Co2 Absorption and V2o5
Co2 Absorption and V2o5
Co2 Absorption and V2o5
Carbonate
Sata K. A. Ajjam
Babylon University ,College of Engineering
Abstract
Several process improvements have recently been incorporated into this Hot
Potassium Carbonate process for CO2 removal. The influence of steel surface
condition and solution chemistry on the critical inhibitor concentration required for
spontaneous passivation of carbon steel in typical solutions of hot potassium
carbonate plant (HPC) was studied. V2O5 solution was used as an inhibitor. It was
found that the critical inhibitor concentration depended on solution composition and
the steel surface condition. An inhibitor concentration of 14-15 g/l required to ensure
spontaneous passivation under all conditions. A minimum level of V+5 ions is required
for inhibition, so that monitoring the V+5 concentrations above 30% of total vanadium
crucial to successfully managing corrosion protection in plant.
CO2
.
.
. V2O5
( spontaneous passivation) / -
( V+5) .
. % ( V+5)
1. Introduction
The removal of CO2 is a crucial step in the manufacture of ammonia. Many
technologies efficiently remove CO2 from the process gas. With so many choices
there is no " one method fits all " solution. Consequently, process designers must
align technological benefits of various techniques with the final product or
downstream plant specifications. Ammonia synthesis catalyst (iron) requires pure
synthesis gas and the tolerance limit of CO+CO2 content in the gas is only 10 ppm. In
this research the available technologies for CO2 removal will be discussed in brief
with particular focus on the Benfield system that is widely used for Ammonia Plants.
1-1 Process description of Benfield system
Sirte Oil Company,1983 et al ; The CO2 content in the process gas downstream
of the low temperature converter (LT) is approximately 18%. Carbon dioxide is a
strong catalyst poison. It also reacts with the ammonia present in the synthesis recycle
gas to ammonium carbamate and ammonium carbonate.
These salt-like products form deposits in the machinery and equipment, and this
might cause serious damage.
Various processes are available for removing the CO2 from the synthesis gas , if
the quantity of CO2 to be absorbed is large, the only efficient method of removing the
CO2 is by a regenerative process using liquid absorbents. One normally differentiates
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Electrical Power
Generation
Sea Water
Sea Water
Desalination
Steam
Production
Natural Gas
Supply
Feed Gas
Compression &
Purification
Primary
Reformer
Produces
H2 & CO2
General Plant
Use: Water,
Steam & Elec.
H2
Conversion
Secondary
N
Reformer Adds 2 Makes Extra
CO
CO H & CO
Nitrogen
2
2
CO2
CO2
H2
H2
Air
Compression
Air
Ammonia
Storage &
Shipping
NH3
Ammonia
Synthesis
H2
N2
N2
CO2
Methanation
H2
Removes
Carbon Oxides N
2
Residual
Carbon Dioxide
Removal
NH3
Urea Synthesis
221
CO2
Regenerator
Wash Drum
Absorber
Feed Gas
Reboiler
222
excellent and in only isolated cases has localized corrosion occurred and this has
been due to direct impingement of solution against carbon steel at higher
velocities.
Freshly made solution from V2O5 is all V +5 valence and in a new plant gives
a very high film forming rate.
The V+5 form is the more active as far as corrosion resistance is concerned.
It is principally the V+5 that is responsible for the protective film that protects
the iron against bicarbonate and CO2 corrosion. To do a good vanadation job it is
important for there to be a reasonably high concentration of V+5.
The reduced form V+4 is probably not capable of building a protective film
by itself. A vanadium film once formed, however, can be maintained by V+4, even
if no V+5 is present. If there is abrasion due to packing movement, dirt in the
solution, etc., the film can be thinned and lost and will not be restored unless some
V+5 is present in the solution. After a time of operation equilibrium is established
between V+5 and V+4 generally with a ratio of about 1. This solution has an
adequate amount of V+5 to insure good operations. There are successful operating
plants where the V+5 /V+4 ratio is as low as 0.05 and no corrosion has resulted after
several years of operation.
1.4. Adjustment of V+5/V+4 Ratio.
Shaw, and Hughes, May 2001 et al; As pointed out in the previous section V+4
is capable of maintaining a corrosion resistant vanadium film but probably is unable
to form a new film in the absence of V+5 .
Thus, while the V+5/V+4 ratio require increasing the V+5 concentration to insure
enough formation of a new protective film. Most typically, a high concentration of
V+5 is needed when a system is revanadated after the towers have been opened and
exposed to air, and flushed with water, or have been allowed to dry. Maintaining a
high V + 5 content is also suitable if an unexplained rapid increase in iron content
occurs since the increase could indicate localized corrosion caused by loss of the
protective film due to abrasion caused by high solution velocity or particulates,
packing movement, etc. Whenever a system is revanadated or it is suspected that a
portion of the protective film has been lost, the V+5 content of the solution should be
increased to 0.7 to 0.8 wt.% as KVO3.
The V+5 content can be increased by adding more vanadium (usually as V2O5) or by
oxidation of V + 4 already in solution.
Addition of more vanadium is not objectionable. A total concentration of KVO3 at
least as high as 1.5 wt.% is acceptable. However, oxidizing the vanadium already in
solution with KNO2 (potassium nitrite) will be significantly more economical.
Potassium nitrite selectively oxidizes V+4 and will not oxidize DEA except if present
in excess for a long period. In addition undesirable reaction products do not remain
in solution; N2 and/or NO evolve as a gas. Oxidants other than potassium nitrite (air,
hydrogen peroxide, etc.) should not be used since they will oxidize DEA.
When KNO2 is to be added, it should be prepared as a water solution in the mixing
sump and then transferred to the circulating solution. The amount of KNO2 added
should be such that 70 to 80% of the vanadium is converted to V+5. Complete
conversion to V+5 should be avoided to prevent any possibility of excess KNO2 slowly
reacting with DEA. The amount of KNO2 required will usually fall between .01 and
0.1 wt. % of the solution inventory, depending on the solution's V + 4 content and
also on the amount of readily oxidized trace components that may be present. The
quantity of KNO2 to be used should be determined by laboratory test prior to addition
to the plant solution. A convenient procedure is to prepare several 50 to 100 ml
223
portions of plant solution to which different amounts of KNO2 are added. The samples
are heated to 90C in closed bottles for 3 hours to insure complete reaction.
The V+4 and V+5 contents of each are then determined using recommended analytical
procedure, by a spectrophotometer of companys laboratories.
1-5 Chemistry of CO2 absorption and regeneration in the benfield system.
Samuel Streizoff, 1981 andAmmonia Plant Manual Sirte Oil Company 1983,et
al; Operationally, Benfield system in (Ammonia plant) has given its fair share of
problems such as high CO2 slip, foaming problem and corrosion problem due to
DEA degradation products and difficulties in keeping the Vanadium in the
oxidized form. It has also been found that when operating at about 105% flow
sheet feed gas rate, the Benfield system becomes the plant bottleneck.
Both high CO2 slip and foaming problem are common problems.
The chemistry of CO2 absorption and regeneration involves reaction of the
promoter with CO2 at the gas liquid interface, regeneration of the promoter in the
bulk solution and its return to interface ( both steps known as [shuttle mechanism]
occur in the absorber ) and release of major quantity of CO2 by flashing and heating
the solution in the Regenerator. The chemical reactions for the above are shown
below.
V +5 + Fe +2 V +4 + Fe +3
As V+5 is yellow and V+4 blue in colour of Benfield solution would be yellowish
green or bluish green depending on which of the contents (V+5 or V+4) is major.
The solution turns to dark colour when DEA gets degraded.
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V +4
g/l
T-V
10-15
g/l
V +5 /
V +4
7.54
6.87
5.9
6.5
5.88
7.48
7.78
6.72
6.06
4.56
14.06
14.08
15.26
14.1
14.66
16.04
14.46
15.2
13.28
13.54
21.6
20.95
21.16
20.6
20.54
23.52
22.24
21.92
19.34
18.1
0.536
0.488
0.386
0.461
0.401
0.466
0.538
0.442
0.456
0.337
The ratio varied from 0.44 (minimum) to 0.57 (maximum). It was decided to
oxidize in steps the tetravalent vanadium so as to get more pentavalent that would
improve the ratio.
First step of oxidize the tetravalent vanadium
Oxidation of V+4 was carried out by injection of air to a batch of solution
and transferring back to system. This step was done to oxidize V+4 to V+5 without
adding KNO2 for a week, but there was no significant effect on the inventory of
the solution when the batch was transferred to the system (see analysis sheet no.2).
Daily Benfield Analysis, (No-2)
V +5
g/l
V +4
g/l
T-V
10-15
g/l
V +5 /
V +4
7.54
6.87
5.9
7.16
6.22
6.35
6.72
14.06
14.08
15.26
14.18
15.16
14.85
15.2
21.6
20.95
21.16
21.38
21.38
21.20
21.92
0.536
0.488
0.386
0.505
0.41
0.427
0.442
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Even then no increase in ratio was observed. At the end of this step the Benfield
solution composition was as follows:
223.10 g/l of K2CO3 , 169.14 g/l of KHCO3 , 18.20 g/l of DEA , 6.56 g/l of
+5
V , 14.44 g/l of V+4, Total -V was 21.00 g/l as KVO3
The above analysis data were carried out in the site laboratories of SOC (Sirte Oil
Company) using recommended analytical procedures, by a spectrophotometer and
other equipments.
The total vanadium content was 1.64 wt% KVO3 against normal value of 1.06
to 1.14%. As higher vanadium content increases the rate of degradation of DEA,
the plant was advised to bring down the content of vanadium to normal value.
Third step of oxidize the tetravalent vanadium
The vanadium content of Benfield solution can be reduced only by draining
the solution. Since the carbonate and DEA strength also will fall off on draining,
addition of potassium carbonate and DEA is necessary immediately after draining.
Therefore, the plant could not start draining the solution immediately and after
mobilizing these chemicals which took 26 days, the draining commenced. The
Benfield solution composition at the commencement of draining was as follows:
220.59 g/l of K2CO3 , 174.17 g/l of KHCO3 , 20.37 g/l of DEA , 5.24 g/l of
V+5, 15.42 g/l of V+4, Total -V was 20.66 g/l KVO3 as V2O5 and 51.8 ppm Fe.
The solution was drained in three installments as the solution strength should
not be depleted fast. Otherwise CO2 slip will shoot up. After draining the first
installment, the strength of the solution was made up with 12 MT (metric ton) of
K2CO3 and 1000 Liters of DEA. On completion of this action the total vanadium
came down to 18 g/l. Then, 70 Kg of KNO2 was added for one week due to low
ratio of V +5 / V +4 (0.289) which improved later to about 0.466 (see to analysis
sheet no.3 ).
Daily Benfield Analysis, (No-3)
V +5
g/l
V +4
g/l
T-V
10-15
g/l
V +5 /
V +4
4.56
4.57
4.78
4.60
4.58
3.97
3.99
4.51
13.54
13.67
13.81
13.67
14.0
14.9
13.56
14.01
18.10
18.24
18.59
18.27
18.58
18.87
17.55
18.52
0.336
0.334
0.346
0.336
0.327
0.266
0.294
0.322
After two weeks the second draining was done with make up of 7.75 MT of
K2CO3 and 440 kg of DEA. Then, the total vanadium dropped to about 16 g/l and the
ratio remained at about 0.90. The final draining was carried out after a few days with
a make up of 3.0 MT K2CO3 and 450 kg of DEA. The total vanadium dropped to (1412 g/l ) and the ratio improved to 1.0 or above 1 .(see to analysis sheet no.4).
226
The foaming problem which was persisting in the plant before draining the
solution, disappeared completely and V +5 /V +4 ratio gradually boosted up to 2.83 over
a week. It facilitated to raise up the plant load to 105% without any problems.
No corrosion was observed in Benfield system since the problem of low V +5 / V +4
ratio was tackled in proper time.
Daily Benfield Analysis, (No-4)
V +5
g/l
V +4
g/l
4.74
7.74
7.81
7.83
7.84
7.89
7.87
8.15
8.24
8.31
12.06
8.69
8.69
8.62
8.59
8.46
8.34
7.63
6.98
6.71
T-V
10-15
g/l
16.8
16.43
16.5
16.45
16.46
16.35
16.21
15.78
15.22
15.02
V +5 /
V +4
0.393
0.890
0.898
0.908
0.912
0.932
0.943
1.068
1.180
1.238
227
4-References.
Ammonia Plant Manual of SOC (Sirte Oil Company)-1983.
Ammonia Plant Safety, AICHE Technical Manual, published by the American
Institute of Chemical Engineers, 1997, 3Park Avenue, New York, NY 10016-5991.
GPSA(Gas Processors Suppliers Association ), ENGINEERING DATA BOOK, 2004
Hydrocarbon Processing/ May 2001, Optimize CO2 Removal, T.P.Shaw, and
P.W.Hughes, Costin Oil, Gas & Process Ltd., Manchester, England.
Technology and Manufacture of Ammonia, Samuel Streizoff, 1901, Copyright @
1981 by John Wily & Sons, Wily-Interscience Publication.
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