Using Heat to Build Pressure in Place of a Compressor 5

[ngarwood]

I am currently involved in the design of a new plant process which uses liquid CO2 as its primary medium. Besides a substantial quantity of liquid CO2, our process also requires about 1 CFM of CO2 gas at 650 psi. The storage tank is set at 300 psi.

Our gas supplier (BOC) has suggested using a gas compressor to achieve this, and a price quote of $45-55,000 was obtained from a compressor manufacturer. Our budget is the driving factor, however, so we are exploring all possible alternatives.

One alternative that has been mentioned is the possibility of loading a quantity of liquid CO2 into one of two pressure vessels and using flange heaters to heat the liquid/gas to 1000 psi and deliver it to the process via a pressure regulator. Valves could be used to alternate between the vessels; while one vessel is delivering gas to the process, the other vessel would heat a fresh charge of liquid CO2 in preparation for the next cycle.

Does anyone have experience with this? Our immediate objective is to evaluate its merit based on cost, but I have limited personal experience engineering things of this nature.

Where should I start?

 

[sshep]

While I have no experience with what you are suggesting, perhaps what we call a gas intensifier (booster) may be economical compared to a conventional compressor. This is a device that uses a higher volume of low pressure gas (usually air or N2) to compress a smaller volume of gas to a high pressure, using a piston assembly. No motors, and usually built not to need lubrication.

Your idea is pretty clever, but intrumentation and control requirements may drive up the cost of that alternative.

best wishes, sshep

 

[Montemayor]

ngarwood:

First, let me acknowledge the detailed and articulate description of your basic data and scope of work. Because of it, I have a good understanding of what you intend and am therefore able to offer my many years of experience in this field. What you propose is not only very doable, it is being done all over the world in as a daily routine exercise. I have done this very technique countless times – forward and backwards. The first important and vital thing you should get busy in doing is obtain a T-S (Temperature – Entropy) diagram for CO2. Depending on your resources or where you are located, you have various options on this. You could also employ the services of the National Institute for Science & Technology at their website:

http://webbook.nist.gov/chemistry/fluid/.

However, in order to employ the NIST website you should be well-versed in Thermodynamics. Now, allow me to explain how the technique works by giving you real-life examples.

The daily process of converting low pressure liquid CO2 (LCO2) to high pressure gaseous CO2 takes place when CO2 producers and distributors use a LCO2 pump to fill high pressure steel cylinders (usually DOT specifications 3AA 1800) with 50 lbs of CO2 and distribute it that way to consumers. This is also the manner in which CO2 fire extinguishers are normally filled. The consumers employ regulators on the cylinder valves to regulate the pressure they want to use the gaseous CO2 at. Bear in mind that I have simplified the overall process and will now detail what is taking place thermodynamically:

1. CO2 is normally produced, stored, and distributed today as a liquefied gas at 250 psig and -8.5 oF in the saturated state (LCO2). This is pumped as a liquid using a fill pump such as the Tomco AH-45 Cylinder Filling Pump produced and sold by Tomco Equipment (

http://www.tomcoequipment.com/pumps.htm).

The cylinder is normally filled with resultant high pressure liquid up to 85-90% of its total volume in order to have a compressible vapor space above the liquid. The resultant high pressure liquid is achieved as a result of the cylinder and its contents reaching ambient temperature after being filled. This thermodynamic step can easily be traced and identified on a T-S diagram for CO2. The resultant saturated pressure in the cylinder will be that corresponding to the contained liquid’s temperature (ambient).

2. The high pressure cylinder is distributed to CO2 gas consumers and is fitted by the consumer with a gas regulator directly on the cylinder’s valve. This regulator feeds CO2 gas to the consumer’s process, in accordance to its consumption, at the pre-set regulated pressure. The action of the regulator is an adiabatic expansion of CO2 gas and, as such, produces a colder, lower pressure gas. This product gas can be warmed with a downstream heater to the desired temperature. Additionally, the consumption of the CO2 gas in the cylinder’s vapor space causes the evaporation of the contained high pressure liquid and leads to a refrigeration effect in the cylinder. If the consumption of the gas vapor space is not excessive, ambient temperature will maintain the cylinder warm and its pressure constant through natural convection currents around the cylinder. If the consumption of CO2 is great, however, then a heat source must be used to vaporize high pressure CO2 liquid and maintain the cylinder’s internal gas vapor pressure. This heat source can be administered as a spray of warm water down the exterior walls of the upright cylinder. This technique is often used by small carbonated beverage bottling plants.

3. By carefully controlling the temperature of the cylinder from excessive heating, you can ensure that the CO2 regulator will receive sufficient high pressure gas to regulate down to your pressure level (650 psig?).

I have also done the same technique in producing dry ice (solid CO2 @ -109 oF) from high pressure liquid CO2 at 1,200 psig and 80 oF. What was industrially done was that “evaporators” were filled with liquid CO2 by flashing the high pressure CO2 into them and recycling the produced, cold vapor with reciprocating compressors that compressed the gas back to 1,200 psig and condensed it with cooling water for further recirculation as feed to the evaporators. This is what I meant by saying I’ve also done the reverse.

What you are describing is nothing more than using a LCO2 “evaporator” designed for an MAWP of approximately 2,000 psig and regulating the gas from this vessel down to your needed gas supply pressure (650 psig?). You could employ existing high pressure CO2 cylinders (or “tubes”, which are of a larger capacity) as your evaporators. Your consumption of 450 lbs/day of CO2 gas might be such that you could fabricate a pressure vessel (36” ID x 8 ft long) with 2:1 ellipsoidal heads and a warm water internal coil that could contain approximately 500 lbs of high pressure liquid CO2 at 85 oF and 1,050 psig. This vessel could easily furnish you a one-day’s supply of CO2 gas. You could keep it filled with a Tomco pump. With the proper safety devices and control instrumentation you should be able to operate very easily and safely. You don’t state your production runs or whether your process is 24-hr/day, so I can’t venture to go further on a possible design. I’ll leave that up to you.

I see no practical reason for making this process any more complicated by using two vessels and switching. I would merely pump the LCO2 in when needed. You may possible use a much reduced capacity Tomco pump for this purpose. I would also not employ any flange heaters or apply heat on the vessel itself. I would use a warm water coil to do the vaporization, and it would probably be located externally, in a simple drum with water overflow. I’ve used this type of vaporizer often on LCO2 storage tanks at customer sites.

I won’t go into further design details because I believe I’ve exhausted your basic data and this post is already long enough to give you a hard idea of what can easily be done on a low-cost budget – and done safely. I hope my experience is of some worth to you in this project.

Art Montemayor