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CARGEN—Co-production of syngas and carbon black

As discussed in the previous sections, the process challenges of carbon formation and low syngas ratio are the most critical hurdles in the implementation of DRM technology on a commercial scale. One approach to overcome the challenge of high carbon formation in DRM is to segregate the carbon formation reaction, and the syngas formation reaction. In this manner, it is possible to systematically improve the C/H ratio that favors desirable operational conditions for industrial operation. From thermodynamic assessments, a novel approach to segregate the two reactions (carbon formation and syngas formation) is proposed which helps in addressing the catalyst deactivation problem and syngas ratio problem. In particular, the proposed reactor system utilizes the benefit of lower operational temperatures and high C/H ratio to selectively produce high quality carbon from CO, and CH4, while utilizing the benefit of high temperature and low C/H ratio to produce syngas as desirable product. These reactions are made to essentially happen in two separate reactors that produce only a single product separately. Figure 5 illustrates the process concept. As the focus of the first reactor is to produce solid carbon, it has been termed as “CARGEN” or CARbon GENerator reactor, the second reactor is the normal reforming process to produce syngas. In short, this process perceives carbon as desirable marketable product instead of a problem that should be avoided. By doing so, two separate products can be obtained from 2 sequential reactors, thereby utilizing the DRM reaction.

In Figure 5, two plots of carbon formation as a function of temperature are shown above both reactor blocks. The blue zone in the first plot referring to CARGEN, is to indicate the operational window of the first reactor in terms of temperature. This operational window defines the most favorable operational zone for a CARGEN reaction. The second plot, showing a green zone, refers to the conditions in which carbon formation is minimal, and pertains to the second reactor. Inherently, in the first system, since

Illustration of novel approach to implement DRM process

Figure 5. Illustration of novel approach to implement DRM process. CARGEN (Or Carbon Generator) indicates the first reactor, which produces solid carbon as a desirable product, while REFORMER indicates the second reactor, which produces

syngas as desnable product.

carbon is the main product, the resultant syngas ratio is relatively high, however, its yield is low because the selectivity towards syngas under low temperatures is relatively low. Additionally, a big portion of the feed is expected to remain unconverted in the first reactor, as thermodynamics under these conditions do not favor 100% conversions under these conditions. Nevertheless, the portion of feed that gets converted will only form solid carbon. In conjunction with the previous section, this method could also be seen as a way to reduce the C/H ratio load on the reactants of the second reactor (Reformer imit), as all the products from the first reactor are essentially fed to the second reactor. The operational window of the second reactor pertains to high temperature conditions beyond 800 °C temperature and are expected to lead towards higher conversions of close of 100%. However, under these conditions, syngas is selectively produced. In this way, the reactor segregation based on the type of product produced helps to make a symbiotic relationship between the two reactors.

Now, will the carbon formation in the first reactor affect the catalyst and the design of the process? Wouldn’t it lead to frequent shutdowns? The answer to these two important questions is that the knowhow for handling “coking” reactions is already established and has been practiced extensively in the chemical industiy. Chemical refineries generally employ an important unit, called a “coker” unit, that converts the residues of the Vacuum Distillation Unit (VDU) into pet-coke and light gases using catalyst and thermal cracking. This unit is known to produce around 400-500 tons per day(tpd) of solid coke in big refineries and utilize well-known fluidized bed concept for their operation. The first reactor will be operated under these conditions and, therefore, not many design challenges are expected. However, while segregating the coke formation and syngas formation reactions, an important problem of the DRM reaction is solved by alleviating its C/H ratio load on “actual” reformer unit, w'hich operates on the product gases of the first reactor.

There are numerous opportunities to further improve the two-reactor setup discussed above. The main opportunity lies in the symbiotic relationship between the two reactors in terms of both energy and mass exchange. Mass exchange has already been discussed earlier, and it is simply the transfer of products from the first reactor to the second reactor for upgradation. As for heat exchange, the opportunity lies in the fact that the two reactors are operated at different operational temperatures. Since the product gases fi om the first reactor are low' in temperature, relative to product gases of the second reactor, there is a possibility of preheating the product gases of the first reactor by the relatively hot gases exiting from the second. As carbon formation tendency increases with an increase in pressure, if the first reactor is operated at high-pressure condition, and second reactor at low pressure (syngas formation favors low- pressure condition), then there is also scope to generate external work. In this, the high-pressure gases from the first reactor will be passed through a gas turbine to derive external w'ork, w'hile low-pressure gases will be sent to the second reactor for reforming reaction. An illustration of this scheme is provided in the Figure 6 below.

Illustration of the symbiotic relationship between the CARGEN and REFORMER reactor in terms of opportunities

Figure 6. Illustration of the symbiotic relationship between the CARGEN and REFORMER reactor in terms of opportunities

presented in terms of mass and heat exchange.

3.4.1 Variants of the two-reactor setup

In this section, two variants of the two-reactor setup process are presented. These alternative designs are “sub-sets” or alternatives to the original process and demonstrate a perspective in to the flexibility of the proposed scheme to meet the different objective functions desirable by end consumer or the downstream process plant. These case studies demonstrate the flexibility of the two processes in producing different products, while utilizing the same energy requirements.

(a) Case Study 1:

This case study demonstrates a situation in which the product gases from the first reactor are fed to the second reactor directly without any pre-treatment or mixing with external feed.

In this, a mixture of CH4, CO, and O, are compressed and fed to the CARGEN reactor at 400 °C and 25 bar to produce solid carbon. The unreacted or partially reacted gases from the first reactor are fed to the second reactor wherein they are converted to syngas at high temperature of 820 °C and 25 bar pressure. Figure 7 provides an illustration of this case study.

As can be seen from Figure 7, the feed to the first reactor is comprised of CH4, CO, and O,. The primary reason behind the utilization of a small quantity of oxygen is to promote an internal combustion reaction that could support the endothermic energy requirements of the DRM process. In situ energy production in the reactor reduces the inefficiencies associated with the jacketed heat transfer, which is a general industrial practice. An energy assessment of the present case scenario indicates a total energy requirement of~ 120 kJ/mol, which is almost 50% that of DRM. In addition to this, the total CO, and CH4 conversion from the overall process is seen to be about 62% and 78%, respectively. Energy reduction at this scale could reduce the CO, footprints associated with fuel combustion tremendously. Additionally, the first reactor is also assessed to operate in Auto-thermal mode if the operational temperature is about 420 °C, indicating that the reactor is self-sustaining in terms of heat duty while at the same time produces a significant quantity of solid carbon. In this particular case scenario, about 0.81 moles of carbon are produced per 1.6 moles of carbon in the feed. This indicates almost 50% carbon capture in the first unit itself. The value-addition from the second reactor could be observed in the fact that it operates onto a “pretreated” and improved C/H ratio of the feed gas and, therefore, produces a higher quality syngas of ratio of 2.8, with an overall yield of 1.18 moles.

(b) Case Study 2:

This case study is slightly different from the previous case as it utilizes an additional feed of methane to the second reactor to manipulate its syngas ratio and yield. Figure 8 below illustrates the block flow diagram of the two-reactor setup under this scheme of operation.

The operational pressure of both the reactors is set to 25 bar, while the two reactors operate at different temperature conditions. The temperature of the first reactor (Or CARGEN reactor) is at 420 °C, while the second reactor is at 820 °C. Similar to the previous case study, the carbon formed in the first reactor at 420 °C and 25 bar is about 0.81, while the syngas produced from the second reactor is of a different quality due to additional methane co-feed. An energy assessment on the overall process suggests that this process would require about 118 kJ/mol of energy (47% of DRM), while producing syngas ratio of 2.7 at a yield of 1.39 moles. The implication of addition of methane is to demonstrate the flexibility of the process in producing different qualities of syngas with the addition of side reactants.

Case Study 1 block flow diagram of the two-reactor setup

Figure 7. Case Study 1 block flow diagram of the two-reactor setup.

Case Study 2 block flow diagram of the two-reactor step

Figure 8. Case Study 2 block flow diagram of the two-reactor step.

In terms of energy requirements, both case 1 and case 2 have almost equivalent performance as both the processes require ~ 120 kJ/mol of energy. However, both the processes differ in terms of quality of syngas produced. The key benefit of the two-reactor operation is, therefore, not just limited to its tendency to overcome carbon formation issue, but also in adjusting the syngas ratio depending upon downstream process requirements. In addition to this, lower energy requirements are also realized due to breakdown of the DRM process into its two constituent reaction sets. Process optimization on different variables of the processes can further improve the performance of the CARGEN reactor system.

DRM+COSORB—Post DRM syngas ratio adjustment

The CARGEN approach produces two products, the syngas and solid carbon. However, if a carbon- free route is desired, the DRM+COSORB process offers an alternative pathway to produce syngas and an additional stream of CO while reducing the overall carbon footprint of syngas production. Figure 9 provides an illustration of the DRM+COSORB process concept. As the DRM syngas has a low syngas ratio (H,/CO < 1), the DRM+COSORB process separates a part of the CO in an absorption process. This helps boost the syngas ratio of the DRM syngas and also produces a CO stream that can be used as a feedstock for production of other petrochemicals. The advantage of this process is that it does not require the production of steam or oxygen, thereby reducing the capital costs of the syngas production unit.

The carbon footprint and operating cost comparison of DRM+COSORB process against commercial processes is discussed in the next section.

Carbon footprint and operating cost comparisons for proposed processes

Based on the approach described in section 2.3, the carbon footprints of the proposed processes of CARGEN and DRM+COSORB are shown in Figure 10. The two CARGEN cases studied have a syngas ratio of about 2.75 and, when compared to the ATR which operates in this region, there is a marked

DRM+COSORB process

Figure 9. DRM+COSORB process.

reduction in the overall CO, emissions. The DRM+COSORB process can be tuned for any syngas ratio, based on the amount of CO captured in the COSORB unit. As indicated, across all syngas ratios, the process has reduced overall CO, emissions.

For a preliminary comparison of the operating costs of these processes, the costs of feedstock (natural gas), fuel for reformer (by natural gas firing) and oxidant production were considered. A selling price of S75/MT was considered for the captured CO and S200/MT was considered for the coke. Spot market prices for coke are in the range of S300/MT and refinery sources quote a price of $100/MT and, hence, a median price of S200/MT was chosen for the analysis presented here. Removal of coke from the catalyst is still a technological challenge and its cost has not been included in the analysis. The cost of syngas production via different competing processes across syngas ratios is shown in Figure 11. Between syngas ratios of 2 and 3, both the studied processes (CARGEN and DRM+COSORB) have competitive operating costs.

Carbon footprint comparison for proposed reforming processes with commercial processes

Figure 10. Carbon footprint comparison for proposed reforming processes with commercial processes.

Operating cost comparison for proposed reforming processes

Figure 11. Operating cost comparison for proposed reforming processes.

 
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