Apr-2007

Expanding FCCU wet gas compressor capacity

Parametric correlations can predict WGC performance curves at different speeds and molecular weights. However, performance curves in a range of speeds and suction pressures must first be made available

Tek Sutikno
Fluor Corporation

Article Summary

Applications of the latest technology for fluid catalytic cracking (FCC) reactors and regenerators can increase the throughput capacity of an existing FCC unit (FCCU) to almost twice its original design capacity.

As depicted in Figure 1, an FCCU converts gas oil to lighter products, mainly gasoline. Catalytically cracked gas oil amounts to about 40% of the gasoline produced in a typical refinery equipped with a FCCU. To meet the increased capacity, the existing air blower can be supplemented with oxygen enrichment, and the main fractionator (MF) internals above the slurry pumparound section can be replaced with structured packing.

With the structured packing, the pressure drop across the MF typically lowers, leading to a higher suction pressure available to the wet gas compressor (WGC). Proper utilisation of this higher pressure can minimise compressor modification and expand its operating capacity and range to meet the expected operating variation of the FCCU. Typically, the compressor speed can be optimally changed to meet the required capacity expansion.

The FCCU WGC is generally centrifugal and discharges to a relatively fixed downstream pressure at all operating capacities. For compressors with steam turbine drivers, speed is varied to maintain the MF overhead pressure. For motor-driven compressors, this overhead pressure is controlled by throttling the WGC’s suction pressure.

In the variable-speed case, the engineer responsible for specifying the WGC needs the performance curves to be at various speeds to assure WGC performance meets the required operating envelope of the FCCU. In the fixed-speed case, the performance curves must be at different speeds for the engineer to select the optimum design speed so that the pressure drop range available to the suction-throttling valve controlling the compressor’s throughput capacity is able to meet the expected FCCU operating envelope.

The molecular weight and flow rate of wet gas from the FCCU MF may change significantly, depending on the operating mode, catalyst type and feed characteristics. In the maximum conversion-to-propylene mode, for example, the mass flow rate through the WGC can increase by as much as 40%. The FCCU WGC often becomes the bottleneck, limiting the unit’s capability to operate in a specific mode intended for economic reasons.

Matching WGC performance to the expected variation in FCCU operation is therefore essential. To accomplish this objective, higher WGC suction pressures available from MF internals replacement can be fully utilised to maximise capacity as well as the operating ranges of the existing WGC. WGC performance curves in a range of speeds and suction pressures must first be made available.

Performance curves
The WGC vendor typically provides a set of performance curves, as shown in Figures 2, 3 and 4. Figure 2 shows the discharge pressure versus the mass flow rate at a constant suction pressure condition. Figures 3 and 4 respectively show the polytropic heads and efficiencies at the base speed in a range of inlet volumetric flow rates (ACFM). These polytropic head and efficiency data can be entered in a commercial process simulation program to predict WGC performance at varying suction pressures. Similar sets of curves in a range of different speeds can obviously be requested from the same vendor. However, these curves may not be available for a while and generally require an engineering study and associated new business contract. The engineer responsible for revamping the WGC could require several weeks to prepare the performance request package for the vendor, as a preliminary performance model needs to be first developed to define the expected performance target and provide the vendor with updated details of the compressor system. For example, the engineer needs to provide the vendor with the latest pressure drop and the cooling duty data of the interstage system, which may have been modified in the past.

After evaluating the first set of performance curves at the different speeds received from the vendor, the engineer often finds it necessary to analyse additional sets of performance curves at specific new speeds for optimisation purposes. Moreover, parametric correlations to predict WGC performance at different speeds will help engineers to optimally define the WGC operating envelope, the driver, and the associated compressor control and surge-protection systems. These correlations are also applicable to other centrifugal compressors similar to the FCC WGC.

Eq 1 is the commonly known correlation between the polytropic head (H) of a centrifugal WGC and its rotational speed (N):

 H2 / H1 = ( N2 / N1 ) 2                              Eq 1

Subscripts:  1 - base speed
                   2 - new speed

The inlet volumetric flow rate (Q) varies with the speed N, as shown in 
Eq 2:

Q2 / Q1 =  N2 / N1                                    Eq 2   

Based on these correlations, Figure 5 (as an example) shows H2 values at N2, calculated from known H1 values at N1, do not fully agree with the H2’s at N2 (or 103.92% of N1) reported by the vendor. One of the reasons for the discrepancy is that Eq 1 applies strictly to the case where the suction pressure remains unchanged and the discharge pressure is allowed to vary with the new speed.

In the case of this FCC WGC, the second-stage discharge pressure is essentially fixed by the set point of a pressure controller located downstream, except that the pressure drop from the WGC discharge to this pressure controller may vary somewhat, depending on the flow rate. For a fixed-speed WGC, the first-stage suction pressure will need to vary with the flow rate to meet the fixed discharge pressure. With the WGC suction pressure changing as the speed varies, Eq 1 is not really suitable for estimating the polytropic heads at a different speed and can only be used for rough estimates.

To predict WGC polytropic heads at a new speed more accurately, the conventional polytropic head equation (Eq 3) is needed to account for the effects of suction pressure as well as gas property variations on WGC performance:      
 
H = Zave(1545 / MW)Ts(( Pd / Ps)/ -1) //  Eq 3

where:
Zave = the average compressibility factor from the inlet and outlet condition
MW = molecular weight,
Ts = inlet temperature in oR
Pd and Ps = discharge and suction pressure respectively in psia
/ =  ( m -1)/m =  (k-1)/k / n
k - Cp/Cv = average for the inlet and outlet conditions
n = polytropic efficiency
Subscripts:  s = suction side
                   d = discharge side