Hybrid Process Flow Diagram for Separation of Fusel Oil into Valuable Components

Abstract

Ethanol production by fermentation results in obtaining, in addition to the main product, ethyl alcohol, by-products and secondary products, which include carbon dioxide, fusel oil, and ester–aldehyde cut. Fusel oil, despite its low yield and the large volume of ethanol production, accumulates at distilleries, which ultimately raises the question of its disposal or the rational use of this by-product. Fusel oil, being a complex mixture, can serve as a source of technical alcohols used in various sectors of the economy, including the food industry, pharmaceuticals, organic synthesis, perfume, and cosmetics industries, as well as the production of paints and varnishes. However, the complexity of using fusel oil lies in its difficult separation. The reason for this is the presence of water, which forms low-boiling azeotropes with aliphatic alcohols. Our study aimed to develop a process flow diagram (PFD) that allows individual components from fusel oil to be obtained without extraneous separating agents (not inherent in fusel oil). This condition is necessary to obtain products labeled as natural for further use in the food, perfume, cosmetic, and pharmaceutical industries. The distinctive feature of this work is that the target product is not only isoamyl alcohol but also all other alcohols present in the composition of fusel oil. To achieve this goal and create a mathematical model, the Aspen Plus V14 application, the Non-Random Two Liquid (NRTL) thermodynamic model, and the Vap-Liq/Liq-Liq phase equilibrium were used. Fusel oil separation was modeled using a continuous separation PFD to obtain ethanol, water, isoamyl alcohol, and raw propanol and butanol cuts. The Sorel and Barbet distillation technique was used to isolate ethanol. The isolation of isopropanol and 1-propanol, as well as isobutanol and 1-butanol, was modeled using the batch distillation method. The isolation of fusel oil components was based on their thermodynamic properties and the selection of appropriate techniques for their separation, such as extraction, distillation, pressure swing distillation, and decantation. The simulation of fusel oil separation PFD showed the possibility of obtaining the components of a complex mixture without separating agents, as discussed earlier. Ethanol corresponds to the quality of rectified ethyl alcohol, and 1-butanol and isoamyl alcohols to anhydrous alcohols, whereas isopropanol (which contains an admixture of ethanol), 1-propanol, and isobutanol are obtained as aqueous solutions of different concentrations of alcohols. However, due to a distillation boundary in the raw propanol and butanol cuts, these mixtures cannot be separated completely, which leads to the production of intermediate fractions. To eliminate intermediate fractions and obtain anhydrous isopropanol, 1-propanol, and isobutanol in the future, it is necessary to solve the dehydration problem of either fusel oil or the propanol–butanol mixture.

1. Introduction

The global production of ethyl alcohol through fermentation has steadily increased each year, having bounced back from the COVID-19 pandemic [1]. Ethyl alcohol is used for various purposes, including food, motor fuels, solvents, and as raw material in chemical and pharmaceutical manufacturing. This wide range of applications ensures a strong demand for ethyl alcohol. The primary driver of production growth is reducing the carbon footprint in the economy. This involves transitioning to cleaner alternative energy sources and decreasing hydrocarbon consumption. The ethanol production process through fermentation results in various naturally occurring by-products and secondary products, including carbon dioxide, fusel oil, and the ester–aldehyde cut [2]. This article discusses how fusel oil can be separated into components to meet the needs of the national economy.

The yield of fusel oil can vary from 1 to 11 L per 1000 L of ethanol, depending on several fermentation conditions, such as the type of raw material used, the temperature, the pH level, aeration, genetic engineering techniques, inorganic nutrients, and carbon dioxide pressure [3,4,5,6,7,8,9,10,11,12,13]. As ethanol production increases, there is a proportional rise in the yield of fusel oils. Fusel oil is a valuable source of industrial alcohols utilized in various industries, including food, perfume, pharmaceuticals, and paint and varnish. They also serve as a source of oxygenates for the production of reformulated gasoline [14,15]. Nevertheless, the primary application of this by-product, owing to its low yield, is for combustion in boiler furnaces at distilleries to generate steam or as a source of isoamyl alcohol [16]. As the major component of fusel oil, isoamyl alcohol is always the target product of the separation process. This component is widely used as a solvent, flotation agent, and source for isoamyl acetate, which is used in perfumery and confectionery as a flavoring and fragrance agent and as a solvent in the paint and varnish industry [17]. This perspective on valuable raw materials stems from the challenges of separation. Fusel oil comprises a complex mixture of aliphatic alcohols, water, and other organic compounds found in insignificant quantities. The mixture is complex due to the large number of components and their physical properties. Many components have boiling points close to each other, making separation by distillation challenging. Additionally, water in fusel oil creates azeotropes with alcohols, including homoazeotropes and heteroazeotropes, as well as binary and ternary azeotropes (see

Table 1. Azeotropes and their properties.

№	Components	Azeotrope Type	Boiling Point, °C	Composition, Mass Fraction
1	Ethanol–Water	Homoazeotrope	78.15	0.9562–0.0438
2	Isopropanol–Water	Homoazeotrope	80.18	0.8727–0.1273
3	1-Propanol–Water	Homoazeotrope	87.67	0.6923–0.3077
4	Isobutanol–Water	Heteroazeotrope	92.89	0.5668–0.4332
5	1-Butanol–Water	Heteroazeotrope	95.91	0.4233–0.5767
6	1-Butanol–3M1B–Water	Heteroazeotrope	95.63	0.1811–0.2741–0.5448
7	2M1B–Water	Heteroazeotrope	94.92	0.5025–0.4975
8	3M1B–Water	Heteroazeotrope	95.71	0.4585–0.5415
9	1-Hexanol–Water	Heteroazeotrope	98.21	0.2793–0.7207

). Table 1 presents the azeotropes of the mixture generated in the Aspen Plus application, utilizing the NRTL (Non-Random Two Liquid) thermodynamic model and Vap-Liq/Liq-Liq phase equilibrium at atmospheric pressure.

The major components of fusel oil are water, ethyl alcohol, and isoamyl alcohol, which consists of isomers of 2-methyl-1-butanol (2M1B) and 3-methyl-1-butanol (3M1B). The most comprehensive data on the composition of fusel oil from various raw materials are provided in articles by Thainara B. Massa et al. and N. Montoya et al. [18]. For this reason, the limited number of publications on this topic primarily focuses on isolating isoamyl alcohol. However, when scaling up production, even though other components, such as isopropanol, 1-propanol, isobutanol, and 1-butanol are present in small amounts, their accumulation can become significant. If these substances are not used properly or processed, they require further disposal.

Several methods and techniques are available for separating mixtures that form one or more azeotropes. Homoazeotropes can be separated using pressure swing distillation or by adding additional substances known as separating agents (entrainer). Another method involves using membranes, which are often combined with distillation. When distillation is supplemented with other methods, this is called a hybrid distillation system. Possible combinations include distillation with adsorption or liquid extraction. On the other hand, heteroazeotropes can be separated using decantation. A hybrid approach, typically known as heteroazeotropic distillation, involves combining a decanter and distillation column [19,20].

The methods proposed for separating fusel oil primarily rely on the abovementioned techniques for separating mixtures with one or more azeotropes. Fusel oil components are isolated through distillation using trayed or packed columns, which can be arranged in various process flow configurations. These methods may be continuous, batch, or a combination of both.

Authors studying the problem of fusel oil separation offer various solutions. For example, in their article, Marcela C. Ferreira et al. consider three possible options for separating fusel oil to obtain isoamyl alcohol: a single-column PFD with a decanter for a side stream, a two-column PFD with a decanter for the first column distillate, and the third option is a decanter, stripper, and distillation column arrangement. The article estimates the total annual cost (TAC) as an efficiency indicator [21]. José de Jesús Mendoza-Pedroza et al. propose using a dividing wall column (DWC) as an alternative to the conventional three-column configuration. In this case, the feedstock is considered to be a mixture of water, ethanol, and isoamyl alcohol with a small amount of other components. TAC is also used as an efficiency criterion [22]. Another five-column PFD produces a greater variety of components, including ethanol, isoamyl alcohol, n-propyl alcohol, and isobutyl alcohol. Isoamyl alcohol itself is utilized for the dehydration of fusel oil using the method of azeotropic–extractive distillation [23]. A method for separating fusel oils using a low-boiling solvent, specifically hexane, in a batch distillation column is also proposed. In this method, the aqueous cut is separated first, followed by a mixture of C2–C4 alcohols, isoamyl alcohol, a high-boiling cut, and the distillation residue.

Other proposals concern the internal devices used in distillation columns. Researchers from the Slovak University of Bratislava, led by Vladimír Báleš et al., conducted experiments to obtain isoamyl alcohol in a packed column under various distillation conditions [24]. In addition to using packaging, researchers from Tianjin University propose a pilot plant that combines batch and continuous distillation to separate the isomers of isoamyl alcohol 2M1B and 3M1B [25].

Our study aims to develop a PFD for separating fusel oil into its constituent components without using extractive and separating agents not inherent in fusel oil. This method will enable us to obtain alcohols free from contaminants, allowing them to be labeled as natural products. Our focus is not only on isolating isoamyl alcohol but also on producing ethanol, isopropanol, 1-propanol, isobutanol, and 1-butanol.

While previous studies on fusel oil separation mentioned in earlier paragraphs and other scientific sources have primarily targeted the isolation of isoamyl alcohol and ethanol, many of these studies did not address the presence of components like isopropanol or 1-butanol in the feedstock [26,27,28]. The presence of these alcohols would have significantly complicated the separation process, not only because of the azeotropes formed but also due to their close boiling temperatures; all of them are low-boiling azeotropes. In our study, the initial mixture presents additional challenges because it includes both isopropanol and 1-butanol together with other components.

It is worth noting that our study was innovative due to the use of a hybrid PFD for fusel oil separation, which employs liquid extraction for the preliminary separation of light components from the feedstock in combination with distillation and decantation processes. Also, the expanded component composition of the fusel oil sample under study allows us to consider the most complex variant of the mixture for separation, which may imply greater flexibility of the proposed PFD in case of a change in the feedstock composition.

2. Materials and Methods

The fusel oil sample used in this study was provided by the Natural Ingredients Ro company. The sample’s qualitative characteristics, determined experimentally, are presented in

Table 2. Physical–chemical characteristics of fusel oil.

Characteristics	UM	Value	Standard Method
Density at 20 °C	kg/m3	829	ASTM D 5501 [30]
Refractive index at 20 °C	-	1.3990	ASTM D 1218 [31]
Kinematic viscosity at 40 °C	cSt	2.6	ASTM D 445 [32]
Pour point	°C	−34	ASTM D 97 [33]
Water content	wt.%	14	ASTM D 4006 [34]
Copper corrosion	-	2c	ASTM D 130 [35]
Stainless steel corrosion	-	No corrosion	ASTM A 380 [36]
Calorific value	kJ/kg	32,858	ASTM D 5865-12 [37]

, while its component composition is detailed in

Table 3. Component composition of fusel oil.

№	Components	Mass Concentration, wt.%
1	Ethanol	9.0
2	Isopropanol	1.0
3	1-Propanol	1.0
4	Isobutanol	1.5
5	1-Butanol	0.5
6	2-Methyl-1-butanol (optically active amyl alcohol)	2.0
7	3-Methyl-1-butanol (isoamyl alcohol)	70.0
8	1-Hexanol	1.0
9	Water	14.0
Total:		100

. The distillation curve according to ASTM D 86 is shown in Figure 1 [29]. The separation process was simulated using the Aspen Plus V14 application, employing the NRTL thermodynamic model and analyzing the Vap-Liq/Liq-Liq phase equilibrium. The binary interaction parameters data, unavailable in the Aspen Plus database, were predicted using the UNIFAC group contribution model. The models for the extractor and distillation columns used in the simulations were based on the Radfrac setup.

The initial condition for the separation of fusel oil was to recover isoamyl alcohol, which is a mixture of isomers of 2-methyl-1-butanol (2M1B) and 3-methyl-1-butanol (3M1B) of no less than 99.6 wt.% of the original mixture. The purity must be at least 99 wt.%, which includes the sum of 2M1B, 3M1B, and 1-hexanol, without using any separating agents. These requirements are essential for the further processing of isoamyl alcohol into isoamyl acetate for the needs of the food industry. Additionally, to scale up the process, the production of isoamyl alcohol must be at least 1250 kg/h to ensure continuous operation [38].

It is proposed to separate fusel oil using a hybrid extraction–distillation PFD to achieve the objectives outlined in the article’s introduction. The entire process flow can be divided into three zones: a preliminary extraction separation zone; a water treatment zone that includes distillation and water-stripping columns, since the process has an excess amount of water; a distillation separation zone features a column for ethanol, two columns for isoamyl alcohol, and another for separating propanols and butanols. The equipment in this setup operates continuously. Additionally, the PFD includes a batch distillation column for separating isopropanol, 1-propanol, isobutanol, and 1-butanol. The proposed PFD is illustrated in Figure 2.

Preliminary separation of the feedstock is performed through liquid extraction, using water as the extractive agent. Ethanol and isopropanol dissolve well in water, which is one of the components of fusel oil, so using water as an extractive agent is obvious. Moreover, water’s ability to separate from organic compounds by distillation can be ideally used to regenerate water and return it for extraction. The optimal ratio of fusel oil to the extractive agent is 1:1. Increasing the water supply to 1:1.2 results in higher water content in the extract. The operational parameters of the extraction process are given in

Table 4. Extraction process operational parameters (E-1).

1	Extractor type E-1	Column
2	Extraction process type	Continuous
3	Number of trays in the E-1 extractor	30
4	Top tray pressure, bar	1.8
5	Top tray temperature, °C	10
6	Extractive agent	Water
7	Extractive agent temperature, °C	10
8	Extractive agent pressure, bar	2
9	Fusel oil pressure, bar	2
10	Fusel oil temperature, °C	10
11	Fusel oil flow rate into the E-1 extractor, kg/h	1740
12	Fusel oil to extractive agent ratio	1:1

.

The fusel oil, pre-separated in this way, leaves the E-1 extractor as an extract and a raffinate stream, with the composition given in

Table 5. Extract and raffinate composition from the E-1 extractor.

Components	Mass Fraction
	Extract	Raffinate
Ethanol	0.0739	1.59 × 10−6
Isopropanol	0.0066	0.0025
1-Propanol	0.0032	0.0078
Isobutanol	0.0022	0.0157
1-Butanol	0.0009	0.0050
2M1B	0.0012	0.0237
3M1B	0.0331	0.8427
1-Hexanol	0.0002	0.0125
Water	0.8787	0.0901
Total:	1	1

. Table 5 shows that the extraction process successfully separates lighter components from the initial raw materials, such as ethanol and isopropanol, while heavier components are separated to a lesser extent. The resulting raffinate is enriched with isomers of isoamyl alcohol.

The extract stream is routed to the water treatment area and separated in a distillation column, C-1. This column produces a distillate, an organic mixture, and a bottom product, water. The distillate is then mixed with an additional amount of ethanol and sent to the C-2 ethanol column. In the ethanol column, ethanol is separated under low pressure as a distillate, while the remaining components of the mixture are collected as a bottom product. Adding more ethanol is based on the distillation theory developed by French scientists Sorel and Barbet, who established methods for purifying alcohol from impurities. This technique allows close-boiling substances, such as ethanol and isopropanol, to be separated within a single column [39,40,41]. The bottom product from the ethanol column is combined with the raffinate and then directed to the first isoamylic column, C-3, which operates under low pressure. The bottom product of the first isoamylic column is already anhydrous isoamyl alcohol. However, the distillate contains nearly 56 wt.% of water and organic components. This distillate is sent to a decanter, D-1, where it splits into aqueous and organic phases. The drain water, having an insignificant content of organic components, is then fed to the water-stripping column C-5 for purification. The resulting vapor phase is condensed in a cooler, after which it is sent to the D-1 decanter. The purified water is then mixed with the BTTM1 stream. After cooling, a portion of the water will be returned to the E-1 extractor, while the remaining excess amount will undergo further purification to ensure it meets disposal requirements for various indicators, such as chemical oxygen demand (COD), biological oxygen demand (BOD), pH level, turbidity, total nitrogen, total phosphorus, and heavy metals [42]. The organic phase, labeled Prod1, is then fed into the second isoamylic column C-4 using a pump. In this column, the remaining isoamyl alcohol is recovered under low pressure. To achieve a precision separation of the C₃-C₄ alcohols cut from isoamyl alcohol and to prevent contamination of the distillate Dist4 with isoamyl alcohol, it is proposed to withdraw a side stream SS1 based on the component profile of the column C-4 trays (tray number 33). To maximize the recovery of isoamyl alcohol using the continuous method, the ethanol column C-2 is sited before the first isoamylic column C-3, and the condensed vapors from the water-stripping column C-5 are returned to the decanter D-1. The bottom products from the first and second isoamylic columns, C-3 and C-4, and the side stream, SS1, from the second isoamylic column, are mixed to produce the final product: isoamyl alcohol. The distillate stream Dist4 from the second isoamylic column C-4, which contains a mixture of C₃–C₄ alcohols, is routed to the propanol column C-6. The mixture is separated under increased pressure into raw propanol and butanol cuts. An additional side stream, SS2, enriched with propyl alcohols, is withdrawn (tray number 4) and mixed with the distillate Dist5 from the propanol column C-6 to separate these cuts precisely. The raw butanol cut is sent to the decanter D-2 for separation into aqueous and organic phases. The separated excess water from this process is directed to the water-stripping column C-5. The raw butanol and propanol cuts will be further separated into components through batch distillation.

Table 6. Operational parameters of water treatment and distillation processes.

№	Parameter/Characteristic	Water Distillation Column C-1	Ethanol Column C-2	First Isoamylic Column C-3	Second Isoamylic Column C-4	Water-Stripping C-5	Propanol Column C-6
1	Number of trays	30	50	30	40	10	36
2	Condenser type	Total	Total	Total	Total	None	Total
3	Valid phase	Vap-Liq	Vap-Liq	Vap-Liq	Vap-Liq	Vap-Liq	Vap-Liq
4	Convergence	Standard	Standard	Standard	Standard	Standard	Azeotropic
5	Distillate rate, kg/h	380	270.2	428	107.4	46	50
6	Reflux ratio	4	7	5	5.5	-	6
7	Feed tray number	2	23	2	21	1	21
8	Pressure, bar	1.6	0.15	0.3	0.3	1	3
9	Temperature at the top of the column, °C	100	37	64	62	90	116
10	Temperature at the bottom of the column, °C	115	56	106	106	102	129

details the operational parameters for the water treatment and distillation processes.

Thus, through the continuous separation method, ethanol, isoamyl alcohol, water, and raw propanol and butanol cuts are obtained. The composition of these products is presented in

Table 7. Component composition of continuously obtained products.

№	Indicator Name	Ethanol	Isoamyl Alcohol	Water	Raw Propanol Cut	Raw Butanol Cut
1	Average Molecular Weight	43.39	88.31	18.02	37.44	46.95
2	Component composition, mass fraction
	Ethanol	0.9562	1.43 × 10−15	8.70 × 10−7	0.0931	1.11 × 10−7
	Isopropanol	0.0036	6.71 × 10−14	7.71 × 10−10	0.3268	3.21 × 10−10
	1-Propanol	2.38 × 10−10	9.32 × 10−11	1.10 × 10−8	0.3327	2.73 × 10−5
	Isobutanol	5.76 × 10−15	2.51 × 10−8	5.18 × 10−9	0.0004	0.6122
	1-Butanol	3.02 × 10−21	7.84 × 10−5	2.56 × 10−7	4.63 × 10−11	0.2019
	2M1B	5.26 × 10−24	0.0274	2.58 × 10−15	4.78 × 10−6	7.15 × 10−11
	3M1B	2.80 × 10−21	0.9588	6.00 × 10−12	9.48 × 10−6	1.00 × 10−8
	1-Hexanol	2.60 × 10−40	0.0137	1.36 × 10−15	0	0
	Water	0.0402	2.42 × 10−12	1	0.2470	0.1859
	Total:	1	1	1	1	1

.

The PFD for the continuous separation of fusel oil is supplemented by a batch distillation column, C-7. This column is designed to separate the raw propanol and butanol cuts, which are mixtures of two low-boiling azeotropes (refer to Table 1). Due to the characteristics of azeotropes, classical distillation methods cannot achieve complete separation. This limitation is clearly illustrated in Figure 3, where the diagrams of ternary mixtures obtained from the Aspen Plus V14 application (NRTL, Vap-Liq-Liq phase equilibrium, atmospheric pressure) are presented. The figure shows distillation boundaries that cannot be crossed and two distinct distillation zones within which separation can occur.

The presence of a distillation boundary when using conventional batch distillation predicts the existence of an intermediate cut containing all the mixture’s components. Batch distillation for separating raw propanol and butanol cuts is justified because the concentration of these components in the feedstock—fusel oil—is only 1 to 1.5 wt.%. This low concentration makes using continuous columns impractical, and batch distillation reduces the equipment needed to separate the original mixtures.

To evaluate the feasibility of obtaining alcohols such as isopropanol, 1-propanol, isobutanol, and 1-butanol, a batch distillation column with 40 trays was selected. The Aspen Plus V14 simulation application was used, utilizing the NRTL thermodynamic model, the Vap-Liq/Liq-Liq phase equilibrium, and the BatchSep setup. Batch distillation allows the process to be divided into operating steps, with each step designed to recover a specific component or mixture of components from the initial charge. The operational parameters for separating the raw propanol cut are detailed in

Table 8. Raw propanol cut primary distillation.

Initial conditions: Number of trays 40     Total reflux Condenser pressure 2 bar   Subcooled temperature 50 °C Mass boil-up rate 500 kg/h   Total initial charge 1255.2 kg
Operating step number	Reflux ratio	Mass boil-up rate, kg/h	Condenser pressure, bar	Liquid distillate receiver number	Ramp time, h	Duration time, h
O-1	Total	500	2			1
O-2	6	1200	2	1	1	3.5
O-3	6	1200	1.1	2	0.9	3

. Primary distillation is performed using the pressure swing distillation method.

The composition of the obtained products is given in

Table 9. Primary raw propanol cut distillation products composition.

№	Component	Distillate 1	Distillate 2	Bottom 1
		Mass Fraction
l	Ethanol	0.1644	0.0908	0.0007
2	Isopropanol	0.7086	0.1981	0.0017
3	1-Propanol	0.0072	0.4539	0.5932
4	Isobutanol	1.67 × 10−9	1.54 × 10−9	0.0014
5	1-Butanol	1.67 × 10−9	1.54 × 10−9	1.76 × 10−10
6	2M1B	1.99 × 10−9	1.83 × 10−9	1.81 × 10−5
7	3M1B	1.99 × 10−9	1.83 × 10−9	3.60 × 10−5
8	1-Hexanol	0	0	0
9	Water	0.1198	0.2572	0.4029
	Total:	1	1	1

.

The data presented in Table 9 indicate that Distillate 1 is composed of a mixture of ethanol, isopropanol, and water. In contrast, Distillate 2 is an intermediate cut that contains ethanol, isopropanol, 1-propanol, and water. The pressure reduction at step O-3 is intended to enhance the enrichment of the bottom with 1-propanol, which is an aqueous solution. Once the primary distillation is complete, the bottom product of the column is emptied. Subsequently, the intermediate cut, Distillate 2, is transferred from the second receiver R-2 back to the still bottoms of the column for repeated distillation. The distillation conditions are outlined in

Table 10. Propanol intermediate cut secondary distillation.

Initial conditions: Number of trays 40     Total reflux Condenser pressure atm   Subcooled temperature 50 °C Mass boil-up rate 240 kg/h   Total initial charge 480.981 kg
Operating step number	Reflux ration	Mass boil-up rate, kg/h	Condenser pressure, bar	Liquid distillate receiver number	Ramp time, h	Duration time, h
O-1	Total	240	1			1
O-2	6	450	0.5	1	1	5.7

.

The composition of the obtained products from secondary distillation is given in

Table 11. Propanol secondary distillation products composition.

№	Component	Distillate 3	Bottom 2
		Mass Fraction
1	Ethanol	0.1404	0.0003
2	Isopropanol	0.3045	0.0040
3	1-Propanol	0.3390	0.6637
4	Isobutanol	2.39 × 10−9	4.35 × 10−9
5	1-Butanol	2.39 × 10−9	4.35 × 10−9
6	2M1B	2.84 × 10−9	5.18 × 10−9
7	3M1B	2.84 × 10−9	5.18 × 10−9
8	1-Hexanol	0	0
9	Water	0.2161	0.3320
	Total:	1	1

.

Table 11 indicates that Distillate 3 is an intermediate cut, while the bottom product is rich in 1-propanol. To obtain the 1-propanol cut, the bottom products from the previous and subsequent distillations must be combined. The raw butanol cut is distilled using the same apparatus as the one used for the propanols. Before introducing the raw butanol cut, the column must be emptied and rinsed. The details of the distillation process are provided in

Table 12. Primary raw butanol cut distillation.

Initial conditions: Number of trays 40      Total reflux Condenser pressure 3 bar   Subcooled temperature 50 °C Mass boil-up rate 500 kg/h   Total initial charge 1022.39 kg
Operating step number	Reflux ration	Mass boil-up rate, kg/h	Condenser pressure, bar	Liquid distillate receiver number	Ramp time, h	Duration time, h
O-1	Total	500	3			1
O-2	4	1010	3	1	1	8
O-3	6	1010	0.7	2	1	1.7

.

During the primary distillation of the raw butanol cut, the products listed in

Table 13. Raw butanol primary distillation products composition.

№	Component	Distillate 1	Distillate 2	Bottom 1
		Mass Fraction
1	Ethanol	1.60 × 10−7	2.48 × 10−9	0
2	Isopropanol	1.31 × 10−9	3.23 × 10−9	0
3	1-Propanol	3.94 × 10−5	3.23 × 10−9	0
4	Isobutanol	0.7443	0.5276	0.0068
5	1-Butanol	4.55 × 10−6	0.4239	0.9928
6	2M1B	1.35 × 10−9	4.74 × 10−9	0
7	3M1B	1.58 × 10−8	4.74 × 10−9	0
8	1-Hexanol	0	0	0
9	Water	0.2556	0.0484	0.0004
	Total:	1	1	1

are obtained.

The data presented in Table 13 show that the headstream, known as Distillate 1, consists of a mixture of isobutanol and water. Distillate 2 is an intermediate cut, while the residue contains 1-butanol. Distillate 1 is sent to a decanter, D-3, to separate excess water under 1 bar of pressure and 45 °C conditions. The collected water will be periodically routed for processing in the water-stripping column C-5. Furthermore, the intermediate cut, Distillate 2, requires secondary distillation. The specific distillation mode is detailed in

Table 14. Butanols intermediate cut secondary distillation.

Initial conditions: Number of trays 40      Total reflux Condenser pressure 1 bar    Subcooled temperature 50 °C Mass boil-up rate 90 kg/h   Total initial charge 185.7 kg
Operating step number	Reflux ration	Mass boil-up rate, kg/h	Condenser pressure, bar	Liquid distillate receiver number	Ramp time, h	Duration time, h
O-1	Total	90	1			1
O-2	6	170	0.7	1	1	4
O-3	6	170	1	1	1	2.5
O-4	6	170	1	2		3

.

As a result of secondary distillation, the following products are obtained, shown in

Table 15. Butanol secondary distillation products.

№	Component	Distillate 3	Distillate 4	Bottom 2
		Mass Fraction
1	Ethanol	1.11 × 10−8	8.29 × 10−9	0
2	Isopropanol	1.45 × 10−8	1.08 × 10−8	0
3	1-Propanol	1.45 × 10−8	1.08 × 10−8	6.84 × 10−18
4	Isobutanol	0.8907	0.4314	0.0021
5	1-Butanol	0.0009	0.5686	0.9979
6	2M1B	1.06 × 10−8	1.59 × 10−8	1.87 × 10−8
7	3M1B	1.06 × 10−8	1.59 × 10−8	1.87 × 10−8
8	1-Hexanol	0	0	0
9	Water	0.1084	2.20 × 10−6	2.85 × 10−8
	Total:	1	1	1

.

After analyzing the composition of the obtained products, it is obvious that Distillate 3 should be routed to decanter D-3, while Distillate 4 will be referred to as the intermediate cut. The bottom product is identified as a cut of 1-butanol. The bottom products from the primary and secondary distillation processes are combined into one stream, designated as 1-butanol.

Table 16. Composition of propanol and butanol cuts.

№	Components	Propanols			Butanols
		Isopropanol	Intermediate Cut	1-Propanol	Isobutanol	Intermediate Cut	1-Butanol
		Mass Fraction
1	Ethanol	0.1644	0.1404	0.0006	1.49 × 10−7	8.29 × 10−9	0
2	Isopropanol	0.7086	0.3045	0.0025	2.83 × 10−9	1.08 × 10−8	0
3	1-Propanol	0.0072	0.3390	0.6172	3.75 × 10−5	1.08 × 10−8	1.84 × 10−18
4	Isobutanol	1.67 × 10−9	2.39 × 10−9	0.0009	0.8178	0.4314	0.0055
5	1-Butanol	1.67 × 10−9	2.39 × 10−9	1.60 × 10−9	0.0001	0.5686	0.9942
6	2M1B	1.99 × 10−9	2.84 × 10−9	1.20 × 10−5	2.51 × 10−9	1.59 × 10−8	5.01 × 10−9
7	3M1B	1.99 × 10−9	2.84 × 10−9	2.37 × 10−5	1.64 × 10−8	1.59 × 10−8	5.01 × 10−9
8	1-Hexanol	0	0	0	0	0	0
9	Water	0.1198	0.2161	0.3788	0.1820	2.20 × 10−6	0.0003
	Total:	1	1	1	1	1	1

presents the qualitative and quantitative characteristics of the final products and intermediate cuts obtained through batch distillation.

Upon analyzing the data in Table 16, we observe that 1-butanol exhibited the best result, with a concentration of the key component exceeding 0.99 mass fraction. Following closely is isobutanol, which has a concentration of nearly 0.82 mass fraction. Isopropanol, however, contains both ethanol and water. Although the 1-propanol cut has higher purity in terms of the key component, it includes a significant amount of water, reaching almost 0.38 mass fraction. Additionally, in both cases, there are intermediate cuts that cannot be completely separated due to the presence of a distillation boundary. These cuts can be returned to the initial raw propanol and butanol cuts for further separation unless they find alternative applications.

To evaluate the results obtained, it is suggested to consider the material balance of the fusel oil separation and the recovery rate of the key components shown in

Table 17. Material balance of the continuous separation process.

№	Balance Item	Flow Rate, kg/h	Potential Content of Key Component, kg/h									Recovery Rate, Mass Fraction
			Ethanol	Isopropanol	1-Propanol	Isobutanol	1-Butanol	2M1B	3M1B	1-Hexanol	Water
Feed:
1	Fusel oil	1740	156.6	17.4	17.4	26.1	8.7	34.8	1218	17.4	243.6	-
2	Water (extractive agent)	1740									1740	-
3	Ethanol (surplus)	112	106.624	0.672							4.704	-
Product:
	Ethanol	270.2	258.354	0.978823							10.8673	0.9689
	Isoamyl alcohol	1270.3					0.0996	34.7998	1218	17.4		Sum of isomers 0.9999
	Raw propanol cut	52.3	4.86929	17.0932	17.3988	0.0197		0.0002	0.0005		12.9183	Sum of isomers 0.9911
	Raw butanol cut	42.5994			0.0012	26.0801	8.5998				7.9182	Sum of isomers 0.9965
	Treated water	1956.6									1956.6	0.8892 *

(*) The actual tent recovery is expected to be 0.9 due to the separation of excess water from isobutanol obtained by batch distillation.

and

Table 18. Material balance of batch distillation of raw propanol and butanol cuts *.

№	Balance Item	Charge/Flow, kg	Potential Content of Key Component, kg									Recovery Rate, Mass Fraction
			Ethanol	Isopropanol	1-Propanol	Isobutanol	1-Butanol	2M1B	3M1B	1-Hexanol	Water
Feed:
1	Raw propanol cut	1255.2	116.863	410.236	417.571	0.4728		0.0060	0.0119		310.039	-
Product:
1	Isopropanol	443.697	72.948	314.394	3.179						53.176	0.7664
2	Intermediate cut	310.738	43.6255	94.6137	105.334						67.165	-
3	1-Propanol	500.766	0.2893	1.2289	309.059	0.4728		0.0060	0.0119		189.698	0.7401
Feed:
1	Raw butanol cut	1022.39	0.0001		0.0279	625.926	206.397				190.039	-
Product:
1	Isobutanol	729.841	0.0001		0.0274	596.871	0.0800				132.863	0.9536
2	Water (drain)	61.1768			0.0005	4.0531	0.0007				57.1225	0.3006
3	Intermediate cut	55.5672				23.9691	31.5979				0.0001	-
4	1-Butanol	175.82				0.9732	174.799				0.0487	0.8469

(*) Due to the low cut yield, a load equal to daily production, i.e., for 24 h, is used to calculate the batch distillation.

.

3. Results

Drawing conclusions based on the results of developing an effective PFD for separating fusel oil into its components, the following key points can be highlighted:

1. The preliminary separation of fusel oil through liquid extraction not only facilitated the further separation of the raw material but also ensured that the fusel oil and its components were not contaminated with other substances.

2. The PFD demonstrated successful results, achieving a recovery rate for the primary target component—isoamyl alcohol—of 0.999 mass fraction.

3. Additionally, a 0.969 mass fraction of ethyl alcohol was isolated from the potential content of the original raw material.

4. A mixture of C₃–C₄ alcohols was separated, resulting in the acquisition of raw propanol and butanol cuts with recovery rates of 0.991 and 0.997 mass fractions, respectively.

5. However, further separation of the raw propanol and butanol cuts through batch distillation encountered challenges due to the presence of low-boiling azeotropes. This resulted in intermediate cuts during distillation, which had to be returned to the original raw cuts unless used for specific purposes, for instance, as solvent. The yield of the intermediate cut was 24.70 wt.% for raw propanols and 5.44 wt.% for raw butanols.

6. Special attention should be given to the water content in the obtained C₃–C₄ alcohol products. Isobutanol and 1-butanol, which form heteroazeotropes, are prone to phase split and water removal. In contrast, isopropanol and 1-propanol form homoazeotropes. Consequently, 1-propanol has a concentration of approximately 62 wt.%.

7. The water produced during the separation process is not overlooked; all water flows are directed to a water treatment zone, where they are purified of organic impurities before being returned to the separation process and further disposal process.

8. The products resulting from the separation process can be characterized by the following parameters: ethanol has a 96.4 vol.% of ethyl alcohol; isoamyl alcohol has a 98.6 wt.% of the sum of isomers; isopropanol has a concentration of 70.9 wt.% of the key component but contains a significant admixture of ethyl alcohol (16.4 wt.%); 1-propanol has a concentration of 61.7 wt.%, with nearly 38 wt.% water content; isobutanol has a concentration of 81.8 wt.% (after decantation) and 18.2 wt.% water; and 1-butanol has a concentration of 99.4 wt.%.

4. Discussion

The separation PFD simulation results indicate that fusel oil can be effectively separated into its constituent components using a hybrid PFD (extraction–distillation). However, separating C₃–C₄ alcohols is complicated by the presence of water, which forms azeotropes. While the butanols’ cuts can be partially separated from some of the water through decantation, the propanols’ cuts cannot be separated by this method due to the formation of homoazeotropes. Therefore, further research should focus on the dehydration of the fusel oil feedstock or propanol–butanol cut (Dist4). Moreover, dehydration of the propanol–butanol cut is preferable due to the smaller number of components and the lower yield of the cut itself compared to the original raw material—fusel oil—which will require less costs. Based on literature data, it is assumed that redistributing the flows of components that form heteroazeotropes may provide a solution for dehydrating the feedstock, thereby addressing the challenge of separating C₃–C₄ alcohols [43]. This approach could eliminate intermediate cuts and yield more concentrated products.

The PFD has shown promising results in isolating and concentrating isoamyl alcohol and other components. Isoamyl alcohol will be used for esterification by acetic acid to synthesize valuable isoamyl acetate. According to analytical companies, from 2021 to 2030, the compound annual growth rate (CAGR) of the isoamyl acetate market is expected to be 5.5% and amount to USD 455.27 million by the end of the specified period [44]. Ethyl alcohol holds potential for technical applications, while isopropyl alcohol is produced with a notable admixture of ethyl alcohol, as both components have close boiling points. However, this mixture may be used as a disinfectant since both components exhibit high antiseptic activity. It is recommended to maintain the total alcohol concentration at 70% [45]. The global isopropanol market is driven by its use as a component of sanitizers, so the expected CAGR between 2023 and 2032 is projected to be 8.1%, rising from the current USD 2.7 billion to USD 5.9 billion in 2032 [46]. However, when the isopropanol product undergoes dehydration, the resulting mixture can be effectively used as an oxygenate additive in gasoline [47]. Solvent or windshield washer fluid can be another application of the mixture. 1-Butanol is used as a solvent in the paint and varnish industry and as a feedstock for producing esters, including plasticizers [48]. The automotive paint market drives 1-butanol consumption. From 2023 to 2031, the market growth (CAGR) is expected to be 6.2%, reaching USD 14.4 billion [49]. Therefore, further studies should explore the dehydration processes for isopropanol, 1-propanol, and isobutanol, mainly focusing on the behavior of their mixtures with water. Addressing this issue will significantly increase the potential applications of the resulting alcohols.

The developed PFD maintains an integrated approach, ensuring all feedstock components are separated and prepared for further use. Even the impurified water can be processed and returned as a make-up for the extractive agent.

Fusel oil recycling fits into the circular economy paradigm because it is a waste product of the agro-industrial complex. Alcohols obtained synthetically by the hydration of olefins or from the oxo synthesis process consume the starting materials produced from petroleum feedstock, while alcohols isolated from fusel oil reduce the carbon footprint, which is relevant today.

In conclusion, the goals outlined in the introduction of this article have been achieved. The simulation results showed that fusel oil was separated into its components without the need for external separating agents. Each component has been analyzed regarding its behavior within a complex mixture and with respect to its potential separation based on the thermodynamic properties of non-ideal solutions. Only substances inherent to the fusel oil, such as water, used as an extractive agent, or ethyl alcohol, used to artificially enhance its concentration, were utilized for separation. Physical methods, including heating, cooling, and pressure changes, were also employed, leading to the system’s effective separation and phase split. These methods have proven effective in the separation process, although further improvements to the proposed PFD may still be explored.