Process for removing hydrogen sulfide from crude petroleum

Abstract: A process for removing hydrogen sulfide from crude petroleum, which comprises contacting the crude petroleum to be freed from hydrogen sulfide with a stripping gas for removing hydrogen sulfide in a hydrogen sulfide stripping column, supplying the spent stripping gas now containing the thus stripped hydrogen sulfide to a hydrogen sulfide absorbing tower, separating the stripping gas from hydrogen sulfide in said absorbing tower by contacting the gas with a hydrogen sulfide absorbing agent to absorb hydrogen sulfide therein and returning the so refreshed hydrogen sulfide stripping gas now freed from hydrogen sulfide to said hydrogen sulfide stripping column under compression at a superatmospheric pressure in order to effect recirculation of the gas.

Description: 2. FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a process for removing hydrogen sulfide from crude pertoleum. A typical prior technique for removing hydrogen sulfide from crude petroleum is the process of so-called cold stripping. Before entering in the details of the present invention, the above prior technique on which the presnet invention has been based should be outlined. As shown in Fig. 2, the cold stripping is carried out by supplying the crude petroleum 101 to be freed from hydrogen sulfide to a hydrogen sulfide stripping column 102, in which the crude petroleum is brought into contact with a stripping gas 103 for removing hydrogen sulfide through a series of trays or plates or over a bulk of packings in counterflow with the gas. Here, hydrogen sulfide contained in the crude petroleum is stripped out by the stripping gas together with the light hydrocarbon gas components of the crude petroleum and is exhausted from the column 102 as a waste gas 104. Here, a superatmospheric pressure is prevailing in the lower part of the column 102. The so stripped oil 105 is then fed to an oil storage tank 106 where the pressure of the oil is releaved to atmospheric pressure and the then liberated gas is exhausted as waste gas 107 and the oil is supplied therefrom to further processing as the treated crude oil 108. In this prior process of cold stripping however, the stripping gas for removing hydrogen sulfide is ever consumed into waste and, in addition, accompaniment of losses in useful light petroleum fractions, such as, propane, isobutane, n-butane, isopentane, n-pentane, hexane and so on, is unavoidable. 3. OBJECT AND SUMMARY OF THE INVENTION The present invention has been achieved basing upon the stand of the technique described above. Thus, the object of the present invention is to provide a novel process for removing hydrogen sulfide from crude petroleum in which there is scarce loss not only in the stripping gas for removing hydrogen sulfide but also in the useful light petroleum fractions of crude petroleum, such as, propane, isobutane and so on. The above object is achieved according to the present invention by a process for removing hydrogen sulfide from crude petroleum, which comprises contacting the crude petroleum to be freed from hydrogen sulfide with a stripping gas for removing hydrogen sulfide in a hydrogen sulfide stripping column, supplying the stripping gas now containing the thus stripped hydrogen sulfide to a hydrogen sulfide absorbing tower, characterized in separating the stripping gas from hydrogen sulfide in said absorbing tower by contacting the gas with a hydrogen sulfide absorbing agent to absorb hydrogen sulfide therein and returning the hydrogen sulfide stripping gas now freed from hydrogen sulfide to said hydrogen sulfide stripping column under compression to a superatmospheric pressure in order to effect recirculation of the gas whereby for stripping off the hydrogen sulfide a stripping gas is used being almost saturated with light petroleum components of the crude petroleum. As the stripping gas for removing hydrogen sulfide to be used in the process according to the present invention, there may be enumerated those which have commonly been employed in the cold stripping process of the prior art, such as for example, natural gases containing methane as the principal component (which may contain N 2 and CO 2 ). As the hydrogen sulfide absorbing agent, there may be incorporated those which have hitherto been used in general, such as, an aqueous solution of, for example, monoethanolamine, monometyl diethanolamine, diethanolamine, diglycolamine and sulfinol. The exhaust gas exhausted from the hydrogen sulfide stripping column consists mainly of the stripping gas for removing hydrogen sulfide and contains now a large amount of hydrogen sulfide stripped out of the crude petroleum and a considerable amount of various light petroleum components of the crude petroluem, such as, propane, isobutane, n-butane, isopentane, n-pentane, hexane and so on, stripped concurrently from the crude petroleum. This exhaust gas is then supplied to the hydrogen sulfide absorbing tower operated with the hydrogen sulfide absorbing agent, such as, an aqueous monoethanolamine solution etc., as mentioned above, so as to cause an effective contact of the gas with the absorbing agent in counterflow, in order to remove therein only hydrogen sulfide from the exhaust gas. The resulting gas mixture freed from hydrogen sulfide is then returned to the hydrogen sulfide stripping tower at a lower portion thereof using a gas recirculator under compression at a superatmospheric pressur, in order to effect recirculation of the gas mixture. In contrast to the prior cold stripping process in which a stripping gas for removing hydrogen sulfide rich in methane and having low content of low boiling hydrocarbon components is used and, hence, a large amount of low boiling petroleum components, such as, propane, isobutane, n-butane, isopentane, n-pentane, hexane etc., are stripped out together with hydrogen sulfide, the process according to the present invention realizes stripping off of hydrogen sulfide using a stripping gas almost saturated with the light petroleum components of the crude petroleum to be subjected to the hydrogen sulfide removal by incorporating recirculation of the stripping gas under separation of the amount of hydrogen sulfide stripped out of the crude peroleum and, hence, no substantial reduction in the useful light petroleum components of the resulting treated crude oil occurs. Moreover, there is substantially no need for replenishing the stripping gas from outside of the system due to recirculated use of the stripping gas for removing hydrogen sulfide. Therefore, it is possible to realize an operation of non-replenishment of the stripping gas for removing hydrogen sulfide from crude petroleum with simultaneous attainment of non-wasting of light petroleum components in the crude petroleum. 4. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic flow line diagram of an embodiment of the apparatus for realizing the process according to the present invention. Fig. 2 is a schematic flow line diagram of a typical apparatus for use for the conventional cold stripping. 5. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS One embodiment of the process according to the present invention is described below with reference to Fig. 1. In this embodiment, the crude petroleum 1 having a content of hydrogen sulfide is supplied to a hydrogen sulfide stripping column 2 and is subjected to stripping by a stripping gas 3 which has been freed from hydrogen sulfide. The exhaust gas 4 exhaused from the stripping column 3, which consists of a gas mixture composed mainly of the stripping gas 3 and of hydrogen sulfide stripped out from the crude petroleum 1 and nearly saturated with light petroleum fractions of the crude petroleum stripped out concurrently, is then conducted to a hydrogen sulfide absorbing tower 9, where hydrogen sulfide is removed from the exhaust gas 4 by absorbing it in a hydrogen sulfide absorbing agent 10. As the hydrogen sulfide absorbing agent, there may be used an aqueous solution of monoethanolamine, monomethyl diethanolamine, diethanolamine, diglycolamine, sulfinol and so on. The bottom discharge 11 of spent hydrogen sulfide absorbing agent containing the absorbed hydrogen sulfide from the absorbing tower 9 is sent by a pump 12 to a regeneration apparatus 14 for regenerating the hydrogen sulfide absorbing agent, where it is freed from hydregen sulfide 15 and the thus regenerated absorbing agent is then returned to the hydrogen sulfide absorbing tower 9. The recirculated stripping gas 16 freed from hydrogen sulfide is passed to a gas recirculator 17, which propels the gas to a lower portion of the stripping column 2 as the refreshed stripping gas under compression upto a predetermined superatmospheric pressure. Some light petroleum components contained in the crude petroleum in supersaturation at the operation condition of the hydrogen sulfide removal system will accumulate in the recirculating stripping gas within the circulation system as an excess gas. The so accumulated excess gas 18 should be extracted out of the system at an adequate period. From the excess gas 18, hydrocarbon components are recovered. The so treated crude oil 5 freed from hydrogen sulfide is supplied to a crude oil tank 6, where the pressure of the oil is releaved to atmospheric pressure and the then liberated gas components are discharged from the tank as waste gas 7. The resulting treated crude oil is supplied therefrom to further processing 8. In Tables 1 and 2 below, the process according to the present invention and the conventional cold stripping process are compared for their particulars, wherein Table 1 gives a comparison as to the running condition of the process and Table 2 represents a comparison as to the practical results of the process. Comparison of Running Condition Items Acc. to the Invention Prior Art Overhead pressure of H 2 S stripping column 0.14 Kg/cm 2 (G) 0.14 Kg/cm 2 (G) Bottom pressure of H 2 S stripping column 0.39 Kg/cm 2 (G) 0.39 Kg/cm 2 (G) Number of plates of H 2 S stripping column 40 40 Pressure in treated crude oil tank 0.07 Kg/cm 2 (G) 0.07 Kg/cm 2 (G) Temperature of crude petroleum 24.7°C 24.7°C Composition of crude petroleum (wt. %) H 2 S 0.196 0.196 CO 2 0.009 0.009 CH 4 0.040 0.040 C 2 H 6 0.374 0.374 C 3 H 8 0.595 0.595 iso-C 4 H 10 0.503 0.503 n-C 4 H 10 1.333 1.333 iso-C 5 H 12 1.297 1.297 n-C 5 H 12 1.382 1.382 C 6+ 94.271 94.271 Sum 100.000 100.000 Consumption of stripping gas (Nm 3 /hr) 0 4350 (calculated as H 2 S-free gas) Treated crude oil 100,000 bbl/Day 100,000 bbl/Day Content of H 2 S in treated crude oil below 50 ppm by weight below 50 ppm by weight Comparison of Results of Process Items Acc. to the Invention Prior Art %-Retention of each component (wt. %) H 2 S 0.196 0.196 CO 2 0.009 0.009 CH 4 0.040 0.040 C 2 H 6 0.374 0.374 C 3 H 8 0.595 0.595 iso-C 4 H 10 0.503 0.503 n-C 4 H 10 1.333 1.333 iso-C 5 H 12 1.297 1.297 n-C 5 H 12 1.382 1.382 C 6+ 94.271 94.271 Product yield (wt.%) 99.41 98.42 H 2 S-free hydrocarbon gas recovered 735 Nm 3 /hr - Consumption of: Combustible gas (Nm 3 /hr) 290 (for reboiler) 4350 (as stripp. gas) Electric power (kW-hr/hr) 150 (for recirculator and pump) - (unnecessary) From Table 2, it is seen that the per cent retention of content of the crude petroleum component is increased from 14.52 % in the conventional process to 64.94 % in the process according to the invention for ethane, from 41.69 % in the conventional process to 86.53 % in the process of the invention for propane, from 68.10 % in the conventional process to 93.82 % in the process of the invention for isobutane, from 77.7 % in the conventional process to 95.64 % in the process of the invention for n-butane, from 90.82 % in the conventional process to 98.24 % in the process of the invention for isopentane and from 93.41 % in the conventional process to 98.66 % in the process of the invention for n-pentane. Basing on these results, the product yield, namely the ratio of the amount of treated crude oil to the amount of original crude petroleum, of 98.43 % in the conventional process was able to increase upto that of 99.41 % in the process according to the present invention. Furthermore, in the process according to thepresent invention, a hydrocarbon gas freed from hydrogen sulfide that has hitherto been discarded without use can be recovered in an amount of 736 Nm 3 /hr at a processing rate of 100,000 barrels per day. As to the consumption of material and energy, the conventional process consumes 4350 Nm 3 /hr of combustible gas (natural gas) as the stripping gas for removing hydrogen sulfide, whereas the process according to the present invention requires a consumption of combustible gas, namely, the recovered hydrocarbon gas, of 290 Nm 3 /hr for the fuel of the reboiler for regenerating the hydrogen sulfide absorbing agent and an electric power of 150 kW-hr/hr for actuating the gas recirculator and pump.

Claims: A process for removing hydrogen sulfide from crude petroleum, comprising contacting the crude petroleum to be freed from hydrogen sulfide with a stripping gas for removing hydrogen sulfide in a hydrogen sulfide stripping column, supplying the spent stripping gas now containing the thus stripped hydrogen sulfide to a hydrogen sulfide absorbing tower, characterized in separating the stripping gas from hydrogen sulfide in said absorbing tower by contacting the gas with a hydrogen sulfide absorbing agent to absorb hydrogen sulfide therein and returning the so refreshed hydrogen sulfide stripping gas now freed from hydrogen sulfide to said hydrogen sulfide stripping column under compression at a superatmospheric pressure in order to effect recirculation of the gas whereby for stripping off the hydrogen sulfide a stripping gas is used being almost saturated with the light petroleum components of the crude petroleum. A process as claimed in Claim 1, wherein a natural gas containing methane as the principal conponent is used for the stripping gas. A process as claimed in Claim 1, wherein an aqueous solution of monoethanolamine is employed for the hydrogen sulfide absorbing agent. A process as claimed in Claim 1, wherein the spent hydrogen sulfide absorbing agent from the hydrogen sulfide absorbing tower is regenerated in a regeneration apparatus. A process as claimed in Claim 1, wherein a part of the recirculating stripping gas for removing hydrogen sulfide is extracted out of the system as an excess gas, from which a hydrocarbon gas is recovered.

Inventors: IIJIMA MASAKI, IIJIMA, MASAKI, IIJIMA, MASAKI, C/O MITSUBISHI JUKOGYO K.K.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: C10G31
IPC codes: C10G31, C10G21