CO2 Removal
INTRODUCTION
Existing solvents
Existing CO2 removal plants operate with different solvents which can be grouped together in two families:
Both families feature important drawbacks such as toxicity and corrosive property. aMDEA® was used in 1971 for the first time by BASF for gas scrubbing application. This solvent is a major improvement compared to the two above families as it is non-corrosive, non toxic and biodegradable. Moreover it lowers substantially the energy requirement in the plant.
Goals of revampings
This document describes all the revamping possibilities provided by CEAMAG for these CO2 removal plants. The revamping projects can be distinguished in 4 groups:
It is worth to note that energy savings on CO2 removal plant may allow for operation at low steam carbon ratio (S/C=3.0) in primary reforming section. This is because energy savings compensate the deficit of heat available in process gas and transferred to the solvent in the stripper gas reboiler. Deficit of heat is due to the reduction of steam condensation because a smaller quantity of steam is contained in the reformed gas. Operation at S/C=3.0 brings further energy savings in the complete ammonia plant.
EXISTING SITUATIONS
Some problems are faced with CO2 removal plants, regardless of solvent type:
Water losses in CO2 gas stream:
The gas stream leaving the top of the stripper contains CO2 gas saturated with water and some other gases. This stream flows through a condenser and a separator pot aiming to recover as much water as possible from the CO2 gas stream before it leaves the unit. This water is recycled to the stripper in order to maintain the solvent concentration within acceptable range. Despite this condenser, the water recovery is often low because the gas stream is not cooled enough (about 70°C). Make-up demineralised water must therefore be injected in the stripper to offset the water losses. The consequences are:
Highly efficient water recovery may be achieved by cooling CO2 gas stream under 50°C. In this case the full quantity of water is recovered under 50°C and recycled to the stripper. As operating temperature of solvent in the stripper ranges between 80°C-120°C, substantial quantity of energy is required to warm up again the recycled water.
Corrosion on strippers
In addition to usual corrosion due to the solvent properties, important corrosion is very often observed at the level of rich solvent inlet on top of stripper columns. Corrosion is mainly due to the pressure let down of feed solvent which in turns results in desorption of a large quantity of CO2 gas. High velocity of the liquid-gas mixture increases substantially the corrosion rate in this area. Corresponding cylindrical part of stripper must therefore be replaced few times during the total lifetime of the column.
Operation features of MEA based CO2 removal plants
MEA (Mono Ethanol Amine) solvent is widely used in the ammonia production process in countries where cost of natural gas was low (USA and CIS countries). In reason of natural gas price increase, energy savings must be carried out on these types of units to maintain the competitiveness of the complete ammonia plant.
Decomposition of MEA
In operation MEA decomposes into various chemical substances among which the following can be stated:
MEA usual colour is yellow and turns brown when it contains iron and copper. Some of the degradation products form resins and polymers that build up on heat exchanger tubes and plug the trays. Purging of solvent is therefore required during operation to avoid build up of decomposed viscous MEA. Purged MEA can be reclaimed in a dedicated unit (filtration, neutralization by diluted caustic soda and re-concentration) or disposed.
MEA losses
Besides of losses in reclaimer, MEA is lost in the absorber and the regenerator because of its high partial pressure (1 mbar in absorber and 2-4 mbar in regenerator). This vaporized MEA is usually recovered by condensate wash on top of equipment but a part of the solvent still remains in vapour phase. Losses of MEA in a 450 000 TPY NH3 plant can be estimated around 10 kg/h Solvent losses must be replaced by fresh solvent and it results in consistent financial costs.
Corrosion
Chemicals generated by decomposition of MEA such as formic acid are very corrosive and result in severe damage of process equipment. As said before, corrosion is also observed on top of stripper and is due to the flash of inlet solvent stream. Top part of stripper has therefore to be replaced from time to time.
MEA concentration
Design MEA concentration normally ranges between 18% and 25%. However it is reduced to 8 – 15% in many plants for the following reasons:
Of course this low concentration has a negative impact on sweet gas composition and energy consumption of the CO2 removal plant.
Miscellaneous problems
Operation features of hot potassium carbonate plants
Hot potassium carbonate solvents (K2CO3) are an alternative to amine based solvents. They are used with appropriate activators (DEA or glycine) and corrosion inhibitors (V2O5 and As2O3) in Benfield, Carsol and Giamarco Vetrocoke processes:
Corrosion
K2CO3 reacts with CO2 to generate the very corrosive KHCO4. Corrosion inhibitors such as vanadium oxide V2O5, arsenic As2O3 or potassium dichromate K2Cr2O7 must therefore be added in large quantity. Quantity of arsenic is, for instance, comprised between 10% to 15% of the total volume of solvent.
Toxicity
The corrosion inhibitors are indeed very toxic and are listed as poisons in all industrial countries. Disposal after use is an important concern.
Activator make-up
All the corrosion inhibitors oxidize DEA or glycine activators. These activators must therefore be made up continuously.
Waste of energy
Temperature of lean and semi-lean solvent fed to CO2 absorber must be low to allow for an effective CO2 absorption and a limited loss of solvent due to vapor pressure. Temperature of solvent must be high in CO2 stripper to allow for a good desorption of CO2. Lean and semi-lean solvents are therefore cooled down by water in heat exchangers before they are fed to the absorber. Consequences are loss of energy and consumption of cooling water.
CEAMAG CAPABILITIES FOR REVAMPING
Some technical solutions for reduction of energy consumption are provided by CEAMAG regardless of type of solvent:
CO2 gas stream split condensation (See fig.1)
 Figure 1: TYPICAL EXAMPLE OF SPLIT CONDENSATION ON CO2 GAS STREAM CEAMAG proposes the split condensation to minimize the water losses in CO2 gas stream without increase of the energy consumption necessary to warm up the recycled water (see Chapter 2 - Water losses in CO2 gas stream): CO2 gas stream flows successively through 2 condensing units (condensing unit = 1 condenser and its associated separation pot) to cool the gas below 50°C and recover 90% of water carried by vapour pressure. The separation pot of the first condensing unit allows recovery of the main part of condensed water at 75°C. Same quantity of water as for the existing case can be recycled to the stripper without additional energy requirement to warm it up. Supplementary water recovered at 50°C in second separation pot is in little quantity. It can be recycled to the stripper to lower the demineralised water make-up. Energy required to warm it up to solvent temperature is minor because water flow is little. For this improvement, CEAMAG provides for an optimisation of the temperature of first condensation in regard to the overall energy balance of the system It is worth to note that in case of use of MDEA solvent, this solution even allows for cancellation of demineralised water make up. Water from the second pot is not recycled but disposed without any need for treatment.
Turbine on rich solvent
In addition to split condensation described above, CEAMAG proposes the installation of a turbine on rich solvent stream leaving the absorber. The pressure let down is normally made by a control valve. It is proposed to recover the energy of let down of solvent (about 35 bar to 4 bar) instead of wasting it in the control valve. Turbine can be connected to an alternator in order to produce some electrical power which can be easily added to the existing plant power network. Alternative could consist in driving a pump directly with this turbine. But in the case of a revamping, it is very unlikely to find an absorbed power matching with driving power from turbine.
Flashing feed galleries (See fig.2)
 Figure 2 CEAMAG recommends the installation of so-called flashing feed galleries on top of strippers at the level of rich solvent inlet. These galleries aim to eliminate or reduced the local corrosion phenomenon described in Chapter 2. The main purpose of these galleries is to act as an internal deflector protecting the walls of the pressure vessel. It is supplied in parts which can be inserted through an existing manhole. It can be easily removed and replaced if corroded. Feed device shall be suited for handling the amount of gas released. The flashing feed galleries must be designed in accordance to specific equipment geometry.
Revamping of MEA based plants with re-use of solvent
Revamping possibilities include:
Revamping of hot potassium based plants with re-use of solvent
Revamping possibilities includes:
Thermo-compression (See fig.3)
 Figure 3 : TYPICAL FLOW DIAGRAM FOR THERMO COMPRESSION TECHNOLOGY Thermo-compression aims to recover as much energy as possible from lean and semi-lean solvents before they are cooled down by water. It results in decrease of energy consumption in process and steam reboilers. Side effects are:
Thermo-compression of water vapor applies on both lean solvent and semi-lean solvent streams. It must be implemented for both strippers. The description principle is presented for the lean solvent and for one stripper: Lean solvent exiting the bottom of the stripper is normally sucked by transfer pump and discharged to the top of absorber via cooling water heat exchanger. With thermo-compression technology, lean solvent is first directed to an expansion drum operating at lower pressure. A certain quantity of water contained in the solvent is vaporized as flash steam (called vapour in the rest of this document). A quantity of CO2 is also desorbed from solvent and released to the flash steam. Carbon dioxide desorption is not instantaneous but controlled by kinetic of reaction
2 x KHCO3 -> K2CO3 + H2O + CO2 ( Reaction 1 )
The expansion drum is designed as desorption column with jet type trays in order to get as close as possible to the equilibrium of the equation 1 and to reduce CO2 fixed in the solvent. The temperature of the lean solvent is reduced during this operation. Heat equivalent to solvent temperature difference is transferred to desorption and vaporization heat of the vapour. Vapour is recompressed by means of a steam ejector and injected back to the stripper. Desorption and vaporization heat are therefore returned back to the process together with the energy of ejector driving steam. Corresponding quantity of heat is therefore not required anymore in reboilers. Exact location of vapour injection into the stripper depends on actual recompression pressure. This pressure must match with gas pressure in stripper at corresponding level. It is worth to note that lean solvent regeneration mainly depends on quantity of heat brought to the stripper regardless of the location at which the heat is injected. It is therefore advised to inject the vapour as high as possible in the stripper because pressure is minimal at top (only 3 or 4 trays must be left above this injection to leave enough time to the vapour to give off its heat to the solvent). Driving steam is a low pressure steam produced from process condensate. Low potential heat available in the process (for instance in flue gas of primary reformer convection section) is utilized to generate the steam. Thermo-compression technology can bring further benefits when revamping includes an increase of capacity:
Nota: multi-flash system is also available with a principle similar to CEAMAG thermo-compression technology. It consists in series of let downs of semi-lean solvent, vapour is also recompressed by LP steam. This system has a high efficiency but requires the installation of large and heavy expansion drums at 15 & 18 meter elevation. This drawback affects very often negatively the feasibility of the project. Conversely expansion drums of CEAMAG thermo-compression technology are relatively small with reduced solvent hold up quantity and they are installed at 6 to 10 meter elevation. Cost of steel structure is limited and does not offset the benefit on energy savings.
Swap to aMDEA® solvent
In reason of the consistent drawbacks of the two previous groups of solvents, a new solvent was developed for acid gas removal. MDEA (Methyl Di Ethanol Amine) solvent is available from the beginning of the years 70’s and was first applied by BASF. It is used more and more because it overcomes many of the constraints of the 2 previous groups of solvents and even bring additional benefits. The swap from current solvent to MDEA is the option the most recommended by CEAMAG. For this reason CEAMAG has tied up with BASF company whose technology is regarded as the best available. Their product is named aMDEA® . "a" stands for "activated", selection of the right activators for each specific case is actually the heart of the know how. This selection is described in further paragraph and this is what makes the real difference between suppliers.
aMDEA® Process features
Toxicity : aMDEA® is a biodegradable and non-toxic solvent (harmful for human body as any amine but not declared as a poison). Corrosion : aMDEA® is much less corrosive than the other amines or Potassium Carbonate. It does not require any toxic corrosion inhibitors and it even allows, in some particular cases, the use of carbon steel for some parts of equipment. Stability : aMDEA® is a highly stable solvent that does not decompose, foaming tendency is therefore reduced. It cancels the need of a reclaimer unit. Monitoring and analysis requirements are minimized. Make up rate : The blow down streams are minor and are only aimed to avoid any build up of catalyst dust or HT and LT shift conversion by-products. Losses are mainly caused by mechanical losses such as pump switches or pipe drainage. Vapour losses are minor. Consequently, make up rate is minor and can evaluated in the range of 3 – 10% of the initial fill per year. High performances : In addition to these trouble free operation features, aMDEA® solvent brings also consistent and measurable economical benefits:
aMDEA® Low CO2 content in the treated gas
The comparison of CO2 content is given below for Carsol, MEA and aMDEA® technologies:
aMDEA® Low energy consumption
Low energy consumption of aMDEA® based CO2 removal plants is a major benefit compared to MEA. For instance a typical Kellogg 1000 MTPD ammonia plant with a MEA based CO2 removal unit has a specific energy consumption of about 205 MJ/kmol CO2., and about 140 MJ/kmol CO2 in case of a Carsol technology. Solvent swap to aMDEA® reduces this energy consumption to 127 MJ/kmol CO2 for the same capacity. Specific energy consumption may even be lower than 100 MJ/kmol CO2 in some cases. The low solvent circulation rate also reduces the consumption of solvent pumps and is not computed above. As said previously this reduction of energy consumption allow reduction of steam carbon ratio in primary reformer mixed feed down to S/C=3.0. Lower duties on reboiler also allows for increase of plant capacity without any modification of equipment. For instance, the calculations on above mentioned Kellogg plant showed that an uprate of 35% of nameplate capacity was possible without any modification of static equipment or pumps.
aMDEA® Solvent characteristics
The aMDEA® solvent systems are aqueous solutions of the high-boiling tertiary Methyl-Di-Ethanol-Amine plus a small amount of activator for enhancing the CO2 absorption rate. aMDEA® combines the advantages of chemical and physical solvents : the high loading capacity of aMDEA® results in low solvent circulation rates, while the activators keep the absorber height to the minimum. The quasi-physical absorption behavior at CO2 partial pressures above 1 bar ensures the energy efficient regeneration of aMDEA®. Activator content can be adjusted in order to modify the nature of the solvent to either a more physical or a more chemical variant:
aMDEA® Typical revamping with aMDEA® swap
Revamping which includes a solvent swap to aMDEA® is made in partnership with BASF. CEAMAG prepares the basis of design for CO2 removal plant revamping taking into account the complete ammonia plant specific conditions. With the help of this basis of design, BASF performs the process calculations on absorber and strippers and selects the most adequate activators for aMDEA®. CEAMAG makes all studies for installation of the flashing feed galleries. These galleries are indeed absolutely necessary with aMDEA® because of the large physical desorption of CO2 when using this solvent. CEAMAG completes the heat & mass balance of the CO2 removal plant when other options such as split condensation or turbines are included in the scope of revamping project. CEAMAG & BASF take part to the site activities such as:
After start-up, CEAMAG & BASF provide also extensive services such as:
Increase of CO2 stream purity
"IMPROVEMENT OF CO2 PURITY" if problem of CO2 stream purity is a concern for a connected urea plant. |
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