Revamps 2025 Issue

temperature as much as 75ºF. Since the catalyst is not being cooled, the amount of quench is minimised, and the coke make is unchanged. This quench acts as a pumparound on the main fractionator. More than 1.0 mil- lion bpd has been used in this service. Spent catalyst stripper Stripper modifications can also be made to reduce the amount of steam being used. If the steam rate is above 3 lb/1,000 lb of catalyst circulated, then better contacting with sufficient stripping stages can reduce this number to 2 lb/1,000 lb of catalyst. The main design variables for a stripper are catalyst resi-

External cyclone

Internal cyclone

Quench

Figure 2 Post-riser quench

dence time, pressure, temperature, and steam rate. If the residence time is too low, the heavier hydrocarbons do not have enough time to desorb from the catalyst. If the time is too long, both catalytic and thermal cracking occur, which can increase the delta coke and dry gas. Usual residence times are about 90 seconds, though some strippers per- form well with 60 seconds. Stripping temperature is a key variable. Raising the tem- perature improves the desorption of hydrocarbons, but the stripper temperature is not independently controlled. The reactor temperature is set, and a corresponding stripper temperature occurs, depending on where the reactor tem- perature is taken and the design of the stripper. Usually, the stripper temperature is a few degrees lower than the reactor. In resid cracking units, poor stripping has been observed when reactor temperatures were low. Many of the heavy crackers were run to produce diesel during the winter sea- son and ran with reactor temperatures of 920ºF to 940ºF. These were typical in gas oil crackers but produced poor results in the resid units. Raising the temperature to at least 970ºF greatly improved the operation by dramat- ically reducing the delta coke. Resid contains nitrogen compounds, which act as temporary poisons due to their bonding with the catalyst’s acid sites. Increasing the reactor temperature is a standard technique for dealing with high nitrogen in the feed for all FCCUs. Aromatic compounds do not desorb as fast as the saturated hydrocarbons. One possible benefit of reduced steam is a lower load on the overhead condensers on the main column. This could allow more feed for the unit. Less steam may provide an economic boon, depending on the cost of steam. In extreme cases, the steam rate was halved without any loss in strip- per performance. A hydrogen reduction in the coke should also increase the liquid yield. Another option is to put the steam into the feed injectors and improve the reactor yields. The hydrodynamics of the

feed system and stripper may provide revamp opportuni- ties. Tracer studies may be helpful with such a study. If the sour water stripper is loaded, then this may provide some relief. Regenerator Afterburn If afterburn is chronic, a revamp may be necessary to cor- rect the problem. The causes could be a low regenerator temperature, a short catalyst bed, or the spent catalyst distribution. Low bed temperatures are caused by low delta coke or short gas contact times. Feed hydrotreating can remove coke precursors, or light feeds may not contain enough large molecules that end up on the catalyst. Short-term solutions include raising the feed temperature, the catalyst activity, or the use of a CO burning promoter. If these do not alleviate the problem, then a revamp should be considered. Raising the bed level may also help. Add additional regenerator length Adding more regenerator length can provide more res- idence time for the air in the regenerator bed. This may prevent the afterburn from occurring in the flue gas line, plenum chamber, and/or the regenerator cyclones. Table 1 shows the data from a commercial unit on two different feeds. This can extend the life of the cyclones and prevent damage to the refractory. Extra bed depth also gives more freeboard to the catalyst, which can reduce catalyst carry- over to the flue gas system. This can reduce flue gas slide valve and orifice chamber erosion, as well as the amount of solids going to the wet gas scrubber or electrostatic precip- itator (ESP). This is especially true for units operating with a superficial velocity >3.2 ft/sec. Whenever the cyclones of the reactor or regenerator are going to be placed, the equipment in that vessel should be examined for possible improvements, as the head removal

Revamps 2025