Separation Processes in Refining and Petrochemistry1

Introduction to Separation Processes

Separation operations are crucial in the refining, gas, and petrochemical industries, serving to prepare feedstocks before chemical transformations and recover desired products afterward. They also purify products or extract unwanted components, such as hydrogen purification or air drying.

These processes are broadly categorized based on the elementary selectivity employed, leveraging differences in various physical or chemical properties of the components to be separated.

I. Processes Based on Vapor-Liquid Equilibria

These are the most common separation processes in the petroleum industry, utilizing differences in volatility between components. Volatility differences are primarily linked to boiling point differences, vapor pressure, or curves on temperature-pressure diagrams.

The liquid-vapor equilibrium coefficient () indicates a component's volatility:

• : Light or volatile component

• : Heavy or non-volatile component

The relative volatility () between two components and often quantifies separation difficulty and is defined as:

$\alpha_{ij} = \frac{K_i}{K_j}" data-type="inline-math"></span>$

This parameter is particularly useful in distillation as it remains nearly constant in industrial columns under specific temperature and pressure conditions.

Flash (Vapor-Liquid Separation)

Flash or vapor-liquid separation is a basic equilibrium operation. While simple, its selectivity is limited, meaning the separation quality is often poor unless there's a very high volatility difference, such as separating hydrogen from hydrocarbons.

• The hydrogen-rich gas will still contain hydrocarbons.

• The liquid hydrocarbon phase will contain some hydrogen.

Non-selective flash operations often require repetition to achieve desired separation, leading to processes like distillation.

Distillation

Distillation is a widely used process in the processing industry that enhances volatility differences through vapor-liquid countercurrent contact and successive contacts between phases. These contacts are achieved using trays or packing (random or structured) within columns.

Key Factors for Distillation Efficiency:

• Number of separation stages: More stages lead to better separation.

• Liquid-vapor traffic: Higher traffic (energy consumption) can improve separation.

Distillation columns can vary significantly in size, from less than one meter to many meters in diameter, and exceeding 30 meters in height. They are prevalent in refineries and petrochemical sites.

Types of Distillation Columns:

• Simple distillation columns: Fractionate mixtures into two products. Commonly used for light cuts (gas and gasolines) and in petrochemistry for components with close volatilities.

• One side draw-off distillation columns: Fractionate mixtures into three products. Very common in refining and petrochemistry.

• Multiple draw-off distillation columns: Fractionate complex mixtures into many cuts. Used for initial crude oil fractionation (atmospheric and vacuum distillation) and complex effluents from various refining units (e.g., FCC, visbreaker, coker, hydrocracker).

Distillation is an energy-intensive process, accounting for about half of the energy consumed in a refinery.

Absorption

Absorption, also based on vapor-liquid equilibrium, processes a gas to extract specific components by dissolving them in an absorption oil (typically a gasoline).

Main Applications:

• Natural gas treatment: Recovers Natural Gas Liquids (NGLs) like C2+ components on production fields.

• Gas plant refining: Recovers LPG from specific gases.

The process occurs in columns similar to distillation columns, with contacts provided by trays or packing. The gas enters the bottom, and the absorption oil enters the top.

Stripping

Stripping is the reverse of absorption, used to treat a liquid (often a hydrocarbon cut containing undesirable volatile components). It extracts volatile components by sweeping them with a vehicle gas (e.g., natural gas, nitrogen, steam).

Main Applications:

• Very common in refining and petrochemical plants, often using steam stripping.

• In some field processing installations to sweeten sour crude oils (remove H2S).

Stripping also occurs in tray or packed columns, where the liquid enters the top and the stripping gas enters the bottom. Absorption and stripping are frequently associated operations in refining and gas treatment.

Azeotropic Distillation

Azeotropic distillation is used to separate azeotropic mixtures, which are common in refining and petrochemistry, especially for hydrocarbon-water hetero-azeotropic mixtures. Water, often present from stripping operations, forms azeotropes with hydrocarbons. This process is commonly found in the manufacturing of organic components like ethanol and solvents.

Examples:

• Separation of water from stripping operations, often collected at the top of distillation columns.

• Hydrocarbon drying.

Extractive Distillation

Extractive distillation is employed for difficult separations involving components with close volatilities but different chemical structures. A liquid solvent with a high affinity for the species to be separated is introduced, creating a "volatility gap" and thus facilitating the separation.

Typical Applications:

• Benzene recovery from C6 cut from a steam cracker.

• Butadiene recovery from C4 cut from a steam cracker.

• Aromatics recovery (particularly Xylenes) from reformate.

The solvent is subsequently recovered in a downstream distillation column.

II. Processes Based on Liquid-Liquid Equilibria

These processes, also known as liquid-liquid extraction, are based on differences in solubility. A liquid feed is contacted with a non-miscible liquid solvent. Components soluble in the solvent are extracted, and the non-soluble fraction is called the raffinate. The extracted components must then be separated from the solvent, typically by distillation.

Liquid-liquid extraction is less common than distillation but is used when distillation is technically or economically unfeasible, especially for separating components with very close volatilities.

Typical Applications in Refining:

• Aromatics and naphthenes extraction from lube base oil using solvents like furfural or NMP (N-methyl pyrrolidone). This is crucial because aromatics and naphthenes negatively impact viscosity index properties.

• Vacuum residue deasphalting: Hydrocarbons are extracted from vacuum residues using light hydrocarbons (e.g., propane, butane, pentane) as solvents, precipitating the asphaltic part. The extracted hydrocarbons, called DAO (Deasphalted Oil), can be further treated or used in cracking units.

Typical Applications in Petrochemistry:

• Aromatics (BTX: Benzene-Toluene-Xylenes) extraction from light cuts (e.g., steam cracker effluent, reformate). Solvents like DEG (Udex process), sulfolane (SHELL process), NMP (LURGI Arosolvan process), and DMSO (IFP process) are used to selectively extract BTX from paraffinic, naphthenic, and olefinic hydrocarbons. This is followed by distillation to separate the solvent and fractionate the aromatics.

III. Processes Based on Chemical Combinations with a Solvent

This category involves processes where a liquid solvent chemically reacts with components to be removed, known as physicochemical absorption. These reactions form a chemical association (e.g., a salt) that is later broken to regenerate the solvent and recover the target components.

The distinction between physicochemical absorption and liquid-liquid extraction here is that physicochemical absorption typically processes a gas with a liquid solvent, whereas liquid-liquid extraction processes a liquid with a liquid solvent.

Most Common Implementations:

• H2S and CO2 removal (acid gas removal): From gas streams in refining and petrochemical industries.

The solvent is typically an alkaline aqueous solution (e.g., DEA, MDEA solutions). The acid components (H2S, CO2) react with the amine (basic properties) to form an amine salt. This "rich amine" is then sent to a regenerator, where increased temperature and lowered pressure break the salt, releasing almost pure H2S or CO2, and regenerating the amine for recycle.

Other treatments, often using basic solutions like caustic soda or potash, remove acidic components (H2S, light mercaptans, HCl) from petroleum cuts or gas flows.

IV. Processes Based on Adsorption Selectivity on Solids

Adsorption is a phenomenon where molecules link with the surface of a solid. This selectivity depends on the shape of the molecules and the type of solid adsorbent. These differences are exploited for separation.

Adsorbent solids are characterized by:

• High specific area: 200 to 1,000 m²/g

• High pore volume: 30 to 50 cm³/g

• Micropore diameter: 3 to 20 Å

Common adsorbents include zeolites (molecular sieves), activated charcoal, activated alumina, and silica gel. Microporous solids with gauged porosity can separate components based on differences in molecular steric hindrance.

Most industrial adsorption processes are regenerative and operate cyclically: adsorption for separation and then desorption of adsorbed components. Regeneration methods include:

• Temperature Swing Adsorption (TSA): Increase in temperature.

• Pressure Swing Adsorption (PSA): Decrease in pressure.

• Displacement: Use of a desorbent that is more adsorbable than the treated components.

Main Applications in Refining and Petrochemical Industries:

1. Dehydration (Drying):

   • Water is adsorbed on silica gel, alumina, or zeolites (most efficient).

   • Common for instrument air, hydrogen, cracked gases, natural gas, LPGs, Jet A1, and certain feedstocks in isomerization units.

2. Desulfurization:

   • Removes H2S, COS, and light mercaptans.

   • Molecular sieves are often used to remove residual sulfur from LPGs, refinery hydrogen, and natural gas.

   • Frequently associated with gas dehydration.

3. Hydrogen Purification:

   • Many streams contain hydrogen mixed with light hydrocarbons, H2S, CO2, CO, H2O.

   • Processes require high-purity hydrogen for hydrotreatment.

   • The PSA process, using zeolites in multiple beds (at least 4), achieves very high hydrogen purity (e.g., from steam reforming or partial oxidation units).

4. Iso/N-Paraffins Fractionation:

   • Found at the outlet of catalytic isomerization units for light gasolines (C5-C6), where non-converted n-paraffins need to be separated to improve octane number.

   • Molecular sieves (5 Å) are used in the liquid phase, preferentially retaining linear n-paraffins due to their structure.

   • The desorbent is butane, which is then separated by distillation. The iso-paraffin rich stream (isomerate) is the final product, and n-paraffins are recycled.

5. Meta/Paraxylene Fractionation:

   • Paraxylene is a crucial petrochemical intermediate, produced alongside meta- and ortho-xylene.

   • Orthoxylene can be separated by super fractionation distillation, but meta-para fractionation is impossible by distillation due to extremely close boiling points and similar volatilities.

   • Adsorbents made of X or Y molecular sieves are used (e.g., UOP Parex process, Axens Eluxyl process).

   • Paraxylene, with its linear structure, is preferentially adsorbed. Desorbents like PDEB (para diethyl benzene) or toluene are used.

   • Metaxylene is recycled to isomerization units.

V. Processes Based on Solid-Liquid Equilibria

These processes exploit differences in crystallization temperature and typically involve two steps:

1. Cooling: The liquid mixture is cooled to selectively crystallize components with higher crystallization temperatures. Additional solvents may be blended to enhance efficiency.

2. Filtration: The produced crystals are separated from the liquid phase.

Industrial Applications:

• Metaxylene / Paraxylene Separation (Historical):

  • Historically, paraxylene was obtained by selectively crystallizing at low temperatures from a meta-paraxylene mixture, followed by continuous filtration.

  • Today, this process has largely been replaced by selective adsorption on molecular sieves.

• Lube Base Oil Dewaxing:

  • This is a significant application in lube base oil manufacturing.

  • After removing aromatics and naphthenes via liquid-liquid extraction, the resulting paraffinic cut solidifies at ambient temperature.

  • Dewaxing removes n-paraffins (waxes) with high crystallization temperatures to produce a liquid product with a sufficiently low pour point.

  • The process involves:

    1. Cooling the feed diluted with a solvent (e.g., MEK-toluene mixture) to selectively crystallize n-paraffins while maintaining lubricant-type hydrocarbons in the liquid phase.

    2. Filtration to separate solid paraffin from the oil-solvent mixture.

    3. Solvent recovery via flashes, distillation, and stripping.

VI. Gas Purification Processes by Membrane Permeation

Membrane permeation processes separate gas components based on their permeability (ability to disseminate through a membrane under pressure). These membranes are typically made of glassy or hemicrystalline polymers.

The driving force for permeation is the difference in partial pressures across the membrane. Molecules are classified as slow, intermediate, or fast depending on their size and diffusivity.

Main Applications:

• Hydrogen purification: This is the primary application in the refining and petrochemical industries. Membranes are used to increase hydrogen purity (e.g., from 70-80% to 90% or more in catalytic reforming units) and recover hydrogen from purge gases of hydrotreatment units, which was previously lost.

• Bulk removal of CO2 and traces of H2S.

• H2O removal.

Factors influencing hydrogen recovery in purge gas include gas composition, feed gas pressure, product permeation pressure, membrane area, and membrane selectivity characteristics.

Types of Membranes Used:

• Spiral Wound Membranes: Modules with flat membranes rolled up around a collector tube. Typical dimensions: 1 meter length, 10-20 cm diameter, 100-500 m²/m³ membrane area per volume unit.

• Hollow Fiber Membranes: Modules with a beam of hollow fibers. Typical dimensions: 1-4 meters length, 10-20 cm diameter, 1000-8000 m²/m³ membrane area per volume unit.

Membrane processes are relatively new in refining and petrochemistry compared to other industries like food or health.