DE2251055A1 - METHOD FOR DRYING AND CLEANING CARBON DIOXYDE - Google Patents

Description

VEGYTERV Vegyimüveket Tervezö Vällalat, Budapest (Hungary)

PROCESS FOR DRYING AND PURIFICATION OF VACUUM DIOXYDE

The invention relates to drying and Cleaning of carbon dioxide-containing, contaminated gas mixtures. ·

With deni processes, gas mixtures are advantageously cleaned which are saturated with water vapor and _{contain at least 85% by volume of CO 2} and whose foreign components do not exceed the following limit values:

H ₂ 2

N ₂ 10

A175-544 / 3. -1-

309 818/0741 ÖRfGWNAL INSPECTED

- ο · -

CH ₄ 10

C ₂ H ₆ 0.1

^C 3 ^H 8 ²

^C Ao ²

C ₅ H ₁₂ a / 2

H ₂ S 0.1

a /: The value for CcH, ₂ includes higher paraffins and the aromatic and cyclic compounds that cause odor.

i ■ *

fertilize included.

With the process according to the invention, carbon dioxide of great purity can be produced; The carbon dioxide content of the product is more than 99.9% by volume. The contaminating admixtures with 4 and more carbon atoms can be completely removed; the propane and hydrogen sulfide content can be reduced to a value of less than 1 mg / kg CO ₂ , the methane content to around 20 mg / kg CO ₂ , the water content to 20 mg / kg: so that in the production of dry ice the Separation of ice is not to be feared. The removal of athane, whose relative volatility in relation to carbon dioxide is hardly greater than 1, cannot be carried out in an economical manner: from a raw gas which contains 0.1% by volume of "C ₁ H-, this can only be done up to one Value of 600 mg / kg removed

will. However, the athan content in the product does not interfere. Of the stated purity is for food and also for welding purposes in full size sufficient and secures the odorlessness of the carbon dioxide, which is generally not achieved with the previously known methods.

There are several adsorption, absorption and distillation processes for drying and purifying carbon dioxide known.

One of the most important of the adsorption processes is Process worth mentioning, in which the crude carbon dioxide through a mechanical oil trap as well as through with activated carbon and silica gel filled adsorber is passed. The inert; n

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Gases are removed by flash distillation / czechoslovakian Patent No. 123 345 /.

Absorption processes using various chemical absorbents are used to purify gases known / e.g. the flue gas cleaning according to GDR patent no. 34 8o5 /.

It is the common disadvantage of adsorption and absorption processes that they are cost and substantial Bring material loss with it, and that adsorbents and absorbents must be regenerated, including a corresponding Device is necessary; in addition, the processes are discontinuous.

It is worth noting _; that the adsorbent activated carbon is hydrophobic and therefore cannot be used as a desiccant. .

The most modern method for separating gas mixtures is distillation. Such a procedure is described in in Austrian Patent No. 186 235. at This process tries to overcome the difficulties encountered during distillation - such as compression the gas mixture, the large column height and the volume requirement as well as the fact that the two components According to the air, two columns are needed for purification - by using a cold gas cooling machine to avoid. However, this process works approximately at atmospheric pressure, at which the carbon dioxide is not in the liquid state; therefore the method is not suitable for the purification of carbon dioxide.

Furthermore, a method for the purification of carbon dioxide by distillation is known, which is used to the Trace impurities of the gas., With which one atomic reactors cools to keep at an acceptably small value / franz. Pat. No. 1 365 580 /. The procedure essentially consists in that the compared to carbon dioxide

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- ti m »

less volatile substances, such as water, iodine and bromine, through adsorption, the substances, on the other hand, their volatility is greater than that of carbon dioxide, can be removed by a distillation process in which a significant one Part of the gas to be cleaned is not passed into the distillation column, but for cooling and for Recirculation is used in a surface condenser, after which it is mixed with the purified carbon dioxide. This greatly deteriorates the cleaning efficiency. If the carbon dioxide is heavily polluted, so it is not sufficient to clean part of the gas flow, because after adding the uncleaned part the Product purity is not satisfactory.

In the most modern known method for cleaning carbon dioxide / French. Pat. No. 1 361 238 / will the water is first removed by adsorption drying, the remaining impurities are removed by distillation. The distillation is carried out in a double column. In the first, the more volatile become under higher pressure Components removed. The reflux condenser in the first column serves as a reboiler for the second. The pressure in the second column is lower. In this column, the less volatile components are removed and that vine product peeled off. To achieve heat economy, a solution is described in which the top product the second column is condensed in the reboiler of the first column. The disadvantage of this method is that there is a forced connection between the removal of the volatile and the less volatile constituents, which only means no additional energy consumption compared to the optimum if the more volatile constituents usable amount of heat is identical to the heat requirement of the second column. The cover the heat demand of the reboiler makes the reflux an important part of the process.

30981 8/0741 '

It is the common mistake of the distillative Procedure that they use too much energy / too large Heat levels, too large a material cycle during the distillation / work and either only little cleaning effect to use large-scale apparatus.

In each of the previously known methods, the use of distillation to remove the in the Gas mixture contained water avoided, probably on the assumption that the reaction leading to equilibrium between water and carbon dioxide or carbonic acid, the relative volatility of the water related to water Carbon dioxide adversely affected.

The aim of the invention is to develop a distillation / rectification process for drying and purification of the carbon dioxide in such a way that the adsorption drying occurs during the distillation of the moist Gas can be omitted, clogging caused by hydrate and ice formation can be avoided, the separation is possible with great sharpness and the number as well as the size of the apparatus and the operating costs can be reduced to a significant extent.

The invention is primarily based on the knowledge that the volatility of carbon dioxide in relation to water is extremely high and therefore drying can be carried out easily and cheaply by distillation. In respect of the fact that carbon dioxide does not form hydrates at temperatures of about 10 ⁰ C and at this temperature the equilibrium vapor pressure is 45 atm of carbon dioxide approximately, the water content of carbon dioxide by distillation can be atm removed under a pressure of more than 45th On the other hand, the distillation is ineffective in the vicinity of the critical pressure, which is 74 atmospheres for carbon dioxide. The distillative drying of the carbon dioxide must therefore be carried out at pressures between 4 5 and 74 atmospheres, advantageously at pressures between 45 and 60 atmospheres.

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be led / s. Fig. 3 /.

The second fundamental finding of the invention is that the vapor-liquid equilibrium constants of the components that contaminate the carbon dioxide under the conditions of distillative purification This information may differ significantly from the information known from the literature. The relative volatility related to carbon dioxide of impurities, which are more volatile than carbon dioxide, is greater, the relative volatility of carbon dioxide, based on the less volatile impurities in comparison to it, is smaller than that due to expected value from references. The smaller the difference used in the distillation, the greater the difference Pressure is. In the course of our own investigations, we found that the value of the vapor-liquid equilibrium constant must be multiplied by a correction factor k for the conditions of the distillation:

K = 2.38-0.02. ρ

ρ is the distillation pressure in atmospheres. The value of k is therefore between 1.2 and 2.3, depending on the distillation pressure. This knowledge made it possible to remove the more volatile impurities - H ₂ , N ₂ > CH ^ - with a distillation in which the total amount of the dripping liquid necessary for rectification is ensured by the inflow of the distillation column / feed / and this inflow to at least that belonging to the distillation pressure Boiling temperature of the pure carbon dioxide and is set to a pressure greater than the distillation pressure. On the basis of this knowledge, the removal of the water at a pressure of ** 5-6O atü in a special column with a reflux ratio of less than 0.2 and the removal of the impurities that are less volatile than carbon dioxide / heavy impurities

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inclinations /, together with a reflux ratio of greater made than 0.2.

The one used to purify the raw carbon dioxide. Device is selected depending on the cleaning task.

When a high level of purification is desired and the raw carbonic acid gas is used in considerable amounts / e.g. more than Contains 100 mg / kg / water, three pillars are used: One only to remove the water, one to remove the other impurities, which are less volatile than carbon dioxide, and one to remove the more volatile impurities compared to carbon dioxide.

If the requirements for product purity are not too high, two pillars are used: one to remove the less volatile compounds compared to carbon dioxide, including water, the second to remove impurities that are more volatile than carbon dioxide. ■ ■

If one of the two groups of impurities is missing - e.g. either the more volatile impurities / Hj, Ν «» CH ₁ V or the water - two columns are also used.

One column is used, if only one Group of contaminants or just the water to be removed. If necessary, two pillars can also be structurally be assembled.

The impurities are in accordance with the invention will consist of the gas mixture removed by the water in the pressure range between 15 and 60 atm and is distilled at least +10 ⁰ C. If impurities are less volatile than carbon dioxide, these are removed together with the water when the water is distilled off. In this case, the distillation is carried out at a reflux ratio greater than 0.2. Are there such impurities that are more volatile?

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than the carbon dioxide, these are distilled, while the amount necessary for rectification drips downwards Liquid is ensured by the inlet of the distillation column, this inlet at least to the the boiling temperature of the pure carbon dioxide belonging to the distillation pressure and to a pressure greater than the distillation pressure, is set.

When carrying out the method according to the invention, it is also possible to proceed in such a way that after removing the water, the impurities through Distillation can be removed in two columns. In one column they are lighter compared to carbon dioxide volatile impurities at a pressure of 10-35 atmospheres, in the second column the less volatile impurities at a pressure which is at least 1 atm higher than that of the first column, but does not exceed 60 atmospheres.

The implementation of the process according to the invention is illustrated by the following examples.

Figure 1 shows the process picture of the execution according to example 1. Figure 2 shows the process picture of the execution according to example 3 * Figure 3 shows the relationship between Pressure and temperature during the distillation of a water-containing gas mixture as well as the limit curve of hydrate formation.

example 1

Information about the raw gas:
Pressure 65 atm

Temperature 25 ⁰ C

Composition:% by volume

N ₂ 2.0

C ₂ ... 0.1

CO ₂ 9H _f Q

C ₃ ... o »i

Cj ^ ψ. ·. 0.%

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The gas is saturated with water vapor. . ■

The raw gas is to be cleaned for food purposes. Accordingly, the light hydrocarbons / methane, Athane, propane / contained in the product up to 0.5% by volume The heavier, odor-causing hydrocarbons or compounds must be completely removed. There are no special requirements with regard to the water content.

A distillation sequence is chosen in which the heavier hydrocarbons, then the lighter ones are removed. The distillation takes place in two columns. Execution is illustrated by Fig. 1.

The crude carbon dioxide is liquefied together with the water and the top product of the column 2 in the cooler group 1. After cooling or liquefaction, the carbon dioxide, the temperature of which is approximately 17 ^° C., is expanded in column 2, approximately 10% of which changes into the vapor phase. The distillation pressure is 55 atmospheres gauge, the head temperature of the column 16 ° C, the bottom temperature 20 ⁰ C, the number of plates 25. The reflux ratio is equal to 0.6. ·

The bottom product of column 2 consists of contaminated carbon dioxide, which propane, in comparison to propane contains less volatile impurities and the water. The top product is partially purified carbon dioxide, which escapes from column 2 in the vapor phase.

In column 2 the sur separation of the carbon dioxide from. maintain the reflux necessary for the less volatile impurities through the partial condenser 3 built into the column. In this surface condenser, part of the coolant held in the circuit / liquid NH ₃ / evaporates, the heat of evaporation causes the heat to be extracted.

Return ratio: 0.6

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Recoiling takes place in the recoiling «♦ · As

heat transfer medium is gaseous ammonia »wel-

# -. Γο ν "■" ■ ■. " Ches by means of the pressure pump 5 through the cooler 6 in the circuit

o> f \ ■ '? ■■ "■. ■' ·· ■■ is held.

The second distillation is suitably carried out at 20 atm "So a pressure where the one hand the relative Flüchigkeit of the components is more favorable"others; - 'hand the associated Gleichgewichtstentperatur from - 20 ⁰ C simultaneously corresponds to the storage temperature of the finished product.

For this purpose, the top product of the column 2 / ρ - 55 atm / in the heat exchanger group 7 is cooled to approximately -5 ° C. by evaporating the ammonia condensed in the boiler 4. The composition of the top product »in% by volume is:

N ₂ 2.0

C ₂ ... o, i

CO ₂ 91 *. 3 5
C ₃ ... 0.05

c _{u +} ...

The water content is below 20 mg / kg.

The subcooling of the liquefied, νorge-cooled product is due to the expansion of the top product "of the column 9 and guaranteed by the heat exchange in the boiler 8.

The pressure of the cooled in this manner, about ⁰ to -20 C, the liquid product is lowered to 20 atm and supplied the product in the upper part of the column. 9 This column has 30 trays. The top product consists of carbon dioxide, which contains methane and, by comparison, more volatile impurities; it is used to precool the feed to column 9.

The bottom product is practically pure liquid carbon dioxide with a ^{temperature of -20 0} C and the following composition:

309818/0 741 6 MOHNAL INSPECTED

C ₁ ... 0.11% by volume

C _2. ·. 0.09 VoI, -%

CO ₂ 99.74 volts%

C -... 0.06% by volume

Water approx. 20 mg / feg

Example 2 '"' '" \

A carbon dioxide is required for welding purposes, whose purity is greater than that described in Example 1. In this case, three-stage distillation is used applied in the following way:

The first column serves as a water separator. The raw gas enters the column at 60 atmospheres pressure and + 25 ^° C. temperature without reducing the pressure. The bottom product of the column consists of water-containing CO "which escapes at around 29 ° C"

The overhead product from escaping in vapor phase at about 24 ⁰ C. Its composition is similar to that of feeding of column 2 in Example 1, but it contains only a little water. This column operates at a reflux ratio of 0.1, the cooling medium of the condenser is water, the heat-transferring medium in the boiler is, for example, water or ammonia gas. The column has 12 trays.

Further purification is carried out as described in Example 1, with the difference that the column 2 is operating at a pressure of 45 atm, and accordingly is the header or bottom temperature 9 ° C and 12 ⁰ C.

The third column works under one pressure. of 20 atmospheres, the composition of the liquid carbon dioxide escaping as bottom product at a temperature of -20 ⁰ C is as follows:

C ₁ ... 0.107% by volume C ₂ ... 0.090% by volume CO ₂ 99.800% by volume C ₃ ... 0.003% by volume

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Water content less than 20 mg / kg.

Example 3

The raw carbon dioxide is supposed to be used for manufacturing be cleaned of dry ice and for visual purposes. The remaining impurities must therefore at most 0.2% by volume, the odor-causing compounds must be completely removed, the water content must be reduced to 20 mg / kg.

Information about the raw gas to be processed:

pressure water »FO atü temperature 25 ⁰ C Composition: VoI .-% H ₂ 0.8 N ₂ 0.2 CH ₁₊ 1.7 L b 0.1 co ₂ - 97.0 ^C 3 ^H 8 0.1 C ₄ H ₁₀ and higher coals hydrogen 0.1 80 mg / kg

The gas contains little water because it is already after the im Example 2 described method was dewatered.

In the following, the distillation is carried out using two columns.

In the interest of lowering the operating costs, an operating pressure that is significantly lower than that of Example 1 is selected, with which better separation and better heat utilization is possible. To achieve this, the order in which the impurities are removed is also reversed. \> {■ <*% '■■,' ,. ■ '

The crude carbon dioxide is liquefied in the cooling group, subcooled and transferred to the top of the column directed. The pressure in column 11 is in the part

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OWOiNAL INSPECTED

13.5 atmospheres above the surface of the heat exchanger 13, in the part below it 16 atmospheres. The material is transported from the upper part of the column 11 through the cooler group 10 into the lower part of the column 11. This is done with the aid of the pump 12, which saves the compression work. The temperature of the material pumped with the pump 12 into the lower part is -25 ⁰ C, its supply

s ammens état is the following: ^H 2 ^M 2 C ₁ . * · - C ₂ . • · CO ₂ -C ₃ . V ..and higher substances H ₂ O

0.2 vol% 0.07 vol%; 99.6TVbI .-% 0.12Vol * -% hlenwas-

0.12% by volume 90 mg / kg.

The cooling of the crude carbon dioxide in the cooler group 10 is carried out in several stages. First it is liquefied by a cooling medium, for example ammonia with a temperature of -10 ⁰ C,. - ·

Then it is first with the expanded top product - which is made from hydrogen, nitrogen and methane Carbon dioxide exists - and then with it that in the manner already mentioned from the one above the heat exchanger 13 located part of the column 11 in the below located part of the pumped material is cooled, where l * t% of the latter become steam. Finally it will be with the escaping from the lower part of the 'column 11 as bottom product, relaxed - little water, propane, butane and in comparison to butane less volatile hydrocarbon Carbon dioxide by-products containing impurities -30 ° C cooled. \

. _: The boiling out necessary in the upper part of column 11 and the reflux necessary in the lower part is »

by appropriate heat transfer by means of the heat exchanger 13 guaranteed in the middle part of the column 11. The heat transfer is optimal when both the value the boiling-back ratio as well as that of the reflux is 0.3.

The cleaned carbon dioxide escapes from the middle part of the column. It is made by the compressor 11 Compressed to 20 atmospheres and transported into the re-boiler 15 attached next to the column. This becomes that recently withdrawn liquid carbon dioxide as a finished product.

The finished product has a temperature of * -20 ⁰ C and consists of practically pure carbon dioxide of the following composition:

C, .... 0.02 Völ * -%

C ₂ ... 0.08 vol-I

CO ₂ 99.80% by volume

C ₃ ... 0.1 vol .- *

C ₁ .... + higher hydrocarbons

H ₂ O 20 »g / kg

The upper part of the column 11 has 30 trays lower part has in the section above the inlet 17, below of the feed s 11 trays.

Example H

The raw carbon dioxide contains hydrogen sulfide in an amount of 200 kg / kg. The cleaning takes place according to example 3 with the difference that in the lower part the column 11 a larger material cycle takes place. To bring about this, the compressor 11 is a Larger amount of carbonic acid transported into the reboiler 15 and the surplus by tapping the from the re-boiler IS coming / in Fig. 2 as a thick arrow / line in liquid state in the part of the

309818/0741 owoinal »specified

Column 11, which is located below the heat exchanger 13, returned. The reflux ratio increased in this way is 4. In order to increase the sharpness of the separation, the number of trays of column 11 is increased to 70. , ■ -. '

In the case described in the example, the hydrogen sulfide content of the product is less than mg / kg, the carbon dioxide content 99.9% by volume.

The advantages of the procedure are summarized the following: .."".,.

Blockage due to hydrate formation or ice formation is avoided, making operation easier to control. The impurities are separated off with great clarity. A plant for the production of highly purified Carbon dioxide can be planned in a reliable way.

The number of devices is fewer than in other known methods. As a result of being close to the optimum internal material cycles are the mass / e.g. Diameter/ the apparatus small. .

- The method enables good heat utilization / heat pump /, which means that the amount of energy used remains low. While in the most recent procedure / according to French Pat 1361238 / must be overcome with the heat pump 46 ° C temperature difference., This temperature difference is only 5 ⁰ C. Example 3 of the inventive method.

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Claims (3)

PATENT CLAIMS 1.) Process for between 10 and 60 atü feasible distillative purification of at least 85% carbon dioxide containing, with water, compared to
Carbon dioxide more volatile substances hydrogen, nitrogen, methane, furthermore with less volatile hydrocarbons than carbon dioxide and hydrogen sulfide or with some of the substances listed above, gas mixtures contaminated, characterized in that the contaminants depending on their type
can be removed from the gas mixture by the water in the pressure range between U5 and 60 atü at least + 10 ⁰ C is distilled, or - the less volatile impurities compared to carbon dioxide during the distillation of the water with this are removed together, whereby the distillation carried out at a reflux ratio greater than 0.2 will and / or - which are more volatile than carbon dioxide.
Impurities are distilled in such a way} that
The column feed is used as the dropping liquid necessary for rectification, this feed being set at least to the boiling point of the pure carbon dioxide associated with the distillation pressure and to a pressure greater than the distillation pressure. 2. The method according to claim 1, characterized in that the impurities are removed after the removal of the water by distillation in two columns, the first in comparison to
Carbon dioxide more volatile impurities are distilled at a pressure of 10-35 atmospheres, in the second the distillation of the less volatile impurities at a pressure which is at least one atmosphere greater than that of the first column, but does not exceed 60 atmospheres 3 0 9 8 18/0741