Gas recompression train

2.1 First cooler temperature T1

The case study of the reduction of the cooler temperature (T1) in the first stage of the gas recompression train is done between 80°C and 10°C. The minimum temperature is dictated by the temperature at which hydrates may form.

Numerical results are obtained for P2 equal to 9.5 bar, P3 to 2.56 bar, T2, Tf, Td to 30°C and Th to 80°C.

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Specifications

Variations in ethane and propane proportion in oil are so small that the TVP is not influenced by the cooling temperature of the first stage of the recompression train.

It is not possible to find a value of T1 which makes the TVP below the specification.

As for the oil, rich gas is not significantly influenced by T1. It is observed on the cricondenbar value.

There is a small increase when T1 is reduced but the variation is not significant (0.07 bar).

Cricondenbar is always below 110 bar.

Production

Reducing the temperature after the cooler of the first stage leads to a small decrease of both oil and gas production (by 1 kmol/h). Theses productions decrease because more aqueous flows are removed from the first inlet separator (SA) and the second one (SB).

2.2 Second cooler temperature T2

The temperature after the second stage cooler (T2) is changed from 60°C to 5°C. The minimum temperature is dictated by the temperature at which hydrates may form.

Numerical results are obtained for P2 equal to 9.5 bar, P3 to 2.56 bar, T1, Tf, Td to 30°C and Th to 80°C.

Specifications

When T2 is reduced, ethane content increases in oil. Since it is a light component, it increases the oil TVP (by 0.05 bar).

As for T1, it is not possible to find a value of temperature that brings the TVP below 0.965 bar.

There are two trends for the cricondenbar when T2 is reduced. It is first increased (between 60°C and 35°C), then it decreases. This behaviour is explained by the i-pentane behaviour, as for the previous parameters. i-pentane flow increases until 35°C, then decreases.

For any value of T2, the cricondenbar is below 110 bar.

Production

Reducing the cooler temperature decreases the amount of vapour in the stream so decreases the vapour flow rate in the separator SE. It affects the gas processing part: less liquid is sent from the separator SF to the second stage of stabilization unit. In the same time, less liquid from the separator SG is sent to the inlet separator SA.

Because of the reduction of temperature, more liquid is produced from SE. This increase does not impact so much the production of oil because of the decrease of liquid flow rates previously

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introduced. Indeed, only a small increase in oil production (5 kmol/h) is observed as well as a small decrease in gas production.

3. Gas processing

3.1 Feed gas cooler temperature Tf

The case study on the temperature after the feed gas cooler Tf was made between 60°C and 15°C. The lower value is set to be above the temperature at which hydrates can form.

Numerical results are obtained for P2 equal to 9.5 bar, P3 to 2.56 bar, T1, T2, Td to 30°C and Th to 80°C.

Specifications

Results show that the TVP is reduced by 0.01 bar when the temperature Tf is reduced from 60°C to 50°C. Below this temperature, oil TVP remains the same.

No value of Tf enables to obtain an oil TVP below the specification.

Globally, the cricondenbar of rich gas decreases (by 0.1 bar) with the decreasing temperature.

However, what is observed is a maximum value around 30°C. i-pentane behaviour can explain the results of cricondenbar.

The rich gas cricondenbar is always below the specification.

Production

Two trends are observed for the production rates of rich gas and oil. In the first range of temperature, from 60°C to 30°C, oil production rises and rich gas production decreases. Below 30°C, the contrary is observed.

It can be noted that the change of shape of the curves takes place at the temperature at which the cricondenbar is maximal. It is also where the vapour content decreases in the feed of the gas scrubber and liquid production from SF increases more drastically.

If the flow rate of liquid from SG is studied (Figure 13), it is observable that in the beginning, it is not affected by the temperature reduction. Then, below 30°C, it decreases which has the consequence to increase the production of rich gas. However, the variations are not significant (less than 3 kmol/h).

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Figure 13: influence of Tf on liquid recycle and on rich gas production

3.2 Temperature before dehydration Td

The influence of the temperature of the gas before the dehydration part (Td) is studied between 100°C and 20°C. As before, the lower limit is the temperature of hydrates formation.

Numerical results are obtained for P2 equal to 9.5 bar, P3 to 2.56 bar, T1, T2, Tf to 30 °C and Th to 80 °C.

Specifications

Oil TVP remains constant until 30°C, then increases (variation of 0.06 bar). It follows the behaviour of propane: its content in oil is constant and then increases.

There is no value of Td that brings the oil TVP below the specification.

Concerning the cricondenbar, the contrary is observed: it remains constant until 30°C, then decreases (variation of 3 bar). Propane content decreases making the cricondenbar to decrease also.

Once again, the cricondenbar is always below 110 bar.

Production

Oil production rises (15 kmol/h) whereas rich gas production decreases. It is logical since more liquid is produced when the temperature is reduced.

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