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Debottlenecking Crude Units

The first unit for which we will discuss different debottlenecking options is the first unit in the refinery- the crude unit. Depending upon the unit configuration, operating conditions, and crude slate, the crude unit can have a variety of bottlenecks. A typical bottleneck is crude charge pressure drop through the preheat train. Sometimes, booster pumps are required to overcome the preheat train pressure drop. Before additional pumps are installed, other options should be considered. Oftentimes, pressure drop can be alleviated inexpensively by depassing the crude side exchanger bundles. Depending on the exchanger design, this may require relatively minor exchanger modifications, but with large payouts. Depassing a 4 tube pass exchanger to 2 passes reduces the pressure drop by approximately a factor of 8. Of course, suitable exchanger velocities must be maintained to prevent excessive exchanger fouling.

Another common limit is the exchanger train design pressure. Bigger impellers cannot be installed in the charge pumps to increase the charge rate because the increased pressure will exceed the exchanger design pressure. Downstream booster pumps can alleviate this problem. An even better solution is to move or install a flash drum upstream in the exchanger train. Some exchanger trains do not have a flash drum, or it is located at the end of the train. Moving or adding a flash drum earlier in the exchanger train has several advantages. First, vapor generated does not have to flow through the rest of the exchanger train reducing the significant two phase pressure drop. Second, moving the flash drum from the end of the exchanger train to the beginning allows the exchanger train to run at the lowest possible pressure above the crude tower pressure. The lower pressure minimizes exchanger design pressure limitations. Sometimes a high pressure is maintained on the flash drum at the end of the exchanger train to minimize flashing, but this is not required. Third, a flash drum reduces the vapor traffic load on the crude tower. For even more debottlenecking, a preflash tower can be installed.

 

Debottlenecking Saturates Gas Plants

Typically part of the crude unit or just downstream is the sats gas plant. As the crude unit is expanded, the sats gas plant capacity may need to be increased. Depending on the unit configuration and bottlenecks, there are many potential bottlenecks and expansion options. One typical limit is flooding in the bottom of the debutanizer. Packing is an option, but may not sufficiently debottleneck the tower since high pressure applications may not show significant capacity increases with packing.

One option that may not be obvious, but can have significant benefits is additional feed preheat. Additional feed preheat reduces the fractionating load on the bottom section of the tower. Also, feed preheat debottlenecks the reboiler. Increasing the feed preheat can increase the tower capacity by over 25% in some cases. This significant increase is much cheaper than building a new tower.

Another Saturates Gas Plant example is in propane production. Propane typically has a vapor pressure specification which limits the allowable ethane content. Plants often run their saturated and unsaturated deethanizers conservatively so that little or no ethane remains in the propane. However, to maximize profitability, the amount of ethane in the propane should be maximized since this allows ethane to be sold at liquid fuel propane price rather than ethane gas price. The effect can be even more dramatic since it unloads the deethanizers. The reduced load upon the deethanizers may allow higher charge rates. At the very least, it reduces energy consumption while maximizing higher valued liquid product sales.

 

Debottlenecking FCCU's and Alky's

FCCU’s are another unit which are typically debottlenecked significantly beyond their original design capacities. Advances in riser technology have allowed higher reactor charge rates and conversion. The main fractionator or the other fractionating towers can often become limiting as reactor capacity is increased. Debottlenecking options in the gas plant have kept pace.

Packing is a typical solution to increasing the fractionating capacity. Packing is a proven application in FCCU main fractionator and other towers. Even if only the pumparound zones are packed, the main fractionator capacity typically can be increased by approximately 15%. Also, the pumparound loadings can be modified to significantly increase column capacity. For example, increasing the slurry pumparound duty can greatly unload the rest of the main fractionator. As with all debottlenecking revamps, the ramifications of the changed unit operation upon the equipment and the product specifications need to be rigorously evaluated.

Most plants with a FCCU have a downstream Alkylation unit. Often the reaction section is designed conservatively, and the charge rate can be increased with little impact upon the alkylate octane. Sometimes the fractionating towers become the limit. Sometimes in tall towers with narrow boiling range feedstocks such as the Alky deisobutanizer or debutanizer, the optimum feed location has a greater effect than normal. For example, optimizing the deisobutanizer feed location can improve tower capacity by up to 10%. Also, high capacity trays have increased Alky deisobutanizer capacity by over 15%.

 

Debottlenecking Catalytic Reformers

Catalytic reformers are another unit which can be debottlenecked beyond their design capacity. Improvements in catalyst activity and dense loading technology have allowed the reactors to handle higher charge rates. The downstream fractionation equipment’s capacity must be increased to keep pace. One such tower is the reformate stabilizer. Improvements in fractionation capacity can be made by installing packing as discussed for FCCU Main fractionators. Fractionation capacity gains can also be achieved by installing high capacity trays. Fractionation capacity increase may then cause the reboiler or overhead condenser to be the limiting item. If so, replacing the exchanger bundle with an extended surface bundle can lead to attractive payouts. In one such example, the steam reboiler was flux limited. Replacing the reboiler bundle with an extended surface bundle increased the reboiler area. Since the shell side had the limiting heat transfer coefficient, the results were dramatic. The tower capacity increased by over 25% for just the cost of a $15,000 exchanger bundle.

Extended surface tubes can increase many other services such as overhead condensers. Extended surface tubes are especially applicable to services in which the shell side has the limiting heat transfer coefficient since only the shell side area is increased. Remember to account for the smaller tube ID’s and hence higher pressure drops.

 

Debottlenecking Hydrotreaters

Hydrotreaters also can be debottlecked. Higher capacity catalyst, activated support balls, dense loading of the reactor can all increase the reactor capacity. Once a unit is built, it often is most profitable to maximize the throughput up to the charge pump hydraulic limits. Oftentimes, the charge pump spare can be turned on to increase the oil charge flow. Larger control valves may reduce the charge circuit pressure drop.

There is usually an incentive to run hydrotreaters at the highest possible pressure up to the equipment design pressures. There must be an acceptable safety margin between the operating pressure and the relief valve set pressure to handle the minor unit pressure swings so that the relief valves do not lift. Furthermore, if conventional relief valves are installed, the operating pressure must be maintained below approximately 90% of the relief valve set pressure so that the relief valves do not leak. Pilot operated relief valves allow operation up to 95% of the relief valve set pressure.

 

Debottlenecking Low Pressure Fractionating Towers

Ancillary units also have room for debottlenecking. For this example, we will use an aromatics recovery unit although the discussion is applicable to any low pressure tower. The extraction section can often be pushed beyond the normal design rates by running at a lower solvent to feed ratio. The downstream towers may then become limiting. In any low pressure tower, the raising the tower pressure may significantly unload the trays and overhead condenser. The reboiler load will increase, but may not become a limit, especially if it is a steam reboiler. Raising the tower pressure can increase the tower capacity in the range of 5-10%.

 

Debottlenecking Hydraulic Limits

One example of hydraulic debottlenecking an aromatics extraction unit was particularly dramatic. A market supply disruption raised the price of benzene dramatically. All efforts were made to increase the unit charge rate and maximize benzene production. However, the benzene rundown line became the unit-limiting item. Operations then decided to turn on the spare pump. This increased the capacity by approximately 10%. There was room for more, however. Operations typically ran down a single rundown line into a day tank to minimize the amount of offspec produced by an upset. Another set of day tanks had a separate day tank. So operations began using the second rundown line in parallel. This greatly debottlenecked the system. At the higher flow rates, the control valve pressure drop became limiting so the trim in the control valve was replaced. The combination of all of these items increased the maximum benzene production rate by over 35%. And the only money spent was for the new control valve trim. The payout for replacing the control valve trim would be measured in minutes !