Customized aMDEA Process for Acid Gas Removal
The operational experience with aMDEA gas scrubbing units shows that they contribute substantially to both the reliability and the economy of the ammonia plant.
R. Hugo, H. Meissner, and R. Welker BASF AG, Ludwigshafen, Germany
Introduction
T
he CO2 removal system represents a major energy consumer in an ammonia plant. An energy-efficient gas scrubbing process is thus sine qua non for economic production. On the other hand, the economics of the ammonia plant also require high on-stream times. Each extra plant shutdown due to a failure of the CO2 removal unit causes a consider- able loss of production and adversely affects plant safety. For these reasons, the selection of BASF's aMDEA process can make a substantial contribution to both the reliability and the economy of the whole ammonia plant.Assuming that U.S. ammonia producers are already well acquainted with aMDEA, this article will briefly recapitulate the applications and process features, which will subsequently be illustrated by a detailed consideration of two examples of retrofitting existing CO2 removal units (with all the inherent system con- straints this task implies.
Process History
The development of the aMDEA process was started in the late 1960s. The first aMDEA unit went on- stream in 1971 with the commissioning of the No. HI ammonia plant in Ludwigshafen, Germany. In the fol- lowing years, eight more BASF plants were fitted with aMDEA gas scrubbing units.
The operational experience with these aMDEA units provided the incentive to license the process from 1982 onward. To date, the aMDEA process has been operated successfully in a total of 66 reference plants worldwide, with some 20 additional units currently being under design or construction.
Applications
The aMDEA process is suitable for a wide range of applications (Figure 1). Besides removing CO2 from ammonia synthesis gas, aMDEA units can be used to purify CO/H2 synthesis gas, to sweeten natural gas,
and for speciality applications such as purification of blast furnace gases.
40% of all aMDEA reference units are to be found in ammonia plants; more than half of these were in turn originally operated with alternative solvents and converted to aMDEA to solve various operating prob- lems encountered with the former solvents.
Process Features
The aMDEA solvent systems are aqueous solutions of the effectively nonvolatile methyldiethanolamine plus a small amount of an activator to enhance the CO2 absorption rate (Figure 2).
Generic MDEA reacts with water and CO2 to yield the corresponding protonated species and bicarbonate (Figure 3). The overall rate of conversion is very low.
The absorption can be accelerated by the fast reaction between CO2 and the activator, a secondary amine, which together form a carbamate. The carbamate in turn reacts with the bulk solvent (aqueous MDEA) transferring its CO2 and thereby being regenerated for further reaction. The activator therefore behaves in a similar manner to a homogeneous liquid catalyst with no net consumption, but rather several reaction-regen- eration cycles along the length of the absorber.
The high loading capacity of MDEA results in low solvent circulation rates, while the activator keeps the absorber height to a minimum. The solvent regenera- tion can be carried out to a large extent simply by flashing the aMDEA solution.
An extra degree of flexibility is achieved by varying the activator concentration (Figure 4). This has the effect of tuning solvent behavior to either a more
"chemical" or a more "physical" character. The highly activated MDEA 06 has more of a chemical solvent nature, that is, good absorption efficiency but energy intensive regeneration, while the weakly activated MDEA 01 has only a moderate absorption efficiency but benefits from an energy efficient regeneration, similar to that for a physical solvent.
Typical application criteria for CO2 removal from ammonia synthesis gas are:
• aMDEA types 02 thru 04 in a standard two-stage unit, characterized by low energy consumption values, high gas purities, and good recovery rates (Figure 5);
• aMDEA types 04 thru 06 in a standard single-stage unit, characterized by reduced investment costs and
very low CO2 slippage in the treated gas (Figure 6).
The number of aMDEA solvent systems combined with a variety of appropriate process configurations ensure a design tailor-made for a given application, be it a revamp or a new plant.
Retrofit of Acid Gas Removal Units
For the "grassroots" design of a new plant, the sol- vent, the configuration, and the process parameters can be customized to meet all production and site requirements. For a revamp, however, the design needs considerable process flexibility to conform to a given application. Many additional constraints result from the equipment already installed and the integra- tion of the unit in the ammonia plant. The following examples illustrate such situations.
The first example involves typical conditions for the U.S. reference plants, all of which entailed conver- sions of ammonia synthesis gas scrubbing units to aMDEA (Figure 7). The second example describes the revamp of a hot potassium plant in Australia.
Conversion from an amine-based solvent
The U.S. references are all ammonia plants with a capacity in the range 650-1,500 mtpd, and all have a background similar to that of the unit portrayed in the following example. The gas scrubbing unit comprises a single-stage absorption and stripper regeneration (see Figure 6) and was originally operated with monoethanolamine (ME A).
This unit has been converted to another amine based system in the context of a capacity increase. The alter- native of achieving a higher plant capacity by an increase of the MEA concentration was ruled out by the operational experience regarding the corrosivity of the MEA solution, especially at higher concentration.
Understandably, the corrosivity of the solution was investigated extensively. Following the swap, solvent analyses showed no untoward heavy metal content in the solution for the first year. Thereafter, some weight loss was observed on the corrosion coupons and sub- sequently a steep increase in the concentration of
Figure 1. Application of BASF's aMDEA-process. Figure 2. Activated MDEA solvent system.
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Figure 7. Plant locations. Figure 8. Damaged tubes from the four exchangers.
heavy metals (Fe, Cr, Ni) in the solution took place.
Corrosion problems occurred especially in the solvent heat exchangers (2 parallel stacks, each of 2 individual exchangers) and in the regenerator (duplicate parallel tray columns) along with several additional shutdowns and the need to replace corroded equipment. The hopes placed in this second solvent thus proved to be unfounded, as the unit still suffered from leaks due to corrosion.
At this point, a straight solution swap to aMDEA was carried out without any plant modifications. In a rapid turnaround, the previous solvent was drained off, the unit cleaned, filled with aMDEA solution and restarted.
Equipment inspection after draining off the previ- ous solvent
No Corrosion was detected in the absorber column.
The stripper columns showed corrosion in the bot- tom sections and between the trays. The corrosion was of a local nature, being characterized by cavities of 2 in. (5 cm) diameter and up to 2/5 in. (1 cm) depth from a height of approximately 6.5 ft (2 m) up to the loca- tion of the packing support grid. The material around the cavities was friable and could easily be removed mechnically. The material within the manhole showed similar corrosion effects.
The stripper reboiler heads were damaged with the pitting corrosion characteristic of hot CO2/water vapor attack.
The four solvent heat exchangers are fitted with stainless (SS) tubing and carbon steel (CS) shells.
Although the expansion valve is located downstream the solvent heat exchangers, degassing of CO2 of the rich solution most probably occurs, which results in a two-phase flow through the exchangers, giving rise to severe corrosive attack: In the exchanger heads (pre- dominantly at the hot end) pitting corrosion was observed over the entire CS material (about 2 mm depth) as well as at the welds (4 mm depth). Even though the tubes were of SS, about 10% leaked.
Corrosion was confined to within the tubes, as the outer surface still appeared to be in good shape. The CS elements used to support the tube bundles were extensively corroded. Figure 8 depicts the damaged
tubes from the four exchangers.
The shell of the lean solution cooler (solvent on shellside) also exhibited wear-and-tear, probably as a result of erosive corrosion.
Special attention must be paid to the feed device of Table 1. Heavy Metal Content of the aMDEA
Solution Months of
Operation 7
11 13 16 20 23 26 29 33
Cr (wppm)
7 1 1 2 3 2 1 4 5
Ni (wppm)
1 1 1 1 2 1 4 6 8
Fe (wppm)
7 5 4 7 6 5 5 6 4
the regenerator when converting a chemical solvent to aMDEA: about 30-50% of the CO2 is released from the rich solution by depressurization. The degassing therefore starts just downstream the pressure relief valve and continues in the upper section of the regen- erator. In a tray column flashing takes place on the top tray. The flashing solution, however, shall not enter the downcomer, as degassing in the downcomer can result in flooding of the column. In case the regenera- tor is fitted with packing, the existing distributor must be adapted for the operation with a flashing feed. At any rate, the disengagement zone above the feed device must be sufficiently high hi order to avoid solu- tion entrainment.
Precleaning and repair work
The system was cleaned section by section: first opening manholes and bottom drains of columns and tanks, then removing orifice meters in the liquid lines after closing off the taps hi impulse pressure line con- nections beforehand and finally flushing the equip-
Figure 9. Corrosion rates in bottom of stripper. Figure 10. Revamp of hot-pot unit in Australia.
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11. aMDEA compared to hot potassium carbonate systems.
ment with water using firehoses and/or water jets. The heat exchanger tube bundles were removed and cleaned with a high pressure water jet. Particularly the tubing in the solvent heat exchangers showed a firm deposit layer inside the tubes, which could neverthe- less be removed with a caustic solution jet. The outer surface of these tubes was covered with a grayish- brown layer (most probably FeCO3) after cleaning, which could also be removed easily.
The corroded equipment surfaces were washed off with a water jet. Corrosion cavities were then hollow- grinded, polished and repaired with a corrosion protec- tion layer (0.6-1 mm thickness) applied by a thermal metal spraying procedure. The bottom section of the stripper columns was covered with metal dust from the spraying procedure, and the trays and the column base had to be cleaned with a vacuum cleaner.
Cleaning procedure prior to startup
The two stripper columns were flooded with con- densate and then drained. This procedure was repeated once more with samples being taken to check for the absence of residual suspended particles in filtration tests.
Afterwards, the whole unit was filled with conden- sate, heated up to 176-194°F (80-90°C) and circulated for 4 h, with about 10% of the flow rate passing through the mechanical filter. The condensate analysis after 4 h circulation showed no foam activity or sus- pended particles, and the sample had a yellowish- brown coloration. The condensate circulation proce- dure was repeated, with the sample showing no decol- oration as a result.
The cleaning efforts can be reduced or enhanced depending on the very situation and the history of each plant. In this case, for instance, the system was not flushed by circulating caustic solution or dilute MDEA solution as usually recommended, especially for the removal of grease or other residues such as cor- rosion inhibitors, rust, and so on. The caustic flushing should, however, be employed when considerable con- struction work has been carried out, if new equipment has been installed or if the entire plant is new.
Startup
The system was checked for leakages and put under nitrogen pressure. The aMDEA premix was introduced from tank containers into the system and diluted with water down to 45wt.% amine concentration. The sol- vent loop was closed, defoamer was added, and the circulating solvent was heated up to the design condi- tions. The gas throughput was set to 75% of the design capacity. Within 3 days, a capacity of 108% could be achieved and the conditions (energy, solvent, flow rate) had been optimized to meet the design CO2 slip of 20 vppm.
Operating experience after the swap
The corrosion attack associated with the previous solvent is clearly indicated by a material loss from the test coupons in the mm per year range (Figure 9).
After the revamp to aMDEA, the corrosion was com- pletely eliminated, without needing to add any corro- sion inhibitor. In contrast to the previous solution, the heavy metal content of the aMDEA solution (as tabu- lated in Table 1) was kept virtually constant in the low ppm range.
Other amine-based systems (such as MEA, DEA, DIPA) have also been successfully converted to aMDEA with considerable energy savings achieved and a complete eradication of earlier corrosion prob- lems. The demands on the flexibility of the solvent, however, are considerably higher if the solvent being replaced is of a completely different nature to an amine solution, as in the following example.
Conversion of a hot potassium carbonate unit The second example deals with the very first revamp of a hot-potassium carbonate unit to aMDEA. The 720 mtpd ammonia plant in Australia comprised a two- stage hot-pot gas scrubbing unit. Toxic arsenic salts were used both for activation and corrosion inhibition.
In recent years, sections of the pipework and packed beds had to be replaced owing to the corrosivity of the hot-pot solution.
The main problems, however, were escalating costs for disposal of the arsenic-containing purge stream and the presence of arsenic in the plant wastewater.
The process configuration and the operating condi-
lions of this Australian hot-pot unit (Figure 10) are significantly different from a standard two-stage aMDEA unit (see Figure 5):
• No lean/semilean solvent heat exchanger.
• Ratio semilean to lean solvent flow rate 4.5 vs. 6-7 for aMDEA.
• Lean solvent temperature of 169°F (76°C) vs.
122°F (50°C) for aMDEA.
• Semilean solvent temperature of 234°F (112°C) vs.
167-185°F (75-85°C) for aMDEA.
Table 2. Three Consecutive Flushing Steps Using NaOH Solution
NaOH (wt.%) 4.0 3.9 1.7
Circulation
00
8 8 6
Max. Arsenic (wppm)
1,900 160
60
• Low pressure flash fitted with packing of about 3 times the height as for aMDEA.
• Density of hot-pot solution approximately 25%
higher compared to aMDEA.
Generally, the main challenge for the solvent con- version of hot-pot units arises from the considerably lower temperature level in the absorber and the lower aMDEA density. Both factors typically lead to bottle- necks due to a lack of cooling capacity (most units do not even have a semilean solution cooler!) and inade- quate capacity in the solvent pumps.
Process modifications
For the above reasons, a straight solution swap (as described in the section on conversion from an amine- based solvent) is not usually feasible. The following modifications had to be carried out for this Australian revamp assignment.
• At full capacity, the suction pressure of the lean solution pump (when operated with aMDEA) would have fallen below the required limit. To provide suffi- cient head for the less dense aMDEA, a lean/rich sol-
vent heat exchanger was installed to cool the lean solution from the stripper bottom and the lean solution pump was operated at higher speeds. A peculiarity of the solvent heat exchanger (in contrast to the lean/semilean solvent heat exchanger of the standard design (see Figure 5)) is the shellside flow of CO2- loaded rich solution and the tubeside flow of the lean solution. This arrangement was chosen in order to take into account the high ratio of rich to lean solution in the operation of the existing unit. CO2 is thus released by heating up the rich solution in the exchanger. A lean solution air cooler instead of a solvent heat exchanger was not a practical alternative due to the higher space requirements, and an additional water cooler was ruled out by the increased water consumption.
• The duty of the existing lean solution cooler was increased by providing cooling water at a lower tem- perature. The lean solution temperature could be reduced from 169°F (76°C) to 158°F (70°C) in this way, thus giving rise to a higher solution loading capacity and a reduced solvent flow rate. By this means, the aMDEA design was rendered compatible with the capacity limits of the existing pumps.
• The hot-pot unit was operated at a process gas tem- perature of 433°F (223°C). As such conditions are beyond the operational experience of aMDEA refer- ence units, it was decided to install a process gas cool- er upstream of the regenerator gas reboiler to reduce the absorber inlet gas temperature down to 329°F (165
°C). A gas inlet temperature of 329°F is still adequate to operate the regenerator properly since the energy consumption is much less for aMDEA. The new heat exchanger is part of a reformer feedstock saturator scheme, thus providing energy recovery from the feed gas.
9 The lean absorber section was fitted with polypropylene packing, which was severely deformed.
The two PP packed beds have now been replaced by 1 in. S.S. TP304 rings.
• Parts of the piping showed corrosion and were replaced by SS TP304 lines.
« About 80% of the released CO2 was previously vented to the atmosphere at 194°F (90°C).
Only 20% of the CO2 was cooled down to 113°F (45
°C) condensed and processed up to foodgrade quality.
An additional CO2 cooler, reflux drum, and reflux
Table 3. Comparison of Operating Data for the Hot-Pot Solution
Hot-Pot Solution aMDEA Solution
Feed gas
CO2 slip Lean solution Semilean solution Absorber bottom Process gas reboiler Steam reboiler
Specific energy consumption
185°F (85°C)
429 psia (29.6 bara) 18 vol.% CO2
<500 vppm 169°F (76°C) 234°F(112°C) 226°F (108°C) in operation in operation
49.9 mbtu/lb mol CO2
185°F 429 psia
18 vol.% CO2
<500 vppm 158°F(70°C) 180°F (82°C) 203°F (95°C) in operation not in use
32.7 mbtu/lb mol CO2
pump were installed parallel to the existing reflux cir- cuit in order to provide a moderate temperature of the CO2 off-gas stream, thus limiting the vapor phase sol- vent losses.
Cleaning of the Unit
The unit had been operated with a hot-pot solution since 1969 using arsenic salts as corrosion inhibitor and activator. It was therefore to be expected that con- siderable amounts of precipitated corrosion products (deposit of iron/arsenic complex) were deposited on equipment surfaces. There were some concerns regarding the fact that aMDEA is known to dissolve such deposits quite well, which might have increased foaming susceptibility and carryover of the solution from the regenerator into the condensate reboiler.
The cleaning procedure was carried out as follows:
« Three consecutive flushing steps using NaOH solution at 194°F (90°C), each of which was broken off after the arsenic content had attained its maximum are shown in Table 2.
• Two consecutive water flushing steps were at 194°F (90°C); the water analysis after the second flushing step gave 60 wppm iron, 7 wppm arsenic, 80 wppm NaOH and 100 wppm solids and the sample exhibited a dark brownish color. The solids were iden-
tified as very fine rust particles (probably iron/arsenic complex) in the size range 5-50 urn; the foam test with aMDEA indicated a significant foaming tendency.
• A further water flushing step was not carried out in order to limit the amount of arsenic contaminated flushing water. It was decided to allow for the higher foaming tendency by running the mechanical filter (5- 10 u,m cartridges) at its maximum throughput from the very beginning of the solvent circulation and employ- ing an appropriate defoamer dosage rate.
Startup
The unit was filled with aMDEA premix and the water content and column levels adjusted by adding condensate.
The solvent circulation was commenced at about 50% of the design flow rate. After 12 h of circulation, the solution had a reddish-brown color and a foam test indicated formation of very stable foam (collapse time more than 5 min). After filtering the aMDEA sample, the color turned light yellow and the foam activity was reduced to the normal values.
An extra dose of defoamer was introduced into the solution loop and the circulation rate increased to 95%
of the design figure: the column levels became unsta- ble and hence a further shot of defoamer was added.
The column levels remained constant and the next foam test showed no significant foaming tendency.
The solution circuit was heated up to the design val- ues with the regenerator steam. The CO2 cooler and reflux pumps were put into operation. The semilean and lean solvent temperatures to the absorber were adjusted at a later stage when the solvent coolers were started up.
Process gas was fed through the plant at 50% capac- ity. Within 12 h, the plant throughput was increased to 80% of the design figure. Process conditions were then optimized at full capacity.
Although the process conditions of a hot-pot system differ significantly from aMDEA units, the aMDEA design for this unit showed a good compatibility with the existing equipment. A comparison of the operating data for the hot-pot solution and the aMDEA design for 100% capacity is tabulated in Table 3.
A summary of the benefits from the 1994 solvent conversion is given in Figure 11:
• The aMDEA solvent does not require the addition of any corrosion inhibitor. Hence, the costly disposal of toxic arsenic-material (inherent for the hot-pot sys- tem) is completely avoided. Even the time-consuming passivation of the equipment prior to a startup of the hot-pot system can be omitted, thus keeping the turn- around periods to a minimum.
• Although the process conditions are not cus- tomized for the aMDEA solvent system, the energy saving is still 35% compared with the hot-pot opera- tion.
• The operator workload is lightened as the intense supervision required for operation of the hot-pot unit is no longer necessary. Precipitation does not occur with aMDEA; thus, heat tracing is not required. The revamped unit is operated with virtually no liquid purge stream, as there is no need to separate decompo- sition products or to purge the solution thereof. The plant monitoring has been reduced to a minimum, with only a few simple analyses remaining to be carried out.
Operational Experience
The flexibility of the aMDEA process also offers attractive operational features.
BASF operates two ammonia plants in Ludwigshafen, Germany: Both plants No. HI and No.
IV, a 1,350 mtpd unit, which have come on-stream in 1971 and 1982, respectively, are equipped with low- energy two-stage aMDEA systems (see Figure 5).
Following a capacity increase some years ago, the gas scrubbing unit of the No. IV plant was being oper- ated at the limits of the certain existing equipment items such as solution pumps and columns. A second plant expansion of 6% was recently carried out with- out any equipment changes or capital costs being involved.
The activator content in the solution was increased slightly, thus leading to a higher absorber efficiency by shifting to more chemical solvent characteristics. As the activator could be added during operation, no shut- down was necessary.
Conclusion
This article deals with a specific range of aMDEA applications, namely the conversion of solvents in existing ammonia plant gas scrubbing units.
Units which are designed to be operated with amine based solvents can most often be converted to aMDEA in a straight solution swap: the old solvent is drained off, the system is cleaned, filled with aMDEA premix, made up with condensate, started up (faute de mieux with the old conditions), and optimized.
The conversion of units designed for solvents with a totally different nature to amine solutions normally requires some equipment changes, however. Hot potassium revamps, for instance, are characterized by a lack of adequate cooling and pump capacities.
Furthermore, the heat integration of the gas scrub- bing unit within the whole ammonia complex can exert constraints determining the operating conditions at certain locations in the unit, such as boiler feedwa- ter heaters (T, dT, heat duties, and so on). This can turn out to be a bottleneck, for instance, due to differ- ent physical and thermal properties of the solvents.
The flexibility of the aMDEA process takes account of such constraints by allowing one to change the funda- mental nature of the solvent. Even if such a revamped unit cannot be operated at the optimum conditions when compared to new, grassroots aMDEA units,
advantages still accrue, that is, no corrosion, energy three illuminating examples of the economic efficiency efficiency, and no purge stream disposal. and reliability of the aMDEA process, which is further The straight solution swap in the U.S., the revamp of characterized by the cardinal virtues of being environ- the hot-pot unit in Australia, and the capacity increase mentally friendly, easy-to-operate, and very forgiving, of BASF's ammonia plant in Ludwigshafen provide