DE2410713B2 - PROCESS FOR OBTAINING ACETYLENE-FREE AETHYLENE - Google Patents

Description

35

The invention relates to a method for obtaining acetylene-free ethylene from a gaseous one Mixture of C2 hydrocarbons by absorption of acetylene in one saturated with ethylene Solvent at pressures of more than 5 ata, the solvent during the entire absorption process at a temperature at or near above the liquefaction temperature of the C2 mixture is maintained, whereupon the ethane from the remaining Ethylene-ethane mixture is separated by rectification.

Today, ethylene is mainly produced through thermal splitting or partial combustion of hydrocarbons generated. The resulting gas mixtures contain about 25 to 40% ethylene, but also Acetylene in amounts up to 2%. The gas mixtures resulting from the cracking process are initially freed from higher-boiling hydrocarbons, soot, etc. and then through cooling and condensation further decomposed, resulting in a C2 fraction which, in addition to ethylene and acetylene, also contains ethane. To the Further processing of the ethylene, e.g. B. on polyethylene, it is necessary to use the ethylene in particular from To free acetylene down to traces in the ppm range.

It is known (DT-PS 9 53 700) _{to treat the C 2} fraction with a solvent for acetylene, in particular acetone, in order to selectively dissolve the acetylene from the gas mixture, after which the remaining ethylene-ethane mixture is further broken down by rectification . In order to get by with a minimum amount of solvent, the acetylene is washed out of the gaseous acetylene-ethylene-ethane mixture at or just above its liquefaction temperature. During the washing process, the detergent is saturated with the individual components of the gas mixture. In this solution heat is released, which leads to a considerable heating of the Waschmutels To remedy this disturbing phenomenon is the solvent, in particular acetone, in the known method with liquid ethylene or ethylene and ethane saturated Absorption is thus with a solution of acetone and liquid ethylene or ethylene-ethane mixture. This precaution ensures that only the relatively small heat of solution of the acetylene is released during the absorption process, which allows the absorption process to be carried out over the entire length of the column at or near the liquefaction temperature of the gas mixture, i.e. to achieve the greatest possible effectiveness.

In this known method, however, it has been found to be disadvantageous that the ethylene is at the dew point or the liquid ethylene is indefinitely soluble in acetone at or below the dew point. That has extremely undesirable consequence that the selectivity of acetone for acetylene is opposite In the vicinity of the dew point, ethylene strongly shifts in favor of acetylene, which means that the Acetone is used as an absorbent in the temperature ranges that are favorable for the solubility of acetylene becomes almost unusable.

It is also known (US-PS 29 09 038), the separation of acetylene from a mixture of Make methane, ethane, acetylene, ethylene, propane and propylene with the help of a liquid-liquid extraction. Such a liquid-liquid extraction, however, has the disadvantage compared to gas scrubbing that only very few solvents suitable for a solution of acetylene can be used, as these solvents, in order to enable a later phase separation of the also liquid accompanying substances of the acetylene, must not be mixable with these at all. The possible uses of such a liquid-liquid extraction are therefore only limited.

It is therefore the object of the present invention to provide a process for obtaining acetylene-free ethylene which is free from these disadvantages and which makes it possible to dissolve acetylene from C ₂ -hydrocarbon mixtures down to traces in the ppm range with a minimum of absorbent expenditure.

According to the invention, this object is achieved in that a solvent is used which is only miscible to a limited extent at the absorption pressure with liquid ethylene or mixtures of liquid ethylene and liquid ethane, the solvent not exceeding the equivalent of 800 Nm ^{3 of} ethylene or an ethylene-ethane mixture per ton of solvent contains.

The invention therefore provides a new teaching for the selection of detergents which, in the case of a detergent saturated with ethylene or ethane, are suitable for the selective washing of acetylene from gas mixtures. According to the invention, for acetylene absorption from C ₂ hydrocarbon mixtures, in particular dimethylformamide (DMF), N-methylpyrrolidone (NMP), mixtures of these two solvents, mixtures of N-methylpyrrolidone and methanol and, last but not least, hexamethylphosphoric triamide

(HMPA) suitable for the absorption of acetylene from C2 hydrocarbons for many years known acetone must be eliminated due to the new approach.

The superiority of the absorbents recognized as advantageous over acetone s? I in some Examples shown in the table below:

PCH ₁ Temp. acetone Solubility C ₂ H ₂ Pc-H ₄ Solubility C ₂ H ₄ ata ( ⁰ C) 36 DMF -71 14th 0.01 / 1 -25 1 140 0.01 / 1 -71 *) 1 53 0.05 / 5 -53 *) 1 44 0.1 / 10 -25 *) 32 0.2 / 20 *) Dew point temperature of the C ₂ H ₄ . 21

NMP

98

40

NMP NMP

85 percent by weight 45 percent by weight

Methanol DMF

15 percent by weight 55 percent by weight

72

42 30th

74

43
31

The table shows that the selectivity of acetone for acetylene over ethylene am Dew point is no longer given. In comparison, the selectivity of dimethylformamide and the of N-methylpyrrolidone obtained.

With regard to the selectivity, the N-methylpyrrolidone behaves very advantageously. This detergent has however, the disadvantage of a relatively high melting point (- 25 ° C), so that it is difficult with this agent, the condition to be met, the absorption at or near above the liquefaction temperature of the Perform C2 mixture. According to the invention, this detergent is therefore preferably not pure Form, but used in a mixture with dimethylformamide or methanol, which mixtures working temperatures Allow down to about -60 ° C, without the selectivity of the N-methylpyrrolidone significantly reduce. The same applies to the solvent hexamethylphosphoric triamide.

The teaching of the absorption carried out at or close to above the condensation temperature of the C ₂ -Gemisehes, refers to a temperature range to about 2O ⁰ C above the liquidus temperature. This means that with a mixture of about 80% ethylene, 19% Äihan and 1% acetylene at a pressure of 20 ata in the range from -5 to -24 ° C, at a pressure of 10 ata in the range from -25 to - 47 ° C should be washed. This information shows that the detergent N-methylpyrrolidone, preferred for washing out the acetylene, can be used with suitable additives in processes which work above 10 atm.

The remaining after the acetylene has been washed out of the gas mixture is made up of ethylene and ethane Existing gas mixture can be worked up by rectification in a simple manner at pressures of more than 5 ata.

The limit for the limited miscibility of the detergent with ethylene or ethane, which is to be regarded as critical according to the invention, was recognized as an amount of no more than the converted 800 Nm ³ , preferably no more than 500 Nm ^{3 of} ethylene or an ethylene-ethane mixture per ton of solvent.

The invention will now be explained on the basis of an exemplary embodiment shown schematically.

_{A C 2} fraction which contains 1% by volume of C2H2 passes through line 1 into the washing column 2. It is gaseous, is under a pressure of 10 ata and is at the dew point temperature (-53 ° C). DMF is used as a detergent, which is precooled in the cooler 3 against propylene cold to the column temperature and fed into the column 2 through line 4. At the pressure applied and at the dew point temperature, 1 t of DMF dissolves the equivalent of 200 Nm ^{3 of a} liquid ethylene-ethane mixture. This amount is generated from the vapors rising in the top of the column with the aid of the condenser 5, which is cooled with ethylene, separated in the separator 6 and returned to the top of the column 2 through the line 7. Here the liquid C2 mixture washes out the DMF portion from the rising gas. In the middle part of the column, DMF and the liquid C _{2 hydrocarbons then mix and wash off the C 2} H ₂ down to the smallest traces at almost dew point temperature in the column part below. The bulk of the C ₂ H ₂ is already absorbed in the lowest part of the column above the feed point of the line 1, where the heat of dissolution of the C ₂ H ₂ results in a temperature 5 to 20 ° C higher than the dew point, i.e. -48 ° C to - 33 ° C. The loaded washing solution is then warmed up in the heat exchanger 8, with ethylene and ethane preferably gassing out. This gas is fed to the washing column 2 through line 9 as return gas. After releasing the washing solution in the expansion valve 10 and after further heating in the heat exchanger 11, the washing solution is transferred through line 12 to a regeneration column 13, where it is freed from acetylene by means of the solvent vapor generated by the steam-operated reboiler 14. The regenerated detergent leaves the regeneration column at the foot, is cooled in the heat exchangers 11, 8 and 3 and then fed to the washing column 2 through line 4.

The vapors withdrawn from the top of the regeneration column 13 are cooled in the cooler 15 against propylene cold, with solvent vapors being separated in liquid form in the separator 16, which are fed back to the column 13 through line 17. Acetylene-rich gas is withdrawn from the top of the separator 16. _{The C 2} H ₂ -free gas mixture flowing overhead from the separator 6 is transferred through line 18 to a rectification column 19, where it is _{broken down into C 2} H ₂ (top product) and (bottom product).

1 sheet of drawings

Claims (4)

Patent claims: 1. A method for obtaining acetylene-free ethylene from a gaseous mixture of (! ^ - hydrocarbons by absorption of the acetylene in a solvent saturated with ethylene tel at pressures of more than 5 ata, the solvent during the entire absorption process at a temperature at or is kept close to above the liquefaction temperature of the C ₂ mixture, whereupon the ethane is separated from the remaining "ethylene-ethane mixture by rectification, characterized in that a solvent is used which is at the absorption pressure with liquid ethylene or mixtures of liquid ethylene and liquid ethane is only miscible to a limited extent, the solvent containing a maximum of no more than the equivalent of 800 Nm ^{3 of} ethylene or an ethylene-ethane mixture per ton of solvent. 2. The method according to claim 1, characterized in that the solvent contains a maximum of no more than the equivalent of 500 Nm ³ ethylene or ethylene-ethane mixture per ton of the solvent. 3. The method according to claim 1, characterized in that the solvent is N-methylpyrrolidone is used. 4. The method according to claim 1, characterized in that a mixture of as a solvent Hexamethylphosphoric triamide and methanol is used.