Introduction

In this section, a detailed description of the distillation process used in the liquor production is clearly described with respect to the theory behind it, reasons why it was chosen as the distillation process and design approach. Distillation process can be categorized either as a batch distillation or continuous distillation. Batch distillation is used mainly in chemical (both biochemical and pharmaceutical) industries while continuous distillation is preferred majorly in separation of bulk chemicals such as petrochemical. Figure 1 shows the schematic representation of the two distillation process.

Batch distillation overview

Distillation is generally considered as a way of separating miscible liquids based on their varied physical properties and purifying the distillates based on waste or unwanted component removal (Rawlings, 2014). According to Simasatitkul et al., (2017), batch distillation consists of a process in which feed is loaded within the boiler while the distillates are recovered from the column top as shown in figure 2. The rationale behind the batch distillation method as a separation method is anchored on the distribution of components to be separated between the liquid and their corresponding vapor phases. At the point of boiling, the vapor phase is created for a particular component depending on it boiling point. During the process, all the heat generated within the reboiler by the coil will be applied in vaporizing the specific component at a particular temperature. It is assumed that no heat is lost during the process and at the same time, all the heat is channeled to only the component. Thus, as expected, the temperature stays constant during vaporization of a given component till all the liquid phase of that component have changed into the vapor phase. Nonetheless, the high temperature also makes the other components to evaporate even if they are not vaporizing yet, thus the vapor will equally be consisting of both the components though at varying proportion. As a consequence, the vapor becomes more of the volatile component (alcohol) while the liquid remaining within the reboiler becomes less of the volatile component Caldeira et al., (2017).

Just as mentioned earlier, distillation can be categorized as either batch or continuous distillation process. The preferences of either of the types is anchored on the unique advantages in one over the other depending on the desired outcomes as well as the chemicals to be separated. Batch distillation is preferred for separating small quantities of mixture with capacity which are smaller than required to justify the requirement of the otherwise expensive continuous distillation process. Secondly, batch distillation is flexible to handle a series of different feedstock resulting in a varied product range. Thirdly, batch distillation method is also attached to the possibility to obtain more than one product from a distillation process which effectively separates the products based on the characteristics. Batch distillation also makes it possible to attain different levels of purity from the same product with maximum elimination of fouling. In addition, batch distillation allows the separation of many products using only one column at a more convenient operation mode as compared to the continuous distillation (Kufer and Hasse, 2017). Nonetheless, batch distillation.

Batch distillation progress control

The separation of liquids in a batch distillation starts with a batch of liquid being charged at the reboiler followed by total reflux operation uniformity of the system. After the system has attained uniformity following total reflux, the overhead product portion is the withdrawn continuously with regard to the desired reflux policy (Rumpunen et al., 2015). The batch distillation method separates the liquids using column which acts as a section enrichment. Within the column, batch distillation column can be controlled through either constant reflux or constant overhead composition which is also called the varying reflux.

In a constant reflux, the overhead composition is varied by setting the reflux at a predetermined value which is then maintained throughout the entire run. In the process the overhead composition must vary in accordance to the changing distillate composition.
In constant overhead composition, on the other hand, varies the reflux by increasing the reflux amount returned to the column. As a consequence, the reboiler components is gradually depleted with time. The reflux ratio grows to maximum value where the receivers are then altered to minimize the reflux.

Batch Distillation Column Design

The fermented product to be separated and purified into alcohol of varying ABV is fed at the reboiler when the operation starts for the constant reflux. The heat content within the boiler is absorbed by the feeds making them to evaporate while generating vapor which then moves within the column till it reaches the condenser. At the condenser, the vapor is converted into liquid through the absorption of heat of vaporization from the vapor. The condensed liquid is collected at the reflux tank from where part of the distillate is redirected to the column in the form of a liquid reflux. The liquid reflux then falls through the column in a counter current manner to the rising vapor. The interaction of vapor and reflux liquid results in the mass transfer hence making the light components within the reflux liquid to rise with the vapor while the heavy counterparts fall together with the liquid.

Mass transfer consequently subjects within the column a profile of varying temperature, concentration and pressures zones. At the upper section of the column, temperature and pressure are relatively lower since the lighter components have a lower boiling points coupled with the fact that the vapor losses pressure as it rises the plate packing within the column. As a consequence, the less volatile compounds tend to concentrate at the column bottom while the more volatile counter parts settles at the column top (Ding et al., 2015).

 

Batch Rectifier

A batch rectifier consists of a reboiler within which heating element is located to provide the vaporization temperature necessary for fractional distillation. As the vapor condenses on the upper part of the column, part of the distillate is directed back to the column as a reflux. The remaining distillate sequentially feeds the receiver tanks. Figure 3 shows the schematic representation of batch rectifier. The batch rectification assumes that within the column, there was only ethanol (spirit) and water components at one atmospheric pressure operation. At this operation description, a maximum of 95% ABV can be achieved hence there was no need for further purification. Only dilution was therefore, a requirement to meet the desired ABV concentration.

            Liquor production optimization

The optimization parameters of the plant mass are linked to the energy balances through the integration of heat. The aim of this optimization in the overall liquor production plant was to reduce the plant operation cost by limiting energy use and correlated cost, yield efficiency increment and minimizing the loss of liquor through the waste from the plant. By maximizing the yield efficiency and reducing the energy use, the cost of production and operation will undoubtedly be comparatively lower hence making the production process as economical and as profitable as desired. In addition, by making the production process to be effective, efficient and produce a sufficient fuel liquor production plant do not just become economical and profitable but also produces a safe and standard liquor as per the desires of the clients as well as per the required standards of production.

            Base case for spirit manufacture from cider

In this liquor production process, spirit is produced from the fermented product following the brewing of cider as the base material. At the end of fermentation process, beer is separated from the solid spirit and water mixture using stripping at the beer column just before subjecting the mixture to distillation. At the beer column, spirit is concentration is about 60% to 70% ABV and is further subjected to the batch rectifier to give an azeotropic water and spirit mixture (Jana and Maiti, 2013). The azeotropic mixture is further subjected to a centrifuge where additional separation of solid particles is which are then collected together with the first components then evaporated to dryness. The resulting solid matter is rich in animal nutritional requirements and are thus sold off as animal feed. This makes the production cost be lower as compared to situation where they are disposed-off as waste.

            The design model

The design of the spirit production from cider was modelled through the use of equations that correlates the parameters especially the volumes of the liquor from the total volume from the original cider fermented. This design was based on the effective mass flow, mass flows of the respective components, fractions of the components and the corresponding temperature within the batch distillation network. Such are the parameters necessary for liquor production optimization. The following are the variable description as per the designed project;

F (Reboiler, Condenser) represents the total mass flow rate from the reboiler to the condenser in kg/s
Fc (j, Reboiler, Condenser) represents the j component mass flow from the reboiler to the condenser in kg/s

X (j, Reboiler, Condenser) represents the mass fraction of the j component between the reboiler and the condenser

T (Reboiler, Condenser) represents the temperature difference in 0C between the reboiler and condenser.

Note that j represents the components of the liquor produced and in this case, the components includes but not limited to starch, glucose, ethanol, water, lactic acid, acetic acid, proteins, glycerol, cell mass cellulose amongst other components. These components are sourced from either the base material or the fermentation or processing stages but are not removable within the waste elimination stages such as sieving and stripping (Harwardt and Marquardt, 2012). For instance, starch, glucose, proteins, cellulose are the products of the base material while cell mass comes from the yeast cells used to initiate the fermentation process (Johri et al., 2011). On the other hand, glycerol, acetic acid, fatty acid and ethanol are the major by-products of the brewing or fermentation process for cider. All these components are assumed to be water soluble with exception of starch, cellulose, oil, cell mass and ash. Gaseous components are not mentioned herein since they are allowed to escape or are rather trapped in different containers and thus are missing within the fermented cider in the form of liquid.

For every component j, component mass flow rate is correlated to the total mass flow rate by;

Fc (j, Reboiler, Condenser) = X (j, Reboiler, Condenser) * F (j, Reboiler, Condenser)

This implies that the total mass flow rate is the effective sum of the individual component mass flow rates. Thus;

F (Reboiler, Condenser) =

            Mass transfer or conversion processes during fermentation

Fermentation process can be explained using equations showing how the individual components changes from time to time and from one component to another till ethanol and water mixture are obtained (Kumar et al., 2013). The following are the major equations relating to the conversions;

Glucose to ethanol conversion equation;

By balancing the masses, it implies that for every one kg of glucose, about 0.5114 kg of ethanol and 0.4885 kg of carbon (iv) oxide gas will be produced. Part of the gas produced will be used in further reaction while the remaining part will be eliminated.

Glucose conversion to acetic acid equation;

While the mole ratio for glucose to acetic acid is 1:3, the mass of the three molecules of acetic acid produced will be equivalent to the mass of the glucose used in making them.

Glucose conversion to succinic acid equation

From the mole ratio, every 1 kg of glucose combine with 0.4885 kg of carbon (iv) oxide gas to produce 1.3191 kg of succinic acid while liberating about 0.1776 kg of Oxygen. There are several equation relating to a series of conversions that occur within the process of fermentation most of which are ignored herein.

McCabe Thiele Graphical Analysis

This analysis technique is necessary for the understanding and comprehending the compositional based changes that occur at the distillation column as separation continues (Mayer et al., 2015). McCabe Thiele method provides a faster and equally easier solution to the binary distillation problem. The method is anchored on the on the material compositional change with regard to the equilibrium line (Wang et al., 2016). The equilibrium line is used to show the composition of the materials variation beneath and over the plates. The McCabe Thiele operating line was plotted on the same graph as the equilibrium line to enable the determination of the equilibrium stages number by the graphical construction. For the spirit (ethanol) water mixture within the distillation column, figure 4 shows the McCabe Thiele graph used to determine the equilibrium stages. The operating line equation was developed from;

Where xD is the distillate composition while R represents the reflux

R is defined as the ratio of flow returned as reflux to the flow of the top product taken off. The rectification column operates at the line which depends on variation of R, hence the required number of equilibrium stages is equally dependent on R. For poorly insulated distillation column, the effective reflux ratio may be greater than the R value (Jana, 2017). This was the basis of lagging the column to enhance the efficiency of liquor production. Irrespective of the design, reflux always falls within the two extreme ends, namely total reflux and minimum reflux. Total reflux occurs when there is no uptake of the products as well as no addition of feeds coupled with all condensate being returned to the column. A relatively fewer stages are required in in the separation process when dealing with total reflux design (Jana, 2017). Fenske equation can be used to calculate the minimum number of stages required for the total reflux where all vapor is assumed to be condensed and then returned as liquid. Fenske equation is given by;

On the other hand, minimum reflux occurs when at least two operating lines have their intersection within the equilibrium curve as shown in figure 5. When the feed is a liquid at the point of boiling, the minimum reflux can be calculated as follows;

Optimum Reflux Ratio

For optimum operation, it is necessary to first understand the effect of varying R value on the required number of plates and the corresponding production cost. Increasing R for instance, makes the diameter of the column bigger and thus equally reduces the capital cost. As a consequence, the number of plates gets smaller with comparatively higher heat exchange to increase the boiling and condensation. On the other hand, decreasing R calls for more stages and corresponding higher capital cost though with relatively less condensation and boiling. Figure 6 shows the variation of reflux ratio as a factor affecting the total cost, operating cost and fixed cost (Penniston et al., 2018).

Production related calculations

In this section, the required calculations are done with respect to the desired liquor output. Third of the amount received from the fermenter was juice and the remaining two thirds were to enter the distillation column. With known composition of the desired alcohol by volume, the original volume of the mixture can be modeled in such a way that the desired parameters are manageable. Suppose that A represents the concentration of the vapor, B represents the concentration of the mixture within the reboiler and X represents the concentration fraction of the most volatile liquid (alcohol) within the reboiler, while its corresponding concentration fraction within the vapor is represented by Y, then;

Since changes within the reboiler as time increases also affects the changes in the vapor concentration. For instance, when distillation process proceeds the concentration of the liquid within the reboiler reduces while the corresponding concentration within the vapor increases. This can be represented mathematically as;

Hence, substituting in the above equation and expanding gives,

When the above immediate equation is rearranged, the following equation emerges;

Note that in the above equation, the subscripts f and i represents the final and initial conditions respectively for liquid within the still reboiler. The average distillate composition Xavr is given by the overall component as;

From the given situation, about 10.3/100 alcohol by volume approximately 95/100 by mass escapes to the top of the distillation column This forms 80/100 alcohol by volume in the distillate product. Therefore, it can be assumed that in the above equation, Bi is 0.95, Bf is 0.8, Xi is 0.103 while Xf is zero. Hence, the average distillate composition Xavr during the first batch of distillation is given by;

= 0.6523

The model was designed to carry out distillation with approximate 80% ABV spirit after which the desired concentration can then be diluted to form a saleable product at around 40% ABV. To calculate the required number of plates within the distillation column to be able to carry the distillation as per the planned concentrations, the following Fenske equation was used;

Note that within the Frenske equation given above, subscripts 1 and 2 represents the corresponding concentrations of the most volatile liquid (alcohol) and the less volatile liquid (water). Hence if X1 is 0.103 then this implies that X2 will be equal to 0.897.

n+1 = 3.4439

n = 3.4439 – 1

n = 2.4439

Since n must be a whole number this can be approximated to be 2.

Component calculations

The variation of the components within the liquid was also subjected to calculations based on the concentration fraction to determine their corresponding variation during the distillation process and at the end of the distillation process. The calculations assumed that the residual components would all separate into the bottom product as they have a much boiling point and will therefore evaporate out of the solution into the bottom stream.

To be continued

 

 

 

 

 

 

 

 

 

 

Appendix A: Figures

Figure 1: Schematic representation of distillation process types; Batch distillation a) and continuous distillation b) (retrieved from Bortz et al., 2017)

Figure 2: Batch distillation feed and distillate flow

Figure 3: Batch rectifier schematic diagram

Figure 4: McCabe Thiele graph

Figure 5: McCabe Thiele graph with two operating lines intersecting at the equilibrium curve

Figure 6: Reflux ratio optimization

 

 

 

 

 

 

 

 

 

 

 

 

 

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