Polyoxometalate-Catalyzed Oxygen Delignification of Kraft Pulp: A

The results of the first pilot-scale kraft pulp oxygen bleaching catalyzed by polyoxometalate are presented. ... The pilot-plant experiments were carr...
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Ind. Eng. Chem. Res. 2004, 43, 7754-7761

Polyoxometalate-Catalyzed Oxygen Delignification of Kraft Pulp: A Pilot-Plant Experience Armindo R. Gaspar, Dmitry V. Evtuguin,* and Carlos Pascoal Neto CICECO/Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal

The results of the first pilot-scale kraft pulp oxygen bleaching catalyzed by polyoxometalate are presented. A modified heptamolybdopentavanadophosphate heteropolyanion (HPA) was used as the catalyst. Scale-up experiments clearly revealed the dependency of the pulp delignification rate on the mass- and heat-transfer phenomena showing a rate constant on the pilot equipment assay twice that on the laboratory reactor. The selectivity of the oxygen-catalyzed bleaching stage (OHPA) was higher than that of the conventional oxygen alkaline bleaching (O). Pilot trials demonstrated similar consumption of chlorine dioxide and the strength properties of pulps bleached by OHPADED and ODED sequences. The chemical oxygen demand, absorbable organic halogens, and toxicity of the effluent streams were assessed for each bleaching stage. No catalyst retention on fully bleached pulp was detected. The practical aspects of the eventual industrial application of polyoxometalate-catalyzed oxygen bleaching are discussed. Introduction Recent research and industrial trends in the pulp bleaching deal essentially with the progressive substitution of environmentally hazardous chlorine-based bleaching chemicals by oxygen-based reagents (O2, O3, and H2O2).1 The main difficulty is associated with insufficient selectivity of the pulp delignification/bleaching with former reactants. Particularly, the oxygen delignification in aqueous alkaline media is now industrially used for the pulp bleaching. However, the progressive application of this technology is hindered by the socalled 50% delignification barrier; i.e., after 50% residual lignin removal, the delignification selectivity becomes insufficient to proceed the bleaching without significant polysaccharide degradation, thus decreasing the pulp quality. This significant disadvantage of the oxygen alkaline bleaching is determined by the autoxidative nature of the lignin oxidation with oxygen. Namely, hydroxyl radicals formed in lignin autoxidation stages stimulate the oxidative degradation of cellulose.1 The radical improvement of oxygen bleaching technology is possible by avoiding the lignin autoxidation by implementation of regenerable oxidation reagents/ catalysts, such as polyoxometalates (POMs), that react with lignin via electron-transfer mechanisms.2-4 POMs, particularly Keggin-type heteropolyanions (HPAs), having properties of reversible oxidation with oxygen, were proposed as regenerable oxidation catalysts for the pulp bleaching with oxygen.3,4 In the POM-catalyzed oxygen bleaching, the oxidation of residual lignin and reoxidation of POM occur in one stage (aerobic oxidation of lignin in the presence of HPA). Because of the almost complete oxidation of dissolved organic matter and because bleaching liquors can be continuously reused, POM-based oxidation processes are considered as a very promising approach toward total effluent free bleaching plants.5 Among a large variety of POMs, the heptamolybdopentavanadophosphate HPA [PMo7V5O40]8- (or HPA-5) modified by the addition of MnII salts at the last phase of the catalyst synthesis (HPA-5-MnII) has so far * To whom correspondence should be addressed. Tel.: +351 234 370693. Fax: +351 234 370084. E-mail: [email protected].

been shown to be the most effective catalyst in oxygen delignification.6 In this work, the results on the first pilot-scale experiment on POM-catalyzed oxygen pulp bleaching of Eucalytus globulus kraft pulp using HPA-5-MnII as the catalyst are presented. The main goals of the delignification/bleaching experiments were to evaluate the practical feasibility of this new process at a large scale and to understand how transfer factors (transport, mixing, heat transfer, etc.) may influence the process. Another objective was to assess the toxicity of the pulp obtained and of the accompanying effluents. The quality parameters of fully bleached pulp were estimated and compared with quality parameters of pulp bleached by conventional sequences. Experimental Section Kraft Pulp. The pilot-plant bleaching trials were performed using industrial E. globulus da kraft pulp supplied by Portucel Soporcel (Figueira Foz, Portugal). The κ number of pulp was 14.4 and the intrinsic viscosity 1160 cm3/g. Catalyst. The catalyst used in the pilot-plant trials (HPA-5-MnII or simply HPA) was prepared by the addition of manganese diacetate salt to the solution of HPA-5 at the last step of its synthesis with a HPA-5/ MnII proportion of 1.5 at a temperature of around 70 °C.7 The obtained 0.2 M solution was diluted to the required concentration with tap water in a plastic 1-m3 tank, just before its use in bleaching. The pH was adjusted to 3.8 by the addition of a sodium hydroxide (2 mol/L) solution. Laboratory and Pilot-Plant Bleaching Experiments. The laboratory bleaching experiments were carried out in a Parr reactor model 4842 (1 L) equipped with an automatic temperature control system, a pressure control system, and a mechanical stirrer, as was described previously.5 The pilot-plant experiments were carried out at the Centre Technique du Papier (CTP, Grenoble, France). The industrial eucalypt kraft pulp was subjected to washing at about pH 4 (WA stage) before the bleaching

10.1021/ie040061w CCC: $27.50 © 2004 American Chemical Society Published on Web 11/02/2004

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Figure 1. General scheme of the bleaching module of CTP pilot-scale facilities with a high-pressure oxygen reactor IV (capacity of 150 kg of dry pulp). Designations for equipment are presented in the text.

in the OHPAD1ED2 sequence (OHPA depicts the oxygen stage catalyzed by HPA) or at about pH 7 (W stage) prior to the OD1′ED2′ sequence. The scheme of the bleaching pilot plant is shown in Figure 1. The pulp from the chest V (ca. 3% consistency) was passed through the twin wire press VII, mixed with reagent in mixer I equipped with a steam injector, and transported by means of a middle consistency (MC) pump II and a dynamic mixer III into the high-pressure reactor IV, where the average pulp consistency was about 10%. Then, the reactor IV was heated to the desired temperature by direct steam injection and filled up with oxygen in the case of the OHPA and O stages. After the pulp treatment, the reactor was degasified and washed with water to discharge the pulp to the intermediate chest V. Then, the pulp suspension (∼3% consistency) was transported by pump VI to the twin-wire press VII, where pulp was dewatered to approximately 30% dry weight and filtrates were collected in the washing effluents tank VIII. The experimental conditions were adjusted to reach about 95% washing efficiency. After washing, the treated pulp was used for the next bleaching step. All bleaching stages were carried out in the same reactor IV. The amount of pulp used was 92 kg in the WAOHPAD1ED2 sequence and 76 kg in the WOD1′ED2′ sequence. More detailed conditions for the bleaching stages are presented in the table captions. Pulp Characterization. The pulps were characterized using a TAPPI T236 CM 85 standard for κ number (related with the lignin content in pulps) and a SCANCM 15:88 standard for intrinsic viscosity (an indicator of the cellulose fiber polymerization degree). The eucalypt kraft pulps bleached by the WAOHPAD1ED2 and WOD1′ED2′ sequences were characterized using ISO standards for °SR, opacity, brightness, bulk, tensile index, and tear index and NP standards for the burst index. Brightness reversion was examined for the final bleached pulps according to the TAPPI T260 standard. Effluent Analyses. The chemical oxygen demand (COD) and absorbable organic halogens (AOX) measurements were carried out according to ISO standards (ISO 6060 and ISO/FDIS 9562, respectively). The toxicity test (Microtox) was carried out by a standard procedure with marine bacteria Photobacterium phos-

Figure 2. General scheme of POM-catalyzed oxygen delignification (POMs are presented as Keggin-type compounds in a polyhedral-filling representation; terminal oxygen atoms are depicted as uncolored balls).

phoreum. These analyses were performed in the Environment Department of CTP. Results and Discussion Fundamentals of POM-Catalyzed Oxygen Delignification. The principles of oxidative catalytic delignification with POMs can be formulated as follows (Figure 2): POMs, having an energetic barrier to lignin oxidation lower than that of dioxygen, oxidize the lignin, and then the reduced form of POM is reoxidized by O2.2-5 Using adequate POMs [knowing that E°(lignin) < E°(POM) < E°(O2)] and the appropriate reaction conditions, the oxidation of lignin and reoxidation of the POMs under aerobic conditions may be performed in one stage.3-5 The final products of the lignin oxidation are mainly carbon dioxide and water. Because of the almost complete oxidation of dissolved organic matter, bleaching liquors can be continuously reused in a “closed-loop” system.5 Among a large variety of POMs, HPA-5, stable under moderate acidic conditions, is adapted specifically for catalytic purposes.4 Lignin oxidation occurs either by VV in the HPA-n composition or by VO2+ ions produced via partial HPA-5 hydrolytic dissociation.7,8 VO2+ ions have a higher redox potential (about +0.90 V at pH 2) HPA-n does (ca. +0.60 V at pH 2).9 This fact plays a key role in the oxidative delignification, because VO2+ ions were considered as very active species in the catalytic oxidation of lignin.8 However, at the same time,

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an excessive release of VO2+ from the parent HPA-n contributes to the oxidative degradation of polysaccharides, decreasing the selectivity of the delignification.10 The partial dissociation of VO2+ ions from the parent HPA can be controlled by varying its concentration, pH, and ionic strength of the solution and the solvent nature.4,11 Recent investigations demonstrated that the modification of HPA-5 by the addition of MnII salt in the last step of the HPA synthesis (MnII-substituted HPA-5 or HPA-5-MnII) showed better selectivity of residual lignin oxidation when compared to unmodified HPA-5. These features were explained by the formation of MnII complexes with lacunary HPA-5, which favored control of vanadium group dissociation during delignification.6 Experiments on oxygen bleaching catalyzed by HPA-5MnII revealed the possibility of achieving 80% delignification of pulp in short bleaching sequences while the viscosity decreased by less than 30%. This fact was decisive for the choice of HPA-5-MnII in pilot-scale experiments. Pilot-Scale POM-Catalyzed Oxygen Delignification vs Laboratory Assay. The delignification selectivity and kinetic profiles during the oxygen bleaching catalyzed by HPA (OHPA stage) in the pilot and laboratory experiments have been compared. The pulp delignification selectivity was assessed as the degree of delignification (decrease in the κ number) achieved with a minimum loss on the pulp viscosity. The profiles of intrinsic viscosity development versus pulp delignification progress for both bleaching assays are presented in Figure 3. The viscosity of unbleached kraft pulp used in the pilot-plant trial was lower than that of pulp bleached in the laboratory. This affected the absolute values of intrinsic viscosities at the same κ number of pulps in two series of assays (including at zero reaction time) but did not change the slope of curves (Figure 3). Thus, the delignification selectivities, in general, during the HPA-catalyzed bleaching in the laboratory- and pilot experiments were similar. Taking into account the kinetic profiles of the oxygen delignification catalyzed by HPA, it is noteworthy that in the pilot plant the delignification rate was improved, compared with the trials in the laboratory (Figure 3). The delignification rate in the OHPA stage (using the κ number as the lignin concentration) showed a pseudo first order, as determined from the plot ln(d[lignin]/dt) vs ln[lignin] (not shown). In the pilot experiment, the delignification rate constant in the OHPA stage was 6.0 × 10-3 min-1, indicating that the catalytic oxygen delignification occurred on the pilot equipment about 2 times faster than on the laboratory reactor, where the delignification rate constant was 3.1 × 10-3 min-1. This can be explained by the better mixing of pulp with the catalyst and with oxygen in the MC pump, MC dynamic mixer, and oxygen reactor IV (Figure 1), when compared to the laboratory experiments where a pulp was mixed moderately by a mechanical stirrer. In addition, the direct contact of pulp with a live steam may have a crucial effect on mass/heat transfer, allowing a better delignification rate. This point needs further investigation. Pilot-Scale POM-Catalyzed Oxygen Delignification vs Alkaline Oxygen Delignification. A comparative study on the oxygen delignification in the presence of HPA-5-MnII (OHPA stage) and in the alkaline medium (O stage), used as a prototype of the conven-

Figure 3. Development of the pulp viscosity (top image) and delignification (bottom image) in the oxygen bleaching catalyzed by HPA-5-MnII in the pilot- and laboratory-scale trials (OHPA stage in the pilot assay: cons. 7.8%, [HPA-5-MnII] ) 1.5 mmol/L, pH 3.8, P°(O2) ) 0.55 MPa, and 95 °C). The denotations “i” and “0” correspond to the initial acid-washed pulp and the pulp mixed with catalyst after the heating period, respectively.

tional process, was carried out in order to compare the kinetics of these two processes and the bleached pulp properties. The temperature in the O stage (105 °C) was higher than that in the OHPA stage (95 °C) because of the poor pulp delignification below 100 °C in the former.3 In the initial treatment period dealing with the reactor loading and with the heating to final temperature, a notable part of the most accessible and reactive residual lignin was removed to a similar extent in the O and OHPA stages (Figure 4). Surprisingly, the significant pulp degradation during the reactor-loading period in the OHPA stage when compared to the O stage was observed (Figure 4). Besides the acid hydrolysis, this fact can be explained by the pulp polysaccharides oxidation with HPA under the conditions of quick oxygen depletion.10 This provoked a catalyst structural instability under acidic conditions, followed by a partial dissociation of VO2+ and VO2+ ions from the parent structure. Such a behavior of HPA was discussed previously in more detail.7 VO2+ ions are much less selective delignification species than HPA, and vanadyl (VO2+) ions can catalyze the hydrogen peroxide decomposition with the release of hydroxyl radicals, nonselective oxidizing species.12 A small amount of hydrogen peroxide can be present in the reaction system HPA/ O2, as a result of two-electron reduction of oxygen by vanadium-deficient HPA emerging after partial decomposition of the parent structure.13 Obviously, these negative features dealing with unexpected pulp viscosity loss might be overcome by

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7757 Table 1. Bleaching Results of Eucalypt Kraft Pulp with WAOHPAD1ED2, WOD1′ED2′, and DEDED Sequences

sequencea

stage

kraft pulp WAOHPAD1ED2 WA OHPA D1 E D2 after reversion WOD1′ED2′ W O D1′ E D2′ after reversion DEDED

intrinsic κ viscosity brightness number (cm3/g) (% ISO) 14.4 13.7 6.6 2.9 2.1

1160 1065 835 800 750 755

13.3 7.7 3.7 3.5

1135 990 960 950 900 985

37.3 40.4 40.5 60.2 67.3 85.1 83.5 39.0 59.3 79.1 80.3 86.7 84.4 89.5

a

Figure 4. Development of the pulp viscosity (top image) and delignification (bottom image) in the pilot-scale oxygen bleaching catalyzed by HPA-5-MnII and in the oxygen alkaline bleaching (OHPA stage: cons. 7.8%, [HPA-5-MnII] ) 1.5 mmol/L, pH 3.8, P°(O2) ) 0.55 MPa, and 95 °C. O stage: cons. 7.5%, [MgSO4] ) 0.1% (o.d. pulp), [NaOH] ) 2.3% (o.d. pulp), pH 11.5, P°(O2) ) 0.55 MPa, and 105 °C). The denotations “i” and “0” correspond to the initial acid-washed pulp and the pulp mixed with catalyst/alkali after the heating period, respectively.

simultaneous HPA and oxygen supply, as was done in the case of the laboratory experiment. Thus, in an eventual industrial application, the catalyst and oxygen should be mixed simultaneously with pulp in order to avoid an excess of reduced HPA in the reaction system. In general, although the temperature applied in the OHPA stage (95 °C) was 10 °C lower than that in the O stage (105 °C), the selectivity and the rate of delignification during the first 45 min in OHPA and O treatments were rather close, demonstrating rate constants similar to those determined from the plot ln(d[lignin]/dt) vs ln[lignin] (6.0 × 10-3 and 5.5 × 10-3 min-1, respectively). The differences in the viscosities of OHPA- and Obleached pulps were on the same order as those in the initial pulps before treatment. When the reaction time exceeded 45 min (corresponding around 40% of the pulp delignification), the delignification selectivity in the case of catalyzed oxygen delignification remained satisfactory while the former dropped drastically during the oxygen alkaline treatment (Figure 4). Thus, the catalytic oxygen delignification showed advantages over the conventional alkaline oxygen process, confirming our previous laboratory results.3,4 This comparison might be even more pronounced if the difficulties in the rigorous control of the HPA concentration during the pilot trial were overcome. Despite preliminary corrections made on the condensed water in the oxygen reactor IV (Figure 1) heated by a live steam, the actual HPA concentration was about 1.5 mmol/L instead of 3 mmol/L optimized

Experimental conditions. WA: pulp suspension consistency 2.8%, pH 3.8 adjusted with H2SO4. OHPA: cons. 7.8%, [HPA-5-MnII] ) 1.5 mmol/L, pH 3.8, P(O2) ) 0.55 MPa, 60 min. and 95 °C. D1: cons. 10.0%, ClO2 charge 12 kg/ton (as active chlorine) o.d. pulp, 30 min, and 70 °C. E: cons. 9.5%, NaOH charge 1.0% o.d. pulp, 60 min, and 65 °C. D2: cons. 10.7%, ClO2 charge 12 kg/ton (as active chlorine) o.d. pulp, 90 min, and 70 °C. W: pulp suspension consistency 2.4%, pH 8.5. O: cons. 7.5%, [MgSO4] ) 0.1% (o.d. pulp), [NaOH] ) 2.3% (o.d. pulp), pH 11.5, P(O2) ) 0.55 MPa, 60 min, and 105 °C. D1′: cons. 8.0%, ClO2 charge 14 kg/ton (as active chlorine) o.d. pulp, 30 min, and 70 °C. E: cons. 10.0%, NaOH charge 1.0% o.d. pulp, 60 min, and 73 °C. D2′: cons. 8.5%, ClO2 charge 12 kg/ton (as active chlorine) o.d. pulp, 90 min, and 70 °C. DEDED: during the day when the industrial unbleached pulp was collected at Soporcel pulp mill, the average total ClO2 charge was 55 kg/ton.

for OHPA in the preliminary laboratory experiments with a pulp consistency of 8-10%. This fact influenced negatively both the rate and selectivity of the delignification because HPA plays the role not only of the catalyst for the lignin oxidation but also of the polysaccharide protector toward autoxidation.4 Despite the mentioned troubles (the existence of HPA mainly in the reduced form at the beginning of the OHPA stage and the low HPA concentration) explaining, at least partially, oxidation selectivities lower than could be expected, the results of the pilot experiment are valid in terms of the general tendencies observed. Characterization of Pulp at Different Bleaching Stages. Oxygen-bleached pulps after both catalytic (OHPA) and alkaline (O) stages were submitted to further bleaching with chlorine dioxide, aiming to estimate their bleaching response in the WAOHPAD1ED2 and WOD1′ED2′ sequences. These data were compared with those obtained for the pulp bleached by the industrial DEDED process (Table 1). However, when the ClO2 charge was lowered proportionally to the κ number decrease in pulps prebleached by OHPA and O (from 55 kg/ton in DEDED to 24 kg/ton in WAOHPAD1ED2 and 26 kg/ton in WOD1′ED2′, respectively), the final brightness of pulps bleached by WAOHPAD1ED2 and WOD1′ED2′ was remarkably lower than that of pulps bleached by the DEDED sequence (Table 1). This fact indicates that the bleachability of pulps with chlorine dioxide deteriorated when the oxygen-prebleached stages had been applied. Similar observations were previously reported comparing the eucalypt kraft pulp bleaching in ODEDED and DEDED sequences for the same brightness.14 Therefore, the chlorine dioxide savings in bleaching trials containing oxygen prebleaching will not be proportional to the dropping of a κ number after the oxygen stage.

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Table 2. Physical Properties of Eucalypt Kraft Pulp Bleached with WAOHPADED, WODED, and DEDED Sequences (65 g/m2) ClO2 charge (as active chlorine), kg/ton pulp viscosity, cm3/g brightness, % ISO revolution PFI, n° beating degree, °SR burst index, kPa‚m2/g tensile index, N‚m/g tear index, mN‚m2/g opacity, %

WAOHPADED

WODED

DEDEDa

24

26

55

755 85.1 1800 30 4.16 62.1 7.75 72.0

900 86.7 2000 30 4.24 64.8 8.20 71.5

980 89.5 1250 30 5.30 77.1 8.6 70.4

a

Industrially bleached DEDED kraft pulp was supplied by Soporcel pulp mill.

It is interesting to note that the bleaching results in terms of the final brightness were slightly less pronounced for OHPA than for the O pulp despite the much better brightness progress in D1 and D2 stages when compared to D1′ and D2′ (Table 1). The main reason for such a bleaching behavior is the significant difference in the brightness of OHPA and O pulps. The poor brightness of the OHPA pulp is determined by a strong oxidation of residual lignin, which, unlike degraded lignin fragments in the oxygen alkaline treatment, was not removed to a great extent under acidic oxidation, contributing significantly to the dark color and the κ number of pulp (Table 1). The contribution of hexenuronic acid (HexA) residues to the κ number is believed to be less important for the OHPA pulp than for the O pulp because the former readily degrade under acidic conditions in the presence of HPA.15 The O pulp eventually contains a lower amount of chromophoric structures originating from the residual lignin and a significant proportion of HexA residues. This explains the better brightness of O-prebleached pulp, possessing a κ number higher than the OHPA pulp in practically all bleaching stages with chlorine dioxide (Table 1). Apparently, the application of alkaline extraction after the OHPA bleaching could be advantageous from the point of view of the bleaching response in subsequent chlorine dioxide stages. The better brightness stability of WAOHPAD1ED2bleached pulp than that of pulp bleached by WOD1′ED2′ (Table 1) could be explained by the lower content of the residual lignin and HexA moieties in the former. As was shown previously, the implementation of the oxygen prebleaching stage to the conventional DEDED bleaching (ODEDED sequence) leads to the more pronounced amounts of HexA residues in the final bleached eucalyptus kraft pulp, thus negatively affecting its brightness stability.14 Physical Properties of Bleached Pulps. Despite the noted differences in the viscosities of pulps bleached by WAOHPAD1ED2 and WOD1′ED2′ sequences (Table 2), their strength properties were very similar. Both pulps showed on average 10-20% lower strength indexes when compared to conventional pulp bleached by DEDED. Similar features with eucalypt pulp strength were previously observed by comparing the ODEDED and DEDED bleaching sequences where around 50% chlorine dioxide savings were achieved by implementation of an oxygen alkaline prebleaching stage.15 The pulp bleached by WAOHPAD1ED2 showed better beatability than the pulp bleached by WOD1′ED2. Effluents Toxicity. The toxicity assessment of effluents was done by Microtox analysis, verifying the changes in luminescence of the marine bacteria P.

Table 3. Toxicity of Effluents from Bleaching Stages in WAOHPAD1ED2 and WOD1′ED2′ Sequences pHi WAOHPAD1ED2 sequence

EC50 (%)a toxic units at 15 min at 15 min

3.9

WA effluent

3.5

>75

75 >75 73.4 >75

75 >75