PTQ Q2 2023 Issue

higher reactor yield and recovery required increasing the debutanised gasoline recycle from 5 kb/d to 25 kb/d. The absorber L/V based on total liquid and vapour inlet stream rates was 1.3, with debutanised gasoline accounting for 22% of total liquid rate. MC overhead liquid C 3 = content was 4.5 mol%; therefore, meeting the gas outlet of 3 mol% C 3 = required more debutanised gasoline, with the equilib- rium K-value for these conditions greater than 1. In addition to the ZSM-5 additive cracking, the MC had a side draw of heavy naphtha product, which lowered absorber liquid rate. MC overhead liquid C 3 = content MC overhead receiver liquid C 3 = composition depends on reactor yields and receiver operating conditions. Recent FCC unit data show receiver pressure and temperature varying from 1 to 30+ psig and 90-140°F. The resultant C 3 = com- position varies from 1-5% or higher. Figure 14 data were based on a 23 psig and 110°F temperature MC receiver, which resulted in a 4.5 mol% C 3 = in the overhead liquid. Absorber operating pressure and temperature were 230 psia and 130°F with a C 3 = K-value of ~1.4. This produces 6.0 mol% C 3 = vapour leaving the absorber feed tray; conse- quently, more debutanised gasoline recycle was needed to meet the recovery goal. Debutanised gasoline recycle Total absorber liquid rate (L) has to be increased to recover the incremental reactor LPG yield. In the previous example, the absorber L/V increased to 1.3 to meet a 98% recov- ery and 3% overhead composition. Debutanised gasoline recycle rate increased to 22% of total liquid rate. This mate- rial has to be recycled through the stripper and debutaniser columns, raising reboiler duty and column vapour/liquid loading. Stripper column Higher C3 /C 4 reactor yields increase stripper column reboiler duty and column internals loading often up to vessel shell limits, even with high-capacity trays. The stripper rejects C 2’ s from the recovered C 3 plus to meet the downstream product specifications. As absorber C3 = recovery increases, so does the C 2 % in the stripper feed stream. Ideally, the stripper reboiler duty is minimised to meet the downstream unit C 2 specification, which maximises C3 = recovery with all other variables constant. Gas plants using both an absorber and stripper column with a HP receiver between the two columns allow sloppier stripper operation without large C 3 = losses to fuel gas due to the HP receiver condensers. Figure 13 shows a stacked absorber/stripper with the stripper outlet vapour flowing directly into the absorber. While these designs are more energy efficient, controlling debutaniser feed C2 content is more difficult without large C3 = losses to fuel gas. Figure 16 shows some basic stripper operating data. Feed comes into the HP receiver with about 12.2 mol% total C 2’ s at 131°F, with a small overhead stream leaving at 138°F with 40 mol% total C 2’ s and a large bottom stream leaving at 214°F and 0.2 mol% C 2’ s. Vapour rate leaving the column is the most sensitive variable to control the reboiler

138

Stripper vapour to HP receiver 40.0% C’s

273

131

Stripper feed from HP receiver

12.2% C’s

Pressure, psia Temperature, ˚F

195

277

Heat input

214

Stripper bottom to debutaniser

0.60% C’s

0.21% C’s

Figure 16 Stripper operating data

Dry gas yield: absorber vapour rate Increasing the reactor C 3 = yield raises vapour rate (V) to the absorber unless the MC overhead receiver pressure increases and/or temperature decreases. Reactor dry gas, inert gases, and C 3 /C 4 yield increases determine the mag- nitude of absorber vapour feed rate increase. Dry gas and inert gases have a major influence on HP receiver vapour rate because the majority do not condense and carry C 3 /C 4 into the absorber. Reactor technology and catalyst changes have reduced dry gas yield, which mitigates the increase in vapour rates from higher C 3= yield. For a fixed dry gas/ inert gas yield, vapour rate to the absorber will increase for material changes to the reactor C 3 = yield at constant high pressure receiver temperature and pressure. Absorber operating temperature Operating temperature depends on the liquid and vapour inlet temperatures, total liquid rate entering the absorber, and additional heat removal, including pre-saturator and inter-cooler exchangers (see Figure 15 ). HP receiver (strip - per feed drum) temperature determines vapour rate and amount of C 3 /C 4 that has to be absorbed to meet C 3 = % recovery goals. MC overhead receiver and debutaniser gas- oline recycle temperatures are set by upstream equipment performance. Temperature rise across the absorber varies from 20-40°F, depending on operating pressure. Reducing vapour temperature into the absorber has the largest effect on the absorber operating temperature and temperature rise because it reduces the amount of LPG that has to be absorbed. Reducing absorber operating temperature by 20°F decreases the K-value by about 15%, which reduces incremental liquid rate needed to maintain recovery. Absorber liquid rate MC overhead liquid and recycled debutaniser gasoline make up total absorber liquid rate. The debutanised gasoline has no C 3 =, whereas the MC overhead liquid has 1.5-5% C 3 =, depending on receiver conditions. Because the C 3 = equilib- rium K-value is almost always greater than 1 in the absorber, debutaniser gasoline recycle must increase to meet high recovery targets. In the Figure 14 example, meeting both

PTQ Q2 2023