Nitrogen production plants HPN INDEX 1. Introduction 1.1. Membrane permeation 1.2. PSA 2. HPN (High Pressure Nitrogen) 3. HPN plant advantages 4. Choice of the process 5. Customer HPN technical form generator 6. Summary table of our HPN plants
1. INTRODUCTION Nitrogen (N2), main component of the air (78%), is at present the most required gas by the world-wide market. For its high chemical inactivity it is used in the gaseous state as an blanketing agent (to prevent explosions, fires and unwanted reactions) and in the recovery of solvents. As a low boiling liquid (ebullition point: -195.8 °C), non toxic and inert it’s one of the primary cryogenic refrigerants. Nitrogen is produced by separating air in its components with cryogenic and non cryogenic systems.
NON CRYOGENIC TECHNIQUES •
Permeation on membranes
•
Adsorption on PSA
Both techniques have been recently introduced and impose economically for low purity nitrogen production on-site plants of small/middle dimensions.
CRYOGENIC TECHNIQUES The Cryogenic air separation process is economic for large capacity plants, high purities and for liquid production. •
The big traditional cryogenic distillation plants, with one (BOC process) or two columns ( modified Linde process), allow the co-production of oxygen and argon (in dedicated other Two columns with a purification system) and they are favourable for large tons.
•
The HPN plants represent the most economic solution for onsite high and very high purity nitrogen productions.
A short description of the two non cryogenic technologies follows on the next sections.
1.1. Permeation on membranes This technology, applied to air separation since the 1980’s, uses the selective permeability of the hollow-fiber polymeric membranes to separate the nitrogen from the oxygen. The membranes now on the market, fabricated with polysulfones, polyimides and polycarbonates permeate oxygen, steam and CO2 faster than nitrogen (with selectivity superior to 7-8) according to the following solution-diffusion process: •
adsorption of the gas on the membrane surface
•
solution of the gas into the membrane
•
diffusion of the gas through the membrane
•
release of the gas by the opposite surface
•
desorption of the gas from the surface
•
In the membrane permeation the compressed air, filtered to remove residual oil, is heated to 40-60°C, and therefore passed in parallel and axially through membrane modules, each consisting in a bundle of thousands of hollow fibres. Nitrogen is concentrated to purities ranging from 95 to 99.5%. The membrane systems are economic to produce gaseous nitrogen at rates from 3 to 1000 Nm3/h, according to the produced purity.
Residue gas
Feed gas
Permeate gas
Figure 1.1 – Membrane Process flow sheet
1.2. PSA ( Pressure Swing Adsorption) The PSA separation system is based on the principle of reversible selective adsorption of oxygen on carbon molecular sieves (CMS). Oxygen is adsorbed from the CMS to a faster rate than nitrogen for the combined effect of polarity and molecular dimensions. Compressed air, filtered and cooled to remove excess moisture, is alternately sent on two CMS beds, where O2, CO2 and steam are selectively adsorbed. When a bed is saturated with oxygen, air flow is turned on the second bed, while the first bed is regenerated by depressurizing (releasing in such a way the adsorbed molecules) using a part of nitro and the process is repeated. Atmospheric Argon is concentrated in the product nitrogen. The PSA systems are convenient for on-site productions lower than 2000 Nm3/h of nitrogen with purities ranging from 95 to 99.9%.
Vent or recirculat. system
Air
Condensate Water or chilled water Steam or regeneration gas
Nitrogen
Figure 1.2
PSA Typical
2 HPN (High Pressure Nitrogen) On site HPN plants are replacing, for their lower cost, the standard cryogenic columns or the supply of liquid by tank trucks in all the operations which require a constant nitrogen high purity flow.
The separation process drawn in the diagrams attached includes the following steps: Compression: air is filtered, compressed and therefore cooled to approximately 10°C in a chiller to reduce the moisture content.
•
Purification: CO2 and residual moisture are removed in a molecular sieves/ activated allumina PSA system, regenerated by heated excess waste gas coming out from the cold-box. •
Separation: It’s carried out in a cold-box filled with perlite or other thermal insulating material.
•
The purified air is cooled to the dew point in a heat exchanger crossed in countercurrent by the streams of nitrogen and waste gas produced and it’s afterward fed on the bottom of an internally refluxed separation column. The oxygen-rich air stream (Waste-gas), coming out from the bottom is expanded to low pressure in a lamination valve, providing (by Joule effect) the cooling energy needed to the reflux. The pure nitrogen goes out from the top of the column at a mean purity of 99.99% and can reach for electronic chip production less than 50 ppb(part per billion) oxygen impurity. A liquid nitrogen injection system ( if available ) ensures working continuity, also reducing the start-up times and the compression energy. • Options: The expander insertion allows to recover the cold necessary for the Co-production of a liquid part (Max 10%).It is possible to produce a large part of nitrogen as liquid including a liquefier unit.
Figure 2.1 HPN- Typical PDF
3 ADVANTAGES OF THE HPN PLANT The nitrogen production with HPN plants has the following advantages: • Low investment costs For ranges from 100 to 10000 Nm3/h of high purity nitrogen (> 99.9%) the HPN plants represent undoubtedly the most economic investment. •
Low energy costs
The HPN plant auto-produces the necessary refrigeration duty. The energy consumption is therefore made up by the compressor, the chiller and the PSA regenerating fluid heating. At final pressures of 3 ¸ 7 bars the average energy consumption is 0.2 ¸ 0.35 kWh/Nm3 of 99.99% nitrogen. •
Low maintenance costs
It refers to very simple plants, made with materials suitable to cryogenic applications and studied to work in continuous for long times. •
Plant flexibility
Each plant is fitted to the customer's requirements. The three sections (compression, purification and separation) are modular and easily fabricated on separate skids, which means great freedom in the choice of investment and remarkable installation and start-up time saving. • Process
flexibility
The specification of purity of the nitrogen can be increased up to 50 ppb of oxygen varying the process conditions. •
Liquid production
It is possible to produce on-line a part of liquid at high purity. •
Monitoring
Purity parameters and the process variables are monitored continuously, with possibility of remote control, optimising the process performances.
4 CHOICE OF THE PROCESS The best technology to adopt is mainly function of the capacity and purity requirements, as highlighted by the below diagram. Other important factors are the following: •
Necessity of liquid product: only the cryogenic plants, with the help of expansion and liquefaction cycles ensure the liquid availability. Therefore, in case of limited uses, the purchase of liquid from large technical gases producers is still the best choice.
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Continuity of the supply and logistic choices: for limited and fluctuating flowrates it must be made a careful comparison between the costs of an on-site plant’s installation and those of liquid nitrogen transport and storage.
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For large capacities (higher than 10000 Nm3/h) the opportunity of coproducing oxygen and argon makes economical the installation of a traditional cryogenic plant competitive. Fig. x: Economic distribution of N2 processes 100 1 ppm 99
98
HPN and cryogenic systems
99.9 97 Nitrogen purity, %
Delivered liquid 99.8 96
99.5 95
PSA 94
95 93
Membrane 92
91
90 90 1
10
100 Nitrogen flow rate (Nm3/h)
1000
10000
5. CUSTOMER HPN TECHNICAL FORM GENERATOR Purity required
Min Composition N2 O2 Ar
Product Flow rate
Min
Pressure required
Min
Liquid required
Normal Max Composition Composition % vol N2 % vol N2 O2 O2 Ar Ar 3
Nm /h
Yes
%
Normal Nm3/h
Max
Normal
Max
Yes
% vol
Nm3/h
%
Yes
%
Comments:
6. Summary table of our HPN plants The range of production of our plant Lin assisted or with a production of liquid nitrogen is:
Production purity Nominal Capacity of plant
N2 %
99.5-99.999995 %
Argon %
0.5% till to 50 ppb
O2
5 ppb-10ppm
Water ppm
<.1
200-18.000 Nmc/h GAN
LIN assisted
With 5-30% LIN
