September 2020

Catalyts

Catalyst breakthrough for ethylene production

In a growing and competitive industry, ethylene producers are turning to catalyst suppliers for solutions to innovate. Until recently, a solution to the difficulties inherent to acetylene hydrogenation in ethylene plants has challenged the catalyst industry. Due to their lower total production cost, “front-end” process flowsheets are now predominantly selected for new projects.

Vianna, C., Adams, D., Clariant Catalysts

In a growing and competitive industry, ethylene producers are turning to catalyst suppliers for solutions to innovate. Until recently, a solution to the difficulties inherent to acetylene hydrogenation in ethylene plants has challenged the catalyst industry. Due to their lower total production cost, “front-end” process flowsheets are now predominantly selected for new projects. However, these flowsheets possess a fundamental process challenge: risk of runaway exothermic reactions in the acetylene reactor(s), with subsequent production outages.

Furthermore, the catalyst can be heavily damaged during runaways. With spare reactors rarely installed, these events can cripple production until a catalyst replacement can be executed. This risk is intrinsic to “front-end” processes since both hydrogen and ethylene are present at high concentrations in raw cracked gas and are prone to react. In 2017, a catalyst with novel chemistry was introduced that addressed the decades-long catalytic challenge of mitigating hydrogenation runaway reaction risk in ethylene production.

Ethylene recovery sequences

Ethylene processes vary in the configuration of product recovery process steps. Primary differences are in the placement of the fractionation columns, resulting in a change in location of the acetylene hydrogenation reactors (Fig. 1).

Fig. 1. Simplified block flow diagram showing tail-end and front-end ethylene recovery sequences.
Fig. 1. Simplified block flow diagram showing tail-end and front-end ethylene recovery sequences.

Two process configurations have gained commercial importance: “tail-end” and “front-end.” The tail-end configuration used in the majority of older ethylene plants places acetylene hydrogenation in the fractionated C2-only cut. This location is downstream of the demethanizer column (i.e., after hydrogen, methane separation), and on the overhead line from the deethanizer. Hydrogen is added back in controlled, stoichiometric amounts, strictly limiting incremental exotherms from ethylene hydrogenation. These reactors are spared and periodically steam-air regenerated in-situ to remove polymer buildup on the catalyst surface.

In contrast, the majority of new plants are designed with a front-end process configuration for reduced capital and energy costs. The acetylene hydrogenation reactor is placed on the cracked gas stream upstream of hydrogen removal, on either the deethanizer or depropanizer overhead line. Therefore, acetylene must be fully hydrogenated in the presence of a large amount of hydrogen (15 mol%–40 mol%), but without triggering a significant exothermic reaction of ethylene.

Front-end catalyst performance challenges

Given the high reactivity of ethylene over palladium hydrogenation catalysts, it has been challenging to design catalysts capable of even steady-state, quantitative removal of acetylene while strictly limiting bulk ethylene hydrogenation. As all plants must operate reliably through a variety of transient conditions, the challenges to catalyst developers and risks to operators are exponentially greater for front-end units than for tail-end ones, due to this inherent risk of hydro-genation runaway.

Cracking furnaces are carefully designed to deliver high ethylene yield and acceptable operational cycles between periodic steam-air decoking. Introducing hydrocarbon feed to a recently decoked furnace creates reactor inlet flow variation, but more challenging to acetylene removal are the related carbon monoxide (CO) concentration spikes in the fresh furnace’s outlet cracked gas.

CO strongly inhibits the activity of palladium catalysts by adsorbing on the active sites, thereby hindering the rate of all reactions—especially the rate of ethylene hydrogenation, which also increases selectivity. Due to this activity decrease, CO spikes require the acetylene hydrogenation reactor to operate at higher temperatures to maintain full acetylene conversion. A loss of conversion would lead to extensive off-specification hydrocarbon flaring, especially if an acetylene leakage spike contaminates the high-volume downstream C2 splitter column. Fig. 2A demonstrates the required increase in operating temperature with CO in feed, and the relatively narrow operating window where ethylene reaction can be minimized and thermal runaway avoided.

The management of furnace swapping, a critical operations skill set, directly impacts the magnitude and timing of the expected CO spike. Reacting to a furnace CO spike can be something of a guessing game; adding too little temperature, or adding too late, may result in lowered conversion and an off-spec incident. Adding too much temperature too soon can lead to a runaway.

Fig. 2. Third-generation catalyst behavior during CO swings in front-end acetylene hydrogenation: (A) CO ppmv spike and (B) CO ppmv drop.
Fig. 2. Third-generation catalyst behavior during CO swings in front-end acetylene hydrogenation: (A) CO ppmv spike and (B) CO ppmv drop.

After the CO spike, operators must respond to a corresponding CO decline, which occurs as fresh furnace tubes passivate. This drop in CO increases apparent catalyst activity and requires a proportional reactor temperature decrease. If the CO concentration drops too quickly to manage a controlled reduction of bed temperatures, catalyst reactivation on top of raised bed temperatures produces additional ethylene exotherm, which may easily trigger a runaway reaction, as shown in Fig. 2B.

Furthermore, new furnace technologies may include coking-resistant tubes for longer cycles, which are reported to produce lower continuous CO levels. The effects of a given CO concentration spike on catalyst activity are magnified at lower absolute CO levels. High-performance furnace tubes in plants that already operate at intrinsically low CO levels (e.g., ethane crackers) provide an almost insurmountable stability challenge for previous generations of catalysts.

Plant startups present a greater problem than steady-state operation when it comes to achieving and maintaining on-spec ethylene product. Minimal startup flowrates are desired to minimize greenhouse gas flaring, but the activity impacts of each incremental flow increase are magnified at initially minimal flows. Furthermore, increased residence time while the catalyst is at operating temperature provides ample kinetic opportunity for increased ethylene hydrogenation and exotherm. In particular, new plant startups with as-yet unconfirmed mechanicals and controls, coupled with inexperienced operation teams, are typically the worst-case scenario for runaway risk.

To overcome difficulties and improve plant reliability, operators of front-end units needed a selective hydrogenation catalyst with a wider separation between acetylene and ethylene reactivity. Such a catalyst would provide a significantly wider operating window of stable performance, as well as improved ability to be proactively operated through CO swings and other transient operations, even at low CO levels or low flows. The net benefit would be a major reduction in the likelihood of (a) lost production and damaged catalyst from exothermic runaways when operating temperatures are too high for transient conditions, and (b) producing and flaring high volumes of acetylene-contaminated product in the reverse scenario.

Development of novel catalyst

Although front-end acetylene hydrogenation is a well-established industrial application, it has remained an important subject of research due to unmet operational needs, mainly with respect to operational stability. After transitioning from early nickel catalysts to the much more selective palladium active metal used today, a series of different promoters and manufacturing methods were developed over time to improve selectivity and operability (Table 1).

High-throughput experimentation and industrially novel chemistry has been applied to develop a new, fourth-generation catalyst for the front-end ethylene process. Ethylene hydrogenation activity has been dramatically reduced, even at temperatures substantially above the normal operating range for complete acetylene removal. Consequently, the new formulation has delivered a breakthrough in operational flexibility and stability, even under upset scenarios. In addition, operators have realized a significant boost in selectivity and economic yield from already high-performing third-generation catalysts.

Fig. 3 depicts a standardized laboratory operational stability test, comparing the operating window of recent catalyst generations from one company.a A proprietary catalystb provides an operating window three times wider than prior catalysts, and a normal acetylene activity level as required for typical design conditions and as a drop-in replacement.

Fig. 3. Operating window test results for different catalyst generations.
Fig. 3. Operating window test results for different catalyst generations.

Enhanced stability to CO fluctuation

Beyond its high selectivity, the catalystb can provide markedly reduced sensitivity to CO swings, even at difficult, very low CO levels. Previous catalysts could tolerate CO concentrations down to 125 ppmv–150 ppmv at the cost of reduced selectivity and increased thermal runaway risk. The novel catalyst sets a new benchmark with stable, high performance maintained to < 50 ppmv CO levels as produced in new ethane cracking furnace technologies (Fig. 4).

Fig. 4. Selectivity comparison of proprietary catayst<sup>b</sup> to prior generations at different CO concentrations.
Fig. 4. Selectivity comparison of proprietary cataystb to prior generations at different CO concentrations.

Catalyst performance in commercial plants

The proprietary catalyst has been in commercial operation since 2017 and has with five operating references in both adiabatic and isothermal reactor systems (including three new world-scale units > 1,500 kilotons/yr).

The stable, high performance of the catalyst in one of the world-scale units is demonstrated in Fig. 5. The unit uses a two-bed adiabatic reactor system and has been onstream 3 yr to date. Low and stable inlet temperatures consistently deliver approximately 95% acetylene conversion in Reactor 1, leading to early consideration of possible future CAPEX savings via fewer reactors. The catalyst is delivering extraordinary selectivity (> 90%) and stability, even at exceptionally low CO levels.

Fig. 5. Proprietary catayst<sup>b</sup> performance in a commercial reactor.
Fig. 5. Proprietary cataystb performance in a commercial reactor.

Fig. 6 compares the commercial selectivity of the catalyst and third-generation catalyst at comparable operating conditions in similar units, but with different CO levels. The new catalyst generation’s selectivity is remarkably higher and more stable, despite the lower CO level, with a very low level of ethylene downgrading to ethane recycle. Based on its demonstrated low decay rate, the service life for the catalystb will be able to exceed various commercial life cycle requirements.

Fig. 6. Commercial selectivity of proprietary catayst<sup>b</sup> vs. third-generation catalyst.
Fig. 6. Commercial selectivity of proprietary cataystb vs. third-generation catalyst.

The robustness and reliability of the catalyst has been confirmed in both normal operational transients and major upsets experienced to date. Fig. 7 illustrates a mechanical preheater failure resulting in a massive inlet temperature swing. Although temperatures oscillated dramatically, no significant ethylene exotherm was experienced. Such an instability would likely have triggered a runaway with prior generations, especially given very low CO levels. The stability of the new catalyst can be operationally leveraged in multiple profitable ways.

Fig. 7. Proprietary catayst<sup>b</sup> behavior during loss of inlet temperature control.
Fig. 7. Proprietary cataystb behavior during loss of inlet temperature control.

Takeaway

The increasing proportion of polymer-grade ethylene produced in front-end units, all feeding multiple purity-sensitive derivatives units, makes the operational reliability of acetylene hydrogenation business-critical. Major consequences of 2-d–3-d outages to restart after a runaway, up to taking a complex offline for a month to replace damaged catalyst, have been commonly experienced in the ethylene industry; a plant shutdown can cost companies more than $1.5 MM/d in profit losses. These risks have been recently mitigated by a novel catalyst chemistry that is quickly changing expectations regarding previously accepted “normal” behavior for front-end catalysts.

With sustainability a priority for all operators, the proprietary catayst can decrease emissions via reduced flaring during faster startups at lower hydrocarbon feeds. Further reductions from avoided off-spec and runaway incidents, no matter how they were previously triggered, are provided by the exceptionally wide separation between acetylene removal and the onset of ethylene reactivity.

The improved economic yield offered by the new catalyst is demonstrable, but its value pales in comparison to the significant stability delivered to plant operations staff and the business risk insurance provided for economic owners. HP

NOTES

a Clariant
b OleMax 260

LITERATURE CITED

  1. Xu, L., W. Spaether, M. Sun, J. Boyer and M. Urbancic, “Maximise ethylene gain and acetylene selective hydrogenation efficiency,” Digital Refining, March 2013.
  2. Cokoja, F., D. Adams and D. Cooper, “Ultra-selective acetylene hydrogenation catalyst developed to boost profitability in ethylene production,” The Catalyst Review, November 2018.
  3. Heasley, B., “Mitigating operations risks: Commercializing new catalyst technology in a world-scale ethylene plant,” AIChE Spring Meeting (EPC), April 2019.
  4. IHS Markit, “Chemicals supply and demand forecast,” (Clariant internal market study).

The Authors

Related Articles

From the Archive

Comments

Comments

{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}