Executive Summary
The following report evaluates the possibility of purifying a gas stream via separation from enhanced oil recovery. The gas stream of CH4 and CO2 as is from the enhanced oil well would be able to be sold for $12.1 million, although with purification a higher profit is possible. Four different separation techniques were analyzed: Cryogenic Distillation, Absorption using Water, Absorption using MDEA, and Gas Separation Membranes. Cryogenic distillation proved the most profitable; producing one sellable stream of CH4 as liquified natural gas, and one sellable stream of CO2. The profit was determined from the revenues from CO2 and CH4, and the cost of the trays, condenser and reboiler. Overall, cryogenic distillation had the highest profit of $56.85 million per year. Figure 1 below shows the comparison of four different separation techniques.
Introduction
Enhanced oil recovery system takes CO2 and pumps it into an old oil field well and then the gas exiting the well will contain CO2 and methane. The mixture of gas exiting the wells usually has high levels of CO2 and low levels of CH4. Typically the gas exiting the well can be sold as is depending on the CH4/CO2 concentration, although, by separating the gasses a larger profit can be acquired.
Our team was tasked with separating the CO2 and CH4 in the exit stream of gas that leaves from an enhanced oil recovery system there are four separation methods our team is focusing on including Cryogenic Distillation, Absorption using Water, Absorption using MDEA, and Gas Separation Membranes. We are looking to find the most profitable separation process with the given feed stream exiting from the well. Our parameters for the stream include a 68% concentration of CO2, a feed pressure of 500 psig, a feed flow of 80 MMSCFD, and an operating time of 4500 hr/yr. If our gas was sold as is with no additional separation it could be sold for a CH4 concentration of 32% as low energy gas for $12.1 million, therefore any separation should be more profitable than if our gas was sold as is. Figure 2 below shows the benefits from purifying CH4.
In contrast the market for CO2 is more specific. Carbon dioxide can be sold for $0.005/SCF (SCF = standard cubic feet, standard=60° F and 1 atm in engineering). Carbon monoxide can be used in a wide range of applications including the production of dry ice and other chemical applications.
Analysis of Cryogenic Distillation
Distillation is a separation technique used to separate mixtures based on the differences of volatilities in a boiling liquid mixture. Distillation techniques are only feasible when the mixture components have widely different boiling points, usually a difference greater than 25 degrees. In this project, cryogenic distillation is used to separate CO2 (bp: -78.46 °C) and CH4 (bp: -161.6 °C) (Engineering ToolBox) at very low temperatures by utilizing their differences in boiling points and maintaining constant refrigeration. The cryogenic distillation of these two components will be analyzed based on number of stages, reflux ratio, column pressure, feed quality, product streams, and costs.
With the given feed parameters in our enhanced oil recovery stream, the McCabe-Thiele approach will be used to design the distillation column for all the lower limits of methane in the product streams. These values include 40% methane for medium energy gas, 70% methane for high energy gas, and 95% methane for pipeline quality gas. Due to the extensive design, liquified natural gas, which is <50 ppm of CO2 in distillate stream, was not considered using the McCabe-Thiele approach, but will be considered later in the report. In addition, tray efficiency was not considered, because the McCabe-Thiele Diagram assumes each tray reaches equilibrium conditions.
McCabe-Thiele Approach
Using the vapor-liquid equilibrium data of CH4 and CO2 at 500 psig, the Txy diagram and xy diagram of the mixture were constructed using ASPEN Plus V12.1, and are shown in Figures 3 and 4.
To create the McCabe-Thiele Diagram, a q-line must be plotted to determine the initial energy of the feed stream. Figure 1 is used to determine the dew temperature of the feed mixture, -53.5567 °C. The latent heat of vaporization and specific heat capacity of the feed mixture must be calculated using the latent heat of vaporization and specific heat capacity at ambient temperature (Engineering ToolBox) and the feed composition. The latent heat of vaporization needed is 15.113 KJ/mol K, and the specific heat capacity is .0371 KJ/molK. q is then calculated to be -0.1928. The q value is then used to create the q-line. The equations for q and q-line are shown below in equations 1 and 2. Figure 5 displays the q-line plotted on the xy diagram.
Using the intersection point of the q-line and the equilibrium line in Figure 3, the minimum reflux ratio was calculated for 40%, 70%, and 95% methane in the product streams and multiplied by 1.5 to find the reflux value. The enrichment and stripping lines were then drawn for each product stream to determine the number of theoretical stages needed and are shown in Figures 6-8. It is important to note that pipeline purity gas operates at 600 psig, and a separate Txy and xy diagram were used to calculate the q-line using the same method.
ASPEN Plus Simulation
For further evaluation, ASPEN Plus V12.1 was used to simulate the design using a RadFrac distillation column. The initial conditions made were formulated from the McCabe-Thiele Diagrams. Figure 9 displays the main flowsheet used.
The initial stream and target distillations were defined, and the simulation was run. In order to reach the target separations, variations of initial conditions from the McCabe-Thiele were required. The results are displayed in Tables 1-4.
Profit Analysis
After the target separations were met, the total costs and revenues of the streams were calculated to determine the profit of the distillation column.
To determine the total costs, the price of the trays, cooling system, and heating system were calculated and summed. The costs of cryogenic refrigeration can be estimated from equation 3 (Estimating Refrigeration Costs at Cryogenic Temperatures), where Tdist is the varying temperatures of the distillate streams in °C. The cost of refrigeration can then be converted into the overall cooling costs shown in equation 4. The heating and tray costs are shown in equations 5 and 6. It is important to note that ASPEN Plus includes the condenser and reboiler in the number of stages needed, so the actual number of trays is subtracted by 2.
To determine the total revenue, the revenue for methane and carbon dioxide streams were calculated for each target separation and summed. The heat of combustion of methane is needed to calculate the revenue of methane and is equal to 890000 J/mol (NIST). All other values are given in the problem statement or were calculated using ASPEN Plus. Note that the price of each product stream ($/MMBTU) varies for each type of product stream, with medium quality gas being the lowest $ amount per MMBTU and liquified natural gas being the highest.
Lastly, the total profit of the distillation column is calculated by subtracting the total costs from the total revenue. The equations to calculate cooling costs, heating costs, tray costs, methane revenue, and carbon dioxide revenue are seen below. An overall summary of the costs, revenue, and profit for each target distillation streams are shown in Tables 5-8.
Once the total profits were calculated for each product stream, the most profitable target distillation. All of the targeted product streams generated a large profit and minimal costs. The product stream is most profitable as a liquified natural gas product, with a net profit of $56.85 million. Given that the feed pressure was already at 500 psig, obtaining a liquified natural gas product operating at 600 psig was obtainable, cost friendly, and generated the most revenue. An overall comparison of each product stream is shown in Figure 8.
Note: Absorption using Water, Absorption using MDEA, and Gas Separation Membranes were done by other group partners George Burdge, Leila Toth and Tara Wahn. The link for the entire project can be found here.
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