KINETICS OF GAS HYDRATE FORMATION FROM PYROLYSIS GAS IN WATER-IN-OIL EMULSION SYSTEM

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1 Proceedngs of the 7th Internatonal Conference on Gas Hydrates (ICGH 211), Ednburgh, Scotland, Unted Kngdom, July 17-21, 211. KINETICS OF GAS HYDRATE FORMATION FROM PYROLYSIS GAS IN WATER-IN-OIL EMULSION SYSTEM Qnglan Ma, Guangn Chen, Xuln Wang, Qang Huang, Changyu Sun, Langyng Yang Hgh Pressure Flud Phase Behavor & Property Research Laboratory Chna Unversty of Petroleum, Beng CHINA ABSTRACT The experments on gas hydrate formaton knetcs of ethylene pyrolyss gas n the water-n-ol emulson were carred out n a batch strred reactor at pressures of 4 MPa and 5 MPa and temperatures of K and K. The nfluence of temperature and pressure on hydrate formaton rate was studed, respectvely. The knetc model by Froozabad et al. coupled wth the Chen-Guo hydrate model was used to predct the rate of gas hydrate formaton for the gas mxtures by usng pure gas data. The knetc algorthm of absorpton-hydraton for the gas mxture n the water-n-ol emulson was developed, whch nvolved vapor-lqud-lqud-hydrate four phase equlbrum. The study revealed that the hydrate formaton rate s much faster n the emulson than that n pure water. The expermental data and the modelng work are of value for the desgn of hydrate separaton reactor. Keywords: Hydrate, Ethylene Pyrolyss Gas, Separaton, Emulson NOMENCLATURE A Parameter n knetc model A' Parameter n Chen-Guo hydrate model [MPa] A Bnary nteracton parameter n Chen-Guo hydrate model [K] a w Actvty of water B' Parameter n Chen-Guo hydrate model [K] C' Parameter n Chen-Guo hydrate model [K] C Langmur constant [MPa -1 ] f Fugacty [MPa] K Parameter n knetc model [kmol m -3 mn -1 ] P Pressure [MPa] r gas consumpton rate [kmol m -3 mn -1 ] T Temperature [K] V volume [m 3 ] X Constant n equaton (1) [MPa -1 ] x Mole fracton of component mole fracton of basc hydrate component formed by hydrate former Y Constant n equaton (1) [K] Z Constant n equaton (1) [K] α Hydrate structure parameter x * β Hydrate structure parameter θ Fracton of lnked cavtes occuped by gas component λ 1 Number of small cavtes per water molecule λ 2 Number of large cavtes per water molecule Subscrpts,, k Component,, k w Water INTRODUCTION The ethylene ndustry s the bass of the petrochemcal ndustry. The demand for the ethylene s ncreasng rapdly wth the development of the economy. However, the refrgeraton system s the bottleneck for extendng the producton capacty usng the exstng equpments because of ts large amount of energy consumpton. Currently, the separaton of methane from ethylene and ethane s mplemented by the dstllaton at hgh pressure and the temperature below -11 degree Celsus, whch needs hgh faclty nvestments and operaton costs n the Correspondng author: Phone: +86 () E-mal: maql@cup.edu.cn

2 separaton of the ethylene pyrolyss gas. However, the separaton of low bolng gas mxture by formng hydrate could be performed around the ce pont and avod deep coolng. Therefore applyng hydrate technque to the ethylene producton could reduce the costs of producton. Gas hydrates, also called clathrate hydrate s a sort of nonstochometrc crystallne compound composed of water and gases wth small-szed molecules [1], such as CH 4, C 2 H 6, CO 2, H 2 S, etc. The hydrate-based technque for separatng gas mxtures s based on the dfference of hydrate formaton characterstcs of varous gas speces. Study of gas hydrate formaton knetcs s of sgnfcance n desgnng the separaton equpment and enhancng the process rate and separaton effcency. Several studes on hydrate formaton knetcs have been publshed n the lterature. The study can be dvded nto two categores: the prmary nucleaton process and the crystal growth process. Gas hydrate nucleaton and growth have been nvestgated by varous expermental and modelng methods. Before 198s, the studes were manly focused on the non-hydrocarbon systems. And then more and more researches on the hydrocarbon systems were reported, such as the study on the hydrate growth of methane and ethane by Vysnauskas et al. [2,3], Englezos et al. [4,5] and Skovborg et al. [6] or the study on the nducton tme and drvng force of crystallzaton of methane and ethane hydrates by Kashchev and Froozabad et al. [7-9]. The most researches reported n the lterature are focused on the smple gas hydrate formaton knetcs, such as one component or bnary systems. Nevertheless, the gas mxtures nvolved n the practcal ethylene producton are almost multcomponent systems. The hydrate technque has not been appled to the practcal ndustral producton at the present tme. The lmtng factors are manly as follows: The natural formaton rate of hydrate s so slow that t could not meet the need of ndustral producton; the sngle-stage separaton effcency s very low; the hydrates naturally formed tend toward aggregaton and pluggng the facltes. We found that f the gas mxture contacted wth the water-n-ol emulson, the hydrate formaton rate and the separaton effcency could be enhanced enormously due to the combned contrbutons of absorpton and hydraton. Furthermore, the hydrate crystals would dsperse nto the emulson n small partcles wthout aggregaton and growng up. In ths work, the expermental data of hydrate formaton rate from the ethylene pyrolyss gas n the water-n-ol emulson were obtaned n a batch reactor. The knetc model by Froozabad et al. [7-9] was modfed and coupled wth the Chen-Guo hydrate model [1] to calculate the gas consumpton rate. An algorthm of knetcs of absorpton and hydraton process was developed, whch nvolved the coexstence of vapor-lqud-lqud-hydrate four phases. EXPERIMENTAL EQUIPMENT AND PROCEDURE Analytcal grade hydrogen and hydrocarbon gases suppled by Befeng Gas Industry Corporaton, Ltd., Chna, were used n preparng the smulated gas mxtures. Water used n the experments was de-onzed and dstlled. Referrng to the typcal pyrolyss gas mxture, a gas mxture was prepared. The expermental apparatus used n ths study has been descrbed n detal n prevous artcles from ths laboratory [11-15]. The apparatus conssted of a cylndrcal transparent sapphre cell (2.54 cm n dameter, effectve volume of 6 cm 3 ) nstalled n an ar bath and equpped wth a magnetc strrer for acceleratng the formaton of hydrate. The formaton of the hydrate crystals n the emulson were observed drectly through the transparent cell wall. The accuracy of temperature and pressure measurement was ±.2 K and ±.25 MPa, respectvely. A typcal experment started by washng the sapphre cell wth water and prepared emulson three tmes, respectvely. Ar was removed from the reactor by a vacuum pump. Then, 1 cm 3 of emulson composed of 5 w% water and 5 w% decane was added nto the cell whch was nstalled n an ar bath. The ar-bath temperature was then tuned to the requred temperature, and then the cell was charged wth feed gas untl the requred pressure was reached. The lqud phase was strred wth a magnetc strrer, and a pressure drop would begn due to the hydrate formaton. Drve the manual pump to reduce the cell volume for keepng the pressure constant. The volume was recorded at regular ntervals untl the system pressure was kept stable for 4 hours.

3 MODELING STUDY Rate of Gas Hydrate Formaton The rate of gas hydrate formaton could be expressed n terms of the gas consumpton rate, whch could be gven as follows [7,8] n r G J (1) G K 1 e RT 1 1 A A 2 RT K 2 2e RT (2) J K 2e exp (3) RT where r s the gas consumpton rate, kmol m -3 mn - 1 ; G s the tme-ndependent growth rate of the separate crystalltes; J denotes the statonary nucleaton rate; and K 1, K 2, A, and n are constants. Combnng K 1 and K 2 to one constant K, the rate of gas consumpton [Eq. (1)] then becomes 1 A RT 2 n r G J Ke RT 1 e (4) Drvng Force The dfference between the chemcal potental of a hydrate buldng unt n the aqueous soluton and n the hydrate crystal [9] was used as the drvng force n ths work. The Chen-Guo two-step hydrate formaton model [1] was used to calculate the dfference of the chemcal potental Δμ. The two-step hydrate formaton mechansm can be nterpreted as follows. The frst step: The formaton of a stochometrc basc hydrate through a quas-chemcal reacton. The basc hydrate s defned as a complex compound formed by complete fllng of the basc cavtes (.e., the large cavty) n the empty hydrate lattce wth guest molecules. The second step: The adsorpton of gas molecules nto the empty small cavtes, resultng n the nonstochometrc property of hydrates. In the second step, only small sze gas molecules (e.g., Ar, N 2, O 2, CH 4, etc.) dssolved n water may move nto the empty small cavtes. On the bass of ths theory, Δμ s calculated as follows * f 1 ln 1 2 x ln RT f n (5) * f x (6) 1 2 f 1 P T 1 w 2 f f exp a (7) f T T exp A B A exp T T C (8) where λ 1 and λ 2 stand for the numbers of small and large cavtes per water molecule, respectvely. λ 1 = 2/46 and λ 2 = 6/46 for the structure I hydrate; λ 1 = 16/136 and λ 2 = 8/136 for the structure II hydrate. x * denotes mole fracton of component n large cavtes, Σx * = 1. The symbol a w s the actvty of water. The vapor-phase fugacty (f ) of component can be calculated by the equaton of state (EOS). β = K MPa -1 for structure I hydrates, and β = K MPa -1 for structure II hydrates. θ represents the fracton of the small cavtes occuped by the gas speces. On the bass of the Langmur adsorpton theory, θ s calculated as follows C f (9) 1 Ck f k k The Langmur constant C s formulated as Y C X exp (1) T Z The constants A ', B ', and C ' n Eq. (8) and the constants X, Y, and Z n Eq. (1) are lsted n Tables 1 and 2. The bnary nteracton coeffcents, A, n Eq. (8) are lsted n Table 3. RESULTS AND DISCUSSION Expermental Results The gas hydrate formaton rates for the synthetc pyrolyss gas n the water-n-ol emulson were measured by recordng the volume reducton at a gven temperature and pressure n the batch r e a c t o r.

4 n / mol component A' 1-9 B' C' / MPa / K / K H 2 CH C 2 H C 2 H C 3 H Table 1. Values of A', B', and C' n Eq. (8) (si) component X 1 5 Y Z / MPa -1 / K / K H CH C 2 H 4 C 2 H 6 C 3 H 6 Table 2. Values of X, Y, and Z n Eq. (1) Component CH 4 N 2 C 2 H Table 3. Bnary Interacton Parameters n Eq. (8) growng tme would be reduced under the low temperature and hgh pressure condtons. In addton, for the water-n-ol system, the solublty of gas n the lqud phase would be enhanced under ths condton. Therefore, the gas molecules transferred nto the hydrate slurry (composed of hydrate and emulson) would be ncreased. To compare the gas hydrate formaton rate of the pyrolyss gas n the water-n-ol emulson and that n pure water, the experment was carred out for the pyrolyss gas n pure water. The expermental results are plotted n Fgure 4. Apparently, the gas hydrate formaton rate was enhanced effectvely n the water-n-ol emulson. The absorpton of gas molecules nto the ol, whch formed the transmcelle wth water, could ncrease the contact of water wth gas molecules. Therefore, more gas molecules could transfer nto the hydrate phase. Furthermore, t was observed through the sapphre-wndowed cell that the hydrate partcles formed n the water-n-ol emulson were dspersed n the emulson phase wthout agglomeraton, whch ndcated that water-n-ol emulson could not only enhance the hydrate formaton rate, but also avod the hydrate agglomeraton and growth. The composton of the gas mxture s lsted n Table 4. The ol-water rato of emulson was 1.. The normal volume rato of gas to lqud (emulson) was 9. To obtan the mole number of gas n the gas phase, the followng equaton was used PV n (11) zrt where the compressblty factor z was calculated by the BWRS equaton of state [16]. V s the volume of the gas phase n the reactor. The nfluences of temperature and pressure on the knetcs of hydrate formaton were tested, respectvely. The nfluences of pressure on gas consumpton are plotted n Fgures 1 and 2. The nfluence of temperature on gas consumpton s plotted n Fgure 3. From the fgures, t can be seen that the hydrate formaton rate ncrease wth the temperature decrease and pressure ncrease. The reason for ths s that the low temperature and hgh pressure are of beneft to the hydrate formaton. The nducton tme of nucleaton and gas mxture H 2 CH 4 C 2 H 4 C 2 H 6 C 3 H 6 Pyrolyss gas Table 4. Composton of Pyrolyss Gas Mxture t / mn Fgure 1. Rate of gas consumpton for pyrolyss gas n the water-n-ol emulson under dfferent pressures at T = K., Expermental data (P = 5. MPa);, Expermental data (P = 4.5 MPa);, Expermental data (P = 4. MPa).

5 n / mol n / mol n / mol.25.2 Expermental data (T = K);, Expermental data (T = K);, Expermental data (T = K);, Expermental data (T = K) t / mn Fgure 2. Rate of gas consumpton for pyrolyss gas n the water-n-ol emulson under dfferent pressures at T = K., Expermental data (P = 5. MPa);, Expermental data (P = 4.5 MPa);, Expermental data (P = 4. MPa). Calculatng Results In ths work, we used Petal-Tea equaton of state (PT EOS) [17] to calculate the fugacty of gas speces. The parameters of hydrate formaton knetcs n Eq. (4) were determned by usng the expermental data n the lteratures, whch are lsted n Table 5. On the bass of the Chen-Guo two-step hydrate formaton mechansm, the second step (the adsorpton of gas molecules nto the empty small cavtes) could progress much faster then the frst step (the formaton of a stochometrc basc hydrate). That means that formaton of basc hydrate s the control step durng the procedure of whole hydrate formaton. The parameters n Eq. (4) for the mxture were determned by followng mxng rules t / mn Fgure 4. Comparson of gas consumpton rate for pyrolyss gas n the water-n-ol emulson and pure water at P = 5. MPa and T = K., Expermental data (water-n-ol emulson);, Expermental data (pure water). K x * K A x * A (12) (13) * where x denotes the mole fracton of gas component n basc hydrate (large cavtes). The value of constant n s between.333 and 1 [8]. In ths work, n was taken to be 1 for pure gas and.5 for the gas mxture t / mn Fgure 3. Rate of gas consumpton for pyrolyss gas n the water-n-ol emulson under dfferent temperatures at P = 5. MPa., Expermental data (T = K);, Expermental data (T = K);, Expermental data (T = K);, K Data System A / kmol m -3 mn source CH [4] C 2 H [15] C 2 H [4] C 3 H Table 5. Parameters n Equaton (4) ths work The parameters n Table 5 were obtaned from the pure water system. In the water-n-ol emulson, the gas solublty and dffuson coeffcent n the lqud phase could be mproved due to the absorpton. The parameter K n Eq. (4) contans

6 the nfluence of the gas solublty and dffuson coeffcent on the hydrate formaton rate n pure water system [7,8]. For water-n-ol emulson system, the parameter, K', was correlated as follows P T.42 K K (14) P T Where P = Pa and T = K. The specfc surface energy of the hydrate/lqud nterface, σ, would be reduced n the water-n-ol emulson. The effectve specfc surface energy of the hydrate/lqud nterface, σ ef, contaned n the parameter A n Eq. (4) s defned [18] by ef (15) The factor ψ s a number between and 1. In ths work, ψ was correlated to be.118 from the expermental data. Therefore, Eq. (4) could be rewrtten for the water-n-ol emulson system as follows r.42 K P P exp RT T T A 2 e RT 1 n (16) The gas hydrate formaton n the emulson ncluded two procedures: absorpton and hydraton, where vapor, lqud hydrocarbon, water and hydrate phase would coexst. The calculaton procedure s summarzed as: (1) Input the gas composton n water-free bass y, the rato of ol to water and the rato of gas to lqud. Input temperature T, pressure P and the tme ntervals Δt. (2) Determne the composton of feed mxture z from the rato of ol to water and the rato of gas to lqud. (3) Based on T, P and z, perform three-phase flash calculaton, obtan the compostons of vapor, hydrocarbon-rch and water-rch phase, y, x A, x B, mole fracton of varous phases, V, L A, L B, and vapor phase fugacty f. (4) From f, calculate the occuped fracton of small cavty, and the mole fracton of basc hydrate, x * by Eq. (6) (1). Then, the hydrate formaton drvng force could be obtaned from Eq. (5). (5) Calculate the hydrate formaton rate r from Eq. (16). Then, calculate the amount of hydrate phase H formed n the tme nterval Δt. (6) From θ, x * and H, calculate the composton of hydrate phase x H and corrected feed composton z by Eq. (17) (19). x (17) * 2 x 1 H x x x (18) z z H x 1 H H (19) (7) Set z = z, repeat steps (3) (6) to calculate the hydrate formaton rate of every next tme nterval. The tested calculatons were mplemented by usng the foregong procedure. The comparson of calculated and expermental hydrate formaton rates under the dfferent condtons are plotted n Fgures 5 and 6. The calculaton results of gas consumpton n the water-n-ol emulson are shown n Fgure 7. It can be seen that the calculatons are n good agreement wth the expermental data. Fgure 8 shows the curves of the mole fractons of hydrogen, methane and ethylene n the gas phase over tme durng the hydrate formaton at temperature of K and pressure of 5. MPa. It could be notced that hydrogen and methane n the gas phase were enrched and most ethylene was transferred nto the hydrate slurry, formed wth hydrate and emulson, whch means the pyrolyss gas could be separated by formng hydrate to recover ethylene. CONCLUSION Expermental data on the knetcs of gas hydrate formaton from a pyrolyss gas n the water-n-ol emulson were presented. The expermental results

7 r / kmol.m -3.mn -1.8 y / % n / mol V / Ltre n the hydrate crystal was used as the drvng force, whch was calculated by Chen-Guo hydrate model. The algorthm was developed to calculate the hydrate formaton rate for the emulson system whch nvolves vapor-lqud-lqud-hydrate four phase coexstence, and s found to descrbe the expermental data very well Fgure 5. Comparson of calculated and expermental hydrate formaton rates at pressure of 4. MPa., Expermental data (T = K);, Expermental data (T = K);, Calculaton results. r / kmol.m -3.mn V / Ltre Fgure 6. Comparson of calculated and expermental hydrate formaton rates at pressure of 5. MPa., Expermental data (T = K);, Expermental data (T = K);, Calculaton results. demonstrated that the rate of gas hydrate formaton could be enhanced enormously n the water-n-ol emulson than n pure water. Furthermore, t was observed that the hydrate partcles would not agglomerate and grow to plug the equpment n the water-n-ol emulson system. The obtaned expermental data are valuable for desgnng ndustral process of separatng pyrolyss gas to recover ethylene by formng hydrates. The knetc model by Froozabad et al. was extended to descrbe the knetcs of gas hydrate formaton from mxtures n the water-n-ol emulson. The dfference between the chemcal potental of a hydrate buldng unt n the aqueous soluton and t / mn Fgure 7. Rate of gas consumpton for pyrolyss gas n the water-n-ol emulson., Expermental data (P = 5 MPa, T = K);, Expermental data (P = 4 MPa, T = K);, Expermental data (P = 5 MPa, T = K);, Expermental data (P = 4 MPa, T = K);, Calculaton results CH 4 H 2 C 2 H t / mn Fgure 8. Mole fractons of hydrogen, methane and ethylene n the gas phase. ACKNOWLEDGE The fnancal support receved from the Natonal Natural Scence Foundaton of Chna (No ) s gratefully acknowledged. REFERENCES

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