Continuous Distillation in Tray Columns

A typical industrial installation for cachaça production is shown in Figure 3.18a. The column is divided in a small rectifying section, composed of two or three trays, and a stripping section, composed of 16 to 18 trays. In contrast to the production of hydrated ethanol, in cachaça distillation there is no side stream for removal of high alcohols (propanol, isopropanol, isobutanol, and isoamyl alcohol). The column is operated with a small refl ux ratio, whose required value is slightly infl uenced by the alcoholic graduation of the wine fed into the column. A larger alcoholic con- centration in the wine decreases the refl ux ratio required for attaining the product specifi cations. The heat source is steam, which in some plants is directly injected at the bottom of the stripping section as “live” steam, so that the use of a reboiler is not

Condenser Condenser 1 Condenser 2 Reboiler Reboiler 1 1 19 21 19 21 Wine Vapor Degassing Wine Stillage (a) (b) Cachaça Stillage Cachaça Liquid return

FIGURE 3.18 Typical industrial confi guration for continuous cachaça production (a) with- out degassing and (b) with degassing.

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always necessary. Nevertheless, in order to reduce the generation of waste products (stillage or vinasse), the best option is to use indirect heating with a reboiler, as is usual in conventional distillation plants.

Practically all ethanol fed into the column is recovered in the distilled stream, being admitted a maximum ethanol content of 0.02% in the bottom product, which corresponds to a loss of approximately 0.3 to 0.6% of the total ethanol amount and usually represents the main source of alcoholic loss in the process. When a stricter control of volatile components in cachaça is required, the degassing process can be a good alternative. This procedure consists in the use of a series of partial condensers in the top of the distillation column, where the vapor portion of each condenser is fed into the following condenser, and the condensed phase of each condenser is returned to the distillation column. At the last condenser of the series, the vapor portion is eliminated through the degassing stream, taking away the major part of the volatile compounds. Figure 3.18b presents the degassing scheme used for this work. As can be seen, only two condensers were used; however, the number of condensers is not limited to this number, with the possibility of using multiple condensers. It should be noted that the degassing factor can be expressed as the ratio of total fl ow of degassing stream to the sum of the fl ow of cachaça and the fl ow of the degassing stream.

The control of acetaldehyde concentration is a good example of the degassing function. This component can easily oxidize to acetic acid during the storage time, increasing the cachaça acidity. Knowing that the volatility of the acetaldehyde is extremely high, making possible the concentration of this component in the top of the distillation column, an increase of the degassing stream can eliminate the major part of the acetaldehyde present in cachaça, minimizing the previously mentioned problem. Because it is used only for product quality control, the value of the degas- sing stream is always very low in order to avoid signifi cant ethanol losses.

The industrial process for continuous cachaça production was simulated using the commercial simulator ASPEN Plus [14]. For this simulation the wine was slightly changed, decreasing the ethanol concentration to 0.0645 g/g, an alcoholic gradua- tion of 8.0 ºGL, but keeping the concentration of all minor components to the values given in Table 3.4. The water content was increased in the exact proportion that the ethanol concentration was reduced. In a fi rst set of simulations, without degassing (Figure 3.18a), the infl uence of the distillate rate and refl ux ratio on the sprits’ alco- holic graduation and on the ethanol loss in the stillage was investigated. The refl ux ratio was varied in the range of 0.001 to 1.5 and the distillate rate from 1000 to 2000 kg/h. The feed rate was fi xed at 10,000 kg/h.

According to Figure 3.19, for higher distillate fl ows, the alcoholic graduation is lower, but still within the range required by legislation, and the refl ux ratio has no infl uence on the distillate concentration. For lower distillate rates, a higher refl ux ratio increases the spirits’ alcohol concentration, even above the required limits. The range of infl uence of the refl ux ratio depends on the distillate rate, being the largest in the case of the lowest distillate rate. The reason for this behavior can be better understood on the basis of Figure 3.20, which shows the loss of ethanol, expressed in terms of that part of the ethanol stream fed into the column that is lost in still- age, as a function of distillate rate and refl ux ratio. As can be seen in this fi gure, for lower distillate rates very high ethanol losses, much above the suggested limits (0.3 to 0.6% of the ethanol amount fed into the column), can be avoided only by

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75 70 65 60 55 Alcoholic graduation / °GL 50 45 40 35 0.0 0.3 1000 kg/h 1200 kg/h 1400 kg/h 1500 kg/h 1700 kg/h 1900 kg/h 2000 kg/h 0.6 Reflux ratio 0.9 1.2 1.5

FIGURE 3.19 Cachaça alcoholic graduation as a function of refl ux ratio and distillate rate.

40 35 30 25 0.0 0.0 0.3 0.6 0.9 1.2 1.5 0.5 1.0 1.5 2.0 2.5 3.0 20 E thanol lo ss / % E thanol lo ss / % 15 10 5 0.0 0.3 0.6 Reflux ratio Reflux Ratio 1000 kg/h 1200 kg/h 1400 kg/h 1500 kg/h 0.9 1.2 1.5

FIGURE 3.20 Ethanol loss in stillage as a function of refl ux ratio and distillate rate.

large refl ux ratios. This means that only spirits with high ethanol concentration will require higher refl ux ratios in order to avoid signifi cant ethanol losses. In fact, taking into account the alcoholic graduations required in the cachaça production, refl ux ratios within the range 0.001 to 0.2 are suffi cient.

Figures 3.21–3.23 show the concentration of minor compounds in the distillate (cachaça). Except for acetic acid, the refl ux ratio has a very low infl uence on the

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minor components’ concentration in cachaça, and for this reason their concentration values are represented only as a function of the distillate rate. The concentrations of light components, such as acetaldehyde and ethyl acetate, decrease for large distillate rates. A similar behavior was observed for the superior alcohols.

160 140 120 100 Conc en tra tion in c achaç a / mg/kg 80 60 40 1000 1200 1400 1600 Cachaça mass flow / kg/h

Acetaldehyde Ethyl acetate

1800 2000

FIGURE 3.21 Acetaldehyde and ethyl acetate concentrations in cachaça as a function of distillate rate. Conc en tra tion in c achaç a / mg/kg

Cachaça mass flow / kg/h 1000 2000 1800 1600 1400 1200 1000 800 600 1200 1400

Total superiors alcohols Isoamyl alcohol

1600 1800 2000

FIGURE 3.22 Isoamyl and superior alcohols concentrations in cachaça as a function of distillate rate.

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In the case of ethyl acetate the concentration in the distillate is always below the legislation limits (see Table 3.3), but in the cases of acetaldehyde and superior alcohols the values seem to be above the required limits for the lower distillate rates. Nevertheless, taking into account the corresponding alcoholic graduation of cachaça and the required refl ux ratios in order to avoid high ethanol losses, even for low dis- tillate rates the legislation limits are not exceeded.

Acetic acid concentration in cachaça increases with the distillate rate and decreases with the refl ux ratio, a behavior usually obtained for heavier components, as is the case of this acid in spirits distillation. The limits required by legislation are easily met for this minor component in all simulated cases (see Figure 3.23).

As indicated in Table 3.3, the legislation strictly defi nes limits for the concentra- tion of minor components, especially for methanol and acetaldehyde. As already explained, these limits are easily met in the case of methanol, provided that the presence of pectin is avoided during the must fermentation. For instance, in all pre- viously simulated cases, the methanol concentration in cachaça was not higher than 1.68 mg/kg, well below the legislation limits. In the case of acetaldehyde it is surely more diffi cult to produce a spirit within the legislation limits. As a consequence of its very high volatility, acetaldehyde will doubtless concentrate in the distillate, so that a higher concentration of this component in the wine means necessarily a risk of exceeding the maximum allowed limit. Besides its deleterious direct effect on the product quality, acetaldehyde can also easily oxidize to acetic acid, increasing the spirits’ acidity.

The effect of acetaldehyde concentration in the wine will be further investigated. A degassing (vapor phase) stream can be used for controlling the presence of light components. This was investigated for a selected case of the prior simulation set,

180 160

Acetic acid concentration / mg/kg

140 120 100 80 60 40 20 0 0.0 0.3 0.6 Reflux ratio 2000 kg/h 1900 kg/h 1700 kg/h 1500 kg/h 1400 kg/h 1200 kg/h 1000 kg/h 0.9 1.2 1.5

FIGURE 3.23 Acetic acid concentration in cachaça as a function of refl ux ratio and distillate rate.

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namely for a distillate rate of 1500 kg/h and refl ux ratio of 0.2. To produce different degassing fl ows, the temperature of condenser 2 was varied from 293.2 to 353.2 K. At the lowest temperature, little degassing was produced, and the opposite effect was observed at the highest temperature. In this way, it was possible to investigate the infl uence of this stream on the acetaldehyde concentration and on the ethanol loss.

Aiming to help in the control of the volatiles’ content in the spirit, the degassing stream can be used when the original concentration of those compounds in the wine leads to a distillate composition in disagreement with the legislation limits. Taking into account the usual content range of acetaldehyde in the wine (see Table 3.1), we increased its content to 26 mg/kg.

In this set of simulation cases, a further component was included in the wine composition, namely carbon dioxide. This compound is important for evaluating the performance of the degassing process, represented by the degassing stream. Carbon dioxide is produced during must fermentation, and it could carry part of the gener- ated ethanol away, increasing the product losses. In order to avoid such losses the industrial fermentation process is performed in a closed vessel and the outlet gas stream is pumped into an absorption column used for recovering the volatile com- ponent. The industrial fermentation vessel is operated at temperatures about 305.2 K and under a slightly positive manometric pressure (6.0–8.0 kPa). Assuming that the light phase inside the vessel is composed of gas saturated with ethanol and water and considering that this gas is, for practical purposes, pure carbon dioxide, the solubility concentration of CO2 in the wine can be easily estimated. Using the NRTL param- eters for ethanol–water interactions and the CO2 Henry constants in ethanol–water solutions given by Dalmolin et al. [59], a solubility around 1100 mg CO2/kg of wine (8.0 ºGL) was estimated.

Using these values for acetaldehyde and carbon dioxide, the water content in wine (see Table 3.4) was correspondingly diminished, and the new composition was used as feed stream in this set of simulations. Figure 3.24 shows the change of acet- aldehyde composition in cachaça as well as the loss of ethanol through the degassing stream as a function of the degassing percentage.

As can be seen in Figure 3.24, the degassing stream makes it possible to control the acetaldehyde concentration in cachaça, but it increases the ethanol loss in the dis- tillation process. Taking into account the alcoholic graduation of cachaça obtained in this case (see Figure 3.25), the maximum allowed limit for acetaldehyde concen- tration, given in Table 3.3, corresponds approximately to 167 mg of acetaldehyde/kg spirit, a value that is obtained using a degassing stream of 0.7% (10.7 kg/h). The cor- responding loss of ethanol is 0.58%, which should be added to the value of loss in the stillage. Although the corresponding impact on the product alcoholic concentration is not signifi cant (see Figure 3.25), the estimated loss of ethanol can attain values larger than the loss obtained in the stillage. For this reason the use of a degassing stream for controlling the volatile concentration in the product is appropriate only in cases when the concentration slightly exceeds the legislation limits. Figure 3.25 indicates that the concentration of other volatile components, for instance ethyl acetate, also decreases.

If the concentration of volatiles is large, an alternative equipment confi guration is required. This scheme is shown in Figure 3.26. Columns A and B correspond to the stripping and enriching sections of the prior scheme, respectively. In column A

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Degassing / %

Ethyl acetate concentration / mg/kg

Alcoholic graduation / °GL 52.1 52.0 54 52 50 48 46 44 42 51.9 51.8 51.7 51.6 51.5 0.30 0.45 0.60 0.75 0.90 1.05 1.20 °GL cachaça Ethyl acetate

FIGURE 3.25 Cachaça alcoholic graduation and its ethyl acetate concentration as a func- tion of degassing factor.

1.5 1.2 0.9 0.6 0.3 Ethanol loss / % Acetaldehyde concentration / mg/kg 0.0 0.30 0.45 0.60 0.75 Degassing / % Ethanol Acetaldehyde 0.90 1.05 1.20 180 176 172 168 164 160 156

FIGURE 3.24 Acetaldehyde concentration in cachaça and ethanol loss as a function of degassing factor.

ethanol is stripped away from the liquid phase, so that the ethanol loss in the stillage is very low. In column B ethanol is concentrated up to the desired spirits gradua- tion. Columns A1 and D are used mainly for concentrating the light components, so that a small stream of distillate at the top of column D allows the control of vola- tile components’ level in cachaça. This byproduct stream is named second alcohol

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Feed Second alcohol Cachaça Stillage A1 A D B

FIGURE 3.26 Alternative industrial plant for continuous cachaça production.

and corresponds to an ethanol stream rich in light components, with concentrations much larger than those allowed by legislation. This by-product stream also contains a small amount of the processed ethanol, but it has commercial value for purposes other than the spirit production.

In this confi guration wine is injected at the top of column A1, which usually con- tains four trays. The vapor phase of column A is directed to column D, which also contains four trays and is operated under high refl ux rates. For this reason ethanol and light components are very concentrated in the distillate of this column, guar- anteeing that a small stream, withdrawn from its top, will be enough to control the

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quality of the main product. Using such a scheme, high quality cachaça can be pro- duced without large ethanol losses, even if the concentration of minor components in the wine is higher than usual.