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Alan R. English

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  • in reply to: FCCU Volumetric expansion #29281

    FCC volume expansion = 100 – (C3+ Liquid Yield)

    C3+ liquid yield, vol% FF = 100*{sum of bpd of all liquid products (C3 through decanted oil) / feed bpd}

    C3+ liquid yield is typically expressed for standard cut yields meaning that the C3 product lost to dry gas is added to the actual C3 recovered.

    For conventional FCC units the value of C3+ liquid will range from 106 – 114 vol% FF. The actual value is dependent on feed quality and conversion. Low hydrogen content feeds (coker gas oils or resids) will have low vakyes high hydrogen content feeds (hydrotreated gas oils or tight oils will have high values). When comparing your unit to others you must be careful to only compare it to units with similar feed. Untreated virgin gas oils typically have a value of 110 to 112 vol% of fresh feed at 650-F conversion between about 75 to 80 vol% FF.

  • in reply to: FCCU gas recovery section #28317

    Your C3 in fuel gas looks to be on the high side of typical, but this doesn’t really tell us how much you are losing. Many refiners also monitor the percentage of total C3 production lost to fuel gas.

    Pressure and absorbent liquid flow rate are the main control variables. Pressure should be maximized for best recovery but this must be weighed against any capacity reduction which might result from reduced wet gas compressor capacity. Absorbent liquids should enter at the lowest temperatures achievable. Any attempt to increase C3 recovery will also increase C2 recovery, forcing a corresponding increase in stripper bottom temperature to control C2 in the LPG product. Therefore, the optimum C3 recovery is a balance between the value of recovered C3, wet gas compressor capacity and stripper energy cost. The best way to identify this optimum is with a rigorous simulation of the entire system (wet gas compressor to debutanizer) using any of the available programs (HYSYS, PetroSIM, PROII, etc.). Close monitoring of the LPG C2 level is also critical. You should operate as close to the product specification as possible as anything lower indicates excessive stripper bottom temperature which would waste energy and reduce C3 recovery.

    A low cost capital improvement would be to recycle debutanizer bottoms to the top of the absorber with the High Pressure Separator liquid routed to a lower tray (perhaps 3 or 5, numbered from the top). This improves recovery of the C3 in the HPS liquid.

  • in reply to: FCCU Severity #27619

    The process conditions you mention are within the range of normal FCC. It is not possible to comment on if these conditions are right for your unit with studying your constraints and feed/product pricing. 550 C reactor temperature is fairly high. This is justified when conversion, olefinic LPG and/or octane have high value. Gas yield will be high which typically means feed rate must be reduced. Also, expect high gasoline diolefins (potential gum). You must determine if the advantages outweigh the costs. 180 C preheat is on the low end of the typical range. Low preheat is used to increase catalyst/oil ratio and conversion as well as to lower regenerator temperature. Going much lower may adversely affect how well your feed injectors work due to increased feed viscosity. 365 C fractionator bottom temperature is reasonable but keep in mind that this temperature does not determine LCO cut point or recovery. Typically, the bottom pool temperature can be controlled separatly from the flash zone temperature by diverting cool slurry pumparound directly to the fractionator bottom. Adjust slurry pumparound to the shed deck and LCO draw rate to optimize fractionation but be certain to maintain sufficient rate to assure that all surfaces in this section remain wet to prevent coking.

  • in reply to: Gas recovery section water washing #26291

    There are several factors that can affect the effectiveness of your anti-corrosion water wash. The system you describe, where water is routed first to high pressure service and then cascaded to lower pressure is an older design and not as effective as cascading water from low to high pressure. Cyanide solubility in water increases with pressure so it is likely that you are capturing cyanide in the high pressure water wash which is then released to travel through the system again when the water is used at a lower pressure. Modern unit design collects water from the main fractionator overhead receiver. Much of this water is recycled to the main column overhead receiver exchanger inlet. Additional fresh water (BFW or SWS water) is added to the recycle stream to control wash water pH and cyanide level. Excess receiver water is used to wash the interstage exchanger. Water from the interstage receiver is used to wash the high pressure exchanger. Water from the high pressure receiver is usually routed to the sour water stripper. It will likely be costly to reverse the direction of your water cascade. Adding more makeup water and/or using chemicals such as ammonium polysulfide will directionally help. Wash water pH and cyanide should be monitored to assure water quality.

  • in reply to: FCCU scheduled maintenance X CO Bolier #25626

    I am familiar with several units that typically operate in partial combustion and occasionally operate without the CO boiler. How this is accomplished depends on the local environmental restrictions. Local regulations regarding the operation of the CO boiler vary but are generally targeted to result in only minimal CO emissions. It the regulations specify a CO level at the boiler stack, you can adjust the degree of CO combustion in the regenerator to achieve it. The addition of CO combustion promoter will likely be required to maintain control of the regenerator temperature and achieve total combustion. Keep in mind that this will increase regenerator temperature and additional steps (lowering preheat and removing heavy feeds) may be required. Since this is intended to be a temporary operation; only for the duration of CO boiler maintanence, a catalyst change is not an option. The addition of inert particles, to act as a heat sink, might be an option to lower regenerator temperature if sufficient solid circulation capacity exists.

    If local regulations do not limit CO emissions at the stack, you might be able to bypass the CO boiler and continue to operate in partial combustion. In this case, you must consider the impact on the surrounding area air quality to assure that no unsafe conditions are created. A computer dispersion model should be used to determine the maximum flue gas CO content that would result in acceptable ground level CO concentrations given prevailing atmospheric conditions and stack height.

    Assuming a safe and legal operation without the CO boiler can be maintained, the decision to keep the FCC running or shutdown with the CO boiler becomes an economic one. Changing feeds, adjusting conditions and adding additives will have a cost. The can be compared to the benefit of a 4 year versus 5 year FCC turnaround cycle to make the final decision.

  • in reply to: FCCU Turbo expansion #25609

    It is very difficult to predict the reasonable period between turnarounds for an expander because experience has varied greatly across the industry. Some practitioners can successfully make a 5 year turnaround while many struggle to keep the expander online for more than a year. Usually, the culprit causing is deposition of catalyst fines on the expander blades which in turn causes vibration. In the past decade much improvment in blade coatings and machine design have been achieved. Your expander manufacturer can likely discuss these options with you. From a process perspective, the key to achieving long run lengths is to always operate the expander at its design point. This is because a properly designed expander will experience uniform flow throughout the flow path which will help prevent deposits. Operating above or below the design point may allow eddy currents and dead spaces to develop which will foster blade erosion or allow deposits to form. This means that the unit should be designed such that some flow to the bypass is required even when the unit is at design feed rate so that control of the flow through the expander can be maintained.

    Routine cleaning of the expander (with rice, walnut shells, etc) is also important. Don’t wait for deposits to become a problem because these can also affect the flow pattern and prevent the cleaning agent from getting where it is needed. Many practitioners clean on a weekly basis.

  • in reply to: COS Formation in FCCU #21889


    COS can be formed in the FCC riser by the reaction of H2S with CO2 to form COS and H2O. Thus, increasing the amount of CO2 in the riser (by increasing inerts carryover from the regenerator or by increasing CO2/CO ratio), or increasing the amount of H2S (higher sulfur feed or higher riser outlet temperature) could increase the formation of COS. Decreasing the amount of H2O in the riser (reduced steam rates) might also enhance COS formation. Generally, controlling operating conditions to inhibit COS formation is not cost effective. COS can be removed from the propylene stream via amine treating or with a bed of absorbent.


    Diolefins and high potential gum values are often seen in FCC gasoline product when riser outlet temperatures (ROT) exceed 1000 F (538 C). They are less commonly observed in the LCO but this is likely only because potential gum is not an LCO product specification.

    Polymer-like formations are often observed in the stripper, debutanizer or LPG tower reboilers when HCO or slurry is used as the heating medium. As Evan suggested, in this case the high tube wall temperatures which result from using hotter-heavier reboiling medium dramatically increase the rate of diolefin polymerization.

    Based on these observations, I believe you have a few courses of action. Reducing ROT or increasing MAT activity or catalyst-to-oil ratio will shift the balance of reactions away from thermal cracking and toward catalytic cracking. I am surprised that you are seeing polymer in the sponge column since this column typically operates at low temperature (less than 100 F (38 C)). Is the oil heated elsewhere where the polymer could form and then be carried into the column after cooling?

    Another possible, but rare, mechanism could occur if you are drastically overdosing liquid metals passivators such as antimony. If your metals passivator deposition efficiency is less than 70% you may be in an overdosing situation. Testing for metals in the deposits is also recommended.

  • in reply to: CatCracker Supervisor #25467

    Failures in the HP condenser can usually be traced back to corrosion and salt deposition. These mechanisms are controlled with water washes at the main fractionator overhead condenser, the LP condenser and the HP condenser. The effectiveness of these washes is monitored by regularly checking the pH, and composition of the water being drawn from the water boots of these same condensers. It is also important to make sure that the rate of water injection is sufficient to assure wetting of all surfaces in the condenser. It sounds like you might have some spots near the end of the tubes where water is not reaching. In such a case you could be measuring good quality water but still seeing some corrosion.

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