Wet gas compressor

The hydrocarbon vapors flow to the wet gas compressor. This gas stream contains not only ethane and lighter gases, but about 95% of the C3 and and about 10% of the naphtha. The phrase wet gas refers to condensable components of the gas stream. 

The hydrocarbon liquid is split. Some is pumped back to the main column as reflux and some is pumped forward to the gas plant Condensed water is also split. Some is pumped back as wash to the overhead condensers and some is pumped away to treating. Some might be used as wash to the wet gas compressor discharge coolers,... 

A small amount of nickel in the FCC feed has a significant influence on the unit operation. In a clean gas oil operation, the hydrogen yield is about 40 standard cubic feet (scf) per barrel of feed (0.07 wi /r ). This is a manageable rate that most units can handle. If the nickel level increases to 1.5 ppm, the hydrogen yield increases up to 100 scf per barrel (0.17 wt%). Note that in a 50,000 barrel/day unit, this corresponds to a mere 16 pounds per day of nickel. Unless the catalyst addition rate is increased or the nickel in the feed is passivated (see Chapter 3), the feed rate or conversion may need to be reduced. The wet gas will become lean and may limit the pumping capacity of the wet gas compressor. 

In most units, the increase in hydrogen make does not increase coke yield the coke yield in a cat cracker is constant (Chapter 5). The coke yield does not go up because other unit constraints, such as the regenerator temperature and/or wet gas compressor, force the operator to reduce charge or severity. High hydrogen yield also affects the recovery of Cj-H components in the gas plant. Hydrogen works as an inert and changes the liquid-vapor ratio in the absorbers. 

The decision to add ZSM-5 depends on the objectives and constraints of the unit. ZSM-5 application will increase load on the wet gas compressor, FCC gas plant, and other downstream units. Most refiners who add ZSM-5 do it on a seasonal basis, again depending on their octane need and unit limitations. 

Operating parameters should be specified. It should be documented which constraints (i.e., blower, wet gas compressor, etc.) the unit is operating against. 

A clear understanding of the pressure balance is extremely imptiriant in squeezing the most out of a unit. Incremental capacity can come from increased catalyst circulation or from altering the differential pressure between the reactor-regenerator to free up the wet gas compressor or air blower loads. One must know how to manipulate the pressure balance to identify the true constraints of the unit. 

The reactor pressure is not directly controlled instead, it floats on the main column overhead receiver, A pressure controller on the overhead receiver controls the wet gas compressor and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor. 

It pushes against two or more constraints rather than one single constraint. It can maximize the air blower and wet gas compressor capacities. 

The pressure balance provides an insight into the hydraulics of catalyst circulation. Performing pressure balance surveys will help the unit engineer identify pinch points. It will also balance two common constraints the air blower and the wet gas compressor. 

Dry gas is an undesirable by-product of the FCC unit excessive yields load up the wet gas compressor (WGC) and are often a constraint. The dry gas yield is primarily due to thermal cracking, metals in the feed, and nonselective catalytic cracking. The main factors that contribute to the increase of dry gas are ... 

Reduction of the catalyst/hydrocarbon time in the riser, coupled with the elimination of post-riser cracking, reduces the saturation of the already produced olefins and allows the refiner to increase the reaction severity. The actions enhance the olefin yields and still operate within the wet gas compressor constraints. Elimination of post-riser residence time (direct connection of the reactor cyclones to the riser) or reducing the temperature in the dilute phase virtually eliminates undesired thermal and nonselective cracking. This reduces dry gas and diolefin yields. 

Cyclone separators are extremely important to the successful operation of the cat cracker. Their performance can impact several factors, including the additional cost of fresh catalyst, extra turnaround maintenance costs, allowable limits on emission of the particulates, incremental energy recovery in the wet gas compressor, and hot gas expander. 

Install a water-wash system for dilution and removal of cyanide from the unit. Cascading wash water from the high-pressure zone back to the main column overhead or to the first-stage wet gas compressor outlet is attractive, but it is better to use overhead water and pump it from low pressure to high pressure. 

Verify the position of the wet gas compressor spillback. Determine if the compressor turbine needs water washing. Trend the level of inert gases in the dry gas. 

The FCC reactor pressure is usually controlled at the suction of the wet gas compressor. The reactor pressure is the wet gas compressor suction pressure plus pressure drop through the main fractionator system. 

Evaluate the trade-off between the air blower capacity and wet gas compressor capacity. Spare horsepower at one can be used to unload the other. Consider ... 

A portion of liquid from the overhead receiver is refluxed back to the tower and the remainder is pumped on to the gas plant. The vapor from the receiver goes to the wet gas compressor. The pressure of the reactor/main fractionator system is usually controlled at the compressor suction. 

Improving overhead cooling will increase the wet gas compressor capacity. Excessive pressure drop or limited cooling in the overhead system decreases the capacity. This can result from ... 

The wet gas compressor is always run to a limit, therefore, increasing the available flow will always benefit the unit. The flow can be increased by ... 

Increasing the gas plant pressure. A 10 psi increase in absorber pressure increases C3 recovery by 2% (Figure 9-10). However, this can reduce the wet gas compressor capacity. Fractionation efficiency decreases as the column pressure increases. 

If the receiver vent is in continuous service, route it back to the wet gas compressor interstage rather than to the suction. Consider adding a chiller on the vent gas. 

Increasing the wet gas compressor capacity and increasing duties through the gas plant can impact the flare system. 

This chapter provided several cost recommendations that, once implemented, would provide cost-effective added value to the operation of the FCC. Examples of such items include tips on debottlenecking the air blower, wet gas compressor, and catalyst circulation. This chapter also discussed the latest technologies regarding the riser termination devices, as well as feed injection systems. Prior to implementing any new technologies, it is critical that the objectives and the limitations of the unit are clearly defined to ensure the expected benefits of the new technology are realized. 

Table X. ZSM-5 Yield Benefits - Constant Octane Wet Gas Compressor Constrained...

The benefits of an improved feed injection system and riser can be lower delta coke, lower regenerator temperature, higher conversion, higher gasoline yield, and increased liquid yield. More feed can be processed and an air blower or wet gas compressor limitation can be relieved. 

The dislodged sections of the demister pad are blown into downstream equipment, as into the suction of a centrifugal wet-gas compressor. 

The only problem with this statement is that it contradicts reality. When we actually increase the molecular weight of a gas, the amp load on the centrifugal wet gas compressor shown in Fig. 28.1, does increase. This seems to contradict the Second Law of Thermodynamics. But the Second Law has never been shown to be wrong. So we have a conflict. Our experience tells us that the amp load on the motor must... 

Liz and I recently had a project to expand the wet-gas compressor capacity of a centrifugal machine in Pasadena, Texas. We ran a pressure survey on the compressor system, as summarized in Fig. 30.2. 

This model cat cracker does not use slide valves to control catalyst circulation. Instead, the lift air (valve V ) is used to transport catalyst from the reactor into the regenerator. The flow rate of catalyst from the regenerator to the reactor is controlled by changing the pressure differential between the reactor and the regenerator using valve V2. Pressure in the reactor is controlled by valve V3 on the suction of the wet gas compressor. 

If valve V3 on the wet gas compressor is less than 90 percent open, the setpoint to the feed flow controller should be increased (i.e., bring in as much feed as the wet gas compressor can handle). 

The HO FCC unit effluent must first be processed in an FCC style main fractionator. The main fractionator must remove catalyst fines from the heavy-oil product. The main fractionator also produces a light cycle oil and an overhead gas that is primarily light hydrocarbons and gasoline. The overhead of the main fractionator can be further processed via a wet-gas compressor. The gas is then stripped with the gasoline absorbed via a lean-oil absorber, followed by amine treatment and finally a caustic wash. The combined effluents are sent to compression and into a series of contaminant removal beds and hydrogenation steps. 

Specific for Fluid Cat Cracker Unit - including regenerator system "Overloaded wet gas compressor for FCCU high hydrogen production/increase in production of light ends. Gas compressor flow reversal [poisoned catalyst]. Gas compressor surge [poisoned catalyst (that causes production of lower MM species)]. "Gas compressor flow reversal [poisoned catalyst]. "Wet gas compressor surge [poisoned catalyst (that causes production of lower MM species)]. ... 

In this technical paper the application of a subsea wet gas compressor has been selected as the case example which is used to demonstrate the key steps in the reliabihty quantification process for novel technology. Before presenting the main steps in the work processes defined in Figure 2, it is important to understand some... 

Figure 4. Illustration of a system with a subsea wet gas compressor.

In the next section in this technical paper each of the main steps in the reliabdily quantification process is described in more detail based on the case example with the subsea wet gas compressors defined previously. 

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