5.7 Phase separation in three phase systems
5.7.2 Experimental and modeling results in three phase systems
The overall separation times under 3Φ conditions near the optimum formulation were within the range of tsep <15 min for all investigated systems [64]. Figure 77 shows the overall separation times in systems with Marlipal 24/70 and 24/90. Note that the separation times are depicted in minutes and not in seconds. The drop sizes at similar agitation speeds already indicate a higher tendency towards coalescence in Marlipal 24/90 systems in comparison to Marlipal 24/70 (cf. Figure 49). This is supported by the lower separation times of Marlipal 24/90 systems at its respective optimum formulation. The overall separation time at T = 86◦C for Marlipal 24/70 is tsep = 70.7 min = 4246 s. If only the swarm sedimentation process according to Figure 76 is considered as an estimation for the separation time, droplet sizes of dorg ≈41µm anddaq ≈112 µm would have to be present in the system.
3φ 3φ
0 100 200 300 400
70 75 80 85 90 95
Separation time tsep[min]
Temperature T [°C]
Marlipal 24/70 Marlipal 24/90
Fig. 77: Overall separation times in Marlipal 24/70 and 24/90 systems in their respective 3Φ conditions (α= 0.5,γ = 0.075, V = 0.3 L, DN60, n = 800 rpm,dst= 0.031 m)
40%
40%
20%
A) Keep dispersed phase fractions constant
B) Keep ratio of dispersed phase to continuous phase constant
80%
conti.
disp.
40% 20%
60% 66%
50% 33%
50%
Fig. 78: Two possibilities to define the dispersed phase fractions for the modeling procedure illustrated for the case of an organic continuous phase
The droplet sizes would have to be much larger than the experience from the drop size measurements sug-gests, where the droplet sizes reached the detection limit of the endoscope. If more realistic drop sizes of daq,org = 10µm based on the drop size distribution results in Figure 49 are assumed, the calculated sedimen-tation times aretsed,org = 16.7 s = 279.4 min andtsed,aq = 567.2 s = 9425 min. These values might be realistic for 2
̄Φ systems, where the separation times are in the range of days, although the assumed droplet sizes might still be too large. Despite the coalescence-inhibition due to the surfactant, the coalescence especially under 3Φ conditions has a significant influence on the phase separation process and the resulting droplet sedimentation.
For theC4E2systems, the change of separation times in 2
̄Φ conditions, at the transition from 2
̄Φ to 3Φ systems and within the 3Φ temperature interval were investigated in [III]. In all cases, the separation times increase at the transition after reaching a minimum in 2
̄Φ conditions. If one of the excess phases is the continuous phase, strong deviations in separation time occur around the transition temperature, which is in accordance with the droplet size results (cf. Fig. 51). The separation times are summarized and discussed in [III]. Within the 3Φ temperature interval, the systems which formed multiple emulsions have a nearly constant separation time, unless the dispersion conditions change from multiple emulsions to single droplets or vice versa. These results again show, that the impact of temperature on interface rigidity and coalescence behavior is less pronounced in C4E2 systems in comparison to the other surfactants [III].
The application of the Henschke model to 3Φ systems is tested inC4E2systems at constant temperature of T
= 83◦C in the same way as in the drop size investigations. Therefore, the dispersion type is influenced by a variation ofαand γ (cf. Fig. 52). A proof-of-concept study and an analysis of fit parameters is performed in this study, while an advanced model validation and the possible application of Q under 3Φ conditions is planned to be published in an upcoming research paper. The adaption of the model to 3Φ conditions in a first step neglects a direct interaction of the phases since the model is solved subsequently for each phase. The dispersed phase fractions are defined with c = continuous phase and i = dispersed phase (i = 1 or 2) according to
φ0,i,norm= φ0,i
φ0,i+φ0,c, (48)
for the dispersed phase i and
φ0,c,norm,i= 1−φ0,1−φ0,2
1−φ0,1−φ0,2+φ0,i, (49)
for the continuous phase, depending on the dispersed phase i. Furthermore, the following condition has to be fulfilled:
1 =φ0,i,norm+φ0,c,norm,i. (50)
The definition ofφ0,i,normresults in an overestimation of the dispersed phase in comparison to the real systems, as schematically depicted in Figure 78 for the case of an organic continuous phase. The case A) with a constant dispersed phase fraction leads to a significantly higher continuous phase volume while case B) which is used here leads to an overestimation of both volumes while their ratio is constant.
Similar to the 2
̄Φ conditions, the observed sedimentation velocity is corrected to achieve the relative sedimen-tation velocity (cf. Eq. 40) by
vs,i =vs,observed,i
φ0,c,norm,i
. (51)
If two phases sediment into the same direction and both coalesce at the bottom or top of the vessel, the height of the completely coalesced upper or lower phase needs to be taken into account for the description of the height of the microemulsion phase viaφ0,mi,corrected =φ0,mi +φ0,aq/org. One obstacle in the experimental analysis is the turbidity of the systems which often prevents the analysis of the sedimentation and coalescence curves of both phases. In most cases only the upper, respectively lower curve can be determined without doubt. Since the Henschke model requires either the initial sedimentation velocity vs,i or vs,observed,i or the Sauter mean diameter of phase i, a simplification was used as illustrated in Figure 79. Since there is a lack of sedimentation curve data points for the organic phase, the dashed line connecting the initial height (h = 0 cm at t = 0 s) with the last data point of the separation for the organic phase leads to vs,approx,i=org is used. It is combined with a correction factor z that accounts for the different heights of the actual and the approximated sedimentation curves:
0 0.05 0.1 0.15 0.2
0 20 40 60
Height h [m]
Time t [s]
org/aq mi/aq
simulation (fit) appro . sedimentation org:
d3,2(t=0) = 567 μm, S2= 0.0065, rv* = 0.0482
mi:
d3,2(t=0) = 394 μm, S2= 0.0063, rv* = 0.2049
Fig. 79: Experimental and fitted phase separation in a 3Φ system with a continuous aqueous phase (water, 1-dodecene, C4E2,α= 0.3,γ= 0.2, T = 83◦C, V = 1.2 L, DN100, n = 700 rpm)
0 0.05 0.1 0.15 0.2
0 20 40 60 80 100 120
Height h [m]
Time t [s]
org/mi aq/mi
simulation (fit) approx. sedim.
org:
d3,2(t=0) = 582 μm, S2= 0.0046, rv* = 0.0944
aq:
d3,2(t=0) = 1195 μm, S2= 0.0026, rv* = 0.1018
0 0.05 0.1 0.15 0.2
0 20 40 60 80 100 120
Height h [m]
Time t [s]
org:
d3,2(t=0) = 516 μm, S2= 0.0066, rv* = 0.0501
aq:
d3,2(t=0) = 890 μm, S2= 0.0024, rv* = 0.1011 γ= 0.34
γ= 0.37
Fig. 80: Experimental and fitted phase separation in a 3Φ system with a continuous microemulsion phase (water, 1-dodecene,C4E2,α= 0.5,γ= 0.34 and 0.37, T = 83◦C, V = 1.2L, DN100, n = 700 rpm)
vs,observed,i=z·vs,approx,i. (52)
The value of z is determined using the experimental values of the Figures 79 and 80 with a step variation of 0.001 between z = 1.0 - 1.7 while applying the model each time and evaluating an error parameter S4(z) for each experiment j that is defined as
S4(z) =∑︂J
j=0|(tsep,sim,j,norm−1)|. (53)
S4(z) defines the error that is made using a specific z. For the aforementioned experiments, a value of z = 1.394 was determined which leads to errors in the range of less than 11% in separation time.
Figure 79 depicts the separation in a dispersion with a continuous aqueous phase without multiple emulsions.
The introduction of the aforementioned modifications leads to a good description of the experiment by the fitted simulation curve. The calculated Sauter mean diameters are higher than the maximum diameters in comparable agitated system (cf. Fig. 56). Hence, an intense coalescence in the first seconds after agitation stop can be assumed.
The fit parametersrv∗ of the two phases deviate by nearly an order of magnitude, but r∗v,org/aq = 0.0482 is in a similar range than r∗v,weighted = 0.0207 from the 2
̄Φ experiments. Higher deviations to rv,weighted∗ occur if the microemulsion is the continuous phase (Fig. 80). The fitting was performed under two different process conditions of γ = 0.34 and 0.37 to investigate the variation of the fitting parameters of both phases. A good description of the systems is possible and the fit parameters barely vary. Again, the calculated Sauter mean diameters during sedimentation d3,2(t=0) = 516 - 1195µm are higher than the droplet sizes under steady state conditions which are below d3,2,ss= 150µm.
0.1 1 10
1 2 3 4 5 6 7 8 9
Norm. separation time tsep,norm[-]
Experiment number [-]
experiment Henschke (Fit) Henschke + Q
org/aq org/mi org/mi
n↑ n↑ n↑
Fig. 81: Experimental, fitted and simulated phase separation of the upper interface in 3Φ systems (cf. Tab. 25)
To test the predictive power of the modified Henschke model, it is applied to different 3Φ systems listed in Table 25. Figure 81 shows the separation time needed to form the upper interface. An organic phase ascends in either continuous aqueous or microemulsion phases. The previous fits are illustrated in combination with results obtained using Q from the 2
̄Φ systems. The simulation leads to good results in org/aq systems due to their similarity to the 2
̄Φ condition, where also organic droplets rise in a continuous aqueous phase. The maximum error for org/aq systems is 30% is comparison to the 23% from 2
̄Φ conditions. However, the deviations are significantly higher in case of the org/mi phase combination. The summand Q does not represent the coalescence of the microemulsion phase well since the separation times are overestimated.
The experimental and simulated separation of the lower phases is illustrated in Figure 82. In these cases, the microemulsion ascends in an aqueous phase or the aqueous phase sediments in a continuous microemulsion phase. Although the fitting procedure itself was successfull, the application of the 2
̄Φ Q underestimates the separation time in all cases. The low interfacial tensions of the aq/mi combinations lead to smaller droplet sizes and/or lower coalescence rates than Q suggests. Although the deviation from the experimental results are high, all calculated values are quite similar which was also the case for the org/mi combinations in Figure 81. This leads to the conclusion that a specific Q for these phase combinations might lead to adequate simulation results.
The last case investigated here is an organic continuous phase with multiple emulsions (Fig. 83). The fitting procedure was performed in two different geometries neglecting the presence of multiple emulsions. The devia-tions in fit parameters are higher than in the case of a continuous microemulsion phase, which is attributed to the multiple emulsions and a smaller number of experimental data points.
In this system, the single aqueous and microemulsion droplets and the multiple emulsion droplets sediment.
Similar to the 2
̄Φ conditions, the first seconds after agitation stop influence the Sauter mean diameters of
0.001 0.01 0.1 1 10
1 2 3 4 5 6 7 8 9
Norm. separationtime tsep,norm[-]
Experiment number [-]
experiment Henschke (Fit) Henschke + Q
mi/aq aq/mi aq/mi
n↑ n↑ n↑
Fig. 82: Experimental, fitted and simulated phase separation of the lower interface in 3Φ systems (cf. Tab. 25)
0 0.05 0.1 0.15 0.2
0 20 40 60 80
Height h [m]
Time t [s]
0 0.05 0.1 0.15 0.2
0 20 40 60 80
Height h [m]
Time t [s]
mi/org aq/org simulation (fit) appro . sedim.
mi:
d3,2(t=0) = 302 μm, S2= 0.0541, rv* = 0.0515 aq:
d3,2(t=0) = 442 μm, S2= 0.0015, rv* = 0.024
mi:
d3,2(t=0) = 275 μm, S2= 0.0487, rv* = 0.0567 aq:
d3,2(t=0) = 430 μm, S2= 0.0023, rv* = 0.0191 DN100
DN150
Fig. 83: Experimental and fitted phase separation in a 3Φ system with a continuous organic phase and multiple emulsions (water, 1-dodecene,C4E2,α= 0.5,γ = 0.2, T = 83◦C, V = 1.2 L (DN100) and V = 2.5 L (DN 150), n = 700 rpm
Table 25: Experiment overview in 3Φ systems (T = 83◦C, DN 100, V = 1.3 L) Exp. No. α[-] γ[-] n [rpm] Exp. No. α[-] γ[-] n [rpm]
1 0.3 0.2 500 6 0.5 0.34 900
2 0.3 0.2 700 7 0.5 0.37 500
3 0.3 0.2 900 8 0.5 0.37 700
4 0.5 0.34 500 9 0.5 0.37 900
5 0.5 0.34 700
the 3Φ systems, as is depicted in [IV] for a multiple emulsion. The overall Sauter mean diameter in systems with the same composition as in Fig. 83 increases by a factor of approximately 2.5 in the first five seconds after agitation stop and the number of droplets-per-droplet also rises. This can induce dense-packed zones formation in a different sequence which can be used as an indicator for the initial dispersion conditions [IV]. In subsequent work, the predictive power of the Henschke model in 3Φ systems should be evaluated with additional experiments. The application of the Henschke model to 3Φ conditions indicates, that the introduction of Q is necessary to account for the deviations between experimental steady state and the Sauter mean diameter during sedimentation. The phase separation can then be used as key experiments that provide information on the dispersed phase fraction, dispersion conditions and the initial drop sizes that occur in agitated systems.
6 Summary and Outlook
Within the last years, the hydroformylation process was established as a promising alternative to industrial approaches. The hydroformylation with water, 1-dodecene and Marlipal 24/70 microemulsion systems can be performed in continuously operated miniplant with a yield of 35%, an n-iso selectivity of 99:1 and less than 1 ppm of catalyst loss [116]. The investigations on drop size distributions and phase separation behavior were used and can also be used in future investigations for process optimization and an extension towards other chemical reactions. In this work, the phase behavior and physical properties of different microemulsion systems were determined as a basis for investigations on dispersion and phase separation. The methodical approach for the identification of the dispersion conditions in agitated systems presented in this work is valid for all microemulsion systems. The description of drop size distributions and dynamic phase separation under 2
̄Φ conditions is possible with population balance equations and the Henschke model, although the impact of different surfactant types usually necessitates system-specific fit parameters. For the Henschke model a modification was introduced to account for the changes in drop diameter in the first seconds after agitation stop. Therefore, steady state Sauter mean diameters or simulations with population balance equations can be used to determine the initial conditions for phase separation simulations and vice versa. In 3Φ systems, a fundamental understanding of the interaction of the process parameters and their influence on drop sizes and phase separation was achieved although the prediction of droplet sizes in these systems will most likely stay a purely academic problem for the next years. The formation of complex droplets and their influence on dispersion and phase separation has been thoroughly investigated in this work and the main influencing parameters were identified. The experimental effort to determine all relevant parameters still exceeds the benefit of the current rough prediction of drop sizes in a limited process parameter range. For most non-ionic surfactant systems, the gas bubble size during reaction most likely is the major influencing factor on the reaction rates. Therefore, the gas/liquid mass transfer should be investigated in future studies. The Henschke model is as very promising approach to describe the phase separation in 3Φ conditions with different dispersion conditions. Fitting parameters can be obtained for all phase combinations and future investigations with an optimized experimental setup can provide data for a thorough model validation.
0 1000 2000 3000 4000 5000
40 50 60 70 80 90 100 Separation time tsep[s]
Temperature T [°C]
pure Na2SO4 methanol
3φ 3φ
2φ 3φ
0 25 50 75 100 125 150 175 200
20 40 60 80 100
Separation time tsep[s]
Temperature T [°C]
pure Na2SO4
3φ 3φ 2φ
a)
b)
Fig. 84: Experimental separation times as a function of temperature with and without additives for Marlipal 24/70 systems (left) andC4E2 (right)
0 50 100 150 200 250 300 350
20 30 40 50 60 70 80 90
Separation time tsep[s]
Temperature T [°C]
with catalyst without catalyst
catalyst pure
0 20 40 60 80 100
75 80 85 90
Separation time tsep[s]
Temperature T [°C]
with catalyst without catalyst with catalyst without catalyst
3φ 3φ
catalyst pure
a) b)
Fig. 85: Separation times inC4E2 systems with and without active catalyst complex.
An aspect which has barely been addressed in this work is the influence of additives such as sodium sulfate (N a2SO4), catalyst complex or methanol on the drop size distributions and separation times. Sodium sulfate was used in the miniplant to support the phase separation process by promoting coalescence in Marlipal 24/70 systems [94, 115]. The catalyst complex is known to be surface-active, since the active species has an amphiphilic molecular structure. Methanol can for example be used in the methoxycarbonylation or similar reactions, although the application of microemulsion systems is not always mandatory [128]. Figure 84 shows separation times for the additive-free system with Marlipal 24/70 and in presence ofN a2SO4 and methanol, respectively.
The influence of additives on the position of the 3Φ areas of the surfactant systems are in accordance with the HLD equation, since the 3Φ area is shifted towards lower temperatures by salt and to higher temperatures by methanol. Furthermore, the separation time is smaller in systems with salt in comparison to the additive-free system. Adding sodium sulfate to theC4E2systems also leads to a decrease of the 3Φ temperature interval, but the effect on separation times is negligible, respectively the separation time slightly increases (Fig. 84, right).
This is one effect where the model character of theC4E2 is not sufficient to describe the qualitative behavior of the longer-chained non-ionic surfactants. Similar results and a deviation of the behavior with Marlipal 24/70 andC4E2were found for the impact of the catalyst complex on phase separation. In Marlipal 24/70 systems, the presence of active catalyst increases the separation time in 3Φ systems according to results in the miniplant [94].
Experiments in C4E2 systems, however, show the opposite result (Fig. 85). At 2
̄Φ conditions, the separation times with catalyst are larger than in systems without surfactant, similar to Marlipal 24/70 systems and the effect of salt addition. However, after transition to 3Φ conditions the catalyst promotes coalescence processes and leads to smaller separation times. The quantitative description of drop size and phase behavior results in presence of additives is a challenge that has barely been investigated.
Acknowledgments
This work is part of the Collaborative Research Centre InPROMPT coordinated by the Technische Universität Berlin. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged (TRR 63).
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