# Patent

## Title
Extraction solvent control for reducing stable emulsions

## Abstract

Disclosed herein are methods for recovering diphosphite-containing compounds from mixtures comprising organic mononitriles and organic dinitriles, using liquid-liquid extraction. Also disclosed are treatments to enhance extractability of the diphosphite-containing compounds.

## Background
RELATED APPLICATION
This application claims benefit to Provisional Application No. 61/578,495 filed Dec. 21, 2011 which is herein incorporated by reference in its entirety.


FIELD OF THE INVENTION
The invention relates to recovery of catalyst and ligand from a hydrocyanation reaction product mixture comprising organic dinitriles using liquid-liquid extraction.
BACKGROUND OF THE INVENTION
It is well known in the art that complexes of nickel with phosphorous-containing ligands are useful as catalysts in hydrocyanation reactions. Such nickel complexes using monodentate phosphites are known to catalyze hydrocyanation of butadiene to produce a mixture of pentenenitriles. These catalysts are also useful in the subsequent hydrocyanation of pentenenitriles to produce adiponitrile, an important intermediate in the production of nylon. It is further known that bidentate phosphite and phosphinite ligands can be used to form nickel-based catalysts to perform such hydrocyanation reactions.
U.S. Pat. No. 3,773,809 describes a process for the recovery of Ni complexes of organic phosphites from a product fluid containing organic nitriles produced by hydrocyanating an ethylenically unsaturated organic mononitrile such as 3-pentenenitrile through extraction of the product fluid with a paraffin or cycloparaffin hydrocarbon solvent. Similarly, U.S. Pat. No. 6,936,171 to Jackson and McKinney discloses a process for recovering diphosphite-containing compounds from streams containing dinitriles.
U.S. Pat. No. 4,339,395 describes the formation of an interfacial rag layer during extended periods of continuous extraction of certain phosphite ligands. The '395 patent notes that the interfacial rag hinders, if not halts, the phase separation. Because the process is operated continuously, the rag must be removed continuously from the interface as it accumulates to avoid interrupting operation. To solve this problem for the disclosed components, the '395 patent discloses the addition of minor amounts of substantially water-free ammonia.
SUMMARY OF THE INVENTION
This process recovers diphosphite-containing compounds from a mixture comprising diphosphite-containing compounds, organic mononitriles and organic dinitriles.
Disclosed is a process for recovering diphosphite-containing compounds from a feed mixture comprising diphosphite-containing compounds, organic mononitriles and organic dinitriles in a multistage countercurrent liquid-liquid extractor with extraction solvent comprising aliphatic hydrocarbon, cycloaliphatic hydrocarbon or a mixture of aliphatic and cycloaliphatic hydrocarbon, comprising:
a) flowing the feed mixture to the first stage of the multistage countercurrent liquid-liquid extractor; and
b) contacting the feed mixture with extraction solvent in the multistage countercurrent liquid-liquid extractor;
wherein the first stage of the multistage countercurrent liquid-liquid extractor comprises a mixing section and a settling section, wherein a light phase separates from a heavy phase in the settling section, wherein a mixed phase comprising both heavy phase and light phase is present in the settling section between the light phase and the heavy phase, wherein the light phase comprises extraction solvent and extracted diphosphite-containing compounds, wherein the heavy phase comprises organic mononitriles and organic dinitriles, wherein at least a portion of the light phase is withdrawn from the settling section and treated to recover diphosphite-containing compounds extracted into the light phase, wherein a first portion of the heavy phase is passed to the second stage of the multistage countercurrent liquid-liquid extractor, and wherein a second portion of the heavy phase is withdrawn from the settling section of the first stage of the multistage countercurrent liquid-liquid extractor and recycled to the settling section of the first stage of the multistage countercurrent liquid-liquid extractor.
The mixing sections of the stages of the multistage counter current liquid-liquid extractor form an intimate mixture of unseparated light and heavy phase. This intimate mixture comprises an emulsion phase. The emulsion phase may or may not comprise particulate solid material. This emulsion phase separates into a light phase and a heavy phase in the settling sections of the stages, including the first stage. Accordingly, the settling sections of the stages will contain at least some emulsion phase located between the upper light phase and the lower heavy phase. This emulsion phase tends to reduce in size over time. However, in some instances settling takes longer than desired or the emulsion phase never fully separates into a light phase and a heavy phase. This separation problem may be particularly troublesome in the first stage of a multistage countercurrent liquid-liquid extractor.
Recycle of heavy phase in the separation section of the first stage has been found to result in enhanced settling of the emulsion phase. For example, this recycle may result in the reduction of the size of the emulsion phase in the settling section, wherein the size of the emulsion phase is based upon the size of the emulsion phase in the absence of recycle of the heavy phase. Enhanced settling in the settling section may also be measured as an increased rate of settling, based upon the rate of settling in the absence of recycle of the heavy phase.
Another problem, which may be solved by recycle of heavy phase is formation of rag and build-up of a rag layer the settling section. Rag formation is discussed in U.S. Pat. No. 4,339,395 and U.S. Pat. No. 7,935,229. Rag comprises particulate solid material, and may be considered to be a form of an emulsion phase, which is particularly stable in the sense that it does not dissipate in a practical amount of time for conducting an extraction process. Rag may form in the mixing section or the settling section of an extraction stage, particularly the first stage of a multistage countercurrent liquid-liquid extractor. In the settling section, the rag forms a layer between the heavy phase and the light phase. The formation of a rag layer in the settling section inhibits proper settling of the heavy phase and the light phase. The formation of a rag layer may also inhibit the extraction of diphosphite-containing compounds from the heavy phase into the light phase. In a worst case scenario, rag can build up to extent of completely filling a separation section, necessitating shut down of the extraction process to clean out the settling section. It has been found that recycle of heavy phase in the settling section may reduce or eliminate the size of a rag layer or reduce its rate of formation, based upon the size and rate of formation of the rag layer in the absence of recycle of the heavy phase.
Accordingly, recycle of heavy phase in the settling section of the first stage of a multistage countercurrent extractor may achieve at least one of the following results: (a) a reduction in the size of an emulsion phase in the settling section, based upon the size of the emulsion phase in the absence of recycle of the heavy phase; (b) an increase in the rate of settling in the settling section, based upon the rate of settling in the absence of recycle of the heavy phase; (c) an increase in the amount of diphosphite-containing compounds in the light phase, based upon the upon the amount of diphosphite-containing compounds in the light phase in the absence of recycle of the heavy phase; (d) a partial or total reduction in the size of a rag layer in the settling section, based upon the size of a rag layer in the settling section in the absence of recycle of the heavy phase; and (e) reduction in the rate of formation of a rag layer in the settling section, based upon the rate of formation of a rag layer in the settling section in the absence of recycle of the heavy phase.
The second portion of the heavy phase, which is recycled in the first stage, may be recycled to the settling section in the absence of an intervening step to remove diphosphite-containing compounds from the heavy phase.
The second portion of the heavy phase, which is recycled in the first stage, may be recycled to the settling section in the absence of passing through another liquid-liquid extraction stage.
The extraction solvent feed from the second stage of the multistage countercurrent liquid-liquid extractor to the first stage of the multistage countercurrent liquid-liquid extractor may comprise at least 1000 ppm, for example, from 2000 to 5000 ppm, of diphosphite-containing compounds. The extraction solvent feed from the second stage may comprise at least 10 ppm, for example, from 20 to 200 ppm, of nickel.
A raffinate recycle ratio (RRR) may be between 0.1 and 0.9, for example, between 0.2 and 0.8, wherein RRR is defined by the ratio of X to Y, wherein X is the mass per unit time of the second portion of the heavy phase recycled to the settling section of the first stage of the multistage countercurrent liquid-liquid extractor, and wherein Y is the mass per unit time of all of the heavy phase withdrawn from the settling section of the first stage of the multistage countercurrent liquid-liquid extractor.
The diphosphite-containing compound may be a Ni complex with a diphosphite ligand selected from the group consisting of:




wherein in I, II and III, R1 is phenyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; or naphthyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; and wherein Z and Z1 are independently selected from the group consisting of structural formulae IV, V, VI, VII, and VIII:




and wherein

R2, R3, R4, R5, R6, R7, R8, and R9 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy;
X is O, S, or CH(R19);
R10 is H or C1 to C12 alkyl;




and wherein

R11 and R12 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy and CO2R13,
R13 is C1 to C12 alkyl, or C6 to C10 aryl unsubstituted or substituted with C1 to C4 alkyl;
Y is O, S, or CH(R14);
R14 is H or C1 to C12 alkyl;




wherein

R15 is selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy and CO2R16,
R16 is C1 to C12 alkyl, or C6 to C10 aryl, unsubstituted or substituted with C1 to C4 alkyl,
and wherein
for structural formulae I through VIII, the C1 to C12 alkyl, and C1 to C12 alkoxy groups may be straight chain or branched.
At least one stage of the extraction may be carried out above 40° C.
At least one stage of extraction may contain a Lewis base.
If at least one stage of extraction contains a Lewis base, the Lewis base may be a monodentate triarylphosphite, wherein the aryl groups are unsubstituted or substituted with alkyl groups having 1 to 12 carbon atoms, and wherein the aryl groups may be interconnected.
The Lewis base may optionally be selected from the group consisting of:
a) anhydrous ammonia, pyridine, alkylamine, dialkylamine, trialkylamine wherein the alkyl groups have 1 to 12 carbon atoms; and
b) polyamine.
If the Lewis base is a polyamine, the polyamine may comprise at least one selected from hexamethylene diamine, and dimers and trimers of hexamethylene diamine, for example, bis-hexamethylene triamine.
The Lewis base may optionally comprise a basic ion exchange resin, for example, Amberlyst 21® resin.
One example of a suitable cyclic alkane extraction solvent is cyclohexane.
At least a portion of the process may be carried out in an extraction column or a mixer-settler.
The feed mixture may be an effluent stream from a hydrocyanation process, for example, a process for hydrocyanating 3-pentenenitrile, a process for the single hydrocyanation of 1,3-butadiene to pentenenitriles or a process for the double hydrocyanation of 1,3-butadiene to adiponitrile.
The first stage of the multistage countercurrent liquid-liquid extractor may take place in an extraction column. The entire column may be considered to be a settling section comprising a mixing section between a heavy phase collection section and a light phase collection section. Heavy phase may be recycled to the mixing section of the extraction column.
The first stage of the multistage countercurrent liquid-liquid extractor may take place in a mixer-settler. The mixer-settler may comprise a settling section which is separate from the mixing section. Recycled heavy stream may be recycled upstream from the point of withdraw of the recycled heavy stream.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing the flow of fluids through a multistage countercurrent liquid-liquid extractor.
FIG. 2 is a diagram showing recycle of light phase yin a settling section of a stage of a multistage countercurrent liquid-liquid extractor.
FIG. 3 is a diagram showing recycle of heavy phase in a settling section of an extraction column.
FIG. 4 is a diagram showing recycle of heavy phase in a settling section of a mixing/settling apparatus having three chambers in the settling section.
FIG. 5 is a graph showing the rag level in a settler during catalyst extraction in continuous operation.



DETAILED DESCRIPTION OF THE INVENTION
The processes of the present invention involve methods for recovering diphosphite-containing compounds from a mixture comprising diphosphite-containing compounds and organic dinitriles, using liquid-liquid extraction.
FIG. 1 is a diagram of a multistage countercurrent liquid-liquid extractor. Lines in FIG. 1 represent flow of materials, rather than any particular type of equipment, such as pipes. Similarly, squares in this diagram represent stages or sections for mixing and settling, rather than any particular type of equipment.
Three stages are depicted in FIG. 1. The first stage is depicted by mixing and settling section 1. The second stage is depicted by mixing and settling section 2. The final stage is depicted by mixing and settling section 3. Gap 30 represents a space where additional stages may be inserted. For example, one or more, for example, from one to four, mixing and settling sections may be inserted in gap 30 between mixing and settling section 2 and mixing and settling section 3.
In FIG. 1, a fresh extraction solvent feed, for example, cyclohexane, is introduced into the multistage countercurrent extractor via line 10. The extraction solvent or light phase exiting from mixing settling section 3 passes through line 12 to the next stage of the multistage extractor. In a multistage countercurrent liquid-liquid extractor having three stages, extraction solvent in line 12 would pass directly into stage 2 via line 14. Extraction solvent from stage 2 passes through line 16 to stage 1. The extraction solvent comprising extracted diphosphite-containing compounds passes out of the stage 1 mixing and settling section through line 18.
A feed comprising diphosphite-containing compounds is fed into the stage 1 mixer and settler via line 20. The feed further comprises a mixture comprising organic mononitriles and dinitriles, which is immiscible with the extraction solvent. In stage 1, a portion of the diphosphite-containing compounds is extracted into the extraction solvent which exits stage 1 via line 18. The immiscible dinitrile and mononitrile mixture or the heavy phase is removed from the stage 1 mixing and settling section by line 22 and is passed into the stage 2 mixing and settling section. A portion of the diphosphite-containing compounds is extracted into the light phase in the stage 2 mixing and settling section. The heavy phase exits the stage 2 mixing and settling section by line 24. Similarly, if there are additional stages in gap 30 shown in FIG. 1, extraction of diphosphite-containing compounds will take place in such intermediate stages in a similar manner to that taking place in stage 2.
After the heavy phase passes through the first stage and any intermediate stages, it passes through the final stage mixing and settling section 3. In particular, the heavy phase is introduced into mixing and setting section 3 through line 26. After passing through the final stage mixing and settling section 3, the heavy phase exits via line 28.
A two-stage multistage countercurrent liquid-liquid extractor is represented in FIG. 1 by mixing and settling sections 1 and 2; lines 14, 16 and 18 showing the direction of extraction solvent flow; and lines 20, 22 and 24 showing the direction of heavy phase flow. In a two-stage multistage counter current liquid-liquid extractor, mixing and settling section 3; lines 10, 12, 26 and 28; and gap 30 are omitted. In the two stage countercurrent liquid-liquid extractor, extraction solvent comprising extracted diphosphite-containing compounds passes from the extractor through line 18, and extracted heavy phase, i.e. raffinate, passes from the extractor through line 24.
Thus, it can be seen that the multistage countercurrent liquid-liquid extractor comprises two or more stages with countercurrent flow of extraction solvent and heavy phase.
FIG. 2 is a diagrammatic representation of one type of a mixing and settling section, also referred to herein as a mixer-settler. This type of mixer-settler may be used in any of the stages shown in FIG. 1. This mixer-settler comprises a mixing section 40 and a settling section 50. The mixing section 40 and the settling section 50 are separate. All of the effluent from the mixing section 40 flows into the settling section 50. Fluid from the mixing section 40 flows through the settling section 50 in a in a horizontal manner, although there is also no restriction of movement of fluids vertically throughout the settling section 50.
An extraction solvent is introduced into the mixing section 40 by line 42. A feed comprising diphosphite-containing compounds is introduced into the mixing section 40 by line 44. Alternatively, the contents of lines 42 and 44 may be combined upstream of the mixing section 40 and introduced into mixing section 40 through a single inlet. These two feeds are mixed in the mixing section 40 to provide a mixed phase comprising an emulsion phase represented in FIG. 2 by shaded area 46.
Line 48 represents the flow of mixed phase 46 from the mixing section 40 into the settling section 50. As depicted in FIG. 2, there are three phases in the settling section 50, including a heavy phase 52, a mixed phase 54, and a light phase 56. The heavy phase 52 is depleted in diphosphite-containing compounds, insofar as it has a lower concentration of diphosphite-containing compounds as compared with the concentration of diphosphite-containing compounds in feed 44, due to the extraction of diphosphite-containing compounds into the light phase 56. Correspondingly, the light phase 56 is enriched in diphosphite-containing compounds, insofar as it has a higher concentration of diphosphite-containing compounds as compared with the concentration of diphosphite-containing compounds in extraction solvent feed 42, due to the extraction of diphosphite-containing compounds into the light phase 56. Heavy phase 52 exits the settling section 50 via line 58. At least a portion of the light phase 56 is removed from the settling section 50 via line 60, and another portion of the light phase 56 may, optionally, be removed from the settling section 50 and recycled to mixing section 40 or settling section 50 through lines not shown in FIG. 2.
Although not shown in FIG. 2, which is diagrammatically shows the flow of fluids, it will be understood that each of the mixing section 40 and the settling section 50 may comprise one or more stages, subsections, compartments or chambers. For example, settling section 50 may include more than one chamber between the point of introduction of the mixed phase 46 through line 48 and the point of withdrawal of light phase and heavy phase through lines 58, 60 and 62. Horizontal extension between the point of introduction of the mixed phase 46 through line 48 and the point of withdrawal of light and heavy phases through lines 58, 60 and 62 promotes settling of the light and heavy phases 56 and 52. The size of the mixed phase 54 may become progressively smaller as fluids settle and flow through the chamber. For example, the final chamber from where fluids are removed may include little or no mixed phase 54. It will further be understood that mixing section 40 may include one or more types of mixing apparatus, such as an impeller, not shown in FIG. 2.
FIG. 3 provides a representation of another type of apparatus for use as a mixing and settling section. The type of apparatus 70 shown in FIG. 3 is referred to herein as an extraction column. This extraction column 70 includes a mixing section 72, a heavy phase collection section 74 and a light phase collection section 76. The entire column 70 may be considered to be a settling section with a mixing section between collection section 74 and collection section 76. In extraction column 70 the mixing section 72 is part of the settling section. An extraction solvent is introduced into column 70 through line 80. A heavier phase comprising a diphosphite-containing compound is introduced into column 70 through line 90. As the light phase passes upward through the column, and the heavy phase passes downward through the column, a mixture of the two phases is formed in mixing section 72. This mixture is represented in FIG. 3 as shaded mixed phase 84. This mixed phase 84 may comprise an emulsion phase. The point of introduction of heavy phase through line 90 should be sufficiently above the point of introduction of the light phase to allow for sufficient mixing of the two phases in the mixing section resulting in the extraction of diphosphite-containing compounds into the light phase. The intimate mixing of light and heavy phase in mixing section 72 may be promoted by mechanical or static mixing apparatus not shown in FIG. 3. For example, mixing section 72 may comprise baffles or perforated plates, not shown in FIG. 3.
The heavy phase 82 settles into collection section 74 and passes out of the column 70 through line 96. Light phase 86 settles in collection section 76 and passes from the column through line 92. Heavy phase 82 settles into collection section 74 and exits column 70 through line 96. A portion of this heavy phase is taken as a side stream through line 94 and is passed into the mixing section 72 as recycle into column 70. Alternatively, line 94 may be taken directly from collection section 74, instead of as a side stream from line 96. In another alternate embodiment, line 94 may flow directly into line 80 or into column 72 at a point near the interface of the mixed phase 84 and the heavy phase 82.
Recycle of heavy phase into the settling section of extraction column 70 increases the downward flow of heavy phase in the settling section. Without being bound by any theory, it is theorized that this increased downward flow may tend to breakup an emulsion phase, which may tend to otherwise form in the settling section. This emulsion phase, when present, may form at the interface of the mixed phase 84 and the heavy phase 82. Accordingly, the point of introduction of the recycled heavy phase, for example, through line 94 as shown in FIG. 3 should be sufficiently above to point where an emulsion phase would form to allow for heavy phase to flow downward through this point.
FIG. 4 provides a representation of a mixer-settler 100 having a multistage settling section. Mixer-settler 100 has a mixing section 110 and a settling section 112. In mixer-settler 100, the mixing section 110 is separate from the settling section 112. The settling section has three compartments, represented in FIG. 4 as sections 114, 116, and 118. These sections are separated by coalescence plates 120. The coalescence plates 120 may be designed to provide flow of separated light and heavy phases between chambers, while restricting the flow of emulsion phase between chambers. A feed comprising a diphosphite-containing compound is passed into the mixing section 110 via line 130. The extraction solvent is introduced into mixing section 110 via line 132. The mixing section 110 includes an impeller 134 mounted on shaft 136 to provide for mechanical mixing of fluids. Mixing of the feeds provides a mixed phase comprising an emulsion phase represented in FIG. 4 by shading 140.
The mixed phase 140 flows into the settling section 112 as an overflow from the mixing section 110. This mixed phase 140 is prevented from flowing directly into the light phase 144 by baffle plate 142. As settling occurs in settling section 112, the mixed phase 140 decreases in volume, the volume of the light phase 144 increases, and the volume of the heavy phase 146 increases. Heavy phase 146 is removed from settling section 112, in particular from chamber 118, via line 152 and light phase 144 is removed from settling section 112, in particular, from chamber 118, via line 150. A portion of the heavy phase removed through line 152 is taken as a side stream through a line not shown in FIG. 4 and is introduced back into the settling section 112 at an appropriate point in section 114. The vertical point of introduction of line 154 into section 114 may be, for example, at or near the interface of the mixed phase 140 and the heavy phase 146.
Recycle of heavy phase 146 into the settling section increases the horizontal flow of heavy phase 146 relative to the horizontal flow of mixed phase 140 and light phase 144 through the settling section 112. Without being bound by any theory, it is theorized that by increasing flow of heavy phase 146 at the interface of the heavy phase 146 and the mixed phase 140 relative to the flow of the mixed phase 140 and the heavy phase 146, in general, the tendency of an emulsion phase to stabilize may be reduced. In particular, it is theorized that the increased horizontal flow of heavy phase 146 may result in mild agitation or shear at the interface of the heavy phase 146 and the mixed phase 140, where a stable emulsion phase might otherwise tend to form. It is also theorized that downward flow of heavy phase through an emulsion phase or rag layer may tend to force the emulsion phase or rag layer downward towards or into the heavy phase, thereby tending to break up the emulsion phase or rag layer.
It is desired to maximize the horizontal displacement of the of the point of withdraw and the point of reentry of the recycle stream. For example, in a multi-chamber settling section, heavy phase 146 may be removed from the chamber, e.g., chamber 118, furthest removed from the point of introduction of the mixed phase 146 from the mixing section 110 into the settling section 112, and the recycled heavy phase 146 may be reintroduced into the settling section 112 at a point near the introduction of the mixed phase 146 from the mixing section 110 into the settling section 112. For example, one point where the recycled heavy phase 146 may be introduced into the settling section 112 is at a point upstream of baffle plate 142, where mixed phase 140 overflows from the mixing section 110 into the settling section 112.
It is desirable for both a mononitrile and a dinitrile to be present in the countercurrent contactor. For a discussion of the role of monodentate and bidentate ligand in extraction of hydrocyanation reactor effluent streams, see U.S. Pat. No. 3,773,809 to Walter and U.S. Pat. No. 6,936,171 to Jackson and McKinney.
For the process disclosed herein, suitable ratios of mononitrile to dinitrile components include 0.01 to 2.5, for example, 0.01 to 1.5, for example 0.65 to 1.5.
Maximum temperature is limited by the volatility of the hydrocarbon solvent utilized, but recovery generally improves as the temperature is increased. Examples of suitable operating ranges are 40° C. to 100° C. and 50° C. to 80° C.
The controlled addition of monophosphite ligands may enhance settling. Examples of monophosphite ligands that may be useful as additives include those disclosed in Drinkard et al U.S. Pat. No. 3,496,215, U.S. Pat. No. 3,496,217, U.S. Pat. No. 3,496,218, U.S. Pat. No. 5,543,536, and published PCT Application WO 01/36429 (BASF).
The addition of Lewis base compounds to a mixture comprising diphosphite-containing compounds, organic mononitriles and organic dinitriles may enhance settling, especially when the mixture comprises a Lewis acid, such as ZnCl2. The addition may take place either before or during an extraction process in a multistage countercurrent extractor. Examples of suitable weak Lewis base compounds include water and alcohols. Suitable stronger Lewis base compounds include hexamethylene diamine, dimers and trimers of hexamethylene diamine, ammonia, aryl- or alkyl amines, such as pyridine or triethylamine, or basic resins such as Amberlyst 21®, a commercially available basic resin made by Rohm and Haas. The addition of Lewis base may reduce or eliminate any inhibiting effect of Lewis acid on catalyst recovery.
The diphosphite-containing compounds extracted by the processes described herein are also referred to herein as bidentate phosphorus-containing ligands. These extracted ligands comprise free ligands (e.g., those which are not complexed to a metal, such as nickel) and those which are complexed to a metal, such as nickel. Accordingly, it will be understood that extraction processes described herein are useful for recovering diphosphite-containing compounds which are metal/ligand complexes, such as a complex of zero valent nickel with at least one ligand comprising a bidentate-phosphorus containing ligand.
Suitable ligands for extraction are bidentate phosphorous-containing ligands selected from the group consisting of bidentate phosphites, and bidentate phosphinites. Preferred ligands are bidentate phosphite ligands.
Diphosphite Ligands
Examples of bidentate phosphite ligands useful in the invention include those having the following structural formulae:




wherein in I, II and III, R1 is phenyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; or naphthyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; and Z and Z1 are independently selected from the group consisting of structural formulae IV, V, VI, VII, and VIII:





and wherein

R2, R3, R4, R5, R6, R7, R8, and R9 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy;
X is O, S, or CH(R10);
R19 is H or C1 to C12 alkyl;




and wherein

and R12 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy; and CO2R13,
R13 is C1 to C12 alkyl or C6 to Co1 aryl, unsubstituted or substituted. with C1 to C4 alkyl;
Y is O, S, or CH(R14);
R14 is H or C1 to C12 alkyl;




wherein

R16 is selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy and CO2R16;
R16 is C1 to C12 alkyl or C6 to C10 aryl, unsubstituted or substituted with C1 to C4 alkyl.
In the structural formulae I through VIII, the C1 to C12 alkyl, and C1 to C12 alkoxy groups may be straight chain or branched.
Another example of a formula of a bidentate phosphite ligand that is useful in the present process is that having the Formula X, shown below




Further examples of bidentate phosphite ligands that are useful in the present process include those having the Formulae XI to XIV, shown below wherein for each formula, R17 is selected from the group consisting of methyl, ethyl or isopropyl, and R18 and R19 are independently selected from H or methyl:




Additional examples of bidentate phosphite ligands that are useful in the present process include a ligand selected from a member of the group represented by Formulae XV and XVI, in which all like reference characters have the same meaning, except as further explicitly limited:




wherein

R41 and R45 are independently selected from the group consisting of C1 to C5 hydrocarbyl, and each of R42, R43, R44, R46, R47 and R48 is independently selected from the group consisting of H and C1 to C4 hydrocarbyl.
For example, the bidentate phosphite ligand can be selected from a member of the group represented by Formula XV and Formula XVI, wherein
R41 is methyl, ethyl, isopropyl or cyclopentyl;
R42 is H or methyl;
R43 is H or a C1 to C4 hydrocarbyl;
R44 is H or methyl;
R45 is methyl, ethyl or isopropyl; and
R46, R47 and R48 are independently selected from the group consisting of H and C1 to C4 hydrocarbyl.
As additional examples, the bidentate phosphite ligand can be selected from a member of the group represented by Formula XV, wherein
R41, R44, and R45 are methyl;
R42, R46, R47 and R48 are H; and
R43 is a C1 to C4 hydrocarbyl;
or
R41 is isopropyl;
R42 is H;
R43 is a C1 to C4 hydrocarbyl;
R44 is H or methyl;
R45 is methyl or ethyl;
R46 and R48 are H or methyl; and
R47 is H, methyl or tertiary-butyl;
or the bidentate phosphite ligand can be selected from a member of the group represented by Formula XVI, wherein
R41 is isopropyl or cyclopentyl;
R45 is methyl or isopropyl; and
R46, R47, and R48 are H.
As yet another example, the bidentate phosphite ligand may be represented by Formula XV, wherein R41 is isopropyl; R42, R46, and R48 are H; and R43, R44, R45, and R47 are methyl.
It will be recognized that Formulae X to XVI are two-dimensional representations of three-dimensional molecules and that rotation about chemical bonds can occur in the molecules to give configurations differing from those shown. For example, rotation about the carbon-carbon bond between the 2- and 2′-positions of the biphenyl, octahydrobinaphthyl, and or binaphthyl bridging groups of Formulae X to XVI, respectively, can bring the two phosphorus atoms of each Formula in closer proximity to one another and can allow the phosphite ligand to bind to nickel in a bidentate fashion. The term “bidentate” is well known in the art and means both phosphorus atoms of the ligand are bonded to a single nickel atom.
Further examples of bidentate phosphite ligands that are useful in the present process include those having the formulae XX to LIII, shown below wherein for each formula, R17 is selected from the group consisting of methyl, ethyl or isopropyl, and R18 and R19 are independently selected from H or methyl:

























Additional suitable bidentate phosphites are of the type disclosed in U.S. Pat. Nos. 5,512,695; 5,512,696; 5,663,369; 5,688,986; 5,723,641; 5,847,101; 5,959,135; 6,120,700; 6,171,996; 6,171,997; 6,399,534; the disclosures of which are incorporated herein by reference. Suitable bidentate phosphinites are of the type disclosed in U.S. Pat. Nos. 5,523,453 and 5,693,843, the disclosures of which are incorporated herein by reference.
Extraction Solvent
Suitable hydrocarbon extraction solvents include paraffins and cycloparaffins (aliphatic and alicyclic hydrocarbons) having a boiling point in the range of about 30° C. to about 135° C., including n-pentane, n-hexane, n-heptane and n-octane, as well as the corresponding branched chain paraffinic hydrocarbons having a boiling point within the range specified. Useful alicyclic hydrocarbons include cyclopentane, cyclohexane and cycloheptane, as well as alkyl substituted alicyclic hydrocarbons having a boiling point within the specified range. Mixtures of hydrocarbons may also be used, such as, for example, mixtures of the hydrocarbons noted above or commercial heptane which contains a number of hydrocarbons in addition to n-heptane. Cyclohexane is the preferred extraction solvent.
The lighter (hydrocarbon) phase recovered from the multistage countercurrent liquid-liquid extractor is directed to suitable equipment to recover catalyst, reactants, etc. for recycle to the hydrocyanation, while the heavier (lower) phase containing dinitriles recovered from the multistage countercurrent liquid-liquid extractor is directed to product recovery after removal of any solids, which may accumulate in the heavier phase. These solids may contain valuable components which may also be recovered, e.g., by the process set forth in U.S. Pat. No. 4,082,811.
EXAMPLES
In the following examples, values for extraction coefficient are the ratio of weight fraction of catalyst in the extract phase (hydrocarbon phase) versus the weight fraction of catalyst in the raffinate phase (organonitrile phase). An increase in extraction coefficient results in greater efficiency in recovering catalyst. As used herein, the terms, light phase, extract phase and hydrocarbon phase, are synonymous. Also, as used herein, the terms, heavy phase, organonitrile phase and raffinate phase, are synonymous.
Example 1
To a 50 mL, jacketed, glass laboratory extractor, equipped with a magnetic stirbar, digital stir-plate, and maintained at 65° C., was charged 10 grams of the product of a pentenenitrile-hydrocyanation reaction, and 10 grams of the extract from the second stage of a mixer-settler cascade, operated in counter-current flow. The extract from the second stage contained approximately 50 ppm nickel and 3100 ppm diphosphite ligand.
The reactor product was approximately:
85% by weight C6 dinitriles
14% by weight C5 mononitriles
1% by weight catalyst components
360 ppm by weight active nickel.
The laboratory reactor was then mixed at 1160 rotations-per-minute, for 20 minutes, and then allowed to settle for 15 minutes. After settling for 15 minutes, a stable emulsion was present throughout the extract phase. Samples were obtained of the extract and raffinate phases of the extractor and analyzed to determine the extent of catalyst extraction. The ratio of active nickel present in the extract phase vs. the raffinate phase was found to be 14.
Example 2
Using the same hydrocyanation reactor product and 2nd stage settler extract as Example 1, a 50 mL, jacketed, glass laboratory extractor, equipped with a magnetic stirbar, digital stir-plate, and maintained at 65° C., was charged 10 grams of the product of a pentene-hydrocyanation reaction, and 10 grams of the extract from the second stage of a mixer-settler cascade, operated in counter-current flow.
The laboratory reactor was then mixed at 1160 rotations-per-minute, for 20 minutes, and then allowed to settle for 15 minutes. After settling for 15 minutes, a stable emulsion was present throughout the extract phase. Then, gentle mixing was applied, approximately 100 rpm, which caused the emulsion to disentrain. Samples were obtained of the extract and raffinate phases of the extractor and analyzed to determine the extent of catalyst extraction. The ratio of active nickel present in the extract phase vs. the raffinate phase was found to be 16.
Examples 1 and 2 illustrate the beneficial effect of applying gentle agitation to the heavy phase of the settling section of the first stage of a multistage countercurrent liquid-liquid extractor, and provide a practical simulation of the effect of recycle of the heavy phase in continuous operation.







TABLE 1







Catalyst and ligand extraction coefficient


for various first stage extraction coefficient recycle ratios.












Example
RRR
Catalyst Recovery (KLL)
Stable Emulsion







1
0
14
Yes



2
1
16
No







KLL = amount of catalyst in the extract/amount of catalyst in the raffinate






Example 3
A three stage countercurrent liquid-liquid extractor, in continuous operation, utilizing the same two feed streams described in Example 1 was operated for a duration of 20 days. Samples were obtained of the extract and raffinate phases of the settling section of the extractor and analyzed to determine the extent of catalyst extraction. The ratio of active nickel present in the extract phase vs. the raffinate phase was found to be 5.6±2. A stable emulsion and rag was present throughout the extract portion of the settling section of the first stage of the extractor. The emulsion and rag was also present to a lesser extent in the settling sections of the second and third stages of the extractor.
Example 4
Example 3 was replicated except that the light-phase was recycled from the settling section back to the mixing section of the first stage of the countercurrent liquid-liquid extractor for a duration of 14 days. The benefits of this type of recycle of a light phase are described in International Publication Number WO 2013/095851. This Example 4 provides a base case for demonstrating improved results as described in Example 5 below.
The ratio of active nickel present in the extract phase vs. the raffinate phase was found to be 8.6±2. Less stable emulsion and rag was present throughout the extract portion of the settling section of the first stage of the extractor than in Example 3.
Example 5
Example 3 was replicated except that both light-phase and heavy-phase were recycled in the first stage of the countercurrent liquid-liquid extractor for a duration of 60 days. Recycle of the light phase took place from the settler to the mixer in the manner described in Example 4. Recycle of the heavy phase took place from the settler and back to the settler in a manner which provided mild agitation to the mixed phase in the settler.
The ratio of active nickel present in the extract phase vs. the raffinate phase was found to be 10.8±2. Less stable emulsion and rag was present throughout the extract portion of the settling section of the extractor than in Example 4.
Results of Examples 3-5 are summarized in Table 2.







TABLE 2







Catalyst extraction coefficients for continuous operation using phase


recycles.











Example
Duration (days)
Phase recycle
KLL
Stable emulsion





3
20
None
 5.6 ± 2
Yes


4
14
Light
 8.6 ± 2
No


5
60
Light and Heavy
10.8 ± 2
No









Examples 3-5 illustrate the beneficial effect of recycling the light phase from the settler back to the mixer, and the heavy phase from the settler and back to the settler, of the settling section of the first stage of a multistage countercurrent liquid-liquid extractor while in continuous operation.
Example 6
Using the same countercurrent liquid-liquid extractor, in continuous operation as in Examples 3-5, the combined level of the raffinate and rag layers in the settling section of the extractor were measured by means of an RF probe (Universal IIITM smart level). The data from the probe is shown in FIG. 5. A rapid change of >2% in less than 2 hour indicates that the rag layer was pushed into the raffinate layer, or the rag re-formed at the interface. When the raffinate recycle in the first stage, from the settler and back to the settler, disturbed the interface, the rag layer was pushed in the raffinate layer and the raffinate and rag layer total % level in the settler of the first stage decreases rapidly as shown in the FIG. 5. When the raffinate recycle no longer disturbed the interface, the rag layer again re-formed at the interface which was measured by the % level in the settler for the raffinate and rag increasing rapidly.
As shown in FIG. 5, the extraction was operated continuously for approximately 25 hours without raffinate recycle, and the rag level (including the thickness of the rag layer and the thickness of the underlying raffinate layer) was between 60% and 70%. After about 25 hours, recycle of raffinate was started. FIG. 5 shows that the rag level rapidly decreased to below 40% with a corresponding increase in the thickness of the extract layer. After about 1 hour, recycle of raffinate was discontinued, and, as shown in FIG. 5, the rag level rapidly increased to at least 60%. However, the rag level did not reach the rag level observed before the first recycle of raffinate. After the first recycle of raffinate was discontinued, the operation of the extraction was continued until the total elapsed time of the experiment reached about 60 hours. At that time, recycle of raffinate was resumed. FIG. 5 again shows that the rag level rapidly decreased to below 40% with a corresponding increase in the thickness of the extract layer. The recycle of raffinate was continued until the total elapsed time of the experiment reached about 77 hours. During this period, the rag level remained substantially constant below 40%. After about 77 hours of total elapsed time, the recycle of raffinate was again discontinued, and, as shown in FIG. 5, the rag level again rapidly increased, this time to a level between 50% and 60%. After about 85 hours of total elapsed time, recycle of raffinate was resumed again. FIG. 5 again shows that the rag level rapidly decreased to below 40%. This level was essentially maintained as recycle of raffinate continued until the end of the experiment at 100 hours of total elapsed time.
Examples 7-11
These Examples 7-11 illustrate that effective catalyst recovery occurs for a mononitrile to dinitrile ratio greater than 0.65.
Five different mixtures comprised of a Ni diphosphite complex, with the diphosphite ligand shown in Structure XX (where R17 is isopropyl, R18 is H, and R19 is methyl), ZnCl2 (equimolar with Ni) and differing in the ratio of mononitrile to dinitrile, were separately liquid-liquid batch extracted with an equal weight of cyane (i.e. cyclohexane). The molar ratio of organic mononitrile to organic dinitrile and the resulting extraction coefficients are shown in the Table 3 below. A compound may be effectively recovered if it has an extraction coefficient of 1 or greater at solvent to feed ratios greater than 1 using a countercurrent multistage extractor.







TABLE 3







Catalyst and ligand extraction coefficients for varying ratios of


mononitriles-to-dinitriles












Catalyst extraction 
Ligand extraction


Example
mononitrile/dinitrile
coefficient
coefficient













7
2.33
1.28
4.09


8
1.85
1.33
8.08


9
1.19
2.02
16.97


10
0.91
2.63
35.99


11
0.57
4.82
49.59









Example 12
This Example demonstrates the effect of hold-up time on the extractability of the diphosphite ligand catalyst.
A mixture comprised predominantly of organic dinitriles and a Ni diphosphite complex, the structure of the diphosphite ligand being shown in Structure XX (where R17 is isopropyl, R18 is H, and R19 is methyl) and ZnCl2 (equimolar with Ni) was divided into two portions. Both portions are liquid-liquid extracted in a three-stage contactor at 40° C., with an equal weight of cyclohexane. Both portions were sampled with time and the progress of the catalyst recovery into the extract phase is shown in Table 4 as the percent of the final steady state value achieved at a given time.







TABLE 4







Concentration of Diphosphite ligand with time in the 


extracting solvent phase.










Time,
% of steady state



minutes
concentration at 40° C.














2
12



4
19



8
34



14
52



30
78



60
100



91
100










Example 13
This Example illustrates the effect of temperature on the extractability of catalyst with last-stage extraction solvent recycle.
A mixture comprised predominantly of organic dinitriles and a Ni diphosphite complex, the structure of the diphosphite ligand being shown in Structure XXIV (where R17 is methyl, R18 is methyl and R19 is H) and ZnCl2 (equimolar with Ni) was divided into three portions. The portions were batch liquid-liquid extracted at 50° C., 65° C. and 80° C., respectively, with an equal weight of n-octane and monitored with time. The results are shown in Table 5.










TABLE 5






% of steady state
% of steady state
% of steady state


Time
at 50° C.
65° C.
80° C.


















2
0.0
0.0
1.8


4
0.0
0.0
1.6


8
0.0
0.0
3.6


14
0.0
0.0
4.3


20
0.0
0.0
3.6


30
0.0
0.0
7.6


60
0.0
1.6
16.3


90
0.7
4.0
48.6









Example 14
This Example demonstrates the effect of adding water in three-stage extraction with cyclohexane recycle in the first stage.
Fifteen grams of a mixture comprised predominantly of organic dinitriles and a Ni diphosphite complex, the structure of the diphosphite ligand being shown in Structure XXIV (where R17 is methyl, R18 is methyl and R19 is H) and ZnCl2 (equimolar with Ni), was extracted in a three-stage continuous extractor at a temperature of 50° C. with an equal weight of cyclohexane for one hour resulting in an catalyst extraction coefficient of 4.3.
To this mixture, 100 microliters of water was added. After continuing to heat and agitate for another hour, the diphosphite Ni extraction coefficient was measured as 13.4—a threefold increase.
Examples 15 and 16
These Examples demonstrate the effect of adding hexamethylene diamine (HMD) to the extraction zone.
Example 1 was repeated except that hexamethylene diamine was added to the product of a pentene-hydrocyanation reaction. To a 50 mL, jacketed, glass laboratory extractor, equipped with a magnetic stirbar, digital stir-plate, and maintained at 65° C., was charged 10 grams of the product of pentene-hydrocyanation reactor product, and 10 grams of the extract from the second stage of a mixer-settler cascade, operated in counter-current flow.
The reactor product was approximately:
85% by weight C6 dinitriles
14% by weight C5 mononitriles
1% by weight catalyst components
360 ppm by weight active nickel.
The laboratory reactor was then mixed at 1160 rotations-per-minute, for 20 minutes, and then allowed to settle for 15 minutes. A stable emulsion was present throughout the extract phase in the absence of the addition of HMD. After 15 minutes of settling, essentially no emulsion phase was present when HMD was added. Samples were obtained of the extract and raffinate phases of the extractor and analyzed to determine the extent of catalyst extraction.







TABLE 6







Effect of hexamethylene diamine on catalyst extraction













Concentration of






HMD added
Catalyst




Example
(ppm)
recovery (KLL)
Stable emulsion
















1
0
14
Yes



15
250
43
No



16
500
80
No

## Claims

1. A process for recovering diphosphite-containing compounds from a feed mixture comprising diphosphite-containing compounds, organic mononitriles and organic dinitriles in a multistage countercurrent liquid-liquid extractor with extraction solvent comprising aliphatic hydrocarbon, cycloaliphatic hydrocarbon or a mixture of aliphatic and cycloaliphatic hydrocarbon, said process comprising:
a) flowing the feed mixture to the first stage of the multistage countercurrent liquid-liquid extractor; and
b) contacting the feed mixture with extraction solvent in the multistage countercurrent liquid-liquid extractor,
wherein the first stage of the multistage countercurrent liquid-liquid extractor comprises a mixing section and a settling section, wherein the mixing section provides a mixed phase comprising a light phase and a heavy phase, wherein a light phase separates from a heavy phase in the settling section, wherein a mixed phase comprising a heavy phase, light phase is present in the settling section between the light phase and the heavy phase, wherein the light phase comprises extraction solvent and extracted diphosphite-containing compounds, wherein the heavy phase comprises organic mononitriles and organic dinitriles, wherein at least a portion of the light phase is withdrawn from the settling section and treated to recover diphosphite-containing compounds extracted into the light phase, wherein a first portion of the heavy phase is passed to the second stage of the multistage countercurrent liquid-liquid extractor, and wherein a second portion of the heavy phase is withdrawn from the settling section of the first stage of the multistage countercurrent liquid-liquid extractor and recycled to the settling section of the first stage of the multistage countercurrent liquid-liquid extractor.

2. The process of claim 1, wherein the recycle of the heavy phase in the settling section of the first stage of the multistage countercurrent liquid-liquid extractor is sufficient to result in enhanced settling of the light phase and heavy phase in the settling section of the first stage.

3. The process of claim 1, wherein the recycle of the heavy phase in the settling section of the first stage of the multistage countercurrent liquid-liquid extractor is sufficient to achieve at least one of the following results: (a) a reduction in the size of an emulsion phase in the settling section, based upon the size of the emulsion phase in the absence of recycle of the heavy phase; (b) an increase in the rate of settling in the settling section, based upon the rate of settling in the absence of recycle of the heavy phase; (c) an increase in the amount of diphosphite-containing compounds in the light phase, based upon the upon the amount of diphosphite-containing compounds in the light phase in the absence of recycle of the heavy phase; (d) a reduction in the size of a rag layer in the settling section, based upon the size of a rag layer in the settling section in the absence of recycle of the heavy phase; and (e) reduction in the rate of formation of a rag layer in the settling section, based upon the rate of formation of a rag layer in the settling section in the absence of recycle of the heavy phase.

4. The process of claim 1, wherein the recycle of the heavy phase in the settling section of the first stage is sufficient to result in improved extraction of diphosphite-containing compounds based upon the extraction of diphosphite compounds in the absence of recycle of the heavy phase.

5. The process of claim 1, wherein the raffinate recycle ratio (RRR) is between 0.1 and 0.9, wherein RRR is defined by the ratio of X to Y, wherein X is the mass per unit time of the second portion of the heavy phase recycled to the settling section of the first stage of the multistage countercurrent liquid-liquid extractor, and wherein Y is the mass per unit time of all of the heavy phase withdrawn from the settling section of the first stage of the multistage countercurrent liquid-liquid extractor.

6. The process of claim 1, wherein the RRR is between 0.2 and 0.8.

7. A process according to claim 1 wherein the diphosphite-containing compound is a Ni complex with a diphosphite ligand selected from the group consisting of:



wherein in I, II and III, R1 is phenyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; or naphthyl, unsubstituted or substituted with one or more C1 to C12 alkyl or C1 to C12 alkoxy groups; and wherein Z and Z1 are independently selected from the group consisting of structural formulae IV, V, VI, VII, and VIII:





and wherein
R2, R3, R4, R5, R6, R7, R8, and R9 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy;
X is O, S, or CH(R10);
R10 is H or C1 to C12 alkyl;





and wherein
R11 and R12 are independently selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy and CO2R13,
R13 is C1 to C12 alkyl, or C6 to C10 aryl unsubstituted or substituted with C1 to C4 alkyl;
Y is O, S, or CH(R14);
R14 is H or C1 to C12 alkyl;





wherein
R15 is selected from the group consisting of H, C1 to C12 alkyl, and C1 to C12 alkoxy and CO2R16,
R16 is C1 to C12 alkyl, or C6 to C10 aryl, unsubstituted or substituted with C1 to C4 alkyl,

and wherein
for structural formulae I through VIII, the C1 to C12 alkyl, and C1 to C12 alkoxy groups may be straight chain or branched.

8. The process of claim 1 wherein at least one stage of the extraction is carried out above 40° C.

9. The process of claim 1, wherein at least one stage contains a Lewis base.

10. The process of claim 9 wherein the Lewis base is a monodentate triarylphosphite wherein the aryl groups are unsubstituted or substituted with alkyl groups having 1 to 12 carbon atoms, and wherein the aryl groups may be interconnected.

11. The process of claim 9 wherein the Lewis base is selected from the group consisting of:
a) anhydrous ammonia, pyridine, alkylamine, dialkylamine, trialkylamine wherein the alkyl groups have 1 to 12 carbon atoms; and
b) polyamine.

12. The process of claim 11 wherein the polyamine comprises at least one selected from hexamethylene diamine and dimers and trimers of hexamethylene diamine.

13. The process of claim 11 wherein the polyamine comprises dimers of hexamethylene diamine.

14. The process of claim 9 wherein the Lewis base compound is Amberlyst 21® resin.

15. The process of claim 1 wherein the extraction solvent is cyclohexane.

16. The process of claim 1 wherein the feed mixture is an effluent stream from a hydrocyanation process.

17. The process of claim 14 wherein the hydrocyanation process includes a 3-pentenenitrile hydrocyanation process.

18. The process of claim 15 wherein the hydrocyanation process includes a 1,3-butadiene hydrocyanation process.

19. The process of claim 1, wherein the first stage of the multistage countercurrent liquid-liquid extractor takes place in an extraction column, wherein the entire column is a settling section comprising a mixing section between a heavy phase collection section and a light phase collection section, and wherein heavy phase is recycled to the mixing section.

20. The process of claim 1, wherein the first stage of the multistage countercurrent liquid-liquid extractor takes place in an mixer-settler, wherein the mixer-settler comprises a settling section which is separate from the mixing section, and wherein recycled heavy stream is recycled upstream from the point of withdraw of the recycled heavy stream.