tailoring coker residence time distributions for improved ... 2005.pdf · tailoring coker residence...

Envision Technologies 1AIChE Spring Conference April 13, 2005Atlanta. Georgia

Tailoring Coker Residence Time Distributions for Improved Liquid Yields:

The Envision Technologies Cross-Flow Coker (ETX)

Robert Pinchuk†

W. Brown†, W. McCaffrey‡, O. Asprino‡, G. Monaghan†

†Envision Technologies Corp.‡ Department of Chemical Engineering and Materials Engineering , University of Alberta


Basic Mechanism of Liquid Product Formation in Coking

•Complete reaction of feed requires 30 -120 seconds, depending upon temperature

•Ideally products are removed within a couple of seconds

•Optimal residence times of the different phases differ by an order of magnitude

Feed(liquid)

Over-Cracked Gases

Gas Phase

Liquid Phase Liquid

Products(gas)

Coke(Solid)

Liquid Products

(gas)

(Product Vaporization)


How to optimize yield of liquid products

• Can improve liquid yields by matching optimal residence times– decouple the two phase so that each residence time can be set

independently

• Lower reaction temperatures will increase liquid yields

What configuration offers the greatest flexibility?


ETX CokerCross-Flow Fluidization

Fluidization gas

Cool solids

Liquid feed

Hot solids

Fluidization gasand reaction products

• Fluidized bed of hot solids provides the energy for the reaction– Solids flow through the reactor horizontally

• The feed is sprayed onto the solids near the solids feed point• Product is recovered along the length of the reactor with the fluidizing gas


Cross-Flow Fluidization

• Commercial fluidization technique most commonly used for drying

• Want to apply this commercial technology to heavy oil processing

• Often referred to as “plug-flow” fluidization because of the solids mixing pattern– this is a major advantage of this

reactor configuration


Perceived Benefits of the Design

1. Independent control over gas and liquid fluxes allows for optimized residence times

2. Plug-flow of the solids eliminates losses of feed due to short-circuiting and reduces the required reactor size

3. Shallow Bed reduces gas phase residence time and overcracking


Residence time distributions

• Cold flow experimental program used to study and confirm benefits of the design

• Process scaled down based on the dimensional analysis

• 0.75 m x 0.75 m square bed– larger than typical experimental bed– close to expected commercial size

5.01 )( pmf

H gdu

=Πmf

exH u

u=Π 2


Solids Mixing

• Horizontal mixing experiments done in a square reactor with no bulk solids flow

• Mixing curves fit with a two dimensional dispersion model to extract dispersion coefficient

• Dispersion coefficients measured between 200 and 20 cm2/s

– Affected by bed depth and fluidization velocity

– Compare well with published data


Solids Residence Time Distribution

• Can predict solids residence time distribution in the ETX reactor by overlaying dispersion on a bulk flow

• Solids residence time distribution approaches that of a plug flow reactor

• Solids carry the reacting feed so the feed residence time distribution is the same as the solids

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2time / mean residence time

Frac

tion

of T

race

r Rel

ease

d fro

m R

eact

or


Gas Phase Mixing

• Experiments performed to establish effect of reactor configuration on over-cracking

• Negative step change experiments done in cold-flow unit

• Model based completely on correlations for bed properties available in the literature with no adjustable parameters.


Effect of Temperature

• Evidence that lower reaction temperatures improves liquid yields

• Sequential processes of liquid production, evolution then degradation

• Decoupling these processes is a common approach taken by many researchers

• Gas side effects are well represented in the literature

• Liquid side dependence on temperature has not been shown

Over-Cracked Gases

Liquid ProductsFeed

Liquid Side• Reaction• Vaporization

Gas Side• Reaction


Liquid Side Effect

• Effect of temperature on liquid side studied in a small quartz reactor

• Athabasca vacuum residue cracked at different temperatures

• Film thickness of 250 microns

• Liquid products condensed weighed and analyzed

• Coke weighed and analyzed

• Non-condensable gasses were not collected

Condenser

Molten Salt Bath

Sweep Gas


Coke Yield

• Reaction temperatures between 450°C and 550°C

• No effect of temperature on coke yield

• Other studies have shown a slight increases in coke yield as temperature is increased ~ 1% over a temperature increase of close to 100°C

– experimental technique was not sensitive enough to see this

14

16

18

20

22

24

26

440 460 480 500 520 540 560Temperature, oC

Cok

e Yi

eld,

%


Gas and Liquid Yields

• Increase in temperature leads to a trade-off between gas and liquids

• 5% increase in liquid yield with a temperature decreases from 530°C to 470°C

• Result seems analogous to gas phase over-cracking but on a much faster time scale

• Actual gas phase over-cracking was not occurring because gas phase temperature was too low and gas phase residence time was too short

0

2

4

6

8

10

12

440 460 480 500 520 540 560Temperature, oC

Gas

Yie

ld, %

67

68

69

70

71

72

73

74

75

76

440 460 480 500 520 540 560Temperature, oC

Liqu

id Y

ield

, %


Product Quality

• Aromatic carbon content and H/C of products was measured

• Product quality was shown to decrease with reaction temperature

• Liquid product quality and yields are not independent

– an increase in the gas yield means that less hydrogen ends up in the liquids 20

22242628303234363840

460 480 500 520 540Temperature (deg-C)

Aro

mat

ic C

arbo

n C

onte

nt (%

)

1.5

1.55

1.6

1.65

1.7

1.75

H/C

(mol

ar ra

tio)

Aromatic Carbon

H/C


Final Hydrogen Distribution

8%16%73%530°C

7%13%78%500°C

8%9%82%470°C

CokeGasLiquid Products

Reaction Temperature

• Final distribution of hydrogen among coking products is significantly affected by reaction temperature


Impact of ETX attributes Solids Residence Time

•Solids mixing in ETX reactor approaches plug-flow

•Feed that exits before [time/reaction time] < 1 is partially wasted

•Current fluid bed coking technology is well mixed

-required to compensate for short-circuiting with larger reactor

•ETX reactor can be made 20 times smaller than a well-mixed reactor

Well -Mixed

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2time / required reaction time

fract

ion

of tr

acer

rele

ased

ETX


Impact of ETX attributesGas phase over-cracking

• Combine the gas phase mixing model with an over-cracking kinetic model

• Models shows effects of reactor configuration on over-cracking

• Model also allows for some extrapolation outside of experimental spacecommercial units

– current fluidized bed cokers lose between 5% and 8% of their products to over-cracking

– allows for some comparison to commercial units

• ETX coker gives 2% increase in liquid yield over Fluid Coking because of the shallow bed


Leveraging the ETX design advantagesHigh Capacity Reactor

• Similar operating conditions as the similar process of fluid coking– Operational confidence– Same amount of coke production

• Solid mixing pattern allows a smaller reactor with increased capacity– 65% increased capacity over a fluid coker– 10,000 bbl/day unit 3 m high, 2 m wide, 6 m long

• Shallow bed provides a yield benefit by reducing over-cracking– Expected 2% increase in product yield from reduced gas phase residence time

• Solids mixing pattern eliminates short-circuiting and losses of partially reacted feed

• Burner temperature 50°C cooler than typical fluid coker– Energy and emission reduction


Further Leveraging the ETX Advantage

• Effect of temperature on liquid yield – Over-cracking model showed the potential for a 5% decrease in over-cracking

losses by reducing the reaction temperature from 530°C to 470 °C – Hot coking experiments showed the potential for a 5% increase in liquid yield,

due to liquid side effects, by reducing the reaction temperature from 530°C to 470 °C

• Combining maximum liquid and gas side effects gives a potential increase in product yield of 9% over current technology (liquid and gas side effects quoted on different basis)

– This yield increase is accompanied by an increase in product quality

• Cost of lower temperatures is reduced reaction rate leading to increased reactor size and decreased capacity

– Reducing reaction temperatures by 10°C can double reactor size requirements


ETX Design can Capture Yield Benefit

• ETX design is already ahead in capacity

• The ETX design can capture 4% of the potential yield increase with no capacity loss compared to fluid coking

0

1

2

3

4

5

6

7

8

470 480 490 500 510 520 530 540Mean Reaction Temperature (deg C)

Yie

ld In

crea

se O

ver F

luid

Cok

ing

(%)

0

50

100

150

200

250

300

Cap

acity

(bbl

/m3/

day)

Fluid CokerCapacity


Benefits of the ETX design

• Cross –flow design allows for independent control over liquid and gas residence times

• Shallow bed reduces gas phase degradation by reducing gas-phase hold-up

• Solids mixing pattern allows for maximum capacity and eliminates losses from short-circuiting– Allows the ETX design to capture 4% liquid yield increase with no loss

of capacity– Increase in liquid yield is accompanied by an increase in product quality

• Application of commercial fluidization technology to upgrading


Acknowledgements

• Industrial Research Assistance Program (IRAP)

• University of Alberta

• National Center for Upgrading Technology (NCUT)

• Coanda Research and Development Corp.

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