Ind. Eng. Chem. Res. 2008, 47, 9573–9580
9573
Recovery of Lemon Essential Oil, Lemon Wax, and Diatomaceous Earth from the Filter Cake of the Lemon Essential Oil Dewaxing Process at Pilot-Plant Scale Carlos A. Correa,† Mo´nica B. Gramajo de Doz,‡ Carlos M. Bonatti,‡ and Horacio N. So´limo*,‡ Departamento de Ingenierı´a de Procesos y Gestio´n Industrial and Departamento de Fı´sica, Facultad de Ciencias Exactas y Tecnologı´a, UniVersidad Nacional de Tucuma´n, AVenida Independencia 1800, 4000 Tucuma´n, Argentina
A pilot-plant scale unit suitable for recovery of lemon essential oil, lemon wax, and diatomaceous earth that exists in the waste filter cake arising from the dewaxing process of the cold-pressed lemon essential oil has been developed for batch operation, using a steam stripping process. It includes steam-distillation of the essential oil contained in the filter cake followed by its liquid-liquid separation from water, cooling and decantation of the nonvolatile residue that remains inside the stripper, and manual separation of lemon wax and diatomaceous earth. Mass and energy balances of the process were also made, which allow for the calculation of the time to exhaust the oil and the consumed vapor required to extract one kilogram of essential oil. The experimental results show that this process is suitable for the separation of these “three components” of the filter cake at pilot-plant scale, minimizing the environment contamination. 1. Introduction Presently, there is an increasing interest for cleaner industrial production. Therefore, waste materials (including byproduct, solid waste, hazardous waste, air emission and/or wastewater discharges) must be minimized. A way to carry out this strategy is to recover all valuable products contained in the industrial effluents.1 Separation processes are vital for the recovery of these valuable products from effluent streams in order to reduce the pollution along with the improvement of the economic performance. Much research was previously performed to recover valuable components from waste streams.2-4 Tucuma´n, located in northwest of Argentina, is an important producer of cold-pressed lemon essential oil.5 It is mainly obtained by using extractor machines, which simultaneously extract oil and juice from the lemon fruit. During the process, the essential oil from the ruptured oil glands in the flavedo is expelled to the outside of the fruit and is washed away from the peel by a timed, vigorous spray of water. The oil is separated (refined) from the emulsion and its accompanying detritus in several steps: (i) the oil passes first through a screening device (finisher) to retain larger particles of lemon rag and pulp; (ii) then, through a high-force centrifuge (desludger); (iii) finally, through a high-speed centrifuge (polisher), which completes the separation of the oil emulsion.5,6 Each step of this global process produces several types of effluents with appreciable quantities of organic compounds, which are not easily amenable to chemical or biological treatment. After being refined, the separate oil is refrigerated (“winterized”) in tall, narrow stainless steel tanks with an approximate capacity of 20000 L at ca. -25 °C for dewaxing (precipitating of natural waxes which are actually solids dissolved in the oil). These substances consist of a mixture of hydrocarbons and esters of high molecular masses, coumarins, sterols, and flavonoids that should be eliminated from the oil because they may damage finished products (such as soft drinks) by sedimenting or making these products cloudy. To accelerate the settling time of the * To whom correspondence should be addressed. Tel.: +54-3814364093, ext. 7857. E-mail:
[email protected]. † Departamento de Ingenierı´a de Procesos y Gestio´n Industrial. ‡ Departamento de Fı´sica.
wax (sometimes as much as 3 weeks), holding tanks have conical bottoms with drain valves at the bottom and at the side, in order to sample the oil at different heights. The oil of the settling tanks must be filtered (or centrifuged) to remove the wax. If a continuous vacuum filter (type Oliver filter)7 is used to retain the wax, a filter cake containing variable amounts of the essential oil, lemon wax, and diatomaceous earth (filter aid) is obtained, which constitutes an industrial waste that cannot be rejected without ulterior treatment, because it is a serious environmental contaminant. The aim of this work is to explore the possibility of recovering all valuable materials contained in the filter cake: lemon essential oil, lemon wax, and diatomaceous earth, minimizing, in this way, the environmental contamination. Some works about the recovery of different materials contained in wastewater of the lemon essential oil process can be found in the literature.8-10 However, as far as we know, no work for this pilot-plant treatment has been published. 2. Materials and Methods 2.1. Feed Material. The filter cake used as the feed was collected from a factory located in Tucuma´n, Argentina. This factory has dewaxing tanks with one valve at the bottom and six side valves, in order to sample the oil at different heights. The cake has different aspects and compositions, depending from which valve the oil is drained. Valves 1-4 provide a “dry” cake, mainly constituted by diatomaceous earth with low concentration of wax, while valves 5 to 7 provide a pasty cake with high content of wax and smaller concentration of diatomaceous earth. All cakes are imbibed in lemon essential oil. This factory only drains the essential oil from valves 4 and 7, obtaining two type of residues. Consequently, the stripper was always fed with a mixture of both types of residues having the mass proportion reported by the factory. 2.2. Equipment. The methodology used for the recovery of all the valuable products consists of several steps: (i) steamdistillation at the local atmospheric pressure of the filter cake, which is placed in a round-bottom tank (stripper) in order to separate the lemon essential oil; (ii) liquid-liquid separation
10.1021/ie7017355 CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
9574 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 1. Operation Parameters for the Steam Distillation Process
Figure 1. Schematic experimental device: (A) stripper; (B) swan neck; (C) condenser; (D) collector cylinder; (E) auxiliary cylinder; (G) tilt car; (H) funnel; (I) beaker; (J) perforated ring; (K) manual platform; (M) manometer; (mt) metric tape; (R) rotameter; (Ti) thermocouples; (Vi) valves.
of the oil from codistilled water; (iii) cooling and decantation of the nonvolatile residue remaining inside the stripper (waxes, condensed vapor, etc.); (iv) manual recovery of wax and diatomaceous earth taking advantage of their different densities, since this nonvolatile residue consists of three layers: solid upper layer (lemon wax), solid lower layer (diatomaceous earth), and liquid middle layer [condensed vapor (water)]. A schematic diagram of the process is shown in Figure 1. It consists of a slightly conical stripper (A) with a total volume of 30 L (32 cm internal diameter and 37 cm height) with a flange at the top attached to a swan neck (B), both thermally isolated with glass wool. A perforated ring (J) on the rounded bottom of the stripper is connected to a steam line and the smallest flange of the swan neck to a condenser (C). AISI 304 stainless steel was used to construct all these parts. The condensed codistilled mixture of water and lemon essential oil discharges in a phase separation device, built with a glass tube (22.4 cm diameter and 120 cm height) (D) provided with a spherical valve (V3) at the bottom. It is connected to a similar auxiliary acrylic tube (E) through a valve system (V4 and V5), in order to help the water separation after decantation and the water discharge, while the valve V2 is used to modify the water flow through the condenser. To measure the magnitudes needed to perform the mass and energy balances of the process, a manometer (M) before valve V1, a rotameter (R) at the water inlet to the condenser, and several thermocouples type J (Ti) attached to a digital thermometer Gefran, model 2308-1-RO with an uncertainty of 0.1 K, were installed. The thermocouple T2 is used to measure the temperature of the vapors. The process was stopped several minutes after reaching the highest temperature, which approximately represents the water boiling temperature at the local atmospheric pressure at the moment of the experiment. The time required to reach this temperature, corresponds to the recovery time for a given fraction of the oil from the filter cake. The height of the oil and water layers were continuously measured using a metric tape (mt) with an uncertainty of 0.1 cm attached to the glass tube (D). Since the cross section of this tube is known and the temperature of each layer was measured, the mass of each phase can be calculated using a calibration curve of density as a function of temperature for water and essential oil with an uncertainty of (0.1 kg. Density was measured with a vibrating tube densimeter KEM DA-300 with a built-in thermostatic unit accurate to 0.01 K, which allows working over the range T ) 277-363 K using degassed
parameters
values
manometer steam pressure before incoming to the stripper (kg · cm-2) local atmospheric pressure during the run (kg · cm-2) experimental temperature of the saturated steam (°C) experimental highest distillation temperature (°C) experimental inlet water temperature to the condenser (°C) experimental outlet water temperature from the condenser (°C)
0.9 0.98 120.0 98.9 21.1 24.2
bidistilled water and dry air as calibrating substances, with an uncertainty of (0.1 kg · m-3. After distillation, the nonvolatile residue remaining inside the stripper is cooled, and the solid lemon wax manually recovered lifting a wire mesh previously installed inside the stripper. To facilitate the transport of the nonvolatile residue to the stainless steel beaker (I) after recovering the wax, the stripper was assembled on a tilt cart (G) that manually circulates on rails to discharge in a funnel (H). This beaker, in turn, is placed on an up and down manually operated platform (K). Upon settling, the diatomaceous earth can be separated from the condensed steam. A platform scales accurate to (0.05 kg was used to weigh the feed and the recovered products in the process. The pilot-plant was constructed with a nominal capacity of approximately 8 kg of filter cake, and it has been designed for batch operation. 3. Results and Discussion 3.1. Steam Distillation. The pilot-plant was operated at the local atmospheric pressure by using steam injection to codistillate the essential oil and water from the cake. Table 1 lists the most important operation parameters of the process. The steam-distilled essential oil is a colorless liquid with a natural fruit aroma characteristic of the lemon from which it is obtained. The lack of color of the steam-distilled essential oil, due to the absence of coumarins, etc., is one of the most important differences with respect to the cold-pressed essential oil, because it leads to very different ultraviolet absorption spectra.11 3.2. Mass and Energy Balances. These two balances are suitable to calculate the time needed to recover a given fraction of the oil and the steam consumed to extract one kilogram of essential oil. To perform both balances, the concentration of lemon essential oil in the feed needs to be determined. This was carried out using a glass Dean-Stark trap suitable for essential oils. A weighed mass of the filter cake (using a PB1502-S Mettler balance accurate to (0.01 g, was used) is introduced in a round-bottom flask with water and steamdistilled. The codistilled essential oil and water were collected in the trap and the mass of the essential oil was calculated measuring both its volume ((0.1 mL), and its temperature, in order to calculate its density by means of the calibration curve. Therefore, the oil concentration can be calculated and it is reported as mass fraction with an uncertainty of (0.005. To avoid confusion, we adopt the capital letter M for the total masses and the lower case letter m for the instantaneous ones. 3.2.1. Mass Balance for the Steam-Distillation Process. Figure 2 shows all the variables involved in the stripper and the phase separation device.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9575
MOo (kg) ) wOoMFo
(1)
MoLW (kg) ) MRLW - wOWMRLW ) (1 - wOW)MRLW
(2)
MDo (kg) ) MFo - (MOo + MoLW)
(3)
o
Figure 2. Stripper and the phase separation device. (S) stripper; (mt) metric tape; (D) diameter of the collect cylinder; (MOF ) experimental mass fed to the process; (hO and hW) instantaneous heights of the upper oil and the bottom aqueous phases, respectively; (T0) temperature of the steam; (G0) average mass flow of the steam; (GD) distilled mass flow; (T) instantaneous ˙ ) lost heat temperature inside the stripper and of the distilled stream; (Q flow; (m) instantaneous (filter cake + condensed steam inside the stripper) mass.
3.2.1.1. Total Mass Balance. Experimental results show that the following conditions are satisfied: (a) the feed is an anhydrous complex mixture composed of diatomaceous earth, lemon wax, and lemon essential oil with a feed mass of MFo, because the water content in the feed never surpassed 0.018 mass percent when it was analyzed with an automatic Mettler DL18 KF titrator; (b) the lost mass in the distillation process is only attributed to the diatomaceous earth and the essential oil, because the lemon wax is completely recovered; (c) the codistilled water is an aqueous oil emulsion with a very low oil mass fraction wW O ; (d) the lemon wax retains a small fraction of the essential oil contained in the feed. Therefore, the following equations should be verified for the feed:
o
o
where MO, MLW, and MD are the masses of the lemon essential oil, lemon wax, and diatomaceous earth contained in the fed mass MFo, while wOW and wOo are the oil mass fractions in the recovered wax and in the feed, respectively. MRLW is the recovered (wax + retained oil) mass. The essential oil and diatomaceous earth mass losses in the process, can be calculated with the following equations: MDL (kg) ) MDo - MDR
(4)
MOL (kg) ) MOo-(MOR + wOWMRLW)
(5)
wOW ) MOL⁄MW
(6)
MOE (kg) ) MOR + MOL ) MOo - wOWMRLW
(7)
where MDL is the diatomaceous earth lost mass, MOL is the essential oil lost mass in the aqueous emulsion; MDR and MOR are the diatomaceous earth and essential oil recovered masses in the process, respectively; while MOE , MW, and wOW are the oil extracted mass, the codistilled water mass, and the oil mass fraction in the aqueous emulsion, respectively. All experimental and calculated values are reported in Table 2. This table also lists the global yield of the process, the recovered lemon essential oil, lemon wax, and diatomaceous earth yields, which are 98.2%, 99.0%, 100%, and 93.3%, respectively, taking into account the filter cake mass fed into the process.
Table 2. Obtained Results for a Typical Steam Distillation Run experimental and calculated magnitudes experimental mass fed to the process (kg) experimental oil mass fraction in the feed essential oil mass in the feed (kg) experimental recovered (wax + oil) mass (kg) experimental oil mass fraction in the recovered wax oil-free wax mass in the feed (kg) diatomaceous earth mass in the feed (kg) experimental codistilled water mass (kg) experimental recovered essential oil mass (kg) experimental recovered diatomaceous earth mass (kg) diatomaceous earth lost mass (kg) essential oil lost mass (kg) oil mass fraction in the aqueous emulsion total oil extracted mass (kg) experimental total condensed vapor inside the stripper (kg) experimental total steam mass used for the process (kg) experimental time to distill the first drop (min) experimental time for the total process (min) average mass flow of the live steam (kg · min-1) final oil extracted fraction from the feed latent heat ratio of essential oil and water (kgwater · kgoil-1) effective steam fraction effectively consumed steam required to extract 1 kg of oil (kgsteam · kgextracted,oil-1) “useful” or “effective” time to distill the heterogeneous mixture (min) experimental room temperature (°C) experimental initial distillation temperature at the local pressure (°C) experimental temperature of the steam (°C) latent heat of the liquid water at 100 °C/kcal · kg-1 heat of vaporization of d-limonene at 100 °C (kcal · mol-1) mean specific heat of water between 96 and 100 °C (kcal · K-1 · kg-1) mean specific heat of the steam between 100 and 120 °C at 1 atm (kcal · K-1 · kg-1) mean apparent specific heat: (stripper + feed) between 22 and 96 °C (kcal · kg-1 · K-1) global yield of the process (mass percent) recovery diatomaceous earth yield (mass percent) recovery essential oil yield (mass percent) recovery lemon wax yieldc (mass percent)
symbol and equation
values
MF wOo MOo ) wOo MFo R MLW wOW o R MW ) (1 - wOW)MW o MDo ) MFo - (MOo + MLW ) MW R MO MDR MDL ) MDo - MDR R MOL ) MOo - (MOR + wOWMLW ) wOW ) MOL /MW E R L MO ) MO + MO MVC MVT t1 tT G0 ) MVT /tT R ) MOE /MOo λOW η ) [MW(1 - wOW) + RMOo λOW]/ (1 - t1/tT)[MW(1 - wOW) + MVC] (ECS/R) te ) ECV · MOo /ηG0 TR Ti T0 λV λO j jPW C j jPV C j jPA C o ηG ) [(MLW + MOR + MDR)/MFo]100 ηD ) (MDR/MDo )100 ηD ) (MOR/MOo )100 ηLW
8.05 ( 0.05 0.615 ( 0.005 4.95 ( 0.05 0.88 ( 0.05 0.03 ( 0.01 0.85 ( 0.05 2.25 ( 0.05 19.4 ( 0.1 4.9 ( 0.1 2.1 ( 0.05 0.15 ( 0.05 0.02 ( 0.05 0.002 ( 0.001 4.92 ( 0.05 8.12 ( 0.05 27.50 ( 0.05 1.60 ( 0.02 81.00 ( 0.02 0.339 ( 0.005 0.994 ( 0.001 0.140 0.742
o
3.24 63.6 22.0 ( 0.1 96.0 ( 0.1 120.0 ( 0.1 540a 75.56 1.006b 0.496c 0.500 98.2 ( 0.1 93.3 ( 0.1 99.0 ( 0.1 100d
a From reference 13. b Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press: Boca Raton, FL, 1988-1989. c Interpolated value from Table F.2 in Smith, J. M.; Van Ness, H. C.; Abbott, M. M. Introduccio´n a la Termodina´mica en Ingenierı´a Quı´mica, 6th ed.; McGraw Hill: Mexico, 2001. d Assumed condition.
9576 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
3.2.2. Energy Balance of the Stripper. Figure 2 shows all streams needed to perform the energy balance of the stripper. In it, G0 is the mass flow of the steam injected to the stripper, with a temperature T0, while m and T are the instantaneous (filter cake + condensed steam) mass inside the stripper and its temperature at a given moment, respectively. On the other hand, GD is the mass flow of the heterogeneous distilled mixture and ˙ is the radiation and convection lost heat flow. Q The nonsteady state thermal balance for the stripper is given by j PW(100 - T) + λV + C j PV(T0 - 100)] ˙ + mCP(dT ⁄ dt) ) G0[C Q
[λWGD(1 - wOE) + λOGDwOE]
j PW(100 - T) + C j PV(T0 - 100)] ) ˙ + mCP(dT ⁄ dt) - G0[C Q (1 - η)G0λV (9) where (1 - η) is the steam fraction condensed inside the stripper, which is used to increase the temperature of the mass m and to compensate the heat losses through the thermal isolation. Therefore, eq 8 can be rewritten as: (10)
where ηG0 is the steam mass flow used to distill the volatile compounds contained in the feed, and η represents the effective steam fraction in the process. 3.2.2.1. Estimation of the Effective Steam Fraction in the Process η. First, we define a new variable R as R ) mOE ⁄ MOo
MVC ) G0t1 + (1 - η)G0(tT - t1) + RMOoλOW
(12)
Since G0 ) MV/tT, eq 12 can be rewritten as T
( )
t1 + RMOoλOW tT
(13)
where MVC and MVT are the condensed steam mass inside the stripper and the total steam mass used in the process, respectively, while t1 and tT are the times required to distill the first drop and for the total process, respectively. λOW is the ratio
t1 + RMOoλOW tT
(14)
However, since MVT ) MVC + MW(1 - wOW), η can be calculated as η)
[MW(1 - wOW) + RMOoλOW]
( )
(15)
t1 1 - [MW(1 - wOW) + MCV ] tT
Note that in eq 13 and 15 R takes its final value ()0.994 for this experiment). Unfortunately, the heat of vaporization for steam-distilled lemon essential oil and some of its main components are unknown. Therefore, the exact value of λOW to be used in eq 15 cannot be obtained. However, this oil is constituted by approximately 95 mass percent of some isomer hydrocarbon terpenes (d-limonene, R-pinene, β-pinene, etc.), which have similar heats of vaporization.13 Among them, d-limonene is the main component of the essential oil with a mass fraction value of 0.7, whose molar heat of vaporization at 100 °C can be calculated fitting the vapor pressure values reported by Stull12 and applying the Antoine’s equation.13 For these reasons, we adopt as the average heat of vaporization for steam-distilled lemon essential oil at 100 °C the value corresponding to d-limonene at this same temperature: λO (kcal · mol-1) ) 75.56.13 Therefore, λOW (kgwater · kgoil -1) ) 0.140. Replacing in eq 15 the values reported in Table 2, gives η ) 0.742. Therefore, only 74.2% of the steam injected into the stripper was useful to distill the essential oil while 25.8% was consumed to heat the feed and to compensate heat losses in the experimental device shown in Figure 1. 3.2.2.2. Mean Apparent Specific Heat of the (Stripper + Fed Mass). To estimate the mean apparent specific heat of j PA, we assume the following the stripper and its fed mass C conditions where term A is the total heat introduced inside the stripper during t1 min; term B is the total accumulated heat inside the (stripper + feed) during t1 min; and term C is the total heat losses during t1:
(11)
which represents the instantaneous extracted essential oil fraction from the feed, whose final value for this experiment is 0.994, as is reported in Table 2. The effective steam fraction in the process η can be estimated from the total condensed steam inside the stripper as follows. MVC is the total condensed steam mass inside the stripper; G0t1 is the condensed steam mass for heating the feed; (1 - η)G0(tT - t1) is the condensed steam mass to compensate heat losses; and RMOo λOW is the condensed steam mass for distillation of the oil:
MVC ) MVTt1 ⁄ tT + (1 - η)MVT 1 -
( )
MVC ) MVT - ηMVT 1 -
(8)
Here, λV, λW, and λO are the latent heats of vaporization of liquid water at 100 °C, of water at temperature T, and of essential oil at temperature T, respectively, while wEO is the instantaneous j jPV, and C j PW are the codistilled oil mass fraction. CP, C instantaneous specific heat of the (filter cake + condensed steam inside the stripper), and the mean specific heats of the steam, and liquid water, respectively. Throughout this work we assume that the steam is condensed at 100 °C rather than 98.9 °C, in order to take into account the influence of the hydrostatic pressure produced by the filter cake over the perforated ring inside the stripper. To eliminate unknown quantities included in eq 8, we introduce the following equality, which defines the variable η:
ηG0λV ) λWGD(1 - wOE) + λOGDwOE
between the vaporization heats of the essential oil and water, both at 100 °C. Therefore:
A V
{G0t1[Cj PW(100 - Ti) + λV + Cj PV(T0 - 100)]} ) B
{(
V
)
}
C
V M j PF + C j PS S (Ti - TR) + q MFo C o MF
j PS and C j PF are the mean where MS is the stripper mass and C specific heats of the materials used to construct the stripper and feed, respectively, while Ti, T0, and TR are the temperatures at the beginning of the distillation process, of the steam, and room temperature, respectively. On the other hand, q is the total heat losses during the heating time t1 and λV is the latent heat of liquid water at 100 °C. Therefore: j PW(100 - Ti) + λV + C j PV(T0 - 100)] ) G0t1[C M q j PF + C j PS S + MFo C (Ti - TR) MFo MFo(Ti - TR)
[
So that, finally:
]
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9577
j PA](Ti - TR) j PV(T0 - 100)] ) MFo[C j PW(100 - Ti) + λV + C G0t1[C (16) j PA is given in Table 2. The estimated mean value of C 4. The Concept of the Effective Consumed Steam ECS. Reordering equation 10:
(
ηG0λV ) λWGDwOE
1 - wOE
λO λW
+
wOE
)
(17)
and using the definition of variable R (eq 11), we can write dmOE ⁄ dt ) MOo dR ⁄ dt ) GDwOE
(18)
This expression gives the distillation rate of the essential oil, which can be obtained from experimental results. Substituting eq 18 in eq 17, gives
(
ηG0λV ) λWMOo
1 - wOE wOE
)
λO + dR ⁄ dt λW
(19)
Figure 3. Total distilled mass mT against the codistilled water mass mW: (b) experimental values. Solid line corresponds to the least-squares fit using eq 33.
But, in view of the Watson correlation,14 λV/λW = 1. This simplification does not introduce a significant error since 100 - T e 5 K always, and it leads to an equation that can be integrated:
∫
te
0
ηG0 dt ) MOo
∫
R
0
(
1 - wOE wOE
+
)
λO dR λW
(20)
Here, te is the time required to distill a fraction R of the MOo kg of essential oil contained in the feed. It is named “useful” or “effective” time. Therefore, the effectively consumed steam (ECS) required to extract a given fraction of essential oil from 1 kg of essential oil contained in the feed can be defined as -1 ) ) ηG0te ⁄ MOo ) ECS (kgsteam · kgfed,oil
∫
R
0
(
1 - wOE wOE
+
)
λO dR λW (21)
If ECS is divided by R, we obtain the kilograms of steam used to extract one kilogram of essential oil. Its value is reported in Table 2. The useful or effective time te can be calculated from equation 21: te ) ECS · MOo ⁄ ηG0
(22)
It is also reported in Table 2. As can be seen in Table 2, the calculated value of the effective time is 63.6 min. This means that the total operation time in this experiment exceeds in 17.4 min the necessary time required to achieve an R value of 0.994. This effective time corresponds to the beginning of the flat portion of the curve in Figure 5. 4.1. Correlation for the Effective Consumed Steam ECS. Steam distillation provides two liquid phases, where the upper phase is constituted by lemon essential oil saturated with water (with a negligible water concentration, due to their very low mutual solubility) while the bottom one is an aqueous oil emulsion with an essential oil mass fraction wOW. As indicated above, these phases are contained within a glass tube that receives the heterogeneous codistilled mixture. This tube has attached to it a calibrated metric tape with a calibration factor FC ) 2.4 cm · L-1, which allows a reading of the oil and water heights as a function of time t. Knowing the cross section of the glass tube and these heights, the volume of each phase can be calculated. The masses of both phases are obtained from
Figure 4. Derivative dmEO/dmW against the codistilled water mass mW. Solid line corresponds to the least-squares fit using eq 37.
their corresponding densities with an estimated uncertainty of (0.1 kg (see Table S.1 in the Supporting Information). The mass balance in the phase separation device for the heterogeneous condensed mixture shows that, at a given time for the water phase: GD(1 - wOE) ) d[mW(1 - wOW)] ⁄ dt but, since wOW,1, the mass balance for water can be written as GD(1 - wOE) ) dmW ⁄ dt
(23)
while, for the oil phase: GDwOE ) dmOE ⁄ dt
(24)
where mEO and mW are the instantaneous masses of the codistilled oil and water, respectively; while GD and wOE are the mass flow of the distilled mixture and the instantaneous oil mass fraction in the condensed heterogeneous mixture, respectively. From eq 23 and 24, we obtain: 1 - wOE wOE
) dmW ⁄ dmOE
(25)
so that, taking into account the definition of the variable R: dR ) dmOE ⁄ MOo we obtain:
(26)
9578 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008
1 - wOE wOE
dR )
d(mW ⁄ MOo) dmOE
dmOE ) d(mW ⁄ MOo)
(27)
Substituting eq 27 in eq 21 gives -1 ECS (kgsteam · kgfed,oil ))
∫
R
0
d(mW ⁄ MOo) +
∫
R
0
λO dR (28) λW
which gives, after integration, -1 ECS (kgsteam · kgfed,oil ) ) (mW ⁄ MOo) + R(λjO ⁄ λW)
(29)
To estimate the ECS required to extract one kilogram of essential oil from the feed, we previously need to reformulate the mass balance for the phase separation device. This can be made as follows for the oil phase: mOE (kg) ) mT - mW
(30)
while, for the water phase: GD(1 - wOE) ) dmW ⁄ dt
Figure 5. Instantaneous oil extracted mass mOE against the codistilled water mass mW: ([) experimental values; (O) calculated values using the integrated equation 37.
(31)
The mass flow of the heterogeneous distilled mixture GD is given by GD (kg · min-1) ) dmT ⁄ dt
(32)
E
where mO, mT, and mW are the instantaneous masses of the distilled oil, total distilled mixture, and distilled water, respectively; while wOE is the instantaneous oil mass fraction in the condensed heterogeneous mixture. Figure 3 shows the experimental instantaneous total distilled mass mT as a function of the experimental instantaneous codistilled water mass mW for the typical run reported in Table 2 and Supporting Information Table S.1. A least-squares regression of the experimental results shown in Figure 3, leads within the 97.5% confidence limits to the following equation: mT (kg) ) (1.15 × 10-3 ( 6 × 10-5)m3W (5.6 × 10-2 ( 2 × 10-3)m2W + (1.91 ( 0.01)mW - (0.10 ( 0.03) (33) Deriving eq 33 with respect to mW and subtracting 1 from the result, we obtain dmT ⁄ dmW - 1 ) (3.4 × 10-3 ( 2 × 10-4)m2W (11.2 × 10-2 ( 4 × 10-3)mW + (0.91 ( 0.01) (34) while, deriving equation 31 with respect to mW leads to dmOE ⁄ dmW ) dmT ⁄ dmW - 1
(35)
Comparing these last two equations gives dmOE ⁄ dmW - 1 ) (3.4 × 10-3 ( 2 × 10-4)m2W (11.2 × 10-2 ( 4 × 10-3)mW + (0.91 ( 0.01) (36) Replacing the experimental mW values (see Supporting Information Table S.1) in eq 36 makes it possible to plot the calculated values of dmOE /dmW against the codistilled water mass mW (Figure 4). A least-squares regression of this plot leads to equation 37, within the 97.5% confidence limits: dmOE ⁄ dmW ) (1.0847 × 10-4 ( 7 × 10-8)m3W + (3.52 × 10-4 ( 2 × 10-6)m2W (0.08656 ( 1 × 10-5)mW + (0.85916 ( 2 × 10-5) (37)
Figure 6. Instantaneous codistilled water mass mW against the oil extracted mass mOE , both divided by MOo .
Figure 4 shows an extreme point with a minimum value at mW (kg) ) 15.26. Therefore, eq 37 is only valid up to this codistilled water mass value. Figure 5 shows the integrated values of equation 37 against mW. The experimental results are also included for comparison. As can be seen, an excellent agreement is obtained. Therefore, the material balance appears to be correct since it represents the experimental results. Figure 5 also shows that the total distilled essential oil mass is 4.92 kg (corresponding to the abscissa of the minimum point in Figure 4) for an R value of 0.994. To calculate the ECS, we return to equation 29. The first term of the right-hand side of this equation is a function of R and its dependence with this variable can be obtained plotting mW/MOo against mOE /MOo ) R. This can be done, transposing the coordinates of Figure 5 and dividing each ordinate and abscissa value by MoO. Figure 6 shows this plot. This correlation, obtained using a trial and error method, is shown in Figure 7. Fitting the results with a nonlinear regression method within the 97.5% confidence limits gave the following equation:
{
mW ⁄ MOo ) (0.148 ( 7 · 10-3) X[(7.2 ( 0.3) -
[
0.010 ( 0.003 X] exp (1 - R)(0.81(0.03) where X ) -ln(1 - R).
]}
+ (0.016 ( 0.005) (38)
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9579
Figure 7. Instantaneous codistilled water mass mW divided by MOo against Z ) X[(7.2 ( 0.3) - X] exp[0.010 ( 0.003/(1 - R)(0.81 ( 0.03)], where X ) - ln(1 - R). Solid line corresponds to the least-squares fit using eq 38.
Figure 7 shows that eq 38 allows the fitting of all the experimental points, when mW/MoO is plotted against Z ) X[(7.2 ( 0.3) - X] exp[0.010 ( 0.003/(1 - R)(0.81 ( 0.03)]. Therefore, taking into account eq 29, the ECS can be estimated for any value of R from the following equation: -1 ) ) R(λjO ⁄ λW) + (0.148 ( 7 × 10-3) × ECS (kgsteam · kgfed,oil 0.010 ( 0.003 X[(7.2 ( 0.3) - X] exp + (1 - R)(0.81(0.03) (0.016 ( 0.005) (39) Equation 39 allows the calculation of the ECS for any R value, and it is valid for 0 < R e 0.999. For example, when R ) 0.994, ECS ) 3.12, which compares well with the experimental value obtained from eq 29, introducing the minimum value of the water mass at the extreme point (mW (kg) ) 15.26) and the filter cake fed mass
{
[
]}
-1 ) ) (15.26 ⁄ 4.95) + 0.994 × 0.140 ) 3.22 ECS (kgsteam · kgfed,oil (40)
The quotient between this value and R represents the number of kilograms of steam required to extract one kilogram of essential oil: -1 ) ) 3.22 ⁄ 0.994 ) 3.24 (41) (ECS ⁄ R) (kgsteam · kgextracted,oil
The estimation of ECS could also be made directly, using our experimental results of the extracted essential oil mass mOE and the codistilled water mass mW, which corresponds to Figure 5. Therefore, Figures 3 and 4 as well as some equations could be omitted. However, in the flat section of Figure 5 some dispersion of the experimental results can be seen when R f 1, because the experimental error for this section is greater than for the other sections of the curve as a consequence of the exhaustion of the essential oil in the fed mass. Therefore, the increments in the oil height are approximately of the same order of magnitude as the experimental error. This introduces serious difficulties in obtaining a safe correlation between ECS and R. 5. Conclusions We present a pilot-plant scale process for batch operation developed to recover all valuable materials contained in the filter cake arising from the dewaxing process of cold-pressed lemon essential oil. The developed process allows the recovery of
lemon essential oil, lemon wax, and diatomaceous earth contained in this cake and significantly reduces the environmental contamination. Its main characteristics are as follows: (1) To extract 1 kg of essential oil achieving an extraction level of 99.4%, 3.24 kg of steam are required. (2) The effective steam fraction was estimated in 74.2% of the steam injected to the stripper. (3) It is easy to manipulate and requires low manpower. (4) It has high efficiency for the recovery of lemon essential oil, lemon wax, and diatomaceous earth. (5) It needs low manometer steam pressure for the stripping process. (6) It does not require expensive facilities. (7) The feeding operations and discharges are simple and fast. (8) All the recovered materials possess industrial interest in different applications. The lemon wax can be employed for the production of wax to polish wood, floors, etc., the lemon essential oil in perfumery, cosmetics, etc., and the diatomaceous earth as load in construction materials, etc. (9) The process allows the transformation of a highly polluting solid waste into a liquid one, which is easer to treat. This wastewater arises from (i) the condensed vapor inside the stripper and (ii) the codistilled water (8.12 and 19.4 kg, respectively, see Table 2). The first wastewater contains suspended diatomaceous earth and wax particles that can be filtered and returned to the process. After filtration, this wastewater can be discharged without any treatment. The second one is an aqueous essential oil saturated solution with a small oil mass fraction, which can be completely eliminated by gas stripping. (10) Although no detailed economic balance was made in this work, the developed process is advantageous from an economic point of view. Furthermore, the reduction of the environmental contamination is an important additional benefit. To verify this statement we made a simple calculation using the number of tons of steam () 3.24 tons, see eq 41) required to extract 1 ton of steam-distilled lemon essential oil and the price of 1 ton of this oil type in the international market (around $11000).15 On the other hand, the cost of 1 ton of vapor at 10 bar using a natural gas boiler is $19-21).16 Therefore, the natural gas cost to produce 1 ton of the oil is $62-68. From this basic economic analysis it follows that there is an important difference between the price of 1 ton of oil in the international market and the natural gas cost required produce it, which surely can compensate other important operating costs (manpower, amortization of the facilities, etc.). On the other hand, the other recovered products (diatomaceous earth and lemon wax) would increase the benefit. Therefore, it is clear that this process is advantageous. Acknowledgment We are indebted to the ANPCyT (Grant, PICTO 2004, No. 633) of Argentina for financial support. We also wish to thank Mr. Jose´ Barrionuevo for the pilot-plant equipment assembly. Supporting Information Available: Experimental results for a typical pilot-plant run for the steam-distillation process, which were used to perform the mass and energy balances. This material is available free of charge via the Internet at http:// pubs.acs.org. Nomenclature j PV ) mean specific heat of the steam (kcal · K-1 · kg-1) C j PA ) mean apparent specific heat of the (stripper + feed) C (kcal · K-1 · kg-1)
9580 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 j PF ) mean specific heat of the feed (kcal · K-1 · kg-1) C j CSP ) mean specific heat of the construction materials of the stripper (kcal · K-1 · kg-1) j PW ) mean specific heat of the liquid water (kcal · K-1 · kg-1) C ECS ) effectively consumed steam required to extract a given fraction of essential oil from 1 kg of essential oil contained in the feed (kgSteam · kgFed,Oil-1) GD ) heterogeneous distilled mixture mass flow (kg · min-1) G0 ) steam mass flow introduced inside the stripper (kg · min-1) m ) instantaneous mass (feed + condensed steam) inside the stripper (kg) mT ) instantaneous total heterogeneous distilled mixture mass (kg) mW ) instantaneous codistilled water mass (kg) mOE ) instantaneous extracted oil mass (kg) MFo ) mass fed to the process (kg) MOo ) mass of the lemon essential oil in the feed (kg) MDo ) mass of the diatomaceous earth in the feed (kg) o MLW ) mass of the lemon wax in the feed (kg) MOE ) extracted oil mass (kg) MOL ) mass of the essential oil lost in the process (kg) MDL ) mass of the diatomaceous earth lost in the process (kg) MW ) total codistilled water mass (kg) MDR ) recovered diatomaceous earth mass (kg) R MLW ) recovered lemon wax mass (kg) R MO ) recovered lemon essential oil mass (kg) MVC ) condensed steam mass inside the stripper (kg) MVT ) total steam mass used to carry out the experiment (kg) MS ) mass of the stripper (kg) q ) total heat losses during the heating time t1 (kcal) ˙ ) lost heat flow in the stripper (kcal · min-1) Q t ) time (min) t1 ) time to distill the first drop (min) te ) “useful” or “effective” time (min) tT ) total time used to carry out the experiment (min) T ) instantaneous temperature inside the stripper (°C) Ti ) temperature at the beginning of the distillation process (°C) T0 ) temperature of the saturated steam (°C) TR ) room temperature (°C) wOE ) instantaneous codistilled oil mass fraction wOW ) oil mass fraction in the aqueous emulsion wOo ) essential oil mass fraction in the feed wOW ) oil mass fraction in the recovered lemon wax Greek letters R ) extracted essential oil fraction η ) effective steam fraction in the process ηG ) global yield of the process ηD ) recovery diatomaceous earth yield ηO ) recovery essential oil yield ηLW ) recovery lemon wax yield λV ) latent heat of vaporization of liquid water at 100 °C (kcal · kg-1)
λW ) latent heat of vaporization of liquid water at T °C (kcal · kg-1) λjjO ) average value of the latent heat of vaporization of lemon essential oil (kcal · kg-1) λOW ) ratio between the average values of the latent heat of vaporization of lemon essential oil and of water, both at 100 °C -1 (kgwater · kgoil )
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ReceiVed for reView December 20, 2007 ReVised manuscript receiVed August 20, 2008 Accepted October 6, 2008 IE7017355
