Mar-2013

Maximise ethylene gain and acetylene selective hydrogenation efficiency

Third-generation stabilised front-end selective acetylene hydrogenation catalysts provide high selectivity, low sensitivity to CO swings and slow deactivation

Ling Xu, Wolf Spaether, Mingyong Sun, Jennifer Boyer and Michael Urbancic
Clariant

Article Summary

Ethylene is one of the most important building blocks in the chemical industry. Its manufacture is a highly competitive global business, and maximising operating profit through various technology improvements is important for all ethylene producers. Based on decades of experience in acetylene selective hydrogenation catalysis, a new generation of front-end acetylene selective hydrogenation catalysts have been developed that offer exceptionable profitability and ease of operation to ethylene manufacturers.^1-4

Front-end acetylene hydrogenation process
Steam cracking of hydrocarbons is the primary method of ethylene production, and acetylene is an inevitable byproduct. Acetylene is a severe poison for downstream polymerisation processes, and conventional distillation cannot reduce its concentration to the necessary levels. Extraction with organic solvents separates acetylene from the ethylene stream, but the acetylene market is too small to install this process in all plants. Instead, the majority of acetylene removal is managed by selective hydrogenation.

Two configurations of acetylene selective hydrogenation are typical — front-end and tail-end — which are primarily differentiated by their positions relative to the cold box in the process layout. In the front-end 
configuration, the acetylene hydrogenation reactor is located before the cold box; in the tail-end it is after the cold box. Additionally, three different designs are applied in front-end hydrogenation — deethaniser, depropaniser and raw gas — depending on the location of the reactors in the flow scheme.

In the deethaniser design, the acetylene hydrogenation converter is located downstream of the deethaniser column and thus contains the entire C2 fraction and lighter components. In the depropaniser design, the acetylene hydrogenation converter is located downstream from the depropaniser, so the feed contains all C3 fraction and lighter components, including methyl acetylene (MA) and propadiene (PD). In the raw gas configuration, the cracked stream enters the hydrogenation reactor after acid gas removal and drying treatment but without any fractionation, and therefore the raw gas feed contains more heavy components, such as C4 and C5 hydrocarbons, including 1,3-butadiene (BD).

Regardless of design, feed to front-end acetylene selective hydrogenation typically contains 0.3-0.8% acetylene, and the converter effluent specification is normally less than 1.0 ppm. The recent trend is to operate the acetylene outlet to less than 0.3 ppm. MAPD and BD in the feeds of depropaniser and raw gas configurations undergo the hydrogenation reaction as well. They are normally not completely converted in the acetylene hydrogenation units but will be further processed in downstream dedicated converters. As a result of relatively clean feed in the deethaniser configuration (without MAPD and BD), the treatment for deethaniser feed typically does not require as high activity as to treat depropaniser feed, which can be easily achieved with the same catalyst at modified operating conditions.

Operational challenges
The first challenge is that the feed in front-end hydrogenation contains an excess of hydrogen, due to the position of the reactor in front of the cold box, where hydrogen and a portion of the methane are separated. Hydrogen levels are 10-40%, which is vastly above the stoichiometric requirement for acetylene hydrogenation. Effective catalysts must have a good selectivity to hydrogenate acetylene to ethylene, but also minimise the hydrogenation of ethylene to ethane to ensure a high yield of ethylene and to reduce the recycle of ethane in the process. In the most severe case, the hydrogenation of ethylene occurs to an extent that temperature runaway will happen, resulting in lost materials and production time, and causing safety and environmental concerns.

Another challenge for front-end hydrogenation operation is fluctuation of CO concentration in the feed. CO functions as an activity inhibitor to hydrogenation as it is adsorbed on the catalyst active sites. On conventional catalysts, when CO increases, higher temperature is required to produce on-specification product. Higher selectivity can be achieved at higher  CO concentrations because it functions as a favourable modifier. However, when CO concentration drops suddenly, more catalyst sites are available and hydrogenation of ethylene occurs more readily. This sudden drop can trigger 
temperature runaway. The third challenge is that increasingly producers are generating hydrogenation feed with lower CO levels, depending on operation, feedstock and processes. The aforementioned favourable function of CO decreases at these low levels, and stable operations become very difficult with conventional catalysts.

Catalyst development and evolution
Catalysts for front-end hydrogenation have been evolving over the past several decades (see Table 1). OleMax 251 is an example of a second-generation catalyst that has been used for more than two decades for all front-end hydrogenation configurations. Catalyst researchers continue the development of front-end hydrogenation catalysts, boosting the performance with each new generation. The third-generation catalysts have been introduced to markets and commercially proven to successfully address the previously mentioned challenges. The future generation is in research and development (R&D), with expectations for further improvements.

Figure 1 compares the operating window laboratory test results for second- and third-generation catalysts at CO levels of 500 ppm and 100 ppm, respectively. The definition of the operating window is the temperature range between acetylene “clean-up” and “runaway.” At the typical industry operating level, 500 ppm CO, the operating window of the third-generation catalyst is more than double that of the second generation. This wide temperature window offers good tolerance to CO fluctuation, while ensuring stable on-spec production.

At 100 ppm CO, the operating window of the second-generation catalyst is very narrow, while the third-generation catalyst enables a wide operating temperature window. Industry discussions note that unstable operations can occur at such low CO levels. The third-
generation catalyst, with its wide operating window at 100 ppm CO comparable to the second generation at 500 ppm CO, is expected to have a much more stable operation under these conditions. Additionally, the wide operating window of the third-generation catalyst enables a faster and smoother startup, as recently verified in commercial operations.

Table 2 compares the CO swing test results on second- and third-generation catalysts. In the laboratory, stable clean-up operations were established at 900 ppm CO, where both catalysts demonstrated high selectivity at 74% and 75%, respectively. However, when CO concentration in the feed was dropped to 300 ppm, the second-generation catalyst experienced a temperature runaway, while the third-generation catalyst had no such temperature excursion.