CA1147272A

CA1147272A - Process for the separation of para-xylene

Info

Publication number: CA1147272A
Application number: CA000348273A
Authority: CA
Inventors: Rudolf J. Maas; Rene M. Visser
Original assignee: Shell Canada Ltd
Current assignee: Shell Canada Ltd
Priority date: 1979-05-01
Filing date: 1980-03-24
Publication date: 1983-05-31
Also published as: JPS55145619A; BR8002652A; EP0018680A1; US4326091A; EP0018680B1; DE3061316D1

Abstract

A B S T R A C T

A PROCESS FOR THE SEPARATION OF PARA-XYLENE

The adsorption selectivity of a crystalline silicate of para-xylene over ethyl benzene is increased by pre-treating the crystalline silicate, or the H-form of the crystalline silicate with a solution of a salt of a polyvalent metal.
The conditions should be such that the charge density and concentration of the metal cation meet a given relationship.

Description

~7;272 This invention relates to a process for the separation of apara-xylene from a mixture consisting substantially of aromatic hydrocarbons with six to nine carbon atoms in the molecule and including para-xylene and ethyl benzene.
Para-xylene is an important base material in the chemical industry.
It is generally required substantially free from other aromatic hydrocarbons and in particular from the isomeric compounds ortho-xylene, meta-xylene and ethyl benzene often produced during its manufacture. Since the four isomeric compounds closely resemble each other as regards their physical and chemical properties, , . , the separation of para-xylene from such a mixture of these four isomers presents a particular problem.
Whilst in the earlier Canadian application 320,847, filed February 5, 1979 by Rudolf J. Maas and Rene M. Visser, it is described to use crystalline r' silicates for the separation of para-xylene from the said mixtures, in practice a large quantity of ethyl benzene is still adsorbed by the silicate. The present invention seeks to improve the selectivity of crystalline silicate for para-xylene over ethyl benzene.
It has now been found that the selectivity for para-xylene can be improved by using a modified crystalline silicate.
Crystalline silicates are characterized as a class of compounds by their thermal stability, their crystallinity which follows from the fact that they all show a clear X-ray powder diffraction pattern, their adsorption be-haviour, and this overall composition.
The invention provides a process for the separation of para-xylene from a mixture substantially consisting of aromatic hydrocarbons with six to nine carbon atoms in the molecule including para-xylene and ethyl benzene, comprising contacting the mixture with crystalline silicate as selective adsorbent of para-xylene.

~ -1- ~

7~Z72 The crystalline silicate, after calcining in air at 500 C:
a) Has an x-ray powder diffraction pattern showing inter alia the reflections given in Table A below:
TABLE A

Source Cu-K~ Wave length 0.15418 nm 29 Relative intensity ''' .
7.8 - 8.2 S
8.7 - 9.1 M
11.8 - 12.1 W
12.4 - 12.7 W
14.6 - 14.9 W
15.4 - 15.7 W
15.8 - 16.1 W
17.6 - 17.9 W
19.2 - 19.5 W
20.2 - 20.6 W
20.7 - 21.1 W
23.1 - 23.4 VS
23.8 - 24.1 S
24.2 - 24.8 M
29.7 - 30.1 M
in which the letters have the following significance:
VS = very strong; S = strong; M = moderate; W = weak.
O is the angle according to Bragg~s law;
b) In the so-called "H-form" after evacuation to 2 x 10 bar at a temperature of 400 C for 16 hours and measured at a hydrocarbon pressure of 8 x 10 bar at a temperature of 100 C it has an adsorption of n-hexane (n-C6H14) of at least 0.8 mmol /g and an adsorption of 2,2-dimethylbutane (2,2 DMB) of at least 0.5 ~7~72 ' mmol./g, and the ratio adsorption of n-hexane adSOrption of 2~2-dimethylbutane should be at least 1.5; and c) Has the following overall composition: y(l.0 + 0.3) M20.y(a Fe203. b A1203).
; SiO2 wherein M is H and/or an alkali metal a + b = l; a ~ 0; b ~ 0 ' OC y ~ 0.1.
~ he process of the invention is also characterized in that the crystalline silicate is modified in order to increase its selectivity for para-xylene by bringing the crystalline silicate into contact with a solution of a salt of a polyvalent metal whose metal cation concentration is m(in gion/l) and charge density is e (e being the charge of the ion, and the radius R of the ion R
being expressed in nonometers) and wherein the product e multiplied by m is at R
least 45, after which the crystalline silicate is filtered, washed and dried or calcined at an elevated temperature so that the salt decomposes to leave the metal cation in the crystalline silicate.
For the adsorption measurements under paragraph b) the silicate must be in its so-called "H-form" as described below.
A modified crystalline silicate shall mean a crystalline silicate which has been brought into contact with a solution of a salt of a polyvalent metal filtered, washed and dried or calcined at elevated temperature, preferablyat between 400 C and 600 C, so that the salt decomposes leaving the metal cationin the silicate. The solution is conveniently, though not necessarily, an aqueous solution.
~he salt may conveniently be one of an organic acid such as a formate or an oxalate, which readily decomposes on heating, or alternatively of an in-organic acid, such as a nitrate which also decomposes without leaving traces of undesirable compounds or elements in the modified crystalline silicate.

~7~272 In order to obtain the increase in the selectivity for para-xylene over ethyl benzene in accordance with the inven~ion, the polyvalent metal cation should have such a charge density e (being the quotient of the charge e and the ; ion radius R, the latter being expressed in nanometres), and a concentration _ in gion/l that the product of the charge density and the concentration amounts to at least 45. Compared with conventional processes, application of the pre-sent invention yields a desorbate which is richer in para-xylene, which may in some cases obviate the need for further purification steps, or certainly render any such steps, for example crystallization, more efficient.
Where the valency of the metal cation is 2, R x m is preferably in excess of 60 in order to obtain a significan-t increase in the selectivity for para-xylene to ethyl benzene SpX~EB. At values of R ~ m of at least 100, a 50%
increase in the selectivity SpX/EB can be obtained at a temperature of 80 C.
Whilst the invention produces a valuable increase in the selectivity SpX/EB where metal cations of a valency of 2 are employed, a still greater im-provement can be obtained where the metal cation has a valency of 3. In this case, an improvement in the selectivlty SpX~EB of 20% is obtained where R x m is greater than 45 and as much as a 100% improvement when R ~ m is at least 115 at a temperature of 80 C.
At lower temperatures, for example, in the liquid phase a selectivity SpX/EB in excess of 5 may be found. These values have been established with respect to a silicate in the Na-form.
However, whilst such increases in the selectivity SpX/EB are very encouraging, it has been found that a pretreatment of the crystalline silicate can still further increase its selectivity. Such a pretreatment involves substitution of hydrogen ions for M ions, which are often sodium ions, present in the originally prepared and calcined silicate. (Such a silicate is said to be in the "H-form"). This may conveniently be performed by bringing the :. ~ ~7~72 crystals into contact with an ammonium salt, or a weak acid. The crystals are then washed and dried. Where an ammonium salt is used they are also heated until the ammonium has decomposed to leave hydrogen ions in the crystal structure.
- Whilst this pretreatment will in itself increase the selectivity for para-xylene, its combination with the modification in accordance with the invention, whereby a metal cation is fixed in the crystalline structure, produces a substantial further improvement in the desired selectivity of some 10% in the case of a crystalline aluminium silicate with a low Al content to more than 70% for a crystalline iron silicate with a high Fe content.
For modifying the crystalline silicate, suitable metal cations may be selected from the earth alkaline metals, rare earth metals, metals of the iron group, manganese, aluminium and gallium. Of these, magnesium and calcium are preferred, and iron, aluminium and lanthanum are most preferred.

- 4a -7~72 The salt of the metal should decompose on heating to leave only the metal cation in the crystalline silicate. Particularly suitable salts are nitrates and oxalates which decompose without depositing any side products which might adversely influence the performance of the crystalline silicate.
Best results are found when the solution of the salt is made as concentrated as possible, say from a 2 molar solution to one of the maximum solubility of the salt in ~uestion.
The unmodified and untreated crystalline silicate preferably has an X-ray diffraction pattern substantially as set out in Table B below:

~7Z~7~

TABLE B
Source Cu-K~ Relative intensity Wave length 0.15418 nm 23 (100. I/Io) description o~ the reflection 8.oo 55 SP
8.90 36 SP
9.10 20 SR
11.95 7 NL
12.55 3 NL
13.25 4 NL
13.95 10 NL

15.55 7 BD

17.75 5 BD
19.35 6 NL
20.40 9 NL
20.90 10 NL
21.80 4 NL
22.25 8 NL
23.25 100~ SP
23.95 45 SP
24.40 27 SP
25.90 11 BD
26.70 9 BD
27.50 NL
29.30 7 NL
29.90 11 BD
31.25 2 NL
32.75 4 NL
34.40 4 NL
36.05 5 BD

37.50 4 BD

o = intensity o~ the strongest separated re~lection occurring in the pattern.

The abbreviations used in Table B to describe the reflections have the following meanings: SP = sharp; SR = shoulder; NL =
normal; BD = broad, 3 is the angle according to Bragg's law.
The pores of the crystalline silicate are generally substantially elliptical in shape and their diameter is between 0.5 and o.6 nanometre (5 and 6 ~).
The adsorbed compounds can be isolated from the adsorbent in various ways. Desorption may, for instance, be effected by heat-ing the adsorbent, by reducing the pressure in the space in which the adsorbent is present or by treating ~e adsorbent with a suitable inert gas or a displacing agent. If solvent desorption is used, toluene will be the most suitable solvent.
The invention will now be further described by way of example. First the preparation of a number of candidate crystal-line silicates will be described:
Silicate A
A crystalline silicate was prepared from a mixture of Fe(N03)3, SiO2, NaOH and ~ (C3H7)4N ~ 0H in water with a molar composition as follows:
5 i2 bFe23 3 ~(C3H7)4N ~oH.Na2o.45oH2o The mixture was heated to 150 C in an autoclave under autogen-ous pressure, at which temperature it was maintained for 24 hours after which it was filtered and washed until its pH was approximately 8. After drying the resulting crystals were calcined for 8 hours at 500 C. This crystalline iron silicate will be referred to as Silicate A.
a) Silicate A was thermally stable to above 600C.
b) It had an X-ray powder diffraction pattern showing the reflections given in Table B above.
c) In the "H-form" at 100 C it has an adsorption of n-C6H14 of 1.22 mmol./g and of 2,2 DMB of o.60 mmol./g.
d) Its chemical formula was:

2 0-011(o.97Fe2o3.o.o3Al2o3).si~2.

`,..~

~7~72 The occurrence of A1203 in the formula can be explained by the presence of up to 500 ppm Al in the SiO2 used in its preparation. Up to 240 ppm Al is also found in the Fe(N03)3 used.
Silicate B
Silicate A was contacted with a 1 molar NH4N03 solution at 100 C for 10 hours (2 x 5 h, which means that the solution was refreshed after 5 hours). The crystals were filtered, washed and then dried for 15 hours at 120C. This treated crystalline iron silicate will be referred to as Silicate B.
Silicate C
; Silicate A was contacted with a 4 molar solution of La(N03)3 at 100 C for 10 hours (2 x 5 h). The crystals were filtered, washed and then dried at 400C for 15 hours. This modified crystalline iron silicate will be referred to as Silicate C.
Silicate D
Silicate B was contacted with a 4 molar solution of La(N03)3 at 100 C for 10 hours (2 x 5 h). The crystals were filtered, washed and then dried at 400 C for 15 hours. This modified crystalline iron silicate will be referred to as Silicate D.
Silicate ~
A reaction mixture was prepared from SiO2, NaN03 and / (C3H7)4N ~OH in water with a molar cGmposition as follows:
29 1sio2 3 0~ (C3H7)4N ~OH-lNa2o.43oH2o.
The mixture was heated to 150C in an autoclave under autogen-ous pressure for 24 hours, then filtered and washed until its pH was below 9. After drying at 120 C, the crystals were calcined at 500 C for 3 hours. This crystalline silicate a) was thermally stable to above 600C;
b) had an X-ray powder diffraction pattern showing inter alia the reflections given in Table B above;
c) in the H-form at 100C it had an adsorption of n-C6H14 of 1.29 mmol./g and of 2,2 DMB of 0.67 mmol./g;
d) its chemical formula was:
0.0003Na20Ø0003Al203.SiG2.

'72 The occurrence of A1203 in the final formula can be ex-plained by the presence of up to 500 ppm Al in the SiO2 used in its preparation. It was then contacted with a 1 molar solution of NH4N03 for 2 hours (2 x 1 h) at 100 C which was followed by drying and at 120 C for 15 hours. This crystalline silicate will be referred to as Silicate E.
Silicate F
Silicate E was contacted with a 1 molar solution of RbN03 for 10 hours (2 x 5 h) at 100C. The crystals were filtered and washed before drying for 15 hours at 400 C. This modified crystalline silicate will be referred to as Silicate F.
Silicate G
Silicate E was contacted with a 1 molar solution of La(N03)3 for 10 hours (2 x 5 h) at 100C. After filtering and washing 15 it was dried for 15 hours at 400C. This modified crystalline silicate will be referred to as Silicate G.
Silicate H
Silicate E was contacted with a 4 molar solution of La(N03)3 for 10 hours (2 x 5 h) at 100C. After filtering and washing 20 it was dried for 14 hours at 400C. This modified crystalline silicate will be referred to as Silicate H.
Silicate I
Silicate B was contacted with a 4 molar solution of Ca(N03)2 for 10 hours (2 x 5 h) at 100 C. After filtering and washing, 25 it was dried for 15 hours at 400C. This modified crystalline ; silicate will be referred to as Silicate I.
Silicate J
Silicate B was contacted with a 2.5 molar solution of Ca(N03)2 for 10 hours (2 x 5 h) at 100 C. After filtering and washing, - 30 it was dried for 15 hours at 400 C. This modified crystalline silicate will be referred to as Silicate J.
Silicate K
A crystalline silicate was prepared from a mixture of Al(N03)3, SiO2, NaOH and L (C3H7)4N 70H in water with a molar composition ~727Z

as follows:
02-8Al23-3 L ( C3H7)4N ~ OH.Na20.450X20.
This mixture was heated to 150C in an autoclave under autogen-ous pressure, at which temperature it was maintained for 24 5 hours, after which it was filtered and washed until its pH was approximately 8. The resulting crystalline silicate after drying and calcining at 500 C for 3 hours will be referred to as Silicate K.
a) Silicate K was thermally stable to above 600 C.
lO b) It ha,d an X-ray powder diffraction pattern showing the reflections given in Table B above.
c) In the H-form its adsorption of n-C6H14 was 1.25 mmol./g and that of 2,2 DMB was 0.62 mmol./g.
d) Its chemical formula was 0.006Na20Ø006A1203.SiO2.
15 Silicate L
Silicate K was contacted with a 1 molar NH4N03 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered, washed and then dried for 15 hours at 120 C. This treated silicate will be referred to as Silicate L.
20 Silicate M
Silicate L was contacted with a o.8 molar Mg(N03)2 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400 C for 15 hours. This modified crystalline silicate will be referred to as Silicate M.
25 Silicate N
Silicate h was contacted with a 2.5 molar Mg(N03)2 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400C for 15 hours. This modified crystalline silicate will be referred to as Silicate N.
30 Silicate O
Silicate L was contacted with a 1 molar Fe(N03)3 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400 C for 15 hours.
This modified crystalline silicate will be referred to as 35 Silicate 0.

;

Silicate P
Silicate L was contacted with a 6 molar Ca(N03)2 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400 C for 15 hours. This modified crystalline silicate will be referred to as Silicate P.
Silicate Q
A crystalline silicate was prepared from a mixture of Al(N03)3, Fe(~03)3, SiO2, NaOH and L (C3H7)4N 70H in water with a molar composition as follows:
2 / 2 3.3/64A1203.3-OL~(C3H7)4N ~ OH Na2450H20-The mixture was heated to 150 C in an autoclave under autogen-ous pressure, at which temperature it was maintained for 24 hours, after which it was filtered and washed until the pH was 8.
After drying the resulting cr~stals were calcined for 8 hours at 500C. This crystalline silicate will be referred to as Silicate Q.
a) Silicate Q was thermally stable to above 600C.
b) It had an X-ray powder diffraction pattern showing the reflections given in Table B above.
` c) In the H-form its adsorption of n-C6H14 was 1.27 mmol./g ; and that of 2,2 DMB was 0.63 mmol./g.
d) Its chemical formula was:
o.oo44Na2o.o.oo44(o.64Fe2o3.o.36Al2o3).sio2.
Silicate R
Silicate Q was contacted with a 1 molar La(N03)3 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400 C for 15 hours. This modified crystalline silicate will be referred to as Silicate R.
Silicate S
Silicate Q was contacted with a 4 molar La(N03)3 solution at 100 C for 10 hours (2 x 5 h). The crystals were filtered and washed and then dried at 400 C for 15 hours. This modified crystalline silicate will be referred to as Silicate S.

~147272 ; 12 EX~IPLE I
Samples of 100 g of each of Silicates A to H inclusive, J and L to 0 inclusive were brought in-to contact with a nitrogen stream at 80C containing para-xylene and ethyl benzene in equal molar proportions, the C8 aromatics having a combined partial pressure of 45 m bar. After equilibrium was reached the s~nples were weighed, and the ratio of para-xylene to ethyl benzene established.
The following results were obtained:
- lO Silicate ~ Para-xylene and Ratio PX/EB
R ethyl benzene adsorbed (%w) A - 8.1 o.8 B - 9.5 1.7 C 118 8.3 1.3 D 118 10.8 3.4 E - 8.9 1.2 F 7 9.5 1.0 G 30 9.5 1.3 H 118 10.5 2.4 J 51 10.0 1.9 L - 9.5 1.5 M 24 9.5 1.4 N 76 10.1 1.9 0 47 10.1 2.0 ~ote: "ratio PX/EB" is the ratio of para-xylene adsorbed to ethyl benzene adsorbed. In the case where the composition of the gas stream remains constant and the quantities of para-xylene and ethyl benzene in the stream are equal the ratio PX/EB is equivalent to ~e selectivity SpX/EB for para-xylene over ethyl benzene for the crystalline silicate.

~,, , ~ 7~72 ; 13 Commentar~
Comparing Silicate C to Silicate A it will be seen that the modification of Silicate C in accordance with ~e invention has improved the ratio PX/EB substantially. Similarly, taking - 5 Silicate D a similar improvement over Silicate B is demonstrated.
Where the product R x m is smaller the effect is less marked, as with Silicate J, although still useful.
Taking Sili~ate E it is seen that the modifications re-sulting in Silicates F and G do not lead to any marked im-provement - the product R x m is too small, and in the case o~
Silicate F, Rb is monovalent. Silicate H in accordance with the invention, however, shows a significant improvement.
Starting from Silicate L, Silicates N and 0, which are in accordance with the invention show a significant improvement, whereas Silicate M (R x m = 24) does not.
EXAMPLE II
Samples of 100 g of each o~ Silicates A, B, D, I, J, L and N to S inclusive were brought separately into contact with a solution of 2,2,4-trimethylpentane at 25 C containing 4%w of para-xylene and ethyl benzene in a ratio of para-xylene to ethyl benzene of 1. After equilibrium was reached the solution was analyzed and the ratio of para-xylene to ethyl benzene adsorbed by the samples thus established.
The following results were obtained:

Silicate (R ~ m) Liquid- Para-xylene Ratio PX/EB SpX/EB
solids and ethyl ratio benzene ad-sorbed (%w) A _ 10.0 10.3 2.7 3.7 B _ 10.1 10.4 3.7 5.7 D 118 13.0 11.8 5.6 9.0 I 81 11.8 10.6 5.2 7.8 J 51 10.2 10.9 4.2 6.6 L _ 10.2 10.3 3.5 5.1 N 76 10.3 10.7 4.2 6.3 0 47 10.5 10.6 4.0 6.3 P 121 9.8 11.2 4.6 7.3 ': Q _ 10.0 9.3 2.7 3.5 R 30 10.0 9 4 2.7 3.6 S 118 10.2 11.0 5.1 7.0 "Ratio PX/EB" is the ratio of para-xylene adsorbed to ethyl benzene adsorbed. "SpX/EB" is the selectivity for para-xylene over ethyl benzene taking account of the reduced proportion of para-xylene in the solution under equilibrium conditions, PX/EB in the absorbed phase i.e. SpX/EB PX/EB in the liquid phase Commentary :
A substantial improvement is ~ound in the selectivity SpX/EB
for Silicate D, I and J over Silicates A and B from which they were derived. Similarly, Silicates N, 0 and P showed an im-provement over Silicate L. Moreover, Silicate R (R ~ m = 30) ; showed hardly any increase over Silicate Q, whereas Silicate S
(R x m = 118) showed a marked increase. It should be noted - that Silicates D, I, J, N, 0, P and S are in accordance with the invention.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the separation of para-xylene from a mixture substantially consisting of aromatic hydrocarbons with six to nine carbon atoms in the molecule including para-xylene and ethyl benzene, comprising contacting the mixture with crystalline silicate as selective adsorbent of para-xylene, which crystalline silicate, after calcining in air at 500°C :
a) has an X-ray powder diffraction pattern showing inter alia the reflections given in Table A below TABLE A

in which the letters have the following significance: VS = very strong; S = strong; M = moderate; W = weak; .theta. is the angle accord-ing to Bragg's law, b) in the H form after evacuation to 2 x 10-9 bar at a temperature of 400°C for 16 hours and measured at a hydrocarbon pressure of 8 x 10-2 bar at a temperature of 100°C, it has an adsorption of n-hexane (n-C6H14) of at least 0.8 mmol/g and an adsorption of 2,2-dimethylbutane of at least 0.5 mmol./g, and the ratio should be at least 1.5;

and c) has the following overall composition:
y(1.0 ? 0.3)M2O.y(aFe2O3.b Al2O3).SiO2 wherein M is H and/or an alkali metal, a + b = 1 a ? 0 b ? 0 0 ? y ? 0.1, characterized in that the crystalline silicate is modified in order to increase its selectivity for para-xylene by bringing the crystalline silicate into contact with a solution of a salt of a polyvalent metal whose metal cation concentration is m(in gion/l) and charge density is ? (e being the charge of the ion, and the radius R of the ion being expressed in nonometers) and wherein the product ? multiplied by m is at least 45, after which the cyrstalline silicate is filtered, washed and dried or calcined at an elevated temperature so that the salt decomposes to leave the metal cation in the crystalline silicate.

2. A process as claimed in claim 1, in which the solution of the salt of the polyvalent metal is an aqueous solution.

3. A process as claimed in claim 1, in which the metal cation has a valency of 2 or 3.

4. A process as claimed in claim l,in which the metal cation has a valency of 2 and that the said product is at least 100.

5. A process as claimed in claim l,in which the said product is at least 115.

6. A process as claimed in claim l,in which the metal is an alkaline earth metal.

7. A process as claimed in claim 1, in which the metal is a rare earth metal.

8. A process as claimed in claim 1, in which the metal is a metal from the iron group.

9. A process as claimed in claim 6, in which metal is either magnesium or calcium.

10. A process as claimed in claim 7, in which the metal is lanthanum.

11. A process as claimed in claim 8, in which the metal is iron.

12. A process as claimed in claim 1, in which the metal is aluminium or gallium.

13. A process as claimed in claim 1 or 2, in which the salt is a salt of an organic acid.

14. A process as claimed in claim 1 or 2, in which the salt is either a formate or an oxalate.

15. A process as claimed in claim 1 or 2, in which the salt is a nitrate.

16. A process as claimed in claim 1 or 2, in which the concentration of the solution of the salt is in excess of a 2 molar solution.

17. A process as claimed in claim 1, in which prior to contacting the crystalline silicate with an aqueous solution of a salt of a polyvalent metal cation, the silicate is put into the H-form.