Apr-2025

Maximising value from refinery off-gases

Case studies examine reactor designs shaped by plant needs and gas composition, demonstrating how ROG purification offers compelling economics

Wolf Spaether, Holli Garret, Kristina Morgan and Felix Schulz
Clariant

Article Summary

In the past, refineries viewed off-gases from fluid catalytic cracking (FCC) units, coker units, and similar sources simply as waste streams, burning them as fuel gas or releasing them through flaring. Today, these off-gases are recognised as valuable resources, containing a rich blend of hydrocarbons, olefins (as much as 30 mol% in off-gas), diolefins, and hydrogen, alongside some undesirable impurities.

Refinery integration with ethylene plants aims to maximise olefin yields. Treating off-gases for removal of critical impurities for the purpose of recovering high-value components such as ethylene, propylene, paraffins, and hydrogen can be a major part of this strategy to significantly boost the plant’s economics while reducing the CO₂ footprint. However, off-gas compositions vary significantly, especially when factoring in the removal of associated impurities. Both catalytic and adsorptive treatments are essential yet challenging to implement.

Currently, across the industry, the proprietary nickel-based OleMax 100 catalyst series treats more than 1,000 metric tons per hour of predominantly refinery-sourced off-gases for nitric oxide, oxygen, acetylene, and heavy metals removal for recovery of hydrocarbon products. This results in more than 300 metric tons per hour of ethylene capacity gained and improved process safety within the downstream cryogenic processing section. In addition to safely removing contaminants, treating and recovering the valuable components from off-gases with adsorbents and catalysts provides added benefits of reducing carbon dioxide and other pollutants emissions that are typically created when used in the refinery fuel gas and flare systems.

Against this backdrop, a focus on experience and results in designing new and revamped off-gas catalytic treatment systems is forthcoming. In some cases, these systems are also known as De-Oxo reactors. Through case studies, specialised catalytic reactor designs shaped by plant needs and gas composition are examined. The examples cover both adsorbent use for removing toxins (mercury, arsine, and phosphine) and catalytic solutions for eliminating nitric oxides, oxygen, and acetylene.

Refinery off-gas positioning
Figure 1 demonstrates the integration of refinery operations into the petrochemical value chain with the major downstream uses of the most important chemical building blocks, ethylene and propylene. The refinery off-gas (ROG) purification section highlighted in green may be integrated with so-called ethylene recovery units (ERU) and/or propylene recovery units (PRU). In some cases, the purified ROG is sent to the separation section of an ethylene plant (steam cracker), including the cryogenic part, which is highly safety-relevant and requires stringent control of critical impurities.

It can also be a standalone unit where typically large volume off-gas streams are treated in dedicated ERU/PRUs to capture the valuable components.

What is refinery offgas?
ROG is derived from several processing sections in varying volumes and compositions. The primary sources are the crude oil distillation CDU (atmospheric and vacuum), coking units (delayed and fluid), and fluidised catalytic cracker (FCC) sections. Other sources may involve hydrocracking, hydrotreating, reforming, and gas processing units. Clariant has come across combined off-gas streams as well as separated fractions (saturated and unsaturated).

Due to the diverse range of feedstocks, ROG compositions vary significantly. However, drawing from three decades of industry experience serving multiple units, Clariant can provide insight into the typical composition patterns it has observed.

It needs to be noted that the analysis of ROG compositions is difficult given the many components and impurities down to ppb levels (see Table 1). In many cases, the refinery and its respective engineering, procurement, and construction (EPC) partners provide expected/simulated ROG compositions in the absence of real analytical data. Trace impurities are rarely analysed and require offline laboratory test equipment unavailable on-site. Clariant has offered to receive real ROG feed samples for trace analysis and testing in its laboratories to support a streamlined design for its clients.

With modernised global exploitation of various quality oil reserves and the increasing focus on circularity, the authors believe that the composition of ROG and other off-gases may change due to more complexity and, most importantly, higher levels of contamination such as heavy metals, alkali metals, sulphur, and nitrogen compounds.

Why ROG purification?
Historically, ROG served primarily as fuel gas for refinery operations, powering furnaces, boilers, and process heaters. However, evolving economic and environmental imperatives have transformed this practice from a practical solution to the efficient use of valuable resources.

ROG streams contain valuable components, including ethylene, propylene, and hydrogen – key elements in modern crude oil-to-chemical (COTC) operations. While stream compositions vary significantly across applications, all require the removal of common contaminants such as oxygen, nitric oxides, and acetylene. These compositional variations dictate specific catalyst, adsorbent, and process configurations for optimal recovery.

Modern uses of ROG include further processing and/or treatment to separate and recover valuable components to be used as chemical building blocks (light olefins) or feedstock (saturated hydrocarbons) for other downstream or integrated processes such as ethylene plants.¹

With refineries producing lesser amounts of transportation fuels today, on-site technologies like FCC or deep catalytic cracking (DCC) units can be updated to produce more olefins. Those streams also require purification before processing and separation. In addition, some technologies can accept heavier streams, specifically converting low-value olefinic, paraffinic, or mixed streams into high-value propylene and ethylene with subsequent purification and separation.²

Although each project is unique in its feed rate, composition, pressure, and integration with other processes, the application can be clustered into three major categories, with the successful implementation of solutions to individual purification challenges in more than 25 projects served, as will be discussed in more detail.

Complete conversion
The first category is the most severe operation with hydrogenating acetylene, propyne (MA), and propadiene (PD) in olefinic streams that contain sulphur and high levels of CO (see Figure 2).

The feed source application, in this case, is processing refinery hydrocarbon feed through cracking furnaces to produce olefinic cracked gas. Cracking mixed hydrocarbons at high temperatures inevitably leads to the generation of C₂ and C₃ acetylenes as byproducts.