PTQ Q2 2024 Issue

Flexible downscaling of MAPD removal from C 3 /C 4 olefin streams

This study simulates selective hydrogenation of a combined C 3 /C 4 cut with 1,3-butadiene, aiming to retain the target products under relevant industrial conditions

Edgar Jordan, Charlotte Fritsch and Joachim Haertlé hte GmbH

R aw olefinic C3/C4 streams in integrated petrochem - ical refineries, mainly the product of naphtha steam crackers or FCC units, usually contain low-weight percentages of acetylene derivatives, known as MAPD compounds. MAPD stands for methylacetylene (propyne, MA) and propadiene (PD), which are the most common impurities; vinylacetylene (but-1-en-3-yne, VA) and ethy - lacetylene (1-butyne, EA) can also be formed. These side products must be removed to protect downstream pro - cesses from severe catalyst poisoning and unwanted side reactions of those highly reactive substances. For instance, polymer-grade propylene must not exceed an MAPD impu - rity level of 1 ppmw. The predominant method for removing these compounds is selective hydrogenation of the triple bonds while preserv - ing the olefinic species. Whereas selective hydrogenation of acetylene in the C2 fraction is performed in the gase - ous phase, C3/C4 selective hydrogenation is predominantly carried out in a liquid state. This is for reasons of energy efficiency and the smaller reactor volume. Another benefit of a liquid state reaction is that greenoil formed as a byproduct during the hydrogenation reaction is washed out of the catalyst bed. Greenoil is an unwanted side product in C3-C4 selective hydrogenation, formed by the oligomerisation of olefins to various heavier unsatu - rated hydrocarbons. This greenoil can lead to catalyst deac - tivation as well as contamination in the unit. Commercial selective hydrogenation processes can be differentiated into front-end and tail-end configurations.

Front-end hydrogenation processes are characterised by an excess of hydrogen (H2) present in the raw product stream from the steam cracking or FCC unit. In contrast, tail-end selective hydrogenation, situated behind the de-methaniser and de-ethaniser columns, is controlled by feeding a stoichiometric amount of hydrogen to the hydro - carbon stream. Whereas for front-end hydrogenation, NiCo catalysts are common, tail-end hydrogenation catalysts are precious metals, mainly based on promoted palladium (Pd). Commercial process licensors and catalyst manufacturers for selective hydrogenation include Axens, UOP, Evonik, Lummus, Linde, KBR, and BASF. Tail-end liquid state selective hydrogenation often incor - porates multistage operation. In the first stage especially, a recycle stream of the product can be integrated to control the acetylene concentration and, in turn, the exothermicity of the reaction. 1-4 For operators of tail-end selective hydrogenation pro - cesses, many aspects of the catalyst and process perfor - mance are important to investigate, such as the long-term stability of the catalyst, the acetylene derivatives conversion level as a function of feed composition, the optimal operating window of the process, and the control of greenoil formation. Experimental set-up The experimental unit was equipped with a flexible up-and-downflow configuration. Liquid feed was pumped by a two-piston syringe pump, whereas the hydrogen flow was controlled by a mass flow controller (see Figure 1 ). The

Bypass

Saturation feed

Upstream pressure control

Downstream pressure control

Downow conguration

Heater/cooler

Reactor

Reaction feed

Upow conguration

Figure 1 Unit set-up for up- and downflow configuration

PTQ Q2 2024