Effect of distillation column reflux condition

Reflux in distillation columns is used to maintain stability in the operating conditions of the stripping section. Slightly subcooled reflux allows the theoretical reflux ratio to be increased without having to raise the amount of reflux fluid. When this reflux fluid enters the stripping section, it causes a large amount of the vapor rising from that section to condense, thereby improving the purity of the product at the top of the column while also ensuring an adequate quantity of such product. One downside, however, is that it increases the heat load on the bottom of the column, resulting in higher energy consumption. If the product has a high added value, then it is still economically viable – it’s much more cost-effective than using saturated liquid reflux!

For distillation units equipped with full-condensation condensers, cold reflux is used in most industrial applications. The main reasons for this are as follows: 1. The vapor at the top of the tower can be completely condensed during this process, thereby reducing vapor emissions and associated losses. 2. It is not easy to control the temperature at the top of the full condensation tower to be at the saturated liquid state. 3. Omitting the cold reflux can increase the theoretical reflux ratio without increasing the reflux volume.

All of the approximately 30 distillation columns in our plant use subcooled liquid reflux; when performing the design calculations for these columns, we assumed subcooled liquid reflux as the operating condition. We are now carrying out modifications to enable feed using saturated liquid, but we have never encountered a situation where saturated liquid reflux is used. If that were the case, the equipment investment for each unit would increase significantly, and the reflux ratio would also have to be raised, which would pose many problems. I wonder in what kind of columns the saturated liquid reflux that the original poster has seen is typically used – could you provide more details?

I learned that the main reason is that the reflow temperature used in this set of process kits is the same at 41 degrees, which found strange. I also have a question: when using sensitivity, should I choose sc-TEMP or T1 for the reflow temperature? No matter which one I select, it results in an error. What’s going on? I need some expert advice. Problem specifications modified at 23:45:41 on Monday, Feb 24, 1997 >> ->; Processing input specifications... ** ERROR WHILE CHECKING INPUT SPECIFICATIONS FOR THE VARIABLE “S-1” IN THE SENSITIVITY BLOCK: This variable is not specified in the block. You can provide an initial estimate in the block or vary another variable. I tried assigning an initial value, but it still doesn’t work; I really don’t know what to do. ->Processing of new specifications completed! Errors occurred while processing input specifications. Last edited by t*nh on 2009-2-25 00:04]

That’s the theory, and my sensitivity analysis also yields the same result. Thank you all for your help. But there’s a problem: why didn’t my calculations converge last night? Yet when I turned on the computer and ran them again this morning, it worked fine How strange!◎◎ SC-REFLUD TEMP (light component content): C 860.95346883.636360.95377881.272730.9540378.909090.95427476.545450.95450674.181820.95472871.818180.9549469.454550.95514467.090910.95533864.727270.95552462.363640.955703600.955874850.953616

May I ask how gas phase loss can be addressed by using bubble point reflux under reduced pressure conditions? Currently, our reflux temperature is 30 degrees Celsius, and there is liquid that has condensed in the buffer tank of the vacuum pump.

If product purity is attempted to be improved by using a lower reflux temperature, this increases the consumption of steam and cooling water; in such cases, it might be better to add a few more trays during the equipment design, although this entails a higher one-time investment.

At present, most distillation columns use external reflux from the condenser at the top of the column, especially in large columns. The advantage of this approach is that it simplifies installation and makes it easier to control the reflux ratio; as a result, the liquid condensed by the condenser is usually subcooled, and this helps to lower the temperature of the exhaust gas downstream of the condenser. However, from a developmental perspective, saturated liquid reflux is more beneficial for energy savings and helps to prevent cold fluids from exerting downward pressure, thereby facilitating control of the temperature at the top of the column. The downside is that this method is generally more suitable for small columns with internal reflux.

What you are referring to is a distillation column equipped with a subcooler. The purpose of this is to ensure that the reflux liquid supplied to the column via the throttle valve is in a saturated state, as required for distillation. When passing through the throttle valve, the pressure drops, and accordingly the saturation temperature also decreases, which causes part of the liquid to vaporize. This results in a reduced amount of reflux liquid entering the column, which is not favorable for distillation. The subcooler helps to reduce the rate of vaporization due to throttling； And it is also possible to recover cold energy in the subcooler.

For methanol distillation, it is advisable to maintain the reflux liquid temperature between 50 and 55 degrees; this ensures an appropriate reflux ratio while preventing waste of heat from the reactor bottom.

How do the tower pressure and temperature change after increasing the reflux? Please explain it from both the practical aspects of factory operations and theoretical perspectives. Thank you.