(76d) Olefin Production Via Reactive Distillation Based Olefin Metathesis

SUMMARY

A metathesis process was developed to efficiently convert low-value olefin streams produced as by-products in olefin production processes into higher value products (ethylene, propylene, and high molecular weight linear olefins).  Pilot plant results indicated that a reactive distillation process will produce high yields of the desired products while minimizing the process energy input.  Bench scale experiments were conducted to supplement the metathesis reaction knowledge base and to develop an optimum catalyst regeneration methodology.  The results from both experimental units served as the framework for an AspenPlus® simulation.

BACKGROUND

High molecular weight (C₁₀-C₁₆) linear olefins, which are in high demand as feedstock for the production of lubricants, surfactants, detergents, and alcohols, are currently generated through an ethylene oligomerization process.  This reaction has a high energy demand and operates under extreme temperatures and pressures.  In addition, the production of ethylene and propylene from existing steam crackers is projected to not meet future demands.  The reactive distillation based metathesis process addresses both these issues by co-producing linear olefins and light olefins.¹

In the process, olefin metathesis occurs under moderate conditions of temperature and pressure and minimal energy demand.  The reaction is a solid-catalyzed liquid-phase reaction that breaks the double bonds in symmetric or asymmetric alkenes and then redistributes the alkene fragments.  The metathesis reaction produces linear internal olefins (LIOs), which when compared to LAOs, provide better product performance in some applications.  Combining the attributes of simultaneous reaction and distillation, the reactive distillation utilizes the olefin metathesis reaction for the selective production of various molecular weight olefins.

METHOD

In order to fully understand the olefin metathesis reactive distillation process, a multi-faceted investigation was required:  bench scale reactor experimentation, pilot plant experimentation, and process simulation.  Each area investigated a different aspect of the process.  Bench scale experimentation yielded important kinetic information about the basic metathesis reactions.  Pilot plant experimentation provided a general picture of the reactive distillation performance and a benchmark for larger production scale operations.  The process simulation aided in the comparison of the pilot plant performance to the simulated process and provided guidance for potential process improvements.

A bench scale apparatus consisting of a single liquid phase up-flow reactor was constructed and used to study specific metathesis reactions utilizing various carbon number feedstocks.  Under controlled space velocity, temperature, and pressure, liquid and vapor product compositions were measured and used to determine reaction conversions.  These reaction conversions were used to compare reactivity of the catalyst relative to feed molecule carbon number and position of the double bond.

A pilot plant process was constructed to allow investigation of the performance of the reactive distillation based olefin metathesis process.  The process involved a multi-phase metathesis reactor within a reactive distillation network.  The pilot plant is in essence three columns stacked on top of each other: a rectifying column at the top, vapor and liquid phase metathesis reactor in the middle, and stripping column on the bottom.  Figure 1 shows a simplified process flow diagram of the pilot plant.

Figure 1 - Process flow diagram of reactive distillation involving olefin metathesis

For the pilot plant experiments, an olefin feed (1-octene) was pre-treated for oxygenate removal and isomerized to a mixed octene feed.  This initial isomerization generated a wider variety of products, as the degree of isomerization in the feed is very predictive of the ultimate bottoms product composition.²  The mixed octenes were then fed into the top of the metathesis reactor as a liquid.  Metathesis occurred generating lighter vapor products and heavier liquid products, which were partitioned into the overhead vapor stream and bottoms product stream.  Typical product stream compositions were C2= to C4= in the overhead vapor stream and C10= to C14= in the bottoms product stream.  Gauze structured packing was used in the rectifying and stripping columns and commercial catalyst was used in both reactors.  Both columns are two inches in diameter with 20 feet of packed height.  Both reactors were six inches in diameter with a catalyst bed of five feet in length.  This process was operated under vacuum to minimize the metathesis reactor and bottoms reboiler temperature, thereby reducing the possibility of oligomerization, polymerization, and skeletal isomerization.

            A process model of the pilot plant process was constructed in AspenPlus®.  The three column pilot plant was modeled as a single distillation column with reactions specified on certain stages in the middle of the column.  Using the metathesis reaction conversions from the bench scale experimentation, quantitative assessments were made for the metathesis reactions based on carbon number and position of the double bond in the reactants.  Yields for specific reactions were estimated and specified for reactions on specific stages of the simulation.  The performance of the simulation was then compared to performance of the pilot plant. 

RESULTS & DISCUSSION

Initial bench scale experimentation was performed with a 1-octene feed into a metathesis catalyst bed under a low temperature reactor temperature (< 200 °F) and weight hourly space velocity (WHSV < 0.5 / hr) similar to conditions in the pilot plant process.  Figure 2 is a gas chromatograph of the heavy liquid product from a 1-octene self-metathesis.

Figure 2 - Gas chromatograph of 1-octene self-metathesis heavy liquid product

A 1-octene self-metathesis has one heavy liquid product:  7-tetradecene.  The detection of two peaks in the tetradecene (C14=) region, suggested the possibility of stereo-isomers: both cis- and trans-7-tetradecene.  Additionally, the detection of peaks outside of C8= and C14= suggested that a small degree of double bond shifting occurred over the metathesis catalyst, resulting in an additional olefinic products.  With increased temperature, this degree of double bond shifting increased thereby exhibiting larger concentrations of alternative olefinic products. 

            Following construction, testing, and brief optimization, a continuous operation of the pilot plant process was performed.  The pilot plant was operated continuously for two separate weeks with each week having a different feed rate (6 lbs/hr and 13 lbs/hr).  The feed to the isomerization reactor was composed of 10 wt% 1-octene and 30 wt % for each 2-octene, 3-octene, and 4-octene.  The metathesis reactor was operated at approximately 200 ^oF and the distillation columns were operated under vacuum with an overhead pressure of 7 psia.  Initially, the process was started up under total reflux and then transitioned into continuous operation.  Table 1 and Table 2 show compositional breakdowns for four locations in the pilot plant process when the process was at steady state.

Table 1 - Pilot plant compositions for a feed rate of 6 lbs/hr (wt %)

Table 2 - Pilot plant compositions for a feed rate of 13 lbs/hr (wt %)

As Table 1 and Table 2 indicate, the impact of doubling the feed rate yielded an increase in vapor traffic and a shift in the column composition profile.  The lower feed rate had higher concentrations of C7= and C8= in the overhead and C8= and C9= in the stripping column bottoms.  Doubling the capacity of the reactor produced a better separation as well as a better reaction yield.  The results indicate that the process will successfully consume moderate molecular weight olefins and produce a combination of light and high molecular weight material.

REFERENCES

1.         Abazajian, Armen Nazar. US Patent 6,515,193: Process for Production of Higher Linear Internal Olefins From Butene, February 4, 2003.

2.         Kawai, Tadashi, et. al.; Metathesis of n-Alkenes over CsNO₃-Re₂O₇-Al₂O₃ Catalyst. Journal of Molecular Catalysis. 1998, 46, 157-172.