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Process design of supercritical CO2 fractionation column

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Supercritical CO2 fluid extraction is based on the selectivity of supercritical CO2 fluid to the solubility of solutes to achieve the separation of material components. Read More: Supercritical carbon dioxide

Due to the complexity of the dissolution of solutes in supercritical CO2 fluids, if different solutes have different solubility in supercritical fluids, they can be separated through multi-stage separation tanks; in the process of equipment design, the design of supercritical CO2 fractionation columns is One of the indispensable contents.

Process design of supercritical CO2 fractionation column
30 supercritical CO2 fractionation column

Read More: Supercritical CO2 Extraction Machine

What is supercritical CO2 fractionation?

Supercritical CO2 fractionation is the process of separating components by utilizing the volatility difference of each component under the same conditions. The key to supercritical CO2 fractionation operation is to master the material balance, vapor-liquid balance, and heat balance in the whole operation process. . These three influence each other and restrict each other. Generally, it is mainly based on the change in the material balance, and the heat balance is adjusted to achieve the vapor-liquid balance. After normal operation, the feed volume, overhead distillate volume, reflux volume, and column bottom output volume should be stable. To this end, it is necessary to observe and control the feed temperature, column top, column bottom temperature, column bottom pressure, and the design of the reboiler.

Read More: CO2 Extraction Vessels: 4 Basics, 6 Special Requirements, 7 Important Parts

Design requirements

It is required that the production capacity of the supercritical CO2 fractionation column to treat the feed liquid is F, and the content of the components to be separated in the feed liquid is xF. It is required that the flow rate of the overhead liquid is D, the content of the component to be separated is xD, the flow rate of the bottom fluid is W, and the content of the component to be separated is xW.

Process Calculation of Auxiliary Equipment

  • Supercritical CO2 Fractionation Column Top Condenser:
    The function of the top condenser of the supercritical CO2 fractionation column is to provide cooling capacity for the condensation of the rising steam. Generally speaking, the supercritical CO2 fractionation column overhead condenser is a partial condenser, that is, only a part of the steam rising in the fractionation column is condensed, and the rest is refluxed into the fractionation column for further processing, which is essentially a heat exchanger. , the process calculation can refer to the process calculation of the aforementioned heat exchanger.
  • Supercritical CO2 fractionation column bottom reboiler:
    The fractionation column bottom reboiler uses steam or other high-temperature medium to heat the material at the bottom of the column to boiling. Its function is to provide heat for the volatilization of volatile components. Its process calculation can refer to the process calculation of the evaporator.
6 factors reduce the cost of supercritical CO2 fractionation
6 factors reduce the cost of supercritical CO2 fractionation

Optimal Design of Supercritical CO2 Fractionation Column

The CO2 fractionation process is characterized by repeated vaporization and condensation, that is, components with different boiling points are separated through a series of fractional vaporization and condensation, so its thermal efficiency is very low. Usually, the energy heated by the reboiler to the fractionation column is more than 95%. Taken away by the cooling water of the top condenser of the CO2 fractionation column, only about 5% of the energy is effectively utilized. Due to the rapid rise of energy prices, people are forced to find ways to reduce energy consumption. How to reduce the energy consumption of the fractionation system has become the focus of chemical design workers. At present, the energy-saving means of the fractionation system are: recovery of sensible heat; recovery of latent heat; reduction of heat loss in the system; reduction of the reflux ratio to reduce the energy demand of the system

How to choose the best reflux ratio

In the process of CO2 fractionation column design, whether the optimal design of the CO2 fractionation system can be realized, the most critical issue is the selection of reflux ratio parameters. The optimal reflux ratio is generally determined by analyzing the relationship between the reflux ratio and the equipment cost and operating cost in the CO2 fractionation system and then determined through economic calculation. When the sum of the equipment cost and the operating cost is the minimum, the corresponding reflux ratio R is is the best reflux ratio.

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