GB2173800A - N-aminoalkyl alkylpiperazine - Google Patents

Abstract

The present invention relates to the novel compound 3-(3-methyl-1-piperazine)-butylamine and to an alkaline salt promoter system which includes 2 to 20% by weight of this N-aminoalkyl alkylpiperazine, 10 to 40% by weight alkali metal salt or hydroxide (e.g. K2 CO3), optionally 2 to 20% by weight amino acid having 4 to 8 carbon atoms, and water. These scrubbing compositions may be used for removing CO2 from gaseous streams containing CO2.

Description

SPECIFICATION N-aminoalkyl alkylpiperazines Recently, it was shown in U.S. Patent No. 4,112,050 that sterically hindered amines are superior to diethanolamine (DEA) as promoters for alkaline salts in the "hot pot" acid gas scrubbing process. U.S. Patent No. 4,094,957 describes an improvement to this process whereby amino acids, especially sterically hindered amino acids, serve to prevent phase separation of the aqueous solution containing sterically hindered amines at high temperatures and low fractional conversions during the acid gas scrubbing process.

One of the preferred sterically hindered amines described in these patents is N-cyclohexyl-1,3-propanediamine. The bulky cyclohexane ring on this diamino compound provides steric hindrance to the carbamate formed at this site thereby favoring the expulsion of CO2 during regeneration, thereby leaving the hindered amine group free to protonate. The primary amino group of this diamino compound assists in maintaining solubility under lean conditions. Under lean conditions when there is insufficient carbonic acid present to protonate the hindered amino group, the molecule would be insoluble were it not for the primary amino group which forms a stable polar carbamate ion. However, even the carbamated primary amino group is insufficient to prevent insolubility of the compound under very lean conditions and an additional additive, as proposed in U.S.Patent No. 4,094,957, an amino acid, is required to maintain solubility of the diamino compound. This amino acid also contributes to additional capacity and faster absorption rates for carbon dioxide, so it therefore acts as a copromoter in addition to solubilizing the sterically hindered diamino compound. Screening studies of available amino acids as possible copromoters for N-cyclohexyl 1,3-propandiamine based on cyclic capacity and rates of absorption ascertained that pipecolinic acid was one of the best amino acid copromoters.

Subsequent studies, however, have demonstrated that the N-cyclohexyl-1 ,3-propanediamine/pipecolinic acid promoter system has several shortcomings. Firstly, N-cyclohexyl-1,3-propanediamine is both chemically unstable and volatile. For example, it degrades into a cyclic urea, especially in the presence of hydrogen sulfide. In fact, the rate of cyclic urea formation has been found to be highly dependent on hydrogen sulfide concentration, a common contaminant of industrial acid gas streams. The cyclic urea formation from this diamine is favored by the stability of the six-membered ring structure of the cyclic urea.In addition to promoter losses due to cyclic urea formation, which may be a serious problem with hydrogen sulfide rich streams, the cyclic urea product has limited solubility, and its separation from solution poses additional problems. Various techniques for coping with this water insoluble cyclic urea have been proposed. See, for example, U.S. Patent Nos. 4,180,548 and 4,183,903. However, these techniques have specific benefits and problems, e.g., specialized equipment is necessary.

Pipecolinic acid also has shortcomings, e.g., it is rather expensive and its picoline precursor is in limited supply.

In view of the commercial potential of using the sterically hindered amino compounds as described and claimed in U.S. Patent Nos. 4,094,957 and 4,112,050, there is a need for finding sterically hindered amino compounds which perform as well as N-cyclohexyl-1,3-propanediamine but do not have the volatility and degradation problems of this compound.

Also, U.S. Patent Nos. 4,094,957 and 4,112,050 disclose various other sterically hindered amines, including specific piperazine compounds. These specifically identified piperazines have been found to be too volatile for economic utilization on large scale acid gas treating facilities. Also, there is a need for finding a less costly replacement for pipecolinic acid which possesses its effectiveness. Preferably, there is a need for finding a single amino compound which performs as well or nearly as well as the N-cycloh exyl-1,3-propane-diaminel pipecolinic acid mixture, but does not suffer the preparative cost, volatility and degradation problems of this mixture. Such a discovery would be of significant technical and economic merit.

Various amino acids have been proposed as promoters for alkaline salts in the "hot pot" gas scrubbing process. For example, British Patent No. 1,305,718 describes the use of beta and gamma amino acids as promoters for alkaline salts in the "hot pot" acid gas treating process. These amino acids, however, are not suitable because the beta-amino acids undergo deamination when heated in aqueous potassium carbonate solutions. The gamma amino acids form insoluble lactams under the same conditions. Also, the alpha-amino acid, N-cyclohexyl glycine, as described in Belgian Patent No. 767,105, forms an insoluble diketopiperazine when heated in aqueous solutions containing potassium carbonate.

It has now been discovered that a certain sterically hindered triamino compound is an excellent promoter for alkaline salts in the "hot pot" acid gas scrubbing process. This amino compound, when used as promoter, not only provides for high carbon dioxide capacity and high rates of carbon dioxide absorption, but does not form undesirable insoluble degradation products as in the case of N-cyclohexyl-1,3propanediamine, the beta and gamma amino acids and the alpha amino acid, N-cyclohexyl glycine. Also, this amino compound is less volatile than N-cyclohexyl-1, 3-propanediamine and the piperazines disclosed in U.S. Patent Nos. 4,094,957 and 4,112,050, thereby the economy of this promoter is greater than the previously employed promoters.

Accordingly, in one embodiment of the present invention, there is provided a process for the removal of CO, from a gaseous stream containing CO, which comprises contacting said gaseous stream (1) in an absorption step with an aqueous absorbing solution comprising (a) a basic alkali metal salt or hydroxide selected from alkali metal bicarbonates, carbonates, hydroxides, borates, phosphates and their mixtures, and (b) an activator or promoter system for said basic alkali metal salt or hydroxide comprising at least an effective amount of N-aminoalkyl alkylpiperazine; and (2) in a desorption and regeneration step, desorbing at least a portion of the absorbed CO, from said absorbing solution.

As another embodiment of the present invention, there is provided an acid gas scrubbing composition comprising: (a) 10 to about 40% by weight of an alkali metal salt or hydroxide, (b) 2 to about 20% by weight of an N-aminoalkyl alkylpiperazine and (c8) the balance, water.

Accordingly, this invention provides the compound 3-(3-methyl-1-piperazino)-butylamine.

The N-aminoalkyl alkylpiperazine promoter may be admixed with certain amino acids as copromoters, preferably an amino acid containing 4 to 8 carbon atoms. The amino acid will preferably comprise a sterically hindered amino acid. Especially preferred amino acids include N-secondary butylglycine, N-isopropyl glycine, N-isopropyl alanine, N-sec. butyl alanine, N-n-butyl glycine and pipecolinic acid.

The N-aminoalkyl alkyipiperazines useful as a promoter in the practice of the present invention has the formula

Also, disclosed is the method for preparing N-aminoalkyl alkyl piperazines.

The triamino piperazines of formula I may be prepared by (1) cyanomethylation or cyanoethylation of a piperazine, followed by hydrogenation of the cyanoalkyl piperazine or (2) reductive condensation of 2,3butane-dione with a triamine (method 2 is used only when R1 is methyl and R2 is hydrogen). By a procedure similar to method 2 it is possible to obtain 2,3-dimethylpiperazine in one step, via reductive condensation of 2,3-butanedione with ethylenediamine. This is a new method to make 2,3-dimethylpiperazine, which so far was made either by hydrogenation of the corresponding 5,6-dihydropyrazine or by condensation of ethylenediamine with 2,3-butanediol.

In general, the aqueous scrubbing solution will comprise an alkaline material comprising a basic alkali metal salt or alkali metal hydroxide selected from Group IA of the Periodic Table of Elements. More preferably, the aqueous scrubbing solution comprises potassium or sodium borate, carbonate, hydroxide, phosphate or bicarbonate. Most preferably, the alkaline material is potassium carbonate.

The alkaline material comprising the basic alkali metal or salt or alkali metal hydroxide may be present in the scrubbing solution in the range from about 10% to about 40% by weight, preferably from 20% to about 35% by weight. The actual amount of alkaline material chosen will be such that the alkaline material and the amino acid activator or promoter system remain in solution throughout the entire cycle of absorption of CO, from the gas stream and desorption of CO, from the solution in the regeneration step.

Likewise, the amount and mole ratio of the amino acids is maintained such that they remain in solution as a single phase throughout the absorption and regeneration steps. Typically, these criteria are met by including from about 2 to about 20% by weight, preferably from 5 to 15% by weight, more preferably, 5 to 10% by weight of this sterically hindered N-amino alkyl alkylpiperazine. Should an amino acid be included, the amino acid can be added in an amount such that the solution contains 2 to about 20% by weight, preferably 5 to about 15% by weight, and more preferably 5 to 10% by weight.

The aqueous scrubbing solution may include a variety of additives typically used in acid gas scrubbing processes, e.g., antifoaming agents, antioxidants, corrosion inhibitors and the like. The amount of these additives will typically be in the range that they are effective for the intended purpose, i.e., an effective amount.

The term acid gas includes CO, alone or in combination with H2S, CS2, HCN, COS and the oxides and sulfur derivatives of C1 to C4 hydrocarbons. These acid gases may be present in trace amounts within a gaseous mixture or in major proportions.

The contacting of the absorbent mixture and the acid gas may take place in any suitable contacting tower. In such processes, the gaseous mixture from which the acid gases are to be removed may be brought into intimate contact with the absorbing solution using conventional means, such as a tower packed with, for example, ceramic rings or with bubble cap plates or sieve plates, or a bubble reactor.

In a preferred mode of practising the invention, the absorption step is conducted by feeding the gaseous mixture into the base of the tower while fresh absorbing solution is fed into the top. The gaseous mixture freed largely from acid gases emerges from the top. Preferably, the temperature of the absorbing solution during the absorption step is in the range from 25 to 20000, and more preferably from 35 to 150 C. Pressures may vary widely; acceptable pressures are between 5 and 2000 psia, (34.5 and 13790 kPa), preferably 100 to 1500 psia (690 to 10343 kPa) and most preferably 200 to 1000 psia (1379 to 6895 kPa) in the absorber. In the desorber, the pressure will range from 5 to 100 psig (134 to 789 kPa). The partial pressure of the acid gas, e.g. CO, in the feed mixture will preferably be in the range from 0.1 to 500 psia (0.69 to 3448 kPa) and more preferably in the range from 1 to 400 psia (6.9 to 2758 kPa). The contacting takes place under conditions, such that the acid gas, e.g., CO2, is absorbed by the solution.

Generally, the countercurrent contacting to remove the acid gas will last for a period of from 0.1 to 60 minutes, preferably 1 to 5 minutes. During absorption, the solution is maintained in a single phase. The triamino piperazine gives low foam in the contacting vessels.

The aqueous absorption solution comprising the alkaline material, the activator system comprising the sterically hindered aminoalkyl piperazine, which is saturated or partially saturated with gases, such as CO2 and H2S may be regenerated so that it may be recycled back to the absorber. The regeneration should also take place in a single liquid phase. Therefore, the presence of the highly water soluble amino acid provides an advantage in this part of the overall acid gas scrubbing process. The regeneration or desorption is accomplished by conventional means, such as pressure reduction, which causes the acid gases to flash off or by passing the solution into a tower of similar construction to that used in the absorption step, at or near the top of the tower, and passing an inert gas such as air or nitrogen or preferably steam up the tower.The temperature of the solution during the regeneration step may be the same as used in the absorption step, i.e., 25 to about 200 C, and preferably 35 to about 150 C. The absorbing solution, after being cleansed of at least a portion of the acid bodies, may be recycled back to the absorbing tower. Makeup absorbent may be added as needed. The use of an amino acid cosolvent, e.g., Nsecondary butyl glycine, enables one to maintain a single phase regardless of the CO2 content in the acid gas.

As a typical example, during desorption, the acid gas, e.g., CO2-rich solution from the high pressure absorber is sent first to a flash chamber where steam and some CO2 are flashed from solution at low pressure. The amount of CO2 flashed off will, in general, be about 35 to 40% of the net CO2 recovered in the flash and stripper. This is increased somewhat, e.g., to 40 to 50%, with the high desorption rate promoter system owing to a closer approach to equilibrium in the flash. Solution from the flash drum is then steam stripped in the packed or plate tower, stripping steam having been generated in the reboiler in the base of the stripper.Pressure in the flash drum and stripper where regeneration occurs is usually 16 to 100 psia, (110 to 690 kPa) preferably 16 to 30 psia (110 to 207 kPa) and the temperature is the range from 25 to 200 C, preferably 35 to 150 C, and more preferably 100 to 140 C. Stripper and flash temperatures will, of course, depend on stripper pressure, thus at 16 to 25 psia (110 to 172 kPa) stripper pressures, the temperature will preferably be about 100 to about 140 C during desorption. Single phase may be maintained and facilitated by use of an amino acid, preferably N-secondary butyl glycine as a copromoter.

In the most preferred embodiment of the present invention, the acid gas, e.g., CO2 is removed from a gaseous stream by means of a process which comprises, in sequential steps, (1) contacting the gaseous stream with a solution comprising 10 to 40 weight percent, preferably 20 to 30 weight percent of potassium carbonate, an activator or promoter system comprising 2 to 20 weight percent, preferably 5 to 15 weight percent, more preferably 5 to 10 weight percent of the sterically hindered aminoalkyl piperazine, as herein defined, and 2 to 20 weight percent, preferably 5 to 15 weight percent, more preferably 5 to 10 weight percent, of the amino acid as herein defined, the balance of said solution being comprised of water, said contacting being conducted at conditions whereby the acid gas is absorbed in said solution, and preferably at a temperature ranging from 25 to 200 C, more preferably from 35 to 150 C and a pressure ranging from 100 to 1500 psig (789 to 10442 kPa) and (2) regenerating said solution at conditions whereby said acid gas is desorbed from said solution. By practicing the present invention, one can operate the process above described at conditions whereby the working capacity, which is the difference in moles of acid gas absorbed in the solution at the termination of steps (1) and (2) based on the moles of potassium carbonate originally present, is greater than obtained under the same operating conditions for removing acid gases from gaseous streams, wherein said same operating conditions do not include an aminoalkyl piperazine as the promoter.In other words, working capacity is defined as follows: CO2 in solution CO2 in solution at completion of less at completion of absorption desorption Which is: Moles of CO2 Absorbed less Moles Residual CO2 Absorbed Initial Moles K2CO2 Initial Moles K2CO2 It should be noted that throughout the specification wherein working capacity is referred to, the term is defined as the difference between CO2 loading in solution at absorption conditions (step 1) and the CO2 loading in solution at regeneration conditions (step 2) each divided by the initial moles of potassium carbonate. The working capacity is equivalent to the thermodynamic cyclic capacity, that is the loading is measured at equilibrium conditions.This working capacity may be obtained from the vapor-liquid equilibrium isotherm, that is, from the relation between the CO2 pressure in the gas and the acid gas, e.g., CO2 loading in the solution at equilibrium at a given temperature. To calculate thermodynamic cyclic capacity, the following parameters must usually be specified: (1) acid gas, e.g., CO2 absorption pressure, (2) acid gas, e.g., CO2 regeneration pressure, (3) temperature of absorption, (4) temperature of regeneration, (5) solution composition, that is weight percent of triamino piperazine and the weight percent of the alkaline salt or hydroxide, for example potassium carbonate, and (6) gas composition.The skilled artisan may conveniently demonstrate the improved process which results by use of the sterically hindered amine a comparison directly with a process wherein the sterically hindered amino compound is not included in the aqueous scrubbing solutions. For example, it will be found when comparing two similar acid gas scrubbing processes (that is similar gas composition, similar scrubbing solution composition, similar pressure and temperature conditions) that when the sterically hindered amines are utilized the difference between the amount of acid gas, e.g., CO2 absorbed at the end of step 1 (absorption step) defined above and step 2 (desorption step) defined above is significantly greater.This significantly increased working capacity is observed even though the scrubbing solution that is being compared comprises an equimolar amount of a prior art amine promoter, such as diethanolamine, 1,6-hexanediamine, etc. It has been found that the use of the aminoalkyl piperazine of the invention provides a working capacity which is at least 15% greater than the working capacity of a scrubbing solution which does not utilize the sterically hindered amine. Working capacity increases of from 20 to 60% may be obtained by use of the sterically hindered amino compound compared to diethanolamine.

Besides increasing working capacity and rates of absorption and desorption, the use of triamino piperazine leads to lower steam consumption during desorption.

Steam requirements are the major part of the energy cost of operating an acid gas, e.g., CO2 scrubbing unit. Substantial reduction in energy, i.e., operating costs will be obtained by the use of the process utilizing the triamino piperazine. Additional savings from new plant investment reduction and debottlenecking of existing plants may also be obtained by the use of the triamino piperazine. The removal of acid gases such as CO2 from gas mixtures is of major industrial importance, particularly the systems which utilize potassium carbonate activated by the unique activator or promoter system of the present invention.

While the sterically hindered amines, as shown in U.S. Patent No. 4,112,050, provide unique benefits in their ability to improve the working capacity in the acid scrubbing process, their efficiency decreases in alkaline "hot pot" (hot potassium carbonate) scrubbing systems at high temperatures and at low concentrations of the acid gas due to phase separation. Therefore, full advantage of these highly effective sterically hindered amines cannot always be utilized at these operating conditions. The addition of an amino acid, as a cosolvent and copromoter as shown in U.S. Patent No. 4,094,957, solves the problem of phase separation and enables a more complete utilization of sterically hindered amino compounds as the alkaline materials activator or promoter.

The absorbing solution of the present invention, as described above, will be comprised of a major proportion of two alkaline materials, e.g., alkali metal salts or hydroxides and a minor proportion of the amino compound activator system. The remainder of the solution will be comprised of water and/or other commonly used additives, such as anti-foaming agents, antioxidants, corrosion inhibitors, etc. Examples of such additives include arsenious anhydride, selenious and tellurous acid, proteins, vanadium oxides, e.g., V2O2, chromates, e.g., K2Cr2O7, etc.

The N-aminoalkyl alkylpiperazine for use in the present invention is:

Many of the amino acids useful in the practice of the present invention are either available commercially or may be prepared by various known procedures. Preferred amino acids are sterically hindered amino acids having 4 to 8 carbon atoms, e.g., N-isopropyl glycine, N-isopropyl alanine, N-sec. butyl glycine, N-sec. butyl alanine, N-n-butyl glycine, N-(2-pentyl)-glycine, and pipecolinic acid.

The invention is illustrated further by the following examples which, however, are not to be taken as limiting in any respect. All parts and percentages, unless expressly stated to be otherwise, are by weight.

Example 1 Synthesis of 3-(3-methyl- 1-piperazino)-butylamine In a 2-1 (7.57 1) Erlenmeyer flask, 500g of 2-methyl-piperazine (4.7 mols) is dissolved in 500 ml of water. 3159 of allyl cyanide (4.7 mols) is added.

The mixture soon becomes homogeneous and reaches a peak temperature of 64 C.

This procedure is repeated the next day. The two batches are combined. After removing H2O and low boilers by distillation at reduced pressure, 13599 of a cut boiling at 116-117 C/.3 mm is obtained. Yield 86%.

This product is charged into a 1-gallon (3.785 1) auto-clave, together with 300 ml of methanol, 15g of KOH dissolved in methanol and 150g of Raney Ni.

Hydrogenation is carried out at room temperature and 850-1700 psi in (5960 kPa to 11821 kPa).

At the end the autoclave is opened, the catalyst is separated by filtration and the filtrate is distilled under reduced pressure. 7429 of product boiling at 137 C/20 mm Hg is obtained.

Found C = 62.4% H = 12.07% N = 23.3% Calc. C = 63.1% H = 12.30% N = 24.6% Example 2 Use of 3-(3-methyl- l-piperazine)-butylamine as an Activator for K2 CO2 in CO2 Removal The apparatus consists of an absorber and a desorber. The absorber is a vessel having a capacity of 2.5 liters and a diameter of 10 cm, equipped with heating jacket, stirrer, thermometer and reflux condenser. A pump removes the liquid from the bottom of the reactor and feeds it back above the liquid through a stainless-steel sparger. The top of the reflux condenser is connected to a U-shaped, openended manometer. The apparatus can be evacuated by means of a pump. Nitrogen and CO2 can be fed to the bottom of the cell through a sparger. CO2, coming from a cylinder, goes first to a 60-liter tank, acting as a ballast, then to a 3-liter wet-test meter.

A recorder is connected to the wet-test meter to give the liters of CO2 absorbed as a function of time.

The desorber is a 1-liter reactor, equipped with stirrer, gas sparger, reflux condenser and thermometer.

Volumes of gases entering and exiting the desorber can be read on a recorder connected to wet-test meters inserted in the desorber inlet and outlet lines.

The following mixture is prepared and put into the absorber: 60.3g of 3-83-methyl-1-piperazino)-butylamine 33.79 of pipecolinic acid 225 g of K2CO2 431 g of H2O The temperature is brought to 80 C. The apparatus is evacuated until the liquid begins to boil. The pump for the circulation of the liquid is regulated at 4 liters/minute. CO2 is admitted to the absorber. The total amount of CO2 absorbed is 42 liters.

The rich solution so obtained is transferred to the desorber and kept there for one hour from the time it begins to boil. 31.3 liters of CO2 is desorbed. No foam is observed in the desorber.

The regenerated solution is cooled, then put back into the absorber. The same operations as described for the initial absorption are carried out. 32.8 liters of C02 is absorbed, of which 11 liters in the first minute.

The rich solution so obtained is regenerated by refluxing it for 1 hour, then put into a 1-liter autoclave equipped with a sapphire window. The solution is brought to 120 C while blowing through it a gaseous mixture containing .2% CO2 and 99.8% of He. When equilbrium is reached, i.e. when the outgoing gas has the same composition as the entering gas, only one phase is present.

Example 3 Volatility experiment The solution used in this experiment is similar to that used in the absorption-desorption test of Example 2, the only difference being that the pipecolinic acid is 3.05%.

The apparatus is a 1-liter flask, equipped with thermometer, magnetic bar, inlet tube for gases and a Dean-Stark trap surmounted by a reflux condenser.

600g of solution is put into the flask and refluxed, while blowing through it a .2%/99.8% CO2/He mixture at a rate of .25 liters/minute.

After one hour at reflux the content of the Dean-Start trap is put back into the flask, then a fresh sample of condensate is taken.

Another sample is taken after a second hour at reflux. The two samples show an amine content of about .11%.

If the experiment is repeated, using N-cyclohexyl-1,3-propanediamine instead of methylpiperazinobutylamine, the condensate contains 1.3% of amine, i.e. the volatility of the triamine is about 1/10 of the volatility of cyclohexyl-propanediamine.

Example 4 Aging experiment The following compounds are charged into a 2-liter stainless-steel bomb: 80.4g of 3-(3-methyl-1-piperazino)-butylamine 459 of pipecolinic acid 4359 of KHCO, 534.5g of H2O The bomb is put into an oven and kept at 1200C for 1000 hours.

Then the product is discharged into a 2-liter flask and refluxed for several hours.

765g of solution is used in an absorption-desorption cycle as described in Example 8.

29.5 liters of CO2 is reabsorbed, of which 8 in the first minute. Comparing with the result of Example 8, it is clear that the aged solution is only slightly inferior to the fresh solution.

Claims (9)

1. 3-(3-methyl-1-piperazino)-butylamine.

2. An aqueous acid gas scrubbing composition comprising: (a) 10 to about 40% by weight of an alkali metal salt or hyroxide, (b) 2 to about 20% by weight of the N-aminoalkylpiperazine claimed in claim 1 and (c) the balance, water.

3. A composition according to claim 2 wherein said alkali metal salt is potassium carbonate.

4. A composition according to any one of claims 1 to 3 which additionally contains 2 to 20% by weight of an amino acid having 4 to 8 carbon atoms per molecule.

5. A composition according to claim 4 which contains 2 to 20% by weight of N-secondary butyl glycine.

6. A composition according to any one of claims 1 to 5 wherein the composition additionally includes an antifoaming agent, an antioxidant and a corrosion inhibitor.

7. A process for the removal of CO2 from a gaseous stream containing CO2 according to claim 1 substantially as hereinbefore described with reference to Example 2.

8. Carbon dioxide-free gaseous streams obtained by the process according to claim 7.

9. An aqueous acid gas scrubbing composition according to claim 2 substantially as hereinbefore described with reference to Examples 2 to 4.