O 2 delignification Kinetics from CSTR and Batch Reactor Data

Many kinetic studies of oxygen delignification have been reported in literature. In the past these studies were mostly performed in batch reactors whereby the caustic and dissolved oxygen concentrations are changing during the reaction. This makes it difficult to determine the reaction order of the different reactants in the rate expressions. Also the lignin content and cellulose degradation of the pulp are only established at the end of an experiment when the sample is removed from the reactor. To overcome these deficiencies, we have adopted a differential reactor system (called Berty reactor) which is used frequently for fluid-solid reaction rate studies. In this continuous stirred tank reactor (CSTR), the dissolved oxygen concentration and the alkali concentration in the feed are kept constant, and the rate of lignin removal is determined from the dissolved lignin concentration in the outflow stream measured by UV-VIS spectroscopy. Experiments were performed at different temperatures (80oC, 90oC, 100oC and 110oC), oxygen pressures (35psig, 55psig, 75psig and 95psig) and caustic concentrations (1.1g/l, 3.3g/l, 5.5g/l and 7.7g/l). The delignification rate is found to be first order in HexA – free residual lignin content. The delignification rate reaction order in [NaOH] and oxygen pressure are 0.6 and 0.5 respectively. The activation energy is 47 kJ/mol. The rate of cellulose degradation is described by two terms: one due to radicals produced by phenolic delignification, and the other due to alkaline hydrolysis INTRODUCTION Oxygen delignification is widely used for lignin removal before bleaching pulp. The well known advantages of oxygen delignification are chemical cost savings, yield retention and improved environmental performance. Better understanding of oxygen delignification kinetics and its relation to cellulose degradation may point to improved operation in industrial practice. Lignin and cellulose degradation occur simultaneously during oxygen delignification. Therefore, a kinetic study of oxygen delignification should include the kinetics of both delignification and cellulose degradation. Normally, delignification is measured by the decrease of Kappa number of pulps. Cellulose degradation is monitored by the decrease in intrinsic viscosity [η] of pulps. However, it is now well known that the kappa number does not correctly represent the amount of residual lignin in the pulp, since hexenuronic acid (HexA) and other non-lignin structures also consume KMnO4 in the kappa number measurement (Li, 1999). In this study, the HexA content will be considered in the kinetics modeling. Another aspect of the present approach is that the kinetics are determined in a differential continuous stirred tank reactor (CSTR) where the dissolved oxygen concentration and the alkali concentration in the feed are kept constant, and the rate of lignin removal is determined from the dissolved lignin concentration in the outflow stream measured by UV-VIS spectroscopy (Ji and van Heiningen, 2006). Kinetic Modeling The kinetics of oxygen delignification is usually presented by a power law equation which includes the influence of the process variables such as reaction temperature, oxygen pressure and caustic concentration (Iribarne and Schroeder, 1997): q n O m L K P OH k dt dK r ] [ ] [ 2 − = − = (1) In equation (1), K is the Kappa number; [OH] is the caustic concentration and [PO2] is the absolute oxygen pressure. The constants m, n, and q are obtained by fitting of the experimental data. The reaction rate coefficient k depends on the temperature and is given by the Arrhenius equation: ) exp( RT E A k A − = (2) In equation (2), EA is the activation energy, R is the gas constant, T is the absolute temperature and A is the frequency factor. A summary of the different power law equations reported in the literature for oxygen delignification is given in Table 1. Table 1 Summary of Kinetics Equations using the Power Law Model Reference [OH ] exponent (m) [O2] or Po2 exponent (n) Kappa Number exponent (q) Activation Energy (kJ/mol) Frequency Factor (A) Agarwal(1998) 7.7 Perng (1997) 0.4 0.5 4.8 60 1.8 Teder (1981) 0.6 0.5 3.2 70 Kovasin (1987) 0.13 0.5 1 18.6 Iribane (1997) 0.7 0.7 2.0 51 3×10 Evans (1979) 1 1.23 1 49.1 10 The reaction orders m, n and the activation energy in Table 1 are difficult to compare because the experiments were done with different pulps. It can also be seen that reaction order in kappa number varies from 1 to 7.7, and the activation energies range from less than 20 to 70 kJ/mol. Cellulose Degradation The cleavage of cellulose polymers was modeled by Iribarne and Schroeder (1997) as the increase in number-average moles of cellulose per gram of pulp (mn). Similarly one can describe the cellulose degradation by the number of cellulose chain scissions during oxygen delignification. Violette and van Heiningen (2003) calculate the number of cellulose chain scissions from the average degree of depolymerization of cellulose (DP) in the pulp at time t=0 and time t=t, i.e. as 1/DPt-1/DP0 DP can be obtained from the intrinsic viscosity [η] by equation (3) (van Heiningen et al, 2003): [ ] 111 . 1 116 65 . 1 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = G H DP η (3) where [η] is intrinsic viscosity of the pulp in cm/g, and G and H are the mass fractions of cellulose and hemicelulose in the pulp. This formula considers the actual weight of cellulose rather than the pulp weight being responsible for the viscosity, and makes a correction for the small contribution of the hemicelluloses to the pulp intrinsic viscosity. The number of moles of cellulose per gram of pulp, mn, can be calculated by equation (4) (Iribane and Schroeder, 1997) as: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ ≅ + = Pulp Gram Moles DP DP m n n n 162 1 18 162 1 (4) In equation (4), the factor of 162 is the molecular weight of the anhydrous glucose unit and 18 is the molecular weight of water. EXPERIMENTAL Oxygen delignification experiments were performed both in a CSTR and a batch Parr reactor. The Parr reactor is a 2-liter horizontal stainless steel reactor obtained from Parr Instruments with an anchor rotating device which wipes the inside of the reactor with Teflon blades. Figure 1 shows a schematic diagram of the Parr reactor set-up. Figure 1 Schematic Diagram of the Horizontal Parr Reactor Set-up The flow diagram of the CSTR setup is shown in Figure 2. Oxygen from a gas cylinder is bubbled into a caustic solution held in a pressurized (130 psig max.) 3 gallon stainless steel container. The container is kept at a desired temperature by an external heating blanket. The actual reactor is a Berty reactor (Autoclave Engineers) with a stationary basket which holds the pulp bed. The nominal volume of the reactor is a 280 ml. It contains a 100 ml basket with a rotor underneath which induces flow through the pulp mat inside the basket. The entire reactor is filled with liquid at the operating pressure, and any gas inside the reactor is vented at the top of the reactor. Oxygen is bubbled overnight through a NaOH solution to obtain a saturated oxygen concentration. Then the reaction is started by feeding the oxygenated caustic solution at constant flow rate and oxygen pressure. The reactor pressure, temperature and outflow rate are recorded every 5 seconds. The UV-VIS absorption of the outflow stream is monitored every 15 seconds. Figure 2. Diagram of CSTR Reactor Setup Raw Materials A commercial unbleached southern pine kraft pulp with an initial Kappa number of 24.4 and intrinsic viscosity of 1189 ml/g was used. Measurements The dissolved lignin concentration was measured using a flow cell and a HP8453 UVVIS spectrophotometer from Agilent. The absorption at 280nm was converted to lignin concentration using the calibration curve made by purified lignin (Indulin AT from Mead-Westvaco, extinction coefficient of 24.8 liter٠g٠cm). The extinction coefficient does not change much during oxygen delignification was verified by mass balance (Ji and vanHeiningen, 2006). (The kappa number of the pulps was measured according to a modified method of the TAPPI standard T236-cm-85. All the chemical dosages were reduced to one tenth of the amount in TAPPI method due to the small amount of samples. Intrinsic viscosity of the pulps was determined following the A.S.T.M. designation D1795-62 (re-approved 1985). The HexA group content was determined after acid hydrolysis of the pulps and UV measurement of the hydrolysis products (2-furoic acid and 5-carboxy-2-furaldehyde) which have a clear absorption peak at 245nm. The HexA content of the final pulp samples generated at 90oC, 3.3g/l NaOH and 75 psig in the CSTR reactor were measured. The HexA content of all these samples falls between 21 and 23 mmol/kg pulp, confirming that HexA is stable during oxygen delignification. Because HexA contributes to the kappa number, the residual lignin content in the pulp should be corrected for the HexA content in the pulp. Typically, 10mmole/kg HexA corresponds to 0.86-1.1 unit of kappa number (Li and Gellerstedt, 2002; Jääskeläinen et al, 2002). In the present study, 10mmol/kg HexA is considered as 1 kappa unit, thus, the residual lignin content in pulp is as: pulp g lignin mg 5 . 1 10 ⎟ ⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = HexA Kappa Lc (5) The pulp in this study has an average value of 22mmol/kg HexA which is equivalent to 2.2 kappa units or 3.3 mg (2.2×1.5 =3.3 mg) lignin. Data Reduction Procedure The CSTR operation was validated by showing that the oxygen delignification kinetics are not influenced by the amount of pulp in the reactor, the feed flow rate, and rotor speed (over 400rpm) (Ji and vanHeiningen, 2006). Most experiments were performed with 4 grams (oven dry basis) of pulp because of the volume of the basket and analysis requirement. The dissolved lignin mass balance for the well mixed reactor during time interval, dt, is: Inflow Outflow + Dissolved by Reaction = Accumulated in Reactor or ) ( ) ( ) ( 0 t dC V dt m t r dt t C r p v = + −φ (6) or p r r m dt t dC V t C t r 1 ) ( ) ( ) ( ⎥⎦ ⎤ ⎢⎣ ⎡ + = φ (7) where r(t) is the rate of delignification (mg lignin/g pulp/min) Фv is the liquid flow rate (ml/min) C(t) is the dissolved lignin concentration (mg lignin/ml) Vr is the reactor volume (ml) mp is the pulp weight (o.d pulp) The reactor volume, Vr,, was determined by a step tracer residence time distribution experiment. Methyl red was used as a tracer. Analysis of the residence time distribution curve showed that the Berty reactor and piping up to the UV-VIS detector could be described by a CSTR of 265 ml and a plug flow volume of 96 ml. This closely agrees with the free volume in the reactor and piping respectively. Therefore, Vr is taken as 265 ml, and the residence time between the Berty reactor and UV detector, td, is v d t φ 96 = (8) Thus the dissolved lignin concentration inside the Berty reactor at time t, C (t), is equal to the concentration measured by UV at time t+td, CL (t+td): ) ( ) ( d L t t C t C + = (9)