Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By addition of extraneous agent – e.g. – solvent – etc.
Reexamination Certificate
2000-07-18
2002-07-02
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By addition of extraneous agent, e.g., solvent, etc.
C585S850000, C585S805000
Reexamination Certificate
active
06414210
ABSTRACT:
The present invention relates to a process for the separation of light olefins, having from 2 to 5 carbon atoms, from paraffins. In particular, light olefins and paraffins may be present in streams coming from ethylene/propylene production plants, either traditional (steam cracking of gaseous charges, secondary streams from FCC for the production of fuels) or innovative (catalytic dehydrogenation of ethane/propane).
The separation process of olefins by the reversible formation of &pgr; complexes with metal salts, and, in particular, 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 various 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 solution 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 hydrogen 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 solution.
The use of H
2
O
2
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 n complexes with metals which have been commercialized, 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 period.
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 (U.S. Pat. No. 2,913,505). This process passed through a pilot-plant level and demonstrative steps, without reaching final commercialization, 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 effects of the process solution and the considerably corrosive nature of fluoborate, which obliges the use of valuable, 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
+
nitrate, stabilizing the solution by the use of hydrogen peroxide 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 solvents to which salts, various types of acids have been added (U.S. Pat. No. 3,347,948; U.S. Pat. No. 4,132,744; U.S. Pat. No. 2,449,793; U.S. Pat. No. 3,189,658), or are directed towards the use of alternative methods for improving the contact between solute and solvent, such as liquid/liquid extraction or functionalized membranes with Ag
+
(U.S. Pat. No. 3,758,603; U.S. Pat. No. 3,758,605; U.S. Pat. No. 3,770,842; U.S. Pat. No. 3,844,735; U.S. Pat. No. 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 absorbed by a particular absorbent (for example AgNO
3
+H
2
O
2
). 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 column to separate the olefins from the absorbent which is recycled to the absorption column.
It has been surprisingly found that by adding a ferric compound (Fe
3+
) to the absorbing solution consisting of a silver salt, instead of hydrogen peroxide as stabilizer of the silver solution, in a reducing environment, the disadvantages 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
Fe
3+
+e
−
→Fe
2+
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 solution.
The advantage of the use of ferric salts particularly lies in the fact that the ferrous ions generated by the reduction 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 before entering the absorber.
The presence of ferric salts, moreover, gives the solution stability properties even at high temperatures so that the stripper can operate under conditions close to atmospheric 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 separation 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 compounds, 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 using various types of schemes which are described hereunder.
One process, with a scheme analogous to the traditional 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 compounds 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 60° C. 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 FIG.
1
.
The stream containing light olefins and paraffins (
1
) is fed to an absorpti
Piovesan Laura
Pirovano Carmen
Sanfilippo Domenico
Saviano Francesco
Griffin Walter D.
Nguyen Tam
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Snamprogetti S.p.A.
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