13 replies to this topic

[Bill Earl]

Hi, this is my first post here.

 

I’m a mechanical engineer, and have designed and built a small CO2 recovery system for a local brewery to collect fermentation CO2, purify it, and liquify it for storage. While most parts of the process are working well, we are having issues with the condensation of the CO2. Our condenser is a brazed plate heat exchanger, using R404A refrigerant on the evaporator side, and is designed (using the manufacturer’s cascade condenser software) to condense CO2 gas at 38 kg/h. In the BPHE, the CO2 gas is de-superheated from 30C, condensed at app. -27C (16 bar-a, 230 psia), and subcooled to app. -35C, before flowing down to a small receiver, with high and low float switches to control flow to the next stage. All parts are well insulated.

 

In recent test runs (using beverage grade CO2, dried to -60C dewpoint, and with venting for inerts), our CO2 condensation rate has only been 10% of the design rate, even though the CO2 pressure and temperatures are consistently in the correct range. We have no explanation for this behaviour yet.

 

The refrigeration system operates in an unstable manner due to under-loading, with its compressor regularly cutting out at suction pressures of app. -8 psig (-60C for R404A). I have read that extreme temperature differences (CO2 at 30C, R404A at -60C in our case) can cause CO2 fog formation in the condenser, although I’m not sure what effect that might have or whether it might be a factor in our low production rate.

 

Piping from the condenser to the receiver is generously sized, and the condensed CO2 in the receiver is sub-cooled, so back-flow of vapour from the receiver to the condenser should not be an issue, at least in theory. However, we will try running a separate vent / equalization line from the receiver back to the condenser inlet to see if that helps.

 

Happy to supply more info if needed. Any ideas? Thanks.

 

[Bobby Strain]

A sketch would help. If you have inerts, you need to vent them, otherwise they will accumulate in the condenser. Then you will suffer symptoms such as you describe.

 

Bobby

[Bill Earl]

Thanks, Bobby. A sketch of our current condenser arrangement is attached.

 

There should be almost no inerts because the system has been thoroughly purged with commercial (beverage grade) CO2, which we are also using for test runs.

 

We are constantly venting about 5% of our flow as gas which will hopefully include inerts based on my understanding that, with BPHEs, inerts are carried downwards with the condensing CO2. The liquid receiver also serves as a disengagement chamber for inerts, with oversize piping from the condenser to allow gas flow back to the vent and condenser. The inerts (basically air) will hopefully favour the vent due to their lower density. The liquid CO2 in the liquid receiver is subcooled by about 8C, so my assumption was there should be minimal back-flow of CO2 vapour from the liquid receiver.

 

This system is currently experimental due to the sparsity of useful information available.

Attached Files

•  CO2 Condenser 1.pdf   27.31KB   560 downloads

Edited by billearl, 09 November 2014 - 02:34 PM.

[Bobby Strain]

Inert gas won't be carried downward if the velocity is too low. Which it probably is in your case. I take it you never achieved your design rate? There are lots of systems producing carbon dioxide from fermenter gas. I never looked closely at the design of the condenser, though. I expect that commercial operations use shell & tube condensers, configured in such manner that is conducive to inert venting. Maybe Katmar can shed some light. He knows about making beer. And probably tasting same, too.

 

Bobby

[Art Montemayor]

Billearl:

 

What you are doing has been successfully carried out probably hundreds and hundreds of times around the world since the early parts of the last century.  The Backus and the Reich Processes were well known then and are still used in their basic form.  This is well explained in the book, “Carbon Dioxide” by Elton Quinn and Charles Jones in 1936 (this is older than I am).  Are you using any of these basic process operations in your purification system?

 

I have designed, built, and modified many CO₂ production units - specifically, some fermentation CO₂ recovery units as well.  I have never used a plate heat exchanger as a CO₂ condenser because of three basic reasons:

• They were not around and available in my time;
• They are not suited mechanically to the basic design of a CO₂ condenser used for the purposes of producing stored product.
• There were simpler, proven, condenser designs available that worked perfectly well.

Bobby Strain is correct, the design most used is a basic shell and tube condenser.  I have used both TEMA BEM, BKU and BEU types.  I have used the shell and tube in both the horizontal and the vertical orientations.  I have never experienced any failure to condense the calculated, design CO₂ flow rates - which have ranged from 100 kg/hr to 2,500 kg/hr.  CO₂ condensation is a unit operation that has been the easiest I have ever encountered in 54 years of experience.  I perceive that you must have a serious design, operation, or installation, problem in your relatively small CO₂ recovery from fermentation.  If done correctly, you should have a smooth, problem-free operation that practically operates with a minimum of attention.

 

Your plate heat exchanger used as a condenser may be “cost effective” (cheaper price) than a shell and tube, but it comes at the expense of some tradeoffs.  A plate unit basically depends on relatively high velocities to attain its known high heat transfer efficiency - and that means it presents problems if there are non-condensables present.  Its configuration does not allow for efficient phase separation to take place.  Please refer to my attached workbook to see how I would install a plate condenser (subject to my personal specifications) to operate as a typical CO₂ condenser.  As Bobby Strain has inferred, you must allow for phase separation and easy, convenient inert gas venting.

 

You state that you are condensing the CO2 at approximately -27 ^oC (-16.6 ^oF) & 16 bar-a, (230 psia), and subcooled to app. -35 ^oC (-31 ^oF).  Again, please refer to my workbook and look at the thermodynamic tables for CO₂.  Liquid CO₂ is normally stored at saturated design conditions of 250 psig and -8.3 ^oF (-22.4 ^oC).  Unless you have a specific need to have the CO₂ at a lower temperature, there is no need to use a refrigerant for the ultra-cold condition of -30 ^oC.  Also, how do you propose to get the liquid CO₂ to the subcooled condition of -35 ^oC?  You cannot “subcool” the saturated liquid without exerting a pressure on it over and above its own vapor pressure.  How do you propose to do this?  (and why?).  For normal liquid CO₂ storage this is never a requirement since the product liquid is always handled as a saturated one - not "subcooled".

 

This is a very simple and field-proven unit operation that has operated for many decades without problems.  Something is wrong with your design (mechanical and/or process) or with your installation/operation.

 Low Pressure CO2 Condensers.xlsx   83.69KB   449 downloads

[Bill Earl]

Thanks, Art, for your well considered comments and advice.

 

I just read the descriptions of the Backus and Reich processes in “Carbon Dioxide”. The Backus process seems to be the basis for all current fermentation CO2 recovery systems I’ve researched (e.g. Wittemann), which are the basis for my design, except for the O2 stripping method. For O2 stripping, I’m attempting to use the method described in the paper “CO2 Recovery: Improved Performance with a Newly Developed System” http://thet.uni-pade...f/CO2_Flens.pdf

 

My objective with this project has been to design and build a CO2 recovery system which was as simple and affordable as possible. My choice of BPHE vs STHE was based on cost as you surmised. I realize the choice of using a brazed plate heat exchanger for condensing CO2 is not ideal compared to a shell and tube type, based mainly on handling of non-condensables, but also on sparsity of BPHE info for this use. BPHEs, however, are used as cascade condensers for condensing CO2 in cascade refrigeration systems, and I did find a mention that non-condensables will normally be carried downward with the condensate (presumably as tiny bubbles).

 

As I mentioned, our current test runs use “pure” (beverage grade) CO2, and I have periodically isolated the condenser and manually vented it, so inerts should not be the current problem. If we find inerts do accumulate in the condenser, we could conceivably detect that by condenser performance and automate this venting method.

 

Subcooling the liquid CO2 was to assure our condenser was cold enough under varying pressure conditions, and also to help with the next step in the process. The small liquid receiver is intended to also serve as a disengagement chamber for inert gas bubbles assuming they are carried downward, and the float switches are to assure no flooding of the condenser. Immediately downstream of the valve is an orifice to drop the pressure from 230 psia to app. 100 psia (-50C). This flows to a “purification” tank, where the flash gas is recycled and the liquid CO2 is partially reboiled using an electric heater, as described in the paper above, to coalesce and separate dissolved gases (specifically O2).

 

I’m not sure I understand your concerns with subcooling the liquid CO2. Any CO2 vapour adjacent to the subcooled liquid will be at 230 psia saturation conditions. I’m simply removing more heat from the liquid CO2 to subcool it.

 

After the “purification” tank, the liquid CO2 is drained to a larger liquid receiver, which when full is pressurized to app. 215 psig to transfer the contents to our large storage tank, which is maintained at app. 200 psig. This transfer method works reasonably well.

 

I realize my design is unconventional in some respects, which we consider experimental, mainly driven by the small scale of the system and trying to keep the capital costs affordable, and we know there are associated risks. At the moment, the condensation issue is critical and we are trying to troubleshoot what is happening. If that fails, we will consider changing to a shell and tube condenser, although we want to retain our current refrigeration unit due to the cost of replacement. One thought which might provide better condensation control would be to use our refrigeration unit to chill glycol to sufficiently low temperature and circulate it through the heat exchanger (BPHE or STHE) to condense CO2. Any experience or thoughts on that?

 

With reference to your diagrams and tables:

• You seem to recommend R143A over R404A, which have similar pressure-temperature curves, with R143A having the advantage of modestly higher vapour pressure at low temperatures. Are there other advantages of R143A?
• You show 18.5 bar-a in your diagram, but we are limited by the pressure rating of our desiccant dryer to 232 psig (17.0 bar-a), although I assume that is of little consequence.
• With the liquid receiver you propose, if we use a constant flow orifice for venting and have a liquid level control valve downstream, it looks like vapour lock would likely result. I did try a similar arrangement to yours first, using a float-operated air vent to release inerts when they built up enough to depress the float, but it didn’t fit well with our batch transfers of condensed CO2 to the purification tank. We switched to the float switch control liquid receiver because we suspected the condenser might be flooding.
• The shell and tube condenser you show is much different than I imagined. I thought the non-condensables always collected and were vented from the highest point in the shell.
• Incidentally, I also have been using the NIST website for my thermodynamic data.

I deeply appreciate your help, and any further comments or questions would be very welcome.

 

[Bill Earl]

To Art Montemayor:

Since reading your post, I have done more research and testing based on your advice. We will certainly change to a shell and tube condenser if we can’t make the brazed plate condenser operate reasonably well.

With regard to your comment that a “plate unit basically depends on relatively high velocities to attain its known high heat transfer efficiency”, I added a vent line from the top of the liquid receiver with a metering valve, venting to atmosphere, per the attached sketch. The purpose was to increase vapour velocity in the condenser, at the expense of dumping excess flow (undoubtedly including the inerts) to atmosphere. This increased the liquid CO2 production dramatically from 10% previously to 40% of design rate. That proves your point, and is encouraging, but is still well below the design rate and wastes app. 20% of the CO2 gas. I plan to do more testing next week to try to improve on this.

Research on shell and tube condensers, not surprisingly, affirms your advice that plate heat exchangers are not suitable for condensing service, and shell and tube heat exchangers are suitable with condensing on the tube side if inerts are present. From looking at your sketches of vertical shell and tube condensers, although I’m not questioning your advice (or supporting advice elsewhere), I have to admit that I don’t understand the advantage of the vertical shell and tube condenser. At low CO2 vapour velocities, what prevents inerts from rising and mixing with the CO2 vapour? At high vapour velocities, to prevent inerts rising and improve heat transfer rates, what prevents significant quantities of CO2 vapour from being vented along with the inerts? This seems to be the same dilemma I’m facing now.

Obviously, I do not feel uncomfortable committing to a shell and tube condenser without a clear understanding of the advantages. The proprietary BPHE cascade condenser software we used seems to have been misleading (at best), and I don’t even have STHE software (which I understand is expensive for occasional use). We would have to commit to an expensive (and long lead time) custom shell and tube condenser designed by others based on our requirements, and based on hope that it will operate as designed. Am I being too cynical?

If we go the shell and tube route, do you consider carbon steel suitable for the low temperatures involved, and do you consider a fixed tubesheet (with carbon steel or copper tubes) suitable? Also, what do you consider to be a reasonable percentage of feed CO2 to vent with the inerts?

Thanks again.

Attached Files

•  CO2 Condenser 2.pdf   31.91KB   443 downloads

Edited by Bill Earl, 22 November 2014 - 03:52 PM.

[Bill Earl]

Does this vertical shell and tube condenser configuration (attached file) make any sense, substituting R404A for water? The non-condensables (mainly O2 and N2) are lighter than CO2, so should tend to collect at the top.

Attached Files

•  Alternative Vertical STHE.pdf   37.32KB   427 downloads

[Art Montemayor]

Bill:

 

If I left you with the impression that I am dead-set against the use of plate heat exchangers as condensers, I apologize for what are bad communications on my part.  I am not against the use of plate exchangers as condensers, but against the use of pre-designed and fabricated plate exchangers used as condensers when they have not been specified for use on fluids that contain non-condensables and require their removal as part of the steady-state operation.  Most of the plate heat exchangers designed and fabricated in the market place do not exhibit any physical design that allows for the efficient removal of non-condensables.  But there are exceptions.  For example, look at the attached Alfa Laval brochure I am attaching and see the difference in the physical and mechanical design.  However, as you can imagine, this special design requires a “special” price - a feature that defeats your objective of low-cost.

 

You have not responded to my question as to whether your condensed product is routed directly to a storage tank.  Most - or all - of the CO₂ condensing I have been involved in over the years has involved using saturated liquid storage tanks directly after condensation.  Note that I use the term “saturated”.  This means that the condensation and storage is done at the temperature and pressure corresponding to the liquid CO₂ at its dew point - and in equilibrium with the vapor space directly above it.  All Liquid CO₂ storage tanks have to have a dedicated vapor space (usually 85 - 90% of the total tank volume) in order to avoid an over-pressure situation.  What this means is that the liquid CO₂ has to be in the saturated condition; it can’t exist in the sub-cooled condition because by definition it exists in equilibrium with its own vapor pressure - which makes it a saturated liquid.  The only way to have liquid CO₂ in the sub-cooled region (as seen in the Mollier Diagram) is to have it at a pressure that exceeds its own vapor pressure at the designated temperature.  And the only way to do that is either impose a mechanical pressure on the liquid while it is totally enclosed (without any vapor space) or to inject a non-condensable gas at a higher pressure directly into the vapor space of the tank.  I don’t think you can tolerate either of these conditions and therefore that is why I stated that you should define how you intend to “sub-cool” the liquid CO₂ (and for what reason?).  In 54 years I have never heard of nor read of anyone condensing and storing liquid CO₂ at sub-cooled conditions.  Since just about 95+% of the industrial applications of CO₂ are in the gaseous state, the justification of storing sub-cooled liquid CO₂ isn’t practical.  Except for the production of Dry Ice, liquid CO₂ is almost always vaporized prior to its use so that any temperature lower than that necessary for saturated storage is considered a waste of good money.

 

I recommend the use of R143A over R404A because the former will refrigerate sufficiently to condense the CO₂ gas while it yields a higher vapor pressure and, consequently, a higher refrigeration compressor suction pressure.  And that is what all refrigeration system operators seek: a suction pressure above atmospheric in order to minimize or eliminate the possible seepage of atmospheric air into the compressor through the shaft seals.  This is what contaminates the refrigerant and forces one to install and maintain a non-condensables purge unit running continuously.  I am partial to using ammonia to refrigerate and condense CO₂.    Additionally, as I’ve previously stated I don’t see any justification to cool the liquid CO₂ any lower than its temperature corresponding to the vapor pressure at approximately 250 psig storage pressure (-8 ^oF).

 

The use of a vertical versus a horizontal shell and tube condenser is one of choice.  But I have never seen a successful installation of a vertical condenser where the condensing fluid is flowed upwards through the tube-side.  Every vertical condenser I’ve seen or worked on has been with the condensing fluid flowing downwards through the tubes.  Here, I would like to have to SRFish (Dale Gulley), one of our Forum members who probably has more expert experience in the design and fabrications of heat transfer equipment than anyone else on the Forum, to join in this discussion and contribute his expert comments and thoughts on your application.

 

You may (or may not) be able to have a fixed tube, shell and tube exchanger successfully operate in your application - depending on the fabricator’s method to allow for the difference in shell and tube expansion and contraction due to the operating temperature.  One way I have eliminated this problem in a cost effective way was to fabricate a TEMA BEU unit with an expansion “dome” built on the top of the shell side.  I condensed the CO₂ in the U-tube bundle, with CO₂ vapor entering the top of the hairpins and I vented the non-condensables directly under the horizontal plate partition in the U-tube bundle’s bonnet.  I did this on a dry ice production facility use to recover the expansion CO₂ vapors from block presses.  This Dry Ice block method collected a lot of atmospheric air every time the press opened and introduced air into the system, making the successful venting of this air important to the recovery of the flashed gas.  The condenser worked very well with CO₂ vent losses that were not considered worth measuring during production.  For your 100 lb/hr unit I can visualize a very small and low-cost U-tube bundle unit made of 3/4” or ½” SS tubes.  The shell side can be A516 carbon steel.

 

Again, my basic questions:

Do you have to have liquid CO₂ produced at a temperature lower than -10 ^oF?  Why?

How do you propose to have the liquid CO₂ “sub-cooled” (colder than its saturated temperature) at a pressure higher than its saturated pressure?

Are you sending your condensed product directly to a storage tank as opposed to elsewhere?

 

 Alfa Laval Compabloc Condenser.pdf   188.96KB   400 downloads

 Selection and Design of Condensers.docx   1.45MB   440 downloads

[Bill Earl]

Art:

Thanks for your additional comments. Please see the attached diagram which adds more detail on the post-condenser process. Our intention is not to store liquid CO2 in a sub-cooled state, but some sub-cooling of liquid CO2 in the condenser provides a temperature indication that we are effectively condensing CO2. It also reduces the percentage of flash CO2 in the next operation of reducing its temperature to -50 C.

As the diagram shows, we are storing liquid CO2 in a saturated state at 14.8 bar-a (200 psig). The reason for the low storage pressure is to allow pressurized transfer of liquid CO2 to the storage tank by periodically diverting some of our process CO2 vapour at app. 215 psig. The reason for the low condensing pressure is a 232 psig pressure rating of certain components of our process, most importantly our desiccant dryer.

I hope that answers your basic questions.

I held off replying pending the completion of another test run on Monday Nov 24 in which I wanted to test a couple ideas which may or may not further improve our production rate in a sustainable way. I would prefer not to detail those ideas until after I test them. However, that test run was postponed due to an unexpected compressor malfunction which requires repair.

Your experienced advice has been very useful in determining our options, and is appreciated. I’ll report our test results when I have them. In the meantime, please feel free to comment further on our process, which we recognize has some experimental aspects, if you choose.

Attached Files

•  CO2 Condenser 3.pdf   38.85KB   369 downloads

[Art Montemayor]

Bill:

 

What I have tried to express is that it is not possible for you to “sub-cool” liquid CO2 without you having a pressure over it that is in excess of the liquid CO2 saturated vapor pressure.   One way to do this is to create and maintain a non-condensable gas over  the liquid CO2 while you are cooling it to a temperature below its saturated temperature.   But you don’t want to tolerate non-condensables in your system – so that is NOT what you want to do.

 

The way we have always purged or rid the system of non-condensables is that we use the very principle of the saturated state:  we cool the compressed gas in the condenser with a refrigerant that is at approximately 10 oF below the desired saturated temperature of the liquid CO2 and let the saturated liquid CO2 drop out of the gas above it and into a storage tank.  If the compressed CO2 were 100% pure, the pressure above the saturated liquid CO2 in the condenser (and in the tank) would always be at the saturated pressure (its vapor pressure at the condensed temperature).  But in real life, the compressed CO2 is NOT PURE.  It is mixed with non-condensables (usually air).   Therefore, what we do is simply purge a portion of the gas mixture that collects above the liquid CO2 in the condenser to rid the system of the non-condensables – preferably at a location where these tend to collect by physical and mechanical choice (see the Alfa-Laval Compubloc brochure I previously submitted).  In this way, we don’t need to monitor the temperature of the liquid CO2.   If the condenser is appropriately designed, the product liquid will always be saturated --- and we KNOW THAT IT IS CONDENSED because the pressure in the condenser is stabilized at the appropriate vapor pressure.  If the CO2 were not condensed while the compressor is operating, the pressure would continuously INCREASE – indicating that the condenser is too small OR that the non-condensables are trapped above the liquid.  By purging a small, continuous stream of non-condensables, you can see that the pressure is stabilized – proving that the remaining CO2 is condensed (assuming that the condenser is adequate in size and design).

 

That is why the design of the condenser is critical for purging of non-condensables – which is why I have tried to impose this fact.

 

At present I am with my grandchildren for the Thanksgiving Holidays and won’t have time to respond until next Monday.  If you wish, you can cantact me by my messenger service found in my background page on the Forum.   There, you can communicate personally.

[Bill Earl]

Thanks, Art. The pressure regulation in our process is not precise enough to detect the presence of non-condensables by an increase in condenser pressure, but we can detect non-condensables by a reduction in the production rate of liquid CO2 using the fill time of the small liquid receiver ("accumulator") at the condenser outlet. I refer to this small liquid receiver as an "accumulator" to distinguish it from the larger liquid receiver later in the process.

We cannot simply condense CO2 at the storage tank pressure because we want to further purify (remove dissolved air) by reboiling a portion of the liquid CO2 at lower temperature (so lower pressure) before transferring the liquid CO2 to our storage tank. Some CO2 vapour is diverted periodically from the condenser inlet to drive the transfer of liquid CO2 from the purification tank (reboiler) to the storage tank, which is maintained at slightly lower pressure than the condenser pressure.

I hope the attached P-h diagram makes this clearer. It also shows de-superheating, condensation, and sub-cooling at constant pressure. I changed the enthalpy scale to correspond to the NIST scale, hence the odd numbering.

What I see as our problem is the lower than expected condensation rate, which as you pointed out, requires higher vapour velocity in a brazed plate heat exchanger, which in turn requires excess vapour flowing through the condenser. How to do this without venting a significant portion of our CO2 is what I am trying to resolve. Even with excess vapour flow, the liquid CO2 exiting the condenser is still sub-cooled by at least 5 C. This is known from our test runs, and helps reduce the quantity of flash CO2 produced in the pressure reduction which follows.

I plan to do another test run this week (postponed from last week due to compressor issues) to test a couple possibilities for increasing our production rate without venting excessive amounts of CO2 vapour. We may yet switch to some configuration of shell and tube condenser if we’re convinced it is the solution.

Attached Files

•  CO2 Recovery - Pressure Enthalpy Diagram.pdf   1.04MB   3046 downloads

[Art Montemayor]

Bill:

 

We are having a communications problem regarding the condensation of gaseous and impure CO₂ as well as a misunderstanding of what the saturated state of a substance is.  I have assumed that you were familiar with phase equilibria, but I now believe from what you write that you are not familiar with this subject.  Your insistence that you can “sub-cool” liquid CO₂ while maintaining it under its saturated pressure is not realistic - at least not in the manner that you show in your sketches.  Your Mollier Diagram sketch is also flawed in that it shows that you can cool the liquid CO₂ below the saturation temperature while maintaining it at it saturation pressure.  That is not possible for the reasons I have stated in previous posts in this thread.  If you refer to the paper that you have referenced (“CO₂ Recovery: Improved Performance with a Newly Developed System”), you will read:

 

“The system is based on the fact that the solubility of the permanent gases decreases with the decreasing temperature of the condensed CO₂.  In the case of liquefying the CO₂ at temperatures below 223 K (-58 ^oF) and pressure between 0.7 (101 psi) and 2.0 MPa (290 psi), the solubility decreases and the permanent gases volatilize from the liquid phase.”

 

The saturated temperatures of liquid CO₂ at 101 and 290 psia are -57 and -3 ^oF respectively.  It is no surprise (nor coincidence) that what the authors are avoiding to say is that they are condensing at -57 ^oF and 101 psia --- WHICH IS THE SATURATED STATE AND NOT THE SUB-COOLED STATE.  The authors are researchers - not engineers - and by profession, they are not practical in thought or in explicit explanations of what they are doing. 

 

Additionally, in order to strip out the dissolved and entrained non-condensables in the Liquid CO₂, the process that has been done since the late 1960’s is the one developed by Pritchard Corporation.  This same process is now also copied (in part) by Wittemann - but Wittemann’s version is not as optimized or efficient in mass and energy balance as Pritchard’s was.  Please refer to the Witteman literature I attach.  I have to submit my literature because you haven’t submitted a process flow diagram or P&ID of your process.  I have details on the Pritchard process - including a heat and material balance I did on it - but I can’t find it in my files at the present time.  I remember that my calculations were very close (less than 5% off) to the actual values of the process.

 

Clearly, the authors are proposing a process of simply producing saturated liquid CO₂ at a lower temperature - and a lower pressure - while “blowing” the undissolved fixed gases in a “purification” tank.  This is a very inefficient way to strip out the gases.  The optimum way of stripping out the dissolved fixed gases is exactly as shown in the Pritchard (and Wittemann) process - use a stripper tower with a reboiler and vent condenser.  There is nothing new or novel here.  This is a standard, generic stripping process that has been proven to work in the field for many years.  Engineers use the principle in stripping towers and even in deaerators for boiler feedwater.  The stripping process works with the liquid CO₂ at the saturated state - and the purification tank the authors mention is also at the saturated conditions.

 

Also refer to the attached papers delivered by Pritchard for additional information.  The Pritchard Process (& Wittemann’s) can deliver liquid CO₂ with a purity of 99.998% by volume - better than what most breweries can analyze.

 

The subject of a proper condenser design for the CO₂ gas stream is, as I’ve stated, a relatively simple one: the condenser should allow for easy mechanical venting of the non-condensables (fixed gases).  I really hope I have driven across my points and experience accumulated over the past 54 years.

Attached Files

•  Purification of Naturally Occurring CO2.docx   341.49KB   384 downloads
•  Brewery Carbon Dioxide Recovery.docx   90.77KB   392 downloads
•  Wittemann Liquid Carbon Dioxide Stripping.docx   597.46KB   519 downloads
•  liquid_co2_stripping_system_literature.pdf   336.49KB   1451 downloads

[Bill Earl]

To Art Montemayor and Bobby Strain,

 

Sorry for the long silence. Your advice and help is deeply appreciated.

 

It helped me recognize the symptoms of inerts in the CO2, and I finally discovered that air was entering the process via float drains during certain periods when compressor suction pressure went slightly negative. Adding check valves with low cracking pressure to the float drains improved performance dramatically, and the process is now performing as designed.

 

Thanks again.

 

Bill Earl