Dynamic Viscoelasticity of Kraft Black Liquor at a High Dry Solid Content with the Addition of Sodium Aluminate
Xiaopeng Yue,a Xin Du,a,b and Yongjian Xu a,*
Black liquor is not only a by-product of the papermaking industry but also an energy source that is often burned at a high solids content in a recovery furnace because of its high combustion efficiency and stability. However, the silicon content of bamboo kraft black liquor (BKBL) is much higher than that of softwood kraft black liquor, and the presence of silicon causes serious problems in the recovery cycle. Sodium aluminate, when used as a desilicating agent during combustion, has an excellent effect on the removal of silicon from BKBL. In this work, the dynamic viscoelasticity of BKBL with the addition of sodium aluminate was studied using a rotational rheometer. The results indicated that the BKBL was a pseudo-plastic fluid. A power-law model and the Cross model accurately described the relationship between the dynamic viscosity and angular frequency. The zero shear rate viscosity of BKBL was relatively high, even at a high temperature. The addition of sodium aluminate increased the viscosity of BKBL when the loading was 1.5 wt.%, but it had the opposite effect when its loading was 0.5 wt.%. With an increase in the angular frequency, the effect of sodium aluminate on the viscosity became less apparent.
Keywords: Dynamic viscoelasticity; Black liquor; Complex viscosity; Dynamic viscosity
Contact information: a: National Demonstration Center for Experimental Light National Demonstration Center, College of Bioresources Chemical and Materials Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, 710021,China; b: College of Mechanical & Electrical Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi Province, 710021, China;
* Corresponding author: firstname.lastname@example.org
Black liquor is an environmentally harmful waste liquor resulting from the kraft pulping process that consists of complex organic and inorganic compounds (Trung et al. 2015). The main organic compounds in black liquor are lignin, polysaccharides, and resinous compounds, which are extracted from papermaking materials. An expensive mixture of cooking chemicals is used to separate cellulose fibers from raw material (Garcia et al. 2009). Hence, the inorganic compounds in black liquor primarily include large amounts of sodium salts and small amounts of calcium, potassium, magnesium, silicon, and iron salts (Liu et al. 2013). To maintain the cost-effectiveness of the pulping process and protect the environment, black liquor is burned in a specially designed furnace to recover the energy and chemicals in an alkali recovery system. Usually, black liquor is concentrated to a solid content above 60 wt.% by multiple effect evaporators before being burned in the recovery furnace. To improve the combustion efficiency and stability, the black liquor can be burned at a higher solids content (Sandquist 1983; Llamas et al.2007). At Hainan Jinhai Pulp and Paper Co., Ltd., the solids content of black liquor is above 80 wt.%, obtained through passivation and crystallization evaporation technology. Other benefits, such as economic and environmental outcomes, including lowering the discharge amount of sulfur dioxide, can be achieved when the black liquor is burned at a high solids content.
Bamboo, which is widely grown in China, is considered a favorable non-wood fiber material for papermaking because of its excellent pulping properties (Sun and Hui 2002). However, the silicon content of bamboo kraft black liquor (BKBL) is much higher than that of softwood pulp black liquor (Cardoso et al. 2009). This characteristic leads to a series of silicon-related problems in the recovery system. More serious silicon scaling occurs in the pipe and evaporation system because the silicon in black liquor reacts with sodium hydroxide (NaOH) to produce sodium silicate (Na2SiO3) during the cooking process (Xu et al. 2015c). The viscosity of black liquor increases due to the higher silicon content, and more energy is needed to pump and atomize the black liquor (Ramamurthy et al. 1993; Karlsson et al. 2013). In addition, the alkali recovery rate decreases with increasing silicon content due to the formation of calcium silicate (CaSiO3) in the lime mud. Recently, a new method to remove silicon from black liquor was proposed (Xu et al. 2015a). Sodium aluminate (NaAlO2), which has typically been used as a desilicating agent, was added to the black liquor before combustion. The Na2SiO3in the black liquor can react with NaAlO2 to produce Na2O∙Al2O3∙4SiO2 during combustion. Then, the combustion product is dissolved in water to form green liquor. The Na2O∙Al2O3∙4SiO2 forms a precipitate, which can be filtered from the green liquor. The decrease in silicon content in green liquor is beneficial to the causticizing process (Xu et al. 2015b).
The rheological properties of black liquor, which strongly depend on temperature, solid content, shear rate, and cooking conditions (Yang et al. 2007; Singh et al. 2016; Zhang and Chen 2016), have a significant impact on the operational factors of a furnace, such as the droplet formation, the power requirements for transport, and drying and swelling characteristics (Zaman and Fricke 1995a). According to the authors’ previous research, BKBL exhibits a shear-thinning phenomenon only when the solid content is above 50 wt.% during the shear process (Xu et al. 2016a). Furthermore, the temperature and solid content have a marked effect on the dynamic viscoelasticity of BKBL (Xu et al. 2016b). However, most studies have focused only on the rheological properties of black liquor at a low or middle solid content. The knowledge of BKBL rheological properties at a high solid content is essential for improving the design and operation of the recovery system.
Considering the conditions, i.e., the temperature of the BKBL from the evaporators is approximately 120 °C and is gradually reduced along the flow direction, this paper focuses on the dynamic viscoelasticity of 70.2 wt.% BKBL and 79.9wt.% BKBL with various sodium aluminate loadings at greater than 70 °C. The storage modulus (G′), loss modulus (G″), dynamic viscosity (η′), and complex viscosity (η*) were studied with a rotational rheometer to explore the viscoelasticity behavior of thick black liquor and the effect of sodium aluminate on the droplet formation of black liquor.
The BKBL was kindly supplied by Chitianhua Pulp and Papermaking Co., Ltd., Guizhou, China. The solids content was 70.2 wt.%. The total alkali content was 25.0 wt.%. The effective alkali was 23.9 g/L. The total sulfur content was 4.43 wt.%. The fuel value was 11.2 MJ/kg. The silicon content was 1.03 wt.%.
Sodium aluminate (NaAlO2) was supplied by Sinopharm Chemical Reagent Co., Ltd., Beijing, China.
The thick black liquor was dried at 80 °C in a vacuum drying oven until the solid concentration reached 79.9 wt.%. Sodium aluminate (0.5 wt.% and 1.5 wt.%, based on the solid weight in black liquor) was added to the BKBL at two different solid contents (70.2 wt.% and 79.9 wt.%). The mixtures were stirred at 260 rpm for 10 min at 40 °C.
The dynamic modulus of BKBL was measured with an AR2000ex rotational rheometer (TA Instruments, New Castle, DE, USA) at 70 °C and 98 °C. The displacement resolution of the rheometer is 0.04 μrad. The jig of the rheometer was a stainless steel parallel plate with a diameter of 25 mm. The measuring space was 700 μm. The angular frequency ranged from 0 to 100 rad/s. Silicone oil, which did not mix with the black liquor and had a negligible effect on torque generation because of its lower density and viscosity, was used to cover the specimens to prevent water evaporation during measurements.
Measuring the shear viscosity of black liquor is difficult, especially at high shear rates, because of viscous heating, whereas the complex viscosity is easier to measure (Zaman and Fricke 1995b). The complex viscosity (η*) is defined as
where η′ is the dynamic viscosity (Pa∙s), η″ is the storage viscosity (Pa∙s), G′ is the storage modulus (Pa), G″ is the loss modulus (Pa), and ω is the angular frequency (rad/s).
The dynamic viscosity data were fitted according to a power-law model (Eq. 2) and the Cross model (Eq. 3), which are often used to describe the relationship between the dynamic viscosity (η′) of black liquor and the angular frequency (ω) (Zaman and Fricke 1995b). The equations are as follows,
where m is a measure of the consistency of the fluid, known as the consistency index (Pa∙sn), n is a measure of the degree of non-Newtonian behavior, known as the power law index, and ω is the angular frequency (rad/s); and,
where η0 is the zero shear rate viscosity (Pa∙s), η∞ is the viscosity of black liquor when ω approaches infinity, λ is a constant parameter (s-1), and α is a dimensionless constant.
RESULTS AND DISCUSSION
Effect of Sodium Aluminate on the Dynamic Modulus of BKBL
Figures 1 and 2 show typical results for the storage modulus (G′) and loss modulus (G″) of BKBL as a function of angular frequency (ω) at different solid contents and temperatures with the addition of sodium aluminate. The storage modulus (G′), which represents the amount of energy that can be stored in a fluid structure during a cycle of oscillation, increased with increasing angular frequency.
Fig. 1. Storage modulus of BKBL with the addition of sodium aluminate at various loadings: (a) 70.2 wt.%; and (b) 79.9 wt.%
At 70 C, the G′ values of thick BKBL with two different solid contents were both greater than those of BKBL when the temperature was 98 C, indicating that the network structure of BKBL was destroyed because of the higher temperature. At the same temperature, the G′ values of BKBL with 1.5 wt.% additive were greater than those of the BKBL without additive. However, the G′ values of BKBL with 0.5 wt.% additive were lower than those of the BKBL without additive. This phenomenon can be explained by consideration of two aspects. On the one hand, the extra sodium ions from sodium aluminate could destroy the network structure of black liquor by breaking the aggregation of lignin molecules (Zaman and Fricke 1996), resulting in a reduction of the energy that can be stored in the structure of BKBL. On the other hand, an increase in the composition of inorganic compounds, providing by the addition of sodium aluminate, could increase the amount of energy stored in the network structure during a cycle of oscillation. Hence, the weakening effect of sodium ions on the structure was judged to be the main cause of the reduction of G′ values when the sodium aluminate loading was 0.5 wt.%. When the loading was 1.5 wt.%, the increase in the composition of inorganic compounds may have played a major role in the increase in G′ values because of the greater amount of sodium aluminate.
The loss modulus (G″) is representative of the amount of energy that is lost by the viscous flow during a cycle of oscillation. As shown in Fig. 2 (a) and (b), G″ increased with increasing angular frequency and was also affected by temperature and sodium aluminate loading. As a representation of viscous dissipation, G″ depends on the strain time (1/ω) and the relaxation time (τ) of the chain segment. At low frequencies, τ was less than 1/ω. There was plenty of time for the chain segment to finish the relaxation process. Hence, the G″ values increased with increasing angular frequency. Raising the temperature accelerated the relaxation process and reduced the viscous dissipation. The sodium ions, which broke the aggregation of lignin, resulting in the reduction of the chain segment length, reduced the viscous dissipation because of the shorter relaxation time. Thus, the G″ values of BKBL with 0.5 wt.% decreased compared with the G″ values of BKBL without the additive. However, the G″ values became larger when the sodium aluminate loading was 1.5 wt.%. One possible reason is that the excess sodium aluminate took up the movement space of the chain segment, which led to the increase in viscous dissipation.