PTQ Q2 2026 Issue

190˚F 13 PSIG

100

Ammonia gas to sulphur plant

1.5 x R min

200˚F

E-2

x = 0.8 F

x = 0.5 F

E-1

P-2

12 13

Sour water feed

x = 0.2 F

90˚F

180˚F

-50

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Feed condition (q)

250˚F

q = 1+ C p (T B - T F )/H v where C p is the specific heat, Btu/lb mole ºF, H v is the molar heat of vaporisation, Btu/lb mole, T B is the boiling point tem - perature, ºF, and T F is the feed temperature, ºF. For a sour water stripper at 15 psig, a feed at 150ºF gives a q of about 1.1. The parameter X F in Figure 1 is the mole fraction of lights in the feed, which for the SWS is about 2-5%, as previously stated. Estimating from Figure 1, the preheat efficiency is approximately zero. In simple words, preheating the feed has a negligible effect on the reboiler duty. This explains the experience reported by Lieberman.⁴ Working with the SWS in Figure 2 , a field test was con - ducted in which the circulating reflux (E-2) was shut off and the feed preheater E-1 was bypassed while maintaining a constant reboiler duty and a constant tower top temper - ature. The effect on the NH₃ in the bottom of the stripper was negligible. Lieberman concluded that the preheater E-1, the circulating reflux loop E-2, and trays 13-16 serve little purpose. He then confirmed this conclusion by describ - ing a successful design that eliminated these unnecessary features. When the stripper bottoms go to a desalter (most com - mon), eliminating the preheater actually saves energy. The desalter typically operates at approximately 250ºF, which is also the stripper bottom temperature. For a 400 gpm SWS, cooling this bottom stream to 150ºF removes 20 MM Btu/h from the desalter water feed. This loss is equivalent to con - densing 20,000 lb/h of steam. Based on a heating value of 1,020 Btu/scf natural gas (Ref 2 in Part 1) and an 80% boiler efficiency, generating 20,000 lb/h of steam takes 24,500 scfh, or 1,032 lb/h of methane, the combustion of which generates 2,840 lb/h of CO 2, or 10,300 metric tons CO2 per year (based on 8,000 hours year accounting for downtime). An average car emits 4.29 metric tons of CO2 per year (Ref 3 in Part 1). So, elim - inating the preheater is equivalent to getting 2,400 cars off the road. In addition to lowering the bespoke carbon footprint, eliminating the pump, piping, and preheater exchanger further reduces the footprint. The preheater is also a con - stant source of headaches due to corrosion, tube leaks, and fouling. Figure 1 Preheat efficiency versus feed thermal condition q, high-volatility system ( α =4)8

155˚F

E-3

Steam

To desalter and hydrotreaters

P-1

Figure 2 Conventional SWS scheme with a feed preheater 4

The downside of eliminating the preheater is increas - ing the possibility of tray damage in the SWS. A sudden step-up of the cold feed rate causes immediate shock con - densation of the steam at the SWS feed zone. The sud - den low pressure generated induces a rush of steam from above and below. A common symptom of this is trays below the feed bent upwards, and those above the feed bent downwards. Two such case studies (22.14 and 22.15) were described in Kister’s Distillation Troubleshooting (Wiley, 2006), and other cases were experienced by the authors. If the pre - heater is eliminated, it is most important to use a heavy- duty (high mechanical strength) tray design and to ensure good control of the feed rate. Good operating procedures and operator training to avoid sudden steps, especially dur - ing start-up and shutdown, are essential. The heavy-duty tray design also strengthens the trays against uplift forces when the base liquid level rises above the reboiler return or the stripping steam inlet. SWS internals SWS are typically characterised by fouling and corrosion issues. Le Grange² lists fouling as the primary challenge in SWS installations and provides detailed descriptions of its sources. Corrosion is listed as another major challenge, and it is essential to fabricate the internals from corrosion-resistant materials. Foulants include the carryover of heavy hydrocarbons into the SWS or their agglomeration in the tower, coke, cata - lyst fines, corrosion products, pyrophoric iron, calcium and magnesium salts, reactions forming elemental sulphur, and polymerisation in the SWS, among others. Despite this, it is surprising to find how many SWS towers (in our experience, about 50%) contain internals that are unsuitable for fouling service. These internals plug, giving poor separation, short

PTQ Q2 2026