METHOD FOR PREPARING A SILVER IMPREGNATION SOLUTION

Abstract

A method for preparing a silver impregnation solution comprises (a) charging a neutralization reactor R1 with an aqueous organic amine; (b) adding oxalic acid powder through a first feeding conduit to the neutralization reactor R1 to obtain an aqueous oxalic acid-organic amine solution; (c) directing the aqueous oxalic acid-organic amine solution from the neutralization reactor to a complexation reactor R2; (d) adding particulate silver oxide through a second feeding conduit to the complexation reactor R2 to obtain a silver impregnation solution; and, optionally, (e) subjecting the silver impregnation solution to filtration. The silver impregnation solution is used for producing a catalyst effective in the oxidative conversion of ethylene to ethylene oxide. The method allows for the preparation of a silver impregnation solution in an efficient and occupationally and environmentally safe way. Security hazards which can occur when oxalic acid and silver oxide are added to an aqueous amine solution using the same powder feeding equipment or the same reactor are avoided.

Claims

1. A method for preparing a silver impregnation solution comprising: (a) charging a neutralization reactor R1 with an aqueous organic amine; (b) adding oxalic acid powder through a first feeding conduit to the neutralization reactor R1 to obtain an aqueous oxalic acid-organic amine solution; (c) directing the aqueous oxalic acid-organic amine solution from the neutralization reactor to a complexation reactor R2; (d) adding particulate silver oxide through a second feeding conduit to the complexation reactor R2 to obtain a silver impregnation solution; and, optionally, (e) subjecting the silver impregnation solution to filtration.

2. The method of claim 1, wherein the oxalic acid to silver molar ratio is in the range of 0.505 to 0.6.

3. The method of claim 1, wherein, prior to step (b), the aqueous organic amine is adjusted to a temperature T.sub.1, wherein
T.sub.1100 C.T.sub.AD-1 wherein T.sub.AD-1 is the maximum adiabatic temperature rise during step (b).

4. The method of claim 1, wherein prior to step (d), the aqueous oxalic acid-organic amine solution is adjusted to a temperature T.sub.2, wherein
T.sub.280 C.T.sub.AD-2 wherein T.sub.AD-2 is the maximum adiabatic temperature rise during step (d).

5. The method of claim 1, wherein the oxalic acid is oxalic acid dihydrate containing 1500 ppmw or less, of sulfate anions.

6. The method of claim 1, wherein the oxalic acid is oxalic acid dihydrate powder having a primary particle size distribution such that at least 75% by volume of the particles have a particle size of 250 m or lower.

7. The method of claim 1, wherein the time period of step (d) does not exceed 3 minutes per kg of added silver oxide, calculated as Ag metal.

8. The method of claim 1, wherein the silver oxide has a water content of up to 20% by weight.

9. The method of claim 1, wherein the silver oxide has a primary particle size distribution such that at least 75% by volume of the particles have a particle size of 250 m or lower

10. The method of claim 1, wherein the silver oxide has a primary particle size distribution such that at least 75% by volume of the particles have a particle size of 50 m or lower, and an agglomerated particle size distribution such that at least 30% by volume is in the form of aggregates having an apparent particle size of at least 50 m.

11. The method of claim 1, wherein the silver oxide has a specific surface area of 0.1 m.sup.2/g to 10 m.sup.2/g.

12. The method of claim 1, wherein the silver oxide contains at most 500 ppmw of trace metals selected from copper, iron, lead, nickel, and sodium.

13. The method of claim 1, wherein the silver oxide contains at most 500 ppmw of the sum of chloride and sulfate anions.

14. The method of claim 1, wherein the final silver impregnation solution contains 24 to 35% by weight of dissolved silver cations.

15. The method of claim 1, wherein the organic amine comprises a vicinal alkylene diamine, preferably ethylenediamine.

16. The method of claim 1, wherein the molar ratio of amine nitrogen atoms of the organic amine to silver cations is at least 2.66.

17. The method of claim 1, wherein aqueous oxalic acid-organic amine solution is withdrawn from the neutralization reactor and directed alternately into a first complexation reactor R2.sub.1; and into a second complexation reactor R2.sub.2.

18. A method for producing a catalyst effective in the oxidative conversion of ethylene to ethylene oxide, the method comprising (i) preparing a silver impregnation solution according to the method of claim 1; (ii) impregnating a refractory support with the silver impregnation solution; and (iii) subjecting the impregnated refractory support to a calcination process.

Description

EXAMPLE 1

Silver Oxalate Reactivity

[0120] To assess the reactivity of Ag.sub.2O powder with oxalic acid dihydrate, the following experiments were devised:

[0121] Sample 1: 1.25 g oxalic acid dihydrate and 1.25 g Ag.sub.2O powder were mixed and allowed to react at room temperature for four hours.

[0122] Sample 2: 1.25 g oxalic acid dihydrate, 1.25 g Ag.sub.2O powder and 0.5 g deionized water were mixed and allowed to react at room temperature for four hours. The deionized water was preheated to 40 C. prior to mixing.

[0123] Both samples showed a visible reaction and were subjected to DSC analysis. The results are shown below:

TABLE-US-00001 Onset temperature [ C.] Heat release [J/g] Sample 1 125 300 Sample 2 85 340

[0124] Thus, oxalic acid dihydrate and Ag.sub.2O powder can form thermally unstable silver oxalate that upon thermal decomposition releases a high amount of energy.

EXAMPLE 2

Determination of Q.SUB.AD-1., cp.SUB.1., Q.SUB.AD-2., and cp.SUB.2

[0125] Reaction enthalpies and heat capacities were measured in a microcalorimeter (RC1e manufactured by Mettler Toledo) as follows. 350.1 g (3.087 mol) 53 wt % ethylenediamine/H.sub.2O solution was charged into the reactor and tempered to 38 C. Then, 120.5 g (0.956 mol) oxalic acid dihydrate was added via a funnel in 5 identical portions of 24.1 g each within 20 sec and stirred for 6min at 38 C. The heat flow during the additions was recorded and Q.sub.AD-1 was calculated. Then, cp.sub.1 was determined.

[0126] Subsequently, 223.5 g (0.965 mol) silver(I)-oxid was added via a funnel in 5 identical portions of 44.7 g within 30 sec and stirred for 6min at 38 C. The heat flow during the additions was recorded and Q.sub.AD-2 was calculated. Finally, cp.sub.2 was determined.

[0127] The results are as follows:

[0128] Q.sub.AD-1=83.5 kJ/batch, cp.sub.1=3,015 kJ/(kg*K), m=0.47 kg

[0129] T.sub.AD-1=Q.sub.AD-1/(m*cp.sub.1)=59 K

[0130] Q.sub.AD-2=62 kJ/batch, cp.sub.2=2,245 kJ/(kg*K), m=0.69 kg

[0131] T.sub.AD-2=Q.sub.AD-2/(m*cp.sub.2)=40 K

PRODUCTION EXAMPLE 1

[0132] 783 kg of an aqueous ethylenediamine solution with an ethylenediamine content of 59 wt % was pumped in a stirring reactor 1. Subsequently the 59 wt % ethylenediamine solution was diluted under stirring with 94 kg of de-ionized water. Next, 26.6 kg of 0.95 wt % KOH solution were added to form an aqueous KOH/ethylenediamine solution. The solution was cooled to a temperature of below 20 C. Then 300 kg of oxalic acid dihydrate (purity 99.6%, particle size distribution is shown in FIG. 1) were added into the stirring reactor 1 stepwise during about 180 minutes under stirring and cooling to control the reaction temperature in the range of 20-25 C. Once the addition of oxalic acid dihydrate was completed and the temperature profile from the addition of the last portion of oxalic acid dihydrate passed a maximum, cooling was terminated and the reaction mixture was stirred further for the next 60 minutes to form an aqueous oxalic acid-ethylenediamine solution.

[0133] Next, 1113 kg of the resulting aqueous oxalic acid-ethylenediamine solution was transferred from the stirring reactor 1 to a stirring reactor 2. The reaction medium was cooled to a temperature below 20 C. Then, 500 kg silver oxide powder (Ag.sub.2O-content 99.90 wt %, moisture content 0.2 wt %, particle size distribution is shown in FIG. 2, BET surface area=0.66 m.sup.2/g), was added during 225 minutes under stirring and cooling to control the reaction temperature in the range of 20-25 C. Once the addition of silver oxide was completed, and the temperature profile from the addition of the last portion of silver oxide passed a maximum, cooling was terminated and the reaction mixture was heated under stirring in a temperature range of about 25-30 C. for the next 3 hours to form an aqueous Ag complex suspension.

[0134] The silver oxide used is commercially available from Ames Goldsmith. Its chemical composition is described below:

TABLE-US-00002 Silver Content as Ag.sub.2O 99.90% moisture content 0.20% chlorides 15 ppm nitrates 100 ppm carbonates 0.25% sulfates 20 ppm copper 20 ppm iron 20 ppm lead 20 ppm nickel 20 ppm sodium 50 ppm other trace metals 20 ppm

[0135] Subsequently, the silver impregnation solution was obtained by passing the reaction mixture through a filtration unit SUPRAdisc II KS 100 Depth Filter Modules available from Pall Corporation, Port Washington, USA to remove a minor amount of undissolved solid. The resulting silver impregnation solution had a density of 1.530 g/ml and a silver content of 29.5 wt %.

PRODUCTION EXAMPLE 2

[0136] 844 kg of an aqueous ethylenediamine solution with an ethylenediamine content of 59.4 wt % was pumped in a stirring reactor 1. Subsequently the 59.4 wt % ethylenediamine solution was diluted under stirring with 113 kg of de-ionized water. Next, 28.8 kg of 0.95 wt % KOH solution were added to form an aqueous KOH/ethylenediamine solution. The solution was cooled to a temperature of below 20 C. Then 325 kg of oxalic acid dihydrate (purity 99.6%, particle size distribution is shown in FIG. 1) were added into the stirring reactor 1 stepwise during about 195 minutes under stirring and cooling to control the reaction temperature in the range of 20-25 C. Once the addition of oxalic acid dihydrate was completed, and the temperature profile from the addition of the last portion of oxalic acid dihydrate passed a maximum, cooling was terminated and the reaction mixture was stirred further for the next 60 minutes to form an aqueous oxalic acid-ethylenediamine solution.

[0137] Next, 1145 kg of the resulting aqueous oxalic acid-ethylenediamine solution was transferred from the stirring reactor 1 to a stirring reactor 2. The reaction medium was cooled to a temperature below 20 C. Then, 530 kg silver oxide powder (Ag.sub.2O-content 99.90 wt %, moisture content 0.2 wt %, particle size distribution is shown in FIG. 2; purity as in Production example 1, BET surface area=0.66 m.sup.2/g), was added during 240 minutes under stirring and cooling to control the reaction temperature in the range of 20-25 C. Once the addition of silver oxide was completed, and the temperature profile from the addition of the last portion of silver oxide passed a maximum, cooling was terminated and the reaction mixture was heated under stirring in a temperature range of about 25-30 C. for the next 6 hours to form an aqueous Ag complex suspension. The applied oxalate to silver molar ratio was 0.49. Despite of the increased heating time, the suspension contained a much higher amount of undissolved solid.

[0138] Subsequently, the reaction mixture was passed through a filtration unit SUPRAdisc II KS 100 Depth Filter Modules available from Pall Corporation, Port Washington, USA to remove to the undissolved solid. However the filtration modules were plugged and it was difficult to obtain a silver complex solution.

[0139] Catalyst Preparation

[0140] Step 1. Preparation of Ag-Containing Intermediate

[0141] 585 kg of a commercially available alpha-alumina support were placed in a vacuum tumble mixer having a volume of 1.8 m.sup.3. The support had a cylindrical geometry with the cylinders having an outer diameter of 9 mm, a length of 9 mm and a wall thickness of 3 mm. The support had a water uptake of 0.55 ml/g and a BET surface area of 2 m.sup.2/g. The support was impregnated with 468 kg of Ag complex solution prepared according to Production Example 1 under a reduced pressure of 50 mbar and at a rate of rotation of 0.5 revolutions/min. Impregnation was carried out at room temperature over a period of 4 hours. The vacuum was then broken and the impregnated support was transferred to a belt calciner. The impregnated material was further heated on a belt calciner at a temperature of 290 C. in nitrogen flow according to calcination parameters described in WO 2012/140614 to form an Ag-containing intermediate product.

[0142] Step 2. Preparation of Final Catalyst

[0143] 328 kg of Ag complex solution prepared according to Production Example 1 were mixed with 13.23 kg of promoter solution I containing Li and S, 14.37 kg of promoter solution II containing Cs and W, and 24.93 kg of promoter solution III containing Re to form Ag impregnation solution.

[0144] The promoter solution I was prepared by dissolving LiNO.sub.3 and (NH.sub.4).sub.2SO.sub.4 in water to form a solution with Li-content of 2.85 wt % and S-content of 0.21 wt %. The promoter solution II was prepared by dissolving CsOH and H.sub.2WO.sub.4 in water to form a solution with Cs-content of 5.3 wt % and W-content of 3.0 wt %. The promoter solution III was prepared by dissolving NH.sub.4ReO.sub.4 in water to form a solution with Re-content of 3.7 wt %.

[0145] 634 kg of Ag-containing intermediate prepared according to Step 1 were impregnated with 357 kg of the Ag impregnation solution under a reduced pressure of 50 mbar and at a rate of rotation of 0.5 revolutions/min. Impregnation was carried out at room temperature over a period of 3 hours. The vacuum was then broken and the impregnated support was transferred to a belt calciner. The impregnated material was further heated on a belt calciner at a temperature of 290 C. in nitrogen flow according to calcination parameters described in WO 2012/140614 to form a final ethylene oxide catalyst.