ADSORBENT KEY TO POLYPROPYLENE CATALYST ACTIVITY

Jean-Claude Detrait, Jacques F. Grootjans
Fina Research
SA Feluy, Belgium

Propylene streams from the refinery fluid catalytic cracking unit and delayed coker contaminants that are poisons to polymerization catalysts.

A new sorbent catalyst, called Prosorb, allows petrochemical producers to clean up these "dirty" propylene streams to a level adequate for use with the new high-yield polymerization catalysts.

The process, called "Triple P" (propylene polishing process), eliminates all common contaminants, including COS, H2S, mercaptans, AsH3, and SbH3. Commercial trials have taken place at two Fina refineries and at a joint venture petrochemical plant.

The refineries used the process to upgrade propylene feed from gasoline value to nearly polymer-grade value. The petrochemical producer was able to increase the activity of its polymerization catalyst and the consistency of the process.

HIGH-YIELD CATALYSTS

The new high-yield propylene polymerization catalysts have increased activity, but have higher sensitivity to poisons. The advent of these catalysts justifies the introduction of new "polishing" steps that reduce the concentration of certain contaminants.

This need is further amplified by the economic attractiveness of recovering propylene from refinery processes, mainly fluid catalytic cracking (FCC) and delayed coking. But, in contrast to propylene from a steam cracker, refinery propylene streams can be severely contaminated, as illustrated in Table 1. This contamination will be aggravated when crude and FCC runs shift to heavier feedstocks.

The main contaminants observed in propylene feedstocks are:

Sulfur-bearing species-H2S, COS, CS2, light mercaptans
Metal hydrides-AsH3, SbH3
Oxides-O2, CO, CO2
Hydrogen.

The similar boiling points of propylene and several of these impurities make their complete removal by fractional distillation difficult. Fina Research has developed and commercialized a process that eliminates most of the potential polymerization poisons down to the levels of detection.

PROCESS CHEMISTRY

In the propylene polishing process, propylene or propane/propylene feedstocks are contacted in the liquid phase at ambient temperature over a sorbent in a continuous downflow, fixed bed-type reactor. The sorbent (Prosorb) was developed by Engelhard Corp.

The proprietary sorbent's unique feature is the combination of a high amount of reduced metal, an open porous structure, and appropriate mechanical properties.

The high metal-loading of Prosorb assures long cycles between sorbent turnarounds.

Its use at exceptionally high space velocities and ambient temperature translates into moderate equipment investment.

The reactions assumed to take place lead to the formation of metal sulfide and metal-arsenic alloy. The ability of the sorbent material to remove COS and AsH3 from hydrocarbon streams is based on the following reaction schemes:

2M + 2COS 2MS + C + CO2 and

MO + COS MS + CO2

M + AsH3 M + As + 3/2H2 and

3MO + 2AsH3 3M + 2As + 3H20

Arsenic formed in both reaction schemes is adsorbed on the support or reacts to form a metallic compound. In addition, other sulfur-containing compounds (such as H2S) also are adsorbed by the sorbent:

 M + H2S MS + H2

The first two reactions indicate that CO disproportionates to carbon and CO2, CO2 being a much lesser poison for polymerization catalysts than CO.

 2CO C + CO2

Impurities similar to arsine that can be present in FCC C3 streams, such as stibine (SbH3), also are retained by the sorbent.

The sorbent also captures molecular oxygen by oxidation of the reduced metal:

 2M + O2 2MO

Another desirable characteristic of the sorbent is its activity as a hydrogenation catalyst, which results in the complete elimination of minute amounts of hydrogen:

 H2 + C3H6 C3H8

These impurities, if not reduced to the desired level, will reduce the yield of the polymerization catalyst. As an example, the yield of a propylene feed containing 205 ppb arsine is 10,000 kg polypropylene/kg catalyst. If the arsine is reduced to 50 ppb, this yield increases to 32,000 kg/kg catalyst.

COS cannot be present during polymerization at concentrations greater than 30 ppb without drastic yield loss. Reduction of COS to 25 ppb, however, can double catalyst activity, depending on the original COS content in the propylene feedstock.

ADSORPTION

The process typically consists of a single sorbent bed in the case of steamcracker propylene, or a dual sorbent bed in case of more contaminated feedstocks (Fig. 1). In the latter case, the sorbent beds normally operate in series, but the piping between the separate vessels is designed so that the fresher sorbent is always in the second, or downstream, position.

In principal, each sorbent bed is sized to bring the product within specification at full throughput in case the other vessel is taken out of service for sorbent replacement.

The sorbent is provided in its reduced state, stabilized with CO2 to avoid pyrophoricity, and it is shipped in stainless steel containers. In closed tanks, the sorbent keeps its properties indefinitely.

In open containers, a slight oxidation can occur. In dry atmospheres, however, this oxidation generally is negligible. Sorbent protected under a high-flash hydrocarbon can be used in the process, provided appropriate handling procedures are implemented. At present, CO2-stabilized material is largely preferred.

PROCESS DESCRIPTION

Liquid feedstock is introduced at ambient temperature to the adsorber in downflow operation. Liquid hourly space velocities (LHSVs) are very high, such that the reactors are compact and can be integrated easily into an existing process.

The process itself is located downstream of a molecular sieve dryer in order to prevent high levels of water from reaching the sorbent. Water deactivates the sorbent temporarily by adsorbing onto the surface and preventing reaction with sulfur species and arsine.

A filter is placed downstream of the system to catch any sorbent fines generated during loading or activation.

The process does not consume any utilities other than nitrogen, which is utilized during loading and conditioning of the sorbent.

The sorbent cannot be regenerated and has to be disposed of by certified companies. Throughout the world, specialized companies offer their services for reclaiming the active metal.

SORBENT COMPARISON

Alternative sorbents are listed in Table 1. Clearly, some are particularly useful for eliminating specific contaminants. For instance, alumina activates the hydrolysis of COS according to the reaction:

 COS + H2O CO2 + H2S

Evidently, this approach requires an additional downstream treatment (amine scrubbing, fractionation, etc.) to remove at least the H2S, which is an extreme polymerization poison.

Other sorbents, like lead oxide, are fairly active for arsine removal but cause problems for disposal and are not attractive for metal reclaiming.

Table 2 summarizes the ultimate capacity of some commercial sorbents at "breakthrough" using feedstocks spiked with COS and AsH3. This capacity dictates the volumes to be used and the expected useful life.

COMMERCIAL EXPERIENCE

The propylene polishing process has been developed through different phases, including:

Pilot plant tests on a slip stream at Fina Raffinaderij, Antwerp
Demonstration plant runs at the Fina Oil & Chemical Co. refinery at La Porte, Tex.

Eventually a full-size plant was built and commissioned at Fina's La Porte site.

AsH3 guard beds were installed and commissioned on both polymerization trains for MonteFina SA, a PetroFina/Himont joint-venture based in Feluy, Belgium. COS treaters based on Himont technology were already existing.

Commissioning was successful, and the plant reported improved polymerization yields and product consistency.

Fina's Antwerp refinery successfully commissioned a dual purification system on its propane/propylene splitter feedstream. Al] performance objectives were comfortably met, as illustrated in Fig. 2, which shows the COS and arsine concentrations at the inlet and outlet of the front adsorber. The concentrations of these species at the outlet of the second adsorber were below the limits of detection.

The sorbent was replaced in the front adsorber in February 1994 when it was still removing arsine and sulfur species. Sulfur and arsenic content throughout the bed were flat profiles of, respectively, 7 wt % and 2.5 wt %. During changeout of the front adsorber, the downstream adsorber continued to produce product within specifications at full rates.

ECONOMICS

The savings generated by the use of the process can be discussed as savings made during refining processes, or savings made during polymer production.

Table 3 indicates the respective investments and savings made for the refining and the polymer applications on the basis of a 200,000 metric tons/year plant. Fig. 3 shows the internal rate of return and simple payback calculated on a 10-year basis.

The elimination of the contaminants from the refinery propylene stream using the process represents for the refiner an upgrade of this feed from gasoline value to a value close to polymer-grade propylene. The resulting profit ranges between $50 and $100/metric ton.

Using the process in a polypropylene plant, on the other hand, enhances the polymerization catalyst activity considerably and increases the consistency of the process. The estimated profit is $210/metric ton of polymer, plus an overall reduction in variable costs of $1 million for improved processing.

In April 1993, Fina Research granted Engelhard the right to license the Triple P process. Engelhard assumes the responsibility of manufacturing and supplying the sorbent at the required specifications.