BENFIELD OPERATING PROBLEMS AND ITS SOLVING
The most commonly reported problem with benfield solution are :
The bicarbonate HCO3- ion is responsible for corrosion damage and also it forms an essential part of the process chemistry, it is not possible to over come the corrosion damage by suppressing its formulation. It was found that by using metavanadate salts (0.01 - 2.0 % wt) has a highly effect in inhibiting corrosion on steel surfaces in K2CO3 system .The metavanadate salts also have additional advantage that they tend to increase the rate of CO2 absorption in the absorber column. In commercial unit today, it is preferred to use alkali metal metavanadates , particularly potassium metavanadate (KVO3) , if desired V2O5 can be added directly to K2CO3 solution.
- Total vanadium, not less than 0.7 wt% as V2O5
- V5+ not less than 0.2 wt% as V2O5
- V5+ during passivation, not less than 0.4 wt% as V2O5
1-1. Passivation Mechanism:
Vanadium is used as a anodic corrosion inhibitor in benfield / DEA solution , to be effective it must be maintained in the V4 / V5 equilibrium state as this corresponds to the free corrosion potential of a steel electrode in the solution being enough to form a passive, tight adherent magnetite film ,which protects the steel from further attack . Figures (1,2) illustrate the benfiled corrosion protection theory. If the vanadium present in the solution is reduced to V3 / V4 equilibrium state (May due to presence of O2 scavengers in BFW, iron Fe +2 in reflux water, CH3OH,) rapid corrosion occurs and V3 will precipitate. The min. recommended level of total vanadium for ammonia plant (30% K2CO3) about 0.35 to 0.45 % V2O5 and about 30 - 50 % of total vanadium present in the V5. Reduction of V5can be countered by addition of oxidizing agents such as KNO2 potassium nitrite to the solution (0.01 to 0.1 % wt).
NO2 - NO + 1/4 O2 + e -
V 4+ V 5+ + e -
Column flooding and dumping due to foaming, it appears that this problem can be solved by use additives such as a polyglycals, silicon or high molecular weight alcohol foam inhibitors, silicon additive greatly increases the column efficiency of both bubble cap and perforated - tray absorbers by improving the bubbling action.
Notice: The chloride ion conc. must be not exceeding 100 PPM other wise chloride stress corrosion -----ing may occur.
Figure ( 1) illustrate the benfiled process corrosion .
Figure ( 2) illustrate the benfiled process corrosion protection theory .
• Long shut-down or new equipment added
- Water flushing, Chemical cleaning, and Full passivation
• Short shut-down, e.g. towers open for inspection
- Full passivation may be needed
• Short shut-down, e.g. towers cooled and inert gas blanketed
- Short re-passivation needed
• Short shut-down, e.g. towers warm and inert gas blanket
- Short re-passivation needed if V5+ concentration is low
• Full passivation
- Recommend 3 to 5 days of hot circulation of solution.
- Monitor vanadium content; V5+ not less than 0.4 wt% as V2O5
• Short re-passivation
- Recommend 8 to 36 hours of hot circulation of solution
- Monitor vanadium content (not less than 8 hours), V5+ not less than 0.4 wt% as V2O5
• Reboiler soaking – 36 hour soak used ONLY for new unit, or if reboiler has a new tube bundle installed.
• System passivation should always be done if new packing or other major new steel was added during shutdown.
2. Degradation of DEA(Thermal ,Oxidative & CO2 Carbomate Degradation )
Amines can be degradable under several conditions such as overheating, regrous oxidation (over dosing of KNO2 or by contact with air at a high temperature), gas components"heavy HCs", lubrications oil and greases, alcohol's, excess and anti foam component added, small fraction of carbamate react to give a non regenerable compound. This degradation product may be lead to several operating problems as, solution darkness, foaming tendency, corrosion, solid formation, solution crystallization, reduction of the valance state of corrosion inhibitor ,interfere with analyses and also reduce the benfield absorption / regeneration capacity. Therefore several improvement was introduced into the traditional hot -Pot. Solution to over come this problem such as by using activated carbon or by replacement the DEA activator by a new activator as described below.
Notice : polyethylene amines are strongly chelating compounds and have the capability to dissolve heavy metals like iron ,nickel and chromium ,this means that no dense protection layer on the surface of vessels and pipes can be formed
3. CARBONATE CARRY - OVER
Carbonate solution carry over causes some operating problem for example pot. Carbonate getting into CO2 comp. Suction and deposits at the second stage inlet, K2CO3 causes crevice corrosion in the condensate system, solution loss and environment impact.
3.1 Causes of Carry Over
1. Liquid foam
2. Inadequate flashing zone & liquid maldistribution.
3. Wash tray (type)and condition
4. Damaged , broken or displaced internals plugged
5. Over capacity
Benfield Foam Test
ZERO to 2 cm Pure water or clean carbonate
2 to 4 cm Non foaming: normal operating Solution
5 to 6 cm Non foaming: Slightly foaming but ok
7 to 10 cm Foaming: Start antifoam addition
10 to 15 cm Heavy foaming: Need rapid antifoam addition. May have to reduce gas and liquid load.
OVER 15 cm
Very heavy foaming: Need to reduce gas and liquid rates, may need “shot-pot” antifoam addition
Excessive amounts added to combat foaming can CAUSE foaming
“Solution is considered foaming if either occurs:
Foam height of 7 cm or more
Foam collapse time of 10 seconds or more”
Commercial the type of washing tray used in this are as bubble cap - tray. Inadequate wash tray or plugged with solid material may causes re-enteriment and reduce the top tower performance.
Vanadium precipitation on the washing tray (analysis as V2O5 /FeO3) due to entrained carbonate solution , acidic wash water on tray , which can be eliminated by controlling the washing tray pH must be over than 7. Therefore K2CO3 added at the washing water inlet (20ppm conc. To increased the pH to 8).