Developing Natural-gas Treatment Systems For Gas Sources With High CO2

This research focuses on the efficiency of the cryogenic carbon capture process. This process can be used instead of other common methods such as solvent-based CO₂ removal, absorbent drying, and cryogenic natural gas liquid extraction systems. The use of fossil fuels continues to increase to meet the power demands of the world.

This increases the concentration of CO₂ in the atmosphere and has been attributed to climate change by climate scientists. Carbon capture technology can play a major role in reducing the threat to the climate. Two major challenges facing this technology include cost and efficiency. Carbon capture in current power plants uses up 30% of the energy, which in turn increases power costs. There are three different categories of carbon capture: precombustion, changes in combustion technology, and post-combustion.

Pre-combustion carbon capture involves the removal of carbon before combustion through gasification processes. Examples of changes in combustion technology include oxyfuel and chemical-looping combustion. These changes, as well as pre-combustion carbon capture, are more logical in new power plants.

Post-combustion carbon capture removes CO₂ following combustion. There are four major post-combustion processes: absorption, adsorption, physical separation, and hybrid solutions.

Absorption processes use solvents that specifically absorb CO₂. High CO₂ partial pressures and low temperatures increase the solubility of CO­₂. Potassium carbonate and amine-based solvents are the two more common absorbents.

Adsorption processes use an exothermic reaction of a solvent to remove CO₂. This reaction is then reversed through an endothermic process. As the gases flow through the fixed bed, the acid gases adsorb onto solid particles.

Physical separation can be completed via a membrane. Membranes are effective at removing high concentrations of acid gases due to the high partial pressures.

Cryogenic carbon capture (CCC) is an example of a hybrid separation process. CCC removes solid CO₂ in desublimating heat exchangers. CCC removes excess CO_{2 ­}from the natural gas streams before the gas is processed.

In this experiment, the pressure ranged from 10 to 400 psi (1 psi = 6.89 kPa) with a constant temperature of -133°C. The highest experimental capture was observed at 206.84 kPa. As the pressure increased, efficiency decreased as methane in the vapor condensed and produced more liquid in which the solid CO₂ could dissolve.

The composition of the natural gas also affects CCC efficiency. As the concentration of methane, ethane, and propane each increase, the amount of CO­₂ in the vapor stream also increases, which reduces the CO₂ capture. The operating costs depend on the market demands and the cost of raw materials. No economic analysis could be done in this experiment due to these possible fluctuations of the market.

The heat exchangers are insulated, which conserves enthalpy, and do not require shaft work. This makes it difficult to use either a first-law definition or a second-law definition. However, a second-law definition was used to define the experimental heat exchanger efficiency at 92.2%.

In conclusion, higher pressures and low methane concentrations in the natural gas result in increased CO₂ capture, with intermediate pressures showing maximum capture efficiency.

This study, Effect of Operating Conditions on Cryogenic Carbon Dioxide Removal was recently published in the journal Energy Technology. The study was published by Farhad Fazlollahi, Mohammad Saeed Safdari and Larry L. Baxter from the Chemical Engineering Department, Brigham Young University, USA.