Claus Process Reactor Simulation

Joel Plawsky, Arun Khuttan, Max Bloomfield

Rensselaer Polytechnic Institute

Troy, NY

1 2015 CFES Annual Conference

Claus Process

2

8H2S + 5O

2® SO

2+ 7S + 8H

2O

• The Claus process is the largest volume gas desulfurizing process and is used to recover elemental sulfur from hydrogen sulfide produced during the hydrodesufurization process.

• H2S is burned and then reduced to form elemental sulfur. Often ammonia is present in the feed and needs to be converted to N2.

Claus Process

3

• The project considered only the first part of the process, the furnace. • The furnace environment is very harsh and temperatures are high. • Acid gases from H2S and often NH3, from hydro-denitrogenation, are

produced.

Claus Process Reactor

4

• Claus furnaces contain a checkerwall to protect a downstream heat exchanger from direct contact with the furnace “flame” and to help mixing.

• Checkerwalls are primarily an obstruction and their lifetime is limited. • Project was designed to determine the effects of introducing a static mixing

element, a Vectorwall™, into the reactor. • Preliminary data suggests the Vectorwall™ provides > 40% improvement in

throughput and yield.

Checkerwall Vectorwall

Claus Process Reactions

5

Reactions and Rate Laws

H

2S +

3

2O

2

k1¾ ®¾ SO

2+ H

2O

r1= k

1P

H2SP

O2

1.5

NH

3+

3

4SO

2

k5¾ ®¾

3

8S

2+

3

2H

2O +

1

2N

2

r5

= k5C

NH3

0.25CSO

2

0.5

NH

3+

3

4O

2

k2¾ ®¾

3

2H

2O +

1

2N

2

r

2= k

2P

NH3

PO

2

0.75

CH4+ 2O

2

k5¾ ®¾ CO

2+ 2H

2O

r

6= k

6C

CH4

0.2 CO

2

1.3

H

2+

1

2O

2

k3¾ ®¾ H

2O

r

3= k

3C

H2

CO

2

H

2+

1

2S

2

k7 f

k7 r

¾ ®¾¾¬ ¾¾¾ H2S

r

7= k

7 fP

H2

PS

2

- k7r

PH

2SP

S2

0.5

CO +

1

2O

2

k4¾ ®¾ CO

2

r

4= k

4C

O2

0.25CCO

CH

2O

0.5

• The basic model was chosen to consist of 7 non-elementary reactions with 11 separate species. Reactions are very fast and highly exothermic. Ammonia reaction is slower and new systems are designed around ammonia destruction.

• The model was designed to solve the fluid mechanics, heat transfer, reaction kinetics, and mass transfer processes governing the behavior of the reactor.

• Thermodynamic properties – NASA polynomial format.

• Transport properties – kinetic theory approximations.

Claus Process Reactor Transport Modeling

• Fluid Mechanics

– Temperatures are high so the system can be assumed to be in the gas phase and behave as an ideal gas mixture.

– Flow rates of reactants are high and any obstruction, like a checkerwall leads to highly turbulent flow. Use k – turbulence model to describe the flow field.

• Heat Transfer

– Assume all heat is generated via the chemical reactions.

– Reactants enter at relatively low temperatures.

– Reactor outer wall is insulated so all heat leaves via convective flow.

– Neglect internal radiative exchange at present.

• Mass Transfer

– Gaseous components form an ideal mixture and diffuse throughout the reactor.

– Process accelerates due to the large temperature increase. 6

Claus Process Reactor Geometry

7

• Proceeded in stages using simplified 1-D, 2-D, and currently full, 3-D geometries.

• A class project solved the kinetics in ideal continuous stirred tank and plug flow reactors.

• Solved the full, coupled system in dispersed plug flow and 2-D representations.

• Reactions are extremely fast and H2S burns as a flame, so finding solutions is difficult and mesh adaptation is critical.

• System on right is > 100,000 elements.

Claus Process Reactor – 1-D Simulation

8

• Modeling of real-world kinetics in a flame-like environment.

– Fractional rate orders are not handled well by most software.

– The large differences in reaction rates require external control of concentrations to insure bounded values and a “soft” landing.

– Solutions need to be approached parametrically. In this case that meant incrementally increasing the heat generation rate.

Gas Velocity Heat Generation Rate

Claus Process Reactor H2S Profiles

9

• Implemented a 2-D formation to provide the first approximation to Vectorwall™ formulations and to compare Vector.

• Great differences in rates and distribution of species depending on the insert geometry. All geometries have same open area for flow.

• Sulfur conversion is greater in the Vectorwall™ reactor.

Hydrogen Sulfide Concentration Profiles

Checkerwall Vectorwall™

Claus Process Reactor Flame Fronts

10

• Reactions actually take place in a flame. Comsol simulation was able to show the flame front.

• Look at different Vectorwall™ configurations. Specifically, whether it is better to have a central opening or central obstruction.

Flame Fronts

(heat

generation rate)

Claus Process Reactor 3-D Geometry

11

• 3-D geometry consists of hexagonally arranged openings with hemispherical heads representing the VectorwallTM Elements.

• Vector elements in the system below are designed to move material in a clockwise direction.

• Each vector element can be individually oriented so intense turbulence can be generated.

Claus Process Reactors 3-D Results - Velocities

12

• Compare three configurations; Straight reactor with choke ring, hex wall reactor, and VectorwallTM reactor. Choke ring system is most common.

• Ideally looking for uniform, plug flow velocity profile.

Vectorwall™ Hex Wall Choke Ring

Claus Process Reactors 3-D Results - Velocities

13

• Streamlines provide a better representation of how the various fuel and oxidant feed streams travel throughout the reactor.

• Vectorwalls introduce swirl downstream, but also introduce mixing upstream.

Vectorwall™ Hex Wall Choke Ring

Green – fuel Blue – air

Claus Process Reactors 3-D Results - Velocities

14

• Vectorwall™ efficiency may be related to how close it is located to the reactant inlet.

• Locating the wall further upstream provides for more dead space upstream of the wall.

• Commercial reactors have a side stream inlet. Mostly inefficient since it does not interact with the main flow. Solution: Feed secondary reactant into Vectorwall™ .

Vectorwall™ Low Side Stream Vectorwall™ High

Claus Process Reactor 3-D Results – Mixing & Reaction

15

• Attempting to assess how to characterize the effect of the Vectorwall™.

• One way to do that is to assess the standard deviation as a function of axial position.

Deviation z( ) = c x, y( ) - cavg

éë

ùû

2

dx dyòò

Top of diverter heads

Vectorwall™ Coal Combustors

16

• Coal combustors also often suffer from inadequate mixing leading to enhanced NOx and CO production.

• Incomplete combustion often results from “roping” where the coal particles accumulate as they follow the flow streamlines.

• Vectorwall™ design can be used to reduce particle accumulation and enhance combustion via enhanced turbulence.

Vectorwall™ Coal Combustors

17

• Reaction set for coal combustion is very similar to Claus. Conditions are not as severe. Tracking particle size is improtant since combustion is a heterogeneous reaction.

Reaction Rate Law (mole/s)

H

2S +

3

2O

2

k1¾ ®¾ SO

2+ H

2O

r1= k

1P

H2SP

O2

1.5

HCN + O

2

k2¾ ®¾ NO + CO +

1

2H

2

r

2= k

2C

HCNC

O2

H

2+

1

2O

2

k3¾ ®¾ H

2O

r3= k

3C

H2

CO

2

CO +

1

2O

2

k4¾ ®¾ CO

2

r

4= k

4C

O2

0.25CCO

CH

2O

0.5

C +

1

2O

2

k5¾ ®¾ CO

r5

= k5

4prp

2Np( ) P

O2

CH4+ 2O

2

k5¾ ®¾ CO

2+ 2H

2O

r

6= k

6C

CH4

0.2 CO

2

1.3

C + CO2

k7¾ ®¾ 2CO

r

7= k

74pr

p

2Np( ) P

CO2

C + H2O

k8¾ ®¾ CO + H

2 r8

= k8

4prp

2Np( ) P

H2O

Coal Process Reactor Plug Flow Reactor

18

• We use a similar process for solving these reactions, beginning with an idealized reactor to insure the set of reactions run and that we can integrate heat generation.

• Coal process leads to a more sedate temperature rise relative to the Claus process.

• Reactions are slower since we are burning a solid particle rather than a gas.

• Inlet temperature into the combustor was set at 1350 K.

Coal Process Reactor Plug Flow Reactor

19

• Heterogeneous burning of the coal particles is the most important process. Modeled as a shrinking core though we don’t actually chart ash creation.

• Set inlet particle size to 75 microns which is broadly representative of commercial systems.

• We can understand how fast the particles burn and how large a combustor is required.

Part

icle

Siz

e (m

)

Conclusions and Future Work

20

• We successfully simulated the Claus process in ideal chemical reactors, in a dispersed, plug flow reactor, in a two-dimensional flow reactor with pseudo-Vector elements, and are finalizing three-dimensional reactor simulations with checkerwall and VectorWallTM inserts.

• The next steps will be: – Complete the 3-D simulations on simple systems to incorporate the full heat

generation rate associated with the reactions. • May be able to understand what we need via parametric sweeps in isothermal

reactors.

– Define the optimal arrangement of Vectorwall™ elements and the optimum location of the Vectorwall™.

– Port the results over for industry use.

• Funding for this project was provided by:

New York State Pollution Prevention Institute