Abstract
Brookfield R.V.T. apparent viscosity measurements of 31.1 to 73º Brix vinasses, with and without nondissolved solids (N.D.S.), were carried out at varying rotation speeds (N) and temperatures (T). A regression analysis of this data was carried out to select the corresponding mathematical models. It was concluded that apparent Brookfield viscosity for low concentration vinasses (up to 52.4º Brix) depends only on linear and quadratic temperature terms. At higher concentrations (66º and 73º Brix) regression models for apparent viscosity, must also include quadratic as well as rotation speed-temperature interaction terms. This behavior is discussed, identifying two types of rheological behaviors and advancing a preliminary hypothesis about the role of solutes, N.D.S. and temperature.
vinasse; Brookfield viscosity; regression analysis
Use of a rotational bench viscometer to study the influence of temperature and agitation speed on vinasse viscosity
L.E. Brossard Perez1, G. Bezzon1, E. Olivares Gómez1 and L.A.B. Cortez1* * To whom correspondence should be addressed
1Universidade Estadual de Campinas, Núcleo Interdisciplinar de Planejamento Energético (NIPE),
Fax (55) (019) 289-4717, Phone (55) (019) 788-7596, CEP 13083-970, Campinas - SP, Brazil.
Universidad de Oriente, Departamento de Ingeniería Química,
Santiago de Cuba, Cuba
(Received: January 10, 1999 ; Accepted: November 7, 1999 )
Abstract - Brookfield R.V.T. apparent viscosity measurements of 31.1 to 73o Brix vinasses, with and without nondissolved solids (N.D.S.), were carried out at varying rotation speeds (N) and temperatures (T). A regression analysis of this data was carried out to select the corresponding mathematical models. It was concluded that apparent Brookfield viscosity for low concentration vinasses (up to 52.4o Brix) depends only on linear and quadratic temperature terms. At higher concentrations (66o and 73o Brix) regression models for apparent viscosity, must also include quadratic as well as rotation speed-temperature interaction terms. This behavior is discussed, identifying two types of rheological behaviors and advancing a preliminary hypothesis about the role of solutes, N.D.S. and temperature.
Keywords: vinasse, Brookfield viscosity, regression analysis.
INTRODUCTION
Vinasse is a by-product of cane juice and/or molasses fermentation in the process of ethanol production process. It consists of an aqueous solution of many organic and inorganic substances, all of a polar nature (Polack et al., 1981). Original vinasse is rather diluted and exhibits a varying amount of nondissolved solids (N.D.S.) of a colloidal nature (Brossard and Cortez, 1996a). The rheology of vinasse has been studied by means of Brookfield R.V.T. measurements for its disposal as vinasse-fuel oil emulsions (Brossard and Cortez, 1996b).
The present contribution is aimed describing variations in the apparent viscosity of vinasse in terms of temperature (T) and rotation speed (N) in Brookfield R.V.T. determinations as well as at advancing an explanation of the observed behavior. Brookfield R.V.T. viscosity data will then be used for a qualitative evaluation of the type of rheological behavior prevailing in the vinasse solutions tested.
The importance of this study is to provide sufficient data to allow engineers to design vinasse in-tube transportation in situations such as the preparation of vinasse in evaporators and combustion. Most data available for vinasse is for low Brix values (up to 5o), at which it behaves mainly like water. However, high Brix vinasse (above 30o) presents non-Newtonian behavior. This observation is critical to pumping design and in-tube transportation, as it is known for other materials in the sugar industry such as molasses. Like that of molasses viscosity, vinasse viscosity is strongly dependent on water content and temperature, and this is the objective of this study.
MATERIALS AND METHODS
Vinasse coming from high test molasses and with a dissolved solids concentration of 30o Brix was concentrated up to 73o Brix using Buchi Rotavapor vacuum equipment. From this concentrated mother vinasse, dilutions were made by adding distilled water in order to prepare vinasse solutions with different concentrations (i.e., 31.1, 44, 52.4, 66 and 73o Brix). Some of these prepared solutions were centrifuged to separate the N.D.S.
Apparent viscosity of vinasse solutions (with and without N.D.S.) was determined at several temperatures and rotation speeds (N) by means of a Brookfield R.V.T. viscometer. For this purpose, cylindrical spindles were used (Nos. 1, 2, 3, and 4).
The resulting values were processed using linear regression analysis, thereby obtaining the corresponding mathematical models. All determinations were made in triplicate, using the mean values for calculations.
RESULTS AND DISCUSSION
Figures 1 to 7 show apparent viscosity values (
) for different temperatures (T) and rotation speed (N), and as can be observed, a curvelike pattern prevails in all cases. Vinasses with 31.1o to 44o Brix concentrations and N.D.S. (Figures 1 to 3) show small variations in with changes in N at a given temperature.
When the vinasse concentration is ³ 66o Brix (Figures 4 to 7), large differences in values are seen when determinations at a specific temperature are carried out at different N. These noted differences become smaller as temperature increases.
A closer look at the viscosities of highly concentrated vinasses (³ 66o Brix) gives additional information. Thus, larger fluctuations in (for a given temperature) at different N are observed when these vinasses contain N.D.S. This can be visualized by comparing Figures 4 and 5 and also Figures 6 and 7. Again an increase in T has a leveling effect. This behavior can be explained in terms of a critical concentration from which point on, vinasse solutions become more structured due to dipole-dipole interactions between their polar solutes. Other noted changes related to temperature effects as well as comments about possible rheological behavior are addressed later in the text.
At relatively low Brix values, N.D.S. present in vinasse have no noticeable effect, probably due to the poorly consolidated associations prevailing in diluted liquid systems.
However, as soon as the above-mentioned, critical concentration is reached and a liquid structure is established, polar N.D.S. nature introduce new linking points, reinforcing the already existing interactions. The final result is an increase in apparent viscosity () accounted for by the presence of N.D.S..
The regression analysis of the experimental data corresponding to Figures 1 to 7 shows that the apparent viscosity of low Brix vinasses (i.e., up to ~ 50o Brix) has linear and quadratic dependence on T with no appearance of significative N terms (Tables 1 and 2). On the other hand, concentrated vinasses (³ 66o Brix) show regression models for with high coefficients of determination (R2) that include additional terms for T-N interactions as well as a quadratic dependence. This is taken as clear evidence of the new nature of concentrated vinasses with and without N.D.S..
As expected, mathematical models obtained for high Brix vinasses present negative T and N linear terms and a positive coefficient for their interaction term. Physically, this means that there is a loosening action in the liquid due to an increase in both individual parameters.
Now, regarding T-N positive interaction, it is evident that when both are lowered at the same time, there is an increase in viscosity.
Even though the Brookfield apparent viscosity is not what could be called a true apparent viscosity (Diaz, 1995), from which a specific rheological model could be inferred, it gives valuable information about the qualitative and quantitative nature of the associations in the liquid under consideration. Thus, the absence of N terms in regression models for the apparent viscosity of vinasse () in the concentration range from 31.1 to 52.4o Brix indicates that does not change when the shear rate (
) assumes different values.
Although, there is not a complete identity between the and N parameters, it is obvious that they are closely and directly related and therefore can be interchangeably used in a qualitative evaluation of the rheology of a liquid system. Newtons law of viscosity (Skelland, 1967) states that the relation between shear stress (
) and shear rate () has a constant value for certain fluids at a definite temperature. This means that any change in causes a proportional change in , while 
remains constant. For Newtonian fluids, temperature is the only parameter affecting . This is precisely what is observed in the above-mentioned vinasse solutions, and in accordance with that, it is also appropriate to consider them as Newtonian fluids.
Returning to the Brookfield viscosity data for 31.1 to 52.4o Brix vinasses (Figures 1 to 3), it is noted that all these curves have one specific slope up to approximately 40o C. From this temperature on, a second segment with a much lower slope is observed. This second segment tends towards a zero slope, indicating an approximation to a constant value of at higher temperatures.
It seems that the structure of these moderately concentrated solutions becomes almost completely disarranged at temperatures slightly above room temperature, and as consequence, is considerably lower and its magnitude change little.
It is also noted that the slope of the first branch of these curves gets steeper with an increase in vinasse concentration. If the T2 term is neglected in the regression models appearing in Table 1 the straight line equations so obtained show an increasing value, for slope (i.e., T term coefficient) with an increase in vinasse concentration.
A reasonable conclusion is that the decreasing effect on caused by an increase in T becomes more accentuated as the concentration of the vinasse solution increases (within the concentration range so far discussed). This phenomenon can be explained by the fact that at a given temperature, the kinetic energy acquired by the system causes greater disorder as the number of particles in the system increases. In this way, of concentrated vinasses is more depressed than that of diluted ones when T is raised.
On the other hand, vinasses with 66 to 73o Brix did not show similar behavior in relation to the form of the curves, although, in accordance with the hypothesis just formulated, the decrease in is considerably sharper than it is for less concentrated vinasses.
Another important difference noted for these concentrated vinasses is that no longer remained constant when N was varied at a given T. Again, a qualitative change in rheological properties could be detected from the Brookfield viscosity data, and in this case, it clearly shows the non-Newtonian nature of concentrated vinasse solutions. As stated previously, this change in rheology can be ascribed, to the complex strengthening of interactions between polar constituents, soluble matter and colloids in vinasse solutions.
CONCLUSIONS
Apparent Brookfield viscosity () of vinasse solutions is basically defined by total amounts of solids in solutions (oBrix) and temperature (T). Regression equations for low Brix vinasses (< 66o Brix) show that can be described as a function of linear and quadratic terms of T. High Brix vinasses (³ 66o Brix) must also include quadratic N and N-T terms in order to obtain a proper mathematical description of their apparent viscosity. The regression equations obtained should help to predict under a rather wide range of conditions. It is important to note that N.D.S. reinforce the liquid structure of vinasse and also increase its viscosity, but only from a critical oBrix on. Through the detailed analysis of the experimental data obtained, Newtonian and non-Newtonian rheological behaviors could be identified. The Brookfield R.V.T. viscosity data allowed observation of different initial viscosity depression patterns depending on temperature and concentration of the vinasse solutions studied.
ACKNOWLEDGEMENTS
The authors thank the State of São Paulo Research Foundation/FAPESP for the financial support that made this publication possible.
NOMENCLATURE
r.p.m.
- Brossard Perez, L.E. and Cortez, L.A.B. Experiments on Vinasse Disposal - Part I: Emulsification with Fuel Oils. Brazilian Journal of Chemical Engineering. vol. 13 no 04, p. 200 (1996a).
- Brossard Perez, L.E. and Cortez L.A.B. Experiments on Vinasse Disposal - Part II: Rheology of Fuel Oil - Vinasse Emulsions. Brazilian Journal of Chemical Engineering. vol. 13, no 4, p. 260 (1996b).
- Diaz, A. Reologia de Fluidos Newtonianos y no Newtonianos. Ed. ISPJAM, Santiago de Cuba (1995).
- Polack, J.A., Day, D.F. and Cho, Y.K. Gasohol from Sugar Cane - Stillage Disposition Report to Department of Natural Resources. DNR Agreement No. 21400-40-33. Audubon Sugar Institute (A.S.I.), Baton Rouge, La. (1981).
- Skelland, A.H.P. Non-Newtonian Flow and Heat Transfer. John Wiley and Sons, New York (1967)
Publication Dates
-
Publication in this collection
06 July 2000 -
Date of issue
June 2000
History
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Accepted
07 Nov 1999 -
Received
10 Jan 1999
