GB1572682A

GB1572682A - Process for the regeneration of an alkanolamine-based absorbent for gases containing co and/or cos

Info

Publication number: GB1572682A
Application number: GB9396/78A
Authority: GB
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1977-03-11
Filing date: 1978-03-09
Publication date: 1980-07-30
Also published as: FR2382922A1; NO146184B; DE2810249A1; FR2382922B1; JPS53113290A; DE2810249C2; NO780825L; JPS6139092B2; NO146184C; NL7702650A; CA1104996A; BR7801458A

Description

(54) PROCESS FOR THE REGENERATION OF AN ALKANOLAMINE-BASED ABSORBENT FOR GASES CONTAINING CO2 AND/OR COS (71) We, SHELL INTERNATION ALE RESEARCH MAATSCHAPPIJ B.V., a company organised under the laws of the The Netherlands, of 30 Carel van Bylandtlaan, The Hague, The Netherlands, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a process for the regeneration of an absorbent for the removal of CO2 and/or COS from gases, which absorbent comprises an aqueous solution of an N- (2- hydroxyalkyl) primary and/or secondary amine and is contaminated with an oxazolidone.

N - (2 - hydroxyalkyl) primary and secondary amines, which contains the group ~NH~C~COH~, are found to form oxazolidones where their aqueous solutions are used as absorbent for CO2, possibly according to the following reaction mechanism:

With COS the alkanolamines probably react as follows:

As an example of an absorbent which is often used for absorbing CO2, COS and H2S from gases, and in which oxazolidone formation -can occur, there may be mentioned an aqueous solution of diisopropanolamine. In this case 3 - (2 hydroxypropyl) - 5 - methyl - 2 oxazolidone is formed. Other examples of alkanolamines of the type to which the invention confines itself are monoethanolamine and diethanolamine, which, too, are used in aqueous solution as absorbents.

Since in such absorbents the formation of oxazolidones may be unacceptable (in some cases the conditions are such that 1% w alkanolamine is converted per hour), a regeneration method has previously been proposed in British patent specification No.

1,118,687, whereby potassium hydroxide is added to the absorbent to be regenerated, as a result of which the oxazolidone is converted into the alkanolamine, with the formation of potassium carbonate.

Disadvantages of this method are that the potassium carbonate must be removed, that expensive chemicals are consumed, that the regeneration can hardly be carried out continuously and that a relatively complicated unit is required.

The present invention provides a process, which does not have the above-mentioned disadvantages and with which the alkanolamine can be recovered from the oxazolidone in a simple way.

According to the invention there is provided a process for the regeneration of an absorbent for the removal of CO2 and/or COS from gases, which absorbent comprises an aqueous solution of an N - (2 - hydroxyalkyl) primary and/or secondary amine and is contaminated with an oxazolidone, in which the absorbent, or an oxazolidone - containing fraction obtained therefrom, is heated to a temperature of at least 200"C at elevated pressure in the presence of water.

It has been found that this heating at elevated pressure converts the oxazolidone into the alkanolamine originally present in the absorbent, with liberation of CO2 and with consumption of water. If the absorbent itself is heated, sufficient water is present for the purpose. If a fraction which is obtained from the absorbent and which does not contain sufficient water is to be heated. some water should be added.

The CO2 formed is preferably removed by steam stripping during the heating at a temperature of at least 2000 C. This is an inexpensive and efficient method, which does not cause contamination of the absorbent and with which the required temperature can easily be reached.

Moreover, steam stripping ensures a continuous discharge of the CO2 formed.

With an adequate pressure control, the use of steam permits the desired quantity of water to be maintained during the conversion of the oxazolidone.

Good results are obtained at heating temperatures in the range of 200 to 3000 C, in particular of 200 to 2500 C. At these temperatures the risk of thermal degradation of the alkanolamines concerned is small.

According to a preferred embodiment heating takes place at a pressure of 1 60 atm. On the one hand, the pressure is then sufficiently high for a good conversion, on the other hand the cost of the required equipment is lower than for much higher pressures.

Absorbents of the type according to the invention are often used in a process for the continuous removal of H2S, COS and CO2 from gases. To this end the gas is often passed in upflow through a column in which a number of contact trays are present and in which absorbent flows from the top downwards over the trays. At the bottom of the column, absorbent loaded with CO2 and 112S is discharged and at the top the purified gas. The loaded absorbent is continuously passed from the bottom of the said column to the top of a stripping column in which it flows downwards and in which it is freed from the CO2 and H2S either by reducing the pressure or by stripping with a stripping gas such as steam. The conditions in the absorption column and in the stripping column are such that CO2 and H2S are taken up and released, respectively. The stripped solvent is continuously recirculated from the bottom of the stripping column to the top of the absorption column. In the stripping column the temperature is kept in general below 175"C.

If the oxazolidone formed in the gas washing column is not removed or converted, a build-up of the oxazolidone content takes place and the absorptive power of the total amount of absorbent gradually decreases.

Very suitably a bleed stream is taken, preferably continuously from the absorbent stream which is recirculated from the bottom of the stripping column to the top of the absorption column, and this bleed stream is regenerated according to the invention.

Because the regeneration according to the invention will be carried out by heating (in general for a relatively long period) it is of advantage to reduce the amount of heat needed by reducing the amount of liquid to be heated. For that reason it is of advantage not to remove oxazolidone from the stream of absorbent to be recirculated from the stripping column to the absorption column as such, but from a fraction of this stream which contains oxazolidone. Such a fraction is obtained very suitably by separation (preferably continuously by means of distillation into a fraction containing oxazolidone and a fraction containing oxazolidone in a lower concentration or not at all. The oxazolidone-containing fraction is treated by the process according to the invention and then recirculated, for instance to the top of the absorption column. In this way an ever increasing oxazolidone content in the total quantity of absorbant is prevented and the oxazolidone content of the absorbent can be set at any desired value without the necessity of treating large quantities of absorbent at high pressure and temperature.

The distillation in which a fraction which contains oxazolidone is obtained is preferably carried out with steam stripping at 100 200 C. The N - (2 - hydroxyalkyl) amine is distilled off and the oxazolidone containing bottom fraction thus obtained contains a reduced quantity of water and alkanolamine as compared with the absorbent.

The invention is further illustrated by the following Example.

EXAMPLE In a number of series of experiments a mixture of water and 3 - (2 hydroxypropyl) - 5 - methyl - 2 oxazolidone was stripped with steam at different temperatures and at elevated pressure for a certain time, samples of the reactor contents being taken during the reaction to be able to determine the course of the conversion of the oxazolidone into diisopropanolamine.

The following procedure was followed: For each experiment a weighed quantity of water and the oxazolidone was introduced into the reactor, which was subsequently flushed with CO2 to expel the air. Then, the temperature, and consequently the pressure, in the reactor was gradually increased within 15 minutes.

The steam that was passed through during the experiment was always passed from the reactor through a condensation vessel, in which a condensed fraction from it was collected. After the desired temperature has been reached samples were taken at regular intervals via a pipe opening into the reactor, whilst the condensed fraction was at the same time discharged. The time when the first sample was taken was always considered as the starting point of an experiment.

In processing the data obtained from the samples the following effects were taken into account: The oxazolidone concentration in the reactor is affected in the following three ways: 1. Part of the oxazolidone is converted into diisopropanolamine; 2. The steam blown through may dilute or concentrate the contents of the reactor; and 3. The steam also makes part of the oxazolidone find its way into the condensed fraction.

Because of effects 2 and 3 the fraction of oxazolidone that was converted into diisopropanolamine had to be determined in a special way: the fraction was calculated from a mass balance which was obtained by analysis of the reactor feed, the samples from the reactor and the condensed fractions. In this determination the oxazolidone from the condensed fractions was always taken to belong to the unconverted part of the oxazolidone.

The results are summarized in the following table.

Time, Pressure, Converted Temp., min. bar fraction OC 225 0.106 450 12 0.148 200 59 0.137 120 23 0.291 225 179 0.608 59 0.069 120 21 0.124 225 179 0.213 25 0.091 65 36 0.328 250 105 0.542 30 0.103 75 35 0.333 250 135 0.509 15 0.040 45 41 0.167 275 16 0.174 33 55 0.356 275 15 0.181 45 57 0.574 275 90 0.827 40 77 0.461 300 The table shows that at elevated pressure and temperature-withing the scope of the invention-a considerable part of the oxazolidone can be convered into amine within a reasonable time.

WHAT WE CLAIM IS: 1. A process for the regeneration of an absorbent for the removal of CO2 and/or COS from gases, which absorbent comprises an aqueous solution of an N - (2 - hydroxyalkyl) primary and/or secondary amine and is contaminated with an oxazolidone, in which the absorbent, or an oxazolidone - containing fraction obtained therefrom is heated to a temperature of at least 200"C at elevated pressure in the presence of water.

2. A process according to claim 1, in which the heating temperature is from 200 to 3000 C.

3. A process according to claim 2, in which the heating temperature is from 2000C to 2500C.

4. A process according to any one of the preceding claims in which the CO2 formed during heating is removed by steam stripping.

5. A process according to any one of the preceding claims in which the heating takes place at a pressure of 1060 atm.

6. A process according to any one of the preceding claims in which an oxazolidone containing fraction is continuously separated from part of the absorbent by distillation, and in that this fraction is heated and in that this fraction is then recirculated.

7. A process according to claim 6 characterized in that the distillation is carried out with steam stripping at 100- 200"C.

8. A process as claimed in claim 1, substantially as described with special reference to the Example.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. first sample was taken was always considered as the starting point of an experiment. In processing the data obtained from the samples the following effects were taken into account: The oxazolidone concentration in the reactor is affected in the following three ways: 1. Part of the oxazolidone is converted into diisopropanolamine; 2. The steam blown through may dilute or concentrate the contents of the reactor; and 3. The steam also makes part of the oxazolidone find its way into the condensed fraction. Because of effects 2 and 3 the fraction of oxazolidone that was converted into diisopropanolamine had to be determined in a special way: the fraction was calculated from a mass balance which was obtained by analysis of the reactor feed, the samples from the reactor and the condensed fractions. In this determination the oxazolidone from the condensed fractions was always taken to belong to the unconverted part of the oxazolidone. The results are summarized in the following table. Time, Pressure, Converted Temp., min. bar fraction OC 225 0.106 450 12 0.148 200 59 0.137 120 23 0.291 225 179 0.608 59 0.069 120 21 0.124 225 179 0.213 25 0.091 65 36 0.328 250 105 0.542 30 0.103 75 35 0.333 250 135 0.509 15 0.040 45 41 0.167 275 16 0.174 33 55 0.356 275 15 0.181 45 57 0.574 275 90 0.827 40 77 0.461 300 The table shows that at elevated pressure and temperature-withing the scope of the invention-a considerable part of the oxazolidone can be convered into amine within a reasonable time. WHAT WE CLAIM IS:

1. A process for the regeneration of an absorbent for the removal of CO2 and/or COS from gases, which absorbent comprises an aqueous solution of an N - (2 - hydroxyalkyl) primary and/or secondary amine and is contaminated with an oxazolidone, in which the absorbent, or an oxazolidone - containing fraction obtained therefrom is heated to a temperature of at least 200"C at elevated pressure in the presence of water.