degradation processes are aggravated, as are any corrosion problems. As clean fuel standards become more and more demanding, a refinery’s consumption of H 2 continues to increase. Considering this, our company installed a new SMR very close to the first. In this case, the technology is top fired, which does not have units to recover CO 2 . In this last stage, taking advantage of the design of the second SMR, which includes an adiabatic pre-reformer for greater operational flexibility (to process different types of feed), the use of a stream of refinery off-gas (ROG) 1,2 as feed to the SMR was investigated. The expectation was to reduce fossil fuel, such as natural gas, consumption and use the ROG that would otherwise be burned and sent to the atmosphere. Additionally, the composition of ROG can fluctuate significantly in a refinery as rates for different units change and, in particular, if a unit goes offline. Typical conditions for different ROG streams in a refinery can be seen in Table 1 . When it comes to incorporating ROG as feed to the SMR, keep in mind a few things for smoother operation:
Typical ROG source
Pressure barg Vol% H 2
Delayed coker off-gas Hydrocracking HP off-gas Hydrodealkylation off-gas
~10
15-30 60-85 50-75 68-88
40-125 25-28 20-30
Catalytic reforming
Catalytic cracking off-gas
~20
~18
Hydrotreater off-gas
20-50
60-80
Table 1
produced as a function of the ROG/natural gas ratio can be seen in Figure 2 . Figure 3 shows how the fuel supplied to the furnace varies, depending on the ROG/natural gas ratio. Here, when the ROG/natural gas ratio increases, there is more H 2 in the feed and fewer hydrocarbons to reform, so the energy required diminishes. As mentioned above, an adiabatic catalytic pre-reformer was installed upstream of the main reformer. The adiabatic pre-reforming process is based on a set of reactions: hydrocarbon steam reforming followed by water-gas exchange reactions and methanisation. Generally, no intermediate products are formed, and complete conversion is usually achieved. The water-gas exchange and methanisation are limited by thermodynamic equilibrium. While steam reforming is strongly endothermic, gas- water shift and methanisation are exothermic. Therefore, the total heat of the reaction can be
• High level of heavy hydrocarbons • High H 2 content • High sulphur levels
The H 2 content in ROG varies between 5 and
10%, up to values of 90%. Therefore, its use in the production of H 2 as a feed to an SMR should be a commitment by case. If the H 2 content is as high as 80%, care must be taken to mix this stream with, for example, natural gas so that the heat required by the SMR is not less than that supplied by the tail gas from the PSA. In our case, H 2 content in the ROG varies from 70-77%, so to have a safe operation and achieve the H 2 flow required by our client, it was mixed with natural gas in different proportions. The relationship between the H 2
1.4 1.2 1.6 1.8 2 2.2 2.6 2.4 2.8
1
0.6 0.8 0.2 0.4
0
0
0.5
1
1.5
2
2.5
3
3.5 4
4.5
ROG/ natural gas
Figure 2 Relationship between the H 2 produced as a function of the ROG/natural gas ratio
www.decarbonisationtechnology.com
55
Powered by FlippingBook