Desalination and recovery of catalysts by electrodialysis

233

Desalination, 79 (1990) 233-247 Elsevier Science Publishers B.V., Amsterdam

Desalination and Recovery of Catalysts by Electrodialysis S. SRIDHAR

Hiils AG, Postjach 1320, 4370 Mad (Germany) (Received October 29 1989; in revised form October 2,199O)

1. INTRODUCI’ION

Hexadiinediol can be produced from propargyl alcohol by way of oxidative coupling employing inorganic salts such as (CuCl and NH&l) as catalysts. The diol can be further hydrogenated to hexanediol which finds its application as the diol component for the synthesis of polymers. However, the low solubility of the diinediol leads to its precipitation in aqueous media. Hence, the reaction can be effected in presence of butanol. The reaction is conducted batchwise in presence of oxygen. After the reaction two liquid phases are present in the reactor, the bottom phase consisting of water, dissolved butanol and most of the salts and a top phase containing butanol and the diol besides a certain amount of water with dissolved salts (Fig. 1). Propargylakohol

Butanoll H,O

12 I-

BUtFUlOl

ll

’

Reaction

Hydrogenation

I01

6

-r 7

Hexanediol

Fig. 1. The process.

ooll-9164/90/$03.50

Distillation

0 1990 Elsevier Science Publishers B.V.

234

The aqueous phase is retained in the reactor and reused in the next batch operation. Only the organic product phase is removed from the reactor (stream 3 in Fig. 1) and led to the electrodialysis step where it is desalinated. Traces of salts still present after this step can be removed in a successive ion exchange stage (not shown in Fig. 1). After desalination the phase emerges as stream 4 to be hydrogenated to hexanediol. The mixture is then distilled to recover the diol as bottoms product and the solvents butanol and water as distillate. The water formed in the initial coupling reaction is purged as the aqueous phase of the distillate. The butanol phase (with dissolved water) is supplemented by fresh butanol to make up for loss in the aqueous phase and led back to the electrodialysis step along with fresh propargyl alcohol (stream 13 in Fig. 1). This mixture is used to pick up the salts from the down stream 3. Hence, the stream 1 from the electrodialysis step consists of all the ingredients for the reaction and is fed to the reactor for the next batch operation. In effect, streams 3 and 1 comprise a loop in which the salts circulate, whereas streams 3, 4, 6, 11, 13 and 1 a loop for the solvents butanol and water. Salt discharge into the environment as well as use of fresh salts are avoided, i.e. electrodialysis effects recovery as well as recycling of the catalysts. On the whole fresh propargyl alcohol is fed into the process and hexanediol is obtained. The salts and solvents are recycled. The water formed in the reaction wiU have to be removed. TABLE I The membrane and the dialysis stack Anion exchange membrane ACLE-SP Cation exchange membrane C66-10F

Tokuyama Soda

Number of chambers per cell Eff. membrane area (cm2) Membrane thickness (mm) Chamber thickness (mm)

2 220 0.2-0.4 0.4

Electrical r-es&tame (ohm. cm2) Anion exchange membrane Cation exchange membrane

15-25 3.8-5.3

One of the primary concerns was the fact that both propargyl alcohol and hexadiinediol are chemically reactive and also affect the skin. The experiments were conducted without allowing leakage and in an atmosphere of nitrogen, but nothing was known about the compatibility of the membranes

235

employed. Moreover, hardly any literature was available concerning electrodialysis in such media with a low water content under 20 wt%. The electrical resistance was expected to be high even in presence of salts; hence a stack with very thin compartments (0.4 mm) was employed. 2.

EXPERIMENTAL SET-UP

The electrodialysis stack was of the classical type with alternating anion and cation exchange membranes arranged in symmetric order so that an eventual current reversal would be feasible. The dimensions and other details are given in Table I. Commercially available membranes were used. The experiments were conducted at 30°C. The electrode chambers were supplied from a reservoir of 1% NH&l solution. The desalination of the product mixture takes place as follows (Fig. 2). The salt-rich mixture (mentioned as “diluate” in Fig. 2) is fed into alternate chambers of the electrodialysis stack. The Cl- ions migrate towards the anode to the left through the anion exchange membranes A, while the cations Cut and NH: migrate to the right through the cation exchange membranes K and towards the cathode. Thus the ions accumulate in the adjacent chambers to be picked up by a mixture of butanol, water and fresh propargyl alcohol (“concentrate” stream in Fig. 2). At the two ends of the stack the anode compartment picks up Cl- ions emerging from the adjacent diluate chamber while the cathode compartment is depleted of Cl’ ions passing into the next concentrate chamber. In effect, the Cl- ions migrate from the cathode chamber via the middle chambers back to the anode chamber. A pair of chambers containing the diluate (initially salt-rich product solution) and the concentrate (initially salt-free reactants) constitute a cell for the desalination. Twenty-five cells were employed between the electrode chambers. A

KA

KA

KA

KA

KA

KA

diluate

Fig. 2. The electrodialysis stack.

236

3.

EXPERIMENTS AND DISCUSSION

3.1. Limiting electrical parameters The amount of salt m transported in unit time across unit surface area of membrane in an electrical field is given by the expression m=-*i c ZF

in which i is the current density, z the valency, F the Faraday constant and C the current yield. The equation suggests application of high current densities in order to increase the salt flux. However, with increasing current density also the diffusional transport of ions from the diluate to the membrane surface through the hydrodynamic boundary layer must increase in order to maintain the necessary supply of ions. For a given solution velocity and temperature the thickness of the boundary layer is constant. The only way the diffusional transport can increase is by way of an increase in the concentration gradient in the layer when the current density is increased (see Fig. 3 illustrating the ion flow). The bulk concentration of C, in the diluate is here taken to be a constant in order to compare the situations at various applied current densities. The ion migration through the membrane is solely determined by the applied EMF and is comparatively rapid. Hence, the depletion at the surface increases with increasing current density. Accordingly the concentration Co at the membrane surface drops ensuring an increasing concentration gradient and diffusional flow. When the value for Co reaches zero (as in Fig. 3), a further increase of the gradient is not possible. At this stage the diffusional transport for the presumed constant bulk concentration becomes a limiting factor for the ion supply and the current density reaches a limiting value. Its further increase leads to a shortage of ions in the vicinity of the membrane, the diffusional transport being insufficient to ensure ion supply to the surface at the desired rate of depletion. This affects electrolysis of water and the current applied is wasted for this undesirable reaction, resulting in a low current yield. This effect can be observed by the sudden increase in electrical resistance. Hence, this upper limit for the current density is to be determined for a pair of diluate and concentrate chambers and the corresponding limiting voltage ascertained.

concentration


+ cathode

anode

distance

Fig. 3. Concentrations

at limiting current density.

2

3

4 cr+/mA

reciprocal current density

Fig. 4. Limiting current density at 35 @/cm.

As the diluate and the concentrate have varying concentrations during the desalination, the limiting values had to be determined for different concentrations in the range corresponding to a specific conductivity of 351,000 S/cm in solutions obtained by an initial desalination experiment. Both the diluate and the concentrate chambers were fed by one and the same solution of known concentration, and the voltage varied. The plot of the

238

reciprocal current density against the specific resistance gives the limiting current density at which the resistance increases due to ion depletion (polarisation, Fig. 4). From such plots the limiting values could be determined, as given in Table II. The limiting voltages have values between 3.5-6.5 V/cell. (The reason for the minimum value at about 140 j.S/cm could not be explained.) In order to avoid the polarisation, a low value of 3 V/cell was chosen for further experiments. Correspondingly also the current densities applied were well below the limiting values.

3.2. Desalination experiments The diluate consisted of the reaction mixture containing 1 wt% salts freed of large suspended particles by way of microfiltration (0.2 CL).The first run at 3 V/cell yielded a solution containing 280 ppm salts after 207 min. The specific conductivity sank from 854 to 12 &j/cm. Ninety-seven percent desalination could be achieved at a current yield of > 99% (see Fig. 5). The amount of concentrate is limited, as the process requires the recirculation of the solvents available from the reaction mixture (cf. Fig. 1). However, the amount could be split into two portions to be employed twice to desalt the same diluate. The next run employed only half the available amount in a first step. The samples were analysed after attaining a value of only 15 @/cm in the diluate in about 200 min and once again before beginning the second step the next day. The second step of electrodialysis employing the second half of the concentrate led to a very low value of 3 pS/cm after 342 min (Fig. 6). Reproduction of the experiment after reduction of the first step to 120 min (at 18 pS/cm) and current reversal before the second step led to the same low value of 3 pS/cm in merely 268 min (Fig. 7). The salt content was 200 wt ppm. In the first step of the run about 97.5% of the salts were removed at a current yield of 94-99%. The next step which amounts to a refining step increased the desalination degree to above 98% at a low yield of around 10% (see Table III). In order to reduce the energy consumption, a further two-step run was carried out at a lower voltage of 2 V/cell. Surprisingly, such a measure does not considerably prolong the desalination: even at the lower voltage a final value of 4 pS/cm (about 200 wt ppm salts) for the diluate could be attained in 4 hours (Fig. 8). An optimal value for the voltage is yet to be determined. The energy requirement at 2 V/cell works out to be about 12.4 kWh/lOO kg hexadiinediol. The salts in the concentrates after all the runs were found to have unchanged catalytic activity for the diinediol production.

ppml

130 290 1000 1900 2100

[wt.

cu

110 220 840 1000 2200

W. wml

NH4

Solution analysis

120 625 2200 3800 6100

W. ppml

cl-

12.6 12.6 12.5 12.5 14.5

W. wml

H2O

30-35 137-145 395-405 625-635 1010-1020

Specific cotlductance bS/cm] 0.86 2.82 6.14 8.64 14.55

190 620 1350 1900 3200

WV

Current density [mA/cm2]

Current strength

Limiting values

The limiting values Stack with 5 cells, flow rate 80-100 llhr, temperature 30°C; 1% NH&l as electrolyte for the electrode chambers. 1 cell = 2 dialysis chambers

TABLE II

3.66 3.46 3.7 4.3 6.6

Voltage [V/cell]

240 10000 I ImAl K IpS/cml

1000

1

1

0

200

100

min

time

Fig. 5. Desalting experiment.

1

0

100

200

I

300 min

time

Fig. 6. Desalting experiment, 2nd run.

There was no significant migration of water between the two streams (Table IV). Only in the third run a certain water flow is to be noted after 120 min. Also propargyl alcohol and diinediol remained confined to their respective streams; e.g. the diol content in the concentrate was below detection (0.1 mol.%).

241

The electrode chambers contained 30-40 wt ppm copper at the end of a run, which can be alluded to leakage through the last anion exchange membrane at the cathode (Fig. 2). However, on a technical scale with 100 or more cells, this leakage would be of no significance. 10000 I [mAI

I

--*-A-

\%

lo-

*-.4-A-*

x.x xx-h. +--x,x

IL 0

100

I

time

.

K -x-x

200

300

mtn

Fig. 7. Desalting experiment, 3rd run.

10000I ImAl K

IpS/cml

1

0

Fig. 8. Desalting experiment

100 at 2 V/cd.

rime

200

min

300

Stage

Initial After 207 min End

Initial After 197 min Initial 2nd step End

Initial After 120 min Initial 2nd step End

Initial After 100 min Initial 2nd step End

Run

1

2

3

4

110

3071 220 220

2872 180 160 130

3100 130 120 100

120 120

2600

8.9

1695 13 14

2400 7 6 10

2300 90 80 90

1600 90 85

590

20

Gl

10

46

7.8

3000 n.a.

25 n.a. n.a.

5.4 20 3043 8200 3.4 13

< 1 3568 2

5900 195 46

3200 3200 3200 3200

< 3 16 16 4 3300 3200 3200 3100

;;o

3300

3 n.a. 40

< 3 30 30 30

64 1090 36 405

35 5800 5000

NH4

54 1060 16 380

< 50 3400 < 50 < 50

< 50 1600 1700

cu

98.4

97.3

98.45

-

-

5.6

ZO

n.a. 267 n.a. 342

6500 6500 6500 6500

94.2

23.86

99.47

> 99.0 12.8

-

> 98.2

-

> 99.0 -

WI

Current yield

97.7

-

97.0

97.5

n.a. 362 n.a. 434

6500 6500 6500 6500

-

-

DesaIination [%I

Yield based on chloride content

6300 n.a.

;;;

n.a.

c

z

6300

Cl

Electrode solution [wt. ppm]

16 3700 2 6

3 3700 14 60

< 3 1300 1700

Cl

[wt. ppm]

< 3 2660 6 26

6300 125 131 43

6500 58 82 24

5000 67 74

NH4

cu

Cl

CU NH4

Concentrate

Diluate [wt. ppm]

Salt analysis @.a. = not analysed)

TABLE III

=

16.6 15.9 17.4 17.1

7.4 7.0 n.a. ma.

8.9 8.5 8.3 8.2

11.8 15.8 16.8 17.5

Initial After 120 min Initial 2nd step After 268 min

Initial After 100 min Initial 2nd step After 276 min

8.0 7.3 n.8. 8.8

17.4 16.3 17.9 18.3

Initial After 197 min Initial 2nd step After 342 min

4

8.5 n.8. 6.6

16.4 16.6 17.7

H3D

Initial After 207 min End

%I

1

Hz0

w.

Stage

Run

0.9 2.0 n.8. n.a.

1.2 1.3 1.4 1.5

2.1 2.5 n.a. 1.4

1.0 n.a. 1.2

P,OH

Propargyl alcohol, n.a.

Diluate (GC)

Analysis of probes H3D = Hexadiinediol, P30H

TABLE IV

75.1 75.1 n.a. n.8.

78.1 72.0 73.5 72.7

72.1 73.9 n.a. 71.4

z

74.1

17.1 19.2 18.2 19.3

17.9 15.5 21.0 21.0

23.0 24.3 23.0 24.0

22.0 23.3 22.8

Hz0

w.

%I

0 0 0 0

ma. 0 ma. 0

n.a. n.a. n.a. 0

n.a. ma. 0

H3D

n.8. n.a. n.8. 66.1 n.8. 71.6 n.a. 63.0 68.5 66.6

ma. n-8. n.8. 9.9 n.a. 12.9 n.a. 16.0 14.5 14.2 ma. 10.8

Zb

;!$I

n.8.

Butanol

n.a. n.a. 17.3

P30H

Concentrate (talc. and GC)

not analysed

Butanol

=

1st step End 2nd step End

1st step End 2nd step End

1st step End 2nd step End

End

Remarks

244

3.3. The signijkance of the current strength In all these experiments the current strengths were below the corresponding limiting values and polarisation was avoided. As a high current density would lead to a quick desalination, low electrical resistance of the solution enabling such high current densities would be of advantage. At the beginning of a run the current strength remained at 1 amp and even increased at first as the initially salt-free concentrate gained in conduc tivity (at this concentration range the conductivity is not linearly dependent on salt concentration). Initially the salt concentration in the diluate is higher than in the concentrate. Hence the concentration difference works in favour of the electrodialysis, but after only 20 min the two streams reached equal concentrations. Hereafter the electrodialysis has an uphill task of transporting the salts against the concentration potential as the concentrate stream gains in salt value while the diluate is being depleted. This condition holds for most of the period of desalination. This detrimental effect is to be expected to be all the more enhanced when only half the volume of the concentrate is employed, as the salt concentration in this stream would rise even more and faster: the streams reached the point of equal conductivity after only about 10 min. In spite of this fact the two-step runs work out to be advantageous. The reason is that the early rise in conductivity of the concentrate lowers the total resistance and allows a high rate of desalination. This effect is illustrated in Fig. 9. kR.cm 60

I /

/

/”

(1 I total

concentrate

(2 I half the concentrate 100 time min

Fig. 9. The electrical resistances.

150

200

245

The total specific resistance (of both streams) which passes through a minimum is always lower in the two-step process. The point of equal concentration is shifted only from 20 to 10 min after the start in a total period of about 250 min for the desalination. The advantage of lower resistance more than compensates for the minor disadvantage of a slightly earlier onset of desalting against a transmembrane concentration gradient. Most of the desalination took place in the first half an hour when the resistance was low and the current strength high. In the case where a solution containing 0.92 wt% salts was desalted employing all the available concentrate, the current strength rose to nearly 1.5 A. In the other cases with 1.1-1.2 wt% initial salt concentration and only half the available concentrate, the current strength was even higher, of the order of 1.9 A at the same voltage.

4.

BATCHWISE OPERATION The

operation of the two-step process on an industrial scale is illustrated in Fig. 10 (stack with two cells). It is assumed that a current reversal after each step is of advantage and is to be provided. The diluate and the concentrate are placed in tanks 1 and 4 respectively. The desalted solution (diluate after dialysis) is led into tank 3 and the salt-rich concentrate is fed from tank 7 to the reaction step for production of hexadiinediol (cf. Fig. 1). from reaction step

Fig. 10

from distillation

step

246

Fig. 10 illustrates the first step with the diluate in tank 2 and the concentrate in tank 5. The diluate enters the chambers A and C, the concentrate enters B and D. The second step is also operated similarly with the other half of the concentrate in tank 6. After this batch the concentrate is emptied from tanks 5 and 6 into 7 and mixed. After filling up tank 2 with fresh diluate from tank 1 and current reversal, the next batch is begun (Fig. 11). The diluate is now led not into chambers A and C but into B and D, while the concentrate enters A and C. The second step of this new batch is operated accordingly, again using tanks 5 and 6 for the concentrate, and so on.

Fig. 11

A disadvantage of this method is the fact that each time a current reversal is undertaken, not only the polarities of the electrodes are reversed but also the feed streams must be led into different chambers. Moreover, the salt-rich concentrate in the dead space of tank 5 or 6 comes into contact with fresh salt-free concentrate being fed in, affecting its desalting capacity. In case these considerations are found to be of grave import, the following procedure can be adopted (Fig. 12). The streams from tanks 2 or 3 are always led into chambers A and C and from tanks 6 or 7 always to chambers B and D, In Fig. 12 the desalted diluate is being led from tank 2 to 4. Tanks 3 and 6 contain the diluate and concentrate respectively and the electrodialysis is in operation. After the first step the next one is carried out with tank 7 containing the concentrate and the diluate still in tank 3. Subsequently, for the next batch operation tank 2 which previously contained the diluate is filled up with half the concentrate available from tank 5, tank 6 which previously contained salt-rich concentrate is filled up with fresh diluate and only the polarities of the electrodes are reversed to effect a current reversal. A new cycle of operation is begun (Fig. 13). The advantage is that tank 2 with desalted d&ate in its dead space comes into contact with fresh salt-free concentrate; the salt-rich fresh diluate enters tank 6 in which the salt-rich concentrate had been stored.

241

from reaction step

from distillation step

diluat

t

to hydrogenation

step

t

to reaction step

Fig. 12. 5

:rc concentrate

2

Fig. 13.

3