CA2059969A1 - Process for removing acid gases from gaseous mixtures - Google Patents

Abstract

PROCESS FOR REMOVING ACID GASES FROM GASEOUS MIXTURES.

ABSTRACT
A process for removing acid gases such as hydrogen sulphide and/or carbon dioxide from gaseous mixtures which contain them, comprising essentially the absorption of the acid gases by a solvent and the regeneration of the spent solvent by stripping, characterised by using a solvent consisting of an aqueous mixture of dimethylethanolamine (DMEA) having a dimethylethanelamine concentration of between 30 and 70% by weight.

Description

PROCESS FOR REMOVING ACID GASES FROI~ GASEOUS MI.YTURES

This invention relates to a process for removing acid gases 3uch as HzS and/or COz from gaseous mixtures by absorption.
The removal of HzS and/or CO2 is a noted problem in industry which has as yet not found an always valid economical and effective solution. Its possible applications are numerous, a main but not exclusive example being the treatment of natural sas. C02 iS an inert component of this gas which can be left in the gas up to essentially the limit imposed by the required calorific value and Wobbe index. Starting with a fePd which can contain it up to some tens of a percentage (by volume), the gas after treatment contains from 1 to 3% of COz.
H2S removal has to be very thorough to ensure a gas which is free of toxic and aggressive components to the user. Starting with a feed which can contain it up to a few percentage (by volume), the gas after treatment must contaln no more than 1-3 ppm of H2S.
The usP of absorption processes (phycical or chemical) is well known in the removal of acid gases such as COz and HzS from gaseous mixtures. For simplicity of description, the processes of the known art can be classified according to the type of solvent used. Specifically, these are:

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a~ Organic compounds of varioua types such as alcohols, amides, lactams, polyglycols, polyethers etc. These types of solvent are used particularly when the acid gas partial pressure is high.
b) Aqueous solutions of primary or secondary amines such as MEA
or DEA. This type of solvent is used when the acid gas partial pressure is low and when very severe specifications have to be satisfied.
c) Aqueous alkaline carbonate solutions possibly activated by substances such as amines, borates, amino acids etc. This type of ~olvent is normally used to remove large quantities of acid gas at high partial pressure. Particularly interesting results can be obtained if activators are also present.
Industrial processes for separating acid gases by absorption are characterised essentially by two values, namely the investment cost and the operating cost per unit of acid gas eliminated.
The investment cost is substantially proportional to the size of the absorption and regeneration columns (including reboiler and condenser) and hence to the solvent throughput to be used.
The operating cost is substantially proportional to the quantity 2Q of heat required to regenerate the solvent. It is also higher the greater the solvent throughput because of the greater energy consumed in pumping it.
The solvents of type a) are characterised by a low operating cost per unit of acid gas removed, but are also characterised by high investment cost particularly at low partial pressure as the acid load is not high.
The solvents of type b) are characterised by a high operating cost ,. ~ : .
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because in the absorption stage they lead to the formation of carbamate, in the presence of Co2. The reverse reaction carried out in the regeneration column is thus strongly endothermic and hence costly.
The solvents of type c) are characterised by lower operating costs than those of type b) as they lead to the formation of bicarbonate, hence requiring a decomposition reaction which is less endothermic and hence les~ costly than that for carbamate.
However, the solvents of type c) involve relatively high investment costs as they have to be used in relatively low concentration to prevent corrosion phenoMena (or alternatively high quality or alloy steels have to be used leading to a considerable increase in investment cost). It can also be shown that solvents of type b) can also suffer from this latter problem.
MEA and DEA solutions are therefore used as 15-25 wt% solutions to prevent serious corrosion due to the high concentration of the respective carbamate, thus involving consequently higher investment costs.
We ha~e now found that the drawbacks of processes of the known ~rt can be overcome by using as solvent an aqueous solution of dimethylethanolamine at suitable concentration.
The process accordin~ to the present invention for removing acid gases such a5 HzS and/or COz from gaseous mixtures which contain them, and comprising essentially the absorption of the acid gases by a solvent and the regeneration of the spent solvent by stripping, is characterised by using a solvent consisting of an aqueous mixture of dimethylethanolamine ~DMA) having a dimethylethanolamine concentration of between 30 and 70% by ~2a~ , weight, and preferably between ~0 and 5.~%.
This compound leads to the formation of a bicarbonate (or a bisulphite if HzS is present) and enables a high concentration aqueous solution to be used without either a corrosion or a transport probleo, even for a relatively low C02 partial pressure.
It should also be noted that the bicarbonate regeneration is not a costly operation.
The process of the invention can purify gaseous mi~tures with an acid gas content in the feed of between 1 and 90% by volume, but is particularly indicated for those mixtures with an acid gas content of between 3 and 60% by volume.
With this process ~n overhead stream from the absorption column is obtained with a C02 content of between 0.5 and ~% by volume.
A scheme for implementing the proce~s according to the invention is described hereinafter by way of non-limiting example with reference to the Figure.
The gas to be treated is fed through the line 1 to the absorber 2, to which the absorbent solution is fed through the line 3.
The treated gas is withdrawn through the line 4. The spent solution is discharged ~rom the bottom 5 and after being depressurized in 6 and preheated in 7 is fed to the regeneration column 8.
From the bottom of the column 8, which is provided with a reboiler 9, the regenerated solution 10 is fed by the pump 11 to the column 2, after being cooled in 7 and 12.
The acid gases 13 leaving the column ~ are cooled in l~ and separated in 15 into a liquid stream 15 (recycled by the pump 17) and an acid gas 18, which is finally removed.

Th~ ga~ and vapour from the top the columns 2 and 8 respectively can be washed with a ismal1 quan~ity of water to prevent any , ~ , , .

h9 solvent loss into the gaseous e~it streams as described by the applicant in U.K Patent Appln. Publication No. 2167738.
The absorption column can be provided with supplementary intermediate heat e~changers if strict temperature control is necessary.
The water lines for amine removal and the supplementary heat exchangers are not shown in the Figure.
Two examples are given below, one of which is comparative, for the purpose of better illustrating the invention but without in any way limiting it.

The process is carried out in a column comprising 4~ two-cap plates and a diameter of 2" using a 50 wt% mi~ture of dimethylethanolamine (DMEA) and water. The natural gas feed (2 Nm3/h) contains 20% of C02 and is at 70 kg/cmZ. Operating at 70C
at the bottom and 50 C on the top plate, the treated gas has a residual COz content of 1% for a solvent flow of 3.5 kg/h Operating on the same feed under the same temperature and pressure conditions in the same column but using a solvent stream consisting of a ~olution of diethanolamine (DEA) (25% by weight) in water, a flow of 7 kg~h is required to obtain a gas containing 1% of C02.
In neither case were direct measurements made of the heat consumption in the regeneration column (44 two-cap plates, ~ = 2 , top pressure = l.2 kg/cm2, bottom temperature = 120 C). However it was calculated that using DEA involves a 30% higher heat consumption than for operation with DMEA.
Operating with DEA consequently involves both a considerable increase in investment cost and an appreciable increase in operating cost due to the greater energy consumed in handling the S solvent.

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Claims (2)

1. A process for removing acid gases such as H2S and/or CO2 from gaseous mixtures which contain them, comprising essentially the absorption of the acid gases by a solvent and the regeneration of the spent solvent, characterised by using a solvent consisting of an aqueous mixture of dimethylethanolamine (DMEA) having a dimethylethanolamine concentration of between 30 and 70% by weight.

2. A process as claimed in claim 1, wherein the concentration of dimethylethanolamine (DMEA) in the aqueous solution is between 40 and 55% by weight.