The invention relates to a method for operating a feed gas processing plant and a feed gas processing plant. In particular, the invention relates to a method for operating a feed gas processing plant, especially a natural gas processing plant

Natural gas is a hydrocarbon gas mixture, which occurs naturally. This naturally occurring gas is referred to as raw natural gas. It mainly consists of methane, but also includes heavier hydrocarbons, such as ethane, propane or butane, and also amounts of acid gases such as carbon dioxide (CO2) and nitrogen.

For certain applications, it is expedient to extract at least some of these componants from the raw natural gas, as these can either reduce the combustion quality of the gas, as is the case with nitrogen, or to be further processed as they are of commercial value, as is the case with heavier hydrocarbons.

Also, it is nessecary to remove or at least sufficiently reduce the CO2 content, if a cryogenic processing of the natural gas is to be affected. For example, for the removal of valuable heavier hydrocarbons or nitrogen, it is advantageous to use cryogenic processes including low temperature fractionation in industrial scale process plants.

Raw natural gas is cleaned in these processing plants by separating impurities such as nitrogen and CC>2 as well as heavier hydrocarbons from the main gas stream of raw gas transported through the processing unit.

During the process of start-up of such a plant configured and adapted for the processing of raw natural gas including removal of nitrogen, cryogenic units must be taken into operation. Such erogenic units comprise cooling cycles, which require refrigerants.

During a start-up phase of a plant, the temperature of a cryogenic unit for the removal of nitrogen must typically be brought down to around -150 to -160 °C using such a refrigerant in the cooling cycle. Typically the cooling cycle works by compressing the refrigerant and afterwards expansion using the Joule-Thomson effect as driving force for the cool-down. As a refrigerant a gas mixture, mainly consisting of raw natural gas, nitrogen and heavier hydrocarbons if necessary is used. However CO2, which is also present in the raw natural gas can easily freeze at relatively high temperatures of around -57 °C, under certain pressure conditions, leading to a clogging up of the cooling cycle during the start-up phase, when the gas is expanded to lower pressures and thus lower temperatures. Generally it is thus necessary to remove CO2 from the raw natural gas by means of amine gas treatment, also known as acid gas removal, in order to produce an essentially CO2 reduced or free gas, which can then be used in refrigerant. Therefore, additional investment costs arise for the acid gas removal.

On the other hand, without an acid gas removal unit, it takes considerable time during the start up phase to provide a sufficient amount of CO2 reduced gas as source for filling the cooling cycle. Using the raw natural gas with higher CO2 content in the refrigerant, the pressure levels have to be adapted in order not to freeze the cycle during cool-down. Once the process is cold enough and prior to continuing the cool- down, C02 has to be continuously purged out of the cycle until the CO2 content is reduced to the desired value. All in all, this leads to uneconomically long procedures for start-up of a cryogenic unit and thus the plant as a whole.

The object of the invention is to facilitate operation of a natural gas processing plant, especially during the start-up phase.

This object is achieved by a method comprising the method steps of claim 1 and a processing plant comprising the features of claim 10.

According to a first aspect of the present invention, there is provided a method for operating a natural gas processing plant comprising a processing unit utilising a cooling cycle, the method comprising the following steps:

providing a liquid refrigerant which is methane rich and has a C02 content lower then a first value,

vapourising the liquid refrigerant to provide a start-up phase gaseous refrigerant in situ by means of a vapourizing unit ,

feeding the start-up phase gaseous refrigerant into the cooling cycle of the processing unit,

feeding a feedraw natural gas to be processed, the C02 content of which is higher than a second value, which is higher than the first value, into a processing path of the processing plant for processing of the feedraw natural gas, the processing comprising reducing the C02 content of at least part of the feedraw natural gas to provide C02 reduced feedraw natural gas, the C02 content of which is reduced to a value below a third value, which is lower than the second value,

feeding at least part of the C02 reduced feedraw natural gas into the cooling cycle of the processing unit.

According to the invention, by providing a sufficiently clean gaseous refrigerant (with reduced C0 ₂ content) during a start-up phase, this phase can be significantly shortened. Also, by initially providing the refrigerant in liquid form, and then vaporising it by means of a vaporiser provided in situ, the costs for providing remotely located plants with an expedient refrigerant are substantially reduced.

Especially, a start up of the process plant can be performed essentially under normal operating pressures. During the start-up of the process, the pressures do not have to to be altered or tightly controlled in order to avoid reaching the freezing point of the CO2 which is mainly a function of pressure and temperature. It is also not necessary to perform any CO2 purges of the cooling cycles, as they are not subjected to any significant levels of CO2. Start up lines within a cryogenic unit can be significantly reduced and simplified. E.g. additional bypasses with valves around critical process equipment can be avoided.

According to the invention, the risk of erroneous operation leading to clogging of cooling cycle due to freezing out of CO2 during a start up phase is prevented.

Also, by providing a liquid refrigerant, the CO2 content of which is sufficiently low, as a backup during subsequent on spec operation of the plant, a cryogenic unit of a processing plant can be maintained in a cold mode even during phases where the plant does not produce sufficient amounts of CO2 reduced gas. In such phases, the liquid refrigerant can also be vaporised using the in situ vapouriser, and the resulting gaseous refrigerant can be fed into the cooling cycle.

According to a preferred embodiment of the invention, the processing further comprises generating a methane rich gas to be outputted from the processing plant. The method according to the invention is especially advantageous when utilized in such processing plant.

Expediently, the methane rich gas is transported to a place of usage remote from the processing plant by pipeline.

According to an especially preferred embodiment, the start-up phase gaseous refrigerant is vaporised liquid natural gas (LNG), essentially comprising methane.

Advantageously, the feed gas, which is fed into the processing plant, is a raw natural gas. Such raw natural gas comprises a number of contaminants, which, according to the invention, can be removed in an advantageous and cost effective manner.

The first value, i.e. the maximum C0 ₂ content of the liquid refrigerant, is

advantageously 0.005 mol-%, 0.05 mol-%, 0.1 mol-% or 0.5 mol- %. Especially, using a start-up phase gaseous refrigerant with a CO2 content level or concentration lower than 0.1 % is especially advantageous in the context of the present invention.

Typically, the second value, i.e. the CO2 content or concentration of a raw natural gas to be processed, is one of the following values: 0.5 mol-%, 1 mol-%, 2 mol- %, 3 mol-%, 4 mol-%, 5 mol-%, 6 mol-%, 7 mol-% or 8 molo- %. Values specially raw natural gas with higher CO2 contents can easily freeze out during cryogenic processing. This problem is addressed in an effective and cost effective manner by the present invention.

Advantageously, the third value, i.e. the value to which the CO2 content or

concentration of the raw natural gas is reduced, is one of the following values: 0.005 mol-%, 0.05 mol-%, 0.1 mol-%, 0.5 mol-%. It is especially advantageous to provide that the first value is essentially equal to the third value, i.e. that the CO2 content of processed raw natural gas fed into the cooling cycle essentially corresponds to the CO2 content of the start-up phase gaseous refrigerant.

According to a further aspect of the invention, there is provided a processing plant configured and adapted for processing a feedgas, comprising

a processing unit including a cooling cycle, a vapourising unit for vapourising a liquid refrigerant, which is methane rich and has a C02 content lower than a first value, adapted to provide a start-up phase gaseous refrigerant in situ,

a feeding means (also referred to as a feed line, or gaseous refrigerant line) for feeding the start-up phase gaseous refrigerant into the cooling cycle the processing unit,

a feeding means (also referred to as a feed line, or feed gas line) for feeding a feed gas to be processed, the C02 content of which is higher than a second value, into a processing path of the processing plant for processing of the feed gas, the

processing comprising reducing the C02 content of at least part of the feedgas to provide C02 reduced feedgas, the C02 content of which is reduced to a value below a third value, which is lower than the second value,

feeding at least part of the C02 reduced feed gas into the cooling cycle of the processing unit

According to a preferred embodiment of the invention, the processing unit is a nitrogen removal unit and/or a LNG liquefaction unit. LNG essentially consists of methane.

Either unit requires cryogenic processing, utilizing a sufficiently CO2 reduced or CO2 free refrigerant.

Advantageously, the processing unit comprises a partition wall fractioning column. Use of such a partition wall fractioning column provides an especially efficient way of generating CO2 reduced natural gas, which can be used as a refrigerant.

The invention will now be further described with reference to the accompanying figures.

Figure 1 shows a schematic diagram of a prior art processing plant for processing natural gas.

Figure 2 shows a schematic diagram of a processing plant for processing a natural gas, with which the invention can advantageously be implemented,

Figure 3 shows a schematic diagram of a further processing plant, with which the invention can advantageously beimplemented, and Figure 4 shows a preferred embodiment of a processing plant in greater detail, with which the invention can advantageously be implemented.

An example of a prior art gas processing plant, is schematically shown in Figure 1 and generally designated 100. Processing plant 100 is configured and adapted to process raw natural gas.

Plant 100 is adapted to process raw natural gas from a gas source 90. Raw natural gas is essentially methane rich, i.e. has a methane content of typically 75 % to 99 %. When extracted from source 90, it also typically comprises water and natural gas condensate, acid gases such as CO2, nitrogen and further (non-methane) heavier hydrocarbons. In the language of the appended claims, such raw natural gas has a CO2 content above a second value.

The processing plant comprises a water and condensate removal unit 110, an acid gas removal unit 120, a dehydration unit 121 , an NGL (natural gas liquid) recovery unit 130 and a nitrogen rejection unit (NRU) 140. It is noted that units 130 and 140 may be provided in reverse order within the plant. If expedient, a mercury removal unit (not shown) can also be provided upstream of unit 120 or downstream of unit 121. Further units typically included in such a plant are not explicitly shown, but will, at least in part, be briefly referenced in the following.

First, the normal ("on-spec") mode of operation of the plant will be described, i.e. during times when input (i.e. raw natural gas) and outputs are on-spec, i.e. the plant produces all desired outputs, especially methane gas, according to its specification. The path the raw natural gas to be processed takes through the plant up to the point where it is output from the plant as methane gas is refered to as processing path.

During this normal mode of operation, raw natural gas from source 90 is transported to water and condensate removal unit 1 10 for removal of free water and natural gas condensate. This waste water including hydrocarbons is usually disposed off as waste water.

The raw gas is subsequently transported to acid gas removal unit 120 for further processing. Here, acid gases such as hydrogen sulfide and CO2 are removed for example by amine treating. Other means of acid removal are also available. The produced off gas is usually burned in conjunction with a thermal oxidizer. In the language of the claims, the C0 ₂ content of the gas is reduced to a value below a third value.

Gas exiting the acid gas removal unit 120 is transported to NGL recovery unit 130. Some state of the art natural gas processing plants are provided with NGL recovery units utilizing a further cryogenic low temperature fractionation process comprising expansion of the gas through a turbo expander and a subsequent distillation in a demethanising fractionating column. A thus recovered NGL stream (designated 134) can then be further processed through a fractionating train comprising a number of distillation towers (not shown).

Raw gas thus processed is then transported to NRU 140 for removal of nitrogen.

NRU 140 is provided as a cryogenic unit using low temperature fractionation for the removal of nitrogen from the gas. This process can be adapted to also remove helium, if desired. NRU 140 is typically provided as a fractionating column comprising an internal or external cooling cycle, through which a methane containing gaseous refrigerant flows. During the normal mode of operation as presently described, it is advantageously possible to utilize raw gas, the CO2 content of which has been sufficiently reduced in acid gas removal unit 120, as gaseous refrigerant. This reduction of CO2 content is necessary to avoid a clogging up of the cooling cycle due to potential freezing out of CO2.

The residue from the NGL recovery unit is the final, sufficiently purified methane or output gas (designated 143), which is then typically pipelined to the end user markets.

The invention can advantageously be implemented in plants where an acid gas removal unit 120 is not provided, e.g. in order to save investment costs. Such a plant 200 is schematically shown in Figure 2. Components or units also included in the prior art plant according to Figure 1 are designated with the same reference numerals, and will not be explicitly introduced or explained again in the following. Units not provided in the prior art plant of Figure 1 , or which are modified over the units as provided in the prior art plant, are designated with adapted reference numerals. Plant 200, corresponding to the prior art plant, is fed with raw natural gas from source 90 and comprises units 110, 121 , and 130 as described above.

Here, an NRU 240 is further configured and adapted to also remove CO2 from the raw gas, so that a specific acid gas removal unit 120 as provided in the prior art plant of Figure 1 may be omitted from the plant. This simultaneous removal of nitrogen and CO2 is achieved by providing the NRU 240 as a partition wall fractioning column, as will be further explained with reference to Figure 4.

During the start up phase of plant 200, there is, initially, no sufficient amount of raw natural gas, the CO2 content of which has been sufficiently reduced, available, as an acid gas removal unit is not provided. If raw gas containing a too high amount of CO2 was used, there would be a realistic danger of a cooling cycle NRU 240 clogging up, which would lead to cumbersome and time consuming regeneration operations.

On the other hand, waiting for a sufficient amount of CO2 reduced raw gas to become available, i.e. to be produced in NRU 240, before reaching normal operation and on- spec production in NRU 240 would lead to any raw natural gas being processed in the plant during this time typically having to be burned off, as its nitrogen content would be too high for further use.

According to the present invention, it is suggested to provide a sufficiently clean gaseous refrigerant for the NRU 240 during this start up phase. According to the invention, this refrigerant is derived from a source, which is independent from the raw natural gas from source 90 being processed.

According to the invention, this refrigerant is initially provided in liquefied form in a container or truck 95, preferably as liquefied natural gas LNG, the CO2 content of which is below a specific value, so that it can be ensured that for example even in case of a cooling down of the NRU 240 during start-up, no freezing out of CO2 will occur.

Preferably, the CO2 content of this LNG is below 0.005 mol-%. In the claims, this value is referred to as a "first value".

Providing the refrigerant in liquefied form is especially advantageous for supplying remotely located natural gas processing plants with refrigerant, as transportation of a liquefied refrigerant such as LNG is significantly easier than the transport of the corresponding amount of refrigerant in gaseous form, due to its volume being around 600 times smaller. This LNG is advantageously provided to plant 100 in containers or trucks 95 as mentioned.

This LNG is then vaporised in a vaporisation unit 96, which is configured and adapted to vaporise the LNG in order to provide a gaseous refrigerant, the CO2 content of which is sufficiently low to ensure that no freezing out will occur, for the cooling cycle of NRU 240 during a start up phase. This gaseous refrigerant is referred to as start up phase gaseous refrigerant. By providing such a refrigerant, the NRU 240 can be started up in a substantially shorter time than in prior art solutions, so that the plant 200 as a whole is able to provide methane gas according to a desired specification, the nitrogen content of which has been sufficiently reduced, in a shorter space of time than prior art plants. Hereby, the amount of raw gas, which for example has to be burned off as it does not conform to specification, can be minimized.

When the plant 200 itself produces a sufficient amout of raw gas, the CO2 amount of which is reduced to a level at which clogging of the cooling cycle of NRU 140 can be safely prevented, this raw gas can be fed into the cooling cycles. In the language of the claims, this gas has a CO2 content below a third value. For example, the start up gaseous refrigerant can be provided and/or the processing of CO2 reduced natural gas can be effected such that the first value and the third value are essentially identical. Hereby, it can be ensured that the CO2 content of refrigerants used is essentially the same during all operating modes of the processing unit.

At the same time, feeding of gaseous coolant from container or truck 95 can be reduced and eventually terminated.

A second embodiment of a natural gas processing plant, with which the invention can advantageously be implemented, is shown in Figure 3, and generally designated 300. Again, components or units already described above are designated using the same reference numerals, and will not be introduced or explained again.

Here, the order of the processing steps in the pretreatment (up to unit 240) is the same as the embodiment of Figure 2. Also, a specific acid gas removal unit is not necessary in this embodiment. The processing plant 300 again comprises a condensate and water removal unit 1 10, a dehydration unit 121 , an NGL recovery unit 130 and a nitrogen rejection unit (NRU) 240, which is, as in the first embodiment of the invention as shown in Figure 2, provided as a partition wall fractioning column for removal of C02 from the raw gas. Additionally, the produced methane rich gas, provided as output from NRU 240, is liquefied in the liquefaction unit 250.

As in the previous embodiment shown in Figure 2, raw natural gas from source 90 is processed in the plant, and water, condensate, heavier hydrocarbons and nitrogen are removed in units 1 10, 121 , 130 and 240.

As in the first embodiment of the invention as shown in Figure 2, after removal of free liquid water and natural gas condensate in unit 1 10 as well as dehydration in unit 121 , the raw gas is transported to NGL recovery unit 130, which can also be configured and adapted to utilize a cryogenic low temperature fractionation process. A thus recovered NGL stream 134 can again, as in the previous embodiment of the invention shown in Figure 2, be further processed through a fractionating train comprising a number of distillation towers (not shown).

Gas exiting NGL recovery unit 130 is then transported to NRU 240. NRU 240 is again provided as a cryogenic unit using low temperature fractionation for the removal of nitrogen from the gas. This process can be adapted to also remove helium, if desired.

According to this embodiment, NRU 240 is configured and adapted to also remove CO2 from the gas, as mentioned above. This simultaneous removal of nitrogen and CO2 is achieved by providing the NRU 240 as a partition wall fractioning column.

The gas entering NRU 240 is processed to reduce its CO2 content to a sufficiently low level to be able to use it in the cooling cycle of NRU 240, as described above in connection with the plant shown in Figure 2.

Additionally, the produced methane rich gas 143 exiting NRU 240 is further processed in a methane liquefaction unit 250. Here the gas is liquefied and as a product LNG is produced (designated 253). In order to deliver the cooling for the process here an external cooling cycle including compression and expansion of gas is used. The refrigerant is a gas mixture, mainly consistent of the raw natural gas, nitrogen and heavier hydrocarbons if neccessary.

As in the first embodiment of the invention as shown in Figure 2, during a start up phase of plant 300, the cooling cycle of NRU 240 is fed with a gaseous refrigerant initially provided in liquid form (container 95) and then vaporised in vaporiser 96. This gas is again referred to as start up phase gaseous refrigerant. Again, the C0 ₂ content of this start up phase gaseous refrigerant is sufficiently low to prevent clogging up of the cooling cycle. When the plant itself produces sufficient amounts of gas, the CO2 content of which is sufficiently low to prevent clogging, this gas is fed into the cooling cycle, i.e. it is especially added to the start up phase gaseous refrigerant present in the cooling cycle.

Be it noted that in the second embodiment the external cooling cycle of the methane liquefaction unit 250, may also be fed with such a start up phase gaseous refrigerant during the start up phase.

A preferred embodiment of a plant comprising a cryogenic unit, especially a NRU comprising a partition wall fractioning column, with which the invention can be advantageously implemented, is shown in Figure 4. It is noted that a more detailed description of the various processes as performed in this plant, albeit without implementation of the present invention, is disclosed in DE 102015001858, the content of which is herewith incorporated by reference. Also, not all processing steps as performed in the plant according to figure 4 will be explained in detail in the following description. Only the steps relevant in connection with explaining the present invention will be further expanded on.

A raw natural gas stream 1 is led through heat exchangers E1 and E2 and partially condensed in these by means of further processing streams. Natural gas stream 2 exiting heat exchanger E2 is separated in separator D1 into a liquid phase 3 and a gaseous phase 4. The former is fed to separation column T 1 via expansion valve V1. Gaseous phase 4 is expanded in expander X1 and also fed to Column T 1 , i.e. into its head section. A partial stream 5 of gaseous phase is refluxed to column T1 after condensation in heat exchanger E2 via expansion valve V4. A high boiler depleted gas fraction 10 is extracted from the head of column T1 and at least partially condensed in heat exchanger E4 and fed to a second column T2 via expansion valve V6.

In column T2 a rectification separation into a methane rich liquid fraction 1 1 , which is extracted from the sump of column T2, and a low boiler rich gaseous fraction 12, which is extracted from the head of column T2, is executed.

The further processing of these fractions 1 1 , 12, leading to products streams 1 T, 12' as shown in Figure 4, will not be further expanded on in the present context.

The second column T2 is provided with a separation wall T, which is provided at least partly in the region within the column at which the high boiler depleted fraction 10 is fed to the column and at which a hydrocarbon rich fraction 26, which will be further described in the following, is extracted. Seperation wall T has the effect that these two fractions do not come into material contact with one another.

The reflux for the second column T2 is generated by an open cooling cycle. The refrigerant of this cooling cycle has a methane content of around 80-85 mol-%. It is known from DE 102015 001 858 A1 to use the hydrocarbon depleted fraction 26 as mentioned above. This is extracted from column T2 via regulating valve V13, vapourized in side condenser E8, heated in heat exchangers E5' and ET, fed to a first stage of refrigerant compressor C1 , and, together with refrigerant stream 23 from the head of column T2, compressed to an intermediate pressure.

After cooling in intermediate cooler E9, the compressed refrigerant is further compressed to the desired cycle pressure in a second stage of compressor C1. After cooling in further cooler E10 the compressed refrigerant 20, after separation into two partial streams, is cooled in heat exchangers ET and E6, and after mixing, fully condensed against partial stream 13 in heat exchanger E5. The fully condensed refrigerant 21 is then fed to buffer container D4. From this, the two refrigerant streams 24,25 are extracted. Stream 24 is subcooled in heat exchanger E5', and then expanded into column T2 via valve V12, while stream 25 after subcooling in heat exchanger E6 is fed to head condenser E7 of column T2. From this head condenser the partial refrigerant stream is extracted via line 23, heated in heat exchanger E6 and then fed to the first stage of condenser C1.

In head condenser E7 and side condenser E8 the refrigerant streams 25 and 24 are vapourized against reflux streams 14 and 15.

Due to the rectification in column T2 as well as the separation wall T, the C0 ₂ concentration in the refrigerant fraction in line 26 can be reduced to the very low values as claimed, which can also be expressed in vppm, wherein values of under 50vppm or even 5 vppm are achievable.

In other words, column T2 receives raw natural gas including CO2, nitrogen (plus possible inert gases) and methane. By means of providing a partition wall T within the column T2, it is possible to withdraw gas, the CO2 content of which is significantly reduced compared to the gas fraction 10 entering the column. Hereby, the majority of CO2 and methane fraction are pushed down into the sump, without contaminating the gas fraction 26. This gas fraction 26 can be fed into the cooling cycles as soon as column T2 has produced a sufficient amount of gas with reduced CO2 content.

However, during start-up no reflux for column T2 is available and the column cannot produce a CO2 reduced fraction. As described before, using this gas phase for the start-up of the cooling cycle has several disadvantages.

Here the invention can advantageously be implemented. Vapourized LNG can be used as refrigerant and for example, introduced into stream 23 upstream of compressor C1. In Figure 4, vaporised LNG is introduced via line 60. The process can then be started- up with the normal process pressures and cooled down until column T2 produces CO2 reduced refrigerant, which can then be withdrawn from the column. Once this CO2 reduced refrigerant is available, the introduction of external vapourized LNG can be reduced and finally be terminated. In order to bring the process to on-spec operation, the refrigerant has to be replaced or adjusted with the correct amount of refrigerant from the column.