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Optimisation of CO2 Capture Plants Using Rectisol Influence of Vacuum Stages and CO2 Pressure Before Compression on Overall Utility Consumption

Rectisol provides CO2 products at low pressures. An increase of the delivery pressures reduces the efficiency of Rectisol but saves at the same time more energy at the CO2 compressor for applications where high pressure CO2 is needed. At specific pressure levels an optimal overall utility consumption will be reached. A further increase in these pressures will lead to higher operating costs.

In the examined plants vacuum stages in the Rectisol are necessary to reach Claus Gas specification of 25 % H2S. Nitrogen stripping would lead to undesired nitrogen in the CO2 product for sequestration. Furthermore flashing at vacuum pressure leads to low temperatures, which reduce methanol circulation rates and energy needed for refrigeration. These savings are bigger than the power consumption of the vacuum blower.

CO2CompressionRectisol

Refrigeration

Raw Syngas

Clean Syngas

Acid Gas

High Pressure CO2

B.L.

Figure 1: Gas Conditioning of a Syngas plant with CO2 capture using Rectisol

Object of examination is a plant with Carbon Capture. After Coal Gasification and Water Gas Shift, sulphur and CO2 are removed in a Rectisol unit. The clean Syngas can be used in units downstream. CO2 is compressed to 80 bar(a) for sequestration. Sulphur components are collected in an Acid Gas stream, which can be treated by a Claus unit. A refrigeration unit is needed for cooling the Rectisol. Raw Gas composition and product specifications are given in Table1 and Table 2.

Table 1: Raw Gas composition of the ref. plant

Component vol% Hydrogen 53 Carbon Dioxide 34 Nitrogen 10 Carbon Monoxide 2 Hydrogen Sulphide 0.5 Argon 0.4 Water Sat. Methane 0.05

Temperature [C] 34 Pressure [bar(a)] 38

Table 2: Product specifications of the ref. plant

Clean Syngas: CO2 Capture 97 %

CO2 Products:

1.2 bar(a) + 3.3 bar(a) H2S < 10 ppmv Compression to 80 bar(a)

(4 stages with intercooling) Acid Gas: H2S > 25 %


Aim of this study is to optimise the overall utility consumption of a Syngas plant with carbon capture using Rectisol. Optimisation of capital costs is not part of this examination. The product specifications of the plant are fixed. Therefore the Raw Gas composition and properties remain constant for the plant variations examined. The Clean Syngas and Claus Gas specifications are also not changed, as well as the sulphur specification of the CO2 product.

LP-CO2 (CO2 Compressor)

MP-CO2 (CO2 Compressor)

GT Fuel Gas

Absorber MP-Flash Reabsorber Hot Regenerator

Raw Syn Gas

Claus Gas(OxyClaus)

Impure Water

Figure 2: Simplified PFD of Rectisol

Two changes in Rectisol (Figure 2) are examined: The first is higher CO2 delivery pressures to compression, i.e. higher pressures in the upper parts of the Reabsorber. The pressures of the two CO2 streams are increased in steps of equal sizes. The second scenario is a Rectisol without a vacuum stage at the bottom of the Reabsorber.

The changes influence Rectisol, Refrigeration for Rectisol, and CO2 Compression. These three units are contained in the battery limits of this study (Figure 1). All other units are not influenced by the changes as long as the product specifications are reached.

CO2 pressure before compression:

Rectisol provides two CO2 streams with different pressures (here: 1.2 bar(a) and 3.3 bar(a)) due to the regeneration principle of flashing. For sequestration these streams are compressed to 80 bar(a) in a compressor with 4 stages and intercooling with cooling water. An increase in the suction pressure of the compressor leads to a considerable decrease of energy consumption. On the other hand, flashing at higher pressures leads to higher utility consumption of the Rectisol unit. Less carbon dioxide is flashed out of the methanol, leading to higher preloading of the main wash methanol and therefore to higher methanol circulation rates. Also, and even more important: Flashing out CO2 cools down the methanol. Therefore flashing at higher pressure leads to higher temperature of the washing methanol. This has to be compensated by increasing the methanol circulation. Furthermore a rise in the CO2 pressures adds to the pressure difference the vacuum blower has to overcome. This is the main source of the increased overall utility consumption.


Starting from low CO2 pressures, increasing pressure first reduces the duty of compression more than the utility consumption of Rectisol rises (Figure 3, Table 3). At CO2 pressures of approximately 2 bar(a) and 4.1 bar(a) for the two streams the overall utility consumption shows a minimum. At further pressure increases the additional power consumption of Rectisol begins to surpass the savings in compression.

92

94

96

98

100

102

0 0.5 1 1.5 2

Pressures of CO2 Products before Compression

Ove

rall

Pla

nt D

uty

[%]

37802

38624

39446

40268

41090

Ove

rall

Pla

nt D

uty

[kW

](e

xem

plar

y fo

r a

Pla

nt o

f 452

000

Nm

/h)

1.2 bars + 3.3 bars 2.2 bars + 4.3 bars 3.2 bars + 5.3 bars

(Rec

tiso

l + R

efri

ger

atio

n +

CO

2 C

om

pre

ssio

n)

Figure 3: Overall plant duty over pressures of CO2 products before compression

Table 3: Relative utility consumptions for variants of Rectisol units in Carbon Capture plants

Reference Plant:

(1.2 bar(a) +3.3 bar(a))

CO2 Pressure Before

Compress. + 0.5 bar(a)

(1.7 bar(a) +3.8 bar(a))

CO2 Pressure Before

Compress. + 0.75 bar(a)

(1.95 bar(a) +4.05 bar(a))

CO2 Pressure Before

Compress. + 1 bar

(2.2 bar(a) +3.3 bar(a))

CO2 Pressure Before

Compress. + 2 bar(a)

(3.2 bar(a) +4.3 bar(a))

Excluding CO2 compressor: Steam 100 % 1.5 % 1.5 % - 1.5 % + 0.4 %

Cooling Water 100 % +1.1 % + 1.2 % + 3.7 % + 13.3 % Refrigeration Duty 100 % 0.6 % 0.7 % 0.8 % 0.2 %

Overall Power Consumption 100 % + 3.6 % + 3.9 % + 8.4 % + 25.5 %

Including CO2 compressor:

Steam 100 % 1.5 % 1.5 % 1.5 % + 0.4 % Cooling Water 100 % 1.9 % 2.0 % 2.1 % 0.5 %

Refrigeration Duty 100 % 0.6 % 0.7 % 0.8 % 0.2 % Overall

Power Consumption 100 % 2.8 % 3.5 % 3,3 % 1.2 %


Vacuum stages:

There are two possibilities to reach the Claus Gas specification with the given Raw Gas. One is to incorporate nitrogen stripping at the lower part of the Reabsorber. This option leads to nitrogen in the CO2 product. The plant examined uses vacuum stages to flash out carbon dioxide and thus enrich sulphur components. A vacuum blower keeps the pressure difference to the rest of the plant.

The idea to save the energy consumption of the blower by skipping the vacuum stages to reduce energy consumption leads to two consequences: First, the specification of the Claus Gas cannot be reached. In the reference plant 7 % H2S in the Claus Gas are observed. Second, like discussed above, flashing at higher pressures (not vacuum) leads to an increase in utility consumption of Rectisol. Altogether skipping the vacuum stages leads to an increase in total energy consumption of 5.7 % in the reference plant (see table 1) and Claus Gas far off specification (7 % instead of 25 % H2S).

Table 4: Relative utility consumptions for variants of Rectisol units in Carbon Capture plants

Reference Plant Without Vacuum Stages Excluding CO2 compressor:

Steam 100 % + 33.1 % Cooling Water 100 % + 30.4 %

Refrigeration Duty 100 % + 80.7 % Overall Power Consumption 100 % + 13.1 %

Including CO2 compressor:

Steam 100 % + 33.1 % Cooling Water 100 % + 14.4 %

Refrigeration Duty 100 % + 80.7 % Overall Power Consumption 100 % + 5.7 %

Claus Gas 25 % max. 7 %

Conclusion:

At low CO2 pressures before compression a pressure increase leads to bigger savings in compression energy than losses in the Rectisol unit through decreased efficiency. At certain pressures (here: 2 bar(a) and 4.1 bar(a)) a minimum overall utility consumption is reached before the efficiency losses in Rectisol surpass the savings in compression.

For the reference plant a vacuum stage is needed to reach the Claus Gas specification. Furthermore flashing at low pressures leads to low temperatures, which support absorption and therefore reduce methanol circulation and refrigeration duty. These savings are bigger than the power consumption of the vacuum blower.