Ind. Eng. Chem. Res. 1998, 37, 2571-2581
2571
REVIEWS Olefin/Paraffin Separations by Reactive Absorption: A Review Douglas J. Safarik* and R. Bruce Eldridge† Separations Research Program, The University of Texas at Austin, Austin, Texas 78758
Light olefins and paraffins are commonly separated by cryogenic distillation. A process based upon reversible chemical complexation, which employs a mass-separating agent rather than an energy-separating agent, presents an attractive alternative to distillation. Use of such a facilitated-transport-assisted process could substantially reduce the capital costs and energy requirements of olefin/paraffin separations. Copper(I) and silver(I) have long been known to form electron donor/acceptor complexes with olefins. Several chemical systems using these transition metals as the selective separating agent have been studied. A review of copper- and silver-based complexing solutions for olefin/paraffin separations via gas/liquid contacting is presented. Introduction Cryogenic distillation has been the dominant technology utilized for light olefin/paraffin separations for many years. Although distillation is reliable and essentially unchallenged in this application, the necessary low temperatures and high pressures make it an energyintensive separation scheme. A U.S. Department of Energy study estimated that 0.12 Quads (1 Quad ) 1015 BTU) of energy is expended annually for cryogenic distillation of olefin/paraffin mixtures (Eldridge, 1993). Furthermore, a large portion of the capital cost of an olefins plant is devoted to the large distillation columns in the separations train. The enormous capital and operating costs associated with cryogenic distillation processes have motivated continuing research into alternative olefin/paraffin separation schemes. Mixtures of light olefins and paraffins produced in the petroleum refining process are often used as refinery fuel (Eldridge, 1993). Recovery of olefins in these streams would be a substantial conservation of resources. Furthermore, federal environmental regulations such as the Clean Air Act will require reduction of hydrocarbon emissions from chemical processing facilities to low levels. Waste hydrocarbon streams from polyolefin processes and polymer storage facilities, which are typically flared, must be dealt with in an environmentally acceptable manner. Processing these small-volume hydrocarbon streams will require selective and economical separation technologies. Separations Based upon Reversible Chemical Complexation A selective and potentially low cost separation process may be achieved through reversible chemical complexation. Separations based upon reversible chemical complexation utilize a mass-separating agent to dis* Address correspondence to this author. Phone: (512) 4717072. E-mail:
[email protected]. † E-mail:
[email protected].
criminate between and separate different molecules (King, 1987). In contrast, distillation employs an energyseparating agent. In a process based on facilitated transport, the feed mixture is contacted with a second phase which carries a reagent that selectively and reversibly complexes the component of interest. After separation of the phases, the weak chemical interaction is reversed by displacement by another species, by temperature swings, and/or by pressure swings, and the unaltered solute is recovered. It is desirable to use an agent that forms a stable yet reversible complex to provide an improvement in selectivity over energyseparating agents. However, the electron donor/acceptor interaction must be weak enough to permit regeneration in an economically acceptable manner. The facilitator must be inert toward undesired components and thermally stable to avoid irreversible loss. Finally, rates of complexation and decomplexation must be rapid to minimize contactor size. Reversible chemical complexation or reaction is well established as a separation technique in analytical chemistry (King, 1987). On an industrial scale, it is employed in gas purification and metals extraction processes. Removal of CO2 and H2S from gas streams with amine solutions is a common industrial use of reactive absorption. Reversible Chemical Complexation for Olefin/ Paraffin Separations An olefin/paraffin separation process based upon reactive absorption has two major potential benefits. Utilization of a mass-separating rather than an energyseparating agent could substantially reduce energy requirements. Furthermore, a selective facilitator with a large olefin capacity and fast reaction rates would permit use of smaller contactors than are currently employed in distillation. The formation of electron donor/acceptor complexes between alkenes and some transition metals has long been known. Olefin-metal complexes were first identi-
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2572 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 1. Dewar-Chatt model of π-bond complexation.
fied in 1827 with the discovery of a platinum(II)ethylene complex known as Zeise’s salt (Long, 1972). The nature of this weak chemical bonding was first satisfactorily explained in 1951 by Dewar (Herberhold, 1974). Using molecular orbital theory applied to silverethylene complexes, Dewar postulated that interactions between the metal’s atomic orbitals and the olefin’s hybrid molecular orbitals were responsible for metalolefin complex stability. Chatt and Duncanson refined Dewar’s description of metal-olefin bonds based upon their study of platinum(II)- and palladium(II)-olefin complexes (Herberhold, 1974). The metal-olefin bonding described by the Dewar-Chatt model is commonly known as π-bond complexation. The Dewar-Chatt description, as applied to Cu(I)or Ag(I)-ethylene complexes, is shown in Figure 1. The complex is formed by double bonding of a Cu(I) or Ag(I) atom with the olefin (Quinn, 1971; Long, 1972; Herberhold, 1974; Yamamoto, 1986; Bochmann, 1994). Both the metal and alkene act as an electron donor and acceptor in the complexation interaction. A σ component of the bond results from overlap of the vacant outermost s atomic orbital of the metal with the full π (bonding) molecular orbital of the olefin. This new molecular orbital, formed by donation of electrons from olefin to metal, has electron density concentrated between the bond members. In Cu(I) and Ag(I) ions, the outermost s orbital is empty because the single electron present in the metal is lost upon ionization to a +1 valence. In nonionizing facilitators, the metal is often bound to an electronegative atom. These electronegative atoms withdraw electron density from the metal, resulting in a partial positive charge and a substantially vacant outermost s orbital. A π component of the metal-olefin bond is formed by backdonation of electrons from the full outer d atomic orbital of the metal to the vacant π* (antibonding) molecular orbital of the olefin. This new molecular orbital has a nodal plane of electron density between the members of the bond. Due to reversibility of their complexes and relatively low cost, silver(I) and copper(I) are the most suitable transition metals for olefin/paraffin separations (Keller et al., 1992). Other transition metals such as Pd(II), Hg(II), and Pt(II) complex with olefins (Herberhold, 1974). However, these facilitators are impractical due
to safety concerns or expense. These agents also form comparatively stable complexes that are difficult to reverse. Reversible π-complex formation in metal-containing solutions can be utilized for selective recovery of olefins from gas streams. For industrial-scale application, a metallic salt solution should exhibit the following properties (Eldridge, 1993; Keller et al., 1992; King, 1987; Herberhold, 1974): (1) A large absorption capacity and high selectivity for olefins. (2) A complexation reaction that is reversible in an economically feasible manner with temperature and/or pressure changes. (3) Rapid complexation and decomplexation reactions. (4) The absence of side reactions which lead to destabilization or irreversible loss of the facilitator or olefin. Of particular concern are the effects of H2, CO, CO2, H2S, and other sulfur compounds, C2H2 and other alkynes, and the solvent. Solutions and Processes for Reactive Olefin/ Paraffin Separations The tendency for formation of metal-olefin π complexes in either aqueous or nonaqueous solutions will be determined by the following factors (Herberhold, 1972): (1) Nature of the Facilitator • The electron donor/acceptor properties, solubility, and degree of ionization of the facilitator will affect the molar and absolute equilibrium capacity of the solution. (2) Nature of the Solvent • The solvent structure and polarity will influence the solubility of both olefin and facilitator as well as the extent of ionization of the facilitator. This ultimately impacts the complexation capabilities of the system. (3) Concentration of the Complexing Agent • The concentration of the complexing agent influences the absorption capacity and olefin selectivity of the solution via salting in and salting out effects. (4) Nature of the Olefin • The olefin’s electron donor/acceptor behavior, molecular weight, and steric hindrance about the double bond(s) dictate π complex stability. (5) Additives to the Solution • The chemical nature and concentration of additives can influence olefin-metal and/or solvent-metal interactions. (6) Temperature and Pressure • Increased temperature discourages the exothermic π complexation reaction. Higher pressures increase molar and absolute absorptivity of the solution. I. Aqueous System Agents. A. Silver. Studies of silver in aqueous solutions have concentrated on AgBF4 and AgNO3, with particular emphasis placed upon silver nitrate. i. Silver Nitrate. Aqueous silver nitrate is the most completely characterized absorbing solution for olefin/ paraffin separations. Keller et al. (1992) have published a thorough review of this system. Their paper included vapor-liquid equilibrium data, process engineering and design considerations, and economic and risk analyses. Silver(I)-olefin complex formation in an aqueous AgNO3 solution has been studied (Herberhold, 1974).
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2573
Figure 2. Molar ethylene absorption by AgBF4 and AgNO3 solutions, according to Baker (1964).
Figure 3. Absolute absorption of cyclohexene by a AgNO3 solution (Herberhold, 1974).
In this system, equilibria of several reactions are possible:
Ag+ + olefin 79 8 [Ag(olefin)]+ K
(1)
[Ag(olefin)]+ + Ag+ 79 8 [Ag2(olefin)]2+ K
(2)
8 [Ag(olefin)2]+ [Ag(olefin)]+ + olefin 79 K
(3)
1
2
3
etc. The stoichiometry of the complex is highly dependent upon silver and olefin concentrations. Olefin-Ag(I) complexes have defined compositions only at low salt concentrations. In dilute solutions, the 1:1 Ag(olefin)+ complex (reaction (1)) predominates. As the silver ion concentration increases, Ag2(olefin)2+ becomes more common, although it can often be ignored since K2 is typically small. Experiment has shown that the Ag(olefin)2+ complex does not form in dilute aqueous AgNO3. Species distribution in concentrated silver nitrate solutions has not been reported in the literature. As shown in Figure 2, the molar absorptivity (moles of olefin absorbed per mole of Ag+) of the solution decreases as the silver nitrate concentration increases (Baker, 1964). However, the total amount of olefin absorbed per solution volume (absolute absorptivity) increases. Decreases in molar absorptivity result from enhanced Ag+/NO3- interactions in concentrated solutions (Herberhold, 1972; Crookes and Woolf, 1973; Featherstone and Sorrie, 1964). These self-complexing interactions restrict the formation of the silver ionolefin complex. The overall ability of the AgNO3 solution to absorb olefins increases slowly with concentration at first, but solutions between 4 and 10 M exhibit a sharp increase in absolute alkene absorption capacity. This is shown in Figure 3. Absorption capacity decreases with increasing temperature. Molar absorption capacity exhibits a Langmuir-type dependence on olefin partial pressure. This is shown in Figure 4 (Keller et al., 1992). Molar absorptivity increases linearly with pressure at low pressures and approaches a maximum at high pressures. This isotherm is characteristic of a reversible chemical reaction in the liquid. Maximum olefin loading depends on
Figure 4. Molar absorption of ethylene in aqueous AgNO3 at 5 and 25 °C (Keller et al., 1992).
temperature and silver nitrate concentration; increases in either reduce molar absorptivity. The equilibrium capacity of aqueous AgNO3 may be altered in several ways. Addition of another alkene to the liquid markedly increases the olefin absorption capacity (Herberhold, 1972; Crookes and Woolf, 1973). Amine salts, such as monobutylamine nitrate, have reportedly increased the olefin absorption capacity of an aqueous silver nitrate solution (Quinn, 1971). Metal tetrafluoroborates (MBF4) or tetrafluoroboric acid (HBF4) can also increase silver nitrate’s ethylene absorption capacity (Baker, 1964). This phenomenon is observed despite the low ethylene affinity demonstrated by solutions containing only metal tetrafluoroborates or tetrafluoroboric acid. In solution, boron is a highly hydrated cation that competes with Ag(I) ions for water of solvation. This competition reduces the hydration of the silver ions, freeing them to complex with olefins. Measurements of silver ion activity coefficients support this hypothesis. The activity coefficient of the silver ion, γAg+, decreases with increasing salt concentration. This corresponds to the decreased moles of C2H4/moles of Ag(I) ratio as Ag+/NO3- interactions limit formation of the desired complex. Addition of HBF4 or MBF4 increases the activity coefficient of Ag(I) by reducing the metal ion’s degree of hydration. As the fluoroborate concentration is increased, silver ion hydration decreases and
2574 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 6. Union Carbide process based upon an aqueous AgNO3 solution (Keller et al., 1992). Figure 5. Effect of HBF4 on molar ethylene absorption by 2 M AgBF4 and AgNO3 solutions (Baker, 1964).
γAg+ rises, strongly favoring the Ag+/C2H4 complex formation. This effect is apparent in Figure 5. Since protons are heavily hydrated in aqueous solution, one might expect that they elicit the same effect as fluoroborates (Herberhold, 1972; Baker, 1964). In the case of the tetrafluoroboric acid additive, both the proton and tetrafluoroborate ion act to reduce silver solvation. However, in many cases the anion introduced with the proton associates with the silver cation, negating any increase in silver activity attributed to reduced solvation. ii. Silver Tetrafluoroborate (AgBF4). Silver ions form π complexes with olefins more easily in tetrafluoroborate solutions than in AgNO3 solutions (Herberhold, 1972). As shown in Figure 2, aqueous AgBF4 can absorb more olefins than AgNO3. As opposed to silver nitrate, the molar absorptivity of alkenes in aqueous AgBF4 increases with the salt concentration. The difference in concentration effects on molar absorptivity can be explained by the activity of Ag(I) ions in the solutions. Silver ion activity coefficients decrease as the AgNO3 concentration increases due to Ag+/NO3- complexing interactions. In silver tetrafluoroborate solutions, however, the boron atom is highly hydrated and competes with silver ions for water molecules of hydration. In other words, the NO3- anion prefers to associate with Ag+, while BF4- prefers to associate with water. Thus, the presence of tetrafluoroborate effectively frees more Ag+ ions to complex with olefins. The absorption capacity of the AgBF4 solution may be increased in several ways. Addition of small quantities of olefin significantly increases the solubility of aromatic compounds (Herberhold, 1972; Quinn, 1971). Metal tetrafluoroborate (MBF4) and tetrafluoroboric acid (HBF4) additives further reduce the solvation of silver ions and increase their activity (Baker, 1964). This increases the molar absorptivity of olefins, as shown in Figure 5. Several researchers have investigated aqueous silver tetrafluoroborate solutions for olefin/paraffin separations, and patents have been issued to DuPont, Imperial Chemical Industries, and Farbenwerke Hoechst (Quinn, 1971). iii. Other Silver Salts. Baker (1964) and Featherstone and Sorrie (1964) have investigated aqueous solutions of silver perchlorate (AgClO4) and silver trifluoroacetate (AgCF3CO2) (Herberhold, 1972). These
salt solutions exhibit ethylene absorptivity greater than that of AgNO3 but less than that of AgBF4. Silver perchlorate has an ethylene molar absorptivity versus salt concentration behavior similar to that of AgBF4, while trifluoroacetate exhibits essentially constant absorptivity regardless of salt concentration. Kinetics of Complexation in Aqueous Silver Salt Solutions. Only one study of π bond complexation kinetics was encountered in the literature (Herberhold, 1972). Ethylene was absorbed into an aqueous silver trifluoroacetate/sodium trifluoroacetate solution. The olefin/silver reaction was found to be bimolecular and reversible, with equilibrium achieved in about 20 min. B. Copper. The copper(I) ion is chemically similar to the silver(I) ion. Consequently, several olefin/paraffin separation schemes employing solid copper(I) adducts and copper solutions have been examined. However, the chemistry of aqueous copper complexing solutions has been studied less intensively than corresponding silver systems. Vapor-liquid equilibria, complexation reaction equilibria, and complexation/decomplexation kinetics for aqueous Cu(I) systems are not discussed in the literature. Thus, no quantitative comparison of the copper and silver systems can be made. Long (1972) has published a thorough examination of the use of solid CuCl and CuBr salts for separation of unsaturates from a hydrocarbon mixture. This fundamental study remains one of the most detailed sources on copper complexation available in the open literature. Aqueous Solution Based Separation Processes. A. Silver. Union Carbide has developed an olefin separation process using aqueous silver nitrate as the absorbing solution (Keller et al., 1992). A flow diagram is provided in Figure 6. The crude gas stream containing ethylene is fed to the bottom of a countercurrent flow packed absorption column. A packed rather than a trayed column is preferred to minimize the solution inventory in the column. This column operates at 240 psia and 30-40 °C. Heat evolved from the exothermic π bond complexation reaction is recovered from the column. The C2H4-laden solution is then fed to a lowpressure vent column, where product ethylene is used to strip small amounts of residual species (such as H2, CH4, C2H6, CO2, CO) from the liquid. These components are typically present in the feed stream and may be physically absorbed or weakly complexed with the silver ions. Vent gas from the second column is compressed and recycled back to the feed gas stream. The vent
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2575
column bottoms liquid, which contains high-purity C2H4, is fed to a reboiled stripping/decomplexation column that operates at 50 °C and subatmospheric pressure. Energy recovered from the absorption column is supplied to the reboiler. The lean silver solution is returned to the absorption column to complete the process. Impurities in the feed may react irreversibly with or destabilize the silver ions (Keller et al., 1992). Of particular concern are hydrogen sulfide and other sulfur compounds, acetylene and other alkynes, carbon monoxide, carbon dioxide, and hydrogen. All aqueous silver(I) solutions are stable in the presence of CO, CO2, and saturated hydrocarbons. These components are only physically absorbed in the solution. Hydrogen gas effects a gradual reduction of Ag(I) ions and subsequent precipitation of metallic silver. Small quantities of an oxidizing agent, commonly hydrogen peroxide, are added to stabilize the solution and prevent silver loss. Nitric acid is also added since it decreases H2O2 requirements via a synergistic effect. The H2O2/HNO3 stabilizer is added just prior to feeding the solvent to the absorption column. Hydrogen sulfide and perhaps other sulfur compounds react irreversibly with silver(I) ions to form silver sulfide precipitates. Keller et al. (1992) suggested the only solution to this problem is pretreatment to reduce sulfur compound concentrations to low levels. Acetylenes react irreversibly with silver(I) ions regardless of the anions present (Keller et al., 1992; Marcinkowsky et al., 1979). Silver acetylides, with the general formula Ag2C2, are formed in this reaction. Solid acetylides are unstable and shock-sensitive; dry silver acetylides are extremely unstable and present a detonation hazard. Acetylide formation also wastes expensive silver ions from the process. Keller et al. (1992) recommended reducing the acetylene concentration to below 1 ppm prior to feeding a gas stream to this process. Even at this low influent concentration, some acetylides are inevitably formed. Silver permanganate, AgMnO4, is added to a small lean solvent sidestream to oxidize the acetylides. This reaction occurs under neutral or slightly acidic conditions at 75 °C and subatmospheric pressure. The silver acetylides are destroyed, producing solid manganese dioxide and carbon dioxide gas. This regeneration unbinds the silver and does not introduce a foreign cation. Farbenwerke Hoechst has worked extensively on an ethylene separation process utilizing aqueous silver tetrafluoroborate with tetrafluoroboric acid additive (Quinn, 1971). This work has been conducted on the experimental, pilot-plant, and commercial scales. The process flow diagram is similar to that shown in Figure 6. Ethylene is absorbed at 1 atm pressure and 20 °C in a packed column. A vent column where CO2, CO, H2, C2H6, C3H8, and other residual gases are stripped from the olefin-laden solvent follows. Finally, pure ethylene is recovered at 30 °C and 0.3 atm pressure in a desorption column. As with the Union Carbide process, feed stream impurities are incompatible with the silver ions (Quinn, 1971). To avert silver acetylide formation, C2H2 must be removed prior to feeding the gas to the absorption column. Although the effect of sulfur compounds was not discussed, it is likely that insoluble Ag2S is formed in their presence. Carbon monoxide and CO2 do not react irreversibly with Ag(I) or destabilize the solution. Hydrogen peroxide was added at the rate of 1 g/kg of ethylene absorbed to prevent reduction of Ag(I) to silver metal by reducing gases.
B. Copper. Berthelot and then Manchat and Brandt first investigated the ethylene absorption properties of copper(I) chloride solutions (Herberhold, 1972). In the 1930s, Synthetic Ammonia and Nitrate Ltd. conducted experimental and pilot-plant work on copper solutions (called “copper liquor”) for selective olefin removal from gas streams (Miller, 1969). However, copper acetylide formation presented a safety hazard and wasted ions from solution. This effort was abandoned, as no practical means of reducing the acetylene concentration to below 1 ppm was available. During World War II, aqueous cuprous ammonium acetate was successfully used for purification of butadiene in the production of synthetic rubber (Blytas, 1992). Aqueous ethanolamine cuprous nitrate was employed by Imperial Chemical Industries (I.C.I.) in one stage of an ethylene plant at Gendorf, Bavaria, from 1958 to 1968 (Miller, 1969). This copper liquor process separated olefins from cracked gases. Selective acetylene hydrogenation reduced the C2H2 concentration to low levels, minimizing copper acetylide formation and therefore enabling use of Cu(I). The Gendorf plant consisted of four stages (Miller, 1969): (1) Steam cracking of naphtha with a 4:1 C2H4/C2H2 product ratio and a 50% yield. (2) Monoethanolamine scrubbing to reduce the CO2 concentration from 14 to 0.4 vol %, followed by selective hydrogenation to reduce the acetylene concentration to 1 ppm from 7 vol %. (3) Separation of C2H4 and C3H6 using an aqueous Cu(I) ethanolamine solution. (4) A Linde unit to obtain pure ethylene from the stage 3 product gas. The I.C.I. olefin recovery process was based upon an absorber/stripper scheme (Miller, 1969). In addition to absorption and decomplexation towers, an intermediate vent column was employed to strip weakly complexed or physically absorbed species from the solution. Table 1 gives plant operation data. Note that the absorption column operated at high pressure, while desorption occurred at atmospheric and subatmospheric pressures. The product olefin gas contained significant quantities of CO, CH4, C2H6, and H2; the process concentrated carbon monoxide. The copper liquor system was also susceptible to contaminants in the feed gas (Miller, 1969). Acetylide formation was largely prevented with selective hydrogenation. The effects of sulfur compounds were not discussed, but insoluble copper sulfides likely form in their presence. Hydrogen, carbon monoxide, and carbon dioxide did not destabilize the solution. A comparison of industrial operation of the I.C.I. CuNO3/ethanolamine and the Farbenwerke Hoechst silver tetrafluoroborate systems is given in Table 2. Important differences between the AgBF4 and CuNO3/ ethanolamine-based systems include (Miller, 1969) the following: (a) Solution Composition. Both solutions are dense (and presumably viscous), but the AgBF4 solution is 50% denser than the copper liquor. This may present mass-transfer and pumping difficulties. (b) Price. The AgBF4-based system is over 60 times more expensive per volume of aqueous solution. A comparison of price per absolute ethylene absorption capacity at the respective operating conditions reveals that AgBF4 is over 8 times more costly.
2576 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 Table 1. Operating Data from the Gendorf Plant (Miller, 1969) pressure (atm, absolute)
temperature (°C)
absorber intermediate flash tower desorption unit I desorption unit II
29 7.5 1.2 0.4
absorber intermediate flash tower desorption units I and II
3500 m3 STP/h 120 m3/h
volume of cracked gas to absorber volume of circulating copper solution inlet gas m3/h H2 CO CH4 C2H6 C2H4 C3H8 C3H6
(STP)
intermediate flash tower mol %
m3/h
(STP)
20 26 50
residual gas
mol %
678 95 949 119 1519 3 137
19.4 2.7 27.1 3.4 43.4 0.1 3.9
8 1 19 2 7
20 2 48 5 18
3
3500
100.0
40
m3/h
(STP)
olefin gas
mol %
655
38.7
7
921 112 5 3 14
53.5 6.5 0.3 0.2 0.8
100.0
1720
100.0
m3/h
(STP)
mol %
5 94 9 5 1507
0.3 5.4 0.5 0.3 86.6
120
6.9
1740
100.0
Table 2. Comparative Data for Industrial Operation of Aqueous AgBF4 and CuNO3/Ethanolamine Olefin Recovery Processes (Miller, 1969) silver fluoroborate solution composition of solution
price absorption capacity practical operating capacity operating pressures operating temperatures desorption 1st stage 2nd stage 3rd stage materials
ethanolamine-copper nitrate solution
1 L contains 6 mol of Ag ) 645 g 9 mol of BF4- ) 879 g 0.55 L of H2O H2O2 addition of 1 g/kg of C2H4 with hydrogen-containing gases specific gravity ≈ 2 1 m3 ) approximately DM 100 000 CO ) 1 L/L of solution C2H4 ) 150 L/L of solution C3H6 ) 150 L/L of solution approximately 90 L of C2H4/L of solution olefin partial pressure up to 2 atm absolute depending upon gas composition absorption to 30 °C desorption to 80 °C 0.3 atm absolute; approx. 33% of pure gas 0.05 atm absolute; approx. 33% of pure gas 0.025 atm absolute; approx. 33% of pure gas up to 40 °C, stainless steel; above 40 °C, Diabon
(c) Absorption Capacity/Practical Operating Capacity. The AgBF4 solution is more selective for ethylene and propylene and absorbs little CO. Furthermore, it has a larger maximum ethylene capacity and an operating capacity over 6 times greater than the ethanolamine Cu(I) nitrate. (d) Operating Pressure. A much higher pressure is required for the copper solution to achieve a high absorption capacity. Desorption also occurs at a higher pressure in the copper-based system. (e) Construction Materials. The silver system requires use of Diabon steel if the temperature exceeds 40 °C. Long (1972), in his extensive review of solid CuCl and CuBr salts for olefin/paraffin separations, presented a brief discussion of a slurry-based copper chloride process. The process envisioned by Long is similar to previously reviewed schemes: a contacting or complexing device, followed by a vent tower, and then a decomplexation tower. More recently, research on olefin/paraffin separations using aqueous solutions has focused on membranes (Ho and Dalyrymple, 1994; Hsiue and Yang, 1993; Yang and
1 L contains 2.36 mol ) 150 g of Cu (170 g of Cu2O) 3.12 mol ) 200 g of NO3- (250 g of NH4NO3) 0.50 L of H2O 7.38 mol ) 450 g of monoethanolamine specific gravity ≈ 1.30 1 m3 ) approximately DM 1500 CO ) 36 L/L of solution C2H4 ) 19 L/L of solution C3H6 ) 4 L/L of solution approximately 13-14 L of CO + olefins/L of solution olefin partial pressure up to 12 atm absolute depending upon gas composition absorption to 30 °C desorption to 50 °C 1.2 atm absolute; approx. 67% of pure gas 0.4 atm absolute; approx. 33% of pure gas standard structural steel
Hsiue, 1996; Funke et al., 1993; Antonio and Tsou, 1993). In these applications, a silver or copper solution is passed through the membrane interior (“liquid membrane”), or the membrane is impregnated with a metallic salt. Metal ions complex olefins on the feed side of the membrane, hasten the diffusion process, and release olefins on the permeate side. Stone and Webster Engineering, in conjunction with British Petroleum Chemicals, has tested a “continuously regenerated” liquid membrane using a silver-based facilitator (Barchas and Wallsgrove, 1996). They are also attempting to develop a copper-based solution which is stable in the copper(I) state, is not reduced by H2 gas, easily and rapidly binds and unbinds olefins, and is not corrosive. Although current emphasis is on a membrane application, Stone and Webster and British Petroleum envision the eventual development of a large-scale tower-based contacting scheme. II. Nonaqueous System Agents. Copper. Nonaqueous copper-based solutions have been proposed for selective separation of olefins. The competitive equilibrium in such systems may be described by the
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2577
following reactions (Blytas, 1992):
2Cu(I) 79 8 Cu0 + Cu(II) K
(4)
Cu(II) + aL 79 8 Cu(II)La K
(5)
8 Cu(I)Lb Cu(I) + bL 79 K
(6)
8 Cu(I)(OL)d Cu(I) + dOL 79 K
(7)
Cu(I)Lb + dOL 79 8 Cu(I)Lb(OL)d K
(8)
1
2
3
4
5
Note: OL represents an olefin ligand and L any other ligand (such as the solvent, an anion, or a stabilizing ligand). In aqueous systems, the cupric state of copper is more stable than the cuprous state (Blytas, 1992). Reaction (4) equilibrium therefore favors the products, resulting in disproportionation of Cu(I) to Cu(II) and copper metal. Copper(II) effectively coordinates ligands classified as hard bases, such as water, in reaction (5). This consumes cupric ions and drives reaction (4) to further disproportionation of cuprous ions. To prevent depletion of Cu(I) by this mechanism, reaction (6) must compete with reaction (5). This can be accomplished via introduction of a stabilizing ligand to retain Cu(I) in the monovalent state. Ammonia, pyridine, and alkanolamines are potential stabilizers. For example, at 25 °C K1 ) 106 in pure water; in an aqueous ammonia solution, K1 ) 10-2. Adding a complexing stabilizing ligand decreases cuprous ion activity. A lower activity indicates decreased interactions with alkene double bonds. Furthermore, hydrocarbons in aqueous systems have large activity coefficients. These nonidealities result in low hydrocarbon solubilities that tend to offset the Cu(I)-olefin affinity. Nonaqueous copper solutions exhibit minimal disproportionation of cuprous ions and do not require addition of a stabilizing ligand (Blytas, 1992). This is because the solvent often acts as a weakly complexing stabilizer. However, an inert (oxidizer-free) atmosphere must be maintained and moisture must be excluded to prevent disproportionation. Use of ionic cuprous salts in nonaqueous solvents for olefin/paraffin separations was first reported in 1968 (Blytas, 1992). Cuprous trifluoroacetate (CuTFA) in propionitrile and aromatic solvents was used to complex olefins. Cuprous nitrate and cuprous sulfate in propionitrile and cuprous chloroalanate in aromatics were also studied. Organic absorbing solutions are typically prepared via oxidation/reduction or double-replacement reactions. The solubility of resulting copper compounds is dictated by three factors (Blytas, 1992): (1) Salt/solvent coordination energy. (2) Polarity of the salt and the solvent. (3) Crystal energy of the salt or its solvates. The coordination energy is the most critical factor. Nearly all cuprous salts are quite soluble in coordinating solvents such as alkyl thiocyanates, alkyl sulfides, amines, and nitriles. Blytas (1992) reported solubilities up to 65 wt %. Only fluoroborate, methylsulfonate, and fluorosulfonate cuprous salts were not highly soluble in nitriles and amines. This is due to their high crystal energy. Solubility in aromatics is generally low. However, cuprous trifluoroacetate (CuTFA) dissolves in aromatics due to its low crystal energy. Cuprous
aluminum tetrachloride (CuAlCl4) and cuprous tetrafluoroborate (CuBF4) form stable complexes with aromatics, leading to high solubility. Such behavior is common of CuX/Lewis acid salts. Typically, though, the coordination strength of aromatics is insufficient to solubilize ionic cuprous salts. The solubility of saturated and unsaturated hydrocarbons is determined by cuprous salt complexing strength and solution polarity (Blytas, 1992). High complexing strength favors a large unsaturate capacity, while high polarity limits hydrocarbon solubility. Organic copper solutions generally exhibit a larger hydrocarbon capacity than aqueous systems. However, if saturated hydrocarbon solubility in the solvent is large, olefin selectivity will be poor. For this reason, a nonaqueous solvent must be selected carefully. Studies by Blytas (1992) show that ionic cuprous salts form stronger Cu(I)-olefin complexes than covalent cuprous salts. Propionitrile solvent/cuprous salt data indicate that the following order of increasing ionic character corresponds to increasing olefin complex stability: chloride < acetate < ethyl sulfonate ≈ phenyl sulfonate < di-tert-butyl salicylate ≈ salicylate < trifluoroacetate < chloroalanate Thus, copper(I) has a greater availability for olefin complexation in solutions of ionic cuprous salts than covalent cuprous salts. Competition between olefin and solvent for Cu(I) coordination sites increases in the order toluene < propionitrile < propylamine. Amine interactions with Cu(I) severely reduced the olefin complexing capacity. Propylamine’s strong interaction with the Lewis acid AlCl3 resulted in the reaction
CuAlCl4 + CH3CH2CH2NH2 f CuCl + CH3CH2CH2NH2AlCl3 Any strongly coordinating solvent may compete with alkenes for complexation sites or in the extreme case react with the cuprous salt. Complexation lowers the activity coefficient of olefins, thus increasing their solubility (Blytas, 1992). Salting out raises paraffin activity coefficients and reduces their solubility. Since capacity and selectivity increase with increasing salt content, these effects apparently intensify with increasing copper concentration. The effect of salt concentration and solvent on the selectivity and capacity of two systems is shown in Figure 7. Figure 8 shows the propylene and propane absorption capacity of a 1:1 molar CuTFA/xylene solution (Blytas, 1992). The olefin absorption isotherm again exhibits a Langmuir-type dependence on partial pressure, signaling the presence of a reversible chemical reaction. This behavior is also observed in other cuprous salt/organic solvent systems. The equilibrium capacity decreases as the temperature increases, indicating an exothermic complexation reaction. Propane absorption increases linearly with pressure, suggesting it is only physically absorbed. The characteristics of these solutions demonstrate their potential for use in low molecular weight olefin/ paraffin separations. This separation is possible in an absorber-stripper scheme such as that employed for aqueous solutions. Separation of heavier olefin/paraffin mixtures is possible by other contacting schemes. Much of the pioneering work in this field by Shell Development Co. (G. C. Blytas) and Exxon (R. B. Long)
2578 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 7. Capacity and selectivity of CuTFA/xylene and CuTFA/ propionitrile (EtCN) solutions. The parameter of the curves is moles of copper salt per moles of solvent (Blytas, 1992).
Figure 8. Equilibrium capacity of C3 hydrocarbons in the CuTFA/ xylene system (Blytas, 1992).
examined liquid/liquid extraction and extractive distillation as well as absorption/stripping-based separations. However, the current discussion focuses on absorption/ stripping applications. Aromatic solvents are suitable carriers of ionic cuprous salts for olefin absorption at low partial pressures (less than 3.5 atm) (Blytas, 1992). As such, aromatics would be appropriate for separation of alkenes from olefin-lean streams. At higher partial pressures (above 3.5 atm), nitrilic solvents are preferred to achieve greater olefin selectivity. Thus, propionitrile is a feasible solvent for olefin recovery from rich cracked gas streams. Although both systems may be regenerated by temperature and/or pressure swings, CuTFAolefin complexes in propionitrile are more easily reversed. To prevent thermal decomposition of CuTFA/propionitrile and CuTFA/xylene solutions, operating temperatures should not exceed 100 °C (Blytas, 1992). Copper nitrate/propionitrile systems must be kept below 95 °C to maintain stability. Copper nitrate in propionitrile exhibits a very low corrosion rate, while corrosion in CuTFA and Cu2SO4 systems can be controlled by addition of a metal salt of an o-hydroxy aromatic carboxylic acid (Blytas, 1972). Davis and Makin (1972) investigated nonaqueous copper systems for use in olefin/paraffin separations by
liquid extraction. They prepared solutions of cuprous tetrafluoroborate, cuprous hexafluorophosphate, cuprous trifluoroacetate, and silver tetrafluoroborate. Cuprous tetrafluoroborate was determined to possess superior π bond complexation characteristics. Copper atoms in CuBF4 could coordinate three or four ligands, while other salts could complex only two. Cuprous tetrafluoroborate solutions are sensitive to oxygen and disproportionate on contact with moisture (Davis and Makin, 1972). However, water ligands can be added to the complex via hydrated copper salts without destabilization. Aromatic solvents such as toluene are poor stabilizers of CuBF4 and necessitate the addition of stabilizing ligands in the absence of olefins. The coordination strength of cuprous ion/ stabilizer complexes cannot exceed that of Cu(I)/olefin complexes since stabilizers must be displaced by olefins in a separation process. Appropriate stabilizing ligands for use in olefin/paraffin separations are, in order of decreasing coordination strength, olefins ≈ ketones > tetrahydrofuran ≈ dioxane > ethers ≈ sulfones > aromatics. Davis and Makin (1972) reported that nitrilic solvents completely displaced hydrocarbon ligands and formed very stable coordination compounds with cuprous ions in a CuBF4 solution. When coordinated with four nitrile ligands, Cu(I) is unable to complex olefins. If only one nitrile ligand is coordinated, olefin complexation occurs and the solution is stable. Cuprous tetrafluoroborate salts in strongly complexing nitrilic solvents such as benzonitrile are stable in the presence of air and water. However, olefins cannot displace the nitrile ligands and therefore cannot complex with the Cu(I) ions. Amine, aldehyde, sulfoxide, and alcohol solvents elicited disproportionation of the cuprous ion. Sulfolane was an excellent solvent as it stabilized Cu(I) yet provided little resistance to ligand exchange with olefins. Hydrocarbon-Cu(I) complex stability in CuBF4 solutions increased in the following order (Davis and Makin, 1972): cyclic and aliphatic paraffins < aromatics < aliphatic diolefins ≈ aliphatic monolefins < acetylenes < cyclic diolefins ≈ cyclic olefins. Paraffins were believed to not complex with the cuprous ion. However, their solubility in organic solvents implies association with the solvent and perhaps secondary association with the copper. Cuprous tetrafluoroborate solutions also complexed nitrogen-, oxygen-, and sulfur-containing organic compounds (Davis and Makin, 1972). The complex strength with Cu(I) increased in the order O < S < N. It was believed that Cu(I) could coordinate a variety of sulfur- and oxygen-containing compounds yet still ligand exchange with olefins. However, contaminants in the feed to organic solvent-based separation processes are of great concern. Oxygen and water destabilize copper(I)/nonaqueous solvent systems and cause disproportionation in the absence of a stabilizing ligand (Blytas, 1992). Hydrogen sulfide may result in the formation of insoluble copper sulfides. Copper acetylides can form if alkynes are present; Cu2C2 precipitates are shock-sensitive. Carbon dioxide, CO, CH4, and H2 can destabilize the solution or be significantly absorbed. Tenneco exploited the stability of cuprous-CO complexes in development of the COSORB process in the early 1970s (Haase et al., 1982; Haase and Walker, 1974). The previously discussed nonaqueous systems can complex CO as well as unsaturates. This may
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2579
impede recovery of pure olefins from gas streams containing carbon monoxide. Long et al. (1971, 1972, 1973, 1979) proposed the use of bimetallic salts in aromatic solvents for reversible complexation of olefins, carbon monoxide, acetylenes, and aromatics. Of particular interest was CuAlCl4 in toluene. The order of stability of ligand coordination with CuAlCl4 was propylene > ethylene > acetylene > CO. Additional work on the CuAlCl4/toluene system by Walker revealed that hydrogen sulfide is also reversibly complexed (Walker, 1987). Pressure reduction, temperature elevation, and/or competitive complexation (ligand exchange) may displace ligands. Alkylation of the aromatic by the olefin can occur in the presence of aluminum chloride compounds. Excess halide salts in the decomplexing zone minimized these Friedel-Crafts reactions. The salt was believed to function by shifting the dissociation equilibrium of CuAlCl4 toward AlCl4-, thus inhibiting the catalytic activity of AlCl3. Long et al. (1973) later proposed blending multi-ring, highboiling aromatic solvents with single-ring, lower boiling aromatics. This decreased solvent volatility with little sacrifice of complexing properties and acceptable increases in viscosity. Horowitz and Tyler (1974) described the use of cuprous halocarboxylic acids in low-volatility aromatic solvents for separation of light olefins from paraffins. Preferred salts were cuprous perfluoroacetate, propionate, and butyrate. The solvent was selected to maximize cuprous salt solubility and minimize solvent losses during decomplexation. Halocarboxylic acids in aromatics reportedly do not promote alkylation of the aromatic with the olefin. Carbon monoxide and acetylenes are also complexed by these solutions. Horowitz and Tyler (1974) do not comment on the reversibility of these complexes but do suggest pretreatment to remove CO and C2H2. The effect of sulfur compounds is not discussed. Both CuAlCH3Cl3 and CuAlC2H5Cl3 in aromatic solvents are described as potential systems for C2-C4 olefin/paraffin separations (Kroll and Long, 1975). Alkynes and carbon monoxide are also reversibly complexed. Decomplexation can occur via ligand exchange or temperature/pressure swings. Halide salts are employed to prevent Friedel-Crafts reactions. Copper(I) carboxylates and sulfonates have been evaluated for selective removal of ethylene from gas streams (Tabler and Johnson, 1977; Cymbaluk et al., 1996). Preferred compounds are cuprous 2-ethylhexanoate and cuprous dodecylbenzenesulfonate in pxylene. These complexing reagents are claimed to exhibit high olefin capacity, low viscosity, easily reversed reactions, and low reactivity toward other molecules. Unlike most nonaqueous systems, the sulfonate was moisture-tolerant. Walker (1985) used cuprous aluminum cyanotrichloride (CuAl(CN)Cl3) in aromatic solvents to reversibly complex ethylene, propylene, carbon monoxide, and acetylene. This solution dissolved large quantities of unsaturated hydrocarbons and CO at atmospheric pressure and ambient temperature. Desorption was accomplished by lowering pressure and/or raising temperature. Like CuAlCl4 in aromatic solvents, CuAl(CN)Cl3 solutions exhibit fast gas/liquid mass-transfer and good heat-transfer characteristics. However, CuAl(CN)Cl3 has a much lower catalytic tendency than
CuAlCl4, and no inhibitor is required to prevent FriedelCrafts reactions. Research at Exxon by Ho et al. (1988) produced a novel complexing solution of cuprous (hexafluoroacetyl)acetonate (diketonate) in a weakly complexing olefinic solvent. Gaseous C2-C5 olefins were separated from paraffins and branched olefins from unbranched olefins via π bond complexation. The solvent stabilizes Cu(I) in the absence of an olefin and prevents disproportionation to Cu0 and Cu(II). During complexation, an olefin displaces the weakly complexing solvent in a ligandexchange process. Complexation with cuprous diketonate was suppressed by increasing steric hindrance at the ligand double bond. Ethylene absorption increased with steric hindrance at the double bond of the olefinic solvent. R-Methylstyrene, which had the greatest steric hindrance of the solvents investigated, permitted the greatest ethylene absorption. Separation factors of 17 for C2H4/C2H6 and 10 for C3H6/C3H8 were observed in R-methylstyrene. Steric hindrance at the double bond of branched olefins suppressed their complexation with cuprous diketonate, forming the basis of linear/branched olefin separations. Recently, Haase (1995) described the use of group I-B metal halide amines for selective removal of gases from multicomponent streams. The preferred metal is copper and the amine either benzylamine or 2-methylpentamethylenediamine. Studies were conducted in pyridine, benzylamine, and 2-methylpentamethylenediamine solvents. The cuprous portion of the treating agent reversibly complexes olefins, CO, oxygen, and alkynes. Acid gases such as CO2, H2S, mercaptans, carbonyl sulfide, and carbon disulfide react with the amine portions of the complex. This system can tolerate moisture although it reduces the efficiency of the process. Copper complexation and amine reactions can be reversed by temperature and/or pressure swings. Nonaqueous Solution Based Separation Processes Tenneco Chemical’s COSORB process utilizes cuprous aluminum tetrachloride (CuAlCl4) in toluene to selectively absorb carbon monoxide (Carbon Monoxide from Lean Gases, 1977; Haase and Walker, 1974; Haase et al., 1982; Kohl and Nielsen, 1997). Hydrogen, N2, CO2, CH4, and O2 are sparingly soluble in toluene but do not destabilize the solution or complex with Cu(I). However, water and sulfur compounds react irreversibly with the absorbent and must be removed upstream of the COSORB process. COSORB utilizes an absorber/stripper contacting scheme (Carbon Monoxide from Lean Gases, 1977; Haase and Walker, 1974; Haase et al., 1982). As seen in Figure 9, carbon monoxide is absorbed at atmospheric pressure and ambient temperature in a countercurrent absorption column. Solvent recovery from the column off-gas prevents excessive toluene losses (Kohl and Nielsen, 1997). The rich solvent enters a flash tank, where physically absorbed components are removed. The vent gas is scrubbed with lean solvent to recover CO and then sent to an aromatics recovery unit. Carbon monoxide is desorbed in a reboiled stripping tower at 135 °C. A carbon monoxide purity of 99.5% is attainable for most feed compositions. Solvent vaporized during decomplexation is recovered by condensation and adsorption. Commercial operation and laboratory tests have indicated minimal corrosion of carbon steel.
2580 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 9. Flow diagram of the COSORB process (Kohl and Nielsen, 1997).
Tenneco developed a variant of COSORB for olefin/ paraffin separations (Gutierrez et al., 1978). The ESEP process recovers ethylene from gas streams with CuAlCl4 in an aromatic solvent. Ethylene is complexed by the Cu(I) atom of the facilitator; increasing the temperature and/or lowering the pressure reverses the reaction. The solution is stable in the presence of H2, CO, CO2, N2, and C2H6, but carbon monoxide and alkynes complex the Cu(I) atom. Copper(I)-CO complexes are reversible; however, the reversibility of Cu(I)-C2H2 complexes was not discussed. Sulfur compounds react irreversibly with the absorbent, and moisture destroys the solution’s complexation abilities. A significant technological advancement incorporated into ESEP was the inhibition of Friedel-Crafts reactions. Through selection of the proper solvent and operating conditions, Tenneco substantially slowed aromatic alkylation by ethylene. The ESEP process is based upon an absorption/ stripping scheme similar to COSORB (Gutierrez et al., 1978). In a demonstration pilot plant, ESEP produced 99.97 mol % C2H4, 0.03% N2, and
