Normally industrial use CO2 is delivered by truck that carries tens of tons of liquid CO2. And the CO2 will be stored in a big vacuum insulated tank which is generally 25 tons approximately. The temperature of CO2 inside of the tank is about minus 20 degree Celsius, and the pressure is between 1.6-2.0Mpa. When the environment temperature change causes the pressure rise, the tank will give off CO2 to cool down itself.
As you know that liquid is much easier to hand than gas. So we are gonna manage to keep CO2 in the state of liquid during the whole process. Because the CO2 is very liable to vaporize due to its high vapor pressure, so we have to strike the balance between pressure and temperature. This is the foundation of how our system works.
We employed a booster pump that will transport the liquid CO2 from CO2 tank to middle tank whose wall was jacketed design, by which the temperature of the middle tank could be controlled by water temperature controller. One tip I have to bring it up here is that the distance between CO2 tank and middle tank has to be as near as possible. Due to the CO2 has the propensity of vaporizing so that the longer distance, the more risky to vaporize.
To do so, the booster pump is recommended to put under the CO2 tank. Even so, the vaporizing still happens occasionally. When the booster pump is not able to pump the CO2 properly, vaporizing probably happened. The discharge valve is designed for this situation, so open it and let out the vaporized CO2 in order to cool down the temperature of the head of the booster pump.
This process has to be done very carefully, because if the discharge valve is opened too much, the CO2 is liable to become dry ice which will block the pipe, and it will take very long time to melt away. To avoid this circumstance happen, the capacity of the booster pump should be ten times bigger than metering pump so that the booster pump does not have to work all the time, only works periodically.
Besides, longer discharge pipe is advised which can prevent the inlet pipe of booster pump from freezing by dry ice. In addition, the insulated piping of the whole system will contribute to the consistence of CO2 transport, also it can save energy.
The purpose of middle tank is to keep the CO2 at a certain temperature (around 25 degree Celsius) in order to maintain the pressure of the whole system is stable and high enough (6.5-7Mpa). These two premises ensure the constant physical property of CO2 and it gets CO2 primed for precise metering into twin screw extruder. As I mentioned before, the density of CO2 varies when temperature changes.
The highest level alarm of CO2 in the middle tank should be no more than 70% of the total volume of the middle tank in case of any incident happened, the power cut in the summer in some hot region, which may cause circumstance temperature of middle tank goes high, then the CO2 may become supercritical state whose behavior is more difficult to control and predict. As a result the volume of CO2 will be expanded greatly. If the free space is not enough, it may get the middle tank in some danger.