Technology

Introduction

Methylacetylene-propadiene (MAPD) selective hydrogenation is a vital purification step in steam cracker complexes and propylene production units. The goal is to remove trace MAPD impurities (propyne and allene) from C₃ hydrocarbon streams to achieve high-purity, polymer-grade propylene, essential for downstream polypropylene and chemical processes.

Liquid-phase hydrogenation in a trickle bed reactor (TBR) is the global standard for achieving polymer-grade propylene by removing MAPD from C₃ streams. It combines low temperature operation, superior catalyst selectivity, catalyst longevity, stable and safe process control, and the ability to efficiently handle the highly exothermic hydrogenation of trace contaminants in a robust industrial setting. 

Most MAPD/C₃ stream selective hydrogenation processes utilize well-established, public domain reactor designs and operating principles, with the primary proprietary differentiation residing in catalyst formulations, promoter systems, and specific operating conditions; therefore, the generic liquid-phase trickle-bed technology described herein represents the industry-standard approach that can be implemented with catalysts from various commercial suppliers.

Figure 1 - Chemistry of C₃ Stream Hydrogenation

Process Overview

Feedstock Characteristics

• Feed: Propylene-dominant C₃ stream from steam cracking or FCC offgas. Typical MAPD in feed: 0.1–2 wt% (often <500 ppm before final purification).
• Impurities: MAPD (propyne + allene), trace dienes, S, As, Hg, oligomers.

Process Objective

• Convert MAPD to propylene with >99.8% removal.
• Minimize loss of propylene via overhydrogenation to propane.
• Prevent formation of oligomers ("green oil") and manage exothermicity.

Catalyst

• Active phase: Palladium (Pd) supported on alumina is industry standard, sometimes with promoters (Ag, Pb, Sn) for increased selectivity.
• Form: Extrudates or spheres, sized to optimize flow and minimize pressure drop.
• Properties: Low green oil formation, long cycle life (typically >12 months before regeneration), stable conversion and selectivity.

Reactor Configuration

• Type: Fixed-bed trickle flow reactor
• Operating regime: Downflow, co-current, two-phase (liquid hydrocarbons + hydrogen gas) through a packed catalyst bed.
• Design: Multiple beds in series may be used for large capacities or staged control.

Equipment Highlights

• Trickle Bed Reactor: Stainless/carbon steel vessel with catalyst bed(s)
• Feed Pumps/Compressors: To maintain phase and pressure
• Hydrogen Feed System: Rate and purity controlled
• Heat Exchangers: For thermal management
• Product Separators: Gas-liquid disengagement, hydrogen recycle
• Online Analyzers: Continuous GC monitoring for MAPD, propylene, propane

Process Flow Description

Feed Preparation:

• Feedstock is filtered and freed from particulates and poisons (especially sulfur, arsenic).
• Liquid phase ensured via feed cooling/pressurization if required.

Hydrogen Addition:

• Controlled injection of high-purity hydrogen, often premixed with feedstream.

Trickle Bed Reactor Operation:

• Feed (liquid) and hydrogen (gas) co-currently pass downward over the catalyst.
• Highly exothermic hydrogenation is moderated by effective liquid flow (thermal mass), reducing risk of runaway/hot spots.
• Most MAPD is selectively hydrogenated to propylene at mild temperature, optimized H₂:MAPD ratio maintains high selectivity.
• Small amounts of propylene are unavoidably converted to propane; optimization minimizes this loss.
• Operating Conditions

  Parameter	Typical Range
  Temperature	30–60°C
  Pressure	15–30 bar
  LHSV	2–10 h⁻¹
  Hydrogen:MAPD	1.2–3 molar ratio (excess)
  H₂ Purity	>95%
  MAPD in Feed	100–2,000 ppm
  MAPD Out	<10 ppm (polymer grade)

• Liquid phase is maintained by adjusting pressure and temperature according to the feed composition.

Product Processing:

• Effluent is degassed; unreacted hydrogen is separated (can be recycled).
• Treated C₃ stream is polished to meet polymer-grade (<10 ppm MAPD) specs.
• Byproducts (trace propane, minimal green oil) are monitored and managed.

Technology Performance

Advantages of Liquid Phase TBRs

• Superior selectivity for MAPD conversion with minimal propylene loss.
• Excellent control of reaction heat, extending catalyst life and preventing reactor fouling.
• Safe, stable, and flexible operation even with MAPD concentration spikes or variable feed.
• Catalyst cycles can exceed a year with proper feed quality and hydrogen management.
• Lower green oil (oligomer) production than vapor-phase alternatives.

Typical Industrial Performance

• MAPD conversion: >99.8%
• Propylene selectivity: >96–98%
• Propane make: <1.5% of propylene
• Final MAPD content: <10 ppm (polymer grade), routinely achieved

References

1. Axens. Olefins elective Hydrogenation
2. Axens. Selective Hydrogenation Catalysts
3. Clariant. Catalysts for Liquid-Phase Selective Hydrogenation of Methylacetylene and Propadiene (MAPD) in "Tail-End" Configurations
4. Sinopec. BC-L-83 MAPD liquid phase selective hydrogenation catalyst
5. Shell. C2-C5 Hydrogenation
6. ABB Lummus. 2003 Technical Bulletin: Industrial^IT for Acetylene/MAPD Hydrogenation.
7. BASF. SELOP^® Technology
8. BASF. 2017 Technical Bulletin: Process catalysts
9. Süd-Chemie. Selective Hydrogenation for Steam Crackers – OleMax®
10. Süd-Chemie. Catalyst Brochure: Future Perspectives. Uploaded to SCRIBD on Aug 06, 2012.
11. D.-J. Vinel. May 13, 2013. Presentation: Alpha-Olefins Production Technologies. Axens Middle East Seminar, Abu Dhabi: Innovative Technologies for Better Competitiveness.
12. Wei Wu and Yu-Lu Li. Oct 6, 2010. Selective Hydrogenation of Methylacetylene and Propadiene in an Industrial Process: A Multiobjective Optimization Approach. Ind. Eng. Chem. Res. 2011, 50, 3, 1453–1459. DOI: 10.1021/ie100994j.
13. Robert P. Arganbright. International Patent WO1994004477A1: Selective hydrogenation of dienes and acetylenes in C₃ streams. Priority Date: Aug 23, 1993. Chemical Research & Licensing Company.
14. Stephen J. Stanley, Gary R. Gildert. United States Patent US6414205B1: Process for the removal of MAPD from hydrocarbon streams. Priority Date: Mar 24, 200. Catalytic Distillation Technologies.
15. Emerentino B. Quadro. Apr 2, 2019. Modelling of an Industrial Trickle Bed Reactor for Mapd Selective Hydrogenation with CFD Assistance. AIChE Annual Meeting (2010).