Alternative concepts

Because most of the improvements pointed out above involve the regeneration process for the glycol, most of the attention in this chapter is devoted to suggesting improvements/alternative processes for the regeneration process.

4.3.1 Alternative 1: Injecting hot stripping gas into the rich glycol stream

Idea: Instead of heating the entire rich glycol solution to boil out the water, hot stripping gas is injected to absorb the water from the glycol.

Figure 4.2: Alternative regeneration process using hot stripping gas

Hot stripping gas is provided by burning a portion of the dehydrated gas together with flash gas absorbed by the glycol. Combustion air is supplied from the surroundings, together with recycled stripping gas, and combusted in a gas burner. The stripping gas is then injected into the glycol stream, and separated out again using a separator. To reduce glycol loss, multiple stages are required, but to minimize weight and volume of the unit, two stages is proposed. The off gas from the separators must also be treated to recover some of the entrained glycol, as well as separating the BTEX from the water.

Advantages:

- Energy consumption is much lower than for a conventional regeneration process, as the stripping effect is more important than the heating effect

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- Compact; can use separators/scrubbers instead of a distillation column - Shorter retention time for the glycol in the regeneration process - Very good heat transmission, since the hot and the cold fluid is mixed

- Less thermal strain on the glycol solution, as lower temperature is sufficient for regeneration Disadvantages:

- Large glycol entrainment needs to be handled - Contamination of the glycol; more filtering is needed

- Compressor is needed for pressurizing combustion air; reduces reliability because of introducing additional rotating equipment

- Larger glycol loss than the conventional regeneration process - Possible degradation of the glycol caused by presence of oxygen 4.3.2 Alternative 2: Using staged injection of hot stripping gas

Idea: Injecting the leanest and hottest stripping gas in front of the last injection stage, recycle the off gas from the last separator and inject it in the preceding stage.

Figure 4.3: Alternative regeneration process using staged injection of hot stripping gas

The hot stripping gas is obtained using the same method as for alternative 1, but instead of dividing the stream into multiple injection stages, all of the stripping gas is injected before the last stage.

From the last separator, the off gas is then routed to the preceding stage where it is reused as stripping gas. To avoid condensation of the stripping gas and eliminate the need for recompression, the last stage must operate at a higher pressure than the preceding stage. The liquid entering the last stage must therefore be pressurized using a pump. The off gas from the first stage needs to be processed to recover some of the entrained glycol, as well as separating the volatile HCs from the water.

Advantages:

- Energy consumption is much lower than for a conventional regeneration unit

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- Compact; can use separators/scrubbers instead of a distillation column - Shorter retention time for the glycol in the regeneration system - Less thermal strain on the glycol

- Very good heat transmission, since the hot and the cold fluid is mixed - Better fit to equilibrium than alternative 1

Disadvantages:

- Requires pressurizing of the glycol between injection/extraction stages - Glycol entrainment must be handled

- More filtering is needed because of contamination of the glycol - Compressor is needed for combustion air

- Possible degradation of the glycol caused by presence of oxygen - Larger glycol loss than conventional regeneration process 4.3.3 Alternative 3: Using an absorber for regeneration Idea: Using stripping gas to regenerate the rich glycol in an absorber.

Figure 4.4: Alternative regeneration process using an absorber for regeneration

The rich glycol enters from the top of the column flowing downwards, and lean stripping gas enters from the bottom flowing upwards. As the gas flows up the column it will absorb water from the glycol, and exit from the top saturated with water (ideally). Lean glycol exits from the bottom.

Possible stripping gases are atmospheric air, nitrogen or hot combustion gases.

Advantages:

- Less energy consumption than conventional regeneration - Can re-use known technology

- Less thermal stress on the glycol solution

- Low gas flow rate results in a very compact design of the absorber - Lower glycol loss than previously described alternatives

Disadvantages:

- Possible problem with oxidation of the glycol if using atmospheric air as stripping gas

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- Requires equipment for regenerating stripping gas if other than atmospheric air is used - Expensive

4.3.4 Alternative 4: Using piping as a heat exchanger for the glycol regeneration unit Idea: Installing a pipe inside another bigger pipe, and exchanging heat from the hot internal lean glycol to the outer rich cold glycol.

Figure 4.5: Using piping as heat exchanger

A pipe is installed in the center of another pipe and support stands welded between the pipe walls at suitable intervals. The lean hot glycol is passed through the inner pipe, while the rich cold glycol is passed through the outer pipe. As the pressure in the inner and outer pipes is almost the same, the pipe walls can be very thin, enhancing heat transfer and reducing costs.

Advantages:

- Saves weight as the inner pipe walls can be thinner

- Saves floor space as no dedicated heat exchanger is needed - Easy to manufacture

Disadvantages:

- Increased pressure drop in the fluid flowing in the outer pipe

- May provide insufficient heat transfer, e.g. very long piping is needed - Difficult to locate leaks in the inner pipe