•The equilibrium data in THI solution based formation water is first investigated.•The 0.55 mass fraction concentration of EG 0.55 mass fraction fills the vacancy of this area.•The testing pressure range from 4.22 MPa to 34.72 MPa was rare in published data.In this paper, the three-phase coexistence points are generated for multicomponent gas hydrate in methanol (MeOH) solution for (0.05, 0.10, 0.15, and 0.35) mass fraction and ethylene glycol (EG) solution for (0.05, 0.10, 0.15, 0.35, 0.40 and 0.55) mass fraction. The phase equilibrium curves of different system were obtained by an isochoric pressure-search method on high pressure apparatus. The phase equilibrium regions of multicomponent gas hydrate were measured using the same composition of natural gas distributed in the South China Sea. And the different concentration solutions were prepared based formation water. The experimental data were measured in a wide range temperature from 267.74 to 298.53 K and a wide range pressure from 4.22 MPa to 34.72 MPa. The results showed that the hydrate phase equilibrium curves shifted to the inhibition region in accordance with the increased inhibitor concentration. In addition, the equilibrium temperature would decrease about 2.7 K when the concentration of MeOH increased 0.05 mass fraction. Besides, the suppression temperature was 1.25 K with the 0.05 mass fraction increase of EG concentration in the range of 0.05 mass fraction to 0.15 mass fraction. While in high EG concentration region, the suppression temperature was 3.3 K with the same increase of EG concentration (0.05 mass fraction).

•The recovery rate of MEG was affected by the temperature rather than HI-121.•HI-121 could be recovered along with MEG under proper recovery conditions.•HI-121 with various polymer chain lengths reduced the TIE of the recovered MEG.•The solution recovered under milder conditions retained good inhibitory performance.•Adding 5 wt% MEG restored the recovered solution performance to original levels.Kinetic hydrate inhibitors (KHIs) combined with thermodynamic inhibitors (THIs) such as monoethylene glycol (MEG) have been good additives for the prevention of hydrate blockages in oil and gas industry operations. The regeneration and recycling of MEG are conventional process steps used to reduce costs. However, the recovery of THIs in the presence of KHIs or the recovery of the KHIs alone has rarely been investigated. In this paper, a series of experiments was designed to study the recovery of both a KHI based poly (N-vinylcaprolactam) and MEG. The results showed that the MEG recovery rate was closely related to the recovery temperature, but was not influenced by the KHI. The MEG recovery rate from solutions consisting of MEG and the KHI was as high as 94.52%, and the KHI was recovered along with the MEG. The polymer structure of the KHI was rarely changed when the recovery temperature was close to its polymerization temperature. The presence of the KHI had a negative impact on the thermodynamic inhibition efficiency of the MEG. The KHI performance of the recovered solution obtained at the KHI polymerization temperature could reach the level of the fresh combination inhibitor, but the recovered solutions obtained at temperatures far above the KHI polymerization temperature demonstrated worse inhibitory performance. The kinetic performance could be restored by adding 5.0 wt% fresh MEG. MEG enabled a subcooling temperature decrease into the range in which KHI which could play its role effectively, leading to the improved kinetic performance of the recovered solution.Download high-res image (175KB)Download full-size image

•Glycine had weak inhibitory effect while cloud enhance the performance of PVCap.•The induction time of PVCap increased 16-fold with the help of glycine.•The rapid growth region of PVCap was avoided in the presence of glycine.•Glycine significantly decrease the cost and improve biodegradability of PVCap.The inhibitory performance of glycine and its synergistic potentiality for poly N-vinylcaprolactam (PVCap) was studied by evaluating subcooling temperature, induction time and crystal growth inhibition respectively. Glycine could not inhibit CH4 hydrate formation alone but it could enhance the inhibitory performance of PVCap. The subcooling temperature of PVCap increased by 4.1 °C and the induction time also increased by 16-fold after blending the glycine with PVCap. Simultaneously, the performance of PVCap inhibiting hydrate crystal growth became more powerful in the presence of glycine. The rapid growth region of PVCap was totally avoided even at 13.5 °C subcooling with the help of glycine, leading crystal growth rate decreasing by 80%. The biggest difference between glycine and common synergists was that 1.0% mass fraction glycine could equivalently replace PVCap in the same amount, leading 40.8% lower cost and 23.4% higher biodegradability. Furthermore, the relationship between outstanding synergistic effect of glycine and its hydrophilic structure was studied.Download high-res image (62KB)Download full-size image

In this work, the effects of tert-butyl peroxy-2-ethylhexanoate (TBPO) and the operating conditions on the separation of CO2 from a CO2/CH4 (67.0 mol %) gas mixture based on hydrate formation have been studied through experiments. The results showed that CO2 separation was effectively achieved by adding TBPO to aqua-solutions. The content of methane in the residual gas hiked when the gas/liquid volume ratio decreased. Methane concentration in the residual gas phase was increased from 67.0% to 89.6%, obtained when the initial gas/liquid volume ratio was 2.23 by one-stage hydrate separation with 26.0 wt % TBPO at 287 K and 2 MPa. The CH4 recovery was 0.91, and the CO2 separation factor was 17.01 by adding 5.0 wt % TBPO. Through the use of a hand pump to keep pressure constant for the residual gas in the vessel to form hydrates, CH4 concentration in the residual gas phase increased to as high as 93.3%.

In this study, mechanical force applied to squeeze poly(sodium acrylate-co-2-hydroxyethyl methacrylate) hydrogels that contained seawater in order to obtain fresh water. By incorporating ionic monomer sodium acrylate (SA) into hydrogels, the salt rejection was significantly enhanced from 27.62% to 64.57% (feed concentration 35.00g/L NaCl solution). As SA’s concentration continuously increased, salt rejection declined due to the change in hydrogel’s matrix structure. Therefore, water recovery raised as the current swelling degree increased. We also measured pore size distribution by applying mercury intrusion porosimetry on each hydrogel sample in the interest of finding out whether the sample SA5/HEMA15 owned multi pore structure, since the result could be good for the desalination performance. After 4 times reused, the hydrogel remained good desalination performance. Although compared to reverse osmosis (RO) and multistage flash distillation (MSF) & multiple effect distillation (MED) the salt rejection of this hydrogel (roughly 64%) seemed low, the hydrogels can be used for forward osmosis and reverse osmosis, as pretreatment of seawater to reduce the energy consumption for the downstream.

Effects of polyvinyl alcohol (PVA) on the adhesion force of tetrahydrofuran (THF) hydrate particles were investigated with the microscopic manipulating technique. The adhesion forces of THF hydrate particles with PVA concentration ranging from 0.1 to 1.0 wt % were measured at atmospheric pressure and −3 °C. The time-influence adhesion force of the 0.1 wt % PVA–THF hydrate particle was measured in 180 min. While the adhesion forces were measured, morphologies of the particles without PVA and with 0.1, 0.5, and 1.0 wt % PVA were observed. The surface property of the particles was investigated using droplets made from 19 wt % THF aqueous solution to contact THF hydrate particles containing 0.1, 0.5, and 1.0 wt % PVA. The adhesion forces of PVA–THF hydrate particles decreased by more than 50% compared to pure THF hydrate particles, which indicates an anti-agglomerating effect of PVA at low concentrations. The adhesion force of the 0.1 wt % PVA–THF hydrate particle was kept small with increasing time, revealing the effects of PVA on stabilizing the particle–particle interaction and maintaining the low adhesion force. Morphologies show roughness on the surface of PVA–THF hydrate particles. The roughness leads to the decrease of the actual contact area between contacting particles, thus lowering the agglomeration tendency between hydrate particles. The reason for the morphological change and the occurrence of roughness is attributed to the hydrogen bonding between PVA molecules and water molecules. PVA was also found to change the surface property of THF hydrate particles by increasing the contact angle and weakening the wettability, which represents a decrease of the particle/medium liquid interfacial energy. Such an effect may alter the capillary bridge forming between the contacting particles. All results suggest that PVA may be a potential anti-agglomerant in lowering the hydrate plugging risk.

In oil and gas exploration and transportation, low dosage hydrate inhibitors (LDHIs) are more favorably utilized to inhibit the formation of hydrates than thermodynamic inhibitors (THs) as a trend. However, there are no industrial products of LDHIs available domestically, and the corresponding application experience is in urgent need. In this paper, a combined hydrate inhibitor (HY-1) was synthesized after a series of reaction condition optimization, and its performance on THF hydrate inhibition was investigated using kinetic hydrate inhibitor evaluation apparatus with 6 cells bathing in air. The results show that when the reaction temperature is 60°C, the reaction time is 6 h, and the monomer: solvent ratio is 1:2, the product has the best kinetic hydrate inhibitor performance on THF hydrate. On these bases, the scale-up production of this combined hydrate inhibitor was carried out. Although the scale-up product (HY-10) performs less effectively on the THF hydrate inhibition than HY-1, it functions better than a commercial product (Inhibex501) during in-house tests. HY-10 was successfully applied to the gas production process. Field trials in northern Shaanxi PetroChina Changqing Oilfield Company (PCOC) show that 2 wt% of HY-10 is effective on natural gas hydrate inhibition. It is found through economic analysis that the use of HY-10 has obvious economical advantage over methanol and Inhibex501.

Phase equilibrium data for the semiclathrate hydrates formed in three three-component systems, the CO2 + tetra-n-butyl ammonium bromide (TBAB) + water system, the CO2 + tetra-n-butyl ammonium chloride (TBAC) + water system, and the CO2 + tetra-n-butyl ammonium fluoride (TBAF) + water system, were measured in the pressure range of (0.40 to 3.77 MPa) and temperature range of (280.2 to 293.5 K) at (2.93·10−3 and 6.17·10−3) mole fraction of tetra-n-butyl ammonium halide. The experimental data were generated using an isochoric pressure-search method. The equilibrium data for the CO2 + TBAB + water system were compared with some experimental data from the literature. The effects of tetra-n-butyl ammonium halide concentration on the stability zone of the semiclathrate hydrates were studied. It was shown that TBAB, TBAC, and TBAF all can enlarge the hydrate stability zone, and as the tetra-n-butyl ammonium halide concentration increases, so does the hydrate stability zone. The three-phase equilibrium pressure of the CO2 + TBAF + water system is lower than others at the same temperature.

Capturing CO2 by forming hydrate is an attractive technology for reducing the greenhouse effect. The most primary challenges are high energy consumption, low hydrate formation rate, and separation efficiency. This work presents efficient capture of CO2 from simulated flue gas (CO2 (16.60 mol %)/N2 binary mixtures) by formation of semiclathrate hydrates at 4.5 and 7.1 °C and feed pressures ranging from 2.19 to 7.31 MPa. The effect of 0.293 mol % tetra-n-butyl ammonium bromide (TBAB) and tetra-n-butyl ammonium fluoride (TBAF) on the hydrate formation rate, reactor space velocity, and CO2 separation efficiency was studied in a 1 L stirred reactor. The results showed the hydrate formation rate constant increased with increasing feed pressure and reached the maximum at 2.82 × 10−7 mol2/(s·J) with TBAB and 8.26 × 10−7 mol2/(s·J) with TBAF. The space velocity of the hydrate reactor increased with increasing feed pressure and reached a maximum of 13.46 h−1 with TBAB and 25.96 h−1 with TBAF. CO2 recovery was about 50%, and the optimum CO2 separation factor with TBAF was 36.98, which was about 4 times higher than that with TBAB in the range of feed pressure. CO2 could be enriched to 90.40 mol % from simulated flue gas under low feed pressure by two stages of hydrate separation with TBAF. The results demonstrated that TBAB, especially TBAF, could accelerate hydrate formation. The space velocity of the hydrate reactor with TBAB or TBAF was higher than that with THF. CO2 could be easily enriched in the hydrate phase by two stages of hydrate separation under gentle conditions.

The replacement of CH4 from its hydrate in quartz sand with 90:10, 70:30, and 50:50 (wCO2:wH2O) carbon dioxide-in-water (C/W) emulsions and liquid CO2 has been performed in a cell with size of ϕ 36 × 200 mm. The above emulsions were formed in a new emulsifier, in which the temperature and pressure were 285.2 K and 30 MPa, respectively, and the emulsions were stable for 7–12 h. The results of replacing showed that 13.1–27.1%, 14.1–25.5%, and 14.6–24.3% of CH4 had been displaced from its hydrate with the above emulsions after 24–96 h of replacement, corresponding to about 1.5 times the CH4 replaced with high-pressure liquid CO2. The results also showed that the replacement rate of CH4 with the above emulsions and liquid CO2 decreased from 0.543, 0.587, 0.608, and 0.348 1/h to 0.083, 0.077, 0.069, and 0.063 1/h with the replacement time increased from 24 to 96 h. It has been indicated by this study that the use of CO2 emulsions is advantageous compared to the use of liquid CO2 in replacing CH4 from its hydrate.

Gas hydrates now are expected to be one of the most important future unconventional energy resources. In this paper, researches on gas hydrate exploitation in laboratory and field were reviewed and discussed from the aspects of energy efficiency. Different exploiting methods and different types of hydrate reservoir were selected to study their effects on energy efficiencies. Both laboratory studies and field tests have shown that the improved technologies can help to increase efficiency for gas hydrate exploitation. And it also showed the trend that gas hydrate exploitation started to change from permafrost to marine. Energy efficiency ratio (EER) and energy return on energy invested (EROI) were introduced as an indicator of efficiency for natural gas hydrate exploitation. An energy-efficient hydrate production process, called “Hydrate Chain Energy System (HCES)”, including treatment of flue gas, replacement of CH4 with CO2, separation of CO2 from CH4, and storage and transportation of CH4 in hydrate form, was proposed for future natural gas hydrate exploitation. In the meanwhile, some problems, such as mechanism of CO2 replacement, mechanism of CO2 separation, CH4 storage and transportation are also needed to be solved for increasing the energy efficiency of gas hydrate exploitation.Download high-res image (124KB)Download full-size imageHydrate Chain Energy System: A novel energy-efficient hydrate production process, including hydrate-based separation, CH4 storage and transportation of in hydrate form.

Coal bed methane (CBM) has a huge potential to be purified to relieve the shortage of natural gas meanwhile to weaken the greenhouse effect. This paper proposed an optimal design strategy for CBM to obtain an integrated process configuration consisting of three each single separation units, membrane, pressure swing absorption, and cryogenics. A superstructure model was established including all possible network configurations which were solved by MINLP. The design strategy optimized the separation unit configuration and operating conditions to satisfy the target of minimum total annual process cost. An example was presented for the separation of CH4/N2 mixtures in coal bed methane (CBM) treatment. The key operation parameters were also studied and they showed the influence to process configurations.

Clathrate hydrate can be used in energy gas storage and transportation, CO2 capture and cool storage etc. However, these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation. In this paper, ZIF-61 (zeolite imidazolate framework, ZIF) was first used in hydrate formation to stimulate hydrate nucleation. As an additive of clathrate hydrate, ZIF-61 promoted obviously the acceleration of tetrahydrofuran (THF) hydrate nucleation. It shortened the induction time of THF hydrate formation from 2–5 h to 0.3–1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.

In oil and gas field, the application of kinetic hydrate inhibitors (KHIs) independently has remained problematic in high subcooling and high water-cut situation. One feasible method to resolve this problem is the combined use of KHIs and some synergists, which would enhance KHIs' inhibitory effect on both hydrate nucleation and hydrate crystal growth. In this study, a novel kind of KHI copolymer poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)s (HGs) is used in conjunction with TBAB to show its high performance on hydrate inhibition. The performance of HGs with different monomer ratios in structure II tetrahydrofuran (THF) hydrate is investigated using kinetic hydrate inhibitor evaluation apparatus by step-cooling method and isothermal cooling method. With the combined gas hydrate inhibitor at the concentration of 1.0 wt%, the induction time of 19 wt% THF solution could be prolonged to 8.5 h at a high subcooling of 6 °C. Finally, the mechanism of HGs inhibiting the formation of gas hydrate is proposed.

Hydrate formation rate and separation effect on the capture of CO2 from binary mixture via forming hydrate with 5 wt% tetra-n-butyl ammonium bromide (TBAB) solution were studied. The results showed that the induction time was 5 min, and the hydrate formation process finished in 1 h at 4.5 °C and 4.01 MPa. The hydrate formation rate constant reached the maximum of 1.84×10−7 mol2/(s·J) with the feed pressure of 7.30 MPa. The CO2 recovery was about 45% in the feed pressure range from 4.30 to 7.30 MPa. Under the feed pressure of 4.30 MPa, the maximum separation factor and CO2 concentration in hydrate phase were 7.3 and 38.2 mol%, respectively. The results demonstrated that TBAB accelerated hydrate formation and enriched CO2 in hydrate phase under the gentle condition.

CO2 capture by hydrate formation is a novel gas separation technology, by which CO2 is selectively engaged in the cages of hydrate and is separated with other gases, based on the differences of phase equilibrium for CO2 and other gases. However, rigorous temperature and pressure, high energy cost and industrialized hydration separator dragged the development of the hydrate based CO2 capture. In this paper, the key problems in CO2 capture from the different sources such as shifted synthesis gas, flue gas and sour natural gas or biogas were analyzed. For shifted synthesis gas and flue gas, its high energy consumption is the barrier, and for the sour natural gas or biogas (CO2/CH4 system), the bottleneck is how to enhance the selectivity of CO2 hydration. For these gases, scale-up is the main difficulty. Also, this paper explored the possibility of separating different gases by selective hydrate formation and reviewed the progress of CO2 separation from shifted synthesis gas, flue gas and sour natural gas or biogas.

Capture of CO2 by hydrate is one of the attractive technologies for reducing greenhouse effect. The primary challenges are the large energy consumption, low hydrate formation rate and separation efficiency. This work presents a new method for capture of CO2 from simulated flue gas [CO2(16.60%, by mole)/N2 binary mixture] by formation of cyclopentane (CP) hydrates at initial temperature of 8.1 °C with the feed pressures from 2.49 to 3.95 MPa. The effect of cyclopentane and cyclopentane/water emulsion on the hydrate formation rate and CO2 separation efficiency was studied in a 1000 ml stirred reactor. The results showed the hydrate formation rate could be increased remarkably with cyclopentane/water emulsion. CO2 could be enriched to 43.97% (by mole) and 35.29% (by mole) from simulated flue gas with cyclopentane and cyclopentane/water (O/W) emulsion, respectively, by one stage hydrate separation under low feed pressure. CO2 separation factor with cyclopentane was 6.18, higher than that with cyclopentane/water emulsion (4.01), in the range of the feed pressure. The results demonstrated that cyclopentane/water emulsion is a good additive for efficient hydrate capture of CO2.

This paper presents an open reversed Brayton cycle with regeneration (ORBCR) using moist air for air conditioning cooled by circulating water, and proves its feasibility through performance simulation. Its refrigeration depends mainly on the sensible heat of air and the latent heat of water vapor, its performance is more efficient than a conventional air-cycle, and the utilization of turbo-machinery makes it possible. The adoption of this cycle will make air-conditioned room more comfortable and reduce initial cost because of the very low temperature air obtained. The sensitivity analyses of coefficient of performance (COP) of the cycle without regeneration and the equivalent coefficient of performance (COPE) of the cycle with regeneration to the efficiency of compressor (ηc) and the efficiency of compressor (ηt), and the results of both cycles are also given. The simulation results show that the COPE of this system depends mainly on the temperature before turbine (T7),ηc andηt, and varies with the wet bulb temperature of the outdoor air (Twet). Humid air is a perfect working fluid for central air conditioning and no cost to the user. The ORBCR is more efficient because of the use of the return air to cool water, and then to cool the air before turbine.