sections 3.1 and 3.2, the optimum ETB catalyst for

a CFB reactor was Ta2O5/Q-6 with particle size 75–150lm. A 2.6 kg

portion of the optimum catalyst was prepared and loaded into the

circulating fluidized bed (CFB) reactor, as shown in Fig. 1. First, to

evaluate catalyst circulation in the CFB reactor, we performed a

cold-bed test without heating the overall CFB reactor prior to

ETB reaction. The cold-bed test in the CFB reactor is important

because we can determine the rate of catalyst circulation from it,

which is connected with the flow rate of the riser gas and the

degrees of opening of the slide valve. The catalyst circulation rate

(CCR) is controlled by two mechanical slide valves located in the

regenerator bottom (5) and stripper bottom (4), as shown in

Fig. 1. When the degree of opening for the slide valve in the bottom

of the regenerator (5), and the gas velocity in the riser (1) were

fixed, the CCR value could be kept constant by controlling the slide

valve in the bottom of the stripper (4) to keep a constant level of 

catalyst in the stripper (2). Therefore, the CCR could be controlled

by the degree of opening of the slide valve in the bottom of the

regenerator (5) alone, which was determined from the measure-

ment of differential pressure between the top and bottom of the

regenerator. A plot of the different pressure of regenerator versus

the catalyst weight loaded into the regenerator is shown in

Fig. 7. The differential pressures increase with the linear increase

in the catalyst weight (up to 1500 g) because the catalyst is filled

from the stand-pipe region of the regenerator, which has a con-

stant pipe diameter. Above 1500 g of catalyst weight, the slope of 

the different pressure per the catalyst weight decreased due to

 Table 3

Catalytic performance of Ta2O5  supported Q-6 catalysts with different particle size in a fixed bed reactor at 350  C, LHSV of 1 h1 at 30 h.

Particle size (lm) EtOH/AA conv. (%) Carbon selectivity (C mol%)

Ethylene Propylene Butene isomers 1,3-

BD

Ethoxy ethane Ethyl acetate Crotonaldehyde Acetic acid Others*

75–150 33.4 3.2 1.7 1.7 75.2 0.3 0.5 0.3 0.4 16.8

75–200 30.1 4.0 1.7 1.7 74.6 0.2 0.6 0.2 0.3 16.8

200–300 31.5 2.7 1.6 1.6 72.0 0.2 0.6 0.3 2.6 17.4

300–500 24.6 4.0 1.5 1.6 71.0 0.9 0.8 0.2 0.3 20.0

* Unidentified compounds mainly consisting of heavier compounds than acetic acid in GC chromatography.

Fig. 7.  Increase of differential pressure between top and bottom of the regenerator

with an increase in the amount of catalyst in the stand pipe region of theregenerator.

Fig. 8.  Decrease in the differential pressure of the regenerator with time for degree

of opening of the slide valve in the regenerator at 10 l/min of riser gas velocity.

Fig. 9.  Effect of riser gas flow rate and degree of opening of the slide valve on thecatalyst circulation rate (CCR).

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