CO2-Amine: CO2 stripping review note

Fundamental points of CO2 absorption and stripping

-Absorption of CO2

In?general,?there?are?three?major?issues?in?CO2?mass?transfer?in?amine?[1]?thermodynamics?controlled?reversible?reaction?[2]?The?reaction?of?CO2?with?water?produces?carbonic?acid,?which?lowers?pH?and?aids?in?the?hydrolysis?of?carbamate,?and?[3]?exothermic?Amine-CO2?reactions?controlled?by?kinetics.?Because?CO2?begins?its?reaction?with?H2O?after?all?of?the?amine?has?been?consumed,?its?solubility?is?reduced?by?its?low?partial?pressure.

-Stripping of CO2

Q = Q sensible heat + Q vaporization heat + Q absorption heat

On the regeneration side, the?cost?of?regeneration?is?a?big?issue.?Depending?on?the?operating?system,?the?reported?heat?of?regeneration?in?CO2-Amine?absorption?ranges?from?2.1?to?11.25?GJ/t. Capturing CO2 raises the cost of electricity production by 70%, while the energy required for regeneration is estimated to be 15-30% of a power plant's output. Minimizing heat for regeneration has become a significant challenge, particularly for improving stripper structure, discovering new solvents, and improving operating conditions.?

The reported heat of regeneration in CO2-Amine absorption is in the range of 2.1-11.25 GJ/t, depending on the operating system. A simple stripper column is estimated to reduce the output of a coal-fired power plant by 25 to 30%.

The heat of regeneration,

Q = Q sensible heat + Q vaporization heat + Q absorption heat

Absorption/stripping with alkanolamine solvents is a cutting-edge technology for removing CO2 from coal-fired power plants' flue gas. The lean solvent fed to the top of the absorber column countercurrently contacting the flue gas and chemically absorbing the CO2. A typical coal-fired power plant emits 12% CO2 in its flue gas, which the absorber captures 90% of, leaving 1.3%. The rich solvent is heated in a cross exchanger before being introduced into the stripper. Heat applied to the rich solvent reverses the reaction, releasing CO2 and allowing the lean solvent to regenerate. The compressed gaseous CO2 is sequestered, and the lean amine solvent is recycled to the absorber. Due to the energy requirements of the reboiler, pumps, and multi-stage compressor, the base case absorption/ stripping flowsheet using Monoethanolamine (MEA) with a simple stripper column is estimated to reduce the output of a coal-fired power plant by 25 to 30%.

A simple stripper

Figure 1

A variety of solvents have been used to capture CO2. Amines are the most commonly used solvents in chemical absorption to capture CO2. Because an amine has at least one OH and amine group in its chemical structure, the OH group can reduce the vapour pressure of the amine due to the formation of H bonds, and because an amine has alkaline properties, it can absorb acidic gases. Monoethanolamine (MEA) is the most widely used amine due to its high solution absorbability, high alkalinity, high reaction rate, regenerability, and low cost. However, some disadvantages have been identified, including high solvent regeneration energy, corrosion, and degradation. Capturing CO2 raises the cost of electricity production by 70%, while the energy required for regeneration is estimated to be 15-30% of a power plant's output. As a result, effectively lowering the cost of electricity has become critical to the success or failure of carbon capture and sequestration (CCS).?

Figure 2

Vapor and liquid contact in the stripper [More details]

Minimizing heat for regeneration has become a significant challenge, particularly for improving stripper structure, discovering new solvents, and improving operating conditions. Rich solvents are heated in the stripper to allow the release of CO2 from scrubbed solutions during the stripping process. The stripping vapour is regenerated in the reboiler and rises through the column to the top of the stripper, containing water vapour, CO2, and small amounts of solvents. The stripping vapour counter-current comes into contact with a rich-loading feed stream, which absorbs energy from the stripping steam and uses it to desorb CO2. The remaining vapour condenses in the overhead condenser at the top of the column. The vapour and liquid contact system is depicted in the figure below.

Figure 3

The heat of the solvent regeneration in the stripper of the CO2 capture process can be described as follows: Q is equal to Q = Q sen + Q vap + Q abs.

where Qsen is the sensitive heat, Qvap is the evaporation heat, and Qabs is the absorption heat. When the relevant thermodynamic data is available, the three terms are generally evaluated separately.

The cost of regeneration is influenced by the solvent used, the structure of the stripping equipment, the operating temperature, and the cost of steam. Many studies have focused on more efficient solvents with lower heat of absorption in order to minimise the heat of regeneration. According to these studies, blended solvents have been extensively studied in the capture of CO2 gas, while new solvents have also been actively tested.

As a result, many researchers have studied blended amines and effective solvents in order to improve regeneration energy. According to some studies, using a split-flow in the gas stripping column can save at least 20% of the energy. If thermodynamic data are available, an investigation of the heat mechanism of regeneration energy can also be explored in order to understand the contribution of individual heat duties. The reported levels for the heat of regeneration are in the range of 2.1-11.25 GJ/t, depending on the operating system. However, no empirical equations are available to predict the heat of regeneration under the given conditions.

Experimental findings

A stripping system is depicted below. A packed column, a reboiler, a condenser, a heat exchanger, and a heating system were all part of it. The diameter of the column was 50 mm. It was stuffed with an 8 8 mm -ring. The packed column stood 800 mm tall, while the condenser stood 500 mm tall. Furthermore, a reboiler (12 L in volume) was heated with silicone oil via heating tubes. A pressure back valve was used as a suitable value at the top and bottom of the column to adjust the pressure in the column, as shown.

Figure 4 [below]

A continuous process was used to effectively investigate the effect of process variables on stripper performance. To begin, the temperature indicators, cooling water circulator, and oil-bath power supply were turned on and set to a predetermined temperature. The rich loading solution was then poured into the reboiler until it flooded. When the oil-bath temperature reached the preset level, the oil-bath pump power supply was activated, and the flow was adjusted using an oil-bath inlet valve. Third, the cooling water's inlet temperature was adjusted to the desired level.

Figure 4

The reboiler was then adjusted to the desired experimental temperature. The experiment began when the temperatures of the cooling water and the reboiler vapour reached the predetermined levels. The rich loading in the storage tank passed through a heat exchanger and into a packed bed, where it came into contact with the vapour rising from the column's bottom. The heat was released to the rich loading as the input solvent after the lean loading was withdrawn at the bottom of the reboiler and passed through the heat exchanger. The lean loading was removed every 30 minutes

Steady-state in the stripper

During the operation, the temperature at individual points was observed and recorded, as shown in Figure 4. As shown in Figure 5, several critical points were recorded, including T1, T2, T6, T7, and T12. When the operating time was greater than 80 minutes, the temperatures remained constant. Furthermore, the distribution of the lean loading was discovered, as shown in Figure 6, and it also remained constant after 80 minutes, which corresponded to Figure 5. In addition, as shown in Figure 7, the temperature distribution in the packed bed was recorded. Except for T05, which was near the location of the solvent input and was prone to perturbation during operation, the distributions approached steady-state operation after 80 minutes. The effect of the solvent input on other points (T01 to T04) was weak, however, because they were far from the solvent input. As a result, after 80 minutes, the system could be said to have transitioned to a steady state operation.

References

Wikipedia

ScienceDirect

Optimization in the Stripping Process of CO2 Gas Using Mixed Amines