Q & A > Question Details
We have one solvent regneration column which has 4 baffle trays and below which there is a stub-in reboiler at the bottom. Also at the bottom, steam sparger is provided(we think same might have been provided to reduce the boiling point of the column bottoms liquid). Column operates at vacuum (-0.2 kg/cm2g). Column bottom temperature is 187 deg c.Fresh solvent is taken out from top and heavies are removed from bottom intermittantly.The stub in reboiler utilises MP steam(16.5 kg/cm2g) while sparger steam enters at 127 deg C & 1.5 kg/cm2g pressure. The problem is, tubes of this bottom stub in reboiler fail every year and many times, tube bundle is replaced. In recent inspection, the bottom two trays were also found to be fallen & accumulated at bottom.Wanted to know, whether is it a good practice to push steam through sparger in the liquid pool over the bundle of stub in reboiler? And, column bottoms liquid is at 187 deg C & steam enters at 127 deg C. Will that cause steam getting superheated and exert forces on tube bundle & trays above? what could be the possible reasons for both failures and what should be done to avoid them in future?Also, would appreciate any good material or information on mechanical stregthening of trays and various calculations involved.
21/08/2015 A: S Banik, Centre for High Technology, sbdr@rediffmail.com
Would appreciate if you can give some detail of the stub-in exchanger. Is it u-tube type? What is the tube length? Are the tubes welded or expanded in the tube sheet. We are also facing similar problem in a u-tube kettle reboiler. Specific volume of sparged steam would increase by 15-20% as it rises through the liquid column. If the sparger is located below the exchanger it can create problem by inducing vibration which may lead to tube failure. It is very difficult to predict vibration of heat exchangers. However suitable preventive design can be considered.
04/06/2015 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Also, develop a safe procedure to nitrogen test the regenerator for leaks. Since this would be a safety issue with liability implications, I cannot suggest a procedure.
04/06/2015 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Turn off the steam to the sparger and leave it off. Periodically check the performance of the feed tank gas blanket system for preventing air from entering the tank.
31/05/2015 A: Pragati Ranjan Giri, Kuwait Integrated Petroleum Industries Company, pragati_giri@yahoo.com
This is new to me to know that steam is being introduced in the pool of liquid. As you correctly said this sparger steam is being introduced to reduce boiling point of liquid by reducing partial pressure of the components. However, this facilitate to vaporize the lighter component(s) from the mixture.
In my opinion, to use steam for such purposes, the steam should be superheated enough to avoid any condensation inside the column; the steam should pass through all the trays of the column and condense only in overhead condenser. This superheated steam to be introduced above the bottom liquid level otherwise steam will carry over liquid droplets from the pool of the liquid to the trays above. This droplet may lead to flooding of the trays/column, and this excess liquid on the tray may increase the pressure drop across trays/section/column. If this pressure drop across trays + the excess hydrostatic pressure on the tray exceed the design load of the trays, it may lead to damage of the trays.
If the density and height of the liquid pool is much enough to create hydrostatic pressure at sparger outlet to produce condensate from this non-superheated steam at bottom of the liquid pool occasionally, this condensate may produce steam when it reaches the reboiler level causing very high sudden steam flow. This quantity of steam may damage reboiler tube / trays.
If there is any provision to collect sample from bottom of the column, you may check it for water condensation. And if the steam sparger is not required, it may be stopped for certain duration and check the impact.
28/05/2015 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I don't know what your solvent system is, but sometimes solvents form corrosive byproducts because they can degrade over time due to excessive time at temperature or from reaction with system contaminants such as oxygen. In other cases the solvent system picks up trace contaminants that are not removed in the solvent regeneration process, and the contaminants themselves are corrosive. Heat stable salt build up is common source of corrosion in amine systems. These HSS can form from exposure to oxygen or from reaction with ionic contaminants in the process.