Catalytic Deoxygenation of Water: Preparation, Deactivation, and

Nov 9, 2009 - Metals supported on ion exchange resins have been studied in .... form, according to reaction 1); (2) addition of PdCl42− solution (obta...
0 downloads 0 Views 1MB Size
Download PDF
Ind. Eng. Chem. Res. 2010, 49, 81–88

81

Catalytic Deoxygenation of Water: Preparation, Deactivation, and Regeneration of Palladium on a Resin Catalyst Martı´n S. Gross, Marı´a L. Pisarello, Karina A. Pierpauli, and Carlos A. Querini* Instituto de InVestigaciones en Cata´lisis y Petroquı´mica (INCAPE), FIQsUniVersidad Nacional del Litoral, CONICET, Santiago del Estero 2654, S3000AOJ Santa Fe, Argentina

Oxygen dissolved in water is a cause of corrosion in heating installations. On the other hand, in the production of beverages such as beer, it leads to the oxidation of organic matter, decreasing the quality of the product. Therefore, its content must be substantially reduced. In this work, the catalytic deoxygenation of water using a palladium catalyst is studied. The catalyst consists of palladium ion-exchanged on a weak-basic macroporous resin. This work addresses the catalytic deactivation and regeneration of this type of catalyst, comparing results obtained in the laboratory with those obtained from an industrial reactor used to deoxygenate water for beer production. Two deactivation mechanisms are observed: (i) In the short term, the deposition of organic material (humic acids) coming with the water is responsible for activity lost; (ii) in the long term, the cause of the irreversible deactivation is the palladium leaching that occurs during the cleaning-in-place procedure, which is carried out with sodium hydroxide solution at 60 °C. This deactivation occurs in the presence of dissolved oxygen and is strongly favored by the presence of chlorine in the water. This study indicates the conditions necessary to regenerate the catalyst and to largely improve the stability during operation-regeneration cycles. 1. Introduction Metals supported on ion exchange resins have been studied in several processes, such as the reduction of nitrates on drinking water,1 the elimination of oxygen dissolved in water,2-4 alkene epoxidation5 and hydroformylation of styrene.6 Particularly, palladium supported on resins has been addressed in several publications and reaction systems7-10 with particular attention on hydrogenations and nitrates reduction.9 Oxygen dissolved in water has been identified as a contributor in the corrosion of components in a large variety of systems. For this reason, there has been a considerable effort in the development of technologies to substantially reduce the oxygen content in water.11 Membrane reactors12 as well as separation using membranes13 have been recently proposed. The water used in the preparation of beverages must be deoxygenated to avoid the oxidation of organic components, which leads to changes of organoleptic properties. Classic procedures to decrease the level of dissolved oxygen in water involve the use of vacuum, heat, or bubbling of CO2 or N2. Generally, a combination of two of these physical procedures is applied, making it possible to reduce the O2 to 20-30 ppb, and even to 10 ppb with the newest technologies. The cost of this type of process is relatively high, mainly due to the need for mild temperatures in the water. An alternative method to efficiently remove oxygen from water is the heterogeneous catalytic reduction with hydrogen, a method developed a few years ago. The catalytic process makes use of palladium supported on a weak anion exchange resin.14,15 The palladium is incorporated in the resin by ion exchange. This process is capable of achieving residual oxygen concentration of less than 10 ppb. Besides, the reaction takes place at room temperature, and therefore the cost of the process is very low. In this process, a hydrogen flow is injected to the water stream at room temperature, in an amount equivalent to the stoichiometric reaction between hydrogen and oxygen. This * E-mail: [email protected].

stream is flowed through a static mixer, where the hydrogen is divided into very small bubbles facilitating its dissolution in water. The total pressure of the system varies between 2 and 3 bar. The reactor is a fixed-bed, down-flow reactor operated at room temperature. Periodically, to avoid microbiological contamination and to eliminate the organic materials deposited on the catalyst which lead to a loss of activity, the catalyst bed is treated with NaOH at temperatures around 60 °C. This procedure is known as cleaning in place (CIP). It has been established that in commercial operation the catalyst deactivates during operation, it being necessary to replace the catalytic bed after some time. The objective of this study is to analyze the deactivation mechanism, the influence of preparation procedure in catalytic activity and stability, and the regeneration procedure. Water deoxygenation is an important technological problem. Even though a commercial process has been developed and used in many applications, catalytic water deoxygenation has not been addressed in the open literature, neither in order to be applied for beverage production using drinking water nor for the treatment of demineralized water for heating circuits. Only short reports can be found related to this process.2-4 2. Experimental Section 2.1. Support Stability. To evaluate the thermal stability of the ion exchange resin, samples of commercial resins were maintained in a 3 wt % NaOH solution in a hot bath at 80 °C during different times. This treatment is performed to simulate the cleaning-in-place procedure, carried out in the industrial application every week in order to preserve the catalytic bed from microbiological growth. After this treatment, the sample was washed with distilled water and was dried as described in the standard ASTM D 2187-92a.16 Then, an HCl adsorption isotherm was carried out for each sample. Adsorption isotherms allow the determination of the total exchange sites (Nmax) and the equilibrium constant (K) of the following exchange reaction:

10.1021/ie9007369 CCC: $40.75  2010 American Chemical Society Published on Web 11/09/2009

82

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

HO-ads + Cl-THO- + Cl-ads

(1)

The adsorption isotherms were determined by titration of the nonadsorbed Cl-, after reaching equilibrium. Typically, the measurement was carried out after a contact time of 24 h. It was assumed that the isotherm follows a Langmuir type isotherm: N)

NmaxKC 1 + KC

(2)

where N represents the number of adsorption sites (equiv/kg) and is calculated as follows: N)

V(C0 - C) M

(3)

V being the volume of acid solution used in the experiment, C0 the initial acid concentration (mol/L), C the acid concentration once the equilibrium was reached (mol/L), and M the weight of resin (kg). The parameters can be evaluated by linearization of eq 2 or directly from nonlinear regression. 2.2. Catalyst Preparation. According to the U.S. patent presented by Oeckl et al.,14 the commercial catalyst consists of a weak anion exchange macroporous resin, with tertiary amine functional groups, charged with palladium. The resin is initially pretreated with diluted hydrochloric acid. The distribution of the metal within the particle of resin depends on this acid concentration. Oeckl et al. found that with an acid concentration of 1.15% an egg-shell distribution of Pd was obtained, while if the acid concentration during the pretreatment was 5%, Pd was homogeneously distributed. Chloride supplied by HCl acted as ion competitor during the ion exchange procedure, in which palladium was introduced into the resin. In this study, the effect of the HCl concentration was studied in the 3-7 wt % range. Several commercially available resins were used. Results obtained with the resins WA30 from Mitsubishi and MWA-1 from Dow Chemicals are presented. Two resins were selected in order to compare their thermal stability and the effect that this might have on the catalytic activity and regenerability, These resins are based on a high porous styrene divinylbenzene (DVB) polymer matrix and are weak anion exchange resins, with tertiary amine functional groups, and therefore have the same structure as the resin used in the commercial process. These resins have a recommended maximum temperature of 60 °C in the free-basic form and 100 °C in the chlorided form. The amount of Pd introduced was 0.3 wt % (based on dried resin), which is similar to the metal content of the commercial catalyst. The preparation procedure, according to the original work of Oeckl et al.,14 consists of the following steps: (1) pretreatment of the resin with HCl 5 wt %, during 30 min, at room temperature (in this pretreatment, the exchange sites are converted from the free basic form, to the chloride form, according to reaction 1); (2) addition of PdCl42- solution (obtained by dissolution of PdCl2 in acidic NaCl solution) while stirring (in this step the palladium complex competes with chloride for the exchange sites, with the following exchange taking place): (Cl-)ads + PdCl42-fCl- + (PdCl42-)ads (3) reduction of the adsorbed PdCl42- complex with hydrazine, after pH adjustment in a value close to pH 14 (in this step

palladium oxidation state changes from 2 to 0; palladium is easily reduced at low temperatures, it being necessary in many cases to go below 0 °C to observe the reduction profile; during the preparation procedure, the hydrazine reduces the palladium to the metallic form (Pd0); during the deoxygenation reaction, since it is carried out in the presence of an excess of hydrogen, the Pd remains in the metallic form, fully reduced); (4) washing the catalyst with water. 2.3. Catalyst Regeneration. After the catalyst was used in several reactions and CIP cycles, regeneration was carried out by reloading Pd, to introduce 0.1, 0.2, and 0.3 wt % Pd, following the same procedure as that used in the preparation of the fresh catalyst. 2.4. Catalyst Characterization. 2.4.1. Metal Loading. The metal and impurities contents of the catalysts were measured after their complete dissolution with a mixture of perchloric and nitric acids. Pd and Fe were measured by atomic absorption spectrometry, and mercury (which could come from the NaOH solution) was measured by flow injection cold vapor atomic absorption spectrometry (FICVAAS). 2.4.2. Temperature-Programmed Analyses. Temperatureprogrammed-oxidation (TPO) analyses were carried out, to determine the temperature at which the surface groups of the resin decompose. The TPO analyses were carried out heating from room temperature to 120-150 °C, which is the limit of thermal stability of the resin. The gases coming out of the cell were transformed into methane, in a reactor that contained a nickel catalyst. This stream was directly and continuously injected to a flame ionization detector (FID). Details of this technique can be found elsewhere.17 This technique is also useful for detecting with high sensitivity if there are some organic residues left on the catalyst during the reaction that can be gasified at low temperature, below the resin decomposition temperature. 2.4.3. Metal Dispersion. Dynamic CO pulse chemisorption measurements were carried out by sending 250 µL pulses of 5% CO/He on dried samples of both fresh and deactivated catalysts, after reduction in H2 for 1 h at 150 °C. CO is detected with a FID, after quantitative methanation in a Ni/kieselghur catalyst at 400 °C, which largely improves the sensitivity. 2.4.4. Microscopy Analysis. Scanning electron microscopy (SEM) of fresh and deactivated samples at 25 KV were carried out using an electron microscope (JEOL JSM 35C). The atomic elemental analyses were obtained using an energy-dispersive X-ray analysis (EDX) system attached to the SEM instrument, to determine the radial distribution of palladium in the resin particle. 2.5. Catalytic Activity. The activity was measured in a fixed bed reactor. Typically, between 3 and 15 cm3 of resin were loaded into the reactor, which is of 1 cm inner diameter. A hydrogen stream is mixed with crude water, to keep the same proportion as that used in the industrial reactor. After this mixing, the stream containing the water and the hydrogen passes through a mixer filled with small particles of polypropylene. The oxygen concentration in the water is measured with a specific electrode, able to determine concentrations in the level of 1 ppb. Experiments were carried out with drinking water without any pretreatment. The water contains approximately 14 ppm chlorides, 30 ppm sulfates, 40 ppm bicarbonates (as CaCO3), with a pH ) 7.5. The activity measurements were carried out at high space velocity, to be in a kinetic-controlled regime. Industrially, the process is run with a space velocity between 1 and 2 min-1 (L

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

83

Figure 1. Adsorption isotherms: (A) WA30 (Mitsubishi); (B) MWA-1 (Dow Chemical), obtained after treatment at 80 °C in 3 wt % NaOH solution during different times. Table 1. Langmuir Isotherm Parameters, as a Function of the Time in 3 wt % NaOH, at 80°C WA30 time, h 2 6 19

K 1285 1281 406

Nmax, equiv/kg 3.07 2.58 2.53

MWA-1 K 409 403 270

Nmax, equiv/kg 1.74 1.67 1.53

of water/((L of catalyst) min)). The activity tests were carried out at a space velocity of 17 min-1. The hydrogen flow rate is adjusted to have a water volume/hydrogen volume ratio equal to 40. This ratio is used both in the laboratory experiments and in the industrial reactor. 2.6. Palladium Leaching during the Cleaning-in-Place Procedure. The chemical stability of the metal function of these catalysts was studied by performing several treatments with 3 wt % NaOH and determining the amount of Pd in the liquid phase by atomic absorption spectroscopy. The effect of temperature, chlorine concentration, and time was studied. 3. Results and Discussion 3.1. Support Stability. Figure 1 shows the adsorption isotherms obtained both with WA-30 and MWA-1 resins. The Cl- relative adsorption is plotted as a function of the final chlorine concentration. It can be seen that both resins lose Clexchange sites as the length of the treatment at 80 °C in NaOH increases, as indicated by the final values of the isotherms. It can also be seen in this figure that the total adsorption capacity of the WA-30 resin is higher. These results agree with the data provided by the manufacturers, the exchange capacity of the MWA-1 being 1.2 equiv/L, and higher than 1.5 equiv/L in the case of WA-30. Taking into account the apparent bed density, which is 0.615 L/kg, the data obtained in our isotherm perfectly match the manufacturer’s information. The slope of the adsorption isotherms at the origin is the adsorption equilibrium constant (K). Figure 1 indicates that the K-values decrease as the time of the treatment in hot NaOH solution increases. As the adsorption constant increases, the strength of the ion adsorption also increases. Therefore, it can be concluded that those sites that adsorb the Cl- with higher strength are the more labile. In Table 1 values of K (adsorption constant) and Nmax (maximum number of exchange sites, equiv/ L) are listed for both resins. These parameters were obtained from the data shown in Figure 1, assuming that the isotherm corresponds to a Langmuir type adsorption. The curves plotted in this figure correspond to the calculated values obtained with the estimated parameters shown in Table 1. It can be seen that there is a very good fit. The isotherm obtained with the fresh WA-30 resin (not shown) is practically equal to that obtained

Figure 2. CO dynamic chemisorption experiments. Deactivated catalyst: from the industrial reactor after 1 year of operation. Fresh Catalysts, concentration of HCl used during the preparation: 7, 5, and 3 wt %.

after 2 h in NaOH. Therefore, this resin displays a good initial thermal stability. When the treatment time was 19 h, the adsorption capacity decreased to 82% of the fresh resin. On the other hand, the MWA-1 resin lost 14% of the adsorption capacity after 2 h of treatment, the final adsorption capacity after 19 h in hot NaOH solution being 79% of the fresh resin. Therefore, it can be concluded that both resins display similar thermal stability regarding the exchange capacity. This is an important point, since the CIP is industrially carried out weekly at temperatures close to 60 °C. 3.2. Catalyst Characterization. Results of CO chemisorption experiments are shown in Figure 2. The pulses coming out of the cell are presented for the fresh catalysts prepared using different concentrations of HCl during Pd loading. Results obtained with the deactivated catalyst are also included. It can be seen that the concentration of HCl affects the amount of chemisorbed CO, and therefore the metal dispersion. When the acid was used at a concentration of 3 wt %, the catalyst chemisorbed nine pulses of CO, while when the acid was more concentrated (7 wt %), the catalyst chemisorbed four pulses, and when prepared with the intermediate value of 5 wt %, the catalyst chemisorbed seven pulses. These numbers correspond to very low values of dispersions but still clearly indicate that the acid solution concentration affects the metal dispersion. Therefore, the difference between the two extreme values is more than 100%. It means that the catalyst with the highest dispersion is the one obtained using the more diluted HCl. It can be expected that the higher the dispersion, the higher the reaction rate. However, as shown below, the effect of this property on the reaction rate is not relevant, at least under the conditions used in this study to evaluate the catalytic activity.

84

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

Figure 3. (A) TPO profiles of fresh and deactivated catalyst; (B) same experiments, but bypassing the methanator.

On the other hand, the result obtained with the deactivated catalyst clearly indicates that there is a negligible amount of exposed palladium atoms, in agreement with the activity results shown below. Figure 3 shows the results of the TPO analyses. The amount of carbon eliminated from the deactivated catalyst is higher than in the case of the fresh catalyst (Figure 3A). This is consistent with the fact that, during operation, humic acids are deposited on the catalyst, and therefore during the TPO analysis these compounds are gasified. The amount of carbon released from these catalysts represents 1.95 wt % in the case of the deactivated catalyst and 1.22% in the case of the fresh catalyst. The signal of these profiles corresponds to CO-CO2 (combustion products of organic material) and hydrocarbons, coming from the decomposition of both the resin surface groups and the humic acids. To distinguish between the CO-CO2 and hydrocarbons, experiments were carried out bypassing the methanator. In this case, the signal corresponds to the hydrocarbons released due to the decomposition of the surface groups of the resin, since CO and CO2 are not detected by FID. Figure 3B shows the results. The fresh catalyst releases a higher amount of hydrocarbons (0.84 wt %) compared to the deactivated catalyst (0.72 wt %), as expected, since the deactivated sample has already been subjected to many thermal treatments during the CIP procedures, and the surface groups that could release hydrocarbon fragments have already been partially decomposed during all the CIP procedures carried out weekly. SEM analyses gave little information regarding the surface morphology (not shown). The main difference observed between the deactivated and the fresh catalyst is that the surface is smoother in a scale of 0.1 µm in the case of the former. This could be due to the effect of the NaOH during the CIP operation. EDX results are shown in Figure 4. It can be seen that the concentration of the HCl used during catalyst preparation practically does not affect the palladium radial distribution. In all cases the egg-shell can be seen, with the Pd concentrated in the 10% outer shell. Since the reaction between hydrogen and oxygen is fast, it can be expected that the best situation regarding the global reaction rate is to have the palladium in the outer surface of the resin particle to avoid internal mass-transfer effects. Therefore, the preparation procedure that was followed in this study is adequate to achieve this objective. On the other hand, the mechanical loss of the palladium particles is most probably higher in this situation. Nevertheless, this was not verified experimentally. Figure 4C shows an SEM photograph of a catalyst particle. It can be distinguished the darker zone in the outer shell that corresponds to the palladium deposition zone. The adsorption isotherms of the fresh and deactivated catalysts in the industrial reactor are shown in Figure 5. The relative adsorption at constant total number of moles is represented as a function of the final concentration of HCl in the solution. There

Figure 4. EDX results: (A) Radial distribution of palladium in the catalyst particle; (B) detail of panel A, left side; (C) SEM photograph of a catalyst particle. Palladium supported on WA-30.

Figure 5. Relative adsorption isotherm of HCl on fresh and deactivated catalysts. Deactivated catalyst sample taken from the industrial reactor after 1 year of operation.

is a difference between the two solids. As expected, results suggest a higher exchange capacity in the fresh catalyst, as compared to the catalyst after more than a year in operation, which has been cleaned with hot NaOH more than 50 times. The maximum adsorption capacity decreased to one-tenth of the initial value (compare Figures 1 and 5). 3.3. Activity Tests. Figure 6 shows the activity of fresh and regenerated catalysts, as a function of the water residence time. For a residence time of 0.15 min, the oxygen conversion is 80%. It is very important to emphasize that the activity displayed by both the fresh and the deactivated catalysts after palladium reincorporation is very similar. This result indicates that it is

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

85

Figure 6. Catalyst activity as a function of residence time: (A) fresh catalyst; (B) regenerated catalyst (0.2 wt % Pd was loaded to the catalyst deactivated in the industrial reactor).

Figure 7. Level of O2 at the reactor outlet. Comparison of fresh, deactivated, and regenerated catalysts.

possible to recover the activity by loading palladium in order to increase the catalyst palladium content up to the initial value. This is an interesting result, since as previously shown the resin lost a very important fraction of the adsorption sites; therefore, it could be assumed that the Pd distribution could be different. However, the results shown in Figure 6 demonstrate that this degradation of the adsorption sites during operation does not significantly affect the preparation procedure and the palladium distribution, since no effect on catalyst activity was observed. Figure 7 shows the activity of fresh, deactivated, and regenerated catalysts. A blank run is included for reference. In this case no catalyst was loaded in the reactor. The oxygen concentration measured after the separation of liquid and hydrogen is approximately 3.5 ppm. The deactivated catalyst has a very low activity, being able to reduce the oxygen concentration only up to 3 ppm. This is in agreement with the CO pulse chemisorption results shown in Figure 2, in which a negligible amount of exposed palladium is detected on the deactivated catalyst, and accordingly, the activity is practically zero. The regenerated catalysts have an activity similar to that of the fresh catalyst, independently of the amount of Pd loaded into the spent catalyst, or the concentration of HCl used during the regeneration. The catalyst labeled as “egg shell” corresponds to the catalyst prepared with 5 wt % HCl, while in the other regenerated catalyst a concentration of 1 wt % was used. It can be observed that at low palladium loading, there is a difference in activity between the catalysts prepared using 1 and 5 wt % HCl, the activity being higher in the case of the latter, which is consistent with the idea that diffusional problems may have an effect in the global reaction rate. Nevertheless, the results shown in Figure 7 indicate that the catalysts regenerated by loading different amounts of palladium, and using different concentrations of HCl, display a similar activity under the conditions used in this study to evaluate the catalytic performance This might be due to the strong and fast adsorption of the PdCl42- ions on

Figure 8. Activity during reaction-regeneration cycles. Deactivated catalyst, after 0.1 wt % Pd addition. Regeneration: treatment with 3 wt % NaOH at 80 °C, 2 h.

the resin, which leads to a preferential accumulation of Pd on the outer surface of the catalytic particle, as shown by EDX analyses (Figure 4). 3.4. Deactivation Mechanism. We have found that there are two different mechanisms of catalyst deactivation. The first one, which takes place during the short-term operation (days), is catalyst fouling. The organic material present in the water is retained by the catalyst bed. This material covers the whole surface of the catalyst particle and leads to a decrease in catalytic activity. Figure 8 shows the results of an experiment carried out during several days. It can be seen that after 3 days in operation, the catalyst activity dropped and therefore the oxygen concentration at the reactor outlet noticeably increased. However, after the treatment with NaOH, the catalyst recovered the activity, and the conversion was again similar to that of the fresh catalyst. Three cycles are shown in Figure 8, showing that after a short-term deactivation, the activity is recovered following the CIP procedure. This is a reversible deactivation, since the CIP process is able to eliminate this deposit and to recover at least partially the activity, as shown in Figure 8. It has to be emphasized that the activity test is carried out at a much higher space velocity than that used in industry, and therefore the expected deactivation rate in the industrial reactor will be slower than that shown in Figure 8. As shown in Figure 3, the TPO analyses show that the deactivated catalyst contains organic material deposited during the reaction, in agreement with the activity behavior displayed by the catalyst in the experiment shown in Figure 8. The second mechanism occurs in the long term (months). The catalyst unloaded from an industrial reactor after 1 year of operation was analyzed to determine the content of several metals that could be in contact with the catalyst during operation, such as iron, lead, cadmium (that could come from the treated

86

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

Figure 9. Palladium content in the NaOH solution used to regenerate the catalyst, in eight consecutive cycles: (A) fresh and deactivated catalyst (0.07 wt % Pd); (B) regenerated catalysts by addition of 0.1, 0.2, and 0.3 wt % Pd.

water), and mercury (that could come from the sodium hydroxide, obtained by the mercury-cell electrolytic process). After dissolution of fresh and deactivated samples of the catalyst in a mixture of perchloric and nitric acids, the analysis of the liquid phase indicated that there were no metals such as iron (less than 0.05 ppm), cadmium (less than 0.01 ppm), or mercury (less than 0.01 ppm) in the deactivated catalyst. The presence of humic acids as the cause of the irreversible catalyst deactivation is not probable, since the CIP procedure, using NaOH, is enough to eliminate this type of compounds. The analysis of the dissolution of the fresh and deactivated samples clearly showed the reason for catalyst deactivation. After 1 year of operation, the catalyst had 0.15 wt % Pd, as compared to 0.39 wt % of the fresh sample. After this year of operation, it was necessary to add fresh catalyst to the reactor. After 5 months, the global composition of the catalyst dropped to 0.07 wt % Pd. This loss of the active phase is the reason for the catalyst irreversible deactivation. This is the reason why the CO pulse chemisorption experiments, shown in Figure 2, indicated that the deactivated catalyst does not chemisorb CO. The CIP procedure was simulated in the laboratory, treating the catalyst with 3 wt % NaOH at 80 °C and analyzing the Pd composition in the liquid phase. Figure 9 shows the results of the CIP experiments for the fresh and the deactivated catalysts, in eight consecutive treatments to the same sample. It is evident that the amount of palladium lost during the CIP process depends on the palladium content, only when comparing the fresh catalyst with the deactivated one. The loss of Pd in the deactivated catalyst is very low, due to the low content of Pd (0.07 wt %). The regenerated catalysts, obtained by incorporating 0.1, 0.2, and 0.3 wt % Pd to the deactivated catalyst, display small differences between them regarding the Pd lost. A very important result is that during the first four CIP cycles, the Pd loss is smaller in the regenerated catalysts than in the fresh one. The conditions used in these tests were more energetic than the conditions used in the industrial reactor, in which a measurement of the palladium content in the liquid after the CIP procedure was approximately 1.5 ppm. The loss of Pd as a function of time of CIP is shown in Figure 10. After a fast initial increase in the Pd content in the solution during the CIP operation, a slower Pd loss can be seen, although it increases almost linearly with the duration of the procedure. Therefore, it is important to optimize this cleaning procedure to ensure that the humic acids are removed from the catalyst, but avoiding unnecessary time of contact between the catalyst and the hot NaOH solution.

Figure 10. Palladium content in the NaOH solution used to regenerate the catalyst, as a function of the treatment duration.

The reactions that may be involved during the CIP involving palladium are the following: 1 Pd0 + O2 + H2O + 4Cl-f2HO- + PdCl42- K ) 2 3.77 × 10-7

(4)

1 Pd0 + O2 + H2OfPd(HO)2 K ) 1.66 × 1011 2

(5)

1 Pd0 + O2fPdOs K ) 1.00 × 10460 2

(6)

PdOs + H2O + 4Cl-f2HO- + PdCl42- K ) 1.00 × 10-465

(7)

Pd2+ + 4Cl-fPdCl42- K ) 1.00 × 1011

(8)

PdOs + H2OfPd(HO)2 K ) 1.00 × 10-271

(9)

Pd(HO)2 + 4Cl-f2HO- + PdCl42- K ) 1.00 × 10-17 (10) Pd(HO)2f2HO- + Pd2+ K ) 1.00 × 10-28

(11)

1 Pd0 + O2 + H2Of2HO- + Pd2+ K ) 1.00 × 10-17 2 (12) A first important conclusion that can be obtained from this thermodynamic analysis, based on this reaction set, is that, in

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010 Table 2. Content of Pd in Different Cleaning-in-Place Tests, Fresh Catalyst temperature, °C

time, h

80 20 80 80 80 80 80,with N2 a 80, with N2 a 80, with H2 b

7 18 8 6 6 6 8 8 8

NaOH, %

Pd concn, ppm -

3, without Cl 3, without Cl3, withoutCl3, without Cl3, with 7 ppm Cl3, with 25 ppm Cl3, without Cl3, with 25 ppm Cl3, without Cl-

2 0.65 4.2 3.9 4.6 14