Nitrogen Removal in Natural Gas System-II |ChemFam #21|

Greetings to everyone! In my previous post, we have already discussed about the need for removal of nitrogen and different methods of removing it in natural gas systems. Today, we shall be looking into some more techniques about removal of nitrogen from natural gas.

WHAT WE SHALL LEARN?

We shall be learning about the removal of nitrogen by other 3 methods than those we have previously discussed. We shall be discussing the adsorptive method, membrane method and hydrate method. We have already discussed a typical arrangement of a Nitrogen removal unit (NRU) of which these 3 are a part and these 3 methods will be our topic of discussion.

Adsorptive Nitrogen Removal

Adsorption is the primary choice after cryogenic distillation. In this case, generally we use Pressure swing adsorption (PSA) to remove high amount of nitrogen which is the adsorbate at a low heat of adsorption. The adsorption pressure is about 10-100 kPa and adsorption temperature ranges from 273-323 K and the heat of adsorption is about 15-22 kJ/mol. The heat of adsorption is significant because this decides how much energy we would require fir the regeneration of the spent adsorbate.

Adsorbents for Nitrogen Removal

In case of nitrogen removal, we found that mostly the adsorbents show equilibrium selectivity for methane over nitrogen. Of course it depends on the type of adsorbents but most of the adsorbents show more selectivity towards methane i.e., nitrogen is purged out while methane is retained. The molecular sieves and titanosilicate show kinetic and equilibrium selectivity for nitrogen.

The pore size distribution is an important factor that describes how selectively we can adsorb the various solutes. Here, we found that the diameter of nitrogen and methane molecules are very close i.e., about 3.64 Å and 3.8 Å respectively. Thus the pore size of adsorbents have to be very carefully made to differentiate these 2 types of molecules.

Image Source

Mechanism of Adsorptive Separation

✓ Thermodynamic equilibrium mechanism

First, we have the thermodynamic equilibrium mechanism which shows the difference in the adsorbate and the surface interaction or adsorbate-packing interactions when the system reaches equilibrium. This basically means that we are allowing all the solute particles to reach equilibrium and then measuring the respective concentrations on the adsorbate. So depending on the adsorbate-adsorbent interaction, various solutes will be adsorbed to different extent at equilibrium. This difference in the extents will determine the separation and the selectivity of the various solutes by the particular adsorbent at equilibrium.

✓ Steric mechanism

Next we have the steric mechanism. In this mechanism, the difference in the size and shape of gas molecules dictate. That means the molecules which are larger than the pore size will not be allowed to the adsorbent whereas those molecules which are smaller than the pore size will be allowed to enter the adsorbent particles and get adsorbed.

✓ Kinetic mechanism

Lastly we have the steric mechanism. It is based on the difference in the rates of diffusion of the various solutes through the adsorbent particle determines their separation. That means in this kinetic separation all or some of the solutes are allowed to pass through the pores of adsorbent, but depending on the rate at which they travel inside the pores will determine their respective adsorption capacities.

Membrane NRU

These membrane units are limited to pretreatment and compression of feed stream to produce the desired driving force. The natural gas is generally is at sufficient pressure but still we may need compression for membrane separation. As we learnt earlier that the separation through membrane depends on the difference in the partial pressure of the particular solute across the membrane. The membrane performance is characterised by the permeability and the selectivity.

©️@splash-of-angs63 (Image drawn by me)

These membranes are not commonly used due to low selectivity of similar sized nitrogen and methane. The selectivity is < 3 for glassy polymer and < 5 for rubbery polymers. The addition of inorganic fillers increases selectivity by making mixed matrix membrane.

Membranes used in NRU

We use asymmetric membranes made from silica, zeolites and carbon molecular sieves and unlike organic (polymeric) membranes, these membranes can operate at high temperature i.e., above 500°C.

The design of such membrane NRU unit is determined by the membrane capacity, feed gas pressure, contaminants in the feed gas and the desired purity of the product.

Membrane performance in NRU

With the increase in selectivity permeation of nitrogen drops. For an increase by an order of magnitude in selectivity, permeation for N₂ decreases by 4.5 orders of magnitude while that for CO₂ by only 2.6 orders of magnitude. Multi-stage membrane module is done to treat natural gas for N₂ concentration between 4 and 30%.

Nitrogen Rejection Through Hydrate Route

This method has been explored since 1930s cor the natural gas separation. Hydrates may be used to produce a lean gas stream with respect to hydrate forming gases. In case of CO₂-N₂-CH₄ mixture separation, CO₂ preferentially forms hydrate. Use of high pressure for hydrate formation makes this process uneconomical. A typical natural gas forms hydrate at high pressure (>30 bar) and low temperature (<20°C). Hydrate promoters are used to reduce the hydrate formation pressure. Whenever we are using promoters that means we are also adding some kind of impurity to the system.

Image Source

There are no commercial scale process operation for NRU. Contacts between gas-liquid phase is important for hydrate formation. These two phases have to be properly mixed in a chamber. In this case bubble column and stirred tank reactors are used. Hydrate formation is exothermic i.e., it is accompanied by release of heat energy. So, to remove the heat, fluidized bed heat exchanger has been proposed. Such beds also prevent hydrate deposition and increase internal heat transfer coefficient.

B I B L I O G R A P H Y

Pros and cons of different Nitrogen Removal Unit (NRU) technology

Department of Cryogenic Engineering Centre

Read my previous Blogs:

Nitrogen Removal in Natural Gas System-I |ChemFam #20|

Acid Gas Removal in Natural Gas System-II |ChemFam #19|

Acid Gas Removal in Natural Gas System-I |ChemFam #18|

Estimation of Water Content in Natural Gas |ChemFam #17|

Membrane Separation in Natural Gas System |ChemFam #16|

Design of distillation column |ChemFam #15|

Separation Technique: Distillation |ChemFam #14|

Transmission Electron Microscope: Principle and Working |ChemFam #13|