First: Some Definitions:
Before we can begin our discussion, we need to clear up two commonly misunderstood words – “Organic” (when referring to an organic compound) and “Volatility” (when referring to chemistry).
These words have specific meanings in our industry, and they are vastly different than their common meanings in our society today, namely “organic” as in “organic food” and “volatility” as in, explosive, unpredictable, or rapidly changing.
So, for the purposes of hydrocarbon extraction, we define these terms again for you here:
In chemistry, a compound is typically considered “organic” when it contains carbon. Millions of organic compounds are currently known to us, and the study of these compounds is the foundation of what we refer to as “organic chemistry. For a more complete definition, visit the Wikipedia Page.
Volatility, in the sense that matters to us in our industry, is defined as the quality describing how readily a substance will vaporize. Substances with “high volatility” are more likely to exist as a vapor, whereas substances with “low volatility” are more often found as either liquids or solids. The term can also describe the tendency of vapors to condense into a solid or liquid, which is particularly useful for understanding fractional distillation. See Wikipedia for more information.
Introduction to Hydrocarbon Extraction
Chemical extraction can be broadly defined as the removal and isolation of target compound(s) from a solution or solid matrix.
Extractions can be performed using physical processes, or more commonly, with a solvent. Physical extractions are often referred to as solvent-less extractions, which most commonly use temperature and pressure as a means for isolating the component(s) of interest.
Solvent-based extractions rely on solubility to obtain the desired compound(s). Hydrocarbon solvents are commonly used for extraction in various industries.
Organic compounds are often extracted from aqueous solutions using a separatory funnel and a hydrocarbon solvent such as hexane; this technique is referred to as liquid-liquid extraction.
Solid-liquid extractions are often used to extract natural compounds from natural sources, such as plants . For this technique, the solvent is used to selectively dissolve target compound(s) leaving behind the undesired, insoluble solid.
This article focuses on solid-liquid extractions using hydrocarbon solvents and discusses the parameters involved in executing a successful, efficient extraction.
To best understand techniques involving hydrocarbon solvents for extraction, the chemical properties of hydrocarbons as well as the chemical terminology and molecular interactions associated with the concept of solubility should be understood and considered.
What is a hydrocarbon?
Hydrocarbons are a class of organic compounds that contain only hydrogen and carbon atoms.
This class of compounds includes the chemical families known as alkanes (contains only single bonds between atoms), alkenes (contains double bonds between some atoms), and alkynes (contain triple bonds between some atoms).
It should be noted that alkynes are not commonly used as solvents due to the high potential of reactivity. Hydrocarbons can be aliphatic (straight-chained), branched, or cyclic. Some examples of hydrocarbons, their respective chemical family, and structural description are provided in Table 1.
Table 1: Examples of Various Hydrocarbons
|Compound Name||Formula||Structure (a)||Family||Structural Description|
(a) Gray atoms = carbon, white atoms = hydrogen
The chemical and structural properties of hydrocarbons discussed above influence the compound’s effectiveness as a solvent.
The selectivity of a hydrocarbon solvent and the solubility of the target compound(s) is affected by these properties.
Hydrocarbons do pose some safety concerns. Most hydrocarbons volatilize easily and therefore present an inhalation hazard. Hydrocarbons are also extremely flammable and proper care must be taken when they are handled.
Solvent-based extractions rely on the chemical concept of solubility. This section briefly describes the molecular interactions associated with solubility and defines the related chemical terminology.
The chosen solvent must be able to form a solution with the target compound(s). A solution is defined as a homogeneous (single-phase) mixture of two or more substances.
The solvent is the major component of a solution. The substance dissolved in the solvent is commonly referred to as the solute .
A solution is considered “saturated” when no more of the solute can be dissolved in the solvent.
The shape and distribution of electrons in a molecule greatly affect the compatibility of solvent and solute. In addition to the chemical properties of the solvent and solute, process parameters such as temperature and pressure can influence solubility and saturation point of a solution. When discussing the solubility of a liquid solute in a solvent, the terms miscible and immiscible are used.
A homogenous solution is observed when two miscible liquids are mixed in any proportion, whereas mixing two immiscible liquids will form two layers (phases).
Considerations When Developing an Extraction Method
As discussed above, solubility is the underlying chemical property that governs solvent-based extractions. This section discusses the molecular interactions that occur during extraction, as well as the process parameters that should be considered when developing an extraction method.
The general rule (“like dissolves like”) is useful for predicting solubility and key to understanding the underlying chemical interactions taking place during solvent-based extractions.
This rule is often applied to polar and non-polar compounds. Accordingly, polar solvents will dissolve polar compounds, and nonpolar solvents will dissolve nonpolar compounds.
Electrons are not evenly distributed in polar molecules, resulting in an electrostatic potential across the molecule; this is referred to as a dipole . Nonpolar molecules share electrons more evenly across the molecule and contain little to no dipole. Dipole-dipole interactions attract polar molecules to each other. Figure 1 (below) illustrates a dipole-dipole interaction between two water molecules:
Figure 1: Dipole-dipole interaction between two water molecules.
In Figure 1, the dipole-dipole interaction is represented by the dotted red line. The partial positive (δ+) and partial negative (δ-) ends of the water molecules are also labeled.
The weaker electrostatic forces, known as van der Waals forces, cause the attraction between nonpolar molecules. These forces are depicted below in Figure 2:
Figure 2: Interaction of two n-pentane molecules via van der Waals forces.
All molecules exhibit van der Waals forces due to the constant movement of electrons. The electron distribution in nonpolar is relatively uniform on a time-average basis, but likely non-uniform at any given instance, resulting in one side of the molecule having a slight excess of electrons.
This creates a weak, temporary dipole causing molecules to attract . Figure 2 shows this interaction between two n-pentane molecules. The red shaded areas in Figure 2 represent the side of the molecule with excess electrons, and the blue shaded areas represent the slightly positive side of the molecule with (temporarily) lower electron density. This type of electrostatic interaction is much weaker than dipole-dipole interactions.
Hydrocarbon solvents are considered nonpolar; however, it is important to realize that polarity is a matter of degree ranging from nonpolar to highly polar . Figure 3 (below) shows some common solvents (not all are hydrocarbons) arranged by polarity.
The colored surfaces surrounding the molecules in Figure 3 represent the electron density across the molecule.
Figure 3: Polarity of some common solvents. Gray atoms = carbon, white atoms = hydrogen, red atoms = oxygen. The yellow arrow indicates the dipole in water.
The blue areas of the electron density surfaces in Figure 3 represent the part of the molecule with lower electron density and exhibit a partial positive charge.
The red areas of the surfaces in Figure 3 designate higher electron density; these areas have a partial negative charge.
Water is highly polar; the yellow arrow indicates the presence of a dipole (+ indicates the partial positively charged area).
When comparing the two hydrocarbons in Figure 3 (n-butane and toluene), the absence of dark blue and red areas is expected due to their nonpolar nature. However, some red shading is visible around the six-membered ring of toluene. This is observed due to the double-bonded carbon atoms in the ring. Hydrocarbon solvents are nonpolar.
When the generalized rule (“like dissolves like”) is considered, one should conclude that hydrocarbon solvents will dissolve nonpolar compounds.
Process and solvent temperature can affect the solubility of a solute in a specific solvent and influence solvent selectivity. Generally, the solubility of most solids in water increases with temperature . This relationship is also observed with complex organic molecules (such as those found in natural products) in hydrocarbon solvents.
For example, the solubility of stearic acid (a fatty acid often found in plant fats) in n-heptane has been shown to increase when the temperature of the solution was increased .
It has also been demonstrated that Paracetamol (commonly known as acetaminophen) is insoluble in toluene cooled to -5°C, but slightly soluble in toluene heated to 30°C .
The solubility of Paracetamol is greatest in moderately polar solvents like ethanol. However, nonpolar hydrocarbon solvents such as benzene, n-heptane, and toluene are considered good solvents for a similar compound: Ibuprofen.
Ibuprofen is soluble in nonpolar and moderately polar solvents; the saturation point generally increases with temperature in these systems as well [4-5]. Controlling the process and solvent temperature dictates which substances are dissolved and extracted from a solid, as well as the saturation point of the solution.
The temperature should also be considered when selecting the solvent, especially if the solvent is to be removed from the extract via evaporation. We discuss this in further detail in the Post-extraction Processing section below.
The processing pressures used during solid-liquid extractions using a hydrocarbon solvent have little to no effect on solubility [4, 6]. However, process pressure is an important parameter to consider when using hydrocarbon solvents at increased temperatures.
As the solvent temperature increases, so will the vapor pressure. When the vapor pressure of a liquid is equal to the pressure in the system, the liquid will boil and exist in the gas phase.
In order to keep a solvent in the liquid phase at elevated temperatures, pressure may need to be applied.
Some hydrocarbon solvents can form a supercritical fluid by manipulating the pressure and temperature of the solvent in the system.
Critical Temperature & Critical Pressure
Critical temperature and critical pressure are the points at which supercritical fluids form, and both requirements must be met: If you’re above critical temperature but below critical pressure, you still won’t have a supercritical fluid. See the phase diagrams shown below in the image section for “visualization”.
If a solvent is heated in a sealed vessel with a constant volume, the pressure will rise as the solvent volatilizes. As the temperature and pressure continue to rise, the density of the vapor phase will also increase.
The increasing temperature reduces the density of the liquid phase. At sufficiently high temperature and pressure, the densities of the liquid and vapor become equal, and a single phase is observed. This phase is considered supercritical and is neither a liquid nor a gas, however, supercritical fluids have the properties of both liquids and gasses .
Supercritical fluids can act as good solvents due to their high selectivity. Supercritical CO2 is commonly used to make decaffeinated coffee by selectively extracting the caffeine from coffee beans .
The Residuum Oil Supercritical Extraction Process (the ROSE process) uses a supercritical hydrocarbon solvent. The ROSE process utilizes supercritical n-pentane to separate desirable oils from asphaltenes during petroleum refining [7-9].
Table 2 lists some hydrocarbon solvents and the critical parameters they need to form a supercritical fluid.
Table 2: Examples of hydrocarbon solvents and critical parameters
|Compound||Critical Temperature (°C)||Critical Pressure (atm)||Reference|
The critical temperature is listed in °C, and the critical pressure is provided in atmospheres (atm) for the compounds in Table 2.
At a minimum, the compound must be heated to the critical temperature while maintaining the critical pressure to form a supercritical fluid.
This data was obtained from the source listed in the Reference column of Table 2.
Preparing Material for Extraction
The following should be considered when preparing the solid matrix for extraction of the target compound:
- Particulate size,
- The temperature of the solid matrix,
- Possible contaminants or unwanted compounds in the solid that may be soluble in the solvent being used, and
- The chemical properties of the target compound(s) for extraction.
If the solid is ground into a pulp or powder, more surface area will be exposed to the solvent, and the amount of extract collected will be greater when compared to an identical extraction method if the solid is left as larger granules or in its natural state.
This may seem like a simple way to increase yield, but that isn’t always the case.
We’ll discuss this further in the example extraction method described in the following section.
- As discussed above in the Temperature discussion of this section, process temperature has a major influence on the saturation point of the solute and selectivity of the solvent. In most cases, it is desirable to allow the solid to reach the target process temperature in order to best maintain the desired extraction parameters.
- When extracting a target compound from a complex solid, such as a natural product, other unwanted compounds present in the solid may also be soluble in the solvent selected for extraction. In order to minimize the extraction of these unwanted compounds, the parameters discussed in this section should be tuned when developing an extraction method.
- Lastly, the chemical properties of the target compound(s), such as polarity and melting point, should be considered. Additionally, understanding the nature of the molecular forces that bind the target compounds and unwanted compounds to the solid matrix will be advantageous when determining how to prepare the solid for the extraction process.
Once the crude extraction has been completed, several variables must be considered in order to obtain a pristine extract.
The presence of H2O in the solvent rich extract may interfere with techniques used during further separations, purification, and isolation of the target compound(s). This is often overlooked when using hydrocarbon solvents, since they are considered nonpolar solvents.
H2O, on the other hand, is highly polar (thus the mixing of these species is not expected). However, small concentrations of water may be dissolved in the hydrocarbon solvent. This is most common during liquid-liquid extraction .
The best practice is to dry the hydrocarbon solvent. The following table (Table 3) lists some common drying agents:
Table 3: Common drying agents (data obtained from )
|Calcium Chloride||Low||High||Hydrocarbons, Halides|
|Potassium carbonate||Medium||Medium||Amines, Esters, Bases, Ketones|
|Molecular Sieves (3-4Å)||High||Extremely High||General|
Highly colored impurities in the crude extract can also interfere with further refinement of the material.
The removal of these impurities is referred to as decolorization. Common decolorizing agents include activated charcoal, alumina, or silica gel.
Activated charcoal is most commonly mixed in the crude extraction solution and removed by filtration. Alumina and silica gel are usually packed in a column. The extract should be diluted and passed through the column .
Common Extraction Techniques and Equipment
This section briefly describes three extraction techniques that often utilize hydrocarbon solvents, as well as the equipment commonly used the carry out the extraction process.
First, a liquid-liquid extraction technique is briefly discussed (recall that the focus of this article is solid-liquid extractions using hydrocarbon solvents). A classical continuous solid-liquid extraction method using a Soxhlet Extractor is also described. Lastly, a closed-loop extraction system (normally used with highly volatile solvents) is described and an example extraction procedure is proposed.
Liquid-liquid Extractions Using a Separatory Funnel
Although the focus of this article is solid-liquid extractions using hydrocarbon solvents, let’s take a moment to briefly discuss liquid-liquid extractions.
Liquid-liquid extractions are most commonly performed using a separatory funnel. This method involves combining two immiscible solvents in the separatory funnel.
The solute is extracted from one solvent to the other, because the solute is more soluble in the second solvent.
Since the solvents are immiscible, two layers (phases) will separate in the funnel. A stopcock is used to carefully drain one of the layers from the separatory funnel.
This method is commonly used on natural products such as plant tissues, which generally have high water content . In this example, an aqueous (water-based) solution containing molecules extracted from plant tissue would be added to a separatory funnel.
A hydrocarbon solvent, such as n-hexane, would then be added to the separatory funnel. The separatory funnel is then sealed and shaken vigorously, allowing the plant tissue (solute) to be transferred to the n-hexane phase. The separatory funnel should be inverted, and the stopcock opened multiple times during the mixing process to relieve the pressure built up while shaking.
Then, the separatory funnel is left to rest upright until the solvents separate into two distinct layers. The aqueous phase should be the bottom layer, because water has a higher density than n-hexane.
Finally, the stopcock is opened, and only the lower (aqueous) phase is allowed to drain. This process is often repeated to maximize the amount of solute extracted. This method is shown in Figure 4:
Figure 4: Liquid-liquid extraction using a separatory funnel.
Figure 4 shows the extraction of a solute (represented as black dots) from a solution containing two solutes (black and red dots).
The distribution coefficient (also known as the partition coefficient) is a constant used to describe the concentration of solute remaining in each phase. The distribution coefficient can be used to maximize the efficiency of liquid-liquid extractions by determining the amount of the second solvent (n-hexane in this example) to use, as well as how many times the process should be repeated [1, 11].
Solid-liquid Extraction Using a Soxhlet Extractor
Soxhlet Extractors are commonly used for continuous solid-liquid extractions. A typical Soxhlet Extractor is shown in Figure 2. Solid material is placed in a thimble and placed in the extractor.
Figure 5: Soxhlet Extractor. Extractor drawing obtained from .
A flask containing a low-boiling point solvent is fixed to the bottom of the extraction apparatus as shown in Figure 5. The flask is heated and vaporized solvent travels up the arm and into a condenser fixed to the top of the extraction apparatus. The vapor condenses and drips into the thimble. The liquid level in the siphon arm rises with the liquid level in the extractor (see Figure 5).
Once the liquid level is high enough, it drains back into the boiling flask, creating a siphon that removes all solvent left in the extractor.
Since the boiling point of the solvent is lower than the compound(s) extracted, only the solvent volatilizes when the solution is drained into the boiling flask. As the cycle continues, the solute becomes concentrated in the boiling flask.
Solid-liquid Extraction Using a Closed-loop Extraction System
Closed-loop extractors improve process efficiency when low-boiling point solvents are used for extraction.
These types of extractors can extract the target compound(s) from a solid or liquid and recover most of the solvent so that it may be used again without exposing the system to the surroundings during the process. Additionally, closed-loop extractors are advantageous when using hydrocarbon solvents because the solvent is contained within the system, improving safety by minimizing operator exposure to solvents and reducing the chance of fire or explosion.
Closed-loop systems also provide greater control of process parameters, such as temperature and pressure, when compared to open systems .
A closed-loop extraction system consists of three major components:
- A solvent reservoir,
- A material column, and
- An extract collection vessel.
The solvent reservoir stores and supplies the solvent being used for extraction and collects the recovered solvent after extraction.
Some solvent reservoirs are jacketed and/or equipped with heat exchangers. The solid material is contained within the material column. Material columns can also be jacketed, allowing for precise temperature control.
Once the solvent has interacted with the material and dissolved the target compound(s), the solution is transferred into the extract collection vessel.
The volatile solvent is distilled in the extract collection vessel and collected in the solvent reservoir, leaving the crude extract in the collection vessel.
Extract collection vessels can also be jacketed and/or contain heat exchangers. Depending on the parameters of the extraction method, many additional components may be added to the system. Some of the most common components added to closed-loop systems include filters, vacuum pumps, heaters, and chillers.
Other, more specialized systems may also have an inert gas source, dewaxing column, and desiccant filter.
A typical solid-liquid extraction using a closed-loop system involves the following general steps:
- Solid material is loaded in the material column,
- The system is sealed, and a vacuum is pulled,
- Liquid solvent is transferred into the material column,
- The solution (solute dissolved in the solvent) is transferred to the extract collection vessel,
- The solvent is distilled and recovered, and lastly,
- The crude extract is obtained from the collection vessel, and the remaining unwanted solid material is disposed of.
Example: Developing an Extraction Method
The purpose of this section is to provide an example of how the information discussed above is applied when developing an extraction method.
For this example, the goal is to develop a method that will extract essential oils from aromatic plant material.
First, the chemical properties of essential oils (the target compounds for extraction) are considered. Essential oils are generally non-polar, volatile compounds making a low-boiling point hydrocarbon solvent, such as n-butane, ideal for extraction.
- Due to the hazards associated with n-butane, a (certified) closed-loop extraction system should be used.
- All columns, vessels, and reservoirs should be constructed of stainless steel.
- All gaskets and hoses used should be constructed of materials compatible with n-butane, such as polytetrafluoroethylene (PFA), Fluorocarbon elastomer (Viton), or nitrile rubber (Buna-n).
Note: Natural rubber and silicone-based materials should not be used with n-butane.
The solvent should be stored in a jacketed reservoir. Chiller fluid from a recirculating chiller is then passed through the reservoir jacket to cool the solvent. A jacketed material column, connected to a chiller, will maintain low processing temperatures.
Performing the extraction at low temperatures allows the solvent to be more selective, by reducing the solubility of fats and waxes in the plant material (see the discussion on Temperature in the Considerations When Developing an Extraction Method Section).
A dewaxing column, consisting of a second jacketed column and particulate filter, should be cooled to low temperatures in order to remove any fats or waxes dissolved during extraction.
A jacketed collection vessel should then be connected to a recirculating heater to increase solvent distillation rate.
The vaporized solvent is then passed through an inline desiccant filter to remove any water picked up during extraction.
The desiccant filter consists of a column filled with 3Å molecular sieves and particulate filters. A recovery pump should be integrated between the desiccant filter and the solvent reservoir. Recovery pumps are used to increase the solvent recovery rate.
An explosion-proof recovery pump must be used when dealing with highly flammable solvents (like n-butane).
Nitrogen gas is used to help push the solvent through the system, because Nitrogen is inert and has a lower density than n-butane. A sketch of the proposed extraction system is shown in Figure 6.
Figure 6: Sketch of a proposed closed-loop extraction system (not drawn to scale).
In order to preserve the volatile essential oils, the plant material is frozen prior to extraction.
The plant material will only be broken up enough to fit into the extraction column, such that the minimal amount of damage will be done to the cell walls of the plant.
If the plant material was ground beforehand, rupturing the cell walls, other undesirable compounds contained within the plant cells could also be dissolved in the solvent.
The procedure for performing the solid-liquid extraction using the closed-loop system described above is as follows:
- Pre-chill the solvent reservoir, material column, and dewaxing column. The dewaxing column should be cooled to lower temperatures than the solvent reservoir and material column.
- Load the frozen plant matter into the material column.
- Seal the system and open all valves except those on the solvent reservoir and nitrogen supply tank, isolating them from all other components.
- Pull a vacuum on the system.
- Once the system is under vacuum, close the valve to the vacuum source.
- Close the valve located between the dewaxing column and desiccant filter.
- Open the liquid valve on the solvent reservoir.
- Open the valve on the inlet of the material column allowing the liquid solvent to pass over the plant matter and collect in the dewaxing column.
- Once the appropriate amount of solvent has been used, close the liquid valve on the reservoir.
- Open the valve on the nitrogen tank. Allow nitrogen gas to flow until no liquid solvent remains in the material column.
- Close the valves on the nitrogen tank and material column inlet.
- Allow the solution cool in the dewaxing column to allow any fats and waxes to precipitate (solidify).
- Open the valve on the outlet of the dewaxing column to transfer the solution to the collection vessel.
- Once all of the solution is in the collection vessel, close the valve on the outlet of the dewaxing column.
- Open the vapor outlet valve on the extract collection vessel.
- Turn on the recovery recirculating heater connected to the collection vessel.
- Turn on the recovery pump and open the vapor inlet valve on the solvent reservoir.
- Once the solvent has been distilled, close the vapor outlet valve on the extract collection vessel and turn off the heater.
- Turn off the recovery pump and close the vapor inlet valve on the solvent reservoir.
- Open the bottom valve on the extract collection vessel to obtain the extract.
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