From main column
CW
CW
CW
VAPOUR to absorber
PC
Overhead receiver
Interstage receiver
High pressure receiver (Stripper feed drum)
LC
LC
LC
Stripper feed
LIQUID to absorber
Figure 5 WGC with inter-cooler: low stage in front and high stage after the inter-condenser
in front of the inter-condenser and three or four wheels in the higher stage section. The two compressor stages have head/flow and efficiency/flow performance curves that depend on the compressor rotor design and look like a centrifugal pump curve (see Figures 6 and 7 ). The X-axis is inlet cubic feet per minute (ICFM), and the Y-axis is poly - tropic head or polytropic efficiency. The WGC develops a fixed polytropic head for a given ICFM flow. The surge point corresponds to minimum ICFM, and the maximum flow rate is stonewall. The compressor stable operating range lies between low flow rate surge point and high rate stonewall with the compressor control system adjusting recycle to avoid surge. Representative polytropic head/flow curves and efficiency/flow curves are essential to evaluate the compressor system and to identify system changes affecting the driver and WGC. Estimating current reactor yields to evaluate existing unit performance is an essential part of increasing LPG yield. If the WGC curves are representative of performance and plant meters are calibrated properly, the current operat - ing point will lie on the performance curve. The measured WGFR in ICFM should have a developed polytropic head that corresponds to the measured pressure increase across
the low and high stages. Equation 1 shows the polytropic head equation. The gas molecular weight (MW) and the compression coefficient ‘n’ depend on the gas composi - tion and compressor polytropic efficiency, which is WGC dependent. Most refiners will not have updated WGC performance curves based on the current wet gas MW. In some cases, feed hydrotreating, reactor technology, and catalyst for - mulation changes have altered the wet gas MW enough to require the manufacturer to generate new curves. Accurate performance curves allow the impacts of operating pres- sure and temperature changes on WGC to be properly assessed.
Equation 1: Polytropic head
Where: MW = molecular weight
Z AVG = average compressibility T 1 = Suction temperature, °R n = Compression coefficient
29,000
30,000
Surge
Surge
28,000
28,000
26,000
27,000
24,000
26,000
22,000
25,000
20,000
24,000
18,000
Stonewall
Stonewall
23,000
16,000
28,000
30,000
32,000
34,000
36,000
29,000
31,000
33,000
35,000
37,000
6,500
7,000 7,500 8,000 8,500 9,000 9,500
10,500 10,000
Volumetric ow, ICFM
Volumetric ow, ICFM
Figure 6 WGC low stage polytropic head/flow curve
Figure 7 WGC high stage polytropic head/flow curve
31
PTQ Q2 2023
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