Process description for the CC28 example simulation
- Last UpdatedSep 19, 2024
- 5 minute read
The process includes three plug flow reactors (PFRs) that model the steam methane reforming (SMR), the high-temperature water-gas shift (HTS), and the low-temperature water-gas shift (LTS) reactions.
We feed natural gas to the process and split it between burner gas and process feed streams.
Since the SMR reaction is endothermic, we must provide heat by using a fired furnace. In this example, we use a single burner (B1) and four independent HX models (HXSMR, HX2A, HX2B, and HX2C) to model the furnace. We use a compressor (K1) to compress the natural gas feed stream before it enters the furnace. Upon entering the furnace, we mix the natural gas feed with high pressure steam and heat it to the target temperature of 850°C by using the flue gas from B1. The natural gas and high-temperature steam react to form H2 in the SMR reactor. We use a flowsheet equation to connect the duty of the SMR reactor to HXSMR to model the required heat input for the reaction.
The shift reactors then convert unreacted CO and H2O in the SMR product stream (Syngas) into CO2 and H2. We use HX3 to cool the syngas to 450°C before it enters the high-temperature shift reactor (HTS). The water-gas shift reaction is exothermic, so we must cool the product stream from the HTS reactor to 200°C before it enters the LTS reactor. The LTS reactor converts most of the remaining CO and H2O in the syngas stream.
We then cool and compress the product stream to separate the remaining water from the hydrocarbons. Since the product stream consists primarily of light hydrocarbons, a knockout drum (V1) is sufficient for this separation. The gas stream from the knockout drum enters the PSA units which we model here with a simple component splitter. The hydrogen product stream leaves the process and the off gas is recycled back to the front of the process as fuel for the burners.
The process also includes extensive heat integration. A feed water header provides cooling water to exchangers HX3B, HX4, HX5, and HX6. HX3A acts as a superheater for the steam fed to the SMR reactor and therefore does not require cooling water. The hot water outlet stream from the utility side of HX5 mixes with enough feed water to produce a saturated steam product on the utility side of HX4. Saturated steam from HX3B, HX4, and HX6 mix in the steam header. We feed much of the steam in this header to HX3A where the effluent stream from the furnace superheats it. We calculate the flowrate of this stream to achieve the desired steam-to-carbon ratio in the SMR (SC_Ratio). We export the remaining steam in the common header from the process, which we can then use in another portion of the process or sell to another unit.
We base the feed rate and composition of the natural gas stream on typical operating conditions for an industrial SMR[1],[2]. We adjust the flowrate of steam to achieve a typical steam-to-carbon ratio in the reactor inlet of 2.75[2]. Higher steam-to-carbon ratios increase the conversion in the SMR but require larger reactors and process piping. Low steam-to-carbon ratios reduce the overall mass flow in the process but result in a higher methane slippage from the SMR. You can reduce this effect by operating the SMR at higher temperatures.
The following tables show the gas source conditions, process conditions, and equipment sizes.
Table 1: Natural gas feed composition
|
Component |
Composition |
|---|---|
|
Hydrogen (H2) |
0.0002 |
|
Nitrogen (N2) |
0.005 |
|
Oxygen (O2) |
0.0 |
|
Carbon Monoxide (CO) |
0.0 |
|
Carbon Dioxide (CO2) |
0.003 |
|
Methane (CH4) |
0.9472 |
|
Ethane (C2H6) |
0.042 |
|
Propane (C3H8) |
0.002 |
|
n-Butane (C4H10) |
0.0004 |
|
n-Pentane (C5H12) |
0.0002 |
|
Water (H2O) |
0.0 |
Table 2: Process conditions
|
Variable |
Description |
Value |
UOM |
|---|---|---|---|
|
NatGas.T |
Temperature of the natural gas that enters the process. |
25 |
°C |
|
NatGas.P |
Pressure of the natural gas that enters the process. |
2 |
bar |
|
Air.T |
Temperature of the air that we feed to the burner. |
24.85 |
°C |
|
FeedWater.T |
Temperature of the fresh feed water that enters the process. |
25 |
°C |
|
FeedWater.P |
Temperature of the fresh feed water that enters the process. |
30 |
bar |
|
SC_Ratio |
The steam-to-carbon ratio of the feed stream that enters the SMR reactor. |
2.75 |
- |
|
B1.xO2 |
The excess oxygen for the combustion reaction in the burner. |
21 |
% |
|
HXSMR.Tso |
The temperature of the shell-side product stream of the HXSMR heat exchanger. |
1100 |
°C |
|
HX2A.Tto |
The temperature of the tube-side product stream of the HX2A heat exchanger. This ensures that the feed stream to the SMR reactor is sufficiently heated. |
850 |
°C |
|
NatGasFeed.F |
The flowrate of the natural gas that we mix with steam and then feed to the SMR reactor. |
65,000 |
Nm3/h |
|
NatGasFeed.P |
The pressure of the natural gas that we mix with steam and then feed to the SMR reactor. |
29 |
bar |
|
HX3A.Tto |
The temperature of the tube-side product stream of the HX3A heat exchanger. This product stream serves as the steam feed stream to the SMR reactor. |
840 |
°C |
|
HX3B.Tso |
The temperature of the shell-side product stream of the HX3B heat exchanger. This ensures that the feed stream to the HTS reactor is sufficiently cooled. |
450 |
°C |
|
HX3B.VFto |
The vapor fraction of the tube-side product stream of the HX3B heat exchanger. The ensures that the cooling water in the tubes exits the heat exchanger as steam. |
1 |
fraction |
|
HX4.Tso |
The temperature of the shell-side product stream of the HX4 heat exchanger. This ensures that the feed stream to the LTS reactor is sufficiently cooled. |
200 |
°C |
|
HX4.VFto |
The vapor fraction of the tube-side product stream of the HX4 heat exchanger. The ensures that the cooling water in the tubes exits the heat exchanger as steam. |
1 |
fraction |
|
HX5.Tso |
The temperature of the shell-side product stream of the HX5 heat exchanger. |
180 |
°C |
|
HX5.VFto |
The vapor fraction of the tube-side product stream of the HX5 heat exchanger. The ensures that the cooling water in the tubes exits the heat exchanger as steam. |
1 |
fraction |
|
HX6.VFto |
The vapor fraction of the tube-side product stream of the HX6 heat exchanger. The ensures that the cooling water in the tubes exits the heat exchanger as steam. |
1 |
fraction |
|
K2.Pr |
The pressure ratio of the K2 compressor. |
1.35 |
fraction |
|
V1Out.z[H2O] |
The water composition of the V1Out stream, which is the product stream of the V1 knockout drum. |
0.025 |
fraction |
|
H2.z[H2] |
The hydrogen composition of the H2 Sink. |
0.9998 |
fraction |
Table 3: Equipment sizes
|
Variable |
Description |
Value |
UOM |
|---|---|---|---|
|
SMR.Nt |
The number of tubes in the SMR reactor. |
200 |
- |
|
SMR.L |
The length of the tubes in the SMR reactor. |
12 |
m |
|
SMR.D |
The internal diameter of the tubes in the SMR reactor. |
0.1016 |
m |
|
SMR.Ne |
The number of elements in the SMR reactor |
13 |
- |
|
SMR.eps |
The catalyst void fraction for the catalyst in the SMR reactor. |
0.528 |
fraction |
|
HTS.Nt |
The number of tubes in the HTS reactor. |
1 |
- |
|
HTS.L |
The length of the tubes in the HTS reactor. |
2.5 |
m |
|
HTS.D |
The internal diameter of the tubes in the HTS reactor. |
1.25 |
m |
|
HTS.Ne |
The number of elements in the HTS reactor |
8 |
- |
|
HTS.eps |
The catalyst void fraction for the catalyst in the HTS reactor. |
0.5 |
fraction |
|
LTS.Nt |
The number of tubes in the LTS reactor. |
1 |
- |
|
LTS.L |
The length of the tubes in the LTS reactor. |
1.5 |
m |
|
LTS.D |
The internal diameter of the tubes in the LTS reactor. |
0.75 |
m |
|
LTS.Ne |
The number of elements in the LTS reactor |
8 |
- |
|
LTS.eps |
The catalyst void fraction for the catalyst in the LTS reactor. |
0.5 |
fraction |
|
V1.L |
The height of the V1 knockout drum. |
2 |
m |
|
V1.D |
The diameter of the V1 knockout drum. |
1 |
m |