Effect of l-Tryptophan in Promoting the Kinetics of Carbon Dioxide Hydrate Formation

Abstract

Carbon dioxide capture and storage (CCS) can play a vital role in overcoming global warming challenges and can help to reduce carbon footprint significantly from the environment. Carbon dioxide (CO2) hydrates are a potential option for CCS, but slow hydrate formation is a technical challenge that needs to be overcome. Kinetic promoters are often added to improve the formation rate of gas hydrates and process efficiency. In this systematic and comprehensive work, the performance of l-tryptophan, an environmentally friendly promoter, as a kinetic promoter for CO2 hydrate formation was evaluated using a stirred tank reactor. The kinetic performance of the CO2 hydrate formation process was evaluated using varying l-tryptophan concentrations (0, 100, 300, and 1000 ppm) under different experimental pressure and temperature conditions. The impact of l-tryptophan was assessed using gas uptake measurements (induction time, hydrate growth rate, and yield) coupled with visual observations through the front and top view of the reactor. With the addition of only 300 ppm l-tryptophan, an average gas uptake of 114 v/v (78% water conversion to hydrates) was achieved at 273.65 K and 3.4 MPa, which was four times that for the case without the promoter. Further increase in l-tryptophan concentration did not result in significant additional improvement. Compared to 277.65 K, experiments conducted at 273.65 and 275.65 K showed much shorter induction times and better gas uptake performance. With such an excellent enhancement effect at a low dosage and being environmentally friendly, l-tryptophan is recommended as a potential kinetic promoter for CO2 hydrate formation that could be applied for CO2 capture and storage via gas hydrate technology.

Amino Acids as Kinetic Promoters for Gas Hydrate Applications: A Mini Review

“…On the basis of induction time and t 90 data, as well as the final gas uptake or water to hydrate conversion achieved, the authors concluded 300 ppm to be the optimum l -tryptophan concentration, at which the amino acid showcased maximum kinetic promotion activity. Of the three concentrations used, 300 ppm was found to return the shortest induction time and t 90 , as well as the highest average water to hydrate conversion of 78% …”

“…Interestingly, there have been only two amino acids so far that have unambiguously exhibited significant kinetic promotion activity for both methane hydrate and CO 2 hydrate formation. These are l -methionine and L -tryptophan. ,, Both l -methionine and l -tryptophan have also been demonstrated to exhibit potent kinetic promotion activity for CH 4 –CO 2 gas mixtures as well. ,,, The hydrophobic amino acids l -leucine and l -valine that exhibit excellent kinetic promotion activity for methane hydrate formation , have also been reported to promote CO 2 hydrate formation; however, in the case of pure CO 2 systems, the kinetic promotion activity imparted by the hydrophobic amino acids is rather weak. , The one clear insight that has been obtained with regard to the kinetic promotion activity/preferences of amino acids is that hydrophobic amino acids are more likely to act as KHPs as compared to hydrophilic amino acids and that this idea is more likely to hold true for methane hydrate formation over CO 2 hydrate formation; l -leucine, l -phenylalanine, l -methionine, and l -tryptophan are all effective promoters for methane hydrate formation, ,,,, whereas only l -methionine and l -tryptophan show the desired kinetic promotion activity for CO 2 hydrate formation. , It is still not explicit as to why only certain hydrophobic amino acids may be able to significantly promote CO 2 hydrate formation kinetics. In the past, authors have talked about various other factors influencing the kinetic promotion activity/preferences of amino acids, such as the presence of aromatic side chains, length of the side chains, and the hydropathy index of the amino acids .…”

“…Here, we discuss the status quo thus far, on the application of amino acids as kinetic hydrate promoters (KHPs). Leading with a representative molecule, hydrophobic l -tryptophan (rapidly gaining repute as a potent KHP for various gas hydrate systems), ,, we further outline perspectives on the plausible mechanisms of action through which amino acids impact the kinetics of hydrate formation. There have been a handful of review papers in the recent past covering the application of amino acids in gas hydrate research; these are listed in Table , given below.…”

Magnesium-Promoted Rapid Nucleation of Carbon Dioxide Hydrates

“…Alternatively, mechanical stirring can significantly decrease the nucleation time; however, it increases the complexity of the high-pressure reactor in terms of plugging and requires energy input. ,, Thermodynamic promoters, which alter the hydrate equilibrium curve, can also influence the induction time for hydrate nucleation. − Thermodynamic promoters are often combined with kinetic promoters to enhance hydrate formation. − However, these promoters again involve the use of chemicals such as tetra- n -butylammonium bromide (TBAB) or tetrahydrofuran (THF), which are often required in large quantities and not considered environmentally friendly. Other methods, such as utilizing hydrate seeds, supersaturation, ice melts, amino acids, electronucleation, and acoustics have also been studied to facilitate the nucleation of hydrates. − Electronucleation has been studied by the authors of this study: even though it can enable fast nucleation, it introduces additional complexities in the reactor …”

Explicating the amino acid effects for methane storage in hydrate form

“…The volumetric gas uptake (v/v) is calculated using the following equation. 47 It is defined as the “volume of gas/volume of water (hydrate + unreacted).” where Δ n t is the mole of gas consumed, n H 2 O is the mole of water used, MW H 2 O is the molecular weight of water, ρ H 2 O is the density of water and MW hydrate , ρ hydrate are the methane hydrate molecular weight and density calculated using the CSMGem application. 3 …”