Energy Systems and Technology

Prof. Dr.-Ing. B. Epple

Otto-Berndt-Straße 2

64287 Darmstadt / Germany

Phone: +49 6151 16 23002

www.est.tu-darmstadt.de

Freiberg Conference

12-16 June 2016, Cologne, Germany

Improving Response Rates of Acid Gas Absorber

Columns C. Heinze, C. Higman, J. Marasigan, B. Epple

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Overview

Background/Motivation

Project Scope

Results

Outlook

8th International Freiberg Conference

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Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Background

Fluctuating renewable sources (wind, solar)

other generating resources must be able to

accommodate

EPRI-Report on IGCC Flexibility

Bottleneck for increased ramp rates is the Acid

Gas Removal (AGR) unit

Hypothesis was developed

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conventional

solar

wind

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Main IGCC Components

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Gasifier Syngas

Cooling

Saturator Gas

Turbine HRSG

Steam

Turbine

Acid Gas

Removal

Sulphur

Recovery

Sulphur or H2SO4

O2

Flue gas

WWT

Waste Water/ Salt

CO Shift

Slag

~

Coal

O2

Coal

Preparation

N2

N2

ASU

CO2

~

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Functionality of AGR

Configuration of Absorber Column and Rectification Column

Absorber:

Chemical or physical solvent absorbs acid gases (H2S and CO2)

Internals can be random or structured packings or trays

Rectification:

Solvent is regenerated by heating and/or flashing

Details can be more complicated, particularly if selectivity (separation of H2S from CO2) is required.

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Offgas

Steam or

waste heat

Raw gas

Clean gas

Regenerator Absorber

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Typical Tray Type Internals

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Downcomer submerged to avoid

gas bypassing

Hold-up Sieve Tray other tray designs like

valve trays are possible

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

AGR Ramping – Current Experience

Solvent flow ratio controlled from gas flow

(proportional)

Ramp up:

Operators limit ramp up speed; typical limit is < 3 %/min

From experience faster ramps lead to sulfur-

breakthrough

Sulfur-breakthrough even occurs with solvent flow kept

at 100%

Ramp down:

Experience with very fast ramp down on multi-gasifier

installations exist.

No syngas quality issues occur.

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FRC102

FR101

LIC102

AR101

LV-101

FV-102

LIC101

LV-102

FRC103

A

B=K x A

FFY101

H2S+COS

FV-103

HHZ

HSD

CLEAN GAS

SOLVENT

SOLVENT

RAW GAS

H2S LOADED SOLVENT

H2S LOADED SOLVENT

35

34

10

11

82

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Hypothesis

In the EPRI-Report, a hypothesis was developed

to explain poor ramp rate capability of acid gas

absorbers.

Hypothesis

1. At higher operating loads the liquid levels (hold-up)

on the trays are higher than at lower loads.

2. During ramp up transient, the additional liquid

inventory must be built up over the whole column.

3. The upper tray levels will increase first, robbing flow

to lower trays, which, being starved, will become

ineffective.

4. This could be solved by additional liquid supply to

lower columns.

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Lean

Solvent

Raw Gas

Clean Gas

Rich

Solvent

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Steady State Modelling in ASPEN PLUS

Absorber with physical solvent and prewash section

fully modelled according to data sheet of a reference

plant.

Good agreement between supplied data and

simulation results for all species.

Correct sulfur concentration was adjusted using an

appropriate tray efficiency (ca. 25% Murphree)

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-1,00%

-0,50%

0,00%

0,50%

1,00%

CO2 CO Hydrogen Methane Methanol Niotrogen

De

viat

ion

[m

ol-

%]

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Dynamic Modelling in ASPEN Dynamics

Qualitatively ASPEN Dynamics can demonstrate the expected H2S concentration

profile over time for a ramp of 40% in 6 min (ca. 6.7 %/min)

Result is independent of absolute values/Murphree efficiency

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Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Dynamic – Weir Equation

For transient gas flow but constant solvent flow

No sulfur peak occurs in standard ASPEN Dynamics

simulation

Simple weir model in ASPEN only considers liquid

height

More sophisticated Stichlmair1

equation implemented

Considers the effect of bubbling and

foaming of solvent caused by the

gas flow

Improvement leads to simulation

results that match the expectations

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0

0,04

0,08

0,12

0,16

30 45 60 75 90

Su

lfu

r co

ncen

trati

on

[p

pm

]

Time [min]

Stichlmair

Standard ASPEN model

1 Stichlmair J.; Dimensionierung des Gas/Flüssigkeit‐Kontaktapparates

Bodenkolonne, Teil 3. Chemie Ingenieur Technik. 1978;50:453-6.

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Developing a Rapid Response Concept

Base case:

Standard arrangement as used in previous

simulations.

Rapid Response Absorber

Top Feed Type:

Provide additional solvent to the column and feed it

into the top.

Bottom Feed Type:

Provide additional solvent to the column and feed it

into an additional inlet halfway down the column.

Additional solvent flow is calculated via a

differential controller (proportional to change

rate)

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FIC102

FI101

FFY101

LIC101

FIC102

FFY101

dFdt

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Results – Performance of the RRA

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0

0,3

0,6

0,9

1,2

1,5

30 45 60 75 90

Su

lfu

r C

on

cen

trati

on

[p

pm

]

Conventional Absorber

Top Feed Type

Bottom Feed Type

0

0,3

0,6

0,9

1,2

1,5

30 45 60 75 90

Su

lfu

r C

on

cen

trati

on

[p

pm

]

Time [min]

Ramp Speed: 3,3 %/min

Ramp Speed: 6,7 %/min

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Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Results – Summary

Root cause of the poor transient behavior is

the hold-up build-up.

Both RRA approaches show promising

results.

With faster ramps, the Bottom Feed Type is

advantageous.

The downcomer only allows a certain solvent

flow. Higher flows will lead to flooding of the top

trays.

The absolute values for sulfur slip must be

considered as provisional at this stage, since

the dynamic model is not yet validated.

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Lean Solvent

Raw Gas

Clean Gas

Rich Solvent

Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Outlook

Model improvements and model additions:

Other tray limitations

Rate-based effects

Adding desorber to the model

Validation of the ASPEN model as well as

experimental data for chemical absorption

Investigation of packed columns

to confirm that the hypothesis

applies here as well

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Energy Systems and Technology Prof. Dr.-Ing. B. Epple

Acknowledgement

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The authors thank EPRI for funding this work!

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