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NOVEL METHOD FOR GAS SEPARATIONS*Chris Wilson** and Miguel J. Bagajewicz

School of Chemical, Biological, and MaterialsUniversity of Oklahoma

*This work was done as part of the capstone Chemical Engineering class at the University of Oklahoma** Capstone Undergraduate students

AbstractRemoval of Nitrogen, CO2 and H2S from gas as well as separation of ethane and NGL is an important step in gas conditioning.

In this work , we present the economics of a new technology for gas conditioning and we compare it with that of using current existing technologies (amine sweetening, PSA for N2, demethanizer and deethanizer plants).

Amine Treating (Gas Sweetening Process)This processes uses an absorber to remove hydrogen sulfide and carbon dioxide from a natural gas stream. An amine flows down the absorber, where it comes into contact with the sour natural gas. The amine absorbs the contaminants, producing a sweet gas that exits the top of the absorber. The amine solution exits the bottom of the absorber (the pink line) and is pumped into the stripper (see the picture) .in this column the amine (rich with H2S and CO2) is heated, causing the contaminants to dissociate from the amine. The contaminants exit the top of the stripper, and the amine is recycled back into the absorber.

Economic comparison This table shows the possible outcomes for the novel process of gas separation. The four rows that are gray are based off of the refrigeration price that is not bulk (cost for 40 thirty pound tanks). The bottom four are under the assumption that if the refrigeration can be bought in large bulk, it should only cost 70% of the semi-bulk value. The current method that was analyzed includes the total costs for amine treating, demethanization, and deethanization. Three trials of the simulations were conducted, due to uncertainty of a major variable. The economic evaluation is set up to allow for the cost to be calculated after research is done to understand the major variable more accurately.

Novel method for gas processingAbove is a representation of the novel method. The technical detail of how the separation occurs is momentarily withheld. This process separates each component into pure methane, pure carbon dioxide and so on. This is one continuous process.

Current ProcessOnly four out of the seven processes used to separate gas are displayed in this flow sheet. The reason being, the novel method developed only performs like these four processes.

Natural Gas ProcessingThe gas passes through a dehydration process where all the free liquid water, and natural gas condensate is removed. Next, the gas goes through an acid gas removal process, ridding the stream of hydrogen sulfide and carbon dioxide causing the stream to be sweet. Thirdly another dehydration process, normally being glycol dehydration, is used to absorb the water vapor. The Stream then any mercury removed from it through chemisorption process using activated carbon. The last contaminant, nitrogen is then removed, normally by a cryogenic process. Methane and C2+ are left to be separated. A cryogenic process, involving a turbo expander is used to liquefy the gas, allowing for distillation of methane. Distillation columns are then used to separate ethane, propane, butane, and any pentane+ hydrocarbons.

Simulation of Demethanizer

SIMSCI’s ProII was used to evaluate the economics of the demethanizer plant. Fluid flows through two heat exchangers, which is cooled by the pump arounds from the column. The stream is then passed through a refrigeration unit, to chill it even more. A turbo expander, drops the temperature to a low enough point so that the stream can be distilled.

Pressure Swing AdsorptionPressure swing adsorption is used in the process to remove nitrogen. The natural gas and the adsorbent enter a high pressure vessel. The nitrogen adsorbs into the adsorbent at the high partial pressure. (b). shows the relationship between fraction of component adsorbed relative to the partial pressure. Higher pressure gives higher adsorption. Once the nitrogen has been removed from the gas stream, the adsorbent is sent to a regeneration mode, which is a low pressure vessel. At this point the nitrogen dissociates and exits the vessel, while the adsorbent is sent back to be used in the high pressure vessel.

Simulation of Amine TreatingA simulation representing the amine treating cycle used to do the economic calculations. The process is explained in the black picture to the left.

ConclusionCurrently the method for processing gas requires 8 different steps to separate natural gas. The annualized cost is $7.49 per 1000 cubic feet of gas. A novel method has been developed, which reduces the annualized cost drastically. This new method has the ability to bring the annualized cost down to $2.17 at best and to $5.80 at worst. Whether the result is better or worst case scenario, this method is more cost effective than the current.

Simulation of DeethanizationThis is the simulation used for the economic analysis of this process

AcknowledgementsAlbert Curtis – Senior Capstone studentNina Wright – Senior Capstone student

Current Process $4,111.45 $6.74 $7.49Average Case Scenario $8,239.58 $1.24 $2.74Best Case Scenario $7,194.03 $0.98 $2.29Worst Case Scenario $16,697.63 $2.75 $5.80Current Process $3,697.91 $6.74 $7.41Average Case Scenario $7,459.32 $1.24 $2.60Best Case Scenario $6,523.00 $0.98 $2.17Worst Case Scenario $14,372.45 $2.75 $5.37

100%

70%

Fixed Capital Investment (per 1000 cubic feet)

Operating Costs (per 1000 cubic feet/day)

Annulized Cost (perday)

Natural Gas ProcessingAmine Treating