PTQ Q2 2023 Issue

Where: A = Edmister’s absorption factor L = Liquid rate to absorber V = Vapour rate to absorber

C 3 = equilibrium K-values

Pressure, psia

Temperature, °F

K = K-value = vapour mol% C 3 =/liquid mol% C 3 =

100

120

140

Increasing reactor C 3 = yields raises vapour rate to absorber unless there is a large decrease in the reactor C 2 minus yields. For example, if the absorber operating pres- sure and temperature remain constant and vapour rate increases by 20% due to higher reactor C 3 /C 4 yields, then absorber liquid rate must increase to maintain recovery unless the equilibrium K-value is reduced. Raising absorber pressure and lowering temperature reduces K. The chal- lenge to recovering incremental C 3 = is to identify the lowest cost while balancing the variables influencing recovery. Absorber operating pressure Operating pressure should be maintained as high as pos- sible without exceeding the limiting vessel MAWP or WGC maximum discharge pressure. System pressure drop between the WGC discharge and the absorber is an important variable when the WGC discharge pressure limits absorber operating pressure. WGC to absorber pres- sure drop includes exchanger, piping, and meter losses. A detailed pressure survey measuring individual piping runs and equipment pressure drops allows potential modifica - tions to be quickly established. Reducing system pressure drop increases the absorber operating pressure, which reduces the C 3 = K-value for a given operating temperature. Lower K-values decrease liquid rate needed to meet the C 3 = recovery target. In some instances, reduced pressure drop increased absorber pressure by 30 psi, which reduced the K-value by 10% and offset some of the potential increase in liquid rates needed to maintain recovery.

162 200 265

0.90 0.77 0.64

1.40 1.18 0.97

1.70 1.42 1.15

2.00 1.68 1.35

Table 2

Absorption factor Today, process models allow rigorous simulations to be performed quickly, but they do not identify the variables that need to be manipulated to recover the C 3 = plus in the absorber. Prior to computers, Edmister’s simple absorption factor equation was used to design absorbers. Edmister’s absorption factor is a constant for a given % C 3 = recovery and a number of absorber theoretical stages. Equation 2 shows the relationship between the liquid rate (L) to the absorber, vapour rate (V) to the absorber, and C 3 = equi- librium K-value (vapour mol% C 3 =/liquid mol% C 3 =). The equilibrium K-value depends on the operating pressure and temperature, with some selected values shown in Table 2 . K-values decrease as operating pressure increases and increase as operating temperature goes up. Raising absorber pressure and reducing temperature decreases the K-value, reducing the amount of liquid needed to recover the C 3 =.

Equation 2:

Debutaniser bottoms

Overhead receiver

Reduces temperature

Main column ovhd liquid

Compressor discharge

Primary absorber

Interstage condensate

Stripper vapour

Reduces temperature

High pressure receiver

Additional cooling

Stripper feed

To sour water stripper

Figure 15 Decreasing absorber operating temperature

PTQ Q2 2023