Process for the removal of highly concentrated carbon dioxide from high-pressure natural gas

Abstract: This invention relates to a process for removing highly concentrated CO 2 from high-pressure natural gas and recovering it in a high-pressure state. This process comprises the absorption step of bringing high-pressure natural gas having a CO 2 partial pressure of 2 kg/cm 2 or greater and a pressure of 30 kg/cm 2 or greater into gas-liquid contact with a regenerated CO 2 -lean absorbing fluid comprising a CO 2 absorbing fluid of which the difference in saturated CO 2 absorption level between 40°C and 120°C is not less than 30 Nm 3 per ton of solvent at a CO 2 partial pressure of 2 kg/cm 2 , whereby highly concentrated CO 2 present in the high-pressure natural gas is absorbed into the CO 2 -lean absorbing fluid to produce refined natural gas having a reduced CO 2 content and a CO 2 -rich absorbing fluid; and the regeneration step of heating the CO 2 -rich absorbing fluid without depressurizing it, whereby high-pressure CO 2 having a pressure of 10 kg/cm 2 or greater is liberated and a CO 2 -lean absorbing fluid is regenerated and recycled for use in the absorption step.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention This invention relates to a process for the removal of highly concentrated carbon dioxide (CO 2 ) from high-pressure natural gas. More particularly, it relates to a process for the removal of highly concentrated CO 2 from high-pressure natural gas whereby, at a natural gas production spot or the like, CO 2 is separated and removed from the collected high-pressure natural gas having a pressure of 30 kg/cm 2 or greater (the term "pressure" as used herein means an absolute pressure) to produce refined natural gas, and whereby the separated CO 2 can be obtained in a relatively high-pressure state which is beneficial for the purpose of injection when it is utilized in tertiary oil recovery or stored permanently in an underground aquifer. 2. Description of the related art It may usually happen that natural gas produced in a gas field contains an appreciable amount of CO 2 . According to the necessity for reducing the cost required to transport such natural gas from its production spot to a remote consumption place, and for adjusting its calorific value to the standard at the consumption place, some CO 2 is previously removed therefrom to produce refined natural gas having a CO 2 content ranging from 2-3 vol.% to ten-odd vol.%. Conventionally, the CO 2 separated by primary refining at the natural gas production spot or in the neighborhood thereof has seldom been utilized. That is, such CO 2 has been dumped directly into the atmosphere or has rarely been used as an injection gas for tertiary oil recovery in an oil field. Accordingly, little consideration has been given to the pressure of the CO 2 separated by the aforesaid refining process. In recent years, global warming due to an increase of atmospheric CO 2 has come to be regarded as a problem. Accordingly, the present situation is such that the CO 2 separated in the above-described manner must be pressurized in order to inject it into an underground aquifer for the purpose of permanent storage or to use it positively for the purpose of tertiary oil recovery. However, in spite of the fact that high-pressure natural gas is treated, the CO 2 separated by a conventionally employed process for the removal of CO 2 from natural gas has a low pressure close to atmospheric pressure. This is disadvantageous in that, for the above-described purpose of permanent storage or tertiary oil recovery, the CO 2 must be pressurized from a low pressure close to atmospheric pressure to a pressure of about 150 kg/cm 2 which is required for injection. SUMMARY OF THE INVENTION As a result of intensive investigations on the above-described problems concerning the removal of CO 2 from natural gas and the disposal and utilization of the separated CO 2 , the present inventors have found that, by employing a specific process using, among various absorbing fluids having the ability to absorb CO 2 , an absorbing fluid having so-called physical absorbing power characterized by the marked temperature dependence of saturated CO 2 absorption level, CO 2 having a much higher pressure than that obtained by conventional processes can be separated at low energy cost with much more simplified equipment than used in conventional systems. The present invention has been completed on the basis of this finding. That is, the present invention provides a method by which carbon dioxide that has conventionally been dumped into the atmosphere during the collection of natural gas can be separated and recovered in a high-pressure state permitting underground dumping in conformance with recent global environmental standards. This method enables the separated carbon dioxide to be dumped into the ground without using a compressor. Alternatively, it also enables the separated carbon dioxide to be returned to the top of an underground oil stratum in an oil field. Thus, the intended object can be accomplished by employing a system of simple construction. According to the present invention, there is provided a process for the removal of highly concentrated CO 2 from high-pressure natural gas which comprises the absorption step of bringing high-pressure natural gas having a CO 2 partial pressure of 2 kg/cm 2 or greater and a pressure of 30 kg/cm 2 or greater into gas-liquid contact with a regenerated CO 2 -lean absorbing fluid comprising a CO 2 absorbing fluid of which the difference in saturated CO 2 absorption level between 40°C and 120°C is not less than 30 Nm 3 per ton of solvent at a CO 2 partial pressure of 2 kg/cm 2 , whereby highly concentrated CO 2 present in the high-pressure natural gas is absorbed into the CO 2 -lean absorbing fluid to produce refined natural gas having a reduced CO 2 content and a CO 2 -rich absorbing fluid; and the regeneration step of heating the CO 2 -rich absorbing fluid without depressurizing it, whereby high-pressure CO 2 having a pressure of 10 kg/cm 2 or greater is liberated and a CO 2 -lean absorbing fluid is regenerated and recycled for use in the absorption step. After having absorbed CO 2 , the CO 2 absorbing fluid used in the present invention is regenerated in the regeneration step where most of the CO 2 is liberated therefrom, and then recycled for use in the absorption step. In the present invention, the CO 2 absorption capacity of the aforesaid CO 2 absorbing fluid must be such that the difference in saturated CO 2 absorption level between 40°C and 120°C is not less than 30 Nm 3 per ton of solvent, preferably not less than 40 Nm 3 per ton of solvent, at a CO 2 partial pressure of 2 kg/cm 2 . Usually, if the temperature and CO 2 partial pressure of a specific absorbing fluid are determined, the saturated CO 2 absorption level shows a definite value based on the saturated CO 2 absorption curve for the specific absorbing fluid, almost regardless of the type of the CO 2 -containing gas. In the present invention, CO 2 is removed from high-pressure natural gas having a pressure of 30 kg/cm 2 or greater by absorbing it into a CO 2 -lean absorbing fluid obtained by regenerating the CO 2 absorbing fluid in the succeeding regeneration step, and the resulting CO 2 -rich absorbing fluid is heated without substantially depressurizing it, so as to liberate CO 2 therefrom. Accordingly, it is preferable to use an absorbing fluid which can absorb CO 2 easily at a relatively low temperature and a lower CO 2 partial pressure than in the regeneration step, and can liberate CO 2 easily when heated in the regeneration step, i.e., at a relatively high temperature and a relatively high CO 2 partial pressure. The temperature dependence of the saturated CO 2 absorption level of an absorbing fluid depends largely on the types of the chemical agent(s) and solvent constituting the absorbing fluid. In the present invention, the difference in saturated CO 2 absorption level between 40°C and 120°C at a certain CO 2 partial pressure (i.e., a CO 2 partial pressure of 2 kg/cm 2 ) is employed as an index to the ability of an absorbing fluid to absorb CO 2 in the absorption step and liberate CO 2 when heated in the regeneration step. Thus, there is used an absorbing fluid of which the difference in saturated CO 2 absorption level between 40°C and 120°C at a CO 2 partial pressure of 2 kg/cm 2 is not less than 30 Nm 3 per ton of solvent and preferably not less than 40 Nm 3 per ton of solvent. No particular limitation is placed on the type of the absorbing fluid, provided that its difference in saturated CO 2 absorption level between 40°C and 120°C at the aforesaid CO 2 partial pressure is not less than 30 Nm 3 per ton of solvent and it is stable at the heating temperature of the regeneration step. Moreover, when the CO 2 partial pressure is 2 kg/cm 2 , the saturated CO 2 absorption level at 40°C serves as an index to the CO 2 absorption capacity of the CO 2 -lean absorbing fluid. In the present invention, it is preferable to use an absorbing fluid of which this absorption level is not less than 30 Nm 3 per ton of solvent and more preferably not less than 40 Nm 3 per ton of solvent. Specific examples of the aforesaid absorbing fluid include an aqueous solution of N-methyldiethanolamine (MDEA), an aqueous solution of triethanolamine, and an aqueous solution of potassium carbonate, as well as these solutions having a CO 2 absorption promoter (e.g., piperazine) added thereto. When it is desired to absorb and remove highly concentrated CO 2 present in natural gas and thereby obtain the separated CO 2 in a high-pressure state suitable for use in tertiary oil recovery or for the underground storage of CO 2 , the process of the present invention has the advantage of simplifying the equipment and reducing the energy cost. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an exemplary system which can be employed to carry out the process for the removal of CO 2 from high-pressure natural gas in accordance with the present invention; and FIG. 2 illustrates an exemplary system which can be employed to carry out a conventional process for the removal of CO 2 from high-pressure natural gas. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS An exemplary system which can be employed to carry out the process of the present invention is specifically described below with reference to FIG. 1. For purposes of comparison, an exemplary system which has conventionally been employed is illustrated in FIG. 2. In FIG. 1, reference numeral 1 designates natural gas; 2, an absorption tower; 3, refined natural gas; 4, a CO 2 -rich absorbing fluid; 5, a heat exchanger; 6, a heater; 7, a regeneration tower; 8, a CO 2 -lean absorbing fluid; 9, a cooler; 10, an overhead condenser; 11, a separating drum; 12, a compressor; and 13, high-pressure CO 2 . By way of example, the conditions for removing CO 2 from natural gas by employing the system of FIG. 1 and using a 45 wt.% aqueous solution of MDEA as absorbing fluid are given below. Natural gas 1 having a CO 2 content of 26 vol.%, a pressure of 58 kg/cm 2 and a temperature of 25°C is fed to the lower part of an absorption tower 2. This absorption tower 2 is packed, for example, with an irregular packing material so that the ascending natural gas will come into efficient gas-liquid contact with a CO 2 -lean absorbing fluid fed to the upper part thereof. The refined natural gas 3 having been freed of CO 2 by contact with the CO 2 -lean absorbing fluid, which now has a CO 2 content of 2 vol.%, a temperature of 50°C and a pressure of 58 kg/cm 2 , is discharged from the top of absorption tower 2. On the other hand, the CO 2 -lean absorbing fluid having absorbed CO 2 turns into a CO 2 -rich absorbing fluid 4, which is transferred to a regeneration step by means of a pump and regenerated therein. The regeneration step essentially comprises a heat exchanger 5, a heater 6 and a regeneration tower 7. CO 2 -rich absorbing fluid 4 is heated in heat exchanger 5 by heat exchange with a hot CO 2 -lean absorbing fluid 8 which will be described later, further heated with heater 6 using steam as a heat source, and then fed to regeneration tower 7. The purpose of regeneration tower 7 is to separate CO 2 , which has been liberated by the aforesaid heating, from the absorbing fluid to produce a CO 2 -lean absorbing fluid. Although an additional heater such as a reboiler is not required, it may be installed as desired. The liberated CO 2 is cooled in overhead condenser 10 and separated from any entrained absorbing fluid in a separating drum 11. This CO 2 , which has a relatively high pressure of 55 kg/cm 2 at about 40°C, is compressed to a pressure of 150 kg/cm 2 by means of a compressor 12. The resulting high-pressure CO 2 is used for purposes of tertiary oil recovery or stored in the earth. On the other hand, the CO 2 -lean absorbing fluid 8 withdrawn from the bottom of regeneration tower 7, which has a temperature of about 140°C, is cooled in heat exchanger 5 by heating CO 2 -rich absorbing fluid 4 as described above, further cooled with a cooler 9 using cooling water, seawater or the like, and then recycled to the upper part of absorption tower 2. Now, the above-described process for the removal of CO 2 from high-pressure natural gas in accordance with the present invention is compared with a conventional system illustrated in FIG. 2. In FIG. 2, the units and elements having the same functions as those shown in FIG. 1 are designated by the same reference numerals. In the system of FIG. 2, natural gas 1 is fed in the same manner as in FIG. 1, and CO 2 is absorbed and removed therefrom under the same conditions as in FIG. 1. The resulting gas is discharged from the top of absorption tower 2 as refined natural gas 3. On the other hand, the resulting CO 2 -rich absorbing fluid 4 is withdrawn by means of a pump and then flashed in a first flash drum 21 to produce the liberated CO 2 32 and the absorbing fluid having a reduced CO 2 content. The latter is further heated with a heater 22 and fed to a second flash drum 23 where it is flashed again. The liberated CO 2 33 is recovered by way of an overhead condenser 24 and a first separating drum 25. Since the CO 2 liberated in second flash drum 23 has approximately atmospheric pressure, it is compressed with a first compressor 26 and combined with the aforesaid liberated CO 2 32. The combined CO 2 is passed through a second separating drum 27, a second compressor 28, a third separating drum 29 and a third compressor 30 to obtain high-pressure CO 2 31 having a pressure of about 150 kg/cm 2 . On the other hand, the CO 2 -lean absorbing fluid 8 obtained at approximately atmospheric pressure is pressurized with a pressurizing pump and cooled with a cooler 34 to produce a CO 2 -lean absorbing fluid 8', which is fed to the upper part of absorption tower 2. It is evident from a comparison of FIGs. 1 and 2 that, in order to obtain the liberated CO 2 having an identical pressure of 150 kg/cm 2 , the process of the present invention illustrated in FIG. 1 brings about a marked simplification of equipment. In particular, it can be seen that the number of compressors involving a rotary driving mechanism requiring troublesome operational management can be largely decreased. Moreover, in the case where the aforesaid aqueous solution of MDEA is used as absorbing fluid, the amounts of energy required for both systems have been calculated from the saturated CO 2 absorption curve for the absorbing fluid, and the results thus obtained are summarized in Table 1. The thermal efficiency of power units is supposed to be 25%. FIG. 1 (Example) FIG. 2 (Comparative Example) Feed rate of natural gas (Nm 3 /H) (25°C; 58 kg/cm 2 ; CO 2 content, 26 vol.%) 37,852 37,852 Discharge rate of refined natural gas (Nm 3 /H) (50°C; 57.9 kg/cm 2 ; CO 2 content, 2 vol.%) 28,310 28,310 Circulation rate of absorbing fluid (T/H) 555 555 Total flow rate of liberated CO 2 (Nm 3 /H) 9,542 9,542 CO 2 pressure at the outlet of first compressor (kg/cm 2 ) (Compressor power, kW) ― 4.8 (450) CO 2 pressure at the outlet of second compressor (kg/cm 2 ) (Compressor power, kW) ― 30 (937) CO 2 pressure at the outlet of third compressor (kg/cm 2 ) (Compressor power, kW) ― 150 (695) CO 2 pressure at the outlet of compressor 12 (kg/cm 2 ) (Compressor power, kW) 150 (181) ― Power other than CO 2 compressor power (kW) 80 1,159 Total power (kW) 261 3,241 Total quantity of heat used to heat CO 2 -rich absorbing fluid (Kcal/H) 10.5x10 6 1.07x10 6 Total power calculated by converting the quantity of heat into power (kW) (thermal efficiency η = 25%) 3,313 3,552

Claims: A process for a removal of highly concentrated carbon dioxide from high-pressure natural gas which comprises an absorption step of bringing high-pressure natural gas having a carbon dioxide partial pressure of 2 kg/cm 2 (absolute pressure) or greater and a pressure of 30 kg/cm 2 (absolute pressure) or greater into gas-liquid contact with a regenerated carbon dioxide-lean absorbing fluid comprising a carbon dioxide absorbing fluid of which a difference in saturated carbon dioxide absorption level between 40°C and 120°C is not less than 30 Nm 3 per ton of carbon dioxide absorbing fluid at a carbon dioxide partial pressure of 2 kg/cm 2 (absolute pressure), whereby highly concentrated carbon dioxide present in the high-pressure natural gas is absorbed into the carbon dioxide-lean absorbing fluid to produce refined natural gas having a reduced carbon dioxide content and a carbon dioxide-rich absorbing fluid; and a regeneration step of heating the carbon dioxide-rich absorbing fluid without depressurizing it, whereby high-pressure carbon dioxide having a pressure of 10 kg/cm 2 (absolute pressure) or greater is liberated and a carbon dioxide-lean absorbing fluid is regenerated and recycled for use in said absorption step. The process of claim 1, characterized in that the liberated high-pressure carbon dioxide having a pressure of 10 kg/cm 2 (absolute pressure) or greater is either dumped into a ground or returned to a top of an underground oil stratum. The process of claims 1 or 2, characterized in that said carbon dioxide absorbing fluid is an aqueous solution of one or more compounds selected from the group consisting of N-methyldiethanolamine, triethanolamine and potassium carbonate, or an aqueous solution thereof further including piperazine. An apparatus for separating highly concentrated CO 2 from high-pressure natural gas (1) to produce high pressure CO 2 (13) which comprises an absorption tower (2) having a natural gas (1) supply at a lower part thereof and a CO 2 lean absorbing fluid (8) supply at an upper part thereof so as to produce a CO 2 -richt absorbing fluid (4) and refined natural gas (3) and the CO 2 -lean absorbing fluid (8); heating means (5) for heating the CO 2 -rich absorbing fluid (4) to regenerate a CO 2 -lean absorbing fluid (8); separation means (11) for separating the CO 2 from any entrained absorbing fluid; compression means (12) for compressing the CO 2 separated from the entrained absorbing fluid; and cooling means (15) for cooling the CO 2 -lean absorbing fluid (8) and recycling it to the upper part of said absorption tower (2).

Inventors: IIJIMA MASAKI, MITSUOKA SHIGEAKI, IIJIMA, MASAKI, MITSUOKA, SHIGEAKI, IIJIMA, MASAKI, C/O MITSUBISHI JUKOGYO K.K., MITSUOKA, SHIGEAKI, C/O MITSUBISHI JUKOGYO K.K.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1996
Main classification: C10L3
IPC codes: B01D53, C10L3