In our previous post, we looked at the physical aspect of the volume addition of two different solvents. In this post, we shall review the chemical aspect.

When 2 liquids of equal volumes chemically react to form products. The final volume obtained is dependent on the molecular arrangement of the product molecules. When the same volumes of 2M NaOH is mixed with 2M HCl, the final volume is more than the original volume of the two liquids. How?

The “vanishing volume” or “appearing volume” is due to intermolecular interactions which produces differences in arrangement of the product molecules in the mixture versus the reactant molecules. Strong interactions result in closer packing (vanishing volume) and vise versa.

When 250 mL of water is added to 250 mL of water or when 250 mL of alcohol is added to 250 mL of alcohol, the final volume will always be 500 mL, as expected. However, when the same water is added to an equal volume of alcohol, the final volume is about 10% less than the original volume of the two liquids. How?

When two liquids are physically mixed, the “vanishing volume” or “appearing volume” depends on whether the intermolecular interactions attract or repel. Which may cause contraction or expansion, respectively in the arrangement of the solvent molecules in the mixture versus the pure substances. The strength of the interactions determine the extent of contraction or expansion. The molecular interactions and arrangements in the mixture determines the outcome.

 

In this post, we explain why carbamate of sterically hindered AMP is unstable relative to that of MEA. The instability of AMP carbamate then leads to an easy break down to release CO2 and amine in the desorption process, thus giving rise to a higher CO2 loading and a lower desorption heat requirement than those of MEA.

As shown on the graphical abstract, AMP structurally has 2 methyl groups (-CH3) on the tertiary carbon adjacent to the nitrogen while in contrast to MEA, the same carbon has only 2 hydrogens attached to it. Methyl groups in AMP are known as electron releasing groups so that they are relaying their electron density to the tertiary carbon and then the nitrogen.

This phenomenon causes AMP carbamate to destabilize by being exposed to

1)increased electron density on the N which is highly desirable for a reaction with proton from amine (H+) to trigger the carbamate breakdown process and

2)increased electron repulsion with the delocalized electrons on the carbonyl group whose effect destabilizes the carbamate and makes the break-down process more favorable.

In comparison to MEA, the 2 hydrogens (-H) on the tertiary carbon do not have the same inductive effect as seen from the methyl groups. As a result, the carbamate nitrogen reactivity is not enhanced by any extra electron density as seen in AMP carbamate. The instability effect triggered by electron repulsion with the carbonyl group also is not there.

The stability of MEA carbamate is therefore attributed to a combination of these 2 factors which then makes the compound break down process harder to occur compared to that of AMP. The CO2 loading and heat requirement as a result are higher than the case of AMP.

Catalysts are employed in various industrial applications such as water purification, food, medical, cosmetology, automotive, gas purification, petroleum and precious metal recovery. Catalyst have the ability of speeding up the overall reaction without undergoing any permanent chemical change, as such, most catalyst are expected to be inert. All catalyst consist of an active phase, a support and a promoter or an inhibiter. When it comes to solid catalysts, one more component is key in obtaining the final catalyst; a binder!

The key role of the binder is to obtain a pelletized form of the final catalyst powder. This ensures the availability of catalysts with strong mechanical integrity for various applications. Binders used in catalyst preparation include silica, alumina, and aluminum phosphate solution. However, literature shows that the type of binder can interfere with the final products to either form new products or absorb some of the products. The pelletizing of a catalyst may or may not include a binder, depending on the particle size of the catalyst powder. Where a binder is necessary, there is a need to consider its effect on the final activity of the catalyst and its overall contribution in the reaction process.

There is therefore the need to have a wholistic look at the catalyst preparation process, that is from the selection of active metals, support, preparation technique, through to the production of the final catalyst pellets to access the possibility of any side reactions that may interfere with the final results.

When designing an absorber column to achieve the maximum efficiency, two things need to be determined, the diameter and the height of the column. In lame man’s terms, the diameter refers to how fat the column whereas the height refers to how tall the column will be. For starters, diameter relates to capacity, whereas height relates to the absorption rate.

In the reaction between amine and CO2, which is typically a gas-liquid phase reaction, enough contact time must be allowed for the liquid and gas to be able to interact sufficiently with each other in order for the reaction to proceed. Thus, this means that the absorber has to have sufficient height in order for the liquid amine to flow down in a manner that will allow enough time for the up flowing gas to contact it. The height of the column determines how long the liquid and gas flows stay in the column for the reaction to proceed. If the column is too short the reaction time will be short. As such, there will be very little reaction going on and as a result the absorber efficiency will decrease. The absorber efficiency refers to how much CO2 has been removed; essentially it is the CO2 capture efficiency of the process.

Let’s talk about the column diameter. The diameter determines how much gas and liquid the column will allow in order to operate efficiently. So essentially the size of the column determines the allowable gas and liquid flow rate. If you design a column that has a very small diameter, it will be difficult to run the process at high gas flow rates. If these gas flows are allowed in such a scenario, there will be a high pressure drop in the column and this will result in liquid hold up and ultimately flood the column. Column flooding is undesirable, because it results in significant solvent losses and also reduces the CO2 capture efficiency.

If your column is tall but has a very small diameter, you will gain on the reaction kinetics, however you lose on capacity, could have flooding issues and consequently will affect your capture efficiency. If you have a column with a big diameter, but too short, you will avoid flooding issues but you lose on the kinetics, which will also affect your CO2 capture efficiency. So, you need to get these parameters right in your design.

Developing an effective carbon dioxide capture system is essential to reducing greenhouse gas emissions and moving toward a cleaner energy future. As an emerging new class of porous solids, metal-organic frameworks (MOFs) adsorbents are particularly promising as CO2 capture materials because they have high internal surface areas, low heat capacity, and adjustable pore functionality enabling the selective adsorption of large quantities of CO2.

MOFs are formed by assembling metal-containing nodes (metal ions or metal-based clusters) that function as structural building units and organic linkers. One gram of the MOFs has an internal surface area equal to 1.5 football (soccer) pitches or two American football fields. The large surface area offers more space for chemical reactions and adsorption of molecules. Consequently, if appropriately designed, a small amount of MOFs can remove enormous CO2 from the exhaust gas produced by fossil fuel combustion.

The capacity to rationally select the metal-organic framework components is expected to allow the affinity of the internal pore surface toward CO2 to be exactly controlled, facilitating materials properties that are optimized for the specific type of CO2 capture to be performed (post-combustion, pre-combustion, or oxy-fuel combustion) and potentially even for the specific power plant in which the capture system is to be installed. For this reason, a crucial effort has been made in recent years to improve the gas separation performance of metal-organic frameworks, and some studies evaluating the prospects of deploying these materials in real-world CO2 capture systems have begun to emerge.

MOFs could be accomplished using much smaller temperature changes than required for other technologies, giving them a significant advantage over conventional ways to capture CO2. (The adsorbed CO2 can then be utilized in other products.) This strategy eliminates the need to divert high-value, high-temperature steam away from power production, avoiding a large increase in the cost of electricity. In the course of these efforts, we also showed that variants of the MOFs could be efficient for the removal of CO2 from other gas mixtures, including biogas, natural gas, and even directly from the air.

 

In general, rate of chemical absorption of CO2 increases with increasing temperature. This is because an increase in temperature will raise the average kinetic energy of the reactant molecules. Therefore, a greater proportion of molecules will have the minimum energy necessary for an effective collision that yields the product. This was observed in our experiments for the absorption temperature regime (40 – 60 ºC) as depicted by the graphical abstract.

A + B <-> C + D (eq. 1)

Nevertheless, in reversible reactions such as absorption of CO2 with amines, described above in equation 1, the kinetics may change and the general observation described beforehand may differ with regards to the temperature under consideration. Hence, the inference that rate of chemical absorption of CO2 increases with increasing temperature is privy to the temperature regime of CO2 absorption. Considering the chemical equation described in the equation 1 above, the forward reaction comprises the reactants A and B. A and B being Amine(s) and CO2 respectively whiles C and D are the resultant products. An interesting phenomenon observed from our experiment showed that beyond 60 ºC, the rate of absorption decreases with increment in temperature. This implies, higher temperature regimes beyond 60 ºC favor the backward reaction, commonly known as desorption (. Hence from our experiments, the temperature range that favors the forward reaction (Absorption) is between (40-60 ºC) and above 60 ºC favors the backward reaction (Desorption) as depicted in the graphical abstract.

In conclusion, with regards to reversible chemical reactions such as absorption of CO2 using amines, the relationship between temperature and rate of absorption is subjective to the regime of temperature of absorption. Favorable temperature regimes for absorption and desorption are 40-60 ºC and above 60 ºC respectively.