using CO 2 gas hydrates-dissociation energy and its application to cell lysis and dewatering

This study investigated the effect of quick freezing method induced by gas hydrate dissociation energy (QFGD) on microbial fracture and related dewatering effect of sludge. Comparison of gas hydrate treatment and rapid freezing caused by gas hydrate treatment-dissociation energy treatment.

Research background

Energy-induced flash freezing of gas hydrates (QFGD) is a recently developed flash freezing method. As chilled water and pressurized gas meet, gas hydrates can form in the pipeline. N-(2-hydroxyethyl)-N-methylpyrrolidinium tetrafluoroborate ([HEMP][BF4]) and N-butyl-N-methylpyrrolidinium tetrafluoroborate ([BMP][BF4]) are applied to the formation of gas hydrates.

Since the formation of gas hydrates is an exothermic process, the significant temperature increase can be the evidence for the formation of gas hydrates. Temperature profile in the case named as CO2 35,300 showed significant temperature increase up to 4.3℃ due to the Joule-Thomson effect, CO2 dissolution and CO2 gas hydrates formation. Temperature profile in the case named as CO2 35,300 showed significant temperature reduction to -0.9℃ due to the Joule-Thomson effect, CO2 dissolution and CO2 gas hydrates dissociation.

Therefore, it was found that the state of formation of CO2 gas hydrates in mud is above 20 bar at 0.2. This was due to a rather large transfer of energy due to the formation and dissociation of CO2 gas hydrates. This means that the release of tightly bound organic materials was caused by Class III processing, including mixing, pressure displacement, CO2 dissolution, and CO2 gas hydrates.

In the case of class III, when large amounts of organic materials were released and CO2 gas hydrates were formed, large amounts of sludge flocs were broken down by CO2 QFGD. The gas hydrate treatment has been used as alternatives to the conventional freezing method.

Figure 3. Steps for London protocol application in South Korea.[2]

Objective of research

Sludge treatment

Therefore, chemical treatment based on surface charge causes the organic materials to be removed and the dewaterability of the sludge to increase. The process frees the water initially positioned inside the cell from the sludge and improves sludge sloughing.

Gas hydrates and its application

When the water is in the gas hydrate stabilization state, the pressure suddenly drops due to formation of gas hydrates and the temperature suddenly increases due to exothermic gas hydrate formation (B→C). Thermodynamic hydrate inhibitor such as tetrahydrofuran(THF)[38], promotes the formation of gas hydrates by shifting the thermodynamic curve. For example, Kim, Ki-Sub et al found out the effect of pyrrolidinium cation-based ionic liquids on the formation of gas hydrates.

After CO2 gas hydrates were formed in seawater at a condition of 29 bar and 280 K, gas hydrates were pelletized. When seawater was pressurized at a temperature of 0.2 ℃ and more than 25 bar of CO2, CO2 gas hydrates were formed in seawater. In this case, as CO2 gas hydrates formed, the temperature increase was due to the Joule-Thomson effect, CO2 dissolution and gas hydrate formation.

Since CO2 gas hydrates were also formed and the mud was pressurized at a higher pressure, the temperature increased more than CO2 20 150. In this case, when the CO2 gas hydrates were dissociated, the temperature decrease was due to the Joule-Thomson effect, re- gasification of CO2 and dissociation of gas hydrates. Since the CO2 gas hydrates were also dissociated and the pressure in the mud was relieved due to the higher pressure, the temperature decreased more significantly than CO2 20 150.

Samples with CO2 gas hydrate formation, such as CO2 20 150 and CO2 35 150 had significantly greater temperature change during QFGD than without CO2. The structure of gas hydrates is similar to that of ice, and CO2 can diffuse into and out of the microbial cell.

Table 2. Comparison of ice and CH 4 gas hydrates[5]

Sludge treatment

Quick-freezing induced by gas hydrates-dissociation energy

When the mud mixture became stable, the scrubbing was done by replacing the atmospheric gas with the guest molecules in the gas layer. After the temperature became stable, the mud mixture was pressurized under reaction pressure conditions.

Analysis

In my study, rapid freezing occurred in classes III and IV (with the formation and dissociation of CO2 gas hydrate). Because Class II is a treatment that involves mixing, pressure displacement, and CO2 dissolution, some sludge particles can be broken down and released as soluble organic materials. Since the distribution of organic materials in class IV was similar to that in class III, a large amount of mud flakes were decomposed similarly to class III.

As more negatively charged sludge flakes increase the electrostatic force, sludge flakes can easily disintegrate. In this study, QFGD makes the surface charge of the sludge more negative. The release of organic materials was detected by the change of osmotic pressure and conductivity in the supernatant. The released organic materials caused by stirring, pressure shifting and CO2 dissolution caused the conductivity of the supernatant to increase.

At the same shear rate, the shear stress in classes III and IV was greater than in classes I and II. Weighing the filtrate with filter time and TTF of sludge samples by the filter test with 0.45 μm filter. a: the weight of the filtrate change over time, b: TTF from the filter test). Because fine particles were produced by sludge disintegration caused by stirring, pressure shifting, and CO2 dissolution, the average particle size in class II was larger than in class I, and the PDIs also increased.

The resistance to pore blockage in class IV was significantly higher than in class II. Image of frozen raw sludge after QFGD (Condition: CO2 (Purity 99.999% from Korea SEM) as guest molecules, 30 bar as reaction pressure, 0.2℃ as stabilized temperature, 4 hours as reaction time).

Figure 12. Gradual separation of supernatant from sludge

Mechanism of QFGD in sludge application

The effect of QFGD on organic materials in sludge

Since sludge is the complex compound with organic materials, sludge disintegration causes organic materials to be released. Therefore, the release of organic materials is the indirect indicator of dewatering in sludge freezing process. Therefore, mechanical dewatering, such as Alum step and Centrifuge 5000 g step cannot separate tightly bound organic materials.

This means that Class II processing, including mixing, pressure displacement, and CO2 dissolution, results in the release of tightly bound organic materials into soluble organic materials. This means that the release of tightly bound organic materials was caused by Class IV processing, including mixing, pressure displacement, CO2. The release of organic materials was detected by changes in sludge properties such as surface charge, osmotic pressure, and conductivity.

This surface loading was due to organic materials released by agitation, pressure displacement, gas dissolution, and QFGD.

Figure 21. Distribution of organic compounds in the concentrated sludge. (The separation strength increased: Alum, Centrifuge 5000 g, 1 st Ultrasonication, 2 nd Ultrasonication)

The effect of QFGD on sludge rheology

Hysteresis loop was formed by collecting shear rate and shear stress with alternating shear stress. And after the maximum tension was reached and the external force was removed, the internal structure was recovered. 23, since mud is non-Newtonian behavior, all mud samples have a non-linear shear rate-shear stress graph.

At the same shear rate, the shear stress during the period of increasing shear stress is greater than during the period of decreasing shear stress. This is due to the elasticity of the stool; mud maintains its state by resisting change. Since changes in the internal structure were affected by changes in the microstructure, it is less sensitive to changes.

Through agitation, pressure change and CO2 dissolution, intact internal structure was destroyed and thixotropy decreased.

The effect of QFGD on sludge dewatering

On the other hand, in the images for class III and IV (with QFGD treatment) for each settling time, the selected volume decreased than other treatment. Settling volume for class III and IV treatment was lower than class I and II for each settling time. As organic matter was released, the viscosity of sludge increased, preventing fluid from flowing through the sludge and filter.

As sludge breaks down and organic materials are released, it causes the viscosity of the sludge to increase and fine particles are released. As sludge decomposition causes organic materials to be released, causing an increase in sludge viscosity and fine particles separated from sludge, the filtrate separation rate increased in Class II. Therefore, QFGD slows down sludge filtration due to increase in sludge viscosity and pore clogging.

5 g of sludge was filtered using a 0.2 μm filter and the weight of the filtrate was recorded over time.

Figure 24. Settlement analysis of the mixture of centrifuged pellet and gravitationally settled supernatant

Understanding of dewatering test based on filtration

Considering the particle size distribution in μm range, since sludge was disintegrated by stirring, pressure shift and CO2 dissolution, sludge can be divided into fine particles within nm range. Due to this disintegration, class II 2 sludge can exhibit particle size peaks in the nm range. The average particle sizes in class III and IV were larger than in class I and II due to sludge disintegration caused by agitation, pressure shift, CO2 dissolution and QFGD in class III, and agitation, pressure shift, CO2.

This means that a sludge cake made in class II can inhibit the passage of filtrate through the cake more than in class III. Since the released organic materials in class II strengthen the bond between the mud and water, it is difficult for the filtrate to be released from the mud flakes. Therefore, in Class II and IV, the main reason for reducing the sludge dewatering effect was the sludge cake.

Since the pore blocking by fine particles is less affected on sludge resistance than sludge cake, the resistance in class IV is relatively more affected by pore blocking than in class II.

Figure 29. Particle size distribution (above) and average particle size (below) of sludge in μm range

The effect of QFGD on disinfection

And since there were red-colored particles, there was some damaged microbial cell in the class I sample. And since there were red colored particles, there was some damaged microbial cell in the class II sample. And since there were green colored particles, there was some intact microbial cell in the class III sample.

And since there were green colored particles, there was some intact microbial cell in the class IV sample. Although the class II sample appears to have more intact microbial cells than damaged microbial cells in the image, there were 60% of intact microbial cells and 40% of damaged microbial cells in the class II sample. Although the class III sample appears to have more damaged microbial cells than intact microbial cells in the image, there were 28% of intact microbial cells and 72% of damaged microbial cells in the class III sample.

Although the class IV sample appears to have more intact microbial cells than damaged microbial cells in image, there were 18% intact microbial cells and 82% damaged microbial cells in class IV sample.