Determination of the number of trays in Aspen simulations
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I have only started working with tower simulations recently. When simulating an acidic water stripping tower (feed rate of 200,000 kg/h, temperature: 95°C, H2S: 2.7%, NH3: 4.2%), there are several issues that I am still not clear about; I would appreciate it if experts could provide some guidance: 1. The number of trays specified in the actual design documents is 32. So, when I do not take into account the hydraulic characteristics of the trays, how many trays should I use in the RADFRAC model? 2. When using the DSTUW model for calculations, I set the temperature at the top of the tower to 130 kp and at the bottom to 170 kp; this setup enables product separation. However, the model suggests that the number of trays required is no more than 10, and the temperature at the top of the tower is less than 60 degrees. This is quite different from the temperature at the top of the tower obtained using the RADFRAC model, which is 113 degrees. Could you tell me why this is the case? 3. When using DST1 to calculate DSTUW, the tower top temperature turned out to be negative. The temperatures in these two models differ from those in RAFRAC; could you please provide some guidance? 3. If, when performing the calculations, I provide data such as the number of trays, the tray spacing, and the tray diameter – for example, 15 theoretical trays with a plate efficiency of 0.7 set in the simulation – then what should the actual number of trays be in the design process? If I don’t set the plate efficiency, is it sufficient to use the conventional minimum of 2 times the number of theoretical plates? The question is too lengthy; sorry, thank you! reward_7ree
This post was last edited by kkiinnggq on 2016-11-8 at 22:46. The actual number of trays is 32, and there are two approaches for simulation. First, the theoretical number of plates is obtained by directly multiplying 32 by the tray efficiency. Enter the number of theoretical plates in Aspen. Secondly, enter 32 for the number of plates; when performing efficiency simulation calculations based on the distribution across each plate, it is not necessary to enter the details of the plate structure first. The theoretical plate efficiency is 1; plate efficiency refers to the actual plate
1. If radfrac is calculated using the rate model, then 32 plates should be sufficient. In the case of the equilibrium model, there is a distinction between the overall tower efficiency and the efficiency of individual plates. If the overall tower efficiency is known, the theoretical number of plates required can be obtained by multiplying 32 by that efficiency. For the efficiency of individual plates, simply enter 32; it’s okay if the efficiency value is the same for all plates. 2 3. First, check whether the input data is correct – gauge pressure, absolute pressure, etc. If everything is correct, it’s still possible that the efficiency is very low; however, with such a large difference in temperature at the bottom of the tower, it’s unclear whether it’s due to an incorrect setting. 4. In reality, it’s 15/0.7; some margin is added here. The minimum of 2 times the theoretical number of plates still represents the theoretical value, not the actual number of plates. The actual number of plates is obtained by dividing 2 times the minimum theoretical number of plates by the actual efficiency.