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Jul-2012

Catalyst stripper improves FCC unit performance

Stripper internals with improved mass transfer characteristics improve the performance of an FCC unit

Rama Rao Marri and Dalip Soni
Lummus Technology, a CB&I Company

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Article Summary

FCC unit performance is dictated by a delicate coke and heat balance1,2 because the reactor produces the necessary amount of coke to satisfy the 
unit heat balance. The heat produced by the combustion of coke in the regenerator supplies the required heat (via circulating catalyst) for the endothermic riser-reactor.

The coke is classified into four types: contaminant coke, catalytic coke, additive coke and catalyst-to-oil coke. Generally, catalytic, contaminant and additive coke production are functions of feed quality, catalyst type and reactor operating temperature. However, catalyst-to-oil coke is a result of the hydrocarbons entrained within the spent catalyst as it enters the regenerator. This coke includes hydrocarbons absorbed on the catalyst surface and within the catalyst pores. It is very important to strip off these hydrocarbons (coke precursors) by employing an efficient stripper. Removal of these hydrocarbons from the catalyst before it enters the regenerator can significantly improve overall performance and hence profitability of the FCC unit. This article discusses the design and implementation of Lummus Technology’s patented ModGrid stripper and the improvements achieved in commercial FCC units.

Coke yield and delta coke
Understanding delta coke and its relationship to coke yield and heat balance is important for assessing the impact of stripper performance on unit profitability:
Coke yield = Catalyst-to-oil x (delta coke) —   
                                                                         (1)

Delta coke [or (CSC-CRC)] α (Trg-Trx) —        (2)

Where:
• Coke yield is expressed in wt% of the fresh feed rate to the FCC unit, which is determined by the heat balance around the reactor and regenerator
• Catalyst-to-oil is the ratio of catalyst circulation rate to fresh feed rate, which is dimensionless. In general, the higher the catalyst-to-oil ratio, the higher the conversion of feed to valuable products
• Delta coke, or (CSC-CRC), is the difference between the coke on spent catalyst (CSC) leaving the stripper and the coke on regenerated catalyst (CRC) leaving the regenerator, which is expressed in wt% of catalyst
• Trg is the catalyst bed temperature in the regenerator
• Trx is the riser or reactor outlet temperature.

The coke yield is essentially set by operating conditions such as the riser-reactor outlet temperature and feed preheat temperature, while the delta coke is directly influenced by catalyst type, feed quality and unit hardware design. To achieve a higher catalyst-to-oil ratio and hence higher conversion, it is necessary to reduce delta coke or reduce Trg for a given Trx. Trg is a function of regenerator efficiency and CSC (amount of coke carried from the stripper to the regenerator). Therefore, it is important that stripper efficiency be maximised to achieve a higher catalyst-to-oil ratio and conversion.

Characteristics of efficient  stripper design

An efficient catalyst stripper design is one that maximises mass transfer between the two phases (the stripping steam flowing up and the fluidised catalyst flowing down the stripper vessel). Hydrocarbons in the catalyst phase need to be replaced with steam. To enhance this mass transfer phenomenon, the stripper internals should have the following characteristics:
• Maximum surface area for mass transfer per unit volume of the stripper vessel
• Maximum cross-sectional area of the vessel available for catalyst and steam to flow through it
• Maximum active volume (ie, no stagnation or dead zones)
• Uniform distribution of catalyst and steam to avoid channelling and by-passing
• Increased contact time and mixing between the two phases
• Excellent fluidisation quality
• Plug flow conditions
• Mechanically robust to withstand service.

ModGrid design features
The ModGrid stripper design has all the characteristics described above and has delivered superior FCC unit performance in several commercial units currently in operation. This design was conceptualised and then developed by conducting large-scale cold-flow model tests that compared the efficiency of this design to the conventional disk-and-doughnut baffle design. The tests were done at various catalyst flux rates to determine the effect of higher 
catalyst flux rates on stripping efficiency with the two systems. Figure 1 compares the efficiencies in the two cases. At all flux rates, the ModGrid stripper design’s efficiency is higher than the conventional design and it does not drop with an increase in catalyst flows. The design has been in commercial operation for more than six years.

Figure 2 shows the typical ModGrid stripper internal. This modular grid consists of baffles that are angled and oriented such that fluidised catalyst can easily and uniformly flow over these baffles like a curtain. The baffles have holes, and steam passes from under the baffles, through the holes and then through the curtain of fluidised catalyst. The contact of steam and catalyst occurs over these baffles, which provide the maximum surface area for mass transfer and hence higher efficiency. The manner in which the baffles are angled and oriented also results in a maximum cross-sectional area available for catalyst flow through the modular grid. This, in turn, lowers steam and catalyst velocities and increases contact time, which further improves mass transfer efficiency. The notches at both the top and bottom edges along the full length of the baffles break up steam bubbles so these bubbles do not increase in size as they flow up through the modular grids.

The baffles that form the modular grid are sized to extend the full width of the stripper diameter. The baffles are organised and built into modular grids for ease of installation and removal. Figure 3 shows that four to six modular grids are grouped to form a section or assembly of modular grids. Each alternating modular grid is oriented so that the catalyst takes a 360-540-degree turn as it flows down the modular grid assembly.


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