It is quite some time I don’t write about utility system. In this post I want to share you some basic knowledge of nitrogen supply system. In December 2015, I prepared several training presentation about nitrogen supply system when I was working in this company. I think it is a good idea to share some of them.
Nitrogen is used for purging activity and blanketing due to its unreacted feature.
Nitrogen is used at different application, depend on its application.
Required nitrogen purity based on its application
I find that nitrogen is widely used commercially, even in small scale. For example in LCNG station or LNG station, nitrogen is provided in small quantity in form of nitrogen bottle. In larger plant, such as gas processing facility, dedicated nitrogen generator is provided. We can say, there are several methods to supply nitrogen.
Nitrogen supply system
Nitrogen can be supplied directly in gas form and from liquid form, then it is regasified into gas form.
Gaseous nitrogen cylinders are designed t hold gas at pressure up to 2200 psig (151.7 barg). Other types can hold gas up to 2485 psig (171.4 barg) and 2640 psig (182.1 barg). The weight of common 2200 psig nitrogen cylinder is about 150 lb (68.2 kg).
Cryogenic Nitrogen Cylinder
Cryogenic nitrogen cylinder come in two forms: with vaporizer and without vaporizer. The first one, nitrogen is drawn in gaseous form, and the second one, nitrogen is withdrawn in from of liquid. These cylinders are insulated, vacuum-jacketed pressure vessel. The operating pressure is up to 350 psig (24.1 barg) with capacity of 80-450 L of liquid.
Nitrogen Vaporizer Package
Nitrogen vaporizer package is also a good choise for nitrogen supply. The package consists of:
- Nitrogen cryogenic vaporizer: vacuum insulated pressure vessel, sized at several days round trip of nitrogen lorry. The tank is equipped with top fill valve, bottom fill valve, pressure control system, pressure gauge, and pressure safety valve.
- Nitrogen vaporizer: it is usually ambient air vaporizer. Due to intermittent operation of nitrogen system, one vaporizer without spare is enough (I have discussed about this with vendor).
- Pressure regulating system and safety system: consists of self actuated control valve to adjust required supplied temperature of gas. And also provided with low-temperature safety system.
I don’t have real experience about nitrogen vaporizing package. However, in our last project, we propose to use this system. We already contact our local vendor, and the system is the same.
After I browsed, I found that there are two famous nitrogen generation systems which are pressure swing adsorption and membrane. I had two real experiences of using PSA type nitrogen generator.
PSA separates nitrogen by adsorption and desorption (regeneration) of oxygen and other gas.
- Adsorption of oxygen (or nitrogen production) occur at high pressure
- Regeneration occur at low pressure
PSA nitrogen generator comprise of two twin pressure vessel, each contains Carbon Molecular Sieve (CMS) which adsorps oxygen so that nitrogen can pass through. Pore size of CMS is designed higher than size of oxygen molecule but lower than nitrogen molecule.
How about membrane type nitrogen generator? Membrane systems operate on selective gas permeation principle. Typical membrane process uses several membrane module, each module consists of thousands of hollow fibers.
How to choose?
Basically there are two main considerations to choose right nitrogen supply system: purity and flow rate.