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STEAM AND HEAT CONSUMPTION DURING CONDITIONING
OF VEGETABLE FEED RAW MATERIALS IN THE PELLETING
PROCESS
Ryszard Kulig
Departament of Machine Operation in Food Industry, Agricultural University in Lublin, Poland
Summary. The paper presents the results of studies on the effects of the parameters of conditioning of selected feed raw materials on the energy consumption of the process. The studies were conducted on a test stand equipped with a microprocessor system measuring steam and heat consumption. The conclusion reached is that the mean values of steam consumption range between 17.03 and 158 kg·t-1, while those of thermal energy consumption – from 41.1 to 516.7 kJ·kg -1. Among the characteristics distinguishing the treated material, specific heat and bulk density had the most significant impact on heat consumption. The lowest mean demand for thermal energy (91.45 kJ·kg-1) was recorded in maize, while the highest (260.4 kJ·kg -1) – in lucerne.
Key words: pelleting, conditioning, thermal energy, specific heat
Nomenclature: a – thermal expansion coefficient, [m^2 ·s -1] c – specific heat, [kJ·kgK-1] E – amount of thermal energy consumed during conditioning, [kJ] Ec – thermal energy consumption, [kJ·kg-1] ms – weight of ground grain exiting the conditioner, [kg] p – calculated significance level, P (^) p – steam pressure, [kPa] r – Pearson’s correlation coefficient R^2 – coefficient of determination Tk – conditioning temperature, [oC] Z – amount of steam consumed during conditioning, [kg] Zp – unit steam consumption, [kg·t-1^ ] αi – assumed significance level λ – specific heat conductivity, [W·mK-1^ ] ρn – bulk density, [kg·(m^3 ) -1]
INTRODUCTION
The amount of steam and heat consumed in the conditioning process depends primarily on the properties and proportions of particular components in feed mixtures – which is related to the mate-
TEKA Kom. Mot. Energ. Roln. – OL PAN, 2007, 7A, 59–
60 Ryszard Kulig
rial temperature after treatment (usually from 50 to 90oC) as well as to the parameters of process steam [6]. According to Smallman [7], the category of raw materials susceptible to conditioning (i.e. easily absorbing humidity and heat) includes grain as well as high-protein materials with a low fiber content. Fibrous and oil raw materials are less susceptible to this process. The intensity of heat and mass exchange during hydrothermal treatment results primarily from the thermo-physical properties of the material, i.e. specific heat, specific heat conductivity and thermal diffusivity [2]. As reported by Kaleta [3], these properties – with respect to feed raw materials – are the subject of numerous studies and analysis. However, there are no detailed data regarding the effect of the above properties on the parameters of the conditioning process. Literature on the subject focuses mostly on the role of specific heat, which is related to the remaining heat values by the following dependence:
a n
c U
O [kJ ·kgK-1] (^) (1)
Providing the value of specific heat for the most frequently used feed raw materials, Kamiński et al. [4] concluded that this characteristic significantly affects the demand for steam during the pel- leting process. It follows that a mixture of raw materials with higher specific heat requires (in order to reach the appropriate temperature for pelleting) a greater addition of water vapor than a mixture of components distinguished by a lower value of this characteristic. The above findings were partially confirmed by Vandewalle et al. [8] in studies on condition- ing lucerne meal and ground maize. These authors found that a higher demand for steam is recorded during the treatment of the first raw material (c = 2.3 kJ·kgK-1) compared to the second, which is characterized by a lower specific heat value (c = 1.9 kJ·kgK-1). Determining the thermal energy flux supplied with water vapor during conditioning allows to establish the process energy balance as well as unit thermal energy expenditures on the pel- leting technological line. An overview of reference data shows that no comprehensive studies have been carried out so far that would resolve all the issues related to this matter. An aspect that should be pointed out is that the information provided by authors is not in fact confirmed by experimental results, which is due to the lack of test stands equipped with appropriate measur- ing devices [1]. Therefore, the purpose of this study was to determine steam consumption and thermal energy expenditures during hydro-thermal conditioning of materials of varied physical and chemical properties, at assumed temperature levels acquired as a result of applying steam at various pressure levels. Despite widespread use of pelleted feeds, so far many process parameters have been de- termined under production conditions in approximation, or certain parameters (such as steam consumption) have been calculated theoretically only. The need to use new, less expensive raw materials, or to prepare new feed recipes, makes it necessary to search for optimal pelleting process parameters. This kind of research may be conducted with the use of the test stand de- signed at the Department of Operation of Food Processing Machinery, University of Agriculture, Lublin [5].
MATERIALS AND METHODS
The raw materials were comprised of ground barley, maize, wheat, oat, pea, lupine and lu- cerne meal. The materials were ground in a hammer mill equipped with a 3 mm mash sieve. After grinding, the materials were brought to a constant moisture content of 14%. The specific heat value (Tab. 1) and bulk density (Tab. 2) were determined for the experimental materials.
62 Ryszard Kulig
RESULTS
Fig. 1. presents the results of steam consumption analysis, as dependent upon the conditioning temperature and steam pressure.
Electrical (pressing) energy consumption during pelleting with steam are shown in Figure 1. Mean values of this parameter varied from 120 to over 200 kJǜ kg -1. The rate of changes in energy consumption was
Pp [kPa]
Zp^
[kg·t
-1]
0
20
40
60
80
100
120
140
160
180
200 250 300 350 400 Pp [kPa]
200 250 300 350 400 Pp [kPa]
200 250 300 350 400 Pp [kPa]
200 250 300 350 400 Pp [kPa]
200 250 300 350 400
wheat maize oat barley pea lupine lucerne
Tk 50 oC (^) Tk 60 oC Tk 70 oC Tk 80 oC Tk 90 oC
Electrical (pressing) energy consumption during pelleting with steam are shown in Figure 1. Mean values of this parameter varied from 120 to over 200 kJ· kg -1. The generated values are within a wide range, from 17.03 to 158.82 kg/t. Changes of similar nature were also recorded for unit thermal energy expenditures (Fig. 3). In this case, the mean values fluctuated between 41.1 and 516.47 kJ·kg-1. Fig. 2 and Fig. 3 show that steam and thermal energy consumption per unit increase along with an increase in temperature during hydrothermal treatment, and that in both cases the depend- ence takes the form of a second degree polynomial (Tables 3 and 4). It should be stressed that the low values of regression coefficients at variable Tk^2 indicate that the effect of temperature on the values of the analyzed parameters within the tested range is close to the linear form.
26, 32, 38, 44, 50, 55, 61, 67, 73, 79, Pp [kPa] Tk [oC] ponad
Zp [kg·t-1]
50
60
70
80
90
200
250
300
350
400
20
40
60
80
100
above
Fig. 2. Dependence of steam consumption (Zp) on conditioning temperature (Tk) and steam pressure (Pp )
STEAM AND HEAT CONSUMPTION DURING CONDITIONING... 63
It was also observed that an increase in steam pressure results in a decrease in steam con- sumption, upon the same values of conditioning temperature. Due to the fact that steam enthalpy at a pressure of 400 kPa (2738 kJ·kg-1) is only slightly under 2% higher than enthalpy corresponding to 200 kPa (2707 kJ·kg -1^ ), while heat and mass exchange takes place under atmospheric conditions, this is most probably a result of greater heat loss taking place when a lower steam pressure is applied.
73, 93, 113, 133, 153, 172, 192, 212, 232, 252, Pp [kPa] Tk [oC] ponad
Ec [kJ·kg-1]
50
60
70
80
90
200
250
300
350
400
50
100
150
200
250
above Fig. 3. Dependence of unit thermal energy expenditures (Ec) on conditioning temperature (Tk) and steam pressure (Pp) (mean values for 7 tested raw materials)
It follows that unit thermal energy expenditures also diminish along with the rise in steam pressure. However, it should be stressed that the rate of these changes is slow, amounting to only a 0.035 unit increase in heat consumption per pressure unit, and is on average 14% lower than the rate of steam consumption increase. The equations describing the effects of conditioning param- eters on the value of unit steam consumption and unit thermal energy expenditures are presented in Tables 3 and 4.
Table 3. Regression equations describing the effect of conditioning temperature (Tk) and steam pressure (Pp) on steam consumption (Zp)
RAW MATERIAL EQUATION^ A^ B^ C^ D^ R
Wheat Zp = ATk2 – BTk - CPp + D 0,0124 0,7796 0,0245 33,24 0,
Maize Zp = ATk2 – CPp + D 0,0069 0,0364 10,17 0,
Barley Zp = ATk2 – BTk - CPp + D 0,0265 2,24 0,0402 80,97 0,
Oat Zp = ATk2 – CPp + D 0,0081 0,0010 5,70 0,
Pea Zp = ATk2 – CPp + D 0,0082 0,0395 7,58 0,
Lupine Zp = ATk2 – BTk - CPp + D 0,0191 1,32 0,0423 49,55 0,
Lucerne Zp = ATk2 – BTk - CPp + D 0,0408 2,95 0,0707 108,19 0,
STEAM AND HEAT CONSUMPTION DURING CONDITIONING... 65
E
c^
[kJ·kg
-1]
wheat maize
pea barley
lupine oat
lucerne
a a
b c
c
d
e
Fig. 5. Dependence of mean unit thermal energy expenditures (Ec) on the type of raw material (mean values calculated for 5 conditioning temperature levels and 5 steam pressure levels)
The statistical analysis based on the obtained results showed that there is a very high positive correlation between steam consumption (r=0.806; p<0.028) as well as heat consumption (r=0.834; p<0.019) and the value of specific heat of the tested raw materials ( c ). However, the predictive value of the obtained regression equations turned out to be insufficient since the values of coefficients of determination amount to, respectively: R^2 = 0.581 for steam consumption and R^2 = 0.636 for heat consumption. As a result, taking into account the above findings as well as the fact that in the conditioning process the material is proportioned by volume, another important parameter affecting steam demand and the value of unit thermal energy expenditures is bulk density (Fig. 6 and 7). It is significant due to the fact that the value of this parameter shows considerable variation depending on the type of raw material (Tab. 2).
29, 35, 42, 48, 54, 61, 67, 74, 80, 86, c [kJ·kgK-1] ponad
Zp [kg·t-1]
100
200
300400
500600
700
1,
1,
1,
1,
2,
2,
2,
30
40
50
60
70
80
90
100
ȡ [kg ·( m 3 )-1] (^) above
Fig. 6. Dependence of steam consumption (Zp) on specific heat capacity (c) and bulk density (ρn ) (mean values for 7 tested raw materials).
66 Ryszard Kulig
61, 82, 104, 125, 147, 168, 190, 211, 233, 254, c [kJ·kgK-1] ponad
Ec [kJ·kg-1]
100
200300
400
500600
700
1,
1,
1,
1,
2,
2,
2,
80
120
160
200
240
280
ȡ [kg ·( m 3 )-1] above
Fig. 7. Dependence of unit thermal energy expenditures (Ec) on specific heat capacity (c) and bulk density (ρn ) (mean values for 7 tested raw materials)
Figures 6 and 7 show that the increase in steam consumption and in unit thermal energy expenditures is directly proportional to the value of specific heat capacity of the raw material, and inversely proportional to bulk density. As a result, the dependences may be presented in the form of linear multiple regression models:
Z (^) p = 45.03 c – 0.063 ρu – 8.19; R^2 = 0.946 (4)
Ec = 159.31c – 0.187 ρu – 82.03; R^2 = 0.964 (5)
CONCLUSIONS
- Steam consumption in the conditioning process is affected by both temperature of hydro- thermal treatment and steam pressure. An increase in conditioning temperature from 50 to 90 o^ C results in an average increase in steam and heat consumption by 330%.
- Along with increasing steam pressure, the efficiency of heat and mass exchange during conditioning increases and, in consequence, the value of unit thermal energy expenditures decreases insignificantly.
- Among the characteristics distinguishing the treated material, specific heat and bulk den- sity have the most significant impact on steam consumption. The obtained dependences take a linear form.
- The lowest demand for heat is noted during the conditioning of ground wheat and maize (no statistically significant differences between the raw materials) and is on average 63% lower than heat consumption recorded for lucerne.
REFERENCES
Briggs J.L., Maier D.E., Watkins B.A., Behnke K.C. 1999: Effect of Ingredients and Procesing Parameters on Pellet Quality. Poultry Sci., 78: 1464-1471.
