e.l. ligero 2003

Dehydration of ethanol with salt extractive distillation*/acomparative analysis between processes with salt recovery

E.L. Ligero, T.M.K. Ravagnani *

Departamento de Engenharia de Sistemas Quımicos, Faculdade de Engenharia Quımica, Universidade Estadual de Campinas, CP 6066, CEP 13083-970

Campinas, Sao Paulo, Brazil

Received 9 January 2002; received in revised form 21 June 2002; accepted 21 June 2002

Abstract

Anhydrous ethanol can be obtained from a dilute aqueous solution of ethanol via extractive distillation with potassium acetate.

Two process flowsheets with salt recovery were proposed. In the first, dilute ethanol is directly fed to a salt extractive distillation

column and, after that, the salt is recovered in a multiple effect evaporator followed by a spray dryer. In the second, the concentrated

ethanol from conventional distillation is fed to a salt extractive distillation column. In this case, salt is recovered in a single spray

dryer. In both processes the recovered salt is recycled to be used in the extractive distillation column. Every component of each

process was rigorously modeled and its behavior was simulated for a wide range of operating conditions. A global simulation was

then carried out. The results show that the second process is more interesting in terms of energy consumption than the first.

Furthermore, it would be easier to implement changes on existing benzene extractive anhydrous ethanol plants to convert them to

more ecologically attractive concentrated ethanol feed processes.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Anhydrous ethanol; Salt extractive distillation; Spray dryer; Multiple effect evaporators

1. Introduction

A highly used technique in industry to separate

azeotropic mixtures such as ethanol�/water is extractive

distillation with liquid separation agents. There are

several liquid separation agents that can be used in

ethanol dehydration such as benzene, pentane, furfural,

ethylene glycol, diethyl ether and toluene. The most

utilized is benzene, which, due to its carcinogenic effect,

has been progressively eliminated.

One alternative process to produce anhydrous ethanol

is the extractive distillation that uses soluble salts as

separation agents. One comprehensive review of the

literature related to the vapor�/liquid equilibrium of salt

systems and the salt extractive distillation was presented

by Furter and Cook [1] and complemented by Furter [2].

The salt extractive distillation is basically similar to

extractive distillation with liquid entrainers. The salt, a

non-volatile component, is introduced at the top or near

the top tray of the distillation column, flows downward

along the column, and is completely removed with the

bottom product [2].

Besides the lower toxicity level of certain salts

comparing with previously cited liquids used in the

anhydrous ethanol production, one of the advantages of

the salt extractive distillation is the production of a

distillate completely free from the separation agent.

Another favorable aspect related to salt distillation is

the high level of energy savings due to the absence of the

evaporation�/condensation cycle of the volatile separa-

tion agent inside the column.These superior properties of salt extractive distillation

against the liquid extractive distillation process encou-

rage the study of soluble salts in ethanol dehydration.

Theoretical and experimental studies on salt extrac-

tive distillation of ethanol�/water mixtures available in

the literature reinforce the likelihood of replacing liquid

separation agents by salt in the production of anhydrous

ethanol.Several authors [3�/9] have demonstrated that it is

possible to obtain ethanol with high purity level by using

the proper salts. However, most of the studies consider* Corresponding author

E-mail address: [email protected] (T.M.K. Ravagnani).

Chemical Engineering and Processing 42 (2003) 543�/552

www.elsevier.com/locate/cep

0255-2701/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 2 5 5 - 2 7 0 1 ( 0 2 ) 0 0 0 7 5 - 2

mailto:[email protected]

only the ethanol production step, without taking into

account the salt recovery step. Likewise, the number of

available process flowsheets with salt recovery proce-

dures is limited in the literature. The studies that takeglobal simulations into account are even scarcer. There

are only qualitative discussions. There is indeed, a great

interest in alternative processes to produce anhydrous

ethanol with addition of salt. Although the knowledge

of the major characteristics of salt extractive column is

of utmost importance to obtain anhydrous ethanol, the

study of the salt recovery step is also very important.

This step accounts for the economic feasibility of theprocess, allowing the salt to remain in a closed circuit

with no need to be continuously introduced in the

system. The global simulation process of anhydrous

ethanol production enables the two steps to be well

known.

Two distinct processes were proposed and analyzed in

this study. Their basic difference is in the way to

produce ethanol: with or without utilization of a pre-concentrating distillation column. The proposed way to

recover salt consists of using an evaporation system to

concentrate the salt solution and a spray dryer to

complete the purification of the salt.

2. Proposed processes

The success of salt extractive distillation to produceanhydrous ethanol is directly related to the salt chosen.

The choice of the correct salt for extractive distillation is

not exclusively limited to its ability to break the

azeotrope, but other factors including toxicity, easy

handling properties resulting from the system and

availability in the marketplace should also be taken

into account. The salt used in both processes was

potassium acetate. It is able to completely eliminateethanol�/water azeotrope under atmospheric pressure

even if it is present in small concentrations in the liquid

phase of the extractive column [8,10].

In the anhydrous ethanol production by the salt

extractive distillation technique, the following two

distinct steps were taken into account: the production

step and the salt recovery step. In the first step, the

dilute ethanol aqueous solution was fed to a saltdistillation column in order to produce 98.9 mol%

anhydrous ethanol. Next, the dilute salt solution pro-

duced at the bottom of this column was fed to the

recovery step, where it was completely dried. Finally, the

recovered salt was reintroduced in the distillation

column within the reflux stream.

The proposed processes for anhydrous ethanol pro-

duction were named according to the aqueous solutionconcentration of ethanol fed in the salt distillation

column as follows: process with dilute ethanol feed

and process with concentrated ethanol feed.

2.1. Process with dilute ethanol feed

In this process, the production of anhydrous ethanol

is carried out in a single distillation column, whichoperates in the presence of potassium acetate. The

recovery step of the salt consists of a multiple effect

evaporation system and of spray drying. The flowsheet

of this process is presented in Fig. 1.

The dilute aqueous ethanol feed is directly introduced

into the salt distillation column. The distillate is

anhydrous ethanol free from the separation agent, while

all the water and all the salt introduced in the columnare removed as bottom product.

The dilute aqueous potassium acetate solution coming

from the salt distillation column is sent to the salt

recovery step. The purpose of the multiple effect

evaporation system is to remove part of the water in

the solution before it is introduced in the spray dryer

where the complete drying of the salt takes place. Thus,

dried salt is completely recycled in the solid state to thetop of the salt distillation column in a way that will not

affect the purity level of the distillate.

2.2. Process with concentrated ethanol feed

The second alternative proposed for the production ofanhydrous ethanol by salt extractive distillation consists

of the use of two distillation columns in the anhydrous

ethanol production step: the first column is the pre-

concentrator column and the second one is the salt

extractive distillation, as shown by the flowsheet in Fig.

2.

In the pre-concentrator column, which operates in the

absence of salt, a large amount of water is removed asthe bottom product and an ethanol�/water mixture is

obtained as the distillate with a composition close to the

azeotrope. This concentrated aqueous ethanol solution

is then sent to the salt distillation column, which

produces a concentrated aqueous solution of potassium

acetate as the bottom stream which is sent to the salt

recovery step.

In the salt recovery step, contrary to the flowsheetpresented in Fig. 1, there is no need to use the

Fig. 1. Process with dilute ethanol feed.

E.L. Ligero, T.M.K. Ravagnani / Chemical Engineering and Processing 42 (2003) 543�/552544

evaporation system since the aqueous salt solution from

the ethanol production step is already concentrated.

Therefore, the final drying of the salt is performed

exclusively in the spray dryer. Similar to the former

process, the potassium acetate is recycled in the solid

state to the salt distillation column and added to the

reflux stream.

3. Simulation of the processes

In the study of alternative processes for ethanoldehydration, each piece of equipment used in the

ethanol production and salt recovery steps is individu-

ally modeled. Subsequently, these devices are connected

between themselves for global simulation. In Figs. 1 and

2 the presence of a recycle stream, generated in the salt

recovery step, would make one consider the need to use

an iterative sequential method to simulate the global

process. However, the fact that the salt used in theextractive distillation is totally recovered in pure state at

the dryer, makes the recycle properties known and

eliminates the need for iterative calculation.

The modelings of the plant components are described

below.

3.1. Salt distillation column

The steady state salt distillation column is designed

with the rigorous method proposed by Naphtali and

Sandholm [11] with proper modifications in order to

take into account the presence of salts in the liquidphase.

In the original Naphtali and Sandholm model [11],

the mass and energy balance equations and the equili-

brium relationships were written for each component in

each stage. Since the salt is totally dissolved in the

distillation column and is not part of the mass transfer

process, the balance equations of the phases are written

only for the volatile components. The mass and energybalance equations in the salt system remain unaffected

in relation to the system of equations describing the

conventional distillation column without salt, presented

by Fredenslund et al. [12]. The required changes due to

the presence of salt were taken into account in the

equilibrium relationships [4]:

Ee;iKe;fV e

le;i

Le �XNsalt

s�1

hsvse;s

�(1�Ee;i)ve�1;i

V e

V e�1

�ve;i

�0 (1)

In the calculation of the pre-concentrating distillationcolumn that operates in the absence of salt, Fig. 2, the

expressions for the equilibrium relationships are iden-

tical to those presented by Fredenslund et al. [12].

The Murphree efficiency, Ee,i on Eq. (1), for ethanol�/

water system, was considered as 70%. For the ethanol�/

water�/potassium acetate system the value adopted was

60% [7].

The Sander et al. [13] model was used to predict theliquid�/vapor equilibrium of the ethanol�/water�/potas-

sium acetate. According to this model, the activity

coefficient of volatile components results in a sum of

two contributions: one of Debye�/Huckel type and the

other of the Uniquac modified model.

The mass and energy balance equations and the

equilibrium relationships make a total of N (2Nsolv�/l)

non-linear algebraic equations, whose resolution bygeneralized Newton Raphson method determines the

volatile components flow rates in the liquid and vapor

phase, the temperature in each stage and the heat

exchange rates involved in the reboiler and condenser.

3.2. The evaporation system

For the process with dilute ethanol feed stream (see

Fig. 1) the salt recovery starts in a multiple effect

evaporation system. Each step of this evaporator isoperated with forced circulation that is proper for

evaporation of salt solutions.

The method used to design the steady state feed

forward multiple effect evaporation system is the one

proposed by Holland [14]. This method takes into

account the boiling point rise effect.

The equations for each effect of the evaporator

consist of the energy balance, convection heat transferflow rate, phase equilibrium and material balance. The

only exception relates to the last effect, which has only

the two first equations since the pressure and composi-

tion of this effect are specified values. These equations

make a total of 4n�/2 non-linear algebraic equations

whose resolution by Broyden method determines the

heat transfer area of each effect and the mass flow of

steam. The variables such as pressure, boiling tempera-tures of pure solvent and of solution, flow, and mass

fraction of concentrated salt solution of each effect also

become known.

Fig. 2. Process with concentrated ethanol feed.

E.L. Ligero, T.M.K. Ravagnani / Chemical Engineering and Processing 42 (2003) 543�/552 545

For the water�/potassium acetate system, the equili-

brium relationships follow the Duhring rule, i.e. the

boiling temperature of the solution is a linear function

of the boiling temperature of pure water, which iscalculated according to the Sander et al. [13] method.

3.3. Spray drying chamber

The design of the spray dryer used in the final drying

process of potassium acetate follows the Gauvin and

Katta model [15]. This model joins the fundamental

principles of the transport phenomena with experimen-

tal evidences. Its major considerations are the assump-

tion of a saturated condition of the drop surface and

neglect the falling-rate drying period, resulting in a

constant temperature on the surface of the particles. Asa quasi-one-dimensional condition, this model does not

take into account the temperature and the humidity

gradients of the drying air in the radial direction, in a

way that these variables remain uniform in a given

cross-section of the chamber. One major criterion of the

spray dryer design is that the largest particle formed at

the atomizer should be dried before it reaches the drying

chamber wall. The drying chamber is made up of anupper cylindrical section in which the height and the

radius have the same length, and a lower conical section

in which the height is the same value of the cylindrical

part diameter. Another important aspect of the chamber

is the fact that the drying air is tangentially introduced

by a single flow into the top of the cylindrical section,

resulting in a co-current flow.

The chosen atomizer was a centrifugal pressurenozzle, especially adequate for viscous solutions. The

atomizer characteristics and the droplet particle size

distribution are those presented by Gauvin and Katta

[15], except that the range of droplet diameter was 80�/

350 mm.

In the model for the spray dryer, the equations of the

three-dimensional movement of the droplets in the

centrifugal and gravitational fields were solved simulta-neously with the equat ions of mass and heat transfer

with a three-dimensional model of the airflow and with

the instantaneous properties of the drying air. The

following considerations were made:

�/ the drag coefficient, CD, and the lift force, FL,

required in the droplets movement equations were

determined using equations proposed, respectively,

by Beard and Pruppaeher [16] and Saffman [17];

�/ the instantaneous particle diameter is obtained con-

sidering the particles as a dense material;�/ Ranz and Marshall [18] correlations were applied in

mass and heat transfer equations.

The three-dimensional model of the airflow in the

inner part of the chamber depends on the chamber

characteristics. In the studied chamber, the air velocity

depends on the place the particle is located, i.e. the

tangential, radial and axial components of the air

velocity change with the zone and region in thechamber. In the nozzle zone, the particles are highly

affected by the atomizing nozzle. The nozzle zone

finishes, according to [19], when the drying air flow

and entrainment flow become equal. The best relation to

express entrainment flow is given by Bennat and

Eisenklam [20]. The free entrainment zone represents

the area in which the particles are freely dragged by the

drying air. In the cylindrical section, this zone is dividedinto two distinct regions that depend on the radial

position: central and annular.

The droplets movement equation together with the

mass and heat transfer and air flow equations form a set

of ordinary differential equations whose solution defines

the dimension of the chamber, the required air flow in

the drying operation and the total quantity of energy

involved in the process. These equations can be solvedusing the Euler explicit integration method.

4. Results

The following results correspond to process simula-

tions of an anhydrous ethanol unit producing 13.5 mol/

s.

4.1. Dilute ethanol feed process

4.1.1. Salt distillation column

In this process, the dilute aqueous ethanol solution is

directly introduced into the column of salt distillation in

which the anhydrous ethanol is obtained as a distillate

[21]. The operating conditions of the atmosphericpressure column are presented in Table 1. The table

also presents the operating conditions of the salt

distillation column used in the concentrated ethanol

feed process.

The potassium acetate, introduced in the salt distilla-

tion column, has its maximum flow determined as a

function of the molar fraction of the salt inside the

column. It should meet three requirements: salt solubi-

Table 1

Operating conditions of salt distillation column

Process type Dilute ethanol feed Concentrated ethanol

feed

Feed flow (mol/s) 555.5 22.22

Feed composition

(mol% ethanol)

2.4 60

Feed temperature (K) 303.15 353.45

Feed component, ethanol�/water; distillate composition, 98.9 mol%

ethanol; distillate flow, 13.5 mol/s.


lity in pure boiling ethanol (10 mol%) [10]; salt solubility

in the pure water (50 mol%) [10] and finally the limits

suggested by the Sander et al. model [13] to predict the

liquid�/vapor equilibrium. In the case of the diluteethanol aqueous feed, the determinant factor to set the

maximum salt flow is the potassium acetate solubility in

anhydrous ethanol at the reflux stream, assumed as 10

mol%.

This column is designed to operate at optimum reflux

ratio. The minimum reflux obtained for the column

studied is equal to 2.1 as shown in Fig. 3. Assuming a

20% increase in the minimum reflux, the optimum refluxratio is equal to 2.5. The figure also shows the effect of

the reflux ratio on the number of column stages for

concentrated ethanol feed process, which will be dis-

cussed later.

The design data for the salt distillation column

operating at optimum reflux are presented in Table 2.

One of the important aspects for the operation of a

salt distillation column is the effect of potassiumacetated composition at the reflux on the distillate

composition. Fig. 4 shows that the molar fraction of

the resulting salt in the reflux should not be smaller than

10% molar, once the ethanol produced is out of the

desired specification, which is 98.9% M. The salt

concentration in the reflux can be reduced by increasing

the number of stages. The simulation results show that

for each 4% reduction in the molar composition ofpotassium acetate in the reflux one more stage in the

column is required to produce the desired ethanol

specification.

Although the feed pre-heating diminishes the reboiler

heat requirements, the increase of the feed temperature

above 330 K does not lead to good results due to the

increase in the minimum reflux ratio and salt flow.

4.1.2. Evaporation system

The bottom product of the salt distillation column,

which is basically formed by an aqueous solution with

3.6 wt.% of potassium acetate, is sent to the evaporation

system as shown in the flowsheet of Fig. 1. The salt

concentration in the last effect outlet stream is limited by

the solubility of the salt in water, and the accepted value

for this KAc concentration is 60 wt.%.

The heat transfer rate obtained in the multiple-effect

evaporator depends on the operating pressure difference

between consecutive effects, which is a function of the

steam fed to the first effect and of the vacuum in the last

effect. All specified variables needed to design the

evaporation system are shown in Table 3.

The relative flexibility of the evaporation system

design regarding the number of effects leads to several

possible configurations. Fig. 5 presents the relation

between the number of effects and its individual heat

transfer area.

The heat transfer area, according to Fig. 5, is high

when the operation is achieved in a single effect. In a

double effect system, however, there is an abrupt fall in

the heat transfer area. The use of a number of effects

higher than two reduces even more the heat transfer

area, but this area reduction becomes increasingly

smaller. Thus, in the simulation of the evaporators

Fig. 3. Effect of reflux ratio on the distillation columns.

Table 2

Design of salt distillation column

Process type Dilute ethanol feed Concentrated ethanol feed

Number of stages 37 35

Optimum reflux ratio 2.5 1.2

Feed stage number 18 5

Salt flow (mol/s) 3.74 1.80

Reboiler duty (W) 4.85�106 3.07�105

Fig. 4. Effect of salt composition at reflux stream on distillate purity.

Table 3

Operating conditions of the evaporation system

Feed Components: water/KAc

Flow: 10.13 kg/s

Composition: 3.6 wt.% of salt

Temperature: 373.45 K

Final solution Components: water/KAc

Composition: 60 wt.% of salt

Steam pressure, 10.1�105 Pa; last effect pressure, 0.11�105 Pa.


with six or more effects, the heat transfer area practi-

cally remains constant.

The relation between steam consumption and thenumber of effects of the evaporation system follow the

pattern shown in Fig. 5. The increase from one to two

effects causes a strong reduction in the vapor consump-

tion, and the use of an evaporator with five or more

effects causes the required amount of vapor to reach a

constant value.

Considering the relation between the heat transfer

area and the steam consumption with number of effects,the simulation results show that the evaporation of

potassium acetate solution in a system with four effects

is the best option. The major variable values obtained in

the design of the evaporation system are presented in

Table 4.

One of the determining factors in the design of the

evaporation system is the final concentration of the

KAc. The results of the different final values of therequired solution concentration in the design of the

evaporation system are listed in Figs. 6 and 7. In the

analyzed concentration range, the heat exchange area is

more strongly affected by the final concentration of the

salt than by the total steam required to achieve

evaporation or by the pressure of the last effect.

4.1.3. Spray dryer

The aqueous potassium acetate solution in the outlet

of the evaporation system constitutes the feed to the

spray dryer as illustrated in the flowsheet in Fig. 1. The

feed of the dryer has its flow and concentration set bythe level of pre-concentration assumed in the evapora-

tion system. The feed temperature depends exclusively

on the operating pressure of the last effect of the

evaporator.The drying of the salt solution, in the form of a spray

in the chamber, occurs by contact with a significant

amount of hot air. Typically, the value of the air outlet

temperature is limited with the purpose to prevent

problems such as combustion, explosion, and thermal

instability of the salt. In the case of the potassium

acetate, it is limited by the salt melting point.

The operating conditions of the spray dryer are shown

in Table 5 for both proposed processes.The criterion used in the design of the dryer is highly

conservative, which leads to a chamber with larger

dimensions than those actually required. The basic

variable values defined in the design of the spray dryer

are listed in Table 6 for both proposed processes.

One essentially important aspect in the spray dryer

design is the influence of the feed concentration that in

this design is equal to the pre-concentration level

specified in the outlet stream of the evaporation system.

The immediate consequence of the increase in the pre-

concentration level of the aqueous potassium acetate

solution in the evaporation system is the reduction of

the feed stream to the spray dryer. Thus, an increase in

salt composition in the outlet of the evaporation system

corresponds to a decrease in the feed flow to the

Fig. 5. Influence of number of effects of evaporator on the heat

transfer area.

Table 4

Design of evaporation system

Number of effect 4

Heat surface of each effect 76 m2

Steam flow 3.36 kg/s

Total heat requirement 2.16�107 W

Fig. 6. Effect of final salt concentration on heat transfer area.

Fig. 7. Effect of last effect pressure on vapor requirement.


chamber. The effect of different salt concentrations in

the outlet of the evaporation system is shown in Fig. 8.

This figure shows that an increase in the pre-concentra-

tion level of the feed salt solution results in chamberswith smaller dimensions. Once the feed flow is lower, the

amount of moisture to be evaporated will decrease.

The height of the column increases for feed solutions

with a concentration level below 40%, although in

higher concentrations the chamber becomes lower. For

the evaporation system, the increase in the demand for

highly concentrated salt solutions will result in larger

heat exchange areas and more steam consumption, oncethe saturated state of the solution is approximated. For

this reason, both the designs of evaporation system and

spray dryer have shown that a concentration of 60% of

potassium acetate in the outlet of the evaporation

system would result in devices with acceptable dimen-

sions. Fig. 8 also shows the behavior of the drying

airflow with the increase of the pre-concentration level

of the solution in the outlet of the evaporation system.Likewise the preceding discussion, a pre-concentration

level of 60 wt.% of potassium acetate in the evaporation

system will result in an acceptable value for the drying

air.

4.2. Process with concentrated ethanol feed

4.2.1. Pre-concentrating distillation column

The other possibility analyzed in the production ofanhydrous ethanol is the use of a pre-concentrating

atmospheric pressure distillation column operating

without salt with the purpose to remove most of the

water in the dilute ethanol feed, as shown in the

flowsheet in Fig. 2.

The feed for this column has the same flow, composi-tion and temperature specifications shown in Table 1.

The purity level of the distillate concentration, however,

is limited by the composition of the ethanol�/water

azeotrope. In order to prevent the use of a large number

of stages in the following salt distillation column, the

ethanol content in the distillate was set to 60 mol% and

its flow was kept at 22.22 mol/s.

The minimum reflux ratio and some combinations ofreflux ratio and number of stages that lead to the

production of a distillate with the desired specifications

are shown in Fig. 3. The results of the pre-concentrating

column operating in the optimum reflux of 1.1 are

presented in Table 7.

4.2.2. Salt distillation column

The feed to the salt distillation column was the

distillate produced in the pre-concentrating column

that was in the saturated vapor state because it was

operating with a partial condenser. The operating

Table 5

Operating conditions of spray dryer


Feed Flow (kg/s) 0.608 0.33

Salt composition (wt.%) 60 52.8

Temperature (K) 335.6 388.6

Product flow (kg/s) 0.36 0.18

Feed solutions, water/potassium acetate; product, potassium acetate. Drying air: inlet moisture: 0.005 kg water/kg dry air; inlet temperature,

650.15 K; outlet temperature, 420.15 K.

Table 6

Design of spray dryer


Dimension Radius (m) 2.2 2.6

Total height (m) 6.6 7.8

Drying air flow (kg/s) 1.81 0.0156

Total heat involved (W) 4.53�105 5.55�105

Fig. 8. Effect of feed salt concentrations on dryer height and air flow

rate.


conditions of the salt distillation column are summar-

ized in Table 1.

Since the feed to the salt distillation column was

concentrated in ethanol, this resulted in a lower quantity

of water inside the column, and therefore, the quantity

of salt introduced in the system is not limited by its

solubility in anhydrous ethanol as in the case of dilute

feed process, but now it is limited by its solubility in the

outlet water solution at the bottom of the column. In

addition to these solubility limits, it is important to

observe the ion concentration limitation imposed by

Sander et al. model [13] to predict the liquid�/vapor

equilibrium of the salt system. For this reason, the

distillate composition of the pre-concentrating column

was fixed with lower purity than azcotrope. Besides, as

the water flow inside the salt distillation column is very

low, the salt flow is also low. This low salt quantity

produces low salt concentration at the top of the column

requiring a large number of stages to accomplish the

separation. The salt distillation column feed with 60

mol% of ethanol results in a column with acceptable

dimensions as can be seen on Table 2 and also avoids

any possibility of salt precipitation inside the column.

Table 8 presents some of the reflux ratios and number

of stages combinations in the salt distillation column to

produce a specified ethanol. The salt flow, which is

calculated assuming reflux potassium acetate of 10

mol%, is also presented in this table with the molar

fractions of potassium acetate resulting at the bottom of

the column.

In all cases presented in Table 8, the use of potassium

acetate flows that result in a 10 mol% content of the salt

in the top of the column eliminates the likelihood of salt

precipitation in a region that is rich in ethanol. Analyz-

ing the potassium acetate composition in the bottom of

the distillation column, it can be observed that in all

situations its value was kept below the maximum

solubility of this salt in pure water, which is 50 mol%.

These conditions also ensure that there will be no

precipitation of salt in the trays of the column that is

rich in water. The applicability limit to predict the

vapor�/liquid equilibrium, however, was exceeded in the

bottom of the column in all cases, once the molarfraction of the potassium acetate in pure water is slightly

above 0.15. In spite of that, the temperatures found at

the bottom of the column are in agreement with the

boiling temperature of the aqueous solution with 15

mol% of potassium acetate that was obtained experi-

mentally.

The minimum reflux ratio for the salt distillation

column is equal to 1.1 as shown in Table 8. Theoperation of the column at the optimum reflux ratio

of 1.3 with a potassium acetate flow of 1.56 mol/s (that

satisfy the model limit in trays rich in water) requires 50

separation stages to produce anhydrous ethanol at the

required specification. This high number of stages does

not justify column operation at optimum reflux ratio.

The effect of the salt feed flow on the distillation column

design was already shown in Table 2.As the composition of the bottom product of this

column is 52.8 wt.% in potassium acetate, the multiple

effect evaporation system is not required to recover the

salt.

4.2.3. Spray dryer

The spray dryer feed is the bottom of the salt

distillation column that consists of a concentrated

aqueous solution of potassium acetate. The drying air

admitted to the chamber presents the same initial and

final conditions considered in the design of the spray

dryer of the dilute ethanol feed process (see Table 5).

The operating conditions of the spray dryer, as well as

the design results are also included in Tables 5 and 6,respectively.

4.3. Comparison between processes

The basic difference between the two alternative

processes for the anhydrous ethanol production by saltextractive distillation is related to the fact that the

aqueous dilute ethanol feed is or is not subjected to a

pre-concentration.

In spite of the process of concentrated ethanol feed

demands two distinct distillation columns to produce

98.9 mol% ethanol, as shown in Table 2, this option

requires lower reflux ratio and lower salt quantities than

Table 7

Design of pre-concentrating distillation column

Number of stages 22

Reflux ratio 1.1

Feed stage number 17

Reboiler duty 4.84�106 W

Table 8

Optimum reflux ratio of salt distillation column

Number of stages Reflux ratio Salt flow (mol/s) Salt concentration at bottom (mol%) T (bottom) (K)

35 1.2 1.80 0.170 388.5

40 1.1 1.65 0.158 387.3

45 1.1 1.65 0.158 388.0


the dilute ethanol feed process. It also results in a

column with lower diameter. In regards to energy

consumption, in the ethanol production step, the salt

distillation column employs only 6% of the total energy.

The major part of the supplied energy (94%) was used to

remove the water in the pre-concentrating column. For

this reason, regardless of the presence of the two

distillation columns in the process of concentrated

ethanol feed, it does not require multiple-effect eva-

porators and the increase in energy consumption in

relation to the dilute ethanol feed process is only 6%.The presence or not of a pre-concentrating distillation

column in the ethanol production step affects directly

the way of performing salt recovery, i.e. the presence or

not of multiple effect evaporators. Considering the

energy consumption in the potassium acetate recovery

step, Tables 4 and 6 show that in the process with dilute

ethanol feed the consumption was 2.21�/107 W, in

which only 2% is required in the spray dryer. Otherwise,

in the process of concentrated ethanol feed, the energy

consumption in the salt recovery step is related only to

the spray dryer and, as shown in Table 6, is equal to

5.55�/105 W.

For this reason, the high level of energy consumption

in the evaporation system of the dilute ethanol feed

process becomes unacceptable. Indeed, the energy con-

sumption to recover salt in the dilute ethanol feed

process accounted for 82% of the total energy consump-

tion in the process, and in the concentrated ethanol feed

process only 9.7% of the total energy was used to

recover the salt.

Besides the previously mentioned energy advantages,

due to the lower equipment size, in this process there is

less demand for utilities like hot air (to spray dryer) and

cool water (to condenser). Steam to evaporator is not

required. But the major advantage of the concentrated

ethanol feed process is that it can be installed easily in

units that are in operation.

Independently of the technique employed to separate

the ethanol�/water mixture, such as azeotropic distilla-

tion or extractive distillation with liquid separation

agents, high energy consumption is required. One of

the various advantages of the extractive distillation with

salt is the reduction of this energy requirement. The

energy consumption obtained in the proposed process is

compared with the ones presented by Black [22]. This

comparison reveals the advantage of the extractive

distillation with potassium acetate to other conventional

processes as indicate in Table 9. This result is in

agreement with the experimental observation of Schmitt

[23] who obtained lesser energy consumption using

extractive distillation with potassium acetate when

compared with the azeotropic distillation with benzene.

5. Conclusion

The use of potassium acetate as an extractive agent in

the production of anhydrous ethanol has proven to be

an effective alternative in replacing highly toxic separa-

tion liquid agents, such as benzene.

Regarding the two possibilities of anhydrous ethanolproduction studied, the simulation results showed that

the concentrated ethanol feed process is the most

advantageous alternative. Although this process needs

two distillation columns, it eliminates the need for a

multiple effect evaporator in the salt recovery step.

Compared with dilute ethanol feed process, it requires

lower reflux ratio, which means lower column diameter

and lower salt consumption. Furthermore, it would beeasier to implement changes on existing benzene ex-

tractive anhydrous ethanol plants to convert them to

more ecologically attractive concentrated ethanol feed

processes.

Appendix A: Nomenclature

CD drag coefficient (�/)

E Murphree tray efficiency (�/)FL lift force (N)

/K/ equilibrium ratio (�/)

l component molar liquid flow (mol/s)

/L/ liquid phase total molar flow of volatiles compo-

nents (mol/s)

N total numbers of stage of distillation column

Nsalt total number of the salt (�/)

Nsolv total number of the volatile component (�/)N total numbers of stage of multiple effect eva-

porator system

/V/ total molar vapor phase flow (mol/s)

/v/ component molar vapor phase flow (mol/s)

vs salt molar flow (mol/s)

T temperature (K)

hs summation of the stoichiometric coefficients of

the salt ions

Subscript and superscript

e generic stage of distillation column (�/)

Table 9

Comparative energy consumption of processes

Distillation process (extractive/azeo-

tropic)

Energy consumption (MJ/kg

ethanol)

Potassium acetate (concentrated

ethanol feed)

9.27

Ethylene glycol 34.06

Pentane 10.87

Benzene 12.15

Diethyl ether 13.59


i generic volatile component (�/)

s generic salt

References

[1] W.F. Furter, R.A. Cook, Salt effect in distillation: a literature

review, Int. J. Heat Mass Transfer 10 (1967) 23�/36.

[2] W.F. Furter, Extractive distillation by salt effect, Adv. Chem. Ser.

115 (1972) 35�/45.

[3] D. Barba, V. Brandini, G. Di Giacomo, Hyperazeotropic ethanol

salted-out by extractive distillation. Theoretical evaluation and

experimental check, Chem. Eng. Sci. 40 (1985) 2287�/2292.

[4] A.P.C. Jimenez, S.P. Ravagnani, Modelado y Simulacion del

Proceso de Destilacion Extractiva Salina del Etanol, Information

Tecnologica 6 (1995) 17�/20.

[5] R.A. Cook, W.F. Furter, Extractive distillation employing a

dissolved salt as separating agent, Can. J. Chem. Eng. 46 (1968)

119�/123.

[6] W.F. Furter, Salt effect in distillation: a literature review II, Can.

J. Chem. Eng. 55 (1977) 229�/239.

[7] D. Schmitt, A. Vogelpohl, Distillation of ethanol�/water solutions

in the presence of potassium acetate, Sep. Sci. Technol. 18 (1983)

547�/554.

[8] E. Vercher, A. Munoz, A. Martinez-Andreu, Isobaric vapor�/

liquid equilibrium data for the ethanol�/water�/potassium acetate

and ethanol�/water�/(potassium acetate/sodium acetate) systems,

J. Chem. Eng. 36 (1991) 274�/277.

[9] E. Martinez de la Ossa, M.A. Galan Serrano, Salt effect on the

composition of alcohol obtained from wine by extractive distilla-

tion, Am. J. Enol. Viticult. 42 (1991) 252�/254.

[10] D. Meranda, W.F. Furter, Vapor�/liquid equilibrium data for

system: ethanol�/water saturated with potassium acetate, Can. J.

Chem. Eng. 5 (1966) 298�/300.

[11] L.M. Naphtali, D.P. Sandholm, Multicomponent separation

calculations by linearization, AlChE J. 17 (1971) 148�/153.

[12] A. Fredenslund, J. Gmehling, P. Rasmussen, Vapor�/Liquid

Equilibria Using Unifac A Group Contribution Method, Elsevier,

Amsterdam, LN, 1977.

[13] B. Sander, A. Fredenslund, P. Rasmussen, Calculation of

vapour�/liquid equilibria in mixed solvent/salt systems using an

extended uniquae equation, Chem. Eng. Sci. 41 (1986) 1171�/

1183.

[14] C.D. Holland, Fundamentals and Modeling of Separation

Processes*/Absorption, Distillation, Evaporation and Extrac-

tion, Prentice Hall, Englewood Cliffs, NJ, 1975.

[15] W.H. Gauvin, S. Katta, Basic concepts of spray dryer design,

AIChE J. 22 (1976) 713�/724.

[16] K.V. Beard, H.R. Pruppacher, A determination of the terminal

velocity and drag of small water drops by means of a wind tunnel,

J. Atmos. Sci. 26 (1969) 1066�/1072.

[17] P.O. Saffmam, The lift on a small sphere in a slow shear flow, J.

Fluid Mech. 22 (1956) 385�/400.

[18] W.K. Ranz, W.R. Marshall, Jr, Evaporation from drops, Chem.

Eng. Prog. 48 (1952) 141�/180.

[19] W.H. Galvin, S. Katta, F.H. Knelmam, Drop trajectory predic-

tions and their importance in the design of spray dryers, Int.

Multiphase Flow 1 (1975) 793�/816.

[20] F.G.S. Benatt, P. Eisenklam, Gaseous entrainment into axisy-

metric liquid sprays, J. Institute Fuel, August (1969) 309�/315.

[21] E.L. Ligero, T.M.K. Ravagnani, Proceso alternativo para la

production de etanol anhidro usando una sal como agente de

separacion, Information Tecnologiea 11 (2000) 31�/39.

[22] C. Black, Distillation modeling of ethanol recovery and dehydra-

tion process for ethanol and gasohol, Chem. Eng. Prog. 76 (1980)

78�/85.

[23] D. Schmiu, Der Einfluss von Salzen auf das Dampf- Fluessig-

keitsgleichgcwicht Binacrer Stoffgemesche und Rektifikation

Azeotroper Gimische bei Zugabe von Salzen, Doctor Thesis,

Karlsruhe University, Germany, 1979.


e.l. ligero 2003

Documents