3 replies to this topic

[oneduke]

Hi...i just want to ask whether we can operate a sieve tray column in vacuum? if we can how we could create the vacuum system?

[Art Montemayor]

oneduke:

In the archives of this Student Forum, you will find a thread titled "Ejectors" that was started July 23, 2003 by Febz Beloy. Read it through and find the resource available to resolve your problem.

If you are interested in the solution offered, follow the instructions and you'll get what you're after. You have to be resourceful and aggressive to be a successful Chemical Engineer.

Art Montemayor
Spring, TX

[siretb]

Yes, you can operate a sieve tray under vacuum. Packing can be a good solution , too because of lower preessure drop, if wetting rate is sufficient.
My main warning about sieve tray would be to have enough tray spacing; from experience, for economical reasons, the tendency to reduce tray spacing goes often too far.

[Art Montemayor]

siretb:

You've brought out a very important and vital point about tray towers. I know this skirts the main question and may introduce another issue, but I feel very strongly about the safety involved (as I'm sure you do too) and since this is a Student Forum, I would take this opportunity to share some experience on the subject you bring up.

Unfortunately for tray towers, an expensive trade-off is involved in their design and application: the vertical distance (height) between the physical trays. In the real-world of process engineering, the reality of what a tray tower can impact and the design parameters required for its successful application are suddenly presented at the time the tower has to undergo internal inspection, repairs, or modifications. It is at this occasion that the impact of the tray spacing hits home. The prime, important factor for a process engineer should be the ability to enable a human being to safely enter, inspect, and repair a tray tower without need for assuming risk or danger - all this while wearing safety equipment and carrying tools and equipment.

Common sense dictates that Manways, ladders and platforms as well as lighting and utility manifolds are required at every point of ingress to the tower. This, of course, becomes expensive and designers often resort to minimizing the external entrance facilities by making hinged or bolted assemblies on the trays that allow personnel to enter at one point and climb (or descend) internally from tray-to-tray. This, theoretically, reduces the external facilities down to two (the same as for a packed tower). However, this is seldom acceptable due to the practical need to have strategic escape facilities in case of accidents, medical needs, emergencies, and other safety measures. What all this then winds up being is that the trays must have a height sufficient so that a person can safely get in and out - as well as repair, weld, tighten, install, etc. within each tray. This "safe" height, then, becomes a serious subject involving the economics of fabricating the tray tower.

Management is always trying to reduce the capital cost; process engineers are always trying to ensure that operation and maintenance can be safely done. The two issues butt head-to-head at this junction. I personally have experienced the entrance and exit of many tray towers in the past as a young boilermaker and later as an engineer - and I prefer to stay out of them! It is my experienced judgement that an average-height human needs approximately 28-30" MINIMUM height to be able to quickly and safely work inside a tray tower. I normally insist on a 36" design height and management normally starts complaining of excessive costs. Bear in mind that they are correct - it costs a lot of money to install 36" manways every six or ten trays. This is a tough issue that touches on the sensitive subject of personnel safety in confinded quarters - one of the leading causes of industrial process plant accidental deaths.

A young engineer should be ready to put his gloves on when this issue comes up and be ready to defend his/her opinions. You must be prepared to fight for the correct solution as regards safe, practical, efficient operation and maintenance in the work place. If you don't take a stand at the outset, you will later pay for the results when you are put in charge of an inspection or tray repair job.

This is a subject that unfortunately is never discussed or mentioned in Unit Operations courses or Laboratories in the university - to the detriment of the students. But it is one of many practical realities that all students will have to confront when they go out to compete in industry. It is not an easy or pleasant subject to resolve, but that's why you're paid the big bucks - as your operators would say.

Art Montemayor
Spring, TX