18 replies to this topic

[J_B]

Hello,

 

I’m currently working on a project which would allow condensation of about 3 t/h of saturated steam at atmospheric pressure produced by a half pipe coil reactor (max allowable pressure 0.5bar).

 

Condenser and sub-cooler will be water-cooled (river water).

 

As for the layout:

• the condenser is to be installed on a platform 8m above grade (platform above the reactor);
• the plate heat exchanger is to be installed at grade level.

 

Condensate would be returned in a “hot well” after being collected in floor gutters.

 

There would not be any control on the steam and condensate side.

Condensate flow would be gravity driven.

 

 

I'm questioning the need of an intermediate condensate tank vented to the atmosphere to prevent pressure built-up upstream the condenser / ensure that the condenser operate at atmospheric pressure.

 

This tank would be placed downstream the condenser and upstream the subcooler.

 

Could you provide me with insights regarding this particular design? Is this tank needed at all?

any help would be much appreciated.

 

Kind regards.

 

J_B

[Pilesar]

I think an 'intermediate condensate tank' would still be downstream of the condenser. I don't know how it would help control pressure. Why not just vent the condenser to atmosphere?

[J_B]

Hello Pilesar,

 

Thanks for your answer. 

 

Please correct me if I am wrong assuming that for my application the [reactor / condenser / condensate tank] system could be assimilated to a [distillation column / total condenser / reflux drum] system with no reflux line to the distillation column.

 

From my understanding, when operated to atmospheric pressure, these reflux drums are vented to the atmosphere and cooling water flow stays constant through the condenser. Therefore, the reflux drum and the top of the distillation column would be at atmospheric pressure.

 

My reasoning was that this layout would be similar to my application with steam / condensate streams instead of vapour / distillate streams.

 

I do not intend to control pressure, but to prevent overpressure upstream of the condenser (if any) given the maximum allowable pressure of the reactor.

 

Regarding your suggestion, venting the condenser to the atmosphere would be totally acceptable indeed. Therefore, a condensate tank vented to the atmosphere would not be required.

 

 

Are there any recommendations / sizing rules for the condenser vent?

 

Are there any specific points to look out for regarding the condensate stream from the condenser to the sub-cooler?

[Pilesar]

A condenser vent as large as the inlet pipe will handle the steam flow in case of cooling water failure. It is difficult to follow the written description of the process. Pictures and diagrams would help if it is important for others to understand. Big words like 'assimilated' confuse me in this context. Are you building this process or is it only virtual? There are more details to a well-designed steam condensing exchanger than you might expect. Inerts in the exchanger can cause many operational headaches. Venting condensers can be tricky since air tends to travel to the tube surface of the condenser because of dynamic forces during condensing. You might need to seal your condenser vent from air backflow by using a water seal. This would have a similar function to a 'P' trap in house sink drains used to separate sewer odors from the house air. You do not necessarily need a separate subcooler if you design the condenser to hold a liquid level in the bottom by use of a weir or external piping loops at specific elevations. This method may require additional heat transfer area, but is usually less expensive to purchase and install than two separate equipment items. Baffling may be designed differently in the subcooling region. If the required subcooling is not extensive, enough subcooling may occur without special design attention just from the condensate lingering on the tubes.

[shvet1]

@J_B

 

Please provide a layout (with equipment elevation) and P&ID/PFD to understand the issue.

[J_B]

@Pilesar

Sorry english is not my first language.

It should be read “would be similar to” instead of “could be assimilated to”.

 

The steam condensing process is virtual for the moment, but reactor operating parameters and steam losses to atmosphere are real.

 

In my first post, I was wondering if the control scheme presented in figure 17 p11/12 of the following document could be modified as followed for my application:

• No pressure control valve
• Non-condensable vent valve would be opened (or absent) to allow direct venting to atmosphere
• No reflux line
• No pump but gravity flow
• Sub-cooler downstream of the drum.

The document: https://rccostello.c...mn Pressure.pdf

 

 

@Pilesar / shvet1

Find enclosed a diagram of the system with appropriate subcooling inside the condenser and no condensate tank. 

 

Attached Files

•  Condenser with subcooling.xlsx   25.3KB   169 downloads

[breizh]

Hi,

I'm confused with the description of your process. What you call steam on your drawing is process vapor, what is the link with half pipe coil?

It could be water vapor with other chemicals and inert and or non-condensable gases. Can you provide figures about the composition of the stream?

Are you sure you can use river water to cool this stream? No regulation in your country? 

By the way don't mention cooling water, better river water instead because it's an open circuit.

Why is your condenser partially flooded? 

Btw you may consider a direct contact condenser.

Edit:

Your process seems to be a batch process with a boil off part, this means your heat surface area is variable (no reflux).

 

 

 

my 2 cents.

Breizh 

[J_B]

Hello,

 

Hi,

I'm confused with the description of your process.

What you call steam on your drawing is process vapor,

Yes, you are right.

 

what is the link with half pipe coil?

There is none. It's the type of reactor involved but this information is not relevant.

The maximum allowable pressure of the reactor shell is 0.5 bar and it is not designed to withstand vacuum.

 

It could be water vapor with other chemicals and inert gases.

Inert gas yes (air) at startup.

At first, the reactor is empty (only air is present) then filled with chemicals and water (feed).

Next is reactor heating which lead to vapour formation.

 

Can you provide figures about the composition of the stream?

The stream exiting the reactor is mainly water vapour.

There are traces of dissolved chemicals in the water vapour (ppm) but i can't provide figures.

 

Are you sure you can use river water to cool this stream? No regulation in your country? 

River water is already the main utility used onsite for existing heat exchangers.

Return temperature of river water has to be limited yes, which will lead to high flow rate through the condenser.

 

By the way don't mention cooling water, better river water instead because it's an open circuit.

Ok thanks.

 

Why is your condenser partially flooded?

To allow for subcooling as suggested.

Extra area is implemented in condenser to provide subcooling. Pipe loop ensure that a necessary height of water remain in the condenser.

 

Btw you may consider a direct contact condenser.

Thanks for the document, i'm not familiar with this type of condenser so i will look into it.

 

 

my 2 cents.

Breizh 

 

[breizh]

Hi,

 

Don't use red color to reply, it's considered to be rude.

 

Breizh  

[latexman]

 

It could be water vapor with other chemicals and inert gases.

Inert gas yes (air) at startup.

 

 

Hi J_B,

 

Air is not considered an inert gas, since the O2 in air supports combustion.  That misunderstanding could be very, very dangerous.

[Pilesar]

The process scheme as drawn in the spreadsheet looks excellent. It shows the external loop I mentioned for subcooling. The vent at the top of the loop is for anti-siphon. The noncondensibles will concentrate above the liquid level in the shell so the shellside vent is shown not located at the top of the shell. This appears to be very good engineering! You might install automated control for that shellside vent if noncondensibles are continual and affect operating capacity. The open pipe to sewer should relieve mechanical overpressure if sized appropriately. Andrew Slolely's article does not seem to apply to your situation.

[shvet1]

1/  Could you provide me with insights regarding this particular design? Is this tank needed at all? 

 

2/ Are there any recommendations / sizing rules for the condenser vent?

 

3/ Are there any specific points to look out for regarding the condensate stream from the condenser to the sub-cooler?

 

1/ No, tank is not needed. Gutter is that very tank. Design is ok.

2/ Dispersion modelling is recommended to assure that vent is located far enough from ventilations intakes and workplaces. For example see EPA ALOHA or DNV Phast.

3/ Add a valve in condensate for seal filling during start-up otherwise there is a risk non-condensable gases will go to gutter. Note that a hand-made fabric plug is able to be used instead with the same result.

 

4/ What is the purpose of a valve located on main vent? There is no a situation an operator should close it.

5/ Why vent nozzle is located not at top of shell? How non-condensable gases will escape this gas pocket? Was this gas pocket considered during heat resistance calculation? Rule of thumb is "gas vs condesation" resistance is "100 vs 1".

6/ Why seal vent is not connected to main vent? Why a separate line is required to break a siphon? Is this a filling line for hose connection?

7/ How an operator will know that seal is filled? We used sight glasses for this purpose. Note that manually filling will help to speed-up a start-up procedure.

8/ A horizontal run on a gravity-driven line is dangerous - you have one upstream of a gutter. Angle is too high - it has a potential to break a seal. Angle is too low - bubbles has a potential to accumulate. I recommend to avoid such design.

9/ A mesh is recommended to protect vent from birds and social insects.

10/ Check if heat exchanger has correct design of baffles inside of the shell to provide non-condensable gases escape.

11/ I would provide a bypass of heat exchanger to be able to take it out of operation for maintenance/repair/cleaning. As per my experience river water is a horrible fluid. We are getting out from tubesheet wood, fishes, bags, seaweeds, condoms an many other exciting stuff despite of a water intake has a proper design and maintenance.

12/ Check that inlet to gutter is guaranteed located above liquid level to avoid siphoning.

13/ If condensate is being produced by sudden portions then check that seal vent diameter (hydraulic resistance) is high enough to avoid seal break.

14/ Check that main vent diameter is high enough to provide backflow of liquid otherwise be ready to recurring rainfall downwind. Note that dedicated devices exist for this purpose.

https://wright-austi...ead-design.html

15/ Note that there is no difference heat exchanger is located at elevation 1 or 8 meters other that the first option is a more safe design as eliminates long-run vertical gravity-driven 2-phase flow.

Edited by shvet1, 20 July 2023 - 12:46 AM.

[Pilesar]

Condensing steam tends to want to form vacuum in the system. If you truly want atmospheric pressure in the reactor, the pressure should self-regulate if sufficient path to atmosphere is always open. Heat transfer area should be carefully considered during design if noncondensibles are purposely allowed to build up in the exchanger for pressure control. Do not assume that my silence in regards to other comments means that I agree with them. I am not engineering this system so do not have to fight for design consensus. We might have some lively discussion if we were all in one room responsible for this project design!

[shvet1]

@J_B

Why heat exchanger is elevated at 8m? Why not 0.8 or 80m? Why 8m?

 

also for info, hope your HE is designed properly

 

ExxonMobil std. IXB

Another important criterion is the maximum allowable cooling water outlet film temperature. The maximum film temperature is the water temperature at the liquid-metal interface occurring at the hot end of the exchanger. A conservative estimate of this film temperature is the tube metal temperature at the hot end of the exchanger. These limits are: 

• Salt water: 140°F (60°C)

• Fresh water: 150°F (65°C)

Note: For box coolers, the maximum cooling water outlet temperature is 150°F (65°C) for both salt water and fresh water. 

If water film temperatures are allowed to exceed these values, severe fouling and corrosion could result. 

 

Shell DEP 31.21.01.12

In closed and open cooling water systems, the cooling water bulk outlet temperatures from individual heat exchangers shall not exceed the values listed in DEP 20.21.00.31-Gen. When approved by the Principal, the outlet temperature from selected heat exchangers in open re-circulated cooling tower water systems can be allowed to rise to 49°C (120°F), provided the maximum tube skin temperature on the cooling water side (considering clean conditions) does not exceed 60°C (140°F).

BP std. 26-10

a. The maximum temperature for cooling water shall be limited to 50°C (120°F) and the maximum wall temperature shall be limited to 60°C (140°F) unless otherwise approved by BP.

 

JGC std. 210-120-1-24

c. To reduce the tendency to foul, the maximum allowable outlet temperature for cooling water is limited as follows:

Sea Water : 50°C

Cooling Tower Water : 60°C

d. Another important criterion is the film temperature on the tube surface including a fouling layer. If the film temperature exceeds the following conditions, severe fouling could result.

Sea Water : 65°C

Cooling Tower Water : 70°C

 

UOP std. DM-EQUIP-2073-401

The cooling water outlet temperature should not be hotter than 120ºF (49ºC) to minimize deposition of solids from the water on the tubes. 

Edited by shvet1, 20 July 2023 - 11:14 PM.

[breizh]

Hi,

No news from the OP! Summer holidays?

Breizh

[shvet1]

You are optimist. Loss of interest seems more credible.

[J_B]

Thanks everyone for the valuable feedback. There are many points to address.

 

Holidays will come soon enough but not today. As for my interest for this subject, it’s still strong and i am sorry for the late reply.

 

I will address some questions / remarks specifically, as it should also answer some others.

 

@Latexman

Very true, my mistake…

 

@Breizh

I missed your edit sorry.

 

1)Your process seems to be a batch process with a boil off part, this means your heat surface area is variable (no reflux).

It is a batch process, but the reactor heat surface area remain constant.

There is a reactor level control during the boil off part. Additional feed is put in the reactor as water evaporate. Reaction is stopped after a given time and products are extracted at the reactor bottom.

 

 

2) What is the driving force and how do you overcome the head loss on shell side, i.e. pressure built up in the reactor > P atm?

Vapour condenses as it flows in the condenser. The difference of specific volume of vapour and condensate forms vacuum which should draw vapour from the reactor toward the condenser. That's my understanding.

Friction loss upstream of the condenser will be limited using appropriate pipe diameter and long radius elbows.

LMTD is about 77 due to using river water.

 

 

@shvet1

Thanks for all of your recommendations.

River water is filtered beforehand. Water velocity in the condenser and limitation of river water return temperature should limit fouling but inspection is done during annual shutdown.

The condenser location is due to limited available space at lower levels and grade level.

 

 

@Pilesar

Thanks for your recommendations.

I hope I've not offended anyone in this thread. The design of such system is of course case specific. You and others are contributing and sharing thoughts about design and operation with limited information that I've provided. I'm grateful.

 

 

 

I’ve looked at different technologies of condenser, discussed with a few manufacturers and checked litterature since starting this thread.

Comparing S&T and platular condenser, the platular ones seems doing ok regarding:

• Head loss on the shell side

With my given flowrates, platular head loss on vapour side is less than 5 mbar and about 0.1 bar in the shell side of the S&T condenser.

• Cleanability
• (Price)

With this technology, the non-condensable vent will have to be located opposite to the vapour inlet, on the upper part of the shell.

The condenser inlet and condenser vent outlet could have the same size to allow vapour to exit the building in case of river water supply failure. Exhaust head should prove useful here.

 

I looked at barometric condenser, but I’d rather not mix vapour/condensate with cold water.

 

As for condensate drainage, pipe routing and pipe diameter would be accommodated past the condensate loop to allow for self-venting. See Fig. 2.b. p3/4 of the following document.

 

 

https://www.google.c...E_&opi=89978449

 

[breizh]

Hi,

I'm still puzzled that you want to use river water, knowing the change of quality over the months, from relatively clean and hot in summer to very turbid in winter, assessment based on 10 years' experience operating soda ash plant in France. Same comments about the use of plates heat exchanger with river water. Troubles for sure and heavy maintenance cost (cleaning service and gaskets).

 

You said you did not want to consider direct contact condenser using quality water then there is another option: Aero condenser.

https://files.charti...rochure-Web.pdf

Good luck 

Breizh 

[shvet1]

Water velocity in the condenser and limitation of river water return temperature should limit fouling but inspection is done during annual shutdown.

 

Tube wall temperature at HE hot end is critical. Cold side outlet temperature alone should be applicable only to preliminary calculations. Proper filtation can become a challenging/expensive option so this minor stage has a potential to make inoperable the whole process unit. I saw so many water filters that did not work because of minor (as designers thought) issues like seaweeds or sand.

 

My thought is not about HE will loss U coefficient and condensate will become overheated some degree. No-no. HE has a potential that water will no be able to go through tubeside and therefore vapors will break through to atmosphere. I have been told about cases when such conditions occur during several days or even hours.

 

A deep study of river water conditions and water intake is highly recommended. The worst case I have encountered was a refinery that have had to install ~200 km of long-distance pipeline to supply water from an another river after such study.

 

Note that there is a high chance that proper conditioning of raw water may become too expensive. Good water source is lucky, more often a production side is located at remote location where no such

Edited by shvet1, 24 July 2023 - 01:58 AM.