Double Eccentric Butterfly Valve & PSA(Pressure Swing
Adsorption)
High
performance butterfly valves,generally refers to double eccentric butterfly
valve and triple eccentric butterfly valve,due to their excellent working
performance and high sealing ability (compared with concentric butterfly valve)
are widely used in petrochemical,power plants,metallurgy,civil heating
fields.Eccentric butterfly valves have the good working performacne to meeting
sever and strict requirements in many different industries.Today, we are going
to introduce the application of eccentric butterfly valve in air separation for
making oxygen,hydrogen,CO etc. At present,the mature air separation system is
PSA(Pressure Swing Adsorption),it need certain number of programmed valves
(valve parts usually double or triple eccentric butterfly valve) in its
pipeline system to control and adjust the medium.
Sinopival
double (and triple) eccentric butterfly valve are widely used in this kind of
PSA system due to the valves' excellent features:
1-Long
service life,on-off 1 million times
2-Super
reliable sealing performance:bidirectional sealing zero bubble
3-Patented
seat design
4-Vacuum
servie ability
5-Both
liminated seat and full metal seat avaialble
6-Available
fully rated to Class 2500lb
7-Anti-blowout
device design
8-Fire-safe
design optional
9-Excellent
flow and throttling characteristics covering services from cryogenic to high
temperature
Following we are going to briefly introduce application of double
(and triple) eccentric butterfly valve in PSA (VPSA) process for making oxygen
gas
PSA(Pressure
Swing Adsorption for Making Oxygen) Introduction
1. The Main Technologies and Comparison of Air Separation & Oxygen Making
Oxygen in industrial production and daily life has a wide range of uses,
the air contains 21% (volume concentration) of oxygen, is the cheapest oxygen
raw materials, so oxygen is generally obtained by air separation.
1.1 Technologies of Air Separation & Oxygen Making
1.1.1 Cryogenic separation process: it’s the traditional oxygen making technology,
the oxygen made in this way has high purity, wide product variety and suitable
for large-scale oxygen making.
1.1.2 Pressure swing adsorption process (PSA): it’s new oxygen making technology,
featured with low investment, low energy consumption, suitable for low purity request,
small & medium scale applications.
1.1.3 Membrane separation process: not yet mature and this kind of
technology are not applied in industry and come into use.
1.2 PSA process features (compared with membrane
separation process)
1.2.1 The PSA process is more simple and does not require complex
pretreatment equipment.
1.2.2 Oxygen purity can reach up to 95%, nitrogen content less than 1%,
the rest is argon.
1.2.3 Oxygen making scale smaller than 10000m3 / h, the consumption
of electricity is lower, smaller investment;
1.2.4 High degree of automatic operation of the device, convenient to open
and stop;
1.2.5 The equipment operation independence is strong, the security is
high;
1.2.6 The device is simple, flexible operation (part of the load is
superior, conversion speed of the load is faster);
1.2.7 Low operating and maintenance costs;
1.2.8 Low cost of civil works, occupy less land.
1.3 Comparison Between
Pressure Swing Adsorption Separation Process and Membrane Separation Process
(refer to following table 1)
Item
|
Membrane Separation Process
|
Pressure Swing Adsorption Separation Process
|
Separation Principle
|
Liquefied air, separate oxygen and nitrogen according to their different
boiling point
|
Raise pressure and adsorption,vacuum pumping and desorption,adopt the
different adsorbability of oxygen and nitrogen to achieve separation
|
Main Characteristics of Equipment
|
Complex technology process,more facilities required,including air
compression system, air precooling system, air purification system, the expansion unit, heat
exchange system and rectification tower etc
|
Simple process, less equipment, including blower ,vacuum pump and the
adsorption tower, etc
|
Process
Characteristics
|
Operated at -160～-190℃ cryogenic environment
|
Normal temperature operating
|
Operation
Characteristics
|
Long start time, usually in 15 to 40 hours, must continuously operate,
can not be intermittent operation,if a short shutdown, restart require long
time.
|
Short start time,generally less than 30 min.,both works for continuous and
intermittent running
|
Maintenance
Characteristics
|
Complex facilities structure,high machining precision,big technical difficulty
of maintenance,and high maintenance cost
|
Sinple facilities structure, small technical difficulty of
maintenance,and low maintenance cost
|
Civil Works and Installation
Characteristics
|
Occupy large land area, high requirement for factories and
infrastructure, high project cost.Long installation cycle, big technical
diffculty, high installation costs.
|
Occupy small land area, no special requirement for factories and
infrastructure,low project cost.Short installation cycle, low installation
costs
|
Oxygen-Making
Cost
|
High electricity consumption for small& medium scale oxygen
making,about 0.5～1.0KW/Nm3
|
Low electricity consumption,about0.32～0.35KW/Nm3
|
Safety
|
The equipment is regulated by the pressure vessel specification. Can
cause local accumulation of hydrocarbons, there is the possibility of an
explosion.
|
Low operating pressure, free from pressure vessel regulations, does not
cause local accumulation of hydrocarbons
|
2. Principle of Pressure
Swing Adsorption Separation for Making Oxygen
2.1 Principle of pressure
swing adsorption separation for making oxygen
The main components in the air are nitrogen and oxygen. By selecting the
adsorbents that have different adsorption selectivities to nitrogen and oxygen,
an appropriate process is designed to separate oxygen from nitrogen and obtain oxygen.
Nitrogen and oxygen both have a quadrupole moment, but the quadrupole
moment（0.31A \u65289X） of nitrogen is much larger than that
（0.10 A\65289X）of oxygen, so the adsorption
capacity of nitrogen on zeolite molecular sieve is stronger than that of oxygen
(stronger acting force between nitrogen and molecular sieve surface, as shown
in Chart 1 below).
Thus, under pressure status when the air is passing through the adsorbent
bed containing the zeolite molecular sieve adsorbent, the nitrogen is adsorbed
by the molecular sieve, while the oxygen is less adsorbed and enriched in the
gas phase and flows out of the adsorbent bed to separate the oxygen gas and the
nitrogen gas to obtain oxygen gas . When the molecular sieve adsorbs nitrogen
to near saturation, shut down air flow and reduce the pressure of the adsorbent
bed. The nitrogen adsorbed by the molecular sieve can be desorbed and the
molecular sieve regenerated and reused. Two or more adsorbent beds to switch
jobs, can continuously produce oxygen.
Argon and oxygen have close boiling point, the two are difficult to
separate, them together in the gas phase to be enriched. Therefore, the
pressure swing adsorption oxygen plant usually can only get 90% ~ 95%
concentration of oxygen (oxygen up limit of 95.6%, the rest is the argon), compared
with concentration of 99.5% obtained from membrane air separation unit , also
called oxygen-rich.
2.2 Brief Facilities Introduction
for Pressure Swing Adsorption Separation for Making Oxygen
From the above principle, it can be seen that the adsorbent bed of
pressure swing adsorption air separation oxygen plant must contain at least two
operation steps: adsorption and desorption. Thus, when there is only one
adsorbent bed, the product oxygen is interrupted. In order to continuously
obtain the product gas, usually two or more adsorbent beds are provided in the
oxygen plant, and additional necessary steps are provided in view of energy saving
and smooth operation.
Each adsorbent bed is typically subjected to repetitive steps of
adsorption, forward depressurization, evacuation or depressurization
regeneration, rinse displacement, and pressure equalization. At the same time,
each adsorbent bed is in different operating steps, under the control of the
computer time switch, so that several adsorption bed in coordination, at the
pace of time are staggered, so that the pressure swing adsorption device can
run smoothly, continuous obtain product gas.
According to the different methods of desorption, pressure swing
adsorption oxygen is divided into two processes (see table 2 above):
2.2.1 PSA process: under pressure adsorption (0.2 ~
0.6MPa), atmospheric pressure desorption. Investment is small, simple
equipment, but high energy consumption, suitable for small-scale oxygen-making
applications.
2.2.2 VPSA process:
atmospheric pressure or slightly higher than atmospheric pressure (0 ~ 50KPa)
under the adsorption, vacuum desorption. Equipment is relatively complex, but
the higher efficiency, lower energy consumption, suitable for large-scale
occasions of oxygen-making.
Table 2 Comparison
Between PSA and VPSA process
Technology
Process
|
Suitable for Oxygen-making Sclae
m3/h
|
Adsorption Pressure
KPa
|
Desorption
Pressure
KPa
|
Concentration of Oxygen %
|
Electricity Consumption KWh/m3
|
Oxygen Rate
%
|
PSA
|
≤200
|
200～600
|
大气压
|
80～93
|
0.7～2
|
30～45
|
VPSA
|
100～10000
|
0～50
|
－45～－80
|
80～95
|
0.3～0.5
|
46～68
|
For the actual separation process, other trace components in the air must
also be taken into account. The adsorption capacity of carbon dioxide and water
on conventional adsorbents is generally much greater than that of nitrogen and
oxygen, and the adsorbent can be adsorbed and removed by the addition of a suitable
adsorbent (or by the oxygen-making adsorbent itself). The number of adsorption
towers required for the oxygen plant depends on the scale of the oxygen
generation, the adsorbent performance and the process design, and the running
stability of the multi-tower operation is relatively better, but the equipment
investment is higher. The current trend is to using highly efficient oxygen
adsorbents ,minimize the number of adsorption towers as less as possible,
employing short operating cycles to improve plant efficiency and to save
investment .
