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Dehydration of natural gas is necessary to prevent the formation of gas hydrates, which can plug valves and other components of a gas pipeline, and also to reduce potential corrosion problems. Water removal can be accomplished as shown in the following schematic diagram: Natural gas containing \(80 \mathrm{lb}_{\mathrm{m}} \mathrm{H}_{2} \mathrm{O} / 10^{6} \mathrm{SCF}\) gas \(\left[\mathrm{SCF}=\mathrm{ft}^{3}(\mathrm{STP})\right]\) enters the bottom of an absorber at a rate of \(4.0 \times 10^{6}\) SCF/day. A liquid stream containing triethylene glycol (TEG, molecular weight \(=150.2\) ) and a small amount of water is fed to the top of the absorber. The absorber operates at 500 psia and \(90^{\circ} \mathrm{F}\). The dried gas leaving the absorber contains \(10 \mathrm{lb}_{\mathrm{m}} \mathrm{H}_{2} \mathrm{O} / 10^{6} \mathrm{SCF}\) gas. The solvent leaving the absorber, which contains all the TEG-water mixture fed to the column plus all the water absorbed from the natural gas, goes to a distillation column. The overhead product stream from the distillation column contains only liquid water. The bottoms product stream, which contains TEG and water, is the stream recycled to the absorber.(a) Draw and completely label a flowchart of this process. Calculate the mass flow rate ( \(\left(\mathrm{b}_{\mathrm{m}} / \mathrm{day}\right)\) and volumetric flow rate (ft \(^{3}\) /day) of the overhead product from the distillation column. (b) The greatest possible amount of dehydration is achieved if the gas leaving the absorption column is in equilibrium with the solvent entering the column. If the Henry's law constant for water in TEG at \(90^{\circ} \mathrm{F}\) is \(0.398 \mathrm{psia} / \mathrm{mol}\) fraction, what is the maximum allowable mole fraction of water in the solvent fed to the absorber?(c) A column of infinite height would be required to achieve equilibrium between the gas and liquid at the top of the absorber. For the desired separation to be achieved in practice, the mole fraction of water in the entering solvent must be less than the value calculated in Part (b). Suppose it is \(80 \%\) of that value and the flow rate of TEG in the recirculating solvent is 37 Ib \(_{\mathrm{m}}\) TEG/lb \(_{\mathrm{m}}\) water absorbed in the column. Calculate the flow rate ( \(\left(\mathrm{b}_{\mathrm{m}} / \mathrm{day}\right)\) of the solvent stream entering the absorber and the mole fraction of water in the solvent stream leaving the absorber. (d) What is the purpose of the distillation column in the process? (Hint: Think about how the process would operate without it.)

Step by step solution

01

Draw a flowchart

Make a flowchart of the process, labeling all incoming and outgoing streams, process units like the absorber and distillation column, and the compositions and flow rates of the streams.

02

Calculate the mass flow rate

The mass flow rate of the overhead product from the distillation column can be calculated by taking the difference between the water content in the gas entering and leaving the absorber, and multiplying it by the gas flow rate.

03

Calculate the volumetric flow rate

The volumetric flow rate of the overhead product can be calculated by dividing the mass flow rate by the density of water.

04

Determine the maximum mole fraction of water

Using Henry's law, the maximum mole fraction of water in the solvent can be calculated by dividing the pressure of water in the gas by Henry's law constant.

05

Calculate the flow rate of the solvent

The flow rate of the solvent entering the absorber can be estimated by dividing the mass flow rate of water absorbed in the column by the flow rate of TEG in the recirculating solvent, assuming the mole fraction of water in the solvent is 80% of the maximum value.

06

Calculate the mole fraction of water in the solvent leaving the absorber

The water balance around the absorber can be used to estimate the mole fraction of water in the solvent leaving the absorber.

07

Describe the purpose of the distillation column

The distillation column serves to separate the TEG-water mixture, removing the absorbed water and recycling the TEG back to the absorber.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Gas Hydrates Prevention

Gas hydrates, often referred to as 'clathrates', are crystalline water-based solids that resemble ice, and they are known to form under high-pressure and low-temperature conditions typically found in natural gas pipelines. The presence of hydrates in these pipelines can be problematic, leading to blockages and potential operational disruptions. To prevent the formation of gas hydrates, natural gas must undergo dehydration, a process where water vapor is removed from the gas before transportation or storage.

One of the most common dehydration methods used in the industry is through the absorption of water using triethylene glycol (TEG), as illustrated in the textbook exercise. The TEG absorbs water from the natural gas in an absorber column, becoming rich in water content. It is then cycled through a distillation column, where the water is removed, allowing the regenerated TEG to be recirculated back for further absorption. This continuous loop is crucial for effective gas hydrates prevention and maintaining the flow of natural gas through pipelines.

By understanding the risks associated with gas hydrates and incorporating systems to remove moisture effectively, significant downtime and maintenance costs can be avoided. The challenge lies in designing these dehydration systems to ensure optimized performance, considering factors such as gas composition, temperature, pressure, and flow rates. Chemical engineers must thus apply rigorous principles when sizing equipment, determining TEG circulation rates, and ensuring the quality of the dehydrated gas meets the required specifications.

Natural Gas Processing

Processing natural gas to make it suitable for commercial and industrial use is a complex series of operations that involve removing impurities and various non-methane hydrocarbons and fluids to produce pipeline-quality dry natural gas. Dehydration of natural gas, which is the main focus of the given exercise, is one of these critical processing steps. This step is essential not only for preventing gas hydrates but also to minimize corrosion in pipelines and processing equipment due to the presence of water.

The use of desiccants such as triethylene glycol in the dehydration process, as showcased in the exercise, is a common practice. The dried gas, after being stripped of water vapor, meets pipeline transportation standards and prevents hydrate formation. High-level engineering decisions must be taken into account when designing such processing systems. These include the selection of appropriate dehydration techniques, the capacity of the processing plant, and the ensured recoverability and purity of the processed gas.

Other important steps in natural gas processing include acid gas removal, mercury removal, and nitrogen rejection, all of which are aimed at ensuring that the final product is safe, efficient to transport, and meets market or regulatory standards. Through the proper application of chemical engineering principles and technology, natural gas can be treated for safe and efficient end-use.

Chemical Engineering Principles

Chemical engineering involves the application of various fundamental principles to innovate and optimize processes within the chemical and energy industries. In the context of natural gas processing, such as the dehydration process described in the exercise, these principles include mass and energy balances, thermodynamics, fluid mechanics, and transport phenomena.

Mass balance is particularly prominent in the problem exercise, where the flow rates and compositions of water in the TEG solvent stream must be calculated based on the water absorbed from the gas. Energy balance considerations would also be crucial in a real-world scenario, especially when it comes to the distillation process, where heat integration could improve efficiency. Thermodynamics enables the understanding and application of phase equilibria (like Henry's law in the exercise) and helps predict the behavior of the gas and absorbent under varying conditions. Fluid mechanics principles aid in designing pipelines and absorbers ensuring that the pressure drops and flow patterns are conducive to an optimum separation process.

Furthermore, chemical engineering principles are not only applied to these technical aspects but are also critical in areas such as safety, environmental impact assessments, and economic feasibility studies. Modern chemical engineers must balance these considerations while adhering to industry standards and achieving the desired process outcomes. By leveraging these principles, engineers aim to design processes that are efficient, sustainable, and operate safely within the defined economic and regulatory frameworks.