Mar-2003

Technology for propylene boosting in steamcrackers

Integration of steamcracker C4/C5 components with a fixed-bed fluid catalytic-cracking reactor employing a shape-selective heterogeneous zeolytic ZSM-5 catalyst

Heinz V Boelt and Stephan Glanz
Linde

Article Summary

Higher worldwide annual growth rates of propylene relative to ethylene predicate interest in increasing propylene-to-ethylene ratios (P/E) in ethylene plants. Ethylene plants, or steamcrackers, are still the main propylene source, with a 70 per cent share in the primary production routes, and are desired from an economical point of view.

In the cracking furnaces, the achievable P/E is limited to a value of approximately 0.65, since at higher ratios the total olefin yield (that is, the sum of ethylene and propylene) drops to an uneconomically low level. In order to overcome this situation, different approaches have been taken to increase the production of propylene from steamcrackers, including:
—  Recycling of C4 and C5 cuts
—  Integration of a metathesis unit
—  Integration of C4/C5 with fluidised catalytic cracking
—  Integration of C4/C5 with fixed-bed catalytic cracking (Propylur).

Using specially formulated catalysts, the proprietary Propylur technology, effectively integrates steamcracker C4/C5 product with fixed-bed catalytic cracking. Propylur has been developed by Lurgi and is exclusively available through Linde for application in steamcrackers.

Feedstocks and products
To cost-effectively increase steamcracker P/E ratios, Propylur units (Figure 1) are designed to operate with different types of feeds typically available in a petrochemical complex (for example, C4 cuts, raffinate I, raffinate II or gasoline fractions). Products include propylene, ethylene and butylenes in an equilibrium composition, which is nearly independent from the applied feedstock .

There are only a small number of feedstock constraints with respect to purity and composition. Feedstocks containing both high or low concentrations of olefins can be processed. Naturally, feedstocks with a higher olefin content are more favourable, since only the contained olefins are converted to the desired products. Paraffins, cycloalkanes, cycloalkenes and aromatics can be contained in the feedstock and pass the reactor with negligible conversion (non-convertibles). Components such as diolefins should be limited to approximately 1.5 wt%, as these contribute to gum and coke formation.

Catalyst function
In Figure 2, a feedstock having its peak concentration at a chain length of C5= (that is, a gasoline fraction or a naphtha) is converted in a single pass by the Propylur catalyst to a product with a peak concentration at a chain length of C3= (that is, propylene). Whereas the olefin distribution in the feedstock is naturally very dependent on the type of applied feed, the product distribution always looks quite similar (defined by the equilibrium).

Reaction, conversion and yields
The principle of a Propylur reaction can be explained in a non-scientific manner as follows:
— By means of a shape-selective heterogeneous zeolytic catalyst of the ZSM-5 type, olefinic C4 and C5 components are cracked to CH2 molecules, which recombine in an equilibrium reaction to C2, C3 and C4 olefins, having a peak at propylene
— In a single-pass operation, a typical reactor product contains 40–45 per cent propylene, 10 per cent ethylene and 30 per cent butylenes
— If the produced butylenes are recycled, propylene yield can be increased to 60 per cent and ethylene yield to 15 per cent.
The given yields are related to the feedstock’s olefins content as shown in Table 1.

Catalyst performance
In accordance with the chosen favorable process conditions (for example, temperature, pressure and steam dilution) the coking tendency in the Propylur reactor is very low. As a result, no continuous catalyst regeneration is required and an advantageous fixed-bed reactor system can be applied.

The cycle length is expected to exceed 1000 hours, which means that catalyst regeneration has to be performed only eight times per year. Based on laboratory pilot plant experience, a catalyst lifetime of more than 15 months is expected.

Catalyst regeneration is performed in situ by means of a nitrogen cycle with a small air make-up for a controlled combustion of the coke deposits on the catalyst. This regeneration procedure represents common industrial practice and is also used for the regeneration of several types of catalysts in different applications.

The process is based on a shape-selective heterogeneous zeolytic catalyst of the ZSM-5 type, which has been optimised for this specific application in co-operation with Süd Chemie AG, a major catalyst supplier. The reactor type is an adiabatic fixed bed, similar to the reactor of a Claus unit. Applied reaction conditions are moderate, with a typical temperature of approximately 500°C and a pressure just above atmospheric pressure (for example, 2 bar).
 
Process description
One of the outstanding characteristics of Propylur is the application of a steam dilution. It reduces the hydrocarbon partial pressure of the reactants, shifting the equilibrium towards the desired product distribution. Furthermore, both coking tendency and gum formation are suppressed. Additional heat required for the slightly endothermic reaction is transported into the reactor.

The liquid C4 or C4/C5 feedstock is first vapourised and mixed with steam. The HC/steam mixture is further heated against reactor effluent and in a fired heater before entering the fixed-bed adiabatic reactor.

By cooling the reactor effluent, the steam is condensed and can be separated together with some gasoline by-products. The remaining vapour is compressed to accomplish C3/C4 separation at reasonable temperatures. The C3- fraction is routed to the battery limit of the unit.