An Integrated Membrane Process for Butenes

processes Article

An Integrated Membrane Process for Butenes Production Leonardo Melone, Adele Brunetti, Enrico Drioli and Giuseppe Barbieri * Institute on Membrane Technology (ITM-CNR), National Research Council c/o, The University of Calabria, Cubo 17C, Via Pietro BUCCI, 87036 Rende CS, Italy; [email protected] (L.M.); [email protected] (A.B.); [email protected] (E.D.) * Correspondence: [email protected]; Tel.: +39-0984-492029 Academic Editor: Fausto Gallucci Received: 11 August 2016; Accepted: 8 November 2016; Published: 15 November 2016

Abstract: Iso-butene is an important material for the production of chemicals and polymers. It can take part in various chemical reactions, such as hydrogenation, oxidation and other additions owing to the presence of a reactive double bond. It is usually obtained as a by-product of a petroleum refinery, by Fluidized Catalytic Cracking (FCC) of naphtha or gas-oil. However, an interesting alternative to iso-butene production is n-butane dehydroisomerization, which allows the direct conversion of n-butane via dehydrogenation and successive isomerization. In this work, a simulation analysis of an integrated membrane system is proposed for the production and recovery of butenes. The dehydroisomerization of n-butane to iso-butene takes place in a membrane reactor where the hydrogen is removed from the reaction side with a Pd/Ag alloys membrane. Afterwards, the retentate and permeate post-processing is performed in membrane separation units for butenes concentration and recovery. Four different process schemes are developed. The performance of each membrane unit is analyzed by appropriately developed performance maps, to identify the operating conditions windows and the membrane permeation properties required to maximize the recovery of the iso-butene produced. An analysis of integrated systems showed a yield of butenes higher than the other reaction products with high butenes recovery in the gas separation section, with values of molar concentration between 75% and 80%. Keywords: integrated process; membranes; iso-butene production

1. Introduction Nowadays, the great demand for increased energy and fine chemicals species has pushes the industry towards more sustainable production and development. Process Intensification (PI) Strategy [1] has acquired an important role in the industrial development, pursuing volume reduction of devices and plant, minimization of energy demand and better raw materials exploitation [2]. The challenge of membrane engineering is to redesign production processes in the logic of PI and this can also be successfully applied in the petrochemical industry. The annual world production of butenes reached 132 million tons [3]. In particular, iso-butene is the most important butene isomer because it is used in many syntheses, such as butyl rubbers and additives for gasoline (e.g., Methyl tert-butyl ether) production. Today, the most widespread process for butenes production is FCC. A hydrocarbon stream is fed into the FCC unit and an olefins-rich stream is recovered as a product stream. This process produces 70% of the world butenes supply. Another important process for butenes production is steam cracking, which satisfies 26% of the world demand. Only 4% of butenes is produced by dehydrogenation and dehydroisomerization of the butane. The latter technologies are very important in PI logic since they produce specific butenes, e.g., iso-butene. Conversely, the aim of FCC and steam cracking is ethylene Processes 2016, 4, 42; doi:10.3390/pr4040042

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Processes 2016, 4, 42

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and propylene production and the butenes are only reaction by-products. The dehydroisomerization system, instead, uses two different reactors in series. The first unit allows the dehydrogenation of the butane, and then the downstream of the reactor containing butenes is fed into the second reactor. Here, the butenes become specific butene isomers, for example iso-butene. Today, four industrial processes are used for butane conversion to butenes. The Oleflex process developed by UOP uses a fluidized bed reactor operated at 2 bar and variable temperature in the range 580–650 ◦ C. STAR, Clariant and Linde-BASF, instead, use packed bed reactors in the same temperature range as the Oleflex process. In particular, the STAR system operates at 5 bar, whereas, the Clariant and Linde-BASF operate at a reaction pressure of 0.3–0.5 bar and 1 bar, respectively [3]. An interesting alternative to the two-step processes [4] is a direct one-step process, allowing the direct conversion of n-butane to iso-butene [5,6]. Recently, various bi-functional catalytic systems were reported in the literature, usually considering Pt supported zeolites, as successful catalysts for such direct conversion [7,8]. Meanwhile, the use of membrane reactors (MRs) was proposed. This integrated approach, which combines the reaction and separation in a single unit, fits the targets of Process Intensification well [9,10]. Furthermore, the possibility of exceeding the equilibrium constraint of traditional reactors (TRs) in reversible reactions such as dehydrogenations is quite appealing [11]. For dehydrogenation-type reactions, the application of MRs, where hydrogen can be removed with high selectivity from the reaction mixture, is an interesting strategy. For these reactions, thermodynamically favored by low temperature, the removal of a product shifts equilibrium, improving the conversion significantly. Various researchers reported on iso-butane dehydrogenation using catalytic MRs [12–14] considering many types of membranes [15] such as γ-alumina, zeolite MFI, Pd/Ag and Pd, dense silica and carbon molecular sieve membranes [16–19]. In most of their studies a conversion above the equilibrium one of a TR was obtained owing to hydrogen removal through the membrane. In particular, the Pd-based membrane, owing to its infinite selectivity toward hydrogen, promotes the recovery of pure hydrogen not requiring further separation steps. Moreover, its removal from the reaction volume shifts the equilibrium conversion according to the Le Chatelier principle. MR equilibrium conversion was evaluated by Marigliano et al. [20], Brunetti et al. [21] and Barbieri et al. [22] taking into account chemical reaction equilibrium and permeative equilibrium through the membrane. They used a reactor in series model to describe the equilibrium of an ideal Pd-based MR for methane steam reforming, adding an extra constraint which takes into account the permeative equilibrium. A similar approach was used by Al Megren et al. [23] for the n-butane dehydroisomerization reaction in a wide range of temperatures, reaction pressures and equilibrium hydrogen partial pressures, by means of a simplified reaction scheme. This analysis showed that the membrane reactor equilibrium conversion (MREC) achievable in a Pd-Ag MR can be up to seven times greater than the TR one, operated in the same conditions. An important issue in olefins production is their separation, usually performed by cryogenic distillation or PSA [24]; thus, the cost of the separation takes ca. 70% of the total capital. Membrane technology can be a valuable alternative to these technologies owing to its modularity, simplicity, lower costs, etc. Membranes will fit in well in the PI strategy and their use is becoming more and more important also in the petrochemical industry [11–13]. Various papers in the open literature refer to the use of polymeric membranes for the separation of hydrocarbons; Pinnau et al. [25] proposed the use of PTMSP membranes for the separation of n-butanes from light species (hydrogen and nitrogen). Sakellaropoulos et al. [26] showed the applicability of the Matrimid membrane in C4 hydrocarbon separation. Yang et al. [27] and Adachi et al. [28] used mixed matrix membranes (MMMs) with Ag+ ions for olefins separation. In this work, a simulation study of an integrated membrane system is proposed for the production and recovery of butenes. The dehydroisomerization of n-butane to iso-butene takes place in a Pd/Ag MR whose permeate is pure in H2 while the retentate is then treated in a membrane gas separation system for butenes concentration and recovery.

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3 of 15 process schemes (cases studies) are proposed considering the use of polymeric Four different process schemes (cases studies) are proposed considering the use of polymeric gas separation membrane units for downstream processing. The performance of each membrane Four different process schemes (cases studies) are proposed consideringof the usemembrane of polymeric gas membrane units fordeveloped downstream processing. The performance each unit unitseparation is analyzed by appropriately performance maps, to identify the operating conditions gas separation membrane units for downstream processing. The performance of each membrane is analyzedand by the appropriately performance to identify the operating conditions windows membranedeveloped permeation properties maps, required to maximize the recovery of the unit is analyzed bymembrane appropriately developedproperties performance maps, totoidentify the operating conditions windows and the permeation required maximize the recovery of the iso-butene produced. windowsproduced. and the membrane permeation properties required to maximize the recovery of the iso-butene iso-butene produced. 2. Materials and Methods 2. Materials and Methods 2. Materials andscheme Methods The process for butenes production and separation is shown in Figure 1. The first block The process scheme for butenes production and separation is shown in Figure 1. The first block “MR” The is the membrane reactor equipped with a Pd-Ag membrane. This stage is used for converting scheme for butenes production and separation is shown in Figure 1. The block “MR” is theprocess membrane reactor equipped with a Pd-Ag membrane. This stage is used forfirst converting n-butane to butenes, recovering, in the meantime, the hydrogen produced through the Pd-Ag “MR” is the membrane reactor equipped with a Pd-Ag membrane. This stage is used for converting n-butane to butenes, recovering, in the meantime, the hydrogen produced through the Pd-Ag membrane. A butenes, membrane gas separation “GS”the is the second block usedthrough for the separation n-butane to recovering, in thesystem meantime, Pd-Ag of membrane. A membrane gas separation system “GS” is hydrogen the secondproduced block used for thethe separation butenes from the other species, such as methane, ethane and hydrogen contained in the retentate A membrane gas separation “GS” is theand second block used for thein separation of of membrane. butenes from the other species, such assystem methane, ethane hydrogen contained the retentate stream exiting the MR. The driving force necessaryethane for theand permeation iscontained promotedinby pressurizing butenes from the other species, such as methane, hydrogen the retentate stream exiting the MR. The driving force necessary for the permeation is promoted by pressurizing the the MR upstream. negligible pressure drop along the MR,isthe retentate is at the stream exiting theConsidering MR. The driving force necessary for the permeation promoted bystream pressurizing MR upstream. Considering negligible pressure drop along the MR, the retentate stream is at the same same pressure as the feed. In negligible addition, pressure for the drop GS, the pressure is negligible the MR upstream. Considering along the MR, drop the retentate stream along is at thethe pressure as the feed. In addition, for the GS, the pressure drop is negligible along the membrane gas membrane gas separation units same pressure as the feed. In[29]. addition, for the GS, the pressure drop is negligible along the separation units [29]. membrane gas separation units [29]. GS MR GS MR Polymeric Membrane Pd-Ag Membrane Polymeric Membrane Pd-Ag Membrane

Figure 1. Membrane process scheme. Figure1. 1. Membrane Membrane process Figure process scheme. scheme.

2.1 Membrane Reactor 2.1Membrane MembraneReactor Reactor 2.1. The membrane reactor contains a Pd-Ag membrane for the selective separation of hydrogen Themembrane membranereactor reactorcontains contains aa Pd-Ag Pd-Ag membrane membrane for selective separation of hydrogen The for the the 2). selective separation hydrogen during the n-butane dehydroisomerization reaction (Figure The MR has a of tube-in-tube during the n-butane dehydroisomerization reaction (Figure 2). The MR has a tube-in-tube during the n-butane dehydroisomerization (Figure 2). The MR has a tube-in-tube configuration configuration with the internal tube beingreaction the Pd-Ag membrane, infinitively selective toward H2, the configuration with the internal tube being the Pd-Ag membrane, infinitively selective toward H2, the with the internal tube being the Pd-Ag membrane, infinitively selective toward H , the external is external is the reactor shell. The reactants are fed in the annulus between the internal 2 surface of the external is the reactor shell. The reactants are fed in the annulus between the internal surface of the the reactor shell. The reactants are fed in the annulus between the internal surface of the shell and shell and the external surface of the membrane, where the catalyst is present in a packed bed shell and the external surface of the membrane, where the catalyst is present in a packed bed the external surface of the membrane, where the catalyst present in aaspacked bed configuration. configuration. on the theissame same side thefeed; feed; thehydrogen hydrogen configuration. The The retentate retentate stream stream is is recovered recovered on side as the the The retentate stream is recovered on the same side as the feed; the hydrogen permeated through the permeated in the the membrane membraneinner innerside. side. permeatedthrough throughthe themembrane membrane is is recovered recovered in membrane is recovered in the membrane inner side.

Figure reactor scheme. Figure2. Membrane reactor Figure 2. Membrane Membrane reactorscheme. scheme.

The n-butane dehydroisomerizationisisanan endothermic reaction with mole number increasing The n-butane dehydroisomerization endothermic reaction with mole number increasing The n-butane dehydroisomerization endothermic reaction with mole number increasing [6]; [6];therefore, therefore, favored by high temperature temperature and The [6]; ititisisfavored high and low low pressure. pressure. The reactionis isschematically schematically therefore, it is favored by highby temperature and low pressure. The reaction isreaction schematically represented represented intwo two steps: the first first step step (Equation being ofof n-butane to to represented steps: (1)) being the thedehydrogenation dehydrogenation n-butane in two steps: inthe first step the (Equation (1))(Equation being the(1)) dehydrogenation of n-butane to n-butene and n-butene and hydrogen; and the second (Equation (2)), the n-butene isomerization to iso-butene. n-butene and the second (2)),isomerization the n-butene to isomerization hydrogen; andhydrogen; the secondand (Equation (2)),(Equation the n-butene iso-butene. to iso-butene. kJ kJkJ kJ ��reaction (25°C) = 130 kJ (1)(1) ∆𝐻𝐻 (25°C) = 130 130 kJ ∆𝐻𝐻 reaction(25°C) �� ◦ ◦ ∆𝐻𝐻 = ∆𝐻𝐻 (25°C) = 130 e e reaction(25 C) = 130 mol reaction ∆ Hreaction ∆ Hreaction (1) (25 C) = 130mol mol mol mol mol kJ �reaction (25°C) = −17 kJ 𝑛𝑛 − Butene ⇄ 𝑖𝑖𝑖𝑖𝑖𝑖 − Butene ∆𝐻𝐻 (2) �reaction (25°C) = −17mol 𝑛𝑛 − Butene ⇄ 𝑖𝑖𝑖𝑖𝑖𝑖 − Butene ∆𝐻𝐻 (2) kJ ereaction n − Butene iso − Butene ∆H (2) (25 ◦ C) = −17 mol mol


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In a Pd-Ag MR, the continuous removal of hydrogen from the reaction volume drives the reaction toward further n-butane conversion promoting butene formation. The mass balances of the dehydroisomerization reaction carried out in the Pd-Ag MR are summarized in Table 1 [23]. Hydrogen was considered in the feed stream to replace the inert gas in the feed stream as reported in paper of Al-Megren et al. [23]. Therefore, the permeation membrane load is significantly higher in the present study. n0 is the initial n-butane moles; m is the initial mole ratio between hydrogen and n-butane; x1 is the n-butane conversion for dehydrogenation (Equation (1)) and x2 the n-butene conversion for isomerization (Equation (2)); hydrogen permeation is accounted for by z. The thermodynamics analysis was carried out for reactive and permeative equilibrium in the MR [20]. The permeative equilibrium occurs when the net hydrogen flux through the membrane is zero so the hydrogen partial pressure of the retentate equals the permeate one (Equation (3)). Marigliano et al. [20] demonstrated how MREC strongly depends on hydrogen equilibrium partial pressure and its effect is more significant than reaction pressure, promoting the conversion even though the reaction is thermodynamically unfavorable. Permeate Retentate PH = PH 2 ,equilibrium 2 ,equilibrium

(3)

Table 1. Stoichiometric table for the reactive and permeative equilibrium in a membrane reactor. Moles Number n-Butane Initial value Reaction I Reaction II Permeation Equilibrium state Total moles

n-Butene

Hydrogen

iso-Butene

Inert

n0 x1 x2 n0 x1 x2

nInert -

n0 · m n0 x1 n0 x1 - n0 x1 x2 -n0 (m + x1 ) z n0 (m + x1 ) (1 − z) n0 x1 (1 − x2 ) n0 [1 + m + x1 − m z] + nInert

n0 −n0 x1 n0 (1 − x1 )

The equilibrium balance (of a TR) is directly deductible from the MR one, when not accounting for the permeation term (z), which is zero in the TR system. Equations (4) and (5) are the equilibrium reaction constant, as a function of temperature and in terms of the species molar fraction. Kequilibrium,1 ( T )

=

−15.8 103 T

nH2 nn−C4 H8 Reaction ntotal nn−C4 H10 P x1 ( m+ x1 )(1−z)(1− x2 ) Reaction

= 5.71 107 e (1− x1 )

=

n 1+m+ x1 −mz+ Inert n0

Kequilibrium,2 ( T ) = 0.207e

1936 T

=

P

niso−C4 H8 x2 = nn−C4 H8 1 − x2

(4)

(5)

It was chosen to operate the reaction at 550 ◦ C, as a good compromise between the positive effect on reaction thermodynamics and kinetics and the Pd-Ag membrane typical operating range [30]. Therefore, Table 2 reports the values at 550 ◦ C selected from the Figure 1 of Inaba et al. [31]. Ten bar was the maximum pressure used in the different schemes, as it allows low operating and a fixed cost of the compressor. Two different feeds were considered: (1) (2)

An equimolecular mixture of n-butane and nitrogen, as in the Al Megren et al. [23] work. A n-butane/hydrogen mixture, as in the thermodynamics analysis reported below.

A pressure of 0.1 bar is uncommon in industrial processes; however, some industrial plants, such as the Clariant one, operate at a reaction pressure of 0.3–0.5 bar. Therefore, a permeate pressure or hydrogen equilibrium pressure of 0.1 bar was coupled to a reaction pressure of 1.5 or 10 bar. Sweep gas can be also used for keeping the hydrogen partial pressure low.


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The n-butane conversion was calculated for the equilibrium condition at all the temperatures and pressures investigated. In particular, MR conversion of Cases 1, 3 and 4 refer to the data reported by Al-Megren et al. [23]. A lower and conservative value equal to 90% of the equilibrium value was used as the actual conversion of the MR. Then, the selectivities reported in Table 2 were used to evaluate the composition of the MR outlet stream. This procedure was applied since the work was aimed to investigate the process as whole and it was not specifically devoted to the modeling of MRs. Liang et al. [18], by using Pd-based MR, demonstrated that an undesirable formation of heavy alkane products on the membrane surface does not take place. Thus, the reaction selectivities of Inaba et al. [31] were considered as reference. Table 2 reports the values measured at 550 ◦ C, over the Cr/H-SSZ-35 catalyst. Table 2. Reactor selectivities for the n-butane conversion [31]. Component Selectivity

C1 -C3 0.26

C4 iso-Butane

1-Butene

iso-Butene

trans-2-Butene

0.02

0.11

0.24

0.2

cis-2-Butene 1,3-Butadiene 0.11

0.06

0.74

2.2. Separation System The multistage gas separation membrane system was simulated by means of performance maps developed by Brunetti et al. [32] and appropriately calculated for the objectives of this work. The model used for developing these maps consists of a 1D dimensionless model for the mixture species in steady-state conditions. More details can be found in [32]. The performance maps are described in terms of two important dimensionless groups: the permeation number θA (Equation (6)) and the pressure ratio φ (Equation (7)). θA =

Permeance A AMembrane PPermeate xFeed QFeed A φ=

PFeed PPermeate

(6)

(7)

The permeation number expresses a comparison between the two main mass transport mechanisms involved: permeation through the membrane and convective transport at the membrane inlet. A high permeation number corresponds to a high residence time of the stream, meaning low permeation through the membrane. The pressure ratio is an index of the maximum permeation driving force for permeation. For given feed composition, membrane properties, module geometry and operating conditions, the solution of the equations system provides the dimensionless flow rate profile for each species. The concentration profile for each species and the recovery in a specific stream (Figure 3) are obtained from these profiles. Figure 3 shows an example of a typical performance map of a membrane gas separation unit developed in this work where the butenes molar fraction is reported against butenes recovery at the permeate side of the membrane unit. Each point of these curves is a specific membrane system characterized by its own operating conditions, feed compositions, etc. All curves (black line) are obtained for a constant value of pressure ratio and show the same trend: the butenes concentration at the permeate side increases as the butenes recovery increases as well. The blue line was drawn for different pressure ratios and the same permeation number. At a given permeation number, the butenes concentration increases when the pressure ratio decreases. The operating conditions, in term of pressure ratio and permeation number, were chosen according to the required composition of the permeate stream. All the permeate streams are less concentrated in the more permeable species at higher permeation number values. The lower the pressure ratio, the higher the concentration of the less permeable species (the butanes permeability is lower compared


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to hydrogen). The permeation number and pressure ratio were discussed in detail along with the case Processes 2016, 4, 42 of 15 studies. The traditional separation system was also simulated, with Aspen Hysys®software6 (Aspen Technology, Inc, Burlington, VT, USA), as reference case considering the use of a flash for separating Hysys® software Technology, Burlington, VT,to USA), as reference case considering the a hydrocarbon liquid(Aspen fraction. The reactorInc, outlet stream has be cooled before entering the membrane use of a flash for separating a hydrocarbon liquid fraction. The reactor outlet stream has to be cooled units; however, polymeric membranes operate at 30–60 ◦ C and also not in a cryogenic range. The flash before entering the membrane units; however, polymeric membranes operate at 30–60 °C and also unit operating on the cooled stream, although it cannot separate it into pure species, it can split this not in a cryogenic range. The flash unit operating on the cooled stream, although it cannot separate it stream into two streams with significantly different composition.

Butenes permeate molar fraction, -

into pure species, it can split this stream into two streams with significantly different composition. 1 0.6 θ=0.05

0.4

θ=0.1

φ=5 φ=15 φ=10

0.2 0

0

0.2

0.4

0.6

0.8

1

Butenes permeate recovery, -

Figure 3. Butenes permeate of butenes butenespermeate permeate recovery different Figure 3. Butenes permeatemolar molarfraction fraction as as aa function function of recovery forfor different pressure ratio andand permeation number. pressure ratio permeation number.

3. Results 3. Results Membrane Reactorforforn-Butane n-ButaneConversion Conversion 3.1. 3.1 Membrane Reactor Figure 4 shows MREC(red (redline) line)and and TREC TREC (black ofof thethe temperature for for Figure 4 shows thethe MREC (blackline) line)asasaafunction function temperature different reaction pressures. The analysis was carried out for an n-butane/hydrogen feed mixture, different reaction pressures. The analysis was carried out for an n-butane/hydrogen feed mixture, with 0.80 initial molar fraction of n-butane. The thermodynamics of the equimolecular mixture of with 0.80 initial molar fraction of n-butane. The thermodynamics of the equimolecular mixture of n-butane and N2 was already analyzed by Al Megren et al. [23] keeping the hydrogen equilibrium n-butane and N2 was already analyzed by Al Megren et al. [23] keeping the hydrogen equilibrium partial pressure always at 0.1 bar. As mentioned, the n-butane dehydroisomerization is an partial pressure always 0.1 occurs bar. Aswith mentioned, theof n-butane dehydroisomerization is an endothermic endothermic reactionatthat an increase mole number; therefore, it is favored by a high reaction that occurs an increase moleinnumber; it is favored by highMR temperature temperature and awith low pressure. As of shown Figure 4,therefore, equilibrium conversions of aboth and TR andincreased a low pressure. As shownHowever, in Figurethe 4,negative equilibrium of both MR TR increased with temperature. effectconversions of pressure appeared quiteand evident on TR withconversion. temperature. However, the negative effect of pressure appeared quite evident on TR conversion. For instance, at 550 °C, the TR conversion was around 0.3 at a reaction pressure of 5 bar, For but instance, at 550 ◦down C, theto TR0.21 conversion was around 0.3 Conversely, at a reactionthe pressure 5 bar, but it decreased it decreased for a pressure of 10 bar. MRECofdoes not depend on reaction pressure (Figure 4)offor equilibrium hydrogen partial pressure of 0.1 bar. 4 shows, down to 0.21 for a pressure 10an bar. Conversely, the MREC does not depend onFigure reaction pressure at the conditions hydrogen considered,partial that MREC wasofalways than TREC, at owing to the (Figure 4) operating for an equilibrium pressure 0.1 bar.higher Figure 4 shows, the operating hydrogen permeation through the Pd-Ag membrane which promoted the n-butane equilibrium conditions considered, that MREC was always higher than TREC, owing to the hydrogen permeation conversion, by removing a product reactionthe volume. through the Pd-Ag membrane whichfrom promoted n-butane equilibrium conversion, by removing The MREC/TREC ratio, used for estimating the improvement of MR with respect to TR, a product from reaction volume. identifies two different regions: one above and one below. In the first (yellow fill), the MREC was The MREC/TREC ratio, used for estimating the improvement of MR with respect to TR, identifies higher than TREC (MREC/TREC > 1) because of the good effect of the hydrogen removal from the two different regions: one above and one below. In the first (yellow fill), the MREC was higher than reaction side. In the second (cyan fill), TREC is higher than MREC (MREC/TREC < 1) since the TREC (MREC/TREC > 1) because of the good effect of the hydrogen removal from the reaction side. back-permeation causes a higher hydrogen concentration in the MR than that in the TR and, In the second (cyan fill), TRECreduction. is higher than MREC (MREC/TREC < 1) in since back-permeation consequently, a conversion The operating condition chosen thisthe work limited the causes a higher hydrogen concentration in the MR than that in the TR and, consequently, a conversion analysis in the first region. For instance, the MREC was significantly higher than TREC reduction. The operating condition chosen this worklower limited in the first region. (MREC/TREC > 30, Figure 5) at 20 bar and in temperature thanthe 450analysis °C. As the temperature For increased, instance, the was significantly than TREC (MREC/TREC 30, Figure 5) at the MREC MREC/TREC ratio tendedhigher to 1. At temperatures higher than> 800 °C, there is 20 no bar ◦ andadvantage temperature As the temperature increased, the MREC/TREC ratio tended to 1. for lower an MRthan since450 bothC. MREC and TREC tend to 1. Two MR higher operating conditions were is chosen for the analysis thesince MR/GS systems. At temperatures than 800 ◦ C, there no advantage for an of MR bothintegrated MREC and TRECAtend to 1.temperature of 550 °C and hydrogen equilibrium partial pressure of 0.1 bar were considered for both;


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Two MR operating conditions were chosen for the analysis of the MR/GS integrated systems. Processes 2016, 42 7 of 15 Processes 2016,4,4, of 15 A temperature of42550 ◦ C and hydrogen equilibrium partial pressure of 0.1 bar were considered 7for both; they differ for reaction pressure of 10 and 1.5 bar as reported in Table 3. The MR retentate compositions they as reported reported in in Table Table 3.3. The The MR MRretentate retentate theydiffer differfor forreaction reaction pressure pressure of of 10 10 and and 1.5 bar as arecompositions reported, hereafter, in thehereafter, figures related to the separation schemes (cases studies). are reported, in the figures related to the separation schemes (cases studies). compositions are reported, hereafter, in the figures related to the separation schemes (cases studies). n Equilibrium -butane, -EquilibriumConversion Conversion of of n-butane,

11 MREC

0.8 0.8

1bar

0.6 0.6

20bar 20bar 10bar 10bar

TREC

0.4 0.4 0.2 0.2

00 200 200

400 600 800 400 600 800 Temperature, °C Temperature, °C

1000 1000

Figure 4.Equilibrium Equilibrium conversionof ofn-butane n-butane in in aaa membrane membrane reactor and traditional reactor Figure 4. 4.Equilibrium conversion reactor(MREC) (MREC) and traditional reactor Figure conversion of n-butane in membrane reactor (MREC) and traditional reactor (TREC) asfunction a function of the temperaturefor fordifferent different feed feed pressures. Hydrogen equilibrium partial (TREC) as a of the temperature pressures. Hydrogen equilibrium partial (TREC) as a function of the temperature for different feed pressures. Hydrogen equilibrium partial 2 = 80:20. pressure: 0.1 bar. Initial molar compositionn-butane: n-butane: H pressure: 0.1 bar. Initial molar composition H = 80:20. pressure: 0.1 bar. Initial molar composition n-butane: H22 = 80:20. 40 40 20 bar 20 bar

MREC/TREC, - MREC/TREC,

30 30 20 20 10 10 0 0

10 bar 10 bar 5 bar 5 bar 1.5 bar 1.5 bar

400 600 800 400 Temperature, 600 800 °C Temperature, °C

1000 1000

Figure 5. MREC/TREC ratio as a function of the temperature. Hydrogen equilibrium partial pressure:

Figure MREC/TREC ratio as as aa function function of Hydrogen partial pressure: Figure 5. 5.MREC/TREC ratio of the the temperature. Hydrogenequilibrium equilibrium partial pressure: 2 =temperature. 80:20. 0.1 bar. Initial molar composition n-butane: H bar. Initialmolar molarcomposition compositionn-butane: n-butane: H H22 == 80:20. 0.10.1 bar. Initial 80:20. 3.2 Plant Schemes for Butenes Production and Capture

3.2Plant PlantSchemes Schemesfor forButenes Butenes Production Production and 3.2. and Capture Capture As mentioned in the previous sections, the Pd-Ag membrane property is that only the hydrogen

mentioned previous sections, the Pd-Ag Pd-Ag membrane property permeates through it.the This allowssections, a pure hydrogen stream to be recovered the only permeate side of AsAs mentioned ininthe previous the membrane propertyisin isthat that onlythe thehydrogen hydrogen permeates through it. This allows a pure stream to to be recovered in the permeatepartial side of the MR. The permeation promoted by ahydrogen driving force equal difference hydrogen permeates through it. Thisisallows a pure hydrogen stream to bethe recovered ininthe permeate side of pressure between the two the MR. The permeation is membrane promoted sides. by a driving force equal to the difference in hydrogen partial the MR. The permeation is promoted by a driving force equal to the difference in hydrogen partial pressure between the two membrane sides.

pressureTable between the two membrane sides. 3. Operating condition of the different membrane systems. Membrane reactor temperature

550 °C. = Gas Separation case study. Table 3. GS Operating conditionunit of in theeach different membrane systems. Membrane reactor temperature Table 3. Operating condition of the different membrane systems. Membrane reactor temperature 550 °C. GS = Gas Separation unit in each case study. ◦ C. GS parameters 550 = Gas Separation Case unit in each1 case study. Operating Study Case Study 2 Case Study 3 Case Study 4 Membraneparameters reactor feed Operating

Case Study 1

Case Study 2

bar 1 Case10 Study

Operating Parameters

pressure Membrane reactor feed

Case Study 2

Case Study 3 Case Study 4 Case Study1.5 3 bar Case Study 4

10 bar 10 bar

Membrane reactor feed pressure

Membrane reactor feed pressure

1.5 bar bar 1.5

Membrane reactor feed molar composition H102=50:50 :N2 =50:50 n-C n-C44H H10 :N22=50:50 =50:50 4 H10 :N2 =50:50 10 :H2 =80:20 10:N n-C4H10:Nn-C 2=50:50 n-C4H10n-C :H42H =80:20 n-C4n-C H104:N

molar composition Membrane reactor feed Gas separation—Feed pressure to GS1 10 bar n-C4H10:N2=50:50 Gas separation—Feed molar composition Separation stages θ 10 bar pressure to GS1 GS1 50 Gas separation—Feed 10 bar GS2 2.5 Separation stages θ φ pressure to GS1

10 bar

15 bar

n-C4H10:H2=80:20 n-C4H10:N2=50:50 φ θ φ θ φ 10 bar 15 bar 10 10 θ

580

10 bar 0.5 φ

GS3 GS1stages Separation GS4 GS2 GS1

θ50 2.5 50

2.5 10

φ 10 10

10580

θ 0.5 580

0.57 5 φ 25

GS2

2.5

10

0.5

10

-

10 5

5

10

15

10 bar

n-C4H10:N2=50:50 θ φ 10 bar 20

10

θ

15 10

bar φ 10

0.1 θ

10

0.25 10 θ 10 10

15φ15 10 15

20-θ 0.1 20

2

10

10

0.1

10

10 bar5φ 10 φ 510

5


8 of 15

The MR retentate thus contains all the reaction products, the unconverted reactants and the eventual inert gases. This stream was fed to a multistage membrane system for the separation and recovery of the different components. Four case studies were explored (Table 3): MR was always operated at 550 ◦ C and for Case Studies 1 and 2 at 10 bar, whereas for Case Studies 3 and 4 at 1.5 bar (see Al Megren et al. [23]). Each case study was constituted by three or four GS units and differed from the others for the operating conditions. Table 3 summarizes, beyond the MR operating conditions, the conditions of the membrane gas separation units expressed as permeation number and pressure ratio. Each GS unit contained polymeric membranes suitable for a defined separation. Table 4 reports permeance and selectivity of the membranes considered in this work for the different species involved. Matrimid is suitable for the separation of hydrogen from the rest of the stream. PTMSP indeed is known to be selective toward butane; therefore, it was used for the separation of paraffins-olefins. For the PSf membrane, only hydrogen, nitrogen, C3 H8 and C4 H10 permeance values were reported in the literature. In analogy, for PSM-Ag+ membranes, only 1-C4 H8 , C4 H10 and 1,3-C4 H6 permeance values were reported. The remaining separation properties were set on the basis of separation properties of similar species. Generally, the permeance values of these membranes are comparable with the ones of other membranes used for other separations. Table 4. Polymeric membrane properties in terms of permeance (nmol·m−2 ·s−1 ·Pa−1 ) and selectivity. Matrimid [26,33,34] Species H2 N2 CH4 C2 H6 C2 H4 C3 H8 C3 H6 trans-2-C4 H8 cis-2-C4 H8 1-C4 H8 iso-C4 H8 n-C4 H10 iso-C4 H10 1,3-C4 H6

Permeance

Selectivity H2 /i-Specie

51.7 2.01 2.01 0.20 2.01 0.088 0.20 0.201 0.201 0.129 0.129 0.088 0.027 0.20

1 26 26 258 26 586 258 258 258 401 401 586 1900 258

PTMSP [27] Permeance

Selectivity n-C4 H10 /i-Specie

725 1020 1020 3660 3660 9240 9240 16,600 31,300 20,100 20,100 28,200 10,800 18,800

38.8 27.5 27.5 7.7 7.7 3.1 3.1 1.7 0.9 1.4 1.4 1 2.6 1.5

PSM-Ag+ [28]

PSf [35] Permeance

Selectivity H2 /i-Specie

Permeance

Selectivity butenes/i-Specie

3.34 0.83 0.83 0.83 0.83 0.016 0.016 0.004 0.004 0.004 0.004 0.004 0.004 0.004

1 4 4 4 4 200 200 800 800 800 800 800 800 800

5.6 5.6 5.6 5.6 28,200 5.6 28,200 28,200 28,200 28,200 28,200 5.6 5.6 14,100

5000 5000 5000 5000 1 5000 1 1 1 1 1 5000 5000 2

3.3. Case Study 1 Figure 6 shows the flow sheet of Case Study 1. An equimolecular n-butane/nitrogen mixture was fed to the MR operated at 550 ◦ C, 10 bar for feed and a permeate pressure of 0.1 bar. In these circumstances, hydrogen recovery in the permeate was 95.2%. The GS system had three membrane gas separation devices in a cascade configuration and the MR retentate pressure of 10 bar was used to promote permeation in all GS units. The permeates were recovered at atmospheric pressure. As mentioned, the pressure drops in the polymeric membrane units were negligible [29]. First (GS1) and second (GS2) separation units were equipped with membranes selective toward non-condensable species (H2 , N2 , etc.): PSf [35] and Matrimid [33,34] membranes, respectively. A permeate stream with high light species concentration was recovered owing to the high selectivity of the membrane. Moreover, the GS1 operating conditions (permeation number of 50 and pressure ratio of 10) were adjusted for maximizing the concentration of the non-condensable species such as nitrogen (N2 molar concentration of 72.17%) in the permeate. This allowed the butenes in the retentate side to be concentrated from 19.56% to 26.18% with a stage cut of 25.5%. The latter was then fed to a second membrane gas separation device (GS2). Matrimid membranes show a different selectivity (Table 4) than PSf toward non-condensable gases (N2 /C4 and H2 /N2 selectivity is 200 and 4, respectively); therefore, it recovers in the permeate almost all of the hydrogen present in the feeds, increasing the concentration of hydrocarbons in the retentate (stage cut of 55.9%). As third separation stage, a membrane unit with a mixed matrix membrane (PSM-Ag+ ) [28] was considered for

ratio of 10) were adjusted for maximizing the concentration of the non-condensable species such as nitrogen (N2 molar concentration of 72.17%) in the permeate. This allowed the butenes in the retentate side to be concentrated from 19.56% to 26.18% with a stage cut of 25.5%. The latter was then fed to a second membrane gas separation device (GS2). Matrimid membranes show a different selectivity (Table 4) than PSf toward non-condensable gases (N2/C4 and H2/N2 selectivity is 200 and 4, Processes 2016, 4, 42 9 of 15 respectively); therefore, it recovers in the permeate almost all of the hydrogen present in the feeds, increasing the concentration of hydrocarbons in the retentate (stage cut of 55.9%). As third +) [28] was considered separation stage,separation. a membrane with a mixedofmatrix membrane (PSM-Ag(as olefins/paraffins Theunit high selectivity the olefins and di-olefins butadiene) produced for olefins/paraffins separation. The high selectivity of the olefins and di-olefins (as butadiene) a permeate stream rich in butenes isomers with a stage cut of 40.07%. In addition, owing to the high produced a permeate stream rich in butenes isomers with a stage cut of 40.07%. In addition, owing to in selectivity (butenes/n-butane of 5000), the olefins recovery was 61.4% with a butenes concentration the high selectivity (butenes/n-butane of 5000), the olefins recovery was 61.4% with a butenes the permeate stream of 91.95% and butadiene of 7.55%. concentration in the permeate stream of 91.95% and butadiene of 7.55%.

n-C4H10 50% N2 50% 23.2 kmol h-1

MR

550°C 10 bar

Pd/Ag Membrane

GS1 MRRetentate 25.44 kmol h-1

Θ=50 φ= 10

PSf Membrane

GS2 18.96 kmol h-1

n-C4H10 64.14% H2 0.15% N2 12.40% Butenes 4.77% iso-C4H10 2.00% C1-C3 16.14% Butadiene 0.40%




Θ=2.5 φ= 10

Matrimid Membrane

GS3 8.36 kmol h-1

Θ=2.5 φ= 10

5.01 kmol h-1

PSM-Ag+ Membrane

0.1 bar

H2 100% MRPermeate 5.6 kmol h-1

6.48 kmol h-1

10.6 kmol h-1

n-C4H10 0.20% H2 2.93% N2 72.17% Butenes 0.24% iso-C4H10 0.01% C1-C3 24.43% Butadiene 0.02% 17.08 kmol h-1

n-C4H10 7.76% H2 0.78% N2 59.42% Butenes 15.48% iso-C4H10 0.47% C1-C3 14.40% Butadiene 1.69% n-C4H10 4.89% H2 1.59% N2 64.27% Butenes 9.70% iso-C4H10 0.29% C1-C3 18.21% Butadiene 1.05%

n-C4H10 0.35% H2 0% N2 0.07% Butenes 91.95% iso-C4H10 0% C1-C3 0.08% Butadiene 7.55%

GS3Permeate 3.35 kmol h-1

Figure 6. Case Study 1: Flow membrane units operating stream molar Figure 6. Case Study 1: Flow sheet, sheet, membrane units operating conditionsconditions and streamand molar concentration. concentration.

3.4. Case Study 2 3.4 Case Study 2 Figure 7 shows the flow sheet of Case Study 2. The MR was operated, similarly to Case Study 1, Figure 7 shows the flow sheet of Case Study 2. The MR was operated, similarly to Case Study 1, but with a different feed mixture, n-butane:hydrogen = 80:20 as in [36]. Analogously to Case Study 1, but with a different feed mixture, n-butane:hydrogen = 80:20 as in [36]. Analogously to Case Study 1,◦ the MR retentate stream was fed to the membrane separation system, after being cooled down to 50 C. the MR retentate stream was fed to the membrane separation system, after being cooled down to A Matrimid membrane unit (Table 4) was used in the first stage for recovering hydrogen and 50 °C. methane from hydrocarbons, retained in the retentate. Different from Case Study 1, here both retentate A Matrimid membrane unit (Table 4) was used in the first stage for recovering hydrogen and and permeate of GS1 were further treated. The GS1 was operated with a pressure ratio of 5 to maintain methane from hydrocarbons, retained in the retentate. Different from Case Study 1, here both the pressure in the permeate stream (stage-cut of 12.75%) at 2 bar necessary for the permeation of retentate and permeate of GS1 were further treated. The GS1 was operated with a pressure ratio of 5 hydrocarbons PTMSP membranes in GS4. This separation stage us to recover and to maintain through the pressure in the permeate[27] stream (stage-cut of 12.75%) at allowed 2 bar necessary for the concentrate hydrocarbons in the permeate reaching a C4 molar of 59.28%stage with allowed 42.31% of permeation of hydrocarbons through PTMSP membranes [27] concentration in GS4. This separation butenes with a stage-cut of 34.2%. us to recover and concentrate hydrocarbons in the permeate reaching a C4 molar concentration of The with retentate stream of GS1, constituted 59.28% 42.31% of butenes withmainly a stage-cut of 34.2%. of hydrocarbons, was fed into a series of two membrane gas separation units, GS2 and GS3. PTMSP (Table 4) membranes were used in each unit. The scope of this cascade separation was to improve the hydrocarbons concentration and recovery. GS2 resulted in a rich C4 stream to be recovered at the permeate side with a molar concentration of 85.94% and stage cut of 51.1%, when the permeation number was 0.5 and pressure ratio was 10. The retentate stream of GS2, still rich in hydrocarbons, was fed to the last membrane gas separation unit, GS3 (stage cut of 55.1%). A pressure ratio 10 and a permeation number 0.57 were selected for improving butenes recovery. Under these circumstances it was possible to obtain a C4 concentration in the permeate of 74.62% with 49.67% of butenes. Combining the permeate streams of GS2, GS3 and GS4, the total C4 olefins concentration was 55.5% with a high recovery, 87.1%, and a high C4 recovery of 88.5%.

85.94% and stage cut of 51.1%, when the permeation number was 0.5 and pressure ratio was 10. The retentate stream of GS2, still rich in hydrocarbons, was fed to the last membrane gas separation unit, GS3 (stage cut of 55.1%). A pressure ratio 10 and a permeation number 0.57 were selected for improving butenes recovery. Under these circumstances it was possible to obtain a C4 concentration in the permeate of 74.62% with 49.67% of butenes. Combining the permeate streams of GS2, GS3 and Processes 2016,total 4, 42C4 olefins concentration was 55.5% with a high recovery, 87.1%, and a high C4 recovery 10 of 15 GS4, the of 88.5%.

n-C4H10 80% H2 20% 21.2 kmol h-1

n-C4H10 18.09% H2 1.11% Butenes 42.33% iso-C4H10 1.28% C1-C3 33.35% Butadiene 3.85% MR

550°C 10 bar

Pd/Ag Membrane

GS1

Θ=580 φ= 5

MRRetentate Matrimid Membrane 20.63 kmol h-1

GS2

18 kmol h-1

n-C4H10 5.35% H2 0.04% Butenes 19.36% iso-C4H10 0.67% C1-C3 72.96% Butadiene 1.62%


n-C4H10 19.69% H2 0.01% Butenes 44.39% iso-C4H10 1.37% C1-C3 30.58% Butadiene 3.96% Θ=0.5 φ= 10

PTMSP Membrane

3.95 kmol h-1 GS3

8.8 kmol h-1

Θ=0.57 φ= 10

PTMSP Membrane

0.1 bar

H2 100%

2.63 kmol h-1

MRPermeate 14.33 kmol h-1 n-C4H10 7.13% H2 8.68% Butenes 27.97% iso-C4H10 0.86% C1-C3 52.27% Butadiene 3.09%

n-C4H10 5.12% H2 11.62% Butenes 20.51% iso-C4H10 0.63% C1-C3 59.86% Butadiene 2.26% 1.73 kmol h-1

GS4

9.2 kmol h-1

4.85 kmol h-1

n-C4H10 27.29% H2 0% Butenes 52.36% iso-C4H10 1.62% C1-C3 14.06% Butadiene 4.67%

n-C4H10 16.95% H2 0% Butenes 49.67% iso-C4H10 1.54% C1-C3 27.38% Butadiene 4.46%

Θ=25 φ= 2

PTMSP Membrane

0.9 kmol h-1 n-C4H10 10.97% H2 3.03% Butenes 42.31% iso-C4H10 1.31% C1-C3 37.69% Butadiene 4.69%


𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏𝐏 𝐆𝐆𝐆𝐆𝐭𝐭𝐭𝐭𝐭𝐭𝐭𝐭𝐭𝐭 14.95 kmol h-1

Figure 7. Case Study 2: Flow membrane units operating stream molar Figure 7. Case Study 2: Flow sheet, sheet, membrane units operating conditionsconditions and streamand molar concentration. concentration.

3.5. Case Study 3 3.5 Case Study 3 For Case Study 3, the MR operating conditions were unchanged with respect to Case Studies For Case Study 3, the MR operating conditions were unchanged with respect to Case Studies 1 1 and 2 except for the reaction pressure, which was 1.5 bar, a typical pressure used in a traditional and 2 except for the reaction pressure, which was 1.5 bar, a typical pressure used in a traditional reactor for butenes production from butane [3]. The MR feed mixture was the same as for Case Study 1, reactor for butenes production from butane [3]. The MR feed mixture was the same as for Case Study an equimolecular stream of nitrogen and n-butane. Owing to the low reaction pressure, the hydrogen 1, an equimolecular stream of nitrogen and n-butane. Owing to the low reaction pressure, the recovery was lower was (66.2%) than Casethan Studies and 2.1 The MR retentate stream contained large hydrogen recovery lower (66.2%) Case1Studies and 2. The MR retentate stream contained amounts of non-condensable gases (ca. 50%) needing separation. For thisFor purpose, it was fed intofed a GS large amounts of non-condensable gases (ca. 50%) needing separation. this purpose, it was ◦ C. The presence of PSf unit with PSf membranes, after being compressed to 15 bar and cooled to 50 into a GS unit with PSf membranes, after being compressed to 15 bar and cooled to 50 °C. The membranes with the high fraction of hydrogen andof nitrogen in the of GS1 the high presence ofcombined PSf membranes combined with the high fraction hydrogen andfeed nitrogen in and the feed of pressure ratio allowed a higher total hydrogen and nitrogen recovery of 95.9% (Figure 8). GS1 and the high pressure ratio allowed a higher total hydrogen and nitrogen recovery of 95.9% A further concentration of this stream, with consequent recovery of the hydrocarbons, (Figure 8). was done by feeding this stream a second gas separation PSf membranes, A further concentration of to this stream,membrane with consequent recoverydevice of the with hydrocarbons, was after re-compression 15 bar. GS2membrane permeation pressure stage cut were done by feeding thisto stream to aThe second gasnumber, separation deviceratio with and PSf membranes, after10, 10re-compression and 87.57%, respectively. ratio of 10 allowed the cut permeate bar to 15 bar. TheThe GS2pressure permeation number, pressure recovering ratio and stage were 10,at101.5 and 87.57%, respectively. The pressure of 10 recovering at 1.5 bar the with the possibility of recycling this ratio stream richallowed in nitrogen to the the inletpermeate of the reactor, but,with although possibilitythis of recycling this was stream in nitrogen to the inletwork. of theThe reactor, but, stream although interesting, configuration notrich further investigated in this retentate was interesting, this configuration was not further investigated in this work. The retentate stream was richer in hydrocarbons compared to the GS1 permeate and after being mixed with the retentate stream of GS1 (Figure 8), was fed to stage GS3 (stage cut of 39.2%) for hydrocarbons separation. Both streams were at 15 bar. In this latter gas separation unit, with a PTMSP membrane, the feed stream contained 39.8% butenes and ca. 77.11% C4 and it was possible to recover ca. 38.7% of C4 olefins at 43.31% molar, operating at pressure ratio of 15 and permeation number of 0.25.

richer in hydrocarbons compared to the GS1 permeate and after being mixed with the retentate stream of GS1 (Figure 8), was fed to stage GS3 (stage cut of 39.2%) for hydrocarbons separation. Both streams were at 15 bar. In this latter gas separation unit, with a PTMSP membrane, the feed stream contained Processes 2016, 39.8% 4, 42 butenes and ca. 77.11% C4 and it was possible to recover ca. 38.7% of C4 olefins 11 ofat15 43.31% molar, operating at pressure ratio of 15 and permeation number of 0.25.

n-C4H10 50% N2 50% 23.2 kmol h-1

n-C4H10 14.95% H2 7.33% N2 42.72% Butenes 18.33% iso-C4H10 0.57% C1-C3 14.44% Butadiene 1.66% MR

550°C 1.5 bar

Pd/Ag Membrane 0.1 bar

n-C4H10 38.54% H2 0.87% N2 5.64% Butenes 47.30% iso-C4H10 1.46% C1-C3 1.89% Butadiene 4.30% GS1

MRRetentate 27.15 kmol h-1

Θ=10 φ= 15

PSf Membrane

n-C4H10 31.57% H2 2.98% N2 15.47% Butenes 39.91% iso-C4H10 1.23% C1-C3 5.26% Butadiene 3.58% Θ=0.25 φ= 15

GS3 10.25 kmol h-1

-1

12.35 kmol h

H2 100% MRPermeate 3.89 kmol h-1 n-C4H10 0.64% H2 11.25% N2 65.22% Butenes 0.75% iso-C4H10 0.02% C1-C3 22.05% Butadiene 0.07%


2.10 kmol h-1

GS2 16.90 kmol h-1

n-C4H10 2.82% H2 10.68% N2 61.86% Butenes 3.23% iso-C4H10 0.10% C1-C3 20.99% Butadiene 0.32% Θ=0.4 φ= 10

PTMSP Membrane

7.50 kmol h-1


GS3Permeate 4.85 kmol h-1

PSf Membrane


14.80 kmol h-1

Figure Study 3: Flow sheet, sheet, membrane units operating conditionsconditions and stream and molarstream concentration. Figure8.8.Case Case Study 3: Flow membrane units operating molar concentration.

3.6. Case Study 4 3.6 Case Study 4 Case Study 4 differs from the previous cases for the presence of a flash separation (traditional unit Case used Studyfor4 separating differs from the previous cases for (Figure the presence a flash separation (traditional operation) the MR retentate stream 9). TheofMR stream fed to the separation ◦ unit operation) used the MR streamat(Figure to the stage was the same asfor forseparating of Case Study 3. Itretentate was operated 550 C 9). andThe 1.5MR bar stream and thefed permeate separation wasThe the feed samestream as for of Case Study 3. It was operated at 550 °Cmixture. and 1.5 In barparticular, and the pressure wasstage 0.1 bar. was a n-butane/nitrogen equimolecular permeate pressure was was 0.1 bar. wasand a n-butane/nitrogen mixture. In the MR retentate stream fed The to a feed heat stream exchanger then compressed equimolecular to 10 bar and cooled down ◦ MR retentate stream was fed to a heat exchanger andtheir thenrecovery compressed to 10 bar and toparticular, 0 C. Thisthe condition allowed hydrocarbons condensation and thus in a liquid fraction, cooled down to 0 °C. This condition allowed hydrocarbons condensation anda thus their recovery in aof whereas hydrogen and nitrogen were recovered in the gaseous phase with molar concentration liquid fraction, whereas and nitrogen were recovered in the gaseous phase with a molar 73.27%. The liquid streamhydrogen was hydrocarbons rich (C 4 ca. 87.37%). Both streams were heated and fed to concentration of 73.27%. The liquid stream was hydrocarbons (C4 ca. Both streams different membrane gas separation units for further separationrich (Figure 9).87.37%). PSf membranes werewere used and fed to different membrane gas separation units further stream separation (Figure 9). PSf inheated GS1 for hydrogen and nitrogen separation; this resulted in afor retentate rich in hydrocarbon membranes used in GS1 for hydrogen and of nitrogen a retentate (ca. 89.5% of were C4 ), the permeate mainly consisted N2 andseparation; H2 (68.53%this andresulted 11.82%,inrespectively). stream rich in hydrocarbon (ca. 89.5% of C 4), the permeate mainly consisted of N2 and H2 (68.53% The GS1 was operated at a pressure ratio of 10 and permeation number of 20 (stage cut of 90.2%). andretentate 11.82%, was respectively). The GS1the was operatedliquid at a pressure andto permeation of The then mixed with vaporized coming ratio from of the10flash be treated number in a second 20 (stage cut of 90.2%). The retentate was then mixed with the vaporized liquid coming from the membrane gas separation device, GS2 (stage cut 12.88%) with PTMSP membranes. This unit was flash to be in ratio a second gas separation device, GS2C(stage cut 12.88%) with PTMSP operated at treated pressure of 5 membrane and permeation number of 0.1 with 4 recovery of 14% and butenes membranes. This unit was operated at pressure ratio of 5 and permeation number of 0.1 with C4 concentration of 47.16%. recovery of 14% and butenes concentration of 47.16%. 3.7. Case Studies Summary Table 5 reports the C4 concentration and recovery of the four case studies. Owing to the higher selectivity of PSM-Ag+ membrane, Case Study 1 showed a product stream with a high olefins concentration, 99.5%. Case Study 2 showed a C4 recovery of 88.5% but lower C4 molar concentration compared to the Case Study 1. Instead, Case Study 4 allowed obtaining a rich C4 stream (99.3%) with lower C4 recovery (14%).

Processes 2016, 4,4,4242 Processes 2016,

n-C4H10 50% N2 50% -1

23.2 kmol h

of 15 12 12 of 15

n-C4H10 14.95% H2 7.33% N2 42.72% Butenes 18.33% iso-C4H10 0.56% C1-C3 14.44% Butadiene 1.67% MR

550°C 1.5 bar

MRRetentate 27.15 kmol h-1

Pd/Ag Membrane 0.1 bar

16.59 kmol h-1


PSf Membrane GS1

18.40 kmol h-1

Flash


10 bar 0°C

8.75 kmol h-1

Θ=20 φ= 10

1.81 kmol h-1 n-C4H10 33.89% H2 1.19% N2 7.33% Butenes 49.67% iso-C4H10 1.53% C1-C3 1.97% Butadiene 4.42%

Permeate

MR 3.89 kmol h-1

H2 100%


10.56 kmol h-1 n-C4H10 37.12% H2 0.28% N2 2.18% Butenes 45.17% iso-C4H10 1.42% C1-C3 9.76% Butadiene 4.07%

GS2

Θ=0.1 φ= 5

PTMSP Membrane

n-C4H10 35.78% H2 0.32% N2 2.48% Butenes 44.87% iso-C4H10 1.42% C1-C3 11.11% Butadiene 4.02% 9.20 kmol h-1

GS2Permeate 1.36 kmol h-1 n-C4H10 46.24% H2 0.01% N2 0.13% Butenes 47.16% iso-C4H10 1.46% C1-C3 0.58% Butadiene 4.42%

Figure 9. Case Study 4: Flow sheet, membrane units operating conditions and stream molar concentration. Figure 9. Case Study 4: Flow sheet, membrane units operating conditions and stream molar concentration. Table 5. Butenes recovery and concentration of the different case studies.

3.5 Case Studies Summary

Case Study 1

Case Study 2

Table 5 reports the 4 concentration n-C and recovery of Feed molar ratio n-CC 4 H10 :N2 = 50:50 4 H10 :H2 = 80:20

Case Study 3

Case Study 4

then-C four case studies. Owing higher n-C4 H10to :N2the = 50:50 4 H10 :N2 = 50:50 Permeate Permeate GS3 GS3 GS2Permeate + Permeate GSStudy C C4membrane, Case C4 total, C4 selectivity of PSM-Ag 1 showed a product stream with a high olefins 4 88.5% C4 recovery, % 34.7% 39.4% 14% MRRetentate MRRetentate MRRetentate MRRetentate C4 C4 C4 C4 concentration, Case Study 2 showed a C4 recovery of 88.5% but lower C4 molar concentration Permeate Permeate Permeate Permeate GS C4 molar 99.5%.GS3 GS3 GS2 total, C C4 C4 C4 4 99.9% 80% 78.4% 99.3% Permeate 1. Instead, Case Permeate Permeate (99.3%) with compared to the%CaseGS3 Study obtaining a rich C4 GS2 stream concentration, GSPermeate totalStudy 4 allowedGS3 Permeate Permeate Permeate Permeate olefins GS3Olefins GS3Olefins GS2Olefins GStotal, Olefins lower CC44 recovery (14%). 61.4% 87.1% 38.7% 12.9% MRRetentate MRRetentate MRRetentate MRRetentate recovery, % Olefins Olefins Olefins Olefins Permeate The production rate is defined as the ratio between the olefins downstream of Permeate Permeate recovered in the Permeate C olefins molar GS GS3Olefins GS3Olefins GS2Olefins 4 total, Olefins 99.5% 55.5% 43.3% 51.6% Permeate GS3Permeate GS3Permeate GS2Permeate concentration, total the system and % n-butane fed to the MR. ItGSdefines the efficiency of the process in terms of the flow Production rate, % 26.6% 44.9% 16.6% 5.5% rate of olefins recovered with respect to the n-butane fed to the MR. Butenes recoved as permeates (8) of rate =as the ratio between the olefins recovered in the downstream The productionProduction rate is defined 𝑛𝑛 − butane fed to Membrane Reactor the system and n-butane fed to the MR. It defines the efficiency of the process in terms of the flow rate The production rate was calculated referring to butenes recovered as products over n-butane of olefins recovered with respect to the n-butane fed to the MR. fed into the system. The production rate needs to be maximized to improve efficiency. Owing to the higher Butenes recovedsystem as permeates Production rate = butenes recovery compared to the other studies, Case Study 2Reactor is the best scheme with a(8) n −case butane fed to Membrane production rate of 44.9%. Instead, Case Study 4, characterized by high C4 concentration, has a very The production rate was calculated referring to butenes recovered as products over n-butane fed low production rate 5.5%. into the system. Industry wants a high recovery (99%) combined with a high concentration (99%). This can be The production rate needs to be maximized to improve system efficiency. Owing to the higher obtained by low temperature distillation (at high pressure). Membranes can help to improve this by butenes recovery compared to the other case studies, Case Study 2 is the best scheme with a production reducing the energy consumption. rate of 44.9%. Instead, Case Study 4, characterized by high C4 concentration, has a very low production rate 5.5%. Industry wants a high recovery (99%) combined with a high concentration (99%). This can be obtained by low temperature distillation (at high pressure). Membranes can help to improve this by reducing the energy consumption.


13 of 15

4. Conclusions In this work, an integrated membrane system for butenes production was proposed and investigated by means of a modeling analysis. The n-butane dehydroisomerization was considered to take place in a Pd-Ag membrane reactor able to promote conversion, meanwhile recovering a pure H2 stream in the permeate. A thermodynamics study revealed the operating conditions where the membrane reactor shows better performance than a traditional reactor. For instance, at 550 ◦ C and 10 bar and 0.1 bar as reaction and permeate pressure, respectively, the membrane reactor equilibrium conversion was five times higher than that of a traditional reactor. The membrane reactor retentate, rich in unconverted reactants, reaction products, inerts, etc. but poor in hydrogen, was post-treated in a multistage membrane separation system. Four different case studies were investigated, analyzing different options in terms of membranes used and operating conditions suitable to maximize a certain separation. Globally, a high butenes recovery corresponded to a lower C4 olefins concentration (55.5%) with respect to Case Study 1, as in Case Study 2 with C4 recovery of 88.5%. In line with this conclusion, Case Study 4 exhibited a C4 concentration of 99.3% and lower C4 recovery (14%). Owing to the high membrane selectivity in the GS3, Case Study 1 is the best in terms of olefins concentration (99.5%). The Case Study 2 proved to be the best in terms of production rate, the ratio of olefins recovered as permeate and the n-butane fed to the integrated system. Author Contributions: All authors equally contributed to the various aspects of the research and in writing and correcting the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations Acronyms FCC GS MR MREC PI TR TREC

Fluidized catalytic cracking Gas separator Membrane reactor Membrane reactor equilibrium conversion Process intensification Traditional reactor Traditional reactor equilibrium conversion

Variables A H K m n n0 P Q T x x1 x2 z

Surface area, m2 Enthalpy, J mol−1 Equilibrium constant, Mole ratio, Mole number, Initial total mole, Pressure, bar Volumetric flow rate, m3 h−1 Temperature, ◦ C Molar fraction, Conversion dehydrogenation reaction, Conversion isomerization reaction, Fraction of hydrogen permeate, -

Greek letters θ φ

Permeation number, Feed/Permeate pressure ratio, -


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