Spd

MCH-n-heptane-aniline LLE group

Luigi Valentino Francesco Pollice Manuel Garofalo

June 4, 2025

Contents

• Introduction
• Process flow diagram
• Design specification
• Degrees of freedom
• Thermodynamic datas
• Resolution algorithm description
• Sets of equations
• Economic assessments
• Sustainable outlook
• Final design evaluation
• High-grade technological alternative: Distillation of n-heptane/methylcyclohexane (extractive mixture using ionic liquids)

Introduction

The designed system is a liquid-liquid extraction (LLE) unit for separating n-heptane (n-C7) and methylcyclohexane (MCH) in a hydrocarbon mixture. The feed, with 50% MCH and 50% n-heptane by mass, enters at 25 °C and 1 atm, with a flow rate of 5000 kg/h. Due to their similar boiling points, distillation is inefficient and energy-intensive.

Pure aniline is chosen as the solvent due to its low miscibility with n-heptane and strong affinity for methylcyclohexane. These properties ensure effective phase separation and high extraction efficiency, minimizing solvent losses and avoiding azeotrope-related limitations. In line with the project guidelines, realistic process fluctuations are considered: ±5% in feed flow rate, ±2% in composition, and ±5°C in temperature.

To address these uncertainties, three operating scenarios are defined: medium load (nominal condition), high load (positive fluctuation), and low load (negative fluctuation). The design is based on the high load condition since it represents the critical scenario. This is a “Type II” system: at the temperature of 25 °C (and even at 20-30°C) both the liquid pairs A-B and A-C are partially miscible, while B dissolves in any proportion in C. For Type II systems, a high degree of insolubility of S in C and C in S will produce a desirable high relative selectivity, but at the expense of solvent capacity.

Overview of liquid-liquid extraction

LLE is a separation technique that relies on solubility differences between phases, making it ideal when volatility-based methods such as distillation are unsuitable. For mixtures like n-heptane and MCH, which have close boiling points, LLE offers a practical solution.

In this case, the feed contacts aniline, which selectively extracts MCH. Two partially immiscible phases form: an extract phase (aniline + MCH) and a raffinate phase (mainly n-heptane). Internal reflux improves efficiency and reduces the number of required stages.

The design goal is to minimize the number of stages while reaching target purities: 90% MCH in the extract and 5% in the raffinate (both solvent-free). Our study focuses on different LLE units:

Perforated trays column

• This type of column uses trays with holes to promote contact between two liquid phases flowing in opposite directions. The denser (typically aqueous) phase flows downward, while the lighter (organic) phase rises. Although it enhances mass transfer, issues like emulsification and entrainment can reduce its efficiency.

Pulsed sieve trays

• A rapid reciprocating motion imparted to the liquid in a tower results in improved mass transfer. Pulsing is uniform across the cross section, and accordingly the height needed to achieve a required extraction is substantially independent of the diameter as long as hydrodynamic similarity is preserved. The rotary valve pulsator consists of two reservoirs each on the suction and discharge of a variable speed centrifugal pump and hooked to a rotating valve.

Rotating-disk column

• This type of column uses rotating disks mounted on a central shaft to create turbulence and enhance contact between the two liquid phases. The rotation improves mixing and mass transfer, while the compartments between disks help reduce axial mixing. It is effective for systems with low interfacial tension or where a high degree of dispersion is needed.

Karr column

• Karr column is a type of liquid-liquid extraction column with a reciprocating agitator. It consists of a vertical shell with static internals, such as plates or baffles. Inside, a central agitator moves up and down to create controlled turbulence. This motion enhances dispersion and mixing of the two liquid phases. The increased contact area improves mass transfer between the phases.

Mixer-settler

• A mixer-settler system consists of a mixing chamber where the two liquid phases are thoroughly mixed, followed by a settling chamber where the phases separate by gravity. This setup allows precise control of contact time and is ideal for batch operations or systems requiring multiple extraction stages.

Process flow diagram

Figure 1: Layout of a liquid–liquid extraction unit for the separation of a binary mixture using aniline as solvent.

Design specification

The design of a new unit requires a clear definition of the constraints and objectives of the process. This includes boundary conditions, mandatory specifications, and desirable performance criteria.

Boundary conditions

Liquid mixture n-heptane – methylcyclohexane

Mandatory design specification

Desirable design specifications

• Number of theoretical stages Minimization
• Specific solvent consumption Reduction
• Specific energy intensity Reduction
• Carbon footprint Reduction
• Specific energy cost Reduction
• Fractional water consumption Reduction

Table 1: Summary of design specifications for the LLE unit.

Preliminary utility cost estimation

An initial cost evaluation revealed the significant economic impact of the solvent used in the extraction process. According to recent market data, the average price of aniline in 2025 is approximately €1497,3/ton [Fig. 2].

Without solvent recovery, the continuous loss of aniline would lead to unsustainable operating costs. However, introducing a recovery unit could greatly reduce solvent consumption and environmental impact. Although distillation-based recovery requires additional energy input, the long-term economic and environmental benefits of minimizing solvent loss are substantial. This approach not only reduces operating costs but also aligns the process with sustainable engineering practices.

Figure 2: Aniline price May 2025

Figure 3: n-heptane prices

Figure 4: Methylcyclohexane price

Table 2: Estimated utility costs under different solvent recovery scenarios.

Degrees of freedom

General definition

Calculating the degrees of freedom in a chemical system is a fundamental aspect of determining how many variables can be independently specified for the system to be completely defined. This approach is based on the Gibbs rule for calculating degrees of freedom and is applied in several unit operations.

Figure 5: Countercurrent multiple contact extraction.

The total number of degrees of freedom (DOF) for a liquid-liquid extraction column can be expressed as:

DOF = Σ_k=1^M (C + 2) − N_I + r   (1)

• M = Number of equilibrium stages
• C = Number of chemical components
• N_I = Number of interconnected streams
• r = Number of additional constraints (e.g., algebraic relations)

Determination of N_I

The number of interconnected streams in a counter-current liquid-liquid extraction column is given by:

N_I = 2(M + 1)   (2)

This accounts for:

• Two internal streams (raffinate and extract) per stage
• Two external feeds (solute feed and solvent)

DOF expression for the system

The total DOF becomes:

DOF = 2(M + 1)(C + 2) + r   (3)

In more detailed formulations (considering specific flow variables and mass balances), the expression may take the form:

DOF = [2(C + 6)]M − 2(M − 1)(C + 2) + 1 + 2M + 2C + 5   (4)

Boundary conditions (BC)

Boundary conditions include:

• Feed specifications (composition, flowrate, temperature, pressure): C + 2
• Solvent specifications: C + 1
• External heat flux and pressure drops per stage: 2M

BC = 2M + (C + 2) + (C + 1) = 2M + 2C + 3   (5)

Effective degrees of freedom

Subtracting the boundary conditions from the total DOF:

Total DOF − BC = [2M(C + 2) + 5] − [2M + 2C + 3] = 2   (6)

This means that two independent variables must be specified to fully define the system.

Common specification choices

Typical specification choices to fix the DOF include:

1. Purity or recovery of one solute in either extract or raffinate
2. Number of equilibrium stages
3. Solvent flow rate

Alternatively, other known process specifications can be used.

Real case considerations

In real-world applications, 3 or 4 additional degrees of freedom are introduced for column design. These include:

• Type of contactor (e.g., packed column, tray column, rotating disc contactor)
• Column diameter
• Column height

Thermodynamic datas

In the thermodynamics of liquid–liquid extraction, no simple limiting theory, such as that of ideal solutions for vapor–liquid equilibrium, exists. Frequently, experimental data are preferred to predictions based on activity-coefficient correlations. Such data can be correlated and extended by activity-coefficient equations such as NRTL or UNIQUAC.

Equilibrium data

For the equilibrium data, we observed that predictive models such as UNIQUAC, NRTL, and others yielded inconsistent and unreliable results for the system under investigation. Due to these discrepancies, we opted to use experimental data instead. Specifically, we referred to the work of Varteressian and Fenske (1937) [fig. 6], who provided detailed equilibrium measurements for the methylcyclohexane–aniline–n-heptane system at 25°C and 1 atm, including tie-line compositions and saturation data.

Figure 6: Vantessarian & Fenskee, (1937)

Figure 7: Aspen Technology, Inc., (2008)

Physical-chemical properties Aspen Plus

For the physical-chemical properties of the ternary mixture, data was obtained from the [fig. 27] database. The system was modeled using the Non-Random Two-Liquid (NRTL) model, which is particularly suitable for highly non-ideal systems such as hydrocarbon–aromatic solvent mixtures.

NRTL Model: This model was adopted for liquid-liquid extraction and distillation simulations. It captures the non-ideal behavior of the system through local composition-based interactions. The main components of the model are:

• The activity coefficient (γ), which depends on binary interaction parameters.
• A non-randomness factor (α), which adjusts the intensity of local molecular interactions.
• An excess Gibbs energy expression, accounting for deviations from ideality.

The choice of the NRTL model was supported by literature sources and Aspen Plus guidelines, confirming its reliability for this specific ternary system. It was used consistently across all design and simulation steps, ensuring internal consistency of the thermodynamic framework.

Resolution algorithm description

The extraction process was analyzed under the assumptions of constant pressure and temperature throughout the column.

Isobaric condition

• Although small pressure drops occur due to liquid flow resistance and column internals, they are generally negligible in liquid-liquid extraction systems. Therefore, a constant pressure assumption is commonly accepted for equilibrium-based analysis.

Isothermal condition

• The process was considered isothermal, as the thermal effects from mixing and heat exchange with the surroundings are minimal. Although temperature fluctuations were specified in the exercise instructions (T = 25°C ± 5°C), these variations were not considered because they have a negligible influence on the system properties and due to the lack of reliable thermodynamic data to model such effects.

Additional assumptions

Phase equilibrium:

• Liquid-liquid equilibrium between the two streams exiting each stage was assumed to be achieved, with mass transfer resistances considered negligible. Real stage efficiencies were approximated by incorporating Murphree efficiency factors, literature data, and calculations to account for non-ideal stage performance.

No entrainment:

• The model assumes negligible entrainment between the solvent-rich phase (aniline) and the raffinate phase (methylcyclohexane–heptane mixture), ensuring sharp phase separation and simplifying mass balance calculations.

Ideal mixing:

• Each liquid phase is considered to be ideally mixed, exhibiting uniform physical properties throughout the