Clathrate Hydrate in Ternary and Quaternary Systems of Carbon

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Clathrate Hydrate in Ternary and Quaternary Systems of Carbon Powered By Docstoc
					    Clathrate Hydrate Formation in Ternary and Quaternary
     Systems of Carbon Dioxide, Water, Tetrahydrofuran and
                            Sodium Chloride




                    Khalik M. Sabil and Cor J. Peters*




                         Delft University of Technology
                           Faculty of Applied Sciences
               Physical Chemistry and Molecular Thermodynamics
                Julianalaan 136, 2628 BL Delft, The Netherlands
                 Phone: +31-15-2782660; Fax: +31-15-2784289;
                          E-mail: C.J.Peters@tudelft.nl           Field Code Changed
                                                                  Formatted




*
    Corresponding author
Abstract


       This work reports on experimental results of the competitive effect on the
hydrate equilibrium pressure of CO2 systems with both sodium chloride (NaCl) and
tetrahydrofuran (THF). For comparison, hydrate equilibrium data for binary H2O +
CO2, ternary H2O + CO2 + THF and ternary H2O + CO2 + NaCl systems also are
reported. All the data presented are measured by using a Cailletet equipment. For the
quaternary system, the concentrations of NaCl and THF in the aqueous phase are
varied while the concentration of carbon dioxide is kept at ~ 0.3 mol% of the overall
concentration. All measurements are made in a pressure and temperature range of 0.5-
7.5 MPa and 270-300 K, respectively. Experimental measurements show that the
addition of NaCl into the CO2-THF mixed hydrate systems reduces the promoting
effect of THF in the range of concentration studied. It is believed that the reduction of
the promoting effect is mainly due to the ionic clustering between ions and water
molecules and salting-out effect. Further investigations are suggested in order to get a
more detailed behavior of the quaternary system.




Introduction


       Carbon dioxide has been known for a long time to be among a number of
molecules that, when physically combined with water under appropriate pressure and
temperature condition, can form gas hydrate. The phase behavior of a system
consisting CO2 in the hydrate-forming region is of importance for many industrial and
natural processes. On the one hand, carbon dioxide and water are part of natural gas
streams and they are also found in oil reservoirs during enhanced oil recovery. In
these cases, formation of hydrate may cause problems during production and
processing. On the other hand, carbon dioxide hydrate formation may be desirable
because it can facilitate separation processes, freezing and refrigeration processes.
These technological interests create the need for phase equilibrium data for systems
containing CO2.


       The use of CO2 hydrate in technological applications has not yet been put into
practice due to several factors. One of these factors is due to high hydrate equilibrium
pressure requirement. For example, the formation pressure carbon dioxide hydrate
which is 2.1 MPa at 277.55 K is still being considered as high for the use of CO2
hydrate as secondary refrigerant in a classical cooling installation [1]. The presence of
some specific organic components such as tetrahydrofuran, tetrahydropyran and 1-3-
dioxolane are able to reduce these pressures and might enable gas hydrates to offer a
practical option. From the experimental results reported in literature, it was concluded
that the maximum pressure reduction occurred at an organic component concentration
of approximately 5.6 mol% relative to water [2]. However, at higher organic
concentrations in the aqueous solution, a steady increase in hydrate equilibrium
pressure was observed, that is, less reduction and eventually inhibition of the hydrate
formation occurs. In contrast, it is well known that the presence of salts like NaCl,
CaCl2 and MgCl2 exerts a strong hydrate inhibiting effect, resulting in a significant
increase of the hydrate equilibrium pressure. One example of practical interest where
salt and hydrate formers may be present simultaneously, is the separation of highly
soluble salts from aqueous solution by eutectic freeze crystallization [3].


       All these new developments in CO2 hydrate research require good insight in
the phase behavior of the related hydrate systems. The main purpose of this work is to
investigate the competing effects of a hydrate promoter with the hydrate inhibiting
effect of NaCl in systems containing CO2 at hydrate forming conditions. The organic
component chosen in the present work is tetrahydrofuran (THF).




Experimental


   The chemicals used in this work are listed in Table 1 and they are used without any
further purification. The Cailletet equipment has been used for measuring the various
phase transitions that occur in the ternary system of H2O + CO2 + THF and quaternary
system of H2O + CO2 + NaCl + THF. In this apparatus, phase equilibria for mixtures
of known overall composition can be determined visually up to pressures as high as 15
MPa. A platinum resistance thermometer (AΣΛ Laboratories) with an accuracy of
0.01 K is used to read the temperature of the sample. The pressure conditions of the
phase transitions are measured with a dead-weight pressure gauge (de Wit), with a
smallest weight of 0.005 MPa. One is referred to Raeissi and Peters [4] for further
details on the Cailletet equipment.


  Table 1: Components used for the experiments with their supplier and purity
 Component                  Supplier                      Purity

 Water                          Own                          Double distillate
 Carbon Dioxide                 Messer Griesheim             0.9995
 Tetrahydrofuran (THF)          J.T. Baker                   >0.99
 Sodium Chloride (NaCl)         J.T. Baker                   0.9996



       The filling of the equilibrium cell (Cailletet tube) with the desired amounts of
liquid and gaseous components is carried out with the aid of a “gas rack apparatus”,
equipped with a high vacuum turbo pump and an ionization vacuum meter. A known
amount of aqueous solution is injected into capillary glass tube. Then the sample is
degassed at high vacuum while the sample is kept frozen with liquid nitrogen.
Additional degassing of the liquid sample is achieved by successive freezing and
melting the sample under high vacuum. A known amount of carbon dioxide is then
dosed volumetrically at known temperature and pressed into the tube using mercury.
The capillary tube is placed into an autoclave, where the mercury reservoir is
connected to a hydraulic oil system that can be pressurized with a screw-type hand-
pump. In this way, the mercury column is acting as both a seal for the sample and
pressure-transmitting medium. The sample can be kept at a constant temperature,
within 0.01 K, by circulating ethanol around the tube with a thermostatic bath
(Lauda).


       For the measurement of hydrate equilibrium dissociation lines i.e. H + Lw + V
→ Lw +V, H + Lw + Lv → Lw + Lv, H + Lw + Lv + V → Lw +Lv + V, the pressure
is fixed at a constant value by means of the dead-weight pressure gauge.
Subsequently, the temperature is elevated in small steps, typically at a rate of 0.5 K
per 10 minutes, until the dissociation of the hydrate phase can be observed. Small
bubbles appear around the hydrate crystals and the temperature is kept constant as
long as these bubbles are observed. When the bubbles disappear and the hydrate phase
is still present, the temperature is increased by small steps of 0.01 K until no hydrate
crystal is observed. The temperature where the hydrate phase disappears is taken as
the phase transition temperature.


               For the measurement of dew points lines, the temperature is fixed at a constant
value and the pressure is adjusted using the dead-weight pressure gauge. The starting
point is at a pressure where a vapor phase (V), liquid water (Lw), liquid organic (Lv)
are present. From this point, the pressure is decreased gradually until the last droplets
of the liquid organic phase disappears, i.e., Lw + Lv + V → Lw + V. For all types of
phase transition measurements, the pressure is corrected for the pressure generated by
the height of the mercury column and the atmospheric pressure as well.




Results and Discussion


           5



           4



           3
   P/MPa




           2
                                                                               1.2 mol% THF
                                                                               3.0 mol% THF
                                                                               5.0 mol% THF
           1
                                                                               7.0 mol% THF
                                                                               THFfree

           0
            274      276     278    280    282     284    286     288    290       292        294
                                                   T/K


Figure 1. H-Lw-V equilibrium data for the ternary system H2O + CO2 + THF for
different concentrations of THF in the water phase.



               Experimental results for the various types of systems are discussed in this
section. Figure 1 represents three-phase H-Lw-V equilibrium lines for the system H2O
+ CO2 + THF at various THF concentrations in the water phase. Data for the THF-
free system of CO2-water are included for comparison. From this figure, it is clearly
shown that the presence of THF shows a significant hydrate pressure reduction up to
THF concentration of about 5-7% in the water phase. The highest pressure reduction
retrieved is approximately 90% compared to the hydrate equilibrium pressure of THF-
free system. The reduction in hydrate equilibrium pressure is promising enough to
study the influence of THF in the presence of NaCl in CO2 hydrate systems.


           4.5
                                                               H2O + CO2 + NaCl (2.3%)
           4.0                                                 H2O + CO2 + NaCl (1.1%)
                                                               H2O + CO2
                                                               H2O + CO2 + THF (5%)
           3.5


           3.0
   P/MPa




           2.5


           2.0


           1.5


           1.0
              265     270       275       280         285     290          295        300
                                                T/K


Figure 2. H-Lw-V equilibrium data of the ternary system of H2O + CO2 + NaCl with
2.3 mol% NaCl in the water phase and the ternary system H2O + CO2 + THF with 5
mol% THF in the water phase.



            The three-phase H-Lw-V hydrate equilibrium data of the ternary systems H2O
+ CO2 + NaCl with 2.3 mol % and 1.1 mol% NaCl in the water phase, respectively
and the ternary system H2O + CO2 + THF with 5 mol% THF in the water phase are
compared to the binary system of H2O + CO2 and presented in Figure 2. The
inhibiting effect of NaCl and the promoting effect of THF can be seen clearly in this
figure: at given pressure, the equilibrium temperature for the H2O + CO2 + NaCl
systems is lower while the equilibrium temperature for the system H2O + CO2 + THF
is higher than that of the binary system H2O + CO2. Furthermore, it is noticed that at
higher salt concentration the equilibrium temperature will be at a lower value. This
behavior is expected since changing salt concentrations changes chemical
environment and properties such as ionic strength that affect the activity coefficients
and the activity of water.



           7.5
                    1 mol% THF
           6.5      3 mol% THF
                    5 mol % THF

           5.5


           4.5
   P/MPa




           3.5


           2.5


           1.5


           0.5
              271     273    275   277   279    281    283    285    287    289    291
                                                T/K


Figure 3. pT-diagram of the quaternary system H2O + CO2 + NaCl + THF. The
concentration of NaCl is 2 mol% and the pT-curves are given for various
concentrations of THF. The solid and dashed lines are the hydrate equilibrium curves
of the systems H2O + CO2 + NaCl and H2O+CO2, respectively.



            Figure 3 represents the primary experimental results of the quaternary system
with 2 mol% NaCl and various concentration of THF in a pT-diagram. In order to
simplify the diagram, each hydrate equilibrium line presented in the diagram is
actually a combination of three-phase H-Lw-V, four-phase H-Lw-Lv-V and three-
phase H-Lw-Lv data, the latter emerging to elevated pressure. In all the systems, the
overall concentration of CO2 is kept constant at approximately 3 mol%. The figure
shows clearly that significant equilibrium pressure can be achieved by the
introduction of small amount of THF in the system. Moreover, at concentrations
lower than 5 mol% of THF, the equilibrium pressure decreases with increasing mole
fraction of THF. In this case, it is shown that the promoting effect of THF is able to
compete with the inhibiting effect of NaCl.
  10.0
               10 mol% NaCl
   9.0         2 mol% NaCl

   8.0

   7.0

   6.0

   5.0

   4.0

   3.0

   2.0

   1.0

   0.0
         274       276        278   280      282      284      286      288       290



Figure 4. pT-diagram of the system with H2O + CO2 + NaCl + THF. The
concentration of THF is 5 mol% and the pT-curves are given for various
concentrations of NaCl. The solid line is the hydrate equilibrium curve of the system
H2O + CO2.


          At higher NaCl concentrations, it is found that THF still reduces the hydrate
equilibrium pressure, but the pressure reduction is lower than at lower concentration
of NaCl. Also, the hydrate stability region decreases significantly especially at higher
pressure as shown in Figure 4. This behavior is expected since salt ionizes in aqueous
solution and interacts with the dipoles of water molecules with a much stronger
coulombic bond, which causes clustering around the water molecules [5]. The
formation of this bond inhibits the formation of hydration since water is more
attracted to ions than to the hydrate structure. Thus, the more ions are presence in the
solution, the larger hydrate inhibiting effect can be noticed. Moreover, a “salting-out”
effect is observed in the systems studied where the increase of NaCl concentration in
the aqueous solution causes a decrease in the solubility of THF in the water. For
further visualization of this behavior, the data for the dew point lines for H2O + CO2 +
THF and H2O + CO2 + THF + NaCl are presented in Figure 5. It can be seen from this
figure that the inclusion of 2 mol% NaCl in the system reduced the pressure
requirement where liquid-liquid phase split occurs in the system, creating a THF-rich
liquids. In addition, at 5 mol% and 10 mol% NaCl in the system, it is not possible to
prepare a single aqueous liquid phase at ambient conditions (≈278 K, 1.00 MPa) with
5 mol% THF.           It is believed that both ion clustering and salting-out cause a
substantially more subcooling required for formation of hydrate. Thus, as the
concentration of NaCl increases, the inhibition effect of NaCl also increases and
reduces the pressure reduction of THF in the quaternary system.



           2.8

           2.6

           2.4

           2.2

           2.0
   P/MPa




           1.8

           1.6

           1.4
                                                                         NaCl free
           1.2                                                           2 mol% NaCl

           1.0
              282   284     286     288    290         292   294   296      298        300
                                                 T/K


Figure 5. The dew point lines, Lw-Lv-V  Lw-V, for H2O + CO2 + THF and H2O +
CO2 + THF + NaCl system. The concentrations of THF and CO2 are kept constant at
5 mol% and 3 mol% respectively.




Conclusion

             The hydrate equilibrium lines for the systems H2O + CO2, H2O +CO2 + THF,
H2O + CO2 + NaCl and H2O + CO2 + THF + NaCl have been measured at different
concentrations of the components involved by using a Cailletet equipment. From the
experimental results, it is concluded that THF, which is soluble in water, shows a
hydrate promoting effect in the range of concentrations studied. Adding NaCl to the
system increases the equilibrium pressure significantly, though the competing effect
of THF is large enough to overrule this effect of NaCl at low concentrations of NaCl.
At higher concentration of NaCl, salting-out effect has been observed in the system,
reducing the solubility of THF in water and promoting a liquid-liquid phase split in
the system.     Further investigation on the competing effect of NaCl and THF is
required in order to obtain a detailed insight in the phase behavior of the quaternary
system in the hydrate-forming region.



References


[1] A. Delahaye, L. Fournaison, S. Marinhas, I. Chatti, J.-P. Petitet, D. Dalmazzone,
W. Fũrst. Ind. Eng. Chem. Res. 2006, 45, 391-397.


[2] M.D. Jager, R.M. de Deugd, R.M.; C.J. Peters, J. de Swaan Arons, E.D. Sloan,
Fluid Phase Equilibr.1999, 165, 209-223.


[3] R.J.C. Vaessen, F. van Der Ham, G.J. Witkamp. Gas Hydrates: Challenges for the
Future. Annals of The New York Academy of Sciences 2000, 912, 483-501.


[4] S. Raeissi, C.J. Peters, J. Supercrit. Fluids 35 (2005) 10-17.


[5] E.D. Sloan Jr., Clathrate Hydrates of Natural Gas 2nd ed., Marcel Dekker, New
York, 1998.