The flash tank is an essential part in any scheme used for the alkalization of cane juices, in which the purification reactions end and the formation of the flock of tricalcium phosphate and other products begins, in the midst of continuous flashing of gases, air and steam of water.
analysis-productive-alkalization-juice-mixed-tank-flashThis simultaneity of operations make up a different and therefore complex sui generis process; This duality of functions makes its technological importance decide on the final results of the purification process as well as on the factory efficiency.
Despite its importance, the information available about the flash effect is really scarce and this limits any initiative of the mill to correct, improve any design or even design a new tank.
This work is intended through a detailed review to provide the basic engineering of a simple, practical and above all safe system that can be used to make modifications to an existing tank to correct problems during operation or simply to make a new design if it were necessary to assume the variations that occur during the operation without affecting the clarification of the juice or the quality of the sugar to be produced.
Key words: Flash tank, alkalization, clarification, flash effect.
ABSTRACT
Flash tank is important piece of any alkalinization procedure, complex processes are going to be happening stable and continuously inside it to conditioning the juice properly before being fed to the clarifiers so all aspects with its construction, design and of course with the operation they will be crucial for the clarification of cane juices.
The information about “flashing effect” into the sugar process are reduced and insufficient; references to indices of capacity, retention time as well as diameter and height relationship for the most common designs are found in literature, this paper will be focused on the mechanisms that occurred during the process of flashing and would improved a practical, simple and safe method as a valuable tool for any modification, improvements or simply to solve any operating troubles in order to make more efficient the clarification process of raw sugarcane juice.
Keywords: Flash tank, alkalinization, clarification, flash effect.
Introduction
The flash tank is the intermediate point of any procedure used for the alkalization of the mixed juice; that according to "Pedrosa Puertas, chapter 3" purification of juices "(1975): pursues with its two basic agents: lime and heat, producing clear, transparent juices and free of suspended solids, through complex reactions and different processes to put them at rest in a suitable clarifier ”.
Complex processes are going to be stably and continuously taking place inside the flash tank to be able to properly condition the juice before being fed to the clarifiers, therefore the aspects related to its construction, design and, of course, the operation will be decisive for the final result of clarification of juices.
Any modification in the alkalization technology, in the heating of the juice or in the clarification capacity will require certain adjustments or modifications in the flash tank so that the efficiency in the clarification process is not affected, however there is no system or calculation method, allowing the modifications made by the mills to be supported by the basic engineering principles of the flash effect applied to the alkalization schemes for cane juices.
Despite the fact that the information available about the flash effect in cane juice alkalization schemes is so important, we only find references about capacity indices, retention time and diameter-height relationships, for the most common or traditional designs. thus limiting any initiative to introduce improvements, modifications or simply try to solve operational problems that make the clarification process more efficient, based on the precept set forth by “Webre (1949) that: clarification is the most important step in the manufacture of sugar”.
For this reason, this work is focused on studying the flash effect in the cane juice alkalization schemes with its mechanisms in order to establish a practical and simple method that can be used as a guide and a mandatory reference to make any modification, extension or even to design a new flash tank adjusted to the conditions of each mill if necessary.
Figure 1 Vertical flash tank.
To carry out this work, we have reviewed the existing literature as well as designs, some modified over and over again until reaching the best operating condition to satisfy the demands of a higher hourly grind or to achieve higher levels of juice quality. In this universe of designs we have classified the tanks in horizontal or vertical according to their arrangement, the horizontal ones despite offering a greater flashing surface with the same geometry turn out to be the least extended, which is why the vertical design with centralized feeding for the development of this work.
Materials and methods
The quality of the sugar and the factory efficiency are linked to the alkalization process, in which a set of chemical reactions between Ca2 + and the impurities present in the juice, will be carried out in the middle of stepwise heating processes until reaching a temperature in the range of 103 to 106 ⁰C in order to obtain the highest possible degree of removal of these non-sugars; deliver a clear, transparent and free of suspended solids juice to the evaporators.
The flash tank is right in the middle of this complex alkalization process, as shown in Figure 2; all the chemical purification reactions initiated in the mixed juice tank will end within it, but simultaneously the process of formation of the flock of tricalcium phosphate will begin, to separate the clear juice in each of the trays or cells of the clarifiers; This simultaneity of operations makes the dynamics within it probably the most complex within the process and perhaps also the least known.
The flash effect is widely used in the chemical industry for vapor recovery in condensate systems, in multiple effect evaporators and even in the recovery of certain components in binary systems, but the most complex application is that presented in the diagrams. of alkalization of sugarcane juices since, in addition to the separation of air occluded in the juice, gases and water vapor, it will be the end point of a reactor where successive transformations will be continuously occurring during operation.
An appropriate tank must allow the degassing of the juice, the flashing of steam as well as the formation of the floccule to be carried out in a perfect balance to assume the hourly grinding in accordance with the quality of the cane that is ground in the mill; For this, any design must meet the following technological requirements:
- Sufficient flow area to facilitate the exit of the occluded air, gases from the alkalization reaction plus all the steam that produces the flashing effect without dragging of juice or cachaça. Vent pipe with sufficient flow area to conduct the flashed flow into the atmosphere and prevent the tank from operating under pressure. Exit of the juice to the clarifiers; dimensioned in such a way that it is evenly distributed in each one of them in laminar regime. Flashing plate with sufficient area to facilitate the exit of gases, steam and aid in floc formation. Volumetric capacity adjusted to the retention time necessary for the flocculation reaction ends within the tank.
Parameters Values Formulas
Fv = Steam flow. 2.51 ton / hr Ec.1
Fj = Alkalized juice flow. 500.00 ton / hr
T2 = Tempe. heater output. T1 = Tempe. of flashing the juice. 103 ºC
100 ºC
Cp = Caloric capacity of the juice. 0.90 Kcal / kg. ºC Fjf = Fj − Fv Ec.2
= Latent heat of vaporization. 538.90 Kcal / kg
Fjf = Flow of cold juice. 497.49 ton / hr Ec. 3
Hv = Average evaporation rate. 185 to 215 kg./hr.m2
Af = Flow area in flash tank. Af1 = 12.85m2
Table 1 Material balance for a flash tank.
Defining the operating conditions right at the entrance of the flash tank will be the first step to establish a balance of mass and energy that allows us to identify the variables involved in this complex process in which as a result three basic equations necessary to calculate the diameter of the flash tank in the particular operating conditions of the mill, which are shown in Table 1, note that in Eq. 3 the term Hv is included, this is nothing more than an index of the measurement of the evaporative mass flow allowable means per unit area for flashing to occur without juice entrainment, according to Oliver Lyle, chapter 13,"Efficient use of steam" (1956) and that in practice is the one who will give us the assurance that the vapor bubble plus the gases produced leave the juice within the flash tank without any dragging of juice or cachaça.
The most obvious sign that the flash tank is not operating properly can be seen in the vent tower or chimney when the mixture of gases with water vapor draws juice and cachazo through the tube as a consequence of a deficit in its flashing area. for those specific operating conditions. This situation, in addition to producing considerable losses in sugar, will prevent the flashing process from ending inside the tank, moving towards the feed chamber or first clarifier tray; where continuous flashing in this area designed and built for decantation is probably the main cause that will determine that the juices in the first tray will run with bagasse and small particles of cachaça in suspension.
The only possible way to avoid this problem is to achieve that the pressure losses through the flashing tube are minimal, for that reason the steam speed cannot exceed two m / sec, according to Falcón F.; Esturo C et al. Page. 104 "Juice purification" (1995). Taking this condition as a limitation, using the results obtained in Eq.1 and making the necessary mathematical adjustments, Eq.4 in Table 2 is obtained, which is what will allow us to obtain the diameter of the vent tube adjusted to the conditions of operation; It is necessary to point out that the results that we will obtain are referred to conditions that set a minimum limit indispensable for normal operation.
Ve = Specific volume of steam. 1,673 m / ton
Vv = Steam speed. 1.5 to 2.0 m / sec Ec.4
Ф = Diameter of the vent tube. Ф1 = 0.923 m
Parameters Values
Table 2 Estimation of the flashing tube diameter.
The distribution of the juice to the clarifiers is cardinal in the operation of the process, since if the system is not adequately dimensioned, some clarifiers will be filled faster than others, which will inevitably induce an imbalance in the flow of clear juice and there will be no retention desired juice inside the flash tank itself, on the other hand, keep in mind that the flashed juice that flows to the clarifiers is a mixture whose variable composition can exceed 50% by weight of solids thanks to the presence of mud in rain, earth or sand in addition to the tricalcium phosphate flocs, an aspect that must be taken into account when estimating the diameter of the juice as well as the slope of the distribution lines towards each of the sections of the clarifiers.
Parameters Values Formulas
HP = Pumping power. HP = 0
∆N = Hydrostatic head.
∆P = Pressure difference. ∆P = 0
HV = dynamic losses. HV ≈ 0
HD = friction losses. HD = 3.90 feet
HDn = Head loss per section.
hf = Friction loss factor for every 100 feet of Let. Let = Total equivalent length, includes the straight section plus all installed accessories.
Leq = Lr + Lac
Eq.5
Eq.6
Eq.7
Eq.8
Table 3 System of equations for the flow balance in the distribution of flashed juice.
In order to estimate the diameters we will establish a balance between the operation level of the flash tank as the starting point and the farthest clarifier as the end point, for this the general equation of the dynamic balance of fluid flow will be used; Eq. 5 of Table 3 and that when evaluating its terms between these points is transformed into Eq. 6 where it is clearly and strictly defined that: “The available hydrostatic load must always be greater than the losses produced by that system so that to each of the clarifiers reaches the amount of juice according to its capacity ”. As an example we have started from a system with three clarifiers of equal capacity distributed uniformly as indicated in Fig. 3.
Using the empirical equation proposed by William and Hazen, p. 27 Cameron Hydraulic Data for the estimation of the losses produced by the flow (hf) through the piping system and taking as a limitation a flow velocity in the range of two to four feet / sec. It is obtained that the minimum hydrostatic load has of being four feet.
Little is said about the flashing surface, however the main purpose of a flashing plate or surface is to facilitate the self-evaporation process as well as to contribute to the conditioning of the tricalcium phosphate flocculation always before the nucleation process that will take place with the addition of the anionic flocculant so that its decantation is much more agile and efficient within the clarifier; This plate is found in the most common designs of clarifier manufacturers, its shape and location is varied, from using the inner section of the tank itself or deflectors in tangential feeding, perforated mesh or simply a plate. A dish that should have an area not greater than 30% of the total area of the flash tank where it will be installed will be considered for this work.some designs feature straight or helical baffles, probably seeking greater retention.
Although the opinions regarding what should be the optimal retention time of the juice inside the tank are varied, all have as a common factor: "the quality of the juice that is processed by the mill", so for example in Cuba the retention time it was around one minute, as indicated in the Indexes of capacity for Cuban oil mills (1971), which is equivalent to a volume of around 25 ft3 per 1,000 TCM, however, in the literature there are some reports whose values reach up to five minutes, however, and only for this work we have considered that a time of two minutes is sufficient for all gases, air and steam to flash out of the juice before leaving the tank, which is basically the beginning which should take precedence when making any modification or designing a new tank.
Once the retention time has been defined, for our example it will be two minutes and knowing the diameter of the flash tap, we proceed to estimate the operation volume as indicated in Eq. 9, so with these data, the geometry or design of the tank and using mathematical algorithms commonly used to measure boiler house equipment, the operation level will be calculated considering the straight section and the lower cone as indicated in Eq. 10 of Table 4.
Parameters Values Formulas
QJ =
Alkalized juice flow 500.00 ton / hr 473.00 m3 / hr
2,100 gpm V = τ QJ
Τ = Retention time 2 min.
V = volume of juice retained 15.68 m3
HJ = Operation level of the tank 1.58 m HJ = STRAIGHT SECTION + CONICAL HSECTION
Ф = Diameter of the flash tank. 4.04 m
Ec. 9
Ec. 10
Table 4 system of equations to calculate the operation level of the flash tank.
Finally, with this procedure, the calculation system for a flash tank adjusted to the mill's own operating conditions has been structured, to know if the results it offers are reproducible and safe, we will establish a comparison with established methods that have traditionally been used in the sugar cane industry.
Results and Discussion
For our purpose, we will take as a point of reference a mill of about 10,000 TCD of capacity whose hourly grinding will continuously send about 500 ton / hr (2,100 gpm) of juice mixedIV with 14.5 ºBrix, to the fractional alkalization system in which the juice after being pre alkalized in the tank for the mixed juice is heated in two steps until reaching a temperature of 103 ºC at the outlet of the rectifying heaters, as indicated in Fig. 1, then substituting these data in each of the corresponding equations is They will obtain the necessary parameters to define the capacity, flow area and height or level of operation of the flash tank for this grinding.
Source Feet per TCH
Capacity Index 0.150
Door 0.200
LA Tromp 0.250
F. Falcón 0.330
Senén Diego 0.330
Fletcher & Smith 0.393
SUGARTECV 0.400
The flow area demanded by this tank will be
Af1 = 12.85m2, she represents a capacity index of 0.330 ft2 by TCHVI, in fact a consistent result in accordance with the indices that are usually managed as shown in Table 2 and the diameter of the flash tap similarly within the range of values that They are reported in the literature by traditional equipment manufacturers and renowned authors. Furthermore, the prediction of the diameter of the flash tank by this system has a behavior very similar to that suggested by important designers and equipment builders, as shown in graph 1, so that Even though its results are conservative, they will be "reliable and safe above all else", it is important to emphasize that the system seeks the "minimum necessary technical conditions".
Graph 1 Behavior of the diameter of the flash tank Vs flow of alkaline juice by three systems.
IV It is considered that the flow of alkalized juice represents 120% in cane.
V For vertical tanks with a fixed height of 2.00 m. VI
TCH: tons of cane ground per hour, hourly ground.
The diameter of the vent tube is as or more important than that of the main tank itself because through it the flow of all gases and steam from this complex process will go out into the atmosphere in order to avoid pressure within the main body. which means that it works "without pressure". Table 3 shows that the minimum diameter necessary for the vent pipe under these conditions will be Ф1 = 0.923 m (36 in), the dimensionless relationship between the Vent / Tank diameters that is obtained will be equal to 0.23 which is what which is usually handled, being another very positive element in the comparison that we are establishing.
The flashing area is estimated as a function of the diameter of the main tank, in this case it is considered that 30% is adequate to facilitate both the flashing process and contribute to floc formation. Its preferential location in the center of the flash tank, some works carried out by sugar mills locate it with a certain displacement but always conserving the proportions indicated above and leaving space to create a sufficient area so that the rest of the gases and vapors emanate from the juice. A report from an online flocculator located just after the flash tank reports positive results in floc formation before entering clarifiers.
The level of operation in some cases debatable, it is logical that this is the case because the constant dynamics inside the tank makes it difficult to measure it and even its physical appreciation, but more than that it is an obligatory and necessary reference to know what the hydraulic load available for the distribution of the flow, only from it it will be possible to estimate with better accuracy what the required diameter should be in each of the flashed juice distribution sections, as indicated in figure 3 where we have considered by way of example an operating mill with three clarifiers of equal capacity for this grinding.
Due to the importance that this aspect has, it is advisable to check the distribution of pipes taking into account the clarification capacities in order to check if the installed diameters correspond to those calculated and also during the disassembly and repair inspection, inspect each line to decide if requires some special work or just be replaced.
Figure 4 shows a tank of vertical and horizontal design with the dimensions that have been obtained from this simulation and although both cases respond to these specific conditions, it happens that during daily operation these are usually modified for this and to know how This design will respond to variations in the hourly grinding or the temperature of the juice at the outlet of the rectifying heaters. We will carry out a new analysis; now taking into account an increase in the hourly milling of 10% as well as two degrees in the temperature at the outlet of the rectifier heaters, then we recalculated again using this data of "peak" values and verified the results in order to know how the design will behave before these extremes.
Figure 4 Flash tank with central power.
The results presented in Table 5 are very eloquent, the body of the tank is capable of withstanding a “grinding peak” of up to 10% since the speeds do not vary much and if it is punctual it will recover its normal operation. On the other hand, it will not react the same way to a "two degree peak in temperature" since the area demand is above 50%, its recovery probably slower and therefore more damaging to the process. This simulation has illustrated how the tank will react to the normal variations that take place during the operation in the harvest, therefore it must be dimensioned to assume these variations without affecting the processes that occur in it so that the appropriate selection of values and their ranges will be of utmost importance to obtain the minimum necessary conditions.
Diameter 500 ton / hr 550 ton / hr 500 ton / hr
103 oC 103 oC 105 oC
Flash tank 12.85 m2 14.13 m2 10% 21.41 m2 66%
Vent pipe 0.923 m 0.960 m 5% 1.12 m 21%
Table 5 Behavior of the tank under peak conditions.
Conclusions
The quality of the sugar to be produced as well as the factory efficiency will be linked to the juice alkalization processes, the majority of which start in the mixed juice tank and have the flash tank as an intermediate point where they will end all the reactions between Ca2 + and the non-sugars present in the juice while the formation of the tricalcium phosphate flocculation begins; therefore, it is necessary that this intermediate point between chemical reactions and the decantation of their products is adjusted so that there is a balance between these two essential operations as the only way to maintain high levels of efficiency in the particular conditions of each process.It is very important to emphasize that sometimes we make certain arrangements in the area in terms of capacity or modifications in the steam use scheme to increase the temperature of the juice at the outlet of the rectifier heaters but we forget the flash tank, it is then that we appreciate that the result of those works and efforts can be doubtful and inconsistent just for the simple fact of not having considered the importance that the flash tank has.
The most appropriate flash tank will be the one capable of efficiently assuming grinding peaks and increases in the temperature of the juice without prejudice to the operations carried out within it, hence the importance of the adequate selection of the data and parameters to be used to any work that is intended to be undertaken or even for a new design.
It is essential to note that any design of flash tank must operate without pressure, for this the vent tube and the flashing area of the tank must be perfectly balanced in capacity and retention time, regardless of any opinion in this regard, as it is the guarantee that the flashing of gases, steam and air occurs within the tank and never in the feed chambers of any clarifier, thus avoiding one, if not the main, cause of the presence of bagasse or other insoluble particles in suspension in the first clarifier tray trays.
Through this work, the flash effect in the juice alkalization schemes in the sugar cane industry has been studied and this has helped us to obtain a simple, practical and safe calculation method, with which any device can measure the capacity of your installed flash tank regardless of its design and, if necessary, carry out the necessary renovations or modifications with a technological support that will avoid successive errors or simply ignore important details, making the works that are carried out offer consistent results that result in a greater factory efficiency with superior quality in the type of sugar to be produced.
Bibliographic references
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