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Maximise ethylene gain and acetylene selective hydrogenation effi ciency

E thylene is one of the most important building blocks in the chemical industry. Its

manufacture is a highly competitive global business, and maximising operating profi t through various technology improvements is impor-tant for all ethylene producers. Based on decades of experience in acetylene selective hydrogenation catalysis, a new generation of front-end acetylene selective hydro-genation catalysts have been developed that off er exceptionable profi tability 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 confi gurations of acetylene selective hydrogenation are typical — front-end and tail-end — which are primarily diff erentiated by their positions relative to the cold box in the process layout. In the front-end confi guration, the acetylene hydro-genation reactor is located before the cold box; in the tail-end it is after the cold box. Additionally, three diff erent designs are applied

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

in front-end hydrogenation — deethaniser, depropaniser and raw gas — depending on the location of the reactors in the fl ow scheme.

In the deethaniser design, the acetylene hydrogenation converter is located downstream of the deeth-aniser 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 acet-ylene (MA) and propadiene (PD). In the raw gas confi guration, the cracked stream enters the hydro-genation 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 hydro-genation typically contains 0.3-0.8% acetylene, and the converter effl u-ent specifi cation 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 confi gurations undergo the hydro-genation reaction as well. They are normally not completely converted in the acetylene hydrogenation units but will be further processed in downstream dedicated convert-ers. As a result of relatively clean feed in the deethaniser confi gura-tion (without MAPD and BD), the treatment for deethaniser feed typi-cally does not require as high activity as to treat depropaniser

feed, which can be easily achieved with the same catalyst at modifi ed operating conditions.

Operational challengesThe fi rst 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 sepa-rated. Hydrogen levels are 10-40%, which is vastly above the stoichio-metric requirement for acetylene hydrogenation. Eff ective catalysts must have a good selectivity to hydrogenate acetylene to ethylene, but also minimise the hydrogena-tion 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 tempera-ture runaway will happen, resulting in lost materials and production time, and causing safety and environmental concerns.

Another challenge for front-end hydrogenation operation is fl uctua-tion 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-specifi cation product. Higher selectivity can be achieved at higher CO concentra-tions because it functions as a favourable modifi er. However, when CO concentration drops suddenly, more catalyst sites are available and hydrogenation of ethylene occurs more readily. This sudden drop can trigger

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generation. The third-generation catalysts have been introduced to markets and commercially proven to successfully address the previ-ously mentioned challenges. The future generation is in research and development (R&D), with expecta-tions for further improvements.

Figure 1 compares the operating window laboratory test results for second- and third-generation cata-lysts at CO levels of 500 ppm and 100 ppm, respectively. The defi ni-tion of the operating window is the temperature range between acety-lene “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 off ers good tolerance to CO fl uctuation, 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 verifi ed in commercial operations.

Table 2 compares the CO swing test results on second- and third-generation catalysts. In the laboratory, stable clean-up

temperature runaway. The third challenge is that increasingly producers are generating hydro-genation feed with lower CO levels, depending on operation, feedstock and processes. The aforementioned favourable function of CO decreases at these low levels, and stable opera-tions become very diffi cult with conventional catalysts.

Catalyst development and evolutionCatalysts for front-end hydrogena-tion 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 confi gurations. Catalyst researchers continue the

development of front-end hydro-genation catalysts, boosting the performance with each new

500 ppm CO 2nd generation

3rd generation

2nd generation

3rd generation

100 ppm CO

ΔT

Rx

inle

t te

mp

erat

ure

Life cycle

Activity retention

Increased life time

Sel

ecti

vity

Life cycle

Selectivity retention

Third generationSecond generation

Figure 1 Operating windows of second- and third-generation catalysts at 500 ppm and 100 ppm CO levels

Figure 2 Stability schemes of second- and third-generation catalysts

Front-end acetylene hydrogenationGeneration zero Ni-based (1950s)1st gen Pd on carrier; non-promoted (1970s)2nd gen Pd on carrier; promoted (typically Ag) (1990s)3rd gen Pd on carrier; promoted and stabilised 4th gen (developmental) Further increased activity/selectivity balance; bigger plant size

Evolution of front-end acetylene hydrogenation catalysts

Table 1

CO 900 ppmv CO 300 ppmv Selectivity, %

2nd gen 74 Runaway3rd gen 75 No runaway

CO 250 ppmv CO 60 ppmv Selectivity, % 2nd gen 28 Runaway3rd gen 74 No runaway

CO swing tests for second- and third-generation catalysts

Table 2


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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 cata-lyst experienced a temperature runaway, while the third-genera-tion catalyst had no such temperature excursion.

Similar, but more aggressive, tests were conducted at CO levels below 300 ppm. At 250 ppm CO, the second-generation catalyst could clean up acetylene with a selectivity of only 28%, while the third-genera-tion catalyst could achieve clean-up while still maintaining high selec-tivity at 74%. Compared to the selectivity data at 900 ppm CO, it can be seen that the new-generation catalyst itself has intrinsic high selectivity for acetylene hydrogena-tion to ethylene, instead of achieving this only with the modifi cation from CO in the second-generation catalyst. Again, a sudden CO drop to 60 ppm did not trigger a temperature runaway on the third-generation catalyst.

Based on numerous laboratory tests and operator feedback, the typical catalyst performances are generalised in schemes shown in Figure 2. The third-generation cata-lyst is more stable and deactivates more slowly, although its initial activity is slightly lower than the second-generation catalyst. The selectivity is maintained at a high level while on stream. Overall, the third-generation catalyst is a stabi-lised catalyst in all aspects.

Performance of third-generationcatalysts Third-generation catalysts are avail-able to fi t a variety of confi gurations. The ongoing high performance of OleMax 252 in a nameplate 600 KTA ethylene plant in Europe is shown in Figure 3. It has a deethaniser isothermal confi guration with a single reactor on stream. The unit has been on stream for more than four years, and production has been stable and on-specifi cation despite CO fl uctuations between 1000 ppm and 400 ppm. The inlet temperature has been slightly adjusted to

compensate for changes in fl ow rate. The long-term trend has been a gradual increase in fl ow rate, but with inlet temperature within a stable range, thus indicating no signifi cant catalyst deactivation. The exceptional performance in this plant is the on-going high selectivity enabled by OleMax 252. It has been maintained at a high level without decline throughout its operation.

Figure 4 shows the performance of another third-generation catalyst, OleMax 253, at a nameplate 1000 KTA ethylene plant in Asia. It has the deethaniser isothermal confi gu-ration with two parallel reactors on stream. This unit has been on stream for more than one year, and performance is stable and on-specifi -cation production is easily achieved. There is a diff erence in operation

2000 400 600 800 1000 1200

Inlet temperature

Flow rateSelectivityInlet temperature

Flow rateSelectivity

0 100 200 300 400

0 100 200 300 400

Selectivity of RxASelectivity of RXB

Methanol temperature

Inlet temperatureA outlet temperature

A feed rate

Figure 3 Commercial performance of OleMax 252 at deethaniser isothermal reactor (the circled artifacts are due to incorrect analyser calibration)

Figure 4 Commercial performance of OleMax 253 at deethaniser isothermal reactors


with three reactors in series. The commercial performance confi rms the laboratory test results. The initial activity of OleMax 254 is slightly lower than the previous generation, thus requiring a higher inlet temper-ature at start of run, but the inlet temperature modifi cation require-ment for this catalyst is much more moderate. The selectivity achieved is more than 20% higher than that achieved from the previous genera-tion, and it declines more slowly.

Economic gainThe benefi ts of the stable perform-ance off ered by the new-generation OleMax series catalysts easily trans-late to a boost in profi tability for operators. Considering the market margin of ethylene and ethane, the economic profi t from the extra ethylene gain by a 20% selectivity increase from the third-generation catalyst was estimated for a 1000 KTA plant using the average production parameters. The profi t from the extra ethylene gain alone is about $2.0 million in the fi rst

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here from the fi rst reference: the inlet feed temperature is not changed when the fl ow rate changes, but the isothermal bath temperature is modifi ed slightly for activity modifi -cation. Stable catalysts off er operators the fl exibility to operate according to their specifi c situations.

The same exceptionally high selectivity is achieved in this plant without any indication of decline. Also, the operator reports that much less green oil is generated compared to the previous second-generation catalyst. Less green oil formation is indirect evidence of high selectivity and is also a good sign for catalyst longevity. The two reactors perform almost identically in terms of inlet temperature, selec-tivity, fl ow rate and pressure drop across the beds. Overall, the opera-tion is much more simplifi ed because of the stabilised catalyst.

Figure 5 shows the performance of another third-generation catalyst, OleMax 254, in a nameplate 500 KTA plant in Asia. The unit has a depropaniser adiabatic confi guration

year. This profi t is expected to increase in the subsequent years due to the stable selectivity enabled with third-generation catalysts compared to a normal declination with second-generation catalysts.

The third-generation catalysts for front-end selective hydrogenation deliver important performance benefi ts including high selectivity, low green oil make, low sensitivity to CO swings and slow deactivation. This performance off ers excellent operability, such as easy startup, minimised risk of off -spec produc-tion, minimised risk of temperature runaway and increased catalyst life. The economic gain through high selectivity is easily calculated and clearly refl ects the profi t improve-ment of ethylene plant operations.

OleMax is a product mark protected by Clariant in many countries.

References1 Urbancic M A, Sun M, Cooper D B, Blankenship S, Hydrocarbon Processing, June 2009.2 Ringelhan C, Urbancic M A, Sun M, Boyer J, Hydrocarbon Engineering, April 2010.3 Blankenship S, Rajesh R, Sun M, Urbancic M A, Zoldak R, Hydrocarbon Processing, June 2012.4 US patent 7,521,393.

Ling Xu is a Technical Manager with Clariant in Munich, Germany. She also serves as Clariant’s global topic expert on front-end acetylene selective hydrogenation. She holds a BS degree in chemistry and BE degree in environmental engineering from Tsinghua University in China and a PhD in chemistry from University of Iowa in USA. Email: [email protected] Spaether is Sales Director for industry group petrochemicals in Europe, Middle East and Africa. He holds a PhD in chemistry at the University of Muenster, and postdoctoral with Malcolm Green at Oxford University. Email: [email protected] Sun is R&D Group Leader for selective hydrogenation catalysts with Clariant. Sun holds a BS degree in chemistry from Nankai University in China and a PhD in technical chemistry from ETH Zurich, Switzerland. Email: [email protected] Boyer is a Senior Research Chemist for selective hydrogenation catalysts with Clariant in Louisville, Kentucky. She holds a BS degree in chemistry from Miami University in Ohio. Email: [email protected] Urbancic is R&D Manager for petrochemical catalysts at Clariant in Louisville, Kentucky. He holds a BS degree in chemistry from Purdue University and a PhD in inorganic chemistry from the University of Illinois.

0 100 200 300 400 500

0 100 200 300 400 500

3rd G cat. selectivity2nd G cat. selectivity

3rd G cat. inlet temperature2nd G cat. inlet temperature

3rd G cat. selectivity2nd G cat. selectivity

Figure 5 Commercial performance of OleMax 254 at depropaniser adiabatic reactors


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