Towards an ultimate fluidized bed stripper

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Powder Technology 158

Towards an ultimate fluidized bed stripper

Ian Rosea, Heping Cuib, Tianzhu Zhangb, Craig McKnighta, John Graceb,*, Xiaotao Bib, Jim Limb

aSyncrude Research, 9421-17 Avenue, Edmonton, Alberta, Canada T6N 1H4bDepartment of Chemical and Biological Engineering, University of British Columbia, Vancouver, Canada V6T 1Z4

Available online 25 May 2005

Abstract

Experiments were conducted in the geometrically- and dynamically-scaled UBC half-column to test different configurations that might

significantly reduce stripper shed fouling and increase run length, while providing little or no decrease in stripping efficiency in two

commercial fluid cokers. The results showed that a series of horizontal gas jets, without baffles, constrained to use no more steam than in the

existing commercial operations, were unable to fully match the stripping efficiency of the strippers with shed internals. Wall baffles were also

of little assistance. However, relatively large crossed-sheds, called ‘‘mega-sheds,’’ combined with a limited number of staggered gas jets,

provide a promising geometry, with more tolerance to fouling and stripping efficiency equivalent to that of the existing commercial units.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Fluidization; Stripper; Fluid coker; Stripping efficiency; Jets; Baffles

1. Introduction

Fluid cokers are employed in the petroleum industry for

thermal conversion of heavy hydrocarbon molecules into

distillate products [1]. The thermal cracking process is

carried out in large fluidized beds where hot solid particles,

introduced in the freeboard region, provide the heat required

for the endothermic cracking reactions and collect solid by-

products on their surfaces. Hydrocarbon feed is injected

through a number of rows of horizontal nozzles in the feed

section. Vaporized reaction products rise through the bed,

counter-current to coke particles, which travel downward

through a ‘‘stripper’’ section in which steam is employed to

remove interstitial hydrocarbon gases. This stripping,

combined with additional solids residence time to further

react any residual surface liquid, minimizes carry-under of

the valuable hydrocarbon product to the burner. Fluid cokers

operate mostly in the turbulent fluidization regime, with net

upflow of the gas and counter-current net downward flow of

the solid particles. The vapour products tend to rise

primarily in the centre of the bed, surrounded by dense,

0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.powtec.2005.04.034

* Corresponding author. Tel.: +1 604 822 3121; fax: +1 604 822 6003.

E-mail address: [email protected] (J. Grace).

descending particles. After passing through the stripper and

a standpipe at the bottom, the coke particles are circulated to

a separate fluidized bed burner for re-heating, prior to re-

entering the fluid coker at an increased temperature to

supply heat to support the endothermic thermal cracking

reactions which take place in the fluid coker.

Strippers are also widely used in fluid catalytic cracking

(FCC) units [2,3]. In both FCC and fluid coker strippers,

horizontal or inclined baffles are employed to increase gas–

solid contacting, to prevent short-circuiting of solids, to

reduce gas bypassing via bubbles at the centre of the column

and to help distribute the gas radially, reducing axial

dispersion and thereby increasing stripping efficiency.

Fouling of the top rows of stripper baffles and of the

standpipe entrance is a major and persistent issue for

commercial fluid cokers, leading to flooding [4], reduced

run length, time-consuming and expensive clean-up during

shutdowns, and constraints on operation. Some success has

been achieved in reducing fouling in recent years by

modifying Syncrude’s commercial fluid cokers based on

findings [5] from the same geometrically- and dynamically-

scaled cold model column as was used for the work

presented in this paper.

Despite the significant improvements that have been

achieved, further steps are needed if the commercial fluid

(2005) 124 – 132

I. Rose et al. / Powder Technology 158 (2005) 124–132 125

cokers are to be able to meet the objectives of reliable and

aggressive longer run length without interruptions due to

flooding and standpipe entrance reversals, while maintain-

ing, or even increasing, throughputs. Hence work has been

undertaken in search of an ‘‘ultimate stripper’’ that would

accomplish these objectives. In this quest, a number of

possible configurations, from strippers with all baffles

replaced by jets to those combining jets and baffles, were

tested and evaluated in an effort to achieve a configuration

with stripping efficiency comparable to the original com-

mercial stripper design, without requiring more total steam,

but with significantly enhanced tolerance to fouling. This

paper reports on the geometries tested and on what has been

accomplished.

2. Experimental

2.1. Equipment and operating conditions

A schematic of the semi-circular Plexiglas geometrically-

scaled fluid coker cold flow model is shown in Fig. 1. All

dimensions were approximately an order of magnitude

smaller than the corresponding dimensions of Syncrude’s

two fluid cokers. Dynamic similarity was achieved in the

stripper section by matching all important dimensionless

groups, based on dimensional analysis [6]. For more details

see Knapper et al. [7].

FLOOR

464

1614

2400

921

1200

1676

Riser

Impingeme

Venturi

(a)

495

305

840

ID = 203

Fig. 1. Overall schematic of semi-circular cold model fluid coker: (a

The riser has a diameter of 0.2 m, while the stripper is

0.61 m in diameter. FCC particles of mean diameter 76 Amand density 1700 kg/m3 were used in the experiments.

Pressurized air was introduced through numerous nozzles

and spargers located in the stripping and feed sections to

fluidize the particles. The flow through each nozzle was

based on that nozzle’s contribution to the total volumetric

flow of gas in the commercial reactors, so that each nozzle

contributed the same fraction of the total flow as its

counterpart in Syncrude’s fluid cokers. With the unit

operating at steady state, the net flow of particles was

downward through the feed section and the stripper. Solids

left the column through a standpipe whose top entry point

was located asymmetrically just below the bottom row of

stripper baffles, with the overall solids circulation rate

controlled by a pinch valve near the bottom of the standpipe.

The solids were then carried pneumatically through a J-bend

and then up through a riser, above which they passed

through a venturi constriction and then impinged on a

curved separation baffle, acting as a low-pressure-drop gas–

solid separator, which removed most of the particles from

the air stream, facilitating their return to the top of the dense

bed in the coker unit. Air from both the riser and the main

reactor, as well as entrained particles, passed through six

primary parallel cyclones after leaving the top of the coker.

Most of the remaining entrained particles were separated by

the cyclones and returned below the surface of the dense bed

through diplegs. A secondary cyclone and two parallel bag

Primary Cyclones

To Secondary Cycloneand Filter Baghouse

Return Diplegs

Standpipe

nt Box

Feed Section

Stripping Section(Containing Baffles)

Transfer Line

(b)

) side view; (b) front view. All dimensions are in millimeters.

Row 2

Row 4

Row 6

Standpipe entrance

Row 3

Row 5

Row 1

Row 8

Row 7

Right side Left side

Row 8

Row 7

Row 6

Row 5

Row 4

Row 3

Row 2

Row 1

Fig. 3. Positions of horizontal jets in cold model column.

I. Rose et al. / Powder Technology 158 (2005) 124–132126

filter houses captured any particles remaining in the exit air

stream before it was vented to the atmosphere.

The solids circulation rate through the system was

determined from the pressure drop across the venturi

constriction at the top of the riser after calibration by

concurrently measuring the solids circulation rate in the

standpipe using a fibre optical probe to measure the solids

concentration and velocity simultaneously [8]. The super-

ficial stripping gas velocity and solids circulation rate were

varied to study their effects on stripping efficiency as

reported below. Except where otherwise specified, the base

conditions (solids circulation rate of 6.67 kg/s, superficial

stripping gas velocity of 0.24 m/s) were used, corresponding

to dynamically-scaled normal operation of the commercial

fluid cokers.

2.2. Stripper configurations

2.2.1. Original commercial design

The original commercial stripper section was equipped

with 8 staggered rows of ‘‘sheds’’ (90- included top angle)

(Fig. 2). The total open area fraction of baffles was 50%.

The vertical distance between the bottom of one row and the

top of the row below in our work was 50 mm. A row of

attrition nozzles is located at the bottom of the feed section.

Six spargers were installed under the lowest two rows of

sheds to provide uniform introduction of air (simulating

steam in the commercial units). In addition, two rows of

stripping nozzles were located at the bottom of the stripper.

The bottom stripping nozzles were also used for all of the

new configurations investigated below. More information

about the original stripper geometry is provided elsewhere

[5].

Row 8

Row 7

Row 6

Row 5

Row 4

Row 3

Row 2 - Spargers

Row 1 - Spargers

Attrition Jets

Standpipe entrance

Fig. 2. Configuration of original commercial stripper. All 8 rows of sheds

have 90- top-included-angles. All dimensions are in millimeters ([5]).

2.2.2. Empty stripper

A stripper without solid internals was first examined in

the cold flow model. All sheds were removed from the

original stripper which matched the commercial design.

Attrition nozzles and spargers were not used. Stripping gas

was added to the stripper only through the stripping nozzles

at the bottom of the stripper. Different gas flow rates were

tested to study the influence of the steam flow rate on

stripping efficiency.

2.2.3. Stripping with gas jets and no baffles

The semi-circular cold flow model was a good system

for screening alternative configurations due to the flexi-

bility of its design. The possibility of using gas jets without

sheds to promote stripping was first explored. Although a

stripper without solid internals would clearly be much less

likely to suffer from fouling problems, its ability to achieve

good stripping efficiency was in doubt. Stripping efficiency

is expected to benefit from uniform residence times of the

particles and effective mass transfer between gas and

particles. These two factors should be promoted by

uniformly distributed, relatively steady solids downward

flow and small bubbles uniformly distributed over the

cross-section.

Mindful of these issues, jets were provided at different

positions along the length of the stripper, as shown in Fig. 3,

with 8 rows of nozzles of 4.6 mm I.D. In order to test the

influence of jet parameters on stripping efficiency, different

configurations were utilized. When certain jets were not in

operation, the corresponding nozzles were retracted to be

flush with the wall of the stripper. The active nozzles were

inserted 70 mm from the column wall for most tests. The

effects of opposing jets versus staggered (i.e. vertically

Table 1

Alternative stripper configurations investigated experimentally in UBC cold model column without baffles in place

Attrition jets on/off Active jets, left side Active jets, right side Jets staggered? Nozzle insertion, mm

On 2 in row 1, 1 in row 2 2 in row 1, 1 in row 2 No 70

On 2 in row 2, 3 in row 4 2 in row 1, 2 in row 3 Yes 70

On 3 in row 3, 3 in row 5 2 in row 2, 2 in row 4 Yes 70

Off 2 in row 1, 3 in row 3, 3 in row 5, 3 in row 7 2 in row 2, 2 in row 4, 2 in row 6 Yes 70



See Fig. 3 for locations of rows of nozzles.


offset) jets, and jets entering the column close to the wall

versus nearer the centreline were examined. The jet

configurations investigated are summarized in Table 1.

Nozzles not used during any test were retracted to the inside

wall of the column and isolated by closing the correspond-

ing valve.

2.2.4. Strippers with internals combined with gas jets

Strippers with internal baffles in combination with

horizontal jets were also tested. The internals were of two

types—wall baffles and so-called ‘‘mega-sheds.’’ Wall

baffles were sloping conical inserts attached to the inner

wall of the column around the entire periphery and sloping

downwards at an angle of 45- to the horizontal. The mega-

sheds were crossed-sheds, each with an angle of 45- to the

horizontal. In plan view (see Fig. 4), these were either like

the upper half of a plus sign (+) or the upper half of an X for

the half-column. For ease of reference, they are called the

Fig. 4. Plan view of mega-sheds: (a) ‘‘+’’ upper mega-shed; (b) ‘‘X’’ lower

mega-shed.

‘‘+-mega-shed’’ and ‘‘X-mega-shed’’, respectively. A front

view is shown in Fig. 5. Both the wall baffles and mega-

sheds were designed to provide open areas of 50% of the

column cross-section, as for the rows of sheds in the original

commercial design. All attrition nozzles and spargers were

removed from the original geometry of commercial design.

The baffles were combined with 18 stripping jets (9 on each

side), using a total air flow very close to the total flow rate

from the attrition nozzles and spargers when the commercial

units were dynamically matched to the cold model. The

above constraint means that there would be no additional

operating cost for more steam if a new configuration of

favourable stripping efficiency could be found. The

insertion depth of all nozzles was 70 mm from the column

wall, the same as the intrusion of the wall baffles. Most

configurations tested are summarized in Table 2.

To determine whether the existing spargers could still be

utilized in place of some of the jets, an alternative

configuration was tested, with two spargers under the

bottom X-mega-shed replacing 6 jets from the outer wall.

The total air flow rate of jets and spargers was maintained

constant throughout all tests with combined baffles and

jets.

Row 1 - Jet x 1

Row 5 - Jet x 2 Row 5- Jet x 2

Row 6 - Jet x 2

Standpipe entrance

Row 3- Jet x 2

Row 1- Jet x 1

Right sideLeft side

Row 2- Jet x 2

+ -MS

X-MS

WB

Row 4- Jet x 2

Row 7- Jet x 2Row7- Jet x 2

Fig. 5. Stripper configuration with two mega-sheds (MS) (one +, one X),

two wall baffles (WB), and 18 horizontal wall jets.

Table 2

Configurations investigated involving combinations of baffles and jets

Number of wall baffles Configuration of mega-sheds Number of jets

3 3 (two +; one X) 18

0 3 (two +; one X) 18

3 None 18

2 2 (one +. One X) 18

0 2 (one +. One X) 18


2.3. Measurement system

Fig. 6 shows the general set-up for the gas tracer

experiments. The injection system was designed to intro-

duce tracer (helium) into the feed section through the

bottom feed nozzles to simulate feed hydrocarbon gas

introduction in the commercial reactors. The helium tracer

was injected continuously at a constant flow rate.

The gas-sampling probe was positioned in the standpipe

below the stripper. The helium sampling system consisted

of a 6.3 mm diameter tube with a 15 Am sintered metal

filter tip, leading to a thermal conductivity detector (TCD).

The signal from the TCD detector passed through an

amplifier into the data acquisition system. LABTECH

Notebook software recorded the TCD signal at a sampling

frequency of 5–10 Hz for periods of 5 min for each

determination and also triggered the solenoid valve. A needle

valve was situated upstream of the detector to dampen signal

fluctuations.

The helium concentration in the standpipe detected by

the TCD sampling system allows entrainment of hydro-

carbon into the standpipe to be estimated, thereby providing

an estimate of stripping efficiency, defined as (1� tracer

underflow/ tracer injection rate) [9]. All measurements were

performed in triplicate, and average values are plotted on the

ordinates of the figures below. The average standard

deviation in the stripping efficiency in replicate determi-

nations was only 0.0096%, establishing that reproducibility

was excellent. The resulting error bars are so small that they

A Reactor

B

PG

SolenoidValve

Trigger Signal

Helium Tracer

Needle Valve

TCD

Stripper

Standpipe

Fig. 6. Set-up and sampling system

have not been included in the figures plotted below. In view

of the small experimental scatter, our data allow real

differentiation among the geometries investigated in terms

of stripping efficiencies. However, it is recognized that

helium is a non-adsorbing tracer, and that the FCC particles

are porous; whereas some hydrocarbon is adsorbed and

carried on the outer surface of the non-porous coke particles

in real fluid cokers. Hence the stripping efficiencies

determined in our cold model are unlikely to give accurate

quantitative measurement of stripping efficiency in the

commercial system, but they should provide a good

indication of the influence of operating conditions and of

the relative merits of different stripper baffle and jet

configurations.

3. Results and discussion

3.1. Quantification of stripper efficiency

Tests were carried out to obtain axial helium concen-

tration profiles through the stripper when helium was

injected through the feed nozzles. The aim was to determine

how much feed tracer descends through the standpipe, and

hence to estimate the ‘‘stripper efficiency’’. Because of

pronounced local gradients and fluctuations in tracer gas

concentration, more consistent and meaningful results were

obtained by determining an overall ‘‘stripping efficiency’’

based on helium detected across the standpipe from:

gStripper ¼ 1� QHe�stp

QHe�tot

ð1Þ

where QHe-tot is the total flow rate of helium injected

through the feed nozzles, m3/s; QHe-stp is the flow rate of

helium down the standpipe in cubic meters per second,

calculated from the helium concentration obtained by the

TCD and the total gas carry-under in the standpipe. The

latter was obtained from a simple correlation involving the

Amplifier

Sample Stream

Reference Stream

Vacuum

Data AcquisitionSystem

for gas tracer experiments.


solids circulation rate, Q =0.000265ms, as detailed else-

where [9].

3.2. Empty stripper

The cold flow unit was first operated without solids

circulation at a low air flow rate. The solids were initially

fluidized at the top of the unit, then stepwise at lower levels,

with the flow of air (representing stripping steam) into the

cone turned on last. Gas bubbles appeared to be well

distributed across the column. There was some coalescence

and movement to the centre of the vessel as the bubbles

rose. Downward solids flow at the wall was, at times,

disrupted by upward bubble flow. With increasing airflow,

solids circulation was initiated, and the bubbles tended to

migrate increasingly into the core of the stripper. The

standpipe entrance began to operate with gas bubbles

disengaging from its top. Solids downward flow at the wall

was much more pronounced during this operation and rarely

disrupted by bubbles. However, at higher stripping gas flow

rates, standpipe entrance reversal occurred, i.e. the standpipe

entrance filled with gas, leading to unstable operation.

As shown in Fig. 7, the stripping efficiency for this

configuration was relatively poor (98.8–99.4%, i.e. 0.6–

1.2% of the injected helium left through the standpipe).

Observations suggest that massive solids downward flow at

the wall caused this relatively poor stripping efficiency. In

effect, there was limited contacting of the stripping gas in

this zone with the emulsion gas entrained downwards by the

descending solids. As shown in Fig. 7, the stripping

efficiency decreased with decreasing stripping gas flow

rate. The empty stripper led to a considerable reduction in

stripping efficiency due to poor interaction between gas and

solids, compared to the original commercial stripper design.

Thus, a stripper without internals was rejected as an option.

98.8%

99.0%

99.2%

99.4%

99.6%

99.8%

100.0%

6 7 8 9 10

Solids circulation flux, kg/s

Str

ipp

ing

Eff

icie

ncy

(%)

Fig. 7. Stripping efficiencies of original commercial stri

3.3. Gas-jet stripper

As shown in Fig. 8, opposing jets led to relatively low

stripping efficiencies. In this case jets opposed each other

and probably directed gas into a developing gas-rich core.

Large bubbles were then formed which rose rapidly up the

centre of the column. Solids rained down the walls,

bypassing the gas flow, leading to high carry-under, spikes

in the TCD signals, and compromised stripper efficiency.

Staggered jets improved this situation considerably. While

significant gas bypassing remained in the core, the gas was

better distributed, improving the stripping efficiency.

However, when the levels of staggered jets were raised,

the stripping efficiency declined to that of the opposing

jets, probably due to the reduction in gas/solids contacting

time.

Adding staggered jets in the upper stripper at the expense

of the normal attrition jets in the commercial geometry did

very little to help stripping efficiency. This is consistent with

previous results where raising the levels of the staggered jets

reduced stripping efficiency. When the same nozzles were

either retracted to the wall or inserted as far as possible (to

340 mm from the inner surface of the wall), the stripping

efficiency improved significantly, presumably due to better

gas/solid contacting.

In summary, the locations and arrangement of jets have

major impacts on the stripping efficiency. Staggered jets

provided better gas distribution and stripping efficiency than

opposed jets. Jets at lower levels contribute more to

improvement of stripping efficiency than jets at higher

levels. However, strippers utilizing no more total steam than

in the original commercial design were unable to match the

stripping efficiency of the original design with baffles.

These lessons were incorporated into alternative designs

(see below), which include internals.

11 12

Original CommercialDesign, Qatt' = 0.0046,Qsp'= 0.0030,Qbst'=0.0047,Us' = 0.75

Empty Stripper,Qatt'=0, Qsp'=0, Qbst' = 0.0047,Us' =0.33

Empty Stripper,Qatt'=0, Qsp'=0, Qbst' = 0.0046,Us' =0.72

Empty Stripper,Qatt'=0.0045,Qsp'=0, Qbst'= 0.0049,Us' = 0.75

pper design and empty stripper without internals.

98.8%

99.0%

99.2%

99.4%

99.6%

99.8%

100.0%

6 7 8 9 10 11 12


Str

ipp

ing

eff

icie

ncy

(%)

Original Commercial Design, Qatt'= 0.0046, Qsp'= 0.0030,Us' = 0.75

a) Opposing Sparger Jets (3L,3R),Qatt' = 0.0047, Qjet' = 0.0030,Us' = 0.75

b) Staggered Jets (5L,4R),Qatt' = 0.0047, Qjet'= 0.0046,Us' = 1.05

c) Higher Staggered Jets (6L,4R),Qatt' = 0.0047, Qjet' = 0.0050,Us' = 1.08

d) Full Staggered Jets (11L,6R), Qatt' = 0, Qjet' = 0.0087 (jetinsertion: 70 mm), Us' = 1.61

e) Full Staggered Jets (11L,6R),Qatt' = 0, Qjet' = 0.0087 (jetinsertion: 6 mm), Us' = 1.61

f) Full Staggered Jets (11L,6R), Qatt' = 0, Qjet' = 0.0087 (jetinsertion: 340 mm), Us' = 1.61

Fig. 8. Comparison of stripping efficiencies of several gas-jet strippers with original commercial stripper design (L: left side jets; R: right side jets).

QbstV =0.0047.


3.4. Stripper with internal baffles combined with jets

A stripper without jets, equipped with only internals

(mega-sheds and wall baffles), was investigated to deter-

mine whether this configuration could provide the same

stripping efficiencies as the original commercial design.

Experimental results showed that when there were no jets,

all mega-shed/wall baffles configurations tested gave

relatively low stripping efficiencies (see Fig. 9). It appears

that without added gas flow, it was not possible to

adequately prevent gas from being entrained by the

98.8%

99.0%

99.2%

99.4%

99.6%

99.8%

100.0%

6 7 8 9 10


Str

ipp

ing

eff

icie

ncy

(%)

Fig. 9. Stripping efficiencies of several strippers with internal baffles only (no jets)

and QspV =0 for configurations a to e.

descending solids. Hence combined internals and jets were

next investigated.

As shown in Fig. 10, strippers with mega-sheds and wall

baffle internals, combined with jets, attained high stripping

efficiencies, very close to those of the original commercial

design, suggesting that such configurations may have

potential. However, Figs. 9 and 10 suggest that wall baffles

contributed little to improving the stripping efficiency. A

stripper equipped with two mega-sheds as the only baffles

achieved essentially the same stripping efficiencies as one

with three mega-sheds and three wall baffles, another with

11 12

Original Commercial Design, Qatt'= 0.0046,Qsp'= 0.0030, Us' = 0.75

a) 3 Mega-Sheds & 3 Wall Baffles,Qjet' = 0, Us' = 0.33

b) 3 Mega-Sheds, Qjet' = 0, Us' = 0.33

c) 3 Wall Baffles,Qjet' = 0, Us' = 0.33

d) 2 Mega-Sheds & 2 Wall Baffles,Qjet' = 0, Us' = 0.33

e) 2 Mega Sheds,Qjet' = 0, Us' = 0.33

, compared with original commercial stripper design. QbstV =0.0047; QattV =0

98.8%

99.0%

99.2%

99.4%

99.6%

99.8%

100.0%

6 7 8 9 10 11 12


Str

ipp

ing

eff

icie

ncy

(%)

Original Commercial Design, Qatt'= 0.0046,Qsp'= 0.0030, Us' = 0.75

a) 3 Mega-Sheds & 3 Wall Baffles,18 jets, Qjet' = 0.0092,Us' =1.64

b) 3 Mega-Sheds, 18 jets, Qjet' = 0.0092,Us' = 1.64

c) 3 Wall Baffles,18 jets, Qjet' = 0.0092,Us' = 1.64

d) 2 Mega-Sheds & 2 Wall Baffles,18 jets, Qjet' = 0.0092,Us' = 1.64

e) 2 Mega-Sheds, 18 jets, Qjet' = 0.0092,Us' = 1.64

Fig. 10. Stripping efficiencies for several modified strippers (18 jets) compared with original commercial stripper design. QbstV =0.0047; QattV =0 and QspV =0 for

configurations a to e.


three mega-sheds, and a third with two mega-sheds and two

wall baffles. The same two mega-shed configuration was

better than the three wall baffle configuration. Therefore, the

configuration with two mega-sheds (one + and one X)

appeared to be a promising geometry for further develop-

ment. (Note that the ordinate scales in Figs. 10 and 11 cover

the same range as for Figs. 7-9 so that readers can compare

the stripping efficiencies readily).

To explore whether spargers should be retained, a two

mega-shed configuration was tested with two spargers under

98.8%

99.0%

99.2%

99.4%

99.6%

99.8%

100.0%

6 7 8 9 10

Solids circulation flux, kg

Str

ipp

ing

eff

icie

ncy

(%)

Fig. 11. Comparison of stripping efficiencies of modified strippe

the crossed branches of the bottom mega-shed, replacing six

jets. It was found that this configuration demonstrated

stripping efficiencies similar to those for both the original

commercial design and the two mega-sheds, as shown in

Fig. 11. Hence the best stripper configuration among those

considered may consist of two mega-sheds, spargers and

horizontal wall jets. Because of the large clearances in this

geometry, this configuration should be much more tolerant

of fouling than the current commercial geometry. Hence it

represents a significant potential improvement. Further work

11 12

/s

Original CommercialDesign, Qatt'= 0.0046,Qsp'= 0.0030,Us' = 0.75

2 Mega-Sheds,18 jets,Qatt'=0,Qjet' = 0.0092,Us' = 1.64

2 Mega-Sheds,12 jets, 2 spargers,Qatt'=0,Qjet'= 0.0061,Qsp'=0.0031,Us' = 1.64

rs with original commercial stripper design. QbstV =0.0047.


will focus on optimization of jets (number, diameter and

location) and mega-shed geometry (width, location, shape

and angle).

4. Conclusions

Different stripper configurations were tested in an effort

to find a geometry which provides stripping efficiencies at

least as good as the current commercial configuration,

without using any additional steam and with greater fouling

tolerance. An empty stripper showed low stripping efficien-

cies compared to the original commercial design. Neither

gas jets alone nor solid internals alone provided adequate

stripping efficiencies. A new crossed-shed (‘‘mega-shed’’)

design greatly contributed to promote stripping efficiencies,

while wall baffles provided little value. A combination of

two mega-sheds with gas jets/spargers attained the same

stripping efficiencies as the original commercial stripper

geometry, without requiring any more steam. Given its

much more open structure, this geometry appears to have

high potential to reduce the adverse effects of fouling,

prevent flooding and increase run length.

Nomenclature

ms Solids circulation rate, kg/s

Q Total volumetric air flow rate in standpipe, m3/s

QattV Total volumetric flow rate through attrition noz-

zles /QSC,–

QbstV Total volumetric flow rate through stripping

nozzles at bottom of stripper /QSC,–

QHe-stp Volumetric He flow rate in standpipe, m3/s

QHe-tot Total volumetric flow rate of injected helium tracer,

m3/s

QjetV Total volumetric flow rate of stripping jets into

stripper /QSC,–

QSC Representative industrial stripper gas flow rate, m3/s

QspV Total volumetric gas flow rate from spargers into

stripper /QSC,–

UsV Superficial gas velocity/representative industrial

superficial velocity in stripper,–

Greek letters

gStripper Stripping efficiency as determined by helium tracer

test,–

Acknowledgements

The authors are grateful to Syncrude Canada Limited for

sponsoring this work and for permission to publish the

results. We acknowledge assistance with experiments from

D. Appelt, D. Burgardt, S. Gillis, L. Hackman and C.

Schroeter. We also thank Chevron Canada Ltd. for provid-

ing the FCC particles.

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Towards an ultimate fluidized bed stripper

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