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Sep-2003

FCC vapour line coking

Causes, penalties and solutions to coking in the FCC reactor vapour line, with regard to inlet nozzle coking affecting unit capacity, conversion and reliability

Christopher F Dean, Saudi Aramco
Scott W Golden, Process Consulting Services
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Article Summary
Coking in the FCC reactor vapour line (VPL) increases the pressure drop between the air blower discharge and the wet gas compressor (Figure 1) suction. As the pressure drop increases, the wet gas compressor suction pressure must be lowered or the regenerator pressure increased to maintain the pressure balance. Moreover, when it occurs at the main column inlet, it has caused coke to accumulate in the bottom of the main column, increased the main column bottoms (MCB) system fouling, caused higher rates of erosion of the MCB pumps, and premature flooding of the main column slurry pumparound (P/A) section internals when column loading is high. With many refiners attempting to process heavier feeds containing more aromatic species, while also employing higher reactor operating temperatures to produce petrochemical feedstocks (for example, light olefins), coking problems in vapour lines have begun to reappear.

In one case documented by Mauleon(1), vapour line coking increased the VPL pressure drop by 6psi when the main column inlet nozzle became partially blocked with coke. Since many FCCs already operate against the air blower or wet gas compressor volume or driver limit, the feed rate or conversion must be reduced to stay within the compressor constraints. However, when it forms in the main column inlet nozzle, many adverse consequences reduce unit profitability. Mauleon and others have previously covered VPL coking, but a review of the causes, penalties and possible solutions, particularly with regard to inlet nozzle coking observed by the Saudi Aramco and PCS authors, should add further knowledge to this growing concern. 

Nozzle coking
Heavier feeds, cold spots along the VPL, low VPL velocity, line configuration near the main column inlet, and main column bottoms liquid being sucked into the VPL have all caused coke to form. Higher boiling point reactor products can condense where there are cold spots, or some reaction products can polymerise to form large molecules that are non-volatile at VPL temperatures. Cold spots attributable to inadequate insulation or high heat loss near fittings such as flanges facilitate condensation. If these liquids have sufficient residence time in the VPL, coke begins to accumulate on the inside of the line. 

Long VPLs, horizontal runs, and lines that are not free-draining to the column are also factors. Once coke is formed, additional coke has a surface where it grows more easily. In several instances, coke has blocked more than 50% of the cross-sectional area of the line (Figure 2), thereby increasing pressure drop by up to 6psi (0.41 bar) or more. In one example, the main column inlet velocity increased to more than 90m/sec through the open portion of the inlet nozzle ring of coke.  

VPL configuration plays a central role in nozzle coking. Long horizontal runs into the main column create a “vena contractor” near the inlet nozzle as vapour rapidly expands into the main column. Liquid becomes trapped in these low-pressure regions where residence time becomes very high. Flow is always from high to low pressure. Therefore, liquid can flow from inside the main column into the VPL. When this happens, a ring of coke is created (Figure 3). In addition, many lines have a 90-degree elbow oriented horizontally before entering the main column. This creates a low-pressure zone in the inside radius, as high-velocity vapour flows faster along the outer radius. Liquid becomes trapped along the inside radius, causing coke to form (Figure 3). This coking pattern is very distinct. Any piping configuration that creates low-pressure regions and is not free-draining produces areas that trap liquid.

Coke precursors
Feedstock, catalyst and reactor hardware all play a role in coke formation and are well documented by McPherson, among others(2). Feedstock properties influence the specific reactor species (aromatic, olefins and so on) formed. High endpoint compounds condense more easily and di-olefins can react to form non-volatile compounds. Catalyst formulations in recent years have resulted in greater usage of high hydrogen-transfer reaction catalysts, because low gasoline boiling-range olefins are desirable and gasoline octane has not been a driving force.

These high hydrogen-transfer catalysts, in conjunction with the heavier aromatic feeds, tend to produce higher boiling-point polynuclear aromatics (PNAs), which are more likely to condense in the VPL. Once these PNAs condense in the VPL, they easily form coke. Some catalyst formulations, such as those formulated with higher matrix activity, are more able to crack some of the very heavy hydrocarbons. This has reduced coking when unconverted liquid was being carried into the VPL from the reactor. However, there have been other cases where high matrix activity has raised the rate of coking, because the heavy cracked species were more inclined to polymerise to form non-volatile compounds in the VPL which lead to coke.

Moreover, reactor feed mixing, riser disengagement and higher reactor temperatures can increase di-olefin formation, thereby producing more reactive species that form non-volatile compounds. Also, reactor conditions that produce higher boiling-point products lead to higher VPL coking rates. Since the VPL closest to the main column is the coldest portion of the line due to heat loss along the line and the large inlet flanges or valves, the main column inlet nozzle creates the most favourable conditions for coke to form in. If additional polymerisation occurs in the reactor outlet, the non-volatile liquids could form in the VPL closest to the main column or in the inlet nozzle.

Increased pressure drop
When the wet gas compressor or air blower is limiting, a higher system pressure drop lowers unit profitability, because the conversion or feed rate must be lowered. Understanding each component of a system pressure drop and its magnitude is essential when optimising or revamping. In one case, a low-pressure unit operated at 3–4psig in the main column overhead receiver, while the regenerator ran at approximately 14psig. Coke formation in the VPL raised the pressure drop by 2psi, thereby reducing wet gas compressor inlet suction pressure to 1psig. With some feeds, lower compressor inlet pressure was possible, because dry gas and hydrogen yields were low. However, many times, higher-pressure drops have caused feed rates to be reduced to stay within the wet gas compressor capacity. Reducing wet gas suction pressure from 4–1psig requires more than 20% additional volume capacity and significant driver power increase. Sometimes regenerator pressure had to be increased, which reduced the air blower capacity.
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