Q & A > Question Details
In our Vacuum Distillation Unit we have a vacuum system consisting of overhead water coolers, two ejectors with condensers and a liquid ring vacuum pump. During the summer time when the cooling water to overhead coolers and ejector’s condensers increase above 23C we observe increased pressure on the top of the vacuum column to approx 5-6 Kpa (from 3 Kpa) which is of course logical. In this situation when we want to increase the temp to the vacuum column in order to maximize yields , the pressure will increase too and the result is quite opposite. The significant fluctuation of temp of cooling water (sometimes during the day it is about 5C) makes it also impossible to optimize yields from the vacuum column. Please advise what to do in this situation in respect to our plant, how we can keep pressure steady and low , how to improve yields in this circumstances from the vacuum column. Maybe you have similar problems and please write how you deal with that. Some recommended literature/web sites on this subject would be very helpful too.
20/07/2012 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
If my understanding of the problem is correct, the back pressure created by the uncondensed vapor is not allowing the increasing the feed temperature to Vacuum column. Please check if there is any condensable vapor left out in the hot well vent. This could be a simple inadequate cooling problem as some of the respondents suggested. If there are no condensible gases left, the problem could be inadequacy of the total system. If the problem is only related to summer time that could be solved by some simple solutions. The following may sound ridiculous however worth giving a look as these can be inexpensive:
a. Check if the cooling water temperature raise during summers can be prevented (insulate the headers ?)
b. Try condensing more by providing additional cooling to the condensers (chilled water tubing around the vapor inlets to the condensers ?). This facility may need to be operated during summers only.
22/07/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
I think the first answer by Mr. Ragsdale pretty well covers the subject.
Replacing the internals in the column to reduce pressure drop is another way to increase recovery of heavy components. Cleaning the flash zone internals before the hot season to lower flash zone pressure drop may also be helpful.
Some crude oils will thermally crack when contacting higher furnace tube wall temperatures, actually decreasing HVGO yields due to increased pressure drop through the column from the large increase in vapor traffic and very large increase in non-condensables reaching the vacuum generators.
It is also worthwhile to inspect and reseal all possible air ingress leaks points to lower amount of non-condensables the vacuum system has to handle.
22/07/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
You have my sympathy. In one plant I worked in for many years, we studied the cooling water system every summer and never spent any money to increase its capacity. One of your solutions is to solve the cooling water supply problem. Another solution is to increase the efficiency of the use of water available to the unit in question, by improving the design. Eliminate the precondenser. Replace the first stage ejector with a larger one to be able to handle the steam not condensed in the removed precondenser. Rearrange the available condensers to handle the larger load downstream of the new ejector. This is my preferred design – no precondenser.
22/07/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
You can try maximizing your LVGO pumparound duty to decrease the volume and temperature of the vapor feeding your ejectors to help off load them.
You also want to make sure that your condensers are functioning properly. Check the cooling water flow to make sure that you are getting adequate cooling water circulation to them. The surface condensers are normally located way up in the air and have trouble getting design flow rate. If you aren't getting adequate flow to them, you may need to consider installing some booster pumps. If you are not back flushing the exchangeres, consider starting a regular back flushing schedule. Also make sure that your cooling towers are performing up to design and that cooling water supply temperature is meeting expectations.
The other factor to think about when trying to increase vac heater outlet temperature, is cracking. If your temperature gets high enough to accelerate cracking for the specific crudes being processed, then the gas make will rise significantly, which can make your column pressure up. I assume that you have all your ejectors on line. If not that is an easy first step to lower pressure.