Technical exchange: Three-component separation simulation of CS2, H2O and air
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There is a gas consisting of CS2 (596kg/h), H2O (16kg/h), and air (115kg/h). The temperature is 39°C and the pressure is 100Pag. It is cooled by a condenser with an area of 16m2 and an outlet temperature of 20°C. What is the composition after cooling? Use chilled water at 10°C and estimate the flow rate at 10t/h. The following error always occurs in the condenser. Why? Using NRTL, the Henry component is not defined, and the phase state gas-liquid is calculated. If it is changed to gas-liquid-liquid, a warning of missing parameters will appear and cannot be calculated.
According to the given conditions, the compositions of CS2, H2O and air are 596kg/h, 16kg/h and 115kg/h respectively, mixed at a temperature of 39°C and a pressure of 100Pag. After cooling through the condenser, the outlet temperature is 20°C. Since CS2 and H2O are organic matter and water vapor, they have lower boiling points. In the condenser, these two components are first condensed into liquids, while the air remains in the gaseous state. The flow rate of chilled water at 10°C is estimated to be 10t/h, which can be circulated outside the condenser to further reduce the temperature of the mixed gas. Ultimately, the cooled composition should include liquid CS2 and H2O, and gaseous air. The specific composition ratio needs to be calculated. Regarding the problem of errors in the condenser, it may be caused by the NRTL method used which does not define the Henry component. Henry's constants are parameters that describe solubility. Without defining them, it is impossible to calculate a gas-liquid system. After changing the simulation conditions to gas-liquid-liquid, a warning of missing parameters will appear again. This may be because more parameters are needed to describe the multiphase system in this mode, and relevant parameters are missing. Therefore, it may be necessary to consult more literature or use other phase equilibrium calculation methods to solve these problems. .
Thank you so much! This simulation has always been inconsistent with production reality. There is also a phenomenon that occurs during simulation: In the simple mode, the outlet temperature is set to 20°C, and the required heat exchange area is found to be 3m3. However, when it is changed to the detailed mode shell and tube, after inputting the actual equipment parameters (15m2), the outlet temperature is higher than 20°C. What could be the problem? Thanks
Under the simple mode calculation, the total heat transfer coefficient of the heat exchanger needs to be estimated and input manually. If not entered, the default total heat transfer coefficient is used. This coefficient is generally relatively large, so the calculated heat exchange area is small. In the detailed mode, the total heat transfer coefficient is calculated based on the mechanical structure and physical parameters of the equipment. Under this material system, the calculated total heat transfer coefficient is estimated to be much smaller than the system's default total heat transfer coefficient in the simple mode, and the calculated outlet temperature may be calculated to be higher.
Do not use air, use real nitrogen and oxygen. In addition, it is not recommended to use NRTL, it is best to use NRTL-RK. There is no problem in using PR for this system. Both gas-liquid and gas-liquid liquid can be calculated. In fact, it should be gas-liquid-liquid. Use the gas-liquid-liquid model to calculate NRTL-RK and PR. The two results are similar. About 400kg of CS2 and 13kg of water are cooled.