PTQ Q2 2023 Issue

limited when increasing LPG yield, even when using the highest capacity internals. Figure 12 shows a PFD of the WGC discharge through gas plant stripper (de-ethaniser) bottoms utilising an HP receiver recontact drum for the stripper vapour. This flow scheme will be used throughout these discussions, although there are several variants. Propylene must be recovered and the C 2 = removed to meet the downstream product mixed C 3 specification. Gas plant C3 = recovery has increased over the years from 85-90% to typically 95-99% today due to lower dry gas and higher gasoline yields. Refiners push - ing C 3 = yields typically want to operate at 98% recovery or higher, which produces 3% or less C 3 = in the gas leaving the absorber. Higher reactor LPG yields increase the WGC discharge flow rate, increase HP receiver (de-ethaniser or strip - per feed drum) condensing duty, and raise gas flow rate along with C 3 /C 4 composition to the absorber. As receiver pressure increases and temperature decreases to manage WGFR, C 3 = content of the MC overhead liquid increases. The absorber must recover more LPG to prevent losses to the fuel gas system. There are many different flow schemes from the one shown in Figure 12, and the specific equipment limits are always unique. Consequently, there is no one size fits all solution to meet reactor LPG yield shift and recovery goals. Furthermore, some flow schemes are more likely to have free-water inside 7,8 the stripper section, resulting in water entrapment which prematurely floods the column. This flooding limits lean oil rate (see Figure 13 ). In some cases, chilled water is used to reduce the absorber temperature because the operating pressure is very low, which further complicates water entrapment and removal. Absorber off-gas mol% C 3 = can be used as a proxy for % recovery and usually varies from 3-6%. A reactor producing 2 wt% C 2 minus dry gas will have approximately 3 mol% C 3 = in the off-gas, leaving the absorber at 98% recovery. Reactor effluent also contains 0.7-1 wt% FCC feed from inert gases carried from the regenerator to the reactor by the circulating catalyst, which has the same effect as dry gas on C 3 = recovery. Stripper bottoms product C 2 = content depends on whether the C 3 = is used for petrochemical ser- vice, like cumene, chemical or polymer grade C3=, or alkyl - ated. The lower the stripper bottoms C 2 = content, the lower the C 3= recovery, all else equal. Refiners seeking to increase reactor C 3 = yields and maintain or increase recovery have to make gas plant modifications. Primary absorber C 3 = recovery improves with higher pressure, higher liquid rate, and lower temperature. Primary absorber pressures vary from a low of 140 psig to above 250 psig. Typical operating pressure is 180-210 psig. Total absorber liquid rate consists of MC overhead liq- uid and debutaniser gasoline, often called lean oil. Higher debutaniser gasoline recycle increases recovery because it has no C 3 = compared to 1.5-5.0% or higher in the MC overhead liquid. Primary absorber operating temperature depends on the MC overhead receiver liquid, vapour from the HP receiver, and lean oil temperatures. Sometimes inter-coolers and

Sponge absorber

Main fractionator overhead liquid

Lean oil

Compressor discharge

Absorber

Stripper

WGC after-condenser knock-out drum

Single tower absorber/stripper system

Sour water

LCO pumparound

Debutaniser feed

Figure 13 Stacked absorber/stripper

a pre-saturator CW exchanger are used to reduce tem- peratures. Because LPG is absorbed, the temperature rise across the absorber is 30°F or more depending on total liquid rate. Absorber C 3 = recovery depends on six variables: pres- sure, reactor dry gas yield/inert gases, absorber operating temperature, liquid rate to the absorber, % C 3 = in the MC overhead liquid, and the amount of debutaniser gasoline recycle. Each FCC unit has unique operating conditions that contribute to C 3= recovery, with specific process and equipment limitations determining the incremental C 3 = that can be produced and recovered. Figure 14 shows some absorber operating data for a recent revamp that increased FCC charge rate, reactor C 3 = yield, % C 3 = recovery, and ulti- mately LPG production. The following sections discuss the variables that influence C3 = recovery.

133

230

Gas out

3% C=

100

Debutanised gasoline

0% C= 110

Pressure, psia Temperature, ˚F

Main column liquid

4.5% C=

5% C=

101

128

117

110

Vapour

234

22% C=

130

13% C=

Figure 14 Absorber operating data

PTQ Q2 2023