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Optimisation of Natural Gas Processing Facility
Vishal Pawar, Chief Engineer - Process, Aker Powergas Pvt. Ltd. and Vishal Shah, Senior Process Engineer, Aker Powergas Pvt. Ltd. Conceptualisation and firming up of process scheme are of paramount importance in the natural gas processing facility. Process engineers should carefully analyse all the parameters i.e. production profile, composition of well fluid, desired end product quality and then conceptualise the overall scheme by carrying out process simulation. The article tends to demonstrate the importance of these things such as conceptualisation, selection of optimum scheme and process simulation using the case study.

Currently majority of the world’s primary energy is supplied by burning fossil fuels i.e. oil, coal and the natural gas collectively. Typically, before burning these fossil fuels are processed either for cleaning it, so that it can be transported by pipeline or for extracting / recovering valuable products. Particularly talking about natural gas, although it’s processing is less complicated than refining of the crude oil, natural gas processing is extremely energy intensive and requires special attention in making choices which can make overall processing to be energy efficient. Natural gas processing, by and large, being an open art technology, there are various concepts and options available for achieving same results. However selecting the one with the right level of capital investment and operating cost is extremely important.

CASE STUDY
In the case study considered for this article, there is an existing processing facility handling the gas right from the choke valve till the exports gas as indicated in the figure 1. The system consists of TEG absorption based gas dehydration and Joule-Thomson (J-T) expansion valve based dew pointing system in order to meet the Exports gas specification. Typical specifications as follows:

• Gas train capacity : 200mmscfd
• Flowing wellhead pressure (upstream of choke valve) : 82barg
• Choke valve downstream pressure : 45barg
• Export gas specification:
• Pressure : 87barg
• Water content : 7lb/mmscf
• HC dew point temperature : 4°C at 38barg

The intent of the study is to understand the existing system, carry out process simulation to achieve energy optimisation of the facility and make the operation energy efficient which will in turn reduce the operating cost.



EXISTING SYSTEM DESCRIPTION
Well fluid from choke valve is fed to high pressure separator where primary separation takes place between gas and liquid (Refer figure 1). The Wet gas from this separator is sent to the Inlet Gas Scrubber to remove entrained free liquid. This wet gas is then dehydrated in the TEG contactor to the dehydration required quality of 7lb/mmscf and sent further to hydrocarbon dew pointing unit. The dehydrated gas from TEG Contactor is chilled in the Gas-Gas Exchanger (5 degree Celsius temperature approach is maintained for optimum Gas-Gas exchanger design) by gas from the Second Low Temperature Separator before entering the First Low Temperature Separator. This gas is then sent to J-T valve designed with required pressure drop. The adiabatic gas expansion results in temperature drop at downstream of the J-T valve. Any liquid condensation resulting from the expansion is collected in the Second Low Temperature Separator. The gas stream from the Second Low Temperature Separator after heating is compressed to 87barg i.e. pipeline delivery pressure.

SIMULATION MODEL SET UP AND VALIDATION
As a first step, the study is carried out by modeling entire facility in the process simulator. HYSYS, which is renowned software for the upstream industry simulations, is used for process modeling. Thermodynamic property package of Peng-Robinson is used. As a part of validation, the results from the simulation model are corroborated with the actual operating data from the plant. It is observed that results are closely matching with the plant data after fine tuning of the simulation.



The modeling of the processing facility and review of generated heat and mass balance provides complete picture about the plant, makes it possible to understand critical areas which should be concentrated for energy optimisation. Simulation model also provides the flexibility to alter some of the process parametres, which can have impact on the energy requirements. These parametres can be adjusted based on the technical judgment to understand the performance of the plant before actually executing the same in the field. In this case study, similar technical analysis is carried out and various opportunities are observed for the energy optimisations, some of them are discussed in the following sections.



ENERGY SAVING OPPORTUNITY 1 – USE OF TURBO EXPANDER One of the obvious options for optimisation, after observing this type of facility is that whether it is possible to use the turbo-expander in place of J-T valve, which can improve the power recovery making operation energy efficient.



Refer figure 2. In this case, chilled gas from Turbo-Expander Knock-Out Drum is directed to the Turbo-Expander where the gas expands as it flows across the turbine blades and cools near the isentropic gas expansion process. Since the expansion is near isentropic, expander outlet temperature is significantly lower than that is achieved by J-T valve. The Joule-Thomson process is a constant enthalpy expansion, whilst the turbo expander process is a near constant entropy expansion.



The effect of constant enthalpy and constant entropy expansion on temperature drop is further elaborated in figure 3 of Mollier chart of Methane. The Mollier chart has pressure on y-axis and enthalpy on x-axis. It also contains constant entropy curve and constant temperature curves. The blue lines are constant entropy curves and red lines are constant temperature curves (refer zoom out section of figure 3).

Consider an example in which Methane gas is at pressure of 60barg and temperature of 30 degree Celsius (Point P1 in Zoom out section). The gas is then expanded to 40barg. When the gas expansion is isenthalpic (through J-T valve), then it follows the constant enthalpy path of straight vertical line (as enthalpy is on X-axis) from point P1 to P3 in Zoom out section. The corresponding temperature for isenthalpic process at 40barg will be 22 degree Celsius (Point P3).



Now if methane gas at same start conditions expands in isentropic way (almost like in Turbo-expander), then it follows the constant entropy curve (blue line) from point P1 to P2 in Zoom out section. For the same gas expansion to 40barg, the corresponding temperature for isentropic expansion will be 0 degree Celsius (Point P2). Thus as can be observed from Mollier chart that when gas expands in isentropic fashion, the temperature drop is higher compare to isenthalpic expansion.

In brief, the same gas specification can be achieved with less pressure drop in a Turbo-Expander compared to a J-T valve and same is evident from the table 1 that more temperature drop can be created per bar of pressure drop created.

Thus as can be observed from the table1, drop in temperature per bar of pressure drop is much more for turbo-expander than that of J-T valve, meaning that, in our application, same specification of the gas can be achieved by dropping lesser pressure if Turbo-expander is used against J-T valve.

In addition to this, the Turbo Expander can recover useful work from the gas expansion process if a re-compressor is required. Turbo expander energy can be utilised in running the re-compressor so that more power recovery can be done. This gas from re-compressor is further compressed to pipeline pressure of 87barg by export compressor. However, as the inlet pressure of the export compressor is much higher, it results in reduction of overall pressure boosting requirements.

The process facility is modeled in the process simulator replacing the J-T valve with the Turbo-expander. Table 2 provides the results in terms of various process parametres and export gas compressor power.

It can be observed that using turbo expander in place of J-T valve for the achieving desired dew pointing of the gas can provide power saving of around ~1500 kW, say around 1.2 million USD per annum, which is substantial. It is worth noting that, although turbo expander requires additional capital expenditure, looking at the significant amount of saving, pay back period can be approximately two years.

ENERGY SAVING OPPORTUNITY 2 – INCREASED FACILITY PRESSURE As a part of further optimisation of processing facility, it is observed that there is a possibility of operating gas plant at higher pressure, since wellhead flowing pressure is much higher. It can be noted that the wellhead flowing pressure is around 82barg; however the pressure at choke valve downstream is reduced to around 45barg. Due to this, it requires additional power for export gas compressor for pushing the gas to the pipeline pressure. In discussions with the facility owner and then analysing the simulation results, it is understood that the processing facility is operated at the lower pressure since there is a possibility of hydrates formation downstream of J-T valve. Gas Hydrate is an ice-like crystalline solid formed from a mixture of water and natural gas, usually methane. Hydrates once formed are extremely difficult to break and can choke piping.

In order to utilise the available wellhead flowing pressure, possibility of increasing the choke valve downstream pressure is evaluated. After carrying out simulation runs for various cases, it is confirmed that as facility pressure is increased, the difference between the actual temperature and hydrate formation temperature decreases raising the possibility of the hydrate formation. Table 3 provides the results with increased facility pressure.

As can be observed from above table, increase in choke valve downstream pressure reduces the margin between the actual temperature and hydrate formation temperature. As per standard engineering practice, the minimum temperature difference between the actual and Hydrate formation temperature is typically maintained as 5 degree Celsius. Considering this requirement, it is not feasible to increase the facility pressure at the given water content of 7 lb/mmscf. Here one should note that Hydrate formation is not only dependent on pressure but it is also a function of water content in the gas. At given pressure, as the water content decreases, the hydrate formation temperature also decreases.

Therefore, existing TEG based dehydration section is evaluated for improving the dehydrated gas quality. After carrying out detail study of dehydration unit, it is observed that water content can be reduced to 5 lb/mmscf by adding the gas stripping section in TEG regenerator. With addition of stripping section, the lean TEG concentration can be increased which in turn provides the dehydration quality of 5 lb/mmscf.



With this higher dehydration quality (5 lb/mmscf ), simulations runs are carried out with increased choke valve downstream pressure. For each run, the difference between the actual temperature and hydrate formation temperature is evaluated and it is tabulated in Table 4.

As can be observed from tables 3 and 4, with minimum temperature margin of 5 degree Celsius between actual and hydrate formation temperature, the choke valve downstream pressure can be increased to 50.7barg for 5 lb/mmscf water content compared to 45barg for 7 lb/mmscf water content. As shown in Case-3, any further increase in the facility pressure further reduces the margin between lowest temperature and hydrate formation temperature.



This increased facility pressure shall reduce the load on Exports Gas compressor. Table 5 provides the results in terms of operating pressure and Exports gas compressor power for increased choke valve downstream pressure.

It can be observed that with higher choke valve downstream pressure can provide power saving of around ~1100 kW which is substantial.



OVERALL SUMMARY
Table 6 provides the export Gas compressor power for energy saving options against base case.

After evaluating the entire Natural gas processing facility, it is observed that replacing the J-T valve with Turbo-expander for desired gas dew pointing can provide potential power saving of ~ 1500 kW. It is also observed that with higher gas dehydration quality of 5 lb/mmscf, the hydrate formation temperature can be reduced which in turn allows higher facility pressure i.e. 50.7barg from 45barg. With this higher pressure, there can be potential power saving of 1100 kW. Considering both the energy saving possibilities, there can be total power saving of ~ 2600 kW (~30 per cent of export gas compressor power), in other words saving of around USD 2 million per annum.

CONCLUSION
In order to optimise the overall process in terms of capital investments and operating cost, one should really understand the impact and correlation of each parametre on overall process. For case study considered in this article, it is observed that selection of correct options / parametres for achieving the desired results is extremely important. Selection of Turbo expander over J-T valve for this particular case saves 1500 kW power. At the same time, tightening of the dehydrated gas quality allows the processing at higher pressure further reducing the power requirement by 1100 kW. Application of both these options can lead to power saving of around 2600 kW which is saving of around 2million USD per annum. Thus as a process engineer, at the stage of conceptualisation, one has to completely understand the system and must attempt to optimise the overall scheme by studying the system in totality.