bump over in vacuum evaporator 1

[ss123]

I am working for a client who has an evaporator operating under vacuum at 70mmHg which suffers from regular bumpovers. The liquor feed is a mixture consisting of nitric acid and water fed at approx 35C and is fed continuously. During operation, the evaporator has no liquid outlet flow and the function of the evaporator is to concentrate the feed by vaporising essentially water and nitric acid.

Could someone explain to me, why bumpovers would occur in such a system?

 

[rmw]

Bumpover is not a word in my vocabulary, but intuitively, I think it must be what I know as carryover, or priming. If so, I have the following questions.

What kind of heat source drives the evaporator, how is it controlled, and how is it drained or trapped, if it is steam. And, if it is steam, what is the temperature and pressure?

I assume you are saying 70mmhg absolute rather than 70mm of vacuum below atmospheric. It can be ambiguous.

Where does the evaporated vapor go?? How is that process controlled?? Are there any flow variations in the vapor outlet. If it is a condenser, what type condenser is it?? If the vapor outlet is varying, the fluctuation in flow, and hence evaporator vessel pressure could be causing a change in the boiling rate of the nitric acid/water solution.

You really haven't given much information, so I am just stabbing in the dark. Give us some more information to work with.

rmw

 

[unclesyd]

You might take a look at the single effect evaporator model at the following site.

http://www.carf-engineering.com/

 

[ss123]

Sorry rmw, I have been too brief. Here are the answers to the questions:
1. I refer to bump over to mean carry over of liquor along with the vapour route.
2. The heat source is heating coils and a jacket. It is heated by a continous supply of 2barg saturated pressure steam. The evaporator is controlled by fixing the pressure and this will fix the liquor temperature to about 60C. Liquor volume is kept constant by varying the feed flow.
3. The vapors leaves the evaporator via an overhead shell and tube condenser and steam ejector.

 

[CARF]

It may be a good idea to check the levels and level controllers with a datalogger. If the level becomes to high (no matter what causes this) you will have carry-over.

CARF

 

[unclesyd]

rmw,
Bumping was very common term in the distillation part of the chemical industry. It generally refereed to a sudden increase in the vapor load in a column especially on a packed column, but it also applied to all other types of distillation columns.
Bumping was far more common when the most of the control instruments were single point pneumatic. It was quite common to have a "bump" on the chart where a straight line was desired.
It was caused by several different things, but the most common one was the loss of pressure control on a vacuum column. Other sources were a drastic change in feed composition, correcting reboiler flooding, or essentially any non-steady state operation that could affect the base composition.
It could be quite destructive on the column internals. Ceramic Raschig rings were especially vulnerable. Even on large vacuum columns, trays, the effect could be devastating.
You have never lived until you are standing beside a 12' x 80' column operating at < 10 mm Hg and 225°C get "bumped" due to sudden influx of low boiling components in the feed.

We saw a rapid decrease in such events with the introduction of hybrid controllers where the loops could be closed and even more so with the electronic controllers with the more complex control schemes and the faster responding instruments.
It was an operator vs the electron. Guess who eventually won the control battle.

Anecdotal:
When I first started work each building had it's own control room lined with vertical panels and the associated controls and recorders, circular and multipoint. The response time of the instruments compared with today's instruments was horrendous. You feel something happening before the board caught anything. Every operator had a callus on his index finger from tapping the gauge glass to see if the needle was moving. To improve the process control the control room had no chairs, except one at the teletype where you got the lab results.
We also had Hg manometers on these panels to monitor tank levels.

 

[rmw]

Thanks, Unclesyd. Most of my evaporation experience has been water oriented. The newer controls you mention would still be driven ragged if the low boiling fractions in the feed were variable, wouldn't they?

ss123,

Just as I thought, carryover or priming.

1. Are you sure your steam is saturated?
2. Is your condensate evacuation equipment from the steam heating coils/jackets functioning correctly. A sluggish trap could give you the type problem you describe, if it was flooding a lot of the heat transfer surface, and then upon opening, suddenly exposing lots of surface area to live steam
3. "the evaporator is controlled by fixing the pressure" Does this mean that there is some type of control valve on the pressure outlet, or is the pressure fixed by the cooling water flow to the heat exchanger on the outlet?

The heat exchanger is obviously a condenser, and the jets remove any non condensables that the condenser doesn't get.

If the cooling water flow to it is not constant, it can cause the evaporator to prime.

Is the condenser flooding? This, just like the jacketing above could cover and expose heat transfer surface, and cause the unit to prime.

The feed flow is the least of my concerns. If anything, the feed would tend to quench the boiling in the evaporator, not cause it to prime.

Is there any mist elimination equipment in the top of the evaporator vessel?? If there is, and it is loading up, that can cause an evaporator to prime.

Carf's point about the level controls is worth checking out.

The process you describe should be very steady state. Something is varying.

Back to fixing the pressure. If that is done with some kind of valving, then that could be your problem. IF that controller is sluggish, and the pressure builds up beyond set point, and the valve moves suddenly to reduce it, the liquid will boil violently as its saturation pressure equalizes, and that can cause things to go "bump" in the middle of the night.

I hope this gives you some things to check out, and that you find your problem. Keep us posted.

rmw

 

[CARF]

Hi all,

Guess rmw covered most of the things to check and consider, so a star for that ; )

As I'm typing now a can hear the beautiful sound of a triple effect evaporator. Sometimes we have carry over when the levels are to high or the fluid is very thin. This we can see in the cooling water circuit as it becomes polluted with product.

No more about the controls and control valves. It took me 5 days to find out that the steam control valve was not closing properly giving aThe most typical cause of cycling in process plants is from valve stiction or hysteresis. Occasionally aggressive PID tuning and sometimes, the equipment design or non-linearities will create a cycle. 30% steam flow when we thought it was closed. After this shocking discovery (why we cannot control this thing ?) I asked strongly to replace this valve. (A taxi driver going to Athens on saturday to get a new valve, installation sunday night ...) Now we are in control.

Lessons learned: check all control valves (so also the level control valves to the effects) the may leak, stick or have hysteresis. The most typical cause of cycling (i.e. oscillations in levels) in process plants is from valve stiction or hysteresis. Occasionally aggressive PID tuning and sometimes, the equipment design or non-linearities will create a cycle.

Hope this helps.

1,2,3 effect model:

http://home.hccnet.nl/m.dijk/pressure_drop_calculator/calcs/calcs.html

CARF

 

[CARF]

Apologies my english and chaotic text, correction:

Hi all,

Guess rmw covered most of the things to check and consider, so a star for that ; )

As I'm typing now I can hear the beautiful sound of a triple effect evaporator. Sometimes we have carry over when the levels are too high or the fluid is very thin. This we can see in the cooling water circuit as it becomes polluted with product.

Now more about the controls and control valves. It took me 5 days to find out that the steam control valve was not closing properly giving 30% steam flow when we thought it was closed. After this shocking discovery (why we cannot control this thing ?) I asked strongly to replace this valve. (A taxi driver going to Athens on saturday to get a new valve, installation sunday night ...) Now we are in control !

Lessons learned: check all control valves (so also the level control valves to the effects) the may leak, stick or have hysteresis. The most typical cause of cycling (i.e. oscillations in levels) in process plants is from control valve stiction or hysteresis. Occasionally aggressive PID tuning and sometimes, the equipment design or non-linearities will create a cycle.

Hope this helps.

1,2,3 effect model:

http://home.hccnet.nl/m.dijk/pressure_drop_calculator/calcs/calcs.html

CARF