Gas-to-Methanol Production at Lower Operating Conditions: Techno-economic Analysis of SMR, ATR, and DMR

Modeling Environment

Three different methanol production routes were modeled and simulated. These pathways differ in how the syngas is being produced; Steam-Methane Reforming (SMR), Auto-thermal Reforming (ATR), and Dry Methane Reforming (DMR) [14]. The methanol plants were modeled and simulated using ASPEN HYSYS V11 software. In setting up each model, the SRK equation of state was used for the syngas production section and the methanol synthesis and purification section.

Optimization of the Process Operating Parameters

The optimization was done using the in-built Aspen HYSYS optimizer tool. The initial simulations were carried out using values from the upper end of the operating ranges as stated in literatures. While monitoring the product yield and quality, the process operating parameters which essentially are the pressures and temperatures point to point through the flow schemes, were reduced to either the lower end of the operating ranges or even much lower provided, an optimum product yield rate and quality was obtained as output. The results of the optimization are shown in Table 2.

Feedstock

The feedstock used for the three simulation cases is a natural gas composition obtained from the flare stack of a typical gas plant in Nigeria. The composition and conditions are shown in Table 1. The gas is sulfur-free; hence, the reason for not including a gas pre-treatment unit in the simulation cases.

The Steam-Methane Reforming (SMR) Pathway

For this case, methanol production was modeled, simulated, and optimized according to the steam-methane reforming reaction path. The feed gas is a 100 MMscf/d supply of natural gas obtained at 30oC and 2 bar. a, which was compressed to 15 bar.a using a single-stage compressor to meet the pressure requirement at the Pre-reformer. Steam at 600oC and 15 bar.a was also charged into the Pre-reformer to react with the feed gas.

The Pre-reformer was modeled using a conversion reactor and was set to operate at 600oC and 15 bar.a to convert the heavier hydrocarbon molecules in the feed gas stream into syngas (hydrogen and carbon monoxide) product according to Equations 1 to 4 while some side reactions also occur (Equation 5 and 6) [15].

$$ \mathrm {C} _ {2} \mathrm {H} _ {6} (\mathrm {g}) + 2 \mathrm {H} _ {2} \mathrm {O} (\mathrm {g}) = 5 \mathrm {H} _ {2} (\mathrm {g}) + 2 \mathrm {C O} (\mathrm {g}); \quad \Delta \mathrm {H} _ {2 9 8 \mathrm {K}} ^ {0} = $$

= +374.3 kJ mol

∆ (1) $$ \mathrm {C} _ {3} \mathrm {H} _ {8} (\mathrm {g}) + 3 \mathrm {H} _ {2} \mathrm {O} (\mathrm {g}) = 7 \mathrm {H} _ {2} (\mathrm {g}) + 3 \mathrm {C O} (\mathrm {g}); \quad \Delta \mathrm {H} _ {2 9 8 \mathrm {K}} ^ {0} = $$

= + 497.7 kJ mol

∆ (2) $$ \mathrm {i} - \mathrm {C} _ {4} \mathrm {H} _ {1 0} (\mathrm {g}) + 4 \mathrm {H} _ {2} \mathrm {O} (\mathrm {g}) = 9 \mathrm {H} _ {2} (\mathrm {g}) + 4 \mathrm {C O} (\mathrm {g}); \quad \Delta \mathrm {H} _ {2 9 8 \mathrm {K}} ^ {0} = $$

= +651.3 kJ mol

∆ $$ \mathrm {n - C} _ {4} \mathrm {H} _ {1 0} (\mathrm {g}) + 4 \mathrm {H} _ {2} \mathrm {O} (\mathrm {g}) = 9 \mathrm {H} _ {2} (\mathrm {g}) + 4 \mathrm {C O} (\mathrm {g}); \Delta \mathrm {H} _ {2 9 8 \mathrm {K}} ^ {0} = + 6 5 1. 3 \mathrm {k J} \mathrm {m o l} $$

= +651.3 kJ mol

∆ (4) ( ) ( ) ( ) ( ) 2 2 2 298K CO g + H O g = CO g + H g H° = -41 kJ/mol ∆ (5) ( ) ( ) ( ) ( ) 2 4 2 298K CO g + 3H g = CH g + H O g H° = -210 kJ/mol ∆ (6) The Reformer reactor was modeled using a HYSYS equilibrium reactor where the methane present in the feed gas stream was reacted with steam in order to yield syngas product at 850oC and 15 bar.a operating condition with respect to Equations 7 and 8 [14]:

$$ \mathrm {C H} _ {4} (\mathrm {g}) + \mathrm {H} _ {2} \mathrm {O} (\mathrm {g}) = \mathrm {C O} (\mathrm {g}) + 3 \mathrm {H} _ {2} (\mathrm {g}) \quad \Delta \mathrm {H} _ {2 9 8 \mathrm {K}} ^ {\circ} $$

= + 206 kJ/mol

∆ (7) ( ) ( ) ( ) ( ) 2 2 2 298K CO g + H O g = CO g + H g H° = - 41 kJ/mol ∆ (8) After cooling the syngas to 60oC using a cooler, liquid effluent water was separated before passing the gaseous stream mixed with a recycled stream through a jacketed plug flow reactor representing the methanol reactor. The methanol synthesis, which occurs at 40°C and 15 bar.a, was modeled to obey Equations 9 to 11 [14].

$$\text{CO}_2(\text{g}) + 3\text{H}_2(\text{g}) = \text{H}_2\text{O}(\text{g}) + \text{CH}_4\text{OH}(\text{g}) \quad \Delta H^\circ_{298K} = -41.2 \text{ kJ/mol}$$

$$\text{CO}(\text{g}) + 2\text{H}_2(\text{g}) = \text{CH}_4\text{OH}(\text{g}) \quad \Delta H^\circ_{298K} = -90.7 \text{ kJ/mol}$$

$$\text{CH}_4(\text{g}) + 1.5\text{O}_2(\text{g}) = \text{CO} + 2\text{H}_2\text{O}(\text{g}) \quad \Delta H^\circ_{298K} = +41.5 \text{ kJ/mol}$$

The equilibrium expression guiding the methanol synthesis is given by Turton, et al. [16].

$$K = 4.8 \times 10^{-13} \exp(11458/T)$$

The product obtained from the Methanol Reactor was then flashed to 2 bar.a. The gas stream was compressed, gathered, and sent back as a recycled stream into the methanol reactor, while the liquid portion was sent to a distillation column in order to purify the methanol product up to 99% quality.

The Auto-thermal Reforming (ATR) Pathway

Following the auto-thermal reforming reaction path, the methanol production for this case was modeled, simulated, and optimized. The same feed gas composition/condition as in the SMR case was also used for this auto-thermal reforming case; 100MMscf/d supply of natural gas at 30°C and 2bar.a.

The feed gas was first compressed to 15 bar.a using a single-stage compressor before sending it to the Pre-reformer. Steam at 600°C and 15 bar.a was also charged into the Pre-reformer to react with the feed gas. The Pre-reformer was modeled using a conversion reactor, set to operate at 600°C and 15 bar.a to convert some heavier hydrocarbon molecules into the feed gas stream. The reactions taking place (Equations 1 to 6) are similar to the pre-reformer reactions of the SMR case [15].

The Reformer reactor was modeled using a HYSYS equilibrium reactor where the methane present in the feed gas stream was reacted with oxygen supplied from the Air Separation Unit (ASU) in order to yield products at 850°C and 15 bar.a operating condition with respect to Equations 13-15 [15].

$$\text{CH}_4(\text{g}) + 1.5\text{O}_2(\text{g}) = \text{CO} + 2\text{H}_2\text{O}(\text{g}) \quad \Delta H^\circ_{298K} = -520 \text{ kJ/mol}$$

$$\text{CO}(\text{g}) + \text{H}_2\text{O}(\text{g}) = \text{CO}_2 + \text{H}_2(\text{g}) \quad \Delta H^\circ_{298K} = -41 \text{ kJ/mol}$$

$$\text{CH}_4(\text{g}) + \text{H}_2\text{O}(\text{g}) = \text{CO}(\text{g}) + 3\text{H}_2(\text{g}) \quad \Delta H^\circ_{298K} = +206 \text{ kJ/mol}$$

After cooling the syngas to 60°C using a cooler, liquid effluent water was separated before passing the gaseous stream mixed with a recycled stream through a jacketed plug flow reactor representing the methanol reactor. The methanol synthesis, which occurs at 40°C and 15 bar.a, was modeled according to Equations 9 – 12, similar to the SMR case [14]. The products from the Methanol Reactor were then flashed to 2 bar.a. The gas stream was compressed, gathered, and sent back as a recycle stream into the methanol reactor, while the liquid portion was sent to a distillation column in order to improve the methanol product quality up to 99% purity.

The Dry Methane Reforming (DMR) Pathway

Should there be a continuous supply of carbon dioxide adequate to be used as feedstock, say from a carbon capture facility, this DMR case considered a methanol production using natural gas and carbon dioxide as feedstock. Using the same feed gas composition/condition as in the SMR and ATR cases, the gas was first compressed to 15 bar.a using a single-stage compressor before feeding the Reformer. Carbon dioxide is supplied at 30°C and 15 bar.a was separately charged into the Reformer for reaction.

The Reformer was modeled using an equilibrium reactor, set to operate at 850°C and 15 bar.a, following Equation 16 to yield an equimolar syngas as a product [14]. A pre-reformer was omitted because the steam required in the pre-reforming reaction is not part of the feedstock for this “dry” case.

$$\text{CH}_4(\text{g}) + \text{CO}_2(\text{g}) = 2\text{CO}(\text{g}) + 2\text{H}_2(\text{g}) \quad \Delta H^\circ_{298K} = +247 \text{ kJ/mol}$$

After cooling the syngas to 60°C using a cooler, the gaseous stream mixed with a recycled stream was sent to a jacketed plug flow reactor representing the methanol reactor. The methanol synthesis, which occurs at 40°C and 15 bar.a, was modeled according to Equations 9 – 12, similar to the SMR and ATR cases [14]. The products obtained from the Methanol Reactor were then flashed to 2 bar.a. The gas stream was compressed, gathered, and sent back as a recycle stream into the methanol reactor while the liquid portion was distilled to obtain a 99% methanol purity.

Costing the SMR, ATR, and DMR Methanol Plants

The three methanol production options involve similar equipment, specifications, and conditions. The summaries of the comparative costs are shown in Table 3-7. The revenues that may be derived from the three pathways are shown in Table 8, while Table 9 shows the payback periods. Besides the size rating of each piece of equipment, the material of construction and the pressure classes were also factored in the equipment costing for realistic results. With part of the process operating at temperatures as high as 850oC, the recommended material of construction nickel alloy [16] was used. The use of the nickel alloy increased the base cost of each applicable piece of equipment due to the more expensive nickel alloy compared to carbon steel (CS) or stainless steel (SS). The ATR option has the highest purchased cost of equipment (PCE) due to the inclusion of the Air Separation Unit and the relatively larger equipment sizes justified by the yield rate, which adds to the overall cost. Alternatively, the DMR option has the lowest purchased equipment cost, mainly due to the absence of the pre-reformer and the air separation unit from the process.

The utility cost for the three methanol production technologies was assumed to be the same. Also, the cooling water supply was considered to be generated on-site from a well or river with no purchase cost.

The revenue generated from the different plants was estimated from the sale of the methanol product alone, as the other effluents (water and purge gas) were considered to be of no economic value. As of the time of this study, the pricing of methanol in the international Asian market was 430 USD/ ton, according to Methanex® [19]. However, an average price over a two-year period was used (306 USD/ton) to estimate sales revenue for this work. This was done to smoothen price fluctuations. The ATR technology had the highest annual revenue at $572,835,672 because its production rate surpasses the other options. The DMR technology gave the least revenue consequently.

The three methanol production options were relatively profitable based on the payback period analysis. However, the ATR production option would pay back invested capital quicker than the other options owing to its higher yield rate versus plant cost. This result is corroborated by the results in the open literature [14, 21], which confirms that the ATR technology is the most viable option for large-scale plants from 5000 MTPD.

Furthermore, several historical works have been conducted on methanol synthesis that forms the basis of comparison with the studies conducted herein. Despite the rigorous optimization studies conducted on the simulations to yield the most economical throughput, most operating parameters still fall within the stated literature ranges. For instance, the syngas reformers were found to be optimal at 850oC and 15bar, corroborating results in the open literature with ranges between 800oC – 1000oC and 15bar – 40bar generally for the different production technologies [14].

For the methanol synthesis, the optimal condition was found at 40oC, and 15 bar, whereas several works in the open literature had practical ranges within 200 – 300oC and 15 – 100 bar [14, 22, 23]. However, according to Turton, et al. [16], the methanol synthesis should thermodynamically be favored at lower operating temperatures around the bounds of 40oC in consequence of the equilibrium expression (Equation 12), which conforms to Le Chatelier’s principle for exothermic reactions. This provides an opportunity for process intensification in methanol production to achieve lower operating conditions than the existing technologies.

To validate the project capital cost in this study, a comparison was made with a proposed methanol project in Nigeria as a baseline. As reported in NCMB [24], this baseline plant was designed for a 10,000 MTPD throughput and evaluated to cost USD 3.5 billion. Scale factors were used to equalize all the production rates to 5000MTPD to compare the project costs with the baseline plant. The result shown in Figure 4 reveals a good closeness with the baseline.

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