OT: CO2 evaporator or vaporizer

Richard J Kinch · 2007-04-05T19:35:35+00:00

I'm looking for a way to vaporize 20 lbs/hour of liquid CO2 from a refrigerated bulk tank and deliver the gas at about 100 psi. Does anyone know of a ready supplier of such a device? I've talked to the industrial suppliers, and their smallest units are 100 kgs/hour for chemical and brewing plants. Seems like they use similar devices for LN2 vaporization in semiconductor processing, maybe at a smaller scale? I've considered improvising something with a tubing coil immersed in a heated water bath, but it isn't clear how you can meter the liquid input for an intermittent draw on the gas out. This is for a small carbonation/bottling plant, running 1200 liters/hour of beverage.

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sylvan butler 19 years ago

Yes.

why? Because as evap increases the pressure, it will push back any liquid trying to enter. It tries to push both ways, but the gaseous regulator on the output prevents pushing that direction.

BUT, if your liquid is CO2, and your evap maintains 70F to 80F, your system is going to be operating at 800-900psi (rough numbers, I don't recall offhand the exact vapor pressure of CO2 at near room temperature). That is likely a significant problem given your spec to operate the bulk liquid CO2 tank at 300psi.

Forget the tiny orifice. It is neither required nor helpful. You just need that check valve, and it must be able to handle liquid CO2 temperature at 1200+psi operating pressure. Note that everything on the vapor side of that check valve needs the the same or better rating. Then for safety you also need an overpressure relief valve at under

1200psi on the vapor side. With that setup the vapor side will operate between just under 300psi (input pressure) as a lower limit and an upper limit of the CO2 vapor pressure at your vaporizer temperature (I'd go for 75f and lower, can probably use the waste heat from the refridgeration unit). Your output regulator at 100psi will be happy with that range of input pressures and maintain your 100psi output. I have no idea where to get a check valve with that rating.

sdb

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Richard J Kinch 19 years ago

No. Why not? Because the pressure does not rise instantly from evaporation. There is a delay due to the finite flow rate of heat into the liquid CO2 and the evaporator. You crack the valve, 300 psi liquid rushes in to nearly fill the evaporator tubing, and then the vaporization begins. If you *have* a check valve, the evap pressure rises to the CO2 saturation vapor pressure (1000 psi or more as the evap is warmed), and liquid slugs into the regulator. If do *not have* a check valve, the extra pressure now present in the evap pushes liquid and gas back into the bulk Dewar tank, raising its pressure and tripping the Dewar overpressure relief. Bad either way!

I think you understand this, at least intuitively, because you say the vaporizer must be able to withstand the saturation vapor pressure of CO2.

If you're willing to build an evaporator for that full pressure, then you don't necessarily need metering, you just need throttling (orifice or capillary tube). But the point was to build an evaporator for *not* more than a few 100's of psi working pressure, with overpressure relief if the metering failed open.

I got a quote today on a 1000 watt vaporizer: $1200. Apparently all it is is 10 feet of stainless tubing with flare fittings, cast into a block of aluminum, with a strip heater (like a silicon rubber tank heater). No metering or restriction, but then it takes full CO2 P_sv.

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sylvan butler 19 years ago

Essentially constant application of pressure... And essentially constant temperature (minor variation of course).

Saturation pressure depends on temperature. If the evap pressure is under 300psi (your bulk liquid pressure) and the temperature is at 70f, then there will be no liquid left in the evap. The check valve begins to open, and yes, evap will be instant and the pressure will rise simply because there is enough heat capacity to instantly vaporize the first liquid that comes in.

Don't run the evap that warm. Keep it 70f or so (thermostat) and the saturation pressure is only about 700psi. I don't see much liquid remaining in the evap, and especially not pooling near the regulator end. With 300psi source pressure, and a check valve, more liquid will not enter the evap until no liquid remains. The evap will be coldest near the cryo tank, and evap will occur more and more the further from that supply.

Yup, definitely need the check valve.

A low pressure evap seems very difficult (I cannot envision it at all) unless you operate at cryo temperatures. At any kind of "normal" temperature, the CO2 will evap until pressure exceeds "few 100's of psi".

"full CO2 P_sv", exactly. Did that one have a check valve or is it expected you will run your entire system at P_sv?

sdb

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sylvan butler 19 years ago

Oh forgot to mention... Maybe when the system is first started after evap pressure is 0 the "liquid rushes in" but in normal operation the pressure differential will be determined by the check valve and that differential will determine the "rush" speed.

I before said "forget the small orifice" and that was most likely wrong. A low pressure differential OR a sufficiently small orifice will limit the flow of liquid such that it will vaporize quickly, and so quickly stopping further flow (due to the check valve).

sdb

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stans4 19 years ago

All this stuff is very amusing. Talking about regulating liquid and all, I mean. Why, I hear you ask? CO2 passes from liquid to SOLID on its way to gas at STP if it's just released through an orfice. Anybody here EVER use a CO2 fire extinguisher? Liquid CO2 is going to have to be superheated past its critical point if you want to bypass the solid phase and avoid freezing up your equipment. Probably the reason the vaporizer is so expensive. Here's some thermo info:

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pressure at critical point, also, another reason the unit is expensive, over 1K psi.

Looks to me like you need at least couple of thousand BTU/hr for the flow rate contemplated. Don't think a squiggle of tubing in a water tank is going to supply that.

Stan

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Richard J Kinch 19 years ago

Less so if you don't impose misconceptions.

We're talking about a 300 psi environment.

Wrong thermo info. I've written them about the erros on that page, but they haven't fixed it.

20 lbs of 0 deg F liquid CO2 requires 2700 BTUs to vaporize and warm. That's the hourly rate.

Why not? We filled a 55-gallon drum with lukewarm water and ran a

20-foot tubing coil thorugh it. It worked great. Starting at 78 deg F we have a reservoir of over 20,000 BTUs before it starts to freeze. And a 1 KW immersion heater will resupply the heat indefinitely. Only problem is having a bulky drum.

As a process refinement we'll instead use the CO2 vaporization via heat-exchanger to pre-chill the incoming beverage to improve the carbonation process parameters.

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