Technology Type

Several separation and extraction processes are commercially used to isolate butenes and butanes from C₄ raffinate streams, especially raffinate-2 (RAF-2), after preliminary butadiene removal or selective hydrogenation and isobutene removal via etherification of isolefins.

RAF-2 Composition and Separation Challenge

Raffinate-2 (RAF-2) is the C₄ residual stream obtained after removal of both 1,3-butadiene and isobutylene from crude C₄, consisting primarily of:

• cis- or trans-2-butene: 50-60 wt%
• 1-butene: 10-15 wt%
• n-butane: ~20 wt%
• Minor amounts of isobutane and other components

The separation challenge arises from the close boiling points of these components, making conventional distillation economically unfeasible due to low relative volatilities.

Commercial Separation Routes and Pathways

Route 1: Extractive Distillation-Based Separation

• Separation Effectuated: This process splits olefins (butenes: 1-butene, 2-butenes) from paraffins (n-butane, isobutane).
• Primary Technology: Extractive distillation using polar solvents to enhance relative volatility between olefins and paraffins.
• Key Process: N-methyl-2-pyrrolidone (NMP), acetonitrile (ACN), dimethylformamide (DMF), or sulfolane-based solvents:
  • Separation Configuration: Two-column system (extractive column + solvent recovery column)
  • Product Recovery: Butenes preferentially dissolve in polar solvent (bottoms), while butanes are recovered overhead

Route 2: Superfractionation Technology

• Separation Effectuated: Fractionation of mixed butenes and butanes, specifically isolating high-purity 1-butene from 2-butenes and butanes.
• Primary Technology: Deep fractionation using extremely high theoretical plate counts (40-250 stages) to achieve sharp separations.
• Process Configuration:
  • Two-Stage Superfractionation: First column separates heavies (2-butenes, n-butane), second column purifies 1-butene
  • Performance: Achieves >99.5 wt% 1-butene purity with ≥80% recovery
  • Integration: Often combined with extractive distillation for final polishing

Route 3: Molecular Sieve Adsorption

• Separation Effectuated: Selective adsorption based on molecular size allows isolation of linear C₄s from branched isomers.
• Primary Technology: Shape-selective adsorption exploiting molecular size differences.
• Mechanism: Utilizes pore size selectivity (typically 8-8.2 Å) where branched molecules are excluded while linear molecules are adsorbed.
• Limitation: This is used more for specialty, small-to-midsize units or where a high Isobutylene removal is desired, rather than as a refinery-scale approach.

Route 4: Divided Wall Distillation

• Separation Effectuated: Simultaneous multi-component separation (isobutane, 1-butene, n-butane/2-butene) in a single shell with an internal partition.
• Primary Technology: Single column with internal dividing wall for simultaneous multi-component separation.
• Process Configuration:
  • Feed: Feed enters first side of divided column
  • Products: Isobutane (top of second side), 1-butene (bottoms of second side), 2-butene/n-butane mixture (bottoms of first side)
  • Advantage: Reduced equipment count compared to conventional multi-column sequences

Route 5: Integrated Isomerization-Fractionation

• Separation Effectuated: Conversion of 2-butenes to 1-butene via catalytic isomerization, followed by fractionation to separate 1-butene (and often butanes).
• Primary Technology: Catalytic isomerization of 2-butene to 1-butene followed by fractionation to separate 1-butene (and often butanes).
• Process Steps:
  • 2-Butene Isomerization: Catalytic conversion to 1-butene using modified Pd catalyst
  • Fractionation: Separation based on boiling point differences between 1-butene and 2-butene/n-butane.

Typical Separation Pathways

Pathway A: Maximum 1-Butene Recovery

RAF-2 → Extractive Distillation → Superfractionation → Extractive Polishing → High-purity 1-butene (>99.5 wt%)

Pathway B: Bulk Olefin/Paraffin Split

RAF-2 → Extractive Distillation → Mixed butenes + n-butane/isobutane streams → Further processing as required

Pathway C: Molecular Sieve Route

RAF-2 → Molecular Sieve Adsorption → 1-butene product + raffinate containing 2-butenes and butanes

Pathway D: Integrated Processing

RAF-2 → Isomerization → Divided Wall Distillation → Multiple product streams for downstream alkylation

Commercial Technology Selection Factors

The choice of separation route depends on:

• Product requirements: Polymer-grade vs. chemical-grade specifications
• Feedstock composition: Olefin content and purity requirements
• Economic optimization: Capital vs. operating cost considerations
• Integration opportunities: Shared utilities and downstream processing requirements

Summary: Commercial RAF-2 processing employs primarily extractive distillation, superfractionation, molecular sieve adsorption, and divided wall distillation technologies in various combinations to achieve the desired separation of 1-butene, 2-butenes, n-butane, and isobutane components, with specific route selection driven by product specifications and economic considerations. Solvent-based and superfractionation routes are the clear commercial standards at scale; molecular sieve and DWC are niche or site-specific deployments.