PTQ Gas 2022 Issue

103.7 999.7

122.0 1536.0

T P

P (overall) psig Flow (vapour) MMSCFD Water (vapour) ppmw

psig Flow (overall) MMSCFD Water (liquid) ppmw

95.0 1536.0 854 0.185

T P

199.51 0.0135

1.686 1.640

psig

ppmw

Water (overall) Flow (overall)

TEG (liquid)

99.9807

wt %

MMSCFD

To liquication

Fraction vapour (overall)

1.0

Stripping gas

TEG makeup CB

Contactor

103.7 999.7 0.16 0.0135

P (overall) psig Flow (vapour) MMSCFD Water (vapour) ppmw

Flash vapour

Stripper

Wet gas

Reboiler

Rich TEG

120.0

T P

Excess water

psig

1000.0 1 200.0

Fraction vapour (overall)

Stahl

Water (overall) Flow (overall)

MMSCFD

T F 393.6

ppmw

2169

Lean TEG

Figure 1 Flowsheet for gas dehydration using TEG and a Stahl column

The feed gas composition (see Table 1 ) was assumed to be pri- marily wet methane with small amounts of other hydrocarbons at 120°F (49°C) and 120 psig (827 kPa). The VLE model used for the simula- tion was that of Bestani and Shing.⁵ Figure 1 shows a schematic of the process flowsheet. The call - outs on the flowsheet display OGT | ProTreat simulated results for key streams. Results show that the TEG coming from the regenerator is only 0.0193 wt% water (99.9807 wt% TEG), which at 50°C (122°F) can produce 0.0125 ppmv water in the dry gas. The tiny stripping gas stream (only 0.16 MMSCFD of the 199.67 MMSCFD dry gas flow) ena - bles the Stahl column to produce extremely dry solvent. To test the sensitivity of dehy - dration level to process variables, the VLE model, stripping gas flow rate, and Stahl column height were varied. In the first test, the strip - ping gas flow rate to the Stahl col - umn was changed between 0.05 and 0.3 MMSCFD with a constant Stahl column packing height of 10m. In addition to the Bestani-Shing model, the Parrish VLE model was also tested. The results in Figure 2 show that the Parrish-Margules model pre-

designed with specifications sim - ilar to existing TEG plants. A Stahl column was added to the regenera- tion section immediately below the reboiler of the conventional regen - erator. A small slipstream, 0.08% of the fully dehydrated gas, was fed to the bottom of the Stahl column, where it acts as very dry stripping gas. The contactor contains 10m of MellapakPlus 452.Y, the stripper contains 3m of 1in metal Pall rings, and the Stahl column contains 10m of Mellapak 250X. None of these columns is particularly tall, and all are well within reasonable flooding levels. The L/G ratios are typical for glycol dehydration units, so an unu - sual hydraulic situation in any of the columns is not expected.

age further evaporation. This is the principle behind the Stahl column. Case study The following case study focuses on whether it is possible in princi- ple to regenerate triethylene glycol (TEG) to a moisture level capable of drying methane to below 0.1 ppmv H 2 O, the generally accepted maximum moisture level recom - mended for gas entering the liq- uefaction section of an LNG train. It goes without saying that this is not possible using only a reboiled regenerator. Our contention, substantiated via simulation, is that a Stahl col - umn can enable reaching the same low water content while keeping temperatures below TEG’s decom - position limit. Recently, Carmody⁴ presented an interesting paper in which he suggested using the approach being described here. However, without access to a mass transfer rate based simulator, his analysis could not connect ideal stages to actual towers with real internals. Here we show that a TEG system alone can achieve dehydra - tion very satisfactorily for gas liq - uefaction in an LNG plant without using molecular sieves. To test this idea, a plant was

Component

Mol%

Water

0.217 (2169 ppmv) 4.989E-3 (50 ppmv)

CO₂

Methane Ethane Propane

89.299 5.684 2.108 0.589 0.340 0.200 0.200 0.549 0.809

N-Butane Isobutane N-Pentane Isopentane

N-Hexane Nitrogen

Table 1

Gas 2022 23