Article pubs.acs.org/IECR
Direct Conversion of n‑Butane to Isobutene in a Membrane Reactor: Thermodynamic Analysis H. A. Al-Megren,† G. Barbieri,*,‡ I. Mirabelli,§ A. Brunetti,‡ E. Drioli,‡,§ and M. C. Al-Kinany† ‡
Institute on Membrane Technology (ITM-CNR), National Research Council c/o The University of Calabria, Cubo 17C, Via Pietro Bucci, 87036 Rende CS, Italy § Chemical Engineering and Materials Department, The University of Calabria, Cubo 45/A, Via Pietro Bucci, 87036 Rende CS, Italy † King Abdulaziz City for Science & Technology, P.O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia ABSTRACT: Isobutene is an important intermediate compound in the petrochemical industry for the production of polymers (butyl rubber, polybutene, and isoprene) and methyl tert-butyl ether. In this work, the n-butane dehydroisomerization reaction in a membrane reactor (MR) was investigated by thermodynamic analysis in a wide range of temperatures, reaction pressures, and equilibrium hydrogen partial pressures, by means of a simplified reaction scheme. The shift of the equilibrium conversion in an MR was evaluated by taking into account the chemical reaction equilibrium and the permeative equilibrium through a 100% hydrogen-selective membrane. The evaluated limits imposed by thermodynamics on an MR are much wider than those of a traditional reactor so that a conversion of about 7 times higher could be obtained over that of the traditional process under a set of operating conditions. This gives a powerful indication on how the use of an MR can extend the thermodynamic limits of this reaction, in terms of conversion, even at thermodynamically unfavorable operating conditions.
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products measured7 were normal butenes, isobutane, and isobutene.
INTRODUCTION Isobutene is an important intermediate in the petrochemical industry; about 650000 barrels were produced in the USA alone in 2011.1 The forecast is for the worldwide demand for isobutene to increase; thus, the interest in further improving its production is great. Furthermore, with the ever-increasing importance of hydrogen, there is extra incentive to research especially dehydrogenative-type reactions of light alkanes because of their high H/C molar ratio. Isobutene, owing to the presence of its reactive double bond, can take part in various chemical reactions, such as hydrogenation, oxidation, and other additions, resulting in a great variety of products. One of the most widely used reactions in the industry is the addition of methanol or ethanol to isobutene, which leads to two well-known fuel additives: methyl tert-butyl ether or ethyl tert-butyl ether. Besides this important application, isobutene is also used in a variety of polymerization reactions, as a monomer or a copolymer for the formation of various products. One such product is, for instance, butyl rubber, a polymer of isobutene and isoprene. Another promising application of isobutene is the production of antioxidants for food.2 Currently, isobutene is obtained on a large scale from crude oil, by petrochemical cracking with the butadiene removed, and from butanes, supplied from natural gas reserves and refinery streams.3,4 Butanes are the preferred raw materials for the production of isobutene, and the most common industrial method is by isomerization and catalytic dehydrogenation5 to isobutene. The current process practiced is a two-step process, comprising n-butane isomerization and successive isobutane dehydrogenation.6 n-Butane dehydroisomerization (reaction III) involves dehydrogenation of n-butane (reaction I) and successive isomerization to isobutene (reaction II). The main © 2013 American Chemical Society
n‐butane = n‐butene + H 2 ΔHreaction(at 25 °C) = 130 kJ mol−1
(I)
n‐butene = isobutene ΔHreaction(at 25 °C) = −17 kJ mol−1
(II)
n‐butane = isobutene + H 2 ΔHreaction(at 25 °C) = 113.7 kJ mol−1
(III)
An interesting alternative is a direct one-step process, allowing direct conversion of n-butane to isobutene.8,9 Some bifunctional catalytic systems, usually considering zeolite (ZSM5 or MCM22)-supported platinum catalysts or sulfate zirconia, have been reported in the literature as successful catalysts for such direct conversion10,11 including the membrane reactors (MRs). Over the years, the focus of the chemical and industrial process has been addressed toward the development and application of integrated processes, combining the reaction and separation into one single unit, pursuing the logic of Process Intensification.12,13 Reduction in the equipment number and size, improvement of the process efficiency, and, hence, a better process economy are some of the expected benefits.14−16 The Special Issue: Enrico Drioli Festschrift Received: Revised: Accepted: Published: 10380
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brium in an MR was described using the same model as that proposed by Barbieri et al.32,33 They used a tank-in-series model to describe the equilibrium of an ideal palladium-based MR for methane steam reforming, adding an extra constraint that takes into account the permeative equilibrium. The possibility of defining how much an MR, with respect to a TR, can broaden the thermodynamic constraints for this reaction was analyzed. In fact, because the equilibrium conversion is the maximum conversion achievable, it can be useful from a thermodynamic point of view to evaluate the gain that the MR allows. The effects of the temperature, reaction pressure, and equilibrium hydrogen pressure on the MR and TR equilibrium conversion were investigated. Equilibrium conversions achievable in MRs and TRs were discussed.
MR application, which combines the reaction and separation in the same unit, meets this goal very well. Moreover, the possibility of exceeding the equilibrium constraint of traditional reactors (TRs) in reversible reactions such as dehydrogenations has attracted wide attention.17 For dehydrogenative-type reactions, the application of MRs, where hydrogen can be removed, with high selectivity, from the reaction mixture, is an interesting strategy. For such reactions, thermodynamically favored by low temperature, the removal of a product can shift the equilibrium, improving the conversion achievable. MRs were investigated, for instance, for the dehydrogenation of ethane, propane, and isobutane, and the authors experimentally demonstrated that conversion of these light hydrocarbons was substantially higher; however, high conversions still require improvements and a higher temperature.18−20 Isobutane dehydrogenation using catalytic MRs was previously studied by a number of researchers.21−23 Liang and Hughes24 studied isobutane dehydrogenation using a palladium/silver composite MR and obtained increases in the selectivity and yield with respect to a fixed-bed TR. The dehydrogenation of isobutene was studied in a carbon molecular sieve membrane, achieving 40% conversion at 500 °C, compared with 32% at a closed-system equilibrium.21 The benefits of higher conversion as a result of coupling a dehydrogenation reaction with a hydrogenation reaction on the permeate side were also shown by Abashar and Al-Rabiah.19 In all of these reactions, hydrogen is produced as a coproduct of another valuable chemical. When these reactions are carried out in an MR, the hydrogen can be recovered at high purity directly on the permeation side, as a valuable chemical. Many types of membranes 25 have been investigated for this type of application. For example, isobutane dehydrogenation has been studied using γ-alumina, zeolite MFI, palladium/silver and palladium, dense silica, and carbon molecular sieve membranes.26−29 In the present work, the exploitation of an MR to carry out the direct conversion of n-butane to isobutene was investigated for the reactive system including reactions I−III. Isobutene production involves many more reactions and chemical species than those included in reactions I−III; the reactions and chemical species lists also depend on the operating conditions adopted and the catalyst used. The simplified reactive scheme allows a first/comprehensive analysis of hydrogen removal on the reactive equilibrium of the reduced chemical system considered. The palladium/silver-based membrane taken into account in this work, owing to its infinite selectivity toward hydrogen, allows the recovery of pure hydrogen on the permeate side; thus, no further separation step is required. For palladiumbased alloy commercial membranes, typical permance values30 are 200−300 μmol m−2 s−1 Pa−0.5 and significantly lower for laboratory-produced membranes.31 An evaluation of the membrane area required for hydrogen transport from one membrane side to the other during the reaction depends on several different factors such as, for instance, the space velocity, catalyst, etc., giving a time-dependent progress of the reaction and permeation. However, these time-related variables cannot directly be used in the analysis proposed in this work even though some comparison will be discussed later on. The shift in equilibrium conversion as a result of the selective extraction of hydrogen in the MR was evaluated by thermodynamic analysis, taking into account the chemical reaction equilibrium and permeative equilibrium32 through the membrane. The equili-
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THERMODYNAMICS n-Butane dehydrogenation is an endothermic reaction occurring with an increment of mole numbers; thus, equilibrium conversion is favored by a high temperature and low pressure. To evaluate the number of moles present at equilibrium, dehydrogenation and isomerization can be considered as a series reaction system. Thermodynamic analysis was performed on the basis of significant assumptions including (1) no occurrence of side reactions and (2) only linear butene and isobutene production. Even though a real reactive system involves more reactions and chemical species, as a first step, a simplified scheme allows comprehensive analysis of the conversion shift owing to selective product removal from the reaction volume. Thermodynamics of a TR. The determination of the number of moles in a TR is based on thermodynamic and stoichiometric calculations. This is a closed reactor, no inlet or outlet streams; it could be a “batch reactor” in the case of perfect mixing. Table 1 summarizes the mass balance for the Table 1. Number of Moles Determination in the Reactive State and at Equilibrium in a TR number of moles n-butane initial state reactive state of reaction I reactive state of reaction II equilibrium state (only after reactions I and II) total number of moles
n0 −n0x
n0(1 − x)
n-butene
isobutene hydrogen
n0x
n0 x
−n0xw
n0 xw
n0 x(1 − w)
n0 x w
n0 x
ntotal = n0(1 + x)
species involved in the reactions, considering the reactions as a series reaction system. x is the n-butane conversion for dehydrogenation (reaction I) and w the n-butene conversion for isomerization (reaction II). At the initial stage, only n0 moles of n-butane are present. Then reaction I converts n0x moles of n-butane to n0x moles of n-butene and n0x moles of hydrogen (line 2). Reaction II converts a fraction w of the total n0x moles of n-butene in isobutene (line 3). The mole numbers of each species at equilibrium, for reactions I and II, are a function of the conversion degrees (x and w) of both reactions. The overall number of moles is increased by reaction I but does not depend on the conversion of reaction II. The mole fractions, at 10381
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each species at equilibrium, reactive and permeative, is a function of the conversion degrees (x and w) of both reactions and the hydrogen fraction permeated (z). Taking into account only the reaction volume, the total number of moles for the MR is lower than that for the TR because part of the hydrogen produced is in the permeation volume (see Table 3, last row). Table 4 reports the mole fractions of all species in reactive and permeating equilibrium states as a function of the three variables characterizing the MR equilibrium.
equilibrium, of all of the species involved in the reactions are reported in Table 2. Table 2. Mole Fractions in the State of Equilibrium of Reactions I and II species
molar fraction
n-butane
n0(1 − x) 1−x = n0(1 + x) 1+x
n-butene
n0x(1 − w) x(1 − w) = n0(1 + x) 1+x
isobutene hydrogen
Table 4. Mole Fraction in the Equilibrium State
n0xw xw = n0(1 + x) 1+x
species
mole fraction
n-butane
n 0x x = n0(1 + x) 1+x
n0(1 − x) 1−x = n0(1 + x − xz) 1 + x − xz
n-butene
n0x(1 − w) x(1 − w) = n0(1 + x − xz) 1 + x − xz
isobutene
n0xw xw = n0(1 + x − xz) 1 + x − xz
hydrogen
n0x(1 − z) x(1 − z) = n0(1 + x − xz) 1 + x − xz
Thermodynamics of an MR. Thermodynamic analysis of an MR is similar to that of a TR. In an MR, the overall reaction unit can be considered as divided into two sectors: one part is the reaction volume, and the other is the permeation volume. Both sections are kept at a constant pressure and closed; hydrogen can be transported only between these two volumes. In this case, the hydrogen selective membrane (e.g., palladium/ silver alloy) allows removal of the hydrogen from the reaction volume. A part of the hydrogen produced permeates through the membrane into the permeation side. As long as the hydrogen partial pressure in the reaction volume is greater than the partial pressure in the permeate side, there is permeating flux. When the partial pressures of hydrogen, on both membrane sides, are equal, no hydrogen permeation happens, and then permeative equilibrium is reached.34 In the permeative equilibrium state, the partial pressure of hydrogen on both membrane sides is also equal to the partial pressure of hydrogen at equilibrium; this condition is expressed by eq 1. Therefore, the thermodynamic equilibrium in an MR also depends on the permeative equilibrium in addition to the reactive equilibrium. equilibirum permeate side reaction side Phydrogen = Phydrogen = Phydrogen
Table 1 (line 3) allows the composition of a TR to be known, for a given initial condition (n0), when the conversions x and w of reactions I and II are evaluated. Table 3 gives the same information for an MR when also the permeating equilibrium state is achieved; hydrogen permeation is accounted for by “z”. The conversions of reactions I and II are calculated through the equilibrium constants also including the constraint introduced by eq 1 for the permeative equilibrium state. Tables 5 and 6 report the expressions of the equilibrium constants as a function of “x”, “w”, and “z” and the reaction Table 5. Equilibrium Constant for Reaction I at Reactive and Permeative Equilibrium, as a Function Also of x, w, and z KP1 =
reaction side reaction side Phydrogen Pn ‐ butene side Pnreaction ‐ butane
(1)
In this case, hydrogen permeates through a 100% hydrogenselective membrane, and it is recovered in the permeation side (Table 3): the MR allows separation from the reaction volume of one of the products. Table 3 reports the reactive states for reactions I and II (see Table 1) and also includes the information for hydrogen permeation through the variable “z”, which is the fraction of hydrogen in the permeation side compared to the total hydrogen produced. The difference with respect to a TR is introduced by the negative term referred to in hydrogen permeation. Therefore, the number of moles of
KP1 =
(2a)
reaction side reaction side yhydrogen yn ‐ butene P reaction side reaction side yn ‐ butane
(2b)
x(1 − z) x(1 − w) 1 + x − xz reaction side KP1 = P 1 + x − xz 1 + x − xz 1−x
(2c)
2
KP1 =
x (1 − z)(1 − w) reaction side P (1 + x − xz)(1 − x)
(2d)
pressures for reactions I and II, respectively; and eq 4 gives the hydrogen partial pressure at the equilibrium state.
Table 3. Number of Moles Determination at Reactive and Permeative Equilibrium in an MR number of moles n-butane Table1 (extract)
initial state reactive state of reaction I reactive state of reaction II
permeation state permeative and reactive equilibrium states total number of moles
n-butene
n0 −n0x
n0(1 − x) n0(1 + x − xz) 10382
isobutene
hydrogen −n0x
n0x −n0xw
n0xw
n0x(1 − w)
n0xw
−n0xz n0x(1 − z)
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consequence, back-permeation occurs. Higher temperatures promote conversion, and thus the partial pressure of hydrogen in the reaction side is higher, thus depleting the backpermeation. As shown in Figure 1, the pressure negatively affects the equilibrium conversion of TR (dashed curves). For instance, at 600 °C and 1 bar, the equilibrium conversion is around 0.64, but it is much lower (i.e., 0.25) at a reaction pressure of 10 bar. On the contrary, the equilibrium conversion of the MR does not change with variation of the reaction pressure at a set equilibrium hydrogen pressure. The solid curve is the equilibrium conversion achievable in an MR at an equilibrium hydrogen partial pressure of 0.1 bar, for any reaction pressure. The comparison of MREC and TREC is shown in Figure 2, for an equilibrium hydrogen partial pressure of 1 bar. As a
Table 6. Equilibrium Constant for Reaction II at Reactive and Permeative Equilibrium, as a Function Also of x, w, and z KP2 =
KP2 =
reaction side Pisobutene side Pnreaction ‐ butene
(3a)
reaction side yisobutene side ynreaction ‐ butene
(3b)
KP2
n0xw = n0x(1 − w)
KP2
w = 1−w
equilibrium Phydrogen = P reaction side
(3c)
(3d)
x(1 − z) 1 + x − xz
(4)
The mathematical set of equations results in three equations (e.g., eqs 2d, 3d, and 4) that can be resolved for obtaining the three variables (x, w, and z) using the equilibrium constants and reaction pressure values as input.
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RESULTS AND DISCUSSION On the basis of the analysis carried out, the effects of the temperature, reaction pressure, and equilibrium hydrogen partial pressure were investigated. Figure 1 shows the n-butane
Figure 2. n-Butane dehydroisomerization (reaction III): equilibrium conversion of MRs (solid line) and TRs (dashed lines) as a function of the temperature, at different reaction pressures. For the MR, equilibrium hydrogen partial pressure = 1 bar.
general trend, in the range of reaction pressure between 1 and 20 bar at a low temperature, the MR equilibrium conversion is lower than the TR one. Moreover, under this condition, the MREC is always lower than the TREC when the reaction pressure in the TR is low: at 1 bar. As already mentioned, the high equilibrium hydrogen partial pressure causes the hydrogen back-permeation, which depletes the equilibrium conversion obtainable in an MR. However, at a higher temperature and reaction pressure, the MREC profile exceeds the TREC, also at high Pequilibrium hydrogen . The temperature at which the MREC exceeds the TREC is also determined by the reaction pressure: it is higher and higher as the pressure is lower and lower, for instance, 480, 525, and 557 °C at 20, 10, and 5 bar, respectively. As already mentioned, n-butane conversion for the MR actually depends on the equilibrium hydrogen partial pressure, as shown in Figure 3. The higher the equilibrium hydrogen partial pressure, the lower the conversion. This is due to the lower amount of hydrogen transported on the other membrane side. Even though, with an increase in this parameter, the MREC profiles seem similar to those shown by the TR, by variation of the reaction pressure (see Figure 1, dashed curves), the effect of the equilibrium hydrogen partial pressure is quite different. For a clearer understanding, Figure 4 shows the nbutane equilibrium conversion as a function of the reaction pressure at a set equilibrium hydrogen partial pressure of 1 bar. The reaction pressure does not affect the MR equilibrium conversion, although the TR significantly undergoes a negative effect of the pressure. It does not change the MR conversion
Figure 1. n-Butane dehydroisomerization (reaction III): equilibrium conversion of MRs (solid line) and TRs (dashed line) as a function of the temperature, at different reaction pressures. For the MR, equilibrium hydrogen partial pressure = 0.1 bar. Symbols refer to experimental measurements with a feed composition (on molar basis) of 10% n-butane, 20% hydrogen, 70% argon: (●) 557 °C, 1.8 bar, WHSV = 9.9 h−1;10 (▼) 532 °C, 1 bar, WHSV = 12.5 h−1;7 (◇) 500 °C, 1 bar, WHSV = 23 h−1;7 (Δ) 550 °C, 1 bar, WHSV = 10 h−1;35 (▶) 500 °C, vacuum mode.33
equilibrium conversion for the overall reaction (reaction III) as a function of the temperature, in an MR and a TR. The equilibrium conversions, for both MR and TR, are higher at a higher temperature because the reaction is endothermic, and the MR does not affect the conversion dependence on the temperature. The MR equilibrium conversion (MREC) is higher than the TR equilibrium conversion (TREC). Only when the temperature is under 400 °C does the TREC exceed the MREC owing to the back-permeation occurring in the MR. In the permeate, the hydrogen partial pressure is, in fact, equal to 1 bar. On the other side of the membrane, which is the reaction volume, the partial pressure of hydrogen is lower than 1 bar owing to the very low value of conversion; as a 10383
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MREC (5) TREC This variable plotted as a function of the temperature (Figure 5) identifies two regions: one where the MREC is higher than
Figure 3. n-Butane dehydroisomerization (reaction III): MREC as a function of the temperature at different equilibrium hydrogen partial pressures.
Figure 5. Comparison of MREC and TREC as a function of the temperature at different reaction pressures. For the MR, the equilibrium hydrogen partial pressure is 1 and 0.1 bar for the leftand right-side sections, respectively.
the TREC and another where the opposite occurs. At a high pressure, the MREC is higher than the TREC because the MR successfully operates. The MR, in fact, can operate at a higher pressure, without “paying” any penalty in terms of conversion as instead happens for the TR. Therefore, the advantages offered by the MR with respect to the TR increase by increasing the reaction pressure. The TREC is greater than the MREC only in the region at low temperature and pressure, where the MR cannot operate because of the back-permeation. In addition, the maximum improvement in MR conversion, compared to the TR one, occurs at a temperature of 625 °C, for each reaction pressure investigated (Figure 5, left side). The curves show a maximum as a function of the temperature owing to the thermodynamic effect on the reaction at both low and high temperature. In fact, at low temperature, the MREC is similar or lower than the TREC because of the thermodynamics and consequently the back-permeation. In the high temperature range, because the reaction is thermodynamically favored, both MREC and TREC tend to a unitary value; thus, there is no large margin for MR advantages. At the best condition (625 °C and 20 bar), the MREC shows a value more than twice that of the TREC. The distance of MREC from TREC is greatly increased for lower equilibrium hydrogen partial pressure. For an equilibrium hydrogen partial pressure of 0.1 bar (Figure 5, right side), in the whole temperature range investigated, the MREC exceeds the TREC, also at low reaction pressure: the MREC is 3−7 times higher than the TREC. In addition, at this lower equilibrium hydrogen partial pressure, the maximum improvement in the MREC is shifted at lower temperatures (500 °C). An MR operating at 500 °C and 20 and 0.1 bar as reaction and equilibrium hydrogen pressures, respectively, allows one to obtain an equilibrium conversion approximately equal to 50%, 7 times higher than that of a TR, equal to ca. 7%. Considering that the equilibrium conversion is the thermodynamic limit, which sets the maximum possible conversion, on the basis of the above analysis, it can be concluded that the use of an MR can powerfully enhance these dehydrogenative-type reactions.
Figure 4. n-Butane dehydroisomerization (reaction III): equilibrium conversion of MRs (solid lines) and TRs (dashed lines) as a function of the reaction pressure, at different temperatures. For the MR, equilibrium hydrogen partial pressure = 1 bar. Symbols refer to experimental measures with a feed composition (on a molar basis) of 10% n-butane, 20% hydrogen, and 70% argon: (●) 557 °C, 1.8 bar, WHSV = 9.9 h−1;10 (▽) 532 °C, 1 bar, WHSV = 12.5 h−1;7 (◆) 500 °C, 1 bar, WHSV = 23 h−1;7 (▲) 550 °C, 1 bar, WHSV = 10 h−1.35
owing to the removal of hydrogen from the reaction volume that overtakes this negative effect (Figure 4). Moreover, the MR allows the same conversion as a TR to be achieved, operating at the same pressure but at a lower temperature. For instance, the MR operating at 550 °C has the same conversion as a TR operating at 625 °C, for a reaction pressure of 20 bar. Figures 1 and 4 also show reaction conversions of experimental measurements,9,28 using two catalysts, for the same feed composition. These values, close to the ones calculated by thermodynamic analysis, confirm the validity of the assumptions of the reaction scheme taken into account in the proposed analysis; that is, the main reaction products are linear butene and isobutene, and the side reactions have a secondary role. For estimation of the best condition allowing the highest reachable conversions in an MR compared to the TR one, the evaluation of the distance between the MREC and TREC, operating at the same conditions, appears of interest. A specific value was used to quantify this distance, as reported in eq 4: the one calculated in equilibrium condition, of the conversion index, already defined27 as the ratio of actual conversion in an MR and that in a TR. 10384
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CONCLUSION The shift in equilibrium conversion of dehydroisomerization reactions as a result of selective extraction of hydrogen was evaluated in a MR by taking into account simultaneously the chemical reaction and permeative equilibria. The n-butane dehydroisomerization is an endothermic reaction, and therefore high temperature raises the equilibrium conversion, also in an MR. However, the MR shows the largest improvement with respect to the TR at high reaction pressure and lowest equilibrium hydrogen partial pressure. The peculiarity in the MR of the selective hydrogen removal allows exploitation of the positive effect of the reaction pressure on the hydrogen permeation and consequently on the conversion, which, on the contrary, in a TR decreases because the reaction has an increase in the number of moles. Moreover, the hydrogen partial pressure at the equilibrium state is another variable affecting the equilibrium conversion in an MR. A lower equilibrium hydrogen partial pressure leads to a higher removal of the amount of hydrogen from the reaction volume, which promotes the equilibrium shift. The analysis performed also provides an estimation of the best conditions, in the range of temperature and pressure investigated, which allows one to achieve the highest reachable conversions in an MR compared to that of the TR one. An MR operating at 500 °C and 20 and 0.1 bar as reaction and equilibrium hydrogen partial pressures, respectively, shows a conversion 7 times higher than that obtainable in a TR.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS King Abdulaziz City for Science & Technology is gratefully acknowledged for funding the project “Membrane reactor for petrochemical processes”.
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LIST OF SYMBOLS n = total number of moles x = n-butane equilibrium conversion w = n-butene equilibrium conversion z = ratio of hydrogen in the permeate side and hydrogen produced KP = equilibrium constant expressed in terms of the partial pressure P = pressure, bar y = molar fraction
Subscript/Superscript
0 = refering to the initial value equilibrium = refering to equilibrium total = refering to the total
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REFERENCES
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