22 replies to this topic

[rabindra]

I have a reboiler in my unit where the LP stream has no control valve. The Condensate line also has no control valve. The condensate is collected in a pot whose level is controlled through a level control valve.

In this type of system, generally the steam flow is controlled by controlling the heat transfer area in the exchanger by controlling the level. In my system the condensate pot is 1.4 m below the reboiler.

Will the system work? How should the operator control the steam flow ?

A PFD of the system is attached herewith.

Rabindra Patel

Attached Files

•  Steam Reboiler.xls   118.5KB   1681 downloads

[pavanayi]

Rabindra,
This topic has been discussed previously. Please use the search to find related discussions like this one.
http://www.cheresour...__fromsearch__1
This post should answer your query.

[rabindra]

Thanks Pavanayi

I still have a doubt whether the reboiler will function if the condensate pot is below the reboiler itself, without any control valve either in steam side or condensate side. What is the controlling factor that will determine the steam flow?

rabindra

[Zauberberg]

Rabindra,

Have a look at the referenced article: http://www.driedger....4_sh/Ce4_sh.pdf

and let us know if you have further questions.

[kkala]

Probably steam flow is motivated by the lower temperature of hydrocarbon fluid (HC) in the reboiler itself. This causes steam condensation and liquid flow by gravity into the drum, which is being emptied by the level control valve. In this sense ingoing steam is self controlled, while no extra control valve is used. Under actual configuration, reboiler will never be filled (totally or partially) by condensate (*). Condensation will decrease when
Δt decreases at low column duty ( i.e. HC temp approaching steam temperature) and increase when Δt increases at higher column duty (i.e. HC temp getting lower than steam temperature).
Imagine 12" inlet line, reboiler side, 3" outlet line, condensate drum and equalization line full of steam. Then HC (of lower than steam temperature) fills the other side of reboiler. Steam is condensed as above, while heated HC goes up into the column by natural circulation.
Above scheme (naif?) does not ensure satisfactory operation under all conditions. It may have to follow rules similar to those empirically established for common reboilers.
Nevertheless I have only once participated in a design of vaporiser; and it could be filled with condensate. So comments / advice welcomed on the above interpretation. It seems not to be a common design case.

(*) On the condition that condensate "drainage" through 3" line is adequate at any case.

Edited by kkala, 30 September 2011 - 03:15 PM.

[pavanayi]

Kostas,
There is a much simpler explanation.
The steam from the inlet line enters the exchanger through a nozzle and expands. The pressure inside the steam chest is a bit less than the actual steam inlet line.
The condensate pot overhead is connected to the inlet steam line which means the pressure in the vapour space is not equal to the steam chest pressure. (similar to the above explanation).
To equalise the pressure difference, the pot is lowered (1.4m). This means the extra hydrostatic pressure on the condensate line roughly equals the pressure drop across the inlet nozzle.
There will be condensate inside the reboiler always, and the rate of reboiling is controlled by increasing/decreasing the exchanger area available, through the LIC of the condensate pot.

Rabindra,
The above mentioned fact that pressure of the steam line is not equal to pressure within the reboiler steam chest should always be kept in mind when designing/operating such schemes.

Edited by pavanayi, 01 October 2011 - 05:30 AM.

[rabindra]

Thanks everyone...My plant is currently under shutdown. When it is in operation I will get back to you for clarification of any other doubts on the topic.

[sheiko]

Rabindra,
I could not open the downloaded files as I'm using my Iphone right now.
But I can recommend you to read Section 11.3.2 of Norman Lieberman's Working Guide To Process Equipment (3rd Ed.). It explains how the steam flow in your system is controlled and points out some pitfalls to avoid.

In addition, Chemical Enginering magazine has just released its September issue which contains an article from Henri Kister and al. untitled "Reboilers Condensate Drums: Silencing Hydraulic Hammer". It includes some design recommendations regarding such systems.

Edited by sheiko, 04 October 2011 - 09:51 PM.

[rabindra]

Refering to the reply above, if I take the tapping of the Pressure equalisation line from the chest of the exhcanger (channel cover on the condensate outlet side) then I can decrease the height of reboiler (or the difference in elevation of reboiler and condensate pot). Otherway, if I maintain the height of reboiler and condensate pot but still take the pressure equalisation line tapping from chest then the condesate line will be partially filled with steam.

Am I right ??

[npp]

It is unclear what is the target of the controller in the orignal post. The target could be the process temperature outlet the reboiler or some thing else.
Two main variables can be used to control are temperature difference or heat transfer area. Varying one of these two can change the heat transfer duty, thus the amount of steam.
Based on the figure given there is no control, because there is no connection between the target and the level control in condensate drum. Even condensate can be backed up in the heat exchange by pressure difference, but no way to vary it if the set point for level controller is fixed.
Could it be other controller in the column, such as no controller for the reboiler is required, it just provides the maximal duty ?

[katmar]

The design in the sketch supplied by Rabindra does not look like it will work to me. There is either missing information, or the plant will not control automatically. Is this a working plant, and you are asking the question just to understand the process, or is this a proposed design?

In my mind the level in the condensate pot must be very nearly the same as the level in the reboiler, or else there must be a bypass on the process side - as described in the article referenced by Zauberberg.

[sheiko]

In my mind the level in the condensate pot must be very nearly the same as the level in the reboiler, or else there must be a bypass on the process side - as described in the article referenced by Zauberberg.

For the level in the condensate pot to represent the level in the reboiler channel head, the equalizing line (or balance line) has to connect the vapor space of the pot to the steam side of the reboiler (that is, to the bottom of the channel head of the reboiler, assuming steam is on tube side).

Edited by sheiko, 02 October 2011 - 10:33 PM.

[katmar]

Sheiko - yes, I agree there must be a balance line. And the levels of the reboiler and condensate pot must be the same.

I can't see that it will make much difference whether the balance line is connected to the steam inlet pipe or to the steam chest. The pressures will be very nearly the same, but because of Bernoulli the pressure in the chest may actually be higher than in the inlet pipe. In the case of a kettle type reboiler there is no steam chest and the inlet piping is the only place to connect the balance line.

[rabindra]

Probably my first post was missing some information.

The reboiler is in the amine regeneration unit, where the steam flow is ratio controlled (with amine flow to the regenerator). The operator give ratio as the input. Based on the input, the flow of steam is calculated and then set point of the level control is given.

I have attached the revised flow scheme.

regards
Rabindra

Attached Files

•  Steam Reboiler.xls   141KB   1041 downloads

[kkala]

Some more clarifications, rabindra, may be useful to (my) understanding on the attached flow scheme.
1. Although steam / amine ratio is controlled, there is no control valve on the inlet steam line. Relevant FIC resets LIC of "Level control valve", so this valve (not an on/off valve) actually controls condensate flow, but also receives input from condensate drum liquid level. In fact condensate / amine ratio is controlled, but care is taken for drum not to loose liquid level. Is it so?
2. What is the action of sending signal from steam flow transmitter (upstream gate valve) to FIC? Does flow rate of inlet steam has an influence on "Level control valve" opening? Is such a function realized in the box of FIC? This is more complex control, but possible (e.g. in boilers).
3. Advice diameters of natural circulation lines to / from reboiler, as well as of steam equalizing line to condensate drum, if possible.

[npp]

The system could work if a change the level in the condensate drum causes a change in exposed area in the reboiler. This could happen if condensate is backed up in the reboiler, such as some of reboiler tubes are submerged under condensate level.
When heat transfer area is reduced, less steam will be condensed. It results in less steam go to the reboiler, even no control valve on steam line.
The question is ”Does condensate back up in the 3” condensate outlet line, such as the level is inside the tube channel?” It could happen if pressure drop in the reboiler tube side is equal to level difference between condensate level in reboiler and in condensate drum. It also requires that condensate line is connected to the drum at a point below the condensate level.
The design seems risky. If the condensate backup level is lower than expected, the control system will not work.

[rabindra]

Sir

My replies are:
1. I think there should be low selector switch (which will override the signal from ratio controller) if the level comes down very low. But it is not present in my system. So I am unsure what is the preventive mechanism to avoid loss in level.

2. I could not understand the question. What I understand is that based on the Amine flow and the ratio input given by operator, the flow of steam is calculated and then it is controlled by the level control valve (I am not sure how ?).

3. Reboiler Amine inlet 10", Reboiler Vapor outlet 16" (2 outlets), Steam Inlet 12", Condensate Outlet 3". Steam equlisation line 1".

I hope i clarify the doubts.

[rabindra]

@npp

I have the same doubt as you since we maintain around 50-60% level in condensate drum. And the condensate entry to the vessel is near the top (above the LP tapping of Level Gauge) there is little chance of condesate backing up in the reboiler.

[katmar]

Rabindra, thanks for all the extra information. It seems the more info you give, the more questions we ask. But this is an interesting situation and I would also like to understand what is really happening.

In the book Distillation Operation by Henry Kister there are several reboiler steam control strategies shown on pages 515-516. In every situation where the condensate flow is controlled (as opposed to controlling the inlet steam) the condensate pot height is set such that the levels in the reboiler and the condensate pot are very much the same. Contrary to what Pavanayi has written above, I believe that the pressure drop through the steam inlet piping (after the valve) will be negligible and the level in the pot will be very close to the level in the reboiler.

So my questions for you now are

1. In post #3 above you said "I still have a doubt whether the reboiler will function if the condensate pot is below the reboiler itself". Is this a working plant or not? Are you having problems with it, or is it just an intellectual exercise?

2. If it is a working plant where does the level in the condensate pot actually sit?

3. Where is the upper tapping point for the level indicator? If it is at a height above the reboiler the level could be controlling in the vent pipe where we expect it to be.

Edit : I see that you have answered my questions 2 & 3 in your reply to npp while I was typing! Can you tell us what the steam flow rate is so that we can get an idea of the pressure drops?

Edited by katmar, 04 October 2011 - 04:50 AM.

[kkala]

My replies are:
1. I think there should be low selector switch (which will override the signal from ratio controller) if the level comes down very low. But it is not present in my system. So I am unsure what is the preventive mechanism to avoid loss in level.
2. I could not understand the question. What I understand is that based on the Amine flow and the ratio input given by operator, the flow of steam is calculated and then it is controlled by the level control valve (I am not sure how ?).
3. Reboiler Amine inlet 10", Reboiler Vapor outlet 16" (2 outlets), Steam Inlet 12", Condensate Outlet 3". Steam equlisation line 1". I hope i clarify the doubts.

A proverb of neighboring Turkey says water cannot be clarified unless it gets turbid. Hopefully it is on the way to clarification now.
Posts indicate that the reboiler installation is existing; has it been in operation? will present shutdown of plant result in some change on its equipment or conditions? Such clarifications are significant, see also question by katmar.
Back to replies, notes to my understanding.
1. Probably "Level control valve" actually controls condensate flow, but also combines input from drum level. This is not so clear in the scheme, but possible (through DCS).
2. Signal from steam flow transmitter to FIC seems to have no intent, unless it brings a "preemptive" action onto "Level control valve" (sophisticated), e.g. opening a bit as soon as steam flow is detected to increase.
3. OK

Edited by kkala, 04 October 2011 - 06:34 AM.

[rabindra]

The system is in place in my refinery and is in operation (currently though we have an M&I shutdown). Just before shutdown, the production people informed me about the fluctuation in steam flow, condensate level as well as the level in the Amine Regenerator. The production guys requested me to put a control valve in the steam inlet line. Before I do that I wanted to convince myself that the present system will not work. I compared it to other reboiling systems, I found that either there was control valve in steam / condensate line or the condensate pot was of the same level as the reboiler. Hence the doubt.

I am yet to analyse the fluctuation in steam flow, reboiler level and amine level in regenerator though. May be a fluctuation in steam header pressure initiated the disturbance ( I dont have data for header pressure).

One more information I forgot to give. There is a level gauge on the tube side of the reboiler. This means that the designer envisaged that the level will back up in the reboiler. Since the level in the condensate pot is never 100%, I think what Pavanyi told in 6th post (liquid head equals to Delta P) is correct.

Edited by rabindra, 04 October 2011 - 12:33 PM.

[S.AHMAD]

Hi Ravi,

• Very interesting discussion and frankly speaking I have not seen such control scheme in 30+ years of my working experience in oil & gas and petrochemical process plant
• I would like to say that this is an innovative feedforward control scheme (one control valve is eliminated – a project cost saving).
• In the conventional scheme, the controlled variable (Feed/steam ratio) is controlled by DIRECT adjustment of the steam flowrate (manipulated variable). However in this scheme, the steam is INDIRECTLY manipulated. I believe that the scheme works like this - Let say more steam is required (Feed/steam ratio is higher than set point), the controller will trigger the level control valve (LCV) to open more, so that more condensate flow and level in the pot reduces and this will increase the surface area in the reboiler. That is more area available.
• Since U, the overall coefficient, and the DeltaT remain constant, more area means more heat transfer (Q = UA deltaT) and hence more steam condenses (since Q = Ms x heat of condensation). Thus LCV will continue opening until the feed/steam ratio is equal to the set point.
• There is a danger of steam breakthrough. In order to avoid this steam breakthrough, there should be minimum level where the LCV will shut once the level reaches the minimum set point.
• My opinion is that, the scheme works fine during steady state and slight variation in feed. However, it may hair-wired when there is significant process upset (big variation in feed flowrate) due to slower response.
• I personally propose to use the proven conventional control scheme and thus you need an additional control valve for the steam flow control.

[npp]

The system has been worked. It is an interesting control system to learn. But we can still ask a question: Does the control system with condensate level control work as it is designed for? "Steam usage is proportional with amine flow to column".
Let us see on the equipment. The duty of reboiler or steam flow can not increase unlimited. The maximum duty will be Q= U*DT*Tot Area. It will be the duty of reboiler if condensate level in the drum does not affect the exposed area.
The regenerator will make the job; even the reboiler duty is higher than required. A small change in lean loading is not remarkable. The only disadvantage is high steam usage.
It can be checked, by comparing the steam flow measured by flow element and signal from amine flow to column. If these two variables are proportional, the control system has worked well.