SULFUR DIOXIDE REMOVAL FROM GAS STREAMS USING SUBSTITUTED PIPERAZINONES

Abstract: SO₂ is removed from a gas stream by absorbing the SO₂ in an aqueous solution containing a 4-(2-­ hydroxyalkyl)-2-piperazinone compound or an alkyl or aryl substituted derivative thereof. These compounds provide absorbent solutions that are less subject to SO₂ loss during the regeneration step and yet effectively remove the SO₂ from the gas stream without interference from other associated gases.

Description: The removal of sulfur-containing gases from gas streams is important in the purification of natural gas before burning it for home and industrial use and in the cleanup of flue gases used in coal-burning industries. One process still widely used today employs the well known limestone scrubbing. The disadvantage of this process is the necessity of disposing of the large volume of solid waste produced. Many compounds have been suggested as absorbents, some of which are selective to H₂S or CO₂. Other are capable of removing a large percent of all the acidic gases present in the gas stream being treated. One system taught in a recent patent, US-A-4,366,134, employs potassium or sodium citrate to selectively remove SO₂ from a gas stream. In another more recent patent, US-A 4,530,704, the removal of SO₂ from a gas stream is accomplished by contacting a gas stream containing it with an aqueous solution of a piperazinone, morpholinone or their N-alkyl substituted derivatives, eg N,N′-dimethyl-2-piperazinone. Each of the compounds employed by the above patents can be regenerated by the conventional method of steam stripping. An improved method for absorbing SO₂ from gas streams, said method providing increased recovery of SO₂, has now been discovered wherein hydroxyalkyl-2-piperazinones are employed as absorbents. The present invention is a process for removing SO₂ from a gas stream containing it, and which may also contain other gases commonly associated with hydrocarbon streams or combustion gases, by employing as an absorbent therefor an aqueous solution of an N-hydroxyalkyl-2-piperazinone or an alkyl or aryl substituted derivative thereof. The removed SO₂ is subsequently thermally desorbed and recovered. The compounds useful as absorbents for removing SO₂ from gas streams are 4-(2-hydroxyalkyl)-2-piperazinones having the formula: wherein R is hydrogen or an alkyl group having 1 or 2 carbon atoms, R₁ is hydrogen, an alkyl group having 1 to 6 carbon atoms or an aryl or an aralkyl group having 6 to 12 carbon atoms and R₂ is hydrogen, an alkyl or hydroxyalkyl group having 1 to 6 carbon atoms or an aryl or aralkyl group having 6 to 12 carbon atoms. Suitable 4-(2-hydroxyalkyl)-2-piperazinones include 4-(2-hydroxyethyl)-2-piperazinone, 4-(2-hydroxyethyl)-1-methyl-2-piperazinone, 4-(2-hydroxyethyl)-3-methyl-2-piperazinone, 4-(2-hydroxyethyl)-5-methyl-2-piperazinone, 3-ethyl-4-(2-hydroxyethyl)-2-piperazinone, 3-ethyl-4(2-hydroxyethyl)-2-piperazinone, 6-ethyl-4-(2-hydroxyethyl)-2-piperazinone, 4-(2-hydroxyethyl)-5,6-dimethyl-2-piperazinone, 1-ethyl-4-(2-hydroxyethyl)-2-piperazinone, 4-(2-hydroxyethyl)-3-phenyl-2-piperazinone, 1,4-bis(2-hydroxyethyl)-2-piperazinone, 4-(2-hydroxypropyl)-2-piperazinone, 4-(2-hydroxybutyl)-2-piperazinone and 4-(2-hydroxypropyl)-6-methyl-2-piperazinone. High boiling absorbents are desirable to prevent loss of the absorbent along with the SO₂ during the regeneration step. The subject compounds possess a much higher boiling point than the N,N′-dialkyl-2-piperazinones employed for that purpose in US-A-4,530,704. For example, while N,N′-dimethyl-2-piperazinone boils at 241°C, 4-(2-hydroxyethyl)-2-piperazinone boils at 450°C, both at atmospheric pressure. The higher boiling point is thought to be due to the formation of a dimer during heating which, of course, occurs during the regeneration step. The compounds of the present invention are usually employed in aqueous solution at a concentration of 0.1 molar up to their saturation concentration in water. The absorbent solution, after use, is thermally regenerated, eg. by passing steam through the solution, and recycled to the absorption step. The absorber can be operated at a temperature of from 0° to 120°C, but is preferably operated at a temperature of 5° to 95°C. Pressures of from about atmospheric (100 kPa) to 3 atmospheres (300 kPa) can be employed but atmospheric pressure is preferably and conveniently employed. Higher temperatures and pressures are not deleterious, but equipment design modifications may be required. The SO₂ concentration of the gas streams being treated may vary from ten ppm up to 45 vol.%. Preparation of the compounds useful in the invention is accomplished by reacting the appropriate 2-piperazinone or substituted derivative thereof with ethylene oxide or ethylene chlorohydrin. An improved method of preparation involves the reaction of an N-hydroxyalkylalkylenediamine with glyoxal or a substituted glyoxal. The following examples illustrate the use of the absorbents in the process of the invention. EXAMPLE 1 The product of the reaction of 30% aqueous glyoxal solution with 70% aqueous 2[(2-aminoethyl)-amino]ethanol is concentrated in a rotary evaporator under full vacuum at a temperature of 100°C. The resulting product (75% 4-(2-hydroxyethyl)2-piperazinone) is distilled under vacuum. The yellowish-brown liquid distillate is dissolved in 1-propanol and a water-propanol azeotrope removed by distillation at 70°C at a final pressure of <5 mm Hg (0.65 kPa). The product is dissolved in acetonitrile solution, then precipitated by chilling to 4°C., filtered and finally crystallized from acetone to obtain a white product (mp 59.5-60°C., bp 450°C). Capillary gas chromatography indicates the white crystalline solid is 99+% pure 4-(2-hydroxyethyl) -2-piperazinone. EXAMPLE 2 An aqueous solution of 10 wt% of the 4-(2-hydroxyethyl)-2-piperazinone (HEP) prepared in Example 1 is evaluated as a selective solvent for absorption and regeneration of sulfur dioxide. In order to demonstrate that 4-(2-hydroxyethyl)-2-piperazinone is capable of being recycled, SO₂ from a synthetic gas mixture is absorbed with the same aqueous solution of HEP and then regenerated through three cycles. The test procedure used is that described in US-A-4,530,704. The synthetic gas mixture consists of approximately 77% nitrogen, 20% carbon dioxide and 3% sulfur dioxide. The synthetic gas mixture is fed into the bottom of a ten-tray Oldershaw column between 30 to 35°C and at about 4.9 mL/min. The aqueous solution of the absorbent is fed into the top of the column at a rate of 5 mL/min. The operating conditions of the absorber column for the 10 wt.% aqueous solution of HEP during the three cycles, the specific composition of the synthetic gas mixture and the analysis of the gas composition after leaving the column are summarized in Table 1. HEP absorbs nearly 100% of the SO₂ from the gas stream in all three cycles. SO₂ ABSORPTION WITH HEP Cycle No. Gas Rate (Liters/min) Pressure (mm Hg) Temperature (°C) Wt% Gas In Wt% Gas Out Preheat Bottom of Column Top of Column N₂ CO₂ SO₂ N₂ CO₂ SO₂ 1 4.90 35 77 31 52 76.94 19.93 3.13 79.0 21.10 0 2 4.95 32 80 35 54 77.30 19.59 3.11 79.70 20.29 0.005 3 4.93 36 78 30 50 77.54 19.46 2.99 79.76 20.24 0.004 The aqueous solution of the HEP is preheated to between 75° and 78°C and then passes through a stripper column. The aqueous solution of the absorbent is fed at the top of the column at a rate of 4 mL/min. under a pressure of 1 mm Hg. The temperature of the liquid at the top of the column and at the bottom of the column is either 98 or 99°C. The steam stripped SO₂ exits the top with some water vapor. The lean absorbent exits the bottom of the stripper column. The operating conditions of the stripper column for the HEP, the weight % SO₂ entering the column and exiting with the absorbent out the bottom of the column and the % SO₂ stripped for the three cycles are summarized in Table II. Seventy-two to seventy-nine percent of the SO₂ absorbed by the HEP solution is stripped. Also, the HEP solution maintains its absorption and regeneration efficiency throughout the 3 cycle test. SO₂ REGENERATION FROM HEP ABSORBENT Cycle No Feed Rate (mL/min) Pressure (mm Hg) Temperature (°C) Wt% SO₂ % SO₂ Stripped Preheat Bottom of Column Top Column Liquid In Liquid Out 1 4 1 78 98 99 3.20 0.91 72 2 4 1 77 99 99 4.22 0.88 79 3 4 1 75 98 99 4.24 1.00 76 Example 3 An aqueous solution of 20 wt% glyoxal is added dropwise over a 3 hour period to an aqueous solution of 30 wt% 2-[(2-aminoethyl)amino]ethanol at 100°C. The hot aqueous reaction mixture is treated with charcoal (NORIT A), filtered and vacuum stripped. Capillary gas chromatography indicates the golden product is 90% pure 4-(2-hydroxyethyl)-2-piperazinone. Example 4 An aqueous solution of 10 wt% of the HEP prepared in Example 3 is evaluated as a selective solvent for absorption and regeneration of sulfur dioxide. Four times, SO₂ from a synthetic gas mixture is absorbed with the same aqueous solution of HEP and then regenerated to demonstrate that HEP can be recycled. The composition of the synthetic gas mixture and operating conditions for the absorber and stripper systems are nearly the same as that employed in Example 2. The operating conditions of the absorber column for the aqueous solution of HEP, the specific composition of the synthetic gas mixture and the analysis of the gas composition leaving the column is summarized in Table III. The HEP absorbed nearly 100% of the SO₂ from the synthetic gas stream in all four cycles. The operating conditions of the stripper column containing the HEP, the weight % SO₂ entering the column and the % SO₂ stripped for the four cycles is given in Table IV. Seventy-four to seventy-eight percent of the SO₂ absorbed by the HEP solution was stripped. The HEP solution maintains its absorption and regeneration efficiency throughout four cycles. SO₂ABSORPTION WITH HEP Cycle No. Feed Rate (mL/min) Gas Rate (Liters/min) Pressure (mm Hg) Temperature (°C) Wt% Gas In Wt% Gas Out Preheat Bottom of Column Top of Column N₂ CO₂ SO₂ N₂ CO₂ SO₂ 1 5 4.90 35 80 32 50 76.24 20.50 3.26 78.28 21.72 0 2 5 4.91 34 80 35 53 76.79 20.53 2.68 79.07 20.93 0.002 3 5 4.92 35 80 34 50 77.01 20.40 2.59 79.24 20.75 0.006 4 5 4.93 34 80 34 52 76.81 20.73 2.46 78.85 21.14 0.006 SO₂ REGENERATION FROM HEP ABSORBENT Cycle No Feed Rate (mL/min) Pressure (mm Hg) Temperature (°C) Wt% SO₂ % SO₂ Stripped Preheat Bottom of Column Top Column Liquid In Liquid Out 1 4 1 77 99 99 3.85 0.88 77 2 4 1 76 99 99 4.70 1.05 78 3 4 1 76 99 99 4.69 1.08 77 4 4 1 75 99 99 4.74 1.22 74 COMPARATIVE EXAMPLE A An aqueous solution of 10 wt% N, N′-dimethyl-2-piperazinone (NNDP) is evaluated as a selective solvent for absorption and regeneration of sulfur dioxide. The composition of the synthetic gas mixture and operating conditions for the absorber and stripper systems are substantially the same as those employed in Example 2 and 4. The operating conditions of the absorber column for the aqueous solution of NNDP, the specific composition of the synthetic gas mixture and the analysis of the gas composition after leaving the column are summarized in Table V. The NNDP absorbed 100% of the SO₂ from the synthetic gas stream. The operating conditions of the stripper column for the NNDP, the weight % SO₂ entering the column and the % SO₂ leaving the column is given in Table VI. Only sixty percent of the SO₂ absorbed by the NNDP solution was stripped. SO₂ ABSORPTION WITH NNDP Feed Rate (mL/min) Gas Rate (Liters/min) Pressure (mm Hg) Temperature (°C) Wt% Gas In Wt% Gas Out Preheat Bottom of Column Top of Column N₂ CO₂ SO₂ N₂ CO₂ SO₂ 5 4.96 35 80 35 57 76.76 19.86 3.38 79.42 20.57 0.0002 SO₂ REGENERATION FROM NNDP ABSORBENT Feed Rate (mL/min) Pressure (mm Hg) Temperature (°C) Wt% SO₂ % SO₂ Stripped Preheat Bottom of Column Top of Column Liquid In Liquid Out 4 1 76 99 100 3.66 1.44 60

Claims: A process for removing sulfur dioxide from a gas stream comprising contacting said stream with an aqueous solution of an absorbent for said sulfur dioxide and subsequently thermally desorbing and recovering said sulfur dioxide, characterized in that the absorbent is an 4-(2-hydroxyalkyl)-2-piperazinone having the formula wherein R is hydrogen or an alkyl group having 1 or 2 carbon atoms, R₁ is hydrogen, an alkyl group having 1 to 6 carbon atoms or an aryl or an aralkyl group having 6 to 12 carbon atoms and R₂ is hydrogen, an alkyl or hydroxyalkyl group having 1 to 6 carbon atoms or an aryl or aralkyl group having 6 to 12 carbon atoms. A process of Claim 1 wherein said absorbent is 4-(2-hydroxyethyl)-2-piperazinone. A process as claimed in Claim 1, wherein said R₂ alkyl group is methyl or ethyl. A process as claimed in Claim 1, wherein said R₂ hydroxyalkyl group is hydroxyethyl. A process as claimed in any one of Claims 1, 3 and 4, wherein at least one R₁ is an alkyl group. A process as claimed in Claim 5, wherein the R₁ alkyl group is methyl or ethyl. A process as claimed in any one of the preceding claims, wherein the aqueous solution contains a concentration of the absorbent compound of at least about 0.1 molar. A process as claimed in any one of the preceding claims, wherein the absorption of the SO₂ from the gas stream is conducted at a temperature of 0° to 120°C. A process as claimed in Claim 8, wherein the absorption of the SO₂ from the gas stream is conducted at a temperature of 5° to 95°C. A process as claimed in any one of the preceding claims, wherein the concentration of SO₂ in the gas stream is 10 ppm up to 45 vol.% of the gas. The use of an aqueous solution of an 4-(2-hydroxyalkyl)-2-piperazinone as defined in any one of Claims 1 to 6 in the process of claim 1.

Inventors: JONES, MARK B., MOTES, JOHN M., POTTER, JAMES L., TREYBIG, DUANE S.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1988
Main classification: B01D53
IPC codes: B01D53, C01B17, C07D241