Title of Invention	PROCESS FOR THE SEPARATION OF LIGHT OLEFINS FROM PARAFFINS
Abstract	Process for the separation of light olefins from paraffins contained in mixtures, optionally also containing hydrogen, comprising bringing said mixtures into contact with an aqueous solution of one or more compounds of silver, preferably silver nitrate, and one or more ferric compounds, preferably ferrric nitrate.

Full Text

PROCESS FOR THE SEPARATION OF LIGHT OLEFINS FROM PARAFFINS. The present invention relates to a process for the separation of light olefins, having from 2 to 5 carbon at¬oms, from paraffins. In particular, light olefins and par¬affins may be present in streams coming from ethyl¬ene/propylene production plants, either traditional (steam cracking of gaseous charges, secondary streams from FCC for the production of fuels) or innovative (catalytic dehydro-genation of ethane/propane). The separation process of olefins by the reversible formation of n complexes with metal salts, and, in particu¬lar, with copper(I) and silver(I) salts, has been known for a long time: some applications go back to the forties", whereas the separation of ethylene with silver fluoborate was effected more or less in the sixties". Since then vari¬ous industrial applications have been developed, which have mainly privileged the use of solutions of copper salts (e.g. CO separation). One of the reasons for which the use of solutions of Ag+ was not successful in the field of the separation of olefins via absorption, although silver solutions have a high transporting capacity of olefins (at least double with respect to solutions of Cu(I) with an equal concentration), is the instability of the solution, which is particularly evident in the presence of a reducing environment. In fact, in reducing atmospheres, the Ag* ion undergoes a reduction reaction to metal Ag: Ag* + e" > Ag. The metal silver generated, separates from the solu¬tion in the form of a particulate reducing the olefin transporting capacity of the absorption solution. The solution which is normally proposed for overcoming the instability of silver solutions, is the addition of hy¬drogen peroxide which, in an acid environment, is capable of oxidizing (re-dissolving) metal Ag to Ag"*" ion, re¬establishing the initial transporting capacity of the solu¬tion. The use of H2O2 is not without disadvantages, among which the most important is the cost of the reagent, as well as its thermal instability. Processes for the recovery of olefins based on the formation of π complexes with metals which have been com¬mercialized, are limited to very few examples ("Separation and Purification Technology", N.N.Li, J.M. Calo-M. Dekker, Inc., 1992): among others, the separation of ethylene with a solution of copper nitrate/ethanolamine, which, initiated during the second world war, only operated for a short pe¬riod. In the fifties", Hoechst developed a process for the recovery of ethylene via absorption, using a concentrated aqueous solution of Ag fluoborate to fluoboric acid (US-2,913,505). This process passed through a pilot-plant level and demonstrative steps, without reaching final commer¬cialization, due to an unsatisfactory economic result. The activity effected by Hoechst illustrated the main problems connected with the use of a silver solution, which are the formation of Ag acetylides (explosive in the dry state) , the reduction of the Ag+ ion with destabilizing ef¬fects of the process solution and the considerably corro¬sive nature of fluoborate, which obliges the use of valu¬able, and therefore costly, metal materials for the plant construction. Union Carbide experimented on a pilot-plant level, the recovery of ethylene with an aqueous solution of Ag"*" ni¬trate, stabilizing the solution by the use of hydrogen per¬oxide and nitric acid. Silver permanganate is also added to the solution to oxidize the possible acetylene contained in the stream to be treated. In addition to these, other processes have also been proposed which do not specifically refer to ethylene, but more generally to the separation of olefins from paraffins. These include the use of both aqueous and non-aqueous sol¬vents to which salts, various types of acids have been added (US-3,347,948; US-4,132,744; US-2,449,793; US-3,189,658), or are directed towards the use of alternative methods for improving the contact between solute and sol¬vent, such as liquid/liquid extraction or functionalized membranes with Ag+ (US-3,758,603; US-3,758,605; US-3,770,842; US-3,844,735; US-3,801,664). The traditional separation process scheme essentially comprises two steps: an absorption step and a stripping step. The stream containing light olefins and paraffins is fed to an absorption column in which the olefins are ab¬sorbed by a particular absorbent (for example AgNOa + H2.O2) . A stream containing the paraffins and hydrogen leaves the head of this column and is sent to a separation step, whereas a stream consisting of the absorbent and olefins leaves the bottom and is, in turn, fed to a stripping col¬umn to separate the olefins from the absorbent which is re¬cycled to the absorption column. It has been surprisingly found that by adding a ferric compound (Fe3+) to the absorbing solution consisting of a silver salt, instead of hydrogen peroxide as stabilizer of the silver solution, in a reducing environment, the disad- vantages relating to the use of hydrogen peroxide are eliminated. It has been observed that in a reducing atmosphere the reaction of the ferric ion to ferrous ion becomes prevalent with respect to the reduction reaction of Ag+; in this way the concentration of silver ions remains unaltered as do the transporting properties of the solu¬tion. The advantage of the use of ferric salts particularly lies in the fact that the ferrous ions generated by the re¬duction reaction can be re-oxidized to ferric ions in the presence of air; consequently by providing an air-insufflation operation downstream of the desorption column, the absorption/desorption solution can be regenerated be¬fore entering the absorber. The presence of ferric salts, moreover, gives the so¬lution stability properties even at high temperatures so that the stripper can operate under conditions close to at¬mospheric conditions with a clear gain with respect to the overall economy of the processes so far proposed (Hoechst and Union Carbide) which use the stripper at sub-atmospheric pressure. The process of the present invention for the separa¬tion of light olefins from paraffins contained in mixtures. optionally also containing hydrogen, essentially comprises bringing these mixtures in contact with an aqueous solution of one or more silver compounds, preferably a silver salt, more preferably silver nitrate, and one or more ferric com¬pounds, preferably a ferric salt, more preferably ferric nitrate. In the aqueous solution, the silver compound is in a concentration preferably ranging from 0.1 to 6 M, more preferably from 1 to 3 M, whereas the ferric compound is in a concentration preferably ranging from 0.1 to 4 M, more preferably from 0.5 to 2 M. More specifically, the process can be carried out us-.ng various types of schemes which are described hereunder. One process, with a scheme analogous to the tradi-:ional ones, essentially comprises two steps: subjecting the mixture of light olefins and paraffins to absorption, whereby the light olefins are absorbed by an absorbent consisting of the aqueous solution of one or more compounds of silver and one or more ferric com¬pounds described above, obtaining a stream containing paraffins and a stream containing the absorbent and light olefins absorbed; subjecting the stream containing the absorbent and the light olefins absorbed to a stripping, whereby the light olefins absorbed are separated from the absorbent, which is recycled to the absorption step. In the absorption step, it is preferable to operate at temperatures ranging from 30°C to 50°C and at pressures ranging from 7 to 50 atm. , whereas in the stripping step, it is preferable to operate at temperatures ranging from eCC to 120°C and at pressures ranging from 0.2 to 2 atm. An illustrative scheme of the two-step process for the separation of light olefins from paraffins is provided in figure 1. The stream containing light olefins and paraffins (1) is fed to an absorption column (A) in which the olefins are absorbed by the absorbing solution (2) consisting of one or more silver salts and one or more ferric salts. A stream (3) containing paraffins leaves the head of this column, and is sent to subsequent processing operations, and a stream (4) consisting of the absorbent and olefins leaves the bottom, and is, in turn, fed to a stripping column (S) to separate the olefins (5) from the absorbent (6). A part of the absorbent solution (7) is removed laterally from the stripper and is sent, by means of the pump (P2), to a Re¬claimer (R) into which air (8) is insufflated in a quantity necessary for re-oxidizing that part of ferrous ions (Fe2+) produced by the reducing action of the hydrogen to ferric ions (Fe2+) . The re-oxidized solution (9) and the bottom stream of the stripper (6) are recycled (2) by means of the pump (PI) to the absorption column (A). A second process, in alternative to the first one, can be essentially carried out by means of the following two steps: subjecting the mixture of light olefins and paraffins, after being at least partially liquefied, to absorption, whereby the light olefins are absorbed by an absorbent consisting of the aqueous solution of one or more silver compounds and one or more ferric compounds described above, obtaining at the head, a stream containing the paraffins and possible inert products, which is sent to a subsequent fractionation step, and at the bottom, a stream containing the absorbent and light olefins ab¬sorbed; subjecting the stream containing .the absorbent and light olefins absorbed to stripping, whereby the light olefins absorbed are separated from the absorbent, which is re¬cycled to the absorption step. The operating conditions of the process steps substan¬tially correspond to the temperature and pressure ranges specified for the process described above. A general illustrative scheme of this alternative separation process is provided in figure 2, The stream containing light olefins and paraffins (1), after being compressed in (K) and at least partially lique- fied in (L) , is fed (2) to an absorption column (A) in which the olefins are absorbed by the absorbent solution (3) consisting of one or more silver salts and one or more ferric salts, A stream (4) containing paraffins and inert products leaves the head of this column, and is sent to a subsequent separation step, whereas a stream (5), consist¬ing of the absorbent and olefins, leaves the bottom and, in turn, is fed to a stripping column (S) to separate the ole¬fins (6) from the absorbent (7). A part of the absorbent solution (8) is removed laterally from the stripper and sent, by means of the pump (P2), to a Reclaimer (R) into which air (9) is insufflated in a quantity necessary for re-oxidizing that part of ferrous ions (Fe2+), produced by the reducing action of the hydrogen, to ferric ions (Fe2+) . The re-oxidized solution (10) and the bottom stream of the stripper (7) are recycled (3) by means of the pump (Pi) to the absorption column (A). A third alternative process, which, regardless of the absorbent solution used, solves the problem which arises when high concentrations of hydrogen are present in the mixtures, can be used. This alternative process proposed differs in that the olefin/paraffin separation is carried out in a liquid-liquid extractor, after separation of the hydrogen and other possible inert products by absorption of the frac- tion of olefins and paraffins in a suitable hydrocarbon. The process, which forms another object of the present invention, for the separation of light olefins from paraf¬fins contained in mixtures containing hydrogen and/or other inert products (N2, CO, etc.) is characterized in that it comprises the following steps: subjecting the mixture to a separation by absorption in a suitable hydrocarbon in order to separate the hydrogen and/or other inert products from the hydrocarbons con¬tained therein; subjecting the mixture separated from the hydrogen and/or other inert products to a liquid/liquid extrac¬tion using a suitable extracting agent, obtaining a stream containing the paraffins and hydrocarbon absorb¬ent and a stream containing the light olefins and ex¬tracting agent; subjecting the stream containing the paraffins and hy¬drocarbon absorbent to a regeneration to obtain a stream essentially containing said hydrocarbon which is recy¬cled to the separation step and a stream essentially containing paraffins; subjecting the stream containing light olefins and ex¬tracting agent to regeneration to obtain a stream essen¬tially containing the extracting agent which is recycled to the liquid-liquid extraction step and a stream essen- tially containing light olefins. The separation step of hydrogen from the hydrocarbons contained in the mixture can be effected in an absorption column preferably operating at temperatures ranging from 30°C to 50°C and at pressures ranging from 7 to 50 atm. The hydrocarbon absorbent used in this step can be se¬lected from those having, at atmospheric pressure, a boil¬ing point ranging from 30 to 220°C. The hydrocarbon absorb¬ents preferably used are those having from 3 to 12 carbon atoms, in particular naphtha and propane, alone or mixed with each other. The extraction step can be carried out in a liq¬uid/liquid extraction column preferably operating at tem¬peratures ranging from 30°C to 70°C and at pressures rang¬ing from 7 to 50 atm. The extracting agent can be selected from the same aqueous solutions of one or more compounds of silver and one or more ferric compounds described above or any solu¬tion that reversibly complexes the olefins such as, for ex¬ample, cupro-ammonia solutions, if the stream fed does not contain high quantities of CO. The regeneration step of the hydrocarbon absorbent can be effected in a stripping column preferably operating at temperatures ranging from SO°C to 230°C and at pressures ranging from 0.5 to 2 atm. The regeneration step of the extracting agent can be carried out in a stripping column, preferably operating at temperatures ranging from 60°C to 120°C and at pressures ranging from 0.2 to 2 atm. If the extracting agent is a so¬lution containing ferric salts, a reclaimer is necessary, as indicated in schemes 1 and 2, in which the re-oxidation operation of the ferrous ions, obtained by the action of hydrogen, to ferric ions, is effected. A general illustrative scheme of the separation proc¬ess described above is provided in figure 3. The charge (1) is fed to the absorption column (A) into which a hydrocarbon absorbent (2), for example naph¬tha, is introduced. A gas stream (3) containing hydrogen leaves the head of the absorption column (A) , whereas a liquid stream (4), containing the heavy hydrocarbon absorb¬ent with the absorbed compounds (paraffins and olefins), is obtained from the bottom and is fed to the lower part of an extraction column (E). A liquid stream (6) essentially con¬taining the hydrocarbon absorbent and paraffins is obtained at the head, with the use of an extraction solvent (5) fed to the upper part of the extraction column (E), and a liq¬uid stream (7) essentially containing the extracting sol¬vent and olefins, from the bottom. The stream (7) is sent to a regeneration column (S) of the solvent, from which olefins (8) are obtained at the head and the regenerated extracting solvent (5) at the bot¬tom, which is recycled to the extraction column (E). The liquid stream (6) is, in turn, fed to a regenera¬tion column of the absorbent (R) obtaining paraffins (9) at the head and the regenerated absorbent (2) at the bottom, which is recycled to the absorption column (A). A fourth alternative process, which, regardless of the absorbing solution used, solves the problem arising when high concentrations of hydrogen are present in the mixtures to be separated and also allows the regeneration apparatus of the hydrocarbon absorbent to be eliminated, can be adopted. The process, a further object of the present inven¬tion, for the separation of light olefins from paraffins contained in mixtures also containing hydrogen and/or other inert products (Na, CO, etc.) is characterized in that it comprises the following steps: subjecting the mixture of light olefins and paraffins, after being at least partially liquefied, to absorption, using a hydrocarbon identical to the paraffinic hydro¬carbon contained in the mixture in a greater quantity, obtaining inert products and/or hydrogen at the head and a stream containing olefins and paraffins also compris¬ing the hydrocarbon absorbent, at the bottom; subjecting the bottom stream to liquid-liquid extraction by means of a suitable extracting agent, obtaining a stream essentially containing paraffins also comprising the hydrocarbon absorbent, at the head, and a stream substantially containing olefins and the extracting agent, at the bottom; subjecting the stream containing olefins and the ex¬tracting agent to stripping, whereby the olefins are separated from the extracting agent, which is recycled to the liquid-liquid extraction step. The extracting agent can be selected from the same aque¬ous solutions of one or more compounds of silver and one or more ferric compounds described above or any solution which reversibly complexes the olefins such as, for example, cu-pro-ammonia solutions. The operating conditions of the process steps enter within the temperature and pressure ranges specified for the process previously described. A general illustrative scheme of the separation proc¬ess described above is provided in figure 4. The stream containing light olefins and paraffins (1), after being compressed in (K) and at least partially lique¬fied in (L) , is fed to an absorption column (A) in which the olefins are absorbed by the absorbent solution (2) con¬sisting of a hydrocarbon which is identical to the paraffin contained in a greater quantity in the starting mixture. A stream (3) containing hydrogen and/or inert products leaves the head of this column, and a stream (4) consisting of olefins, paraffins and the same paraffin absorbent, from the bottom, which is fed to the lower part of an extraction column (E) . A liquid stream (6) essentially containing paraffins and the same paraffin absorbent, is obtained at the head, with the use of an extraction solvent (5) fed to the upper part of the extraction column (E) , and a liquid stream (7) essentially containing the extracting solvent and olefins, from the bottom. The bottom stream (7) is sent to a stripping column (S), from which olefins (8) are obtained at the head and the regenerated extracting solution (5) at the bottom, which is recycled to the extraction column (E). The following three examples are provided for a better illustration of the invention but in no way limit its scope. EXAMPLE 1 In a plant for the production of 500,000 t/year of ethylene, the product must be recovered from a stream of 13,625 kmoles/h of crude gas containing 17.1 mol. % of eth¬ylene, 52.3% of ethane, 22.4% of hydrogen, 2.5% of CO, 3.1% of CH4, 0.5% of propane and 2.2% of butane. With the pro¬posed process schematized in figure 1, this stream, after being compressed at 31.4 bar, is put in contact, in a col- umn operating at 40°C, with 148,000 kmol/h of a liquid stream, consisting of a 6M aqueous solution of AgNOs and liM of Fe(N03)3, to form an Ag-olefin salt complex which leaves the bottom of the column, whereas the gaseous stream with¬out the olefin fraction (11,341 kmol/h with the following mol. % composition: ethylene 0.2, ethane 62.8, H2O 0.2, H2 26.9, CO 3, CH4 3.7, propane 0.6, butane 2.7) leaves the head. As the saline solution physically absorbs a portion of the paraffinic fraction and inert products, this solu¬tion, in order to increase the purity of the ethylene pro¬duced, is subjected to two successive expansions (at 15 bar and 65°C and at 7 bar and 90°C), whose vapour phase is com¬pressed to be sent to the absorption column, whereas the liquid phase is further depressurized to 0.9 bar before be¬ing sent to the regeneration column. The gaseous stream which leaves the head of this column has a flow-rate of 2,631 kmol/h and consists of ethylene with a very high pu¬rity (>99.99%) saturated with water. The regenerated saline solution leaves the bottom of the column and is sent, by means of a pump, to the head of the absorption column to restart its cycle; part of this solution, before re¬entering the cycle, is sent to a system into which air is insufflated, to re-oxidize the part of ferrous ions formed by the reducing action of hydrogen. EXAMPLE 2 23.4 kmol/h of ethylene are to be recovered from a stream of 284.2 kmol/h having the following mol. % compo¬sition: ethylene 8.71, ethane 51.07, H2O 0.39, N2 0.23, CO 0.52, CO2 2.14, CH4 32.83, propane 1.27, propylene 2.84 us¬ing the process scheme described in figure 1. After elimi¬nating the content of CO2, which in this case is an unde-sired compound in the stream of ethylene produced, with a caustic washing, the gas is compressed at 16 bar and then fed to the bottom of a column in which it comes into con¬tact, in countercurrent, with a stream of 3,100 kmol/h of a 4M aqueous solution of AgNOs. This column operates at 16 bar, 40°C and a stream leaves its head, whose flow-rate is 248.3 kmol/h and mol. % composition: ethylene 0.55, ethane 58.46, H2 0.44, N2 0.27, CO 0.60, CO2 0.00, CH4 37.58, pro¬pane 1.46, propylene 0.18, H2O 0.46; the solution of AgNOa containing the Ag-olefin complex, leaves the bottom of the column and, after a first expansion at a pressure of 1.5 bar to remove the paraffins and inert products physically absorbed, which are then recompressed to be fed again to the absorption column, it is preheated to 90°C and sent to the head of the regeneration column which operates at 0.7 bar. The gas leaving the head of the regenerator is cooled and compressed to eliminate the saturation water and is sent as product to the subsequent processing operations. Its quantity is 31.1 kmol/h and its mol. % composition: ethylene 75.14, ethane 24.47, N2 0.27, CO 2,232 kmol/h of ethylene are to be recovered from a stream of 17,393 kmol/h of a gas having the following mol. % composition: ethane 40.54, H2 19.61, ethylene 13.08, CH4 25.65, propane 0.01, propylene 0.06, butane 0.01, butene 0.11, higher olefins 0.06, CO 0.87, using the process scheme of figure 3. This gas, after being compressed at 35 bar, is sent to the bottom of an absorption column in which it meets, in countercurrent, a stream of 7,640 m3/h of "virgin naphtha". A gas essentially consisting of the frac¬tion of inert gases contained in the charge, leaves the head of the column, more precisely the molar percentage composition of this gas is the following: ethane 1.67, H2 93.35, ethylene 0.56, CH4 2.19, CO 2.23. The stream of naphtha charged with olefinic and paraffinic hydrocarbons leaves the bottom, and is sent to the bottom of a liq¬uid/liquid extraction column in which it is put in contact with 2,880 m3/h of extracting liquid, consisting of a 6M aqueous solution of AgNOs, fed to the head of the column. Two liquid phases leave the column, more precisely the ex¬tracting solution charged with olefins in the form of an Ag/olefin complex, leaves the bottom, whereas the naphtha charged with the paraffinic fraction alone, leaves the head. The extracting solution, after being depressurized at 0.7 bar, enters the head of the regeneration column from which the following products leave: from the head the prod¬uct stream (2,271 kmol/h of gas with the following molar percentage composition: ethylene 98.28, propylene 0.47, bu-tene 0.83, higher olefins 0.42); from the bottom the regen¬erated extracting solution which is sent, by means of a pump, to the head of the extraction column to restart the extraction cycle. The naphtha, charged with the paraffinic fraction alone, is, in turn, sent to a column which oper¬ates at a pressure that is slightly higher than atmospheric pressure (1.3 bar) in which the stripping takes place of the absorbed hydrocarbons, which leave the head (12,362 kmol/h of gas with the following composition: ethane 56.66, H2 6.76, ethylene 0.22, CH4 35.60, propane 0.02, butane 0.01, CO 0.72). The regenerated naphtha is then pumped to be sent to the head of the absorption column and subse¬quently restart the absorption cycle. WE CLAIM: 1. A process for the separation of light olefins from paraffins contained in mixtures containing hydrogen, comprising bringing said mixtures into contact with an aqueous solution of one or more silver compounds and one or more ferric compounds, wherein in the aqueous solution, the silver compound is in a concentration ranging from 0.1 to 6 M and the ferric compound is in a concentration ranging from 0.1 to 4M. 2. The process as claimed in claim 1, wherein the silver compound is a salt. 3. The process as claimed in claim 1, wherein the silver salt is silver nitrate. 4. The process as claimed in claim 1, wherein the ferric compound is a salt. 5. The process as claimed in claim 1, wherein the ferric salt is ferric nitrate. 6. The process as claimed in claim 5, wherein the silver compound is in a concentration ranging from 1 to 3 M and the ferric compound is in a concentration ranging from 0.5 to 2 M. 7. A process for the separation of light olefins from paraffins contained in mixtures also containing hydrogen and/or other inert products characterized in that it comprises the following steps: subjecting the mixture to a separation by absorption in a suitable hydrocarbon in order to separate the hydrogen and/or other inert products from the hydrocarbons contained therein; subjecting the mixture separated from the hydrogen and/or other inert products to a liquid/hquid extraction using a suitable extracting agent obtaining a stream containing paraffins and the hydrocarbon absorbent and a stream containing the light olefins and extracting agent; subjecting the stream containing paraffins and hydrocarbon absorbent to regeneration in order to obtain a stream essentially containing said hydrocarbon, which is recycled to the separation step, and a stream essentially containing paraffins; subjecting the stream containing light olefins and the extracting agent to regeneration in order to obtain a stream essentially containing the extracting agent, which is recycled to the liquid-liquid extraction step, and a stream essentially containing light olefins. 8. The process as claimed in at least one of the claims from 1 to 7, wherein the mixtures also contain hydrogen, characterized in that it comprises the following steps: subjecting the mixture to separation by means of known hydrocarbon absorbent in order to separate the hydrogen from the hydrocarbons contained therein; subjecting the mixture separated from the hydrogen to a liquid/liquid extraction using a known extracting agent consisting of the aqueous solution of one or more compounds of silver and one or more ferric compounds, obtaining a stream containing light paraffins and the hydrocarbon absorbent and a stream containing light olefins and the extracting agent; subjecting the stream containing paraffins and hydrocarbon absorbent to a regeneration to obtain a stream essentially containing the hydrocarbon absorbent, which is recycled to the separation step and a stream essentially containing paraffins; subjecting the stream containing light olefins and the extracting agent to a regeneration to obtain a stream essentially containing the extracting agent, which is recycled to the extraction step and a stream essentially containing light olefins. 9. The process for the separation of light olefins as claimed in claims 1 to 6, which comprises the following steps: subjecting the mixture of light olefins and paraffins, after being at least partially liquefied, to an absorption by means of a hydrocarbon identical to the paraffinic hydrocarbon contained in the mixture in a greater quantity, obtaining, at the head, inert products and/or hydrogen and, at the bottom, a stream containing olefins and paraffins also comprising the hydrocarbon absorbent; subjecting the bottom stream to a liquid-liquid extraction using an extracting agent consisting of the aqueous solution of one or more compounds of silver and one or more ferric compounds, obtaining at the head, a stream substantially containing paraffins and, at the bottom, a stream substantially containing olefins and the extracting agent; subjecting the stream containing olefins and the extracting agent to stripping whereby the olefins are separated from the extracting agent

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