IE68770B1 - Process for drying hydrocarbons its application to the preparation of chloromethanes - Google Patents

Abstract

Process for drying a mixture containing at least one hydrocarbon, hydrochloric acid and water, characterised in that it is placed in contact with a desiccant chosen from (i) anhydrous metal sulphates, chlorides or perchlorates or (ii) phosphorus pentoxide until the desiccant has retained most of the water. This is applied to the synthesis of chloromethanes to dry the methyl chloride leaving the head of the CH3Cl/higher chloromethanes separating column.

Description

The present invention relates to a process for drying hydrocarbons and to its application to the production of chloromethanes.

The invention relates more particularly to optionally 5 halogenated hydrocarbons which contain water and hydrochloric acid. Drying of perchloroethylene using a solution of calcium chloride (Chem. Abstracts. Vol. 99,177849d) and in the absence of hydrochloric acid has already been described. The drying of chloroform CHC13 and of carbon tetrachloride CC14 after their purification by extraction with water (Chem.

Abstracts, Vol. 62,2227e) with calcium chloride (CaCl2), but always in the absence of hydrochloric acid, has also been described. The problem with which the present invention relates is that of drying hydrocarbons containing hydrochloric acid and water without removing the hydrochloric acid. This problem is encountered in the synthesis of chloromethanes as they leave the chlorination reactor, where the chloromethanes are mixed with hydrochloric acid (each time a hydrogen is substituted by a chlorine, a mole of hydrochloric acid is formed as a by-product) and with water which is introduced into the process as an impurity in the starting materials. The water must be removed to prevent its accumulation in this process and also to avoid blocking the V - 3 pipelines with ice and hydrates. The hydrochloric acid can be recovered and then reclaimed by conversion into chlorine in a Deacon reaction or an oxychlorination reaction. If water is removed from this mixture of chloromethanes by condensation then there is a risk, due to the high solubility of hydrochloric acid in water, of obtaining a solution of hydrochloric acid in water which is subsequently difficult to separate. There is the same risk if it is desired to separate the water from this mixture using a drying agent.

Surprisingly, we have found that it is possible to dry selectively hydrocarbons containing hydrochloric acid and water while retaining only water.

The invention therefore provides a process for drying a mixture containing at least one hydrocarbon, hydrochloric acid and water, characterized in that it is brought into contact with a drying agent chosen from (i) anhydrous metal sulphates, chlorides or perchlorates or (ii) phosphorus pentoxide, until the drying agent has retained a major proportion of the water.

Although the invention relates to any hydrocarbon, it is advantageously applied to benzene and its alkylated or polyalkylated derivatives, that is to say benzene substituted by one or more linear or branched hydrocarbon chains, each containing up to 8 carbon atoms. For example, benzene, toluene, xylene, isopropylbenzene, styrene and ethylbenzene may be mentioned. The invention also relates to halogenated hydrocarbons.

The halogenated hydrocarbon may contain fluorine, chlorine or bromine or more than one, for example two or three, of these elements, and may be saturated or unsaturated, that is to say may contain one or more double bonds or one or more triple bonds, or any combination of these possibilities. The halogenated hydrocarbon preferably contains 1 to 4 carbon atoms. The invention is particularly useful in the case of chloromethanes and in the case of chlorinated hydrocarbons containing two carbon atoms. Among the chlorinated hydrocarbons containing two carbons, reference is made, in particular, to 1,2-dichloroethane, vinyl chloride, 1,1,1 trichloroethane, 1,1,2-trichloroethane, trichloroethylene and perchloroethylene. The halogenated hydrocarbon may be a mixture of a number of halogenated hydrocarbons; it may be present with a solvent. The quantity of hydrochloric acid is immaterial as is the quantity of water. However, the quantity of water advantageously does not exceed 1 % and preferably 0.1 %, by weight.

Larger quantities of water can be present, but the invention is generally no longer of economic interest.· « In fact, when a few per cent or more of water is present it is simpler to employ conventional separations such as distillation or an extraction, and then to employ the present invention.

The mixture may be gaseous or liquid or partially gaseous. The temperature and the pressure are immaterial. The invention can be used for drying the mixture whatever the temperature and the pressure at which it is available, without it being necessary to modify them. The mixture may also contain other products, for example chlorine.

The drying agent is generally in solid form and can be easily separated from the mixture. It is typically in the form of a powder or of granules in a stationary bed or a * fluid bed. Its function is to retain only water, and not hydrochloric acid or optionally chlorine when it is present and, naturally, not the halogenated hydrocarbon. It should be an anhydrous salt which quite obviously must not react chemically with hydrochloric acid. Metal sulphates, chlorides and perchlorates are suitable. It is possible, for example, to employ calcium sulphate, sodium sulphate, copper sulphate, zinc chloride, calcium chloride, barium perchlorate or magnesium perchlorate.

Calcium chloride is advantageously employed. The to quality of the drying agent largely depends on the quantity of water which is to remain in the mixture. · A calcium chloride whose water content is from 0 to 25 %, and preferably from 0 to 12 %, by weight is advantageously employed.

To obtain a mixture containing not more than a few 5 ppm of water it is necessary to employ an essentially anhydrous drying agent. For example, to obtain a mixture containing not more than 10 ppm of water, it is necessary to employ a calcium chloride such that its water content generally does not exceed 5 % by weight.

As for the quantity of drying agent, this depends on the total quantity of water which is to be retained. For example, when working in a stationary bed, the drying agent, which is desirably near the entry of the flow of mixture to be dried, becomes saturated with water, and so on in the case of the whole bed. To ensure proper drying, it suffices that enough anhydrous drying agent, not yet saturated with water, should remain. The residence time of the mixture to be dried with the drying agent is immaterial. It is advantageously not more than 10 minutes and preferably from 1 to 5 minutes.

Employing much longer residence times is possible, but is generally not necessary.

Long residence times correspond to a large volume of drying agent. This is a good precaution for obtaining proper drying, but this large volume can cause pressure drops which are incompatible with the remainder of the process. A specialist can very easily choose the best compromise.

The present invention also provides a process for the synthesis of chloromethanes, characterized in that a mixture containing at least one chloromethane, hydrochloric acid and water is brought into contact at any point in the process with a drying agent chosen from (i) anhydrous metal sulphates, chlorides or perchlorates or (ii) phosphorus pentoxide.

The synthesis of chloromethanes usually consists in preparing methyl chloride (CH3Cl) and then, by chlorination, the higher chloromethanes: methylene chloride (CH2C12), chloroform (CHC13) and carbon tetrachloride (CC14). CH3C1 is obtained by chlorination of methane or by hydrochlorination of methanol and then this CH3C1, alone or mixed with a proportion of higher chloromethanes, is subjected to a chlorination using liquid or gaseous chlorine. At the exit of the chlorination reactor a mixture of chloromethanes, hydrochloric acid, a little water and possibly a little chlorine is obtained.

The water is neither a reactant nor a product, but is present as an impurity in the starting materials such as chlorine, and some of it can remain in the CH3C1 originating from the methanol hydrochlorination reaction.

The HC1 can be separated by distilling this mixture through the so-called HCl column; advantage can be taken of a favourable concentration profile of the traces of chlorine in this column so as to exhaust these traces of chlorine, for example with the aid of UV lamps, by completing the chlorination. A mixture of chloromethanes, water and still a little hydrochloric acid which is poorly separated in the HCl column is obtained as tailings. This mixture is usually subjected to a series of distillations to separate the various chloromethanes which form the output and a proportion is recycled to the chlorination reactor to adjust the proportions of the various chloromethanes.

In a first column (which follows the HCl column), called a methyl chloride column, the CH3CI and the major proportion of the water can be taken off overhead, together with the traces of HCl and of chlorine which are still present after the HCl column. At the bottom of this CH3C1 column there is a mixture of higher chloromethanes which can be separated into its various components by distillation.

Although its boiling point is higher than that of the higher chloromethanes, the traces of water leave overhead because of various partial azeotropes and of the formation of more or less stable CH3C1 hydrates.

Such a process for the synthesis of chloromethanes is described in EP 128,818, GB 2,158,067, GB 2,181,132 and GB 1,456,568. The present invention is advantageously applied to methyl chloride obtained from the hydrochlorination of methanol or from the chlorination of methane and to the CH3CI originating from the CH3CI column, either to the entire flow or only to the proportion recycled towards the chlorination reactor. An advantage of the present invention is that it removes water to prevent its accumulation in the process and also to prevent corrosion problems. In fact, the simultaneous presence of water, HCl and possibly chlorine at the head of the CH3CI column leads to corrosion. According to a preferred form of the invention, the mixture leaving overhead from the CH3CI column is dried before the condenser and the reflux pot.

The present invention is illustrated, merely by way of example, with reference to the accompanying drawing in which Figure l shows a preferred diagrammatic form of carrying out the invention. (20) is a chlorination reactor, (30) an HCl separation column, (40) a CH3CI column, (50) a device for drying and (60) a device for separating the higher chloromethanes. The CH3CI originating from the * hydrochlorination (not shown) is introduced at (1), the chlorine at (2) and the recycled chloromethanes at (3). The 'v output (4) is distilled in (30) and the HCl is collected at (5) and then the tailings, containing the chloromethanes, water and a little HCl, are fed into the column (40) via the pipe (6). (7) represents CH3CI output, (8) a reflux and (9) recycle. The higher chloromethanes are separated in (60); (10), (11) and (12) represent the output of CH2CI2, CHCI3 and CCI4 respectively. (9), (13), (14) and (15) represent CH3C1, CH2C12, CHC13and CCI4, which are combined in (3) and are recycled to the reactor (20).

The drying agent is preferably calcium chloride. The mixture to be dried leaving overhead from the CH3CI column is generally at a temperature of 5 °C to 60°C and preferably 20 to 50’C. Its pressure is advantageously from 1 to 12 bars absolute and preferably from 4 to 10 bars absolute.

The quantity of water can vary within wide limits; it is advantageously no greater than 0.5 % by weight and preferably from 50 to 500 ppm. As for the quantity of HCl, its content depends on the efficiency of the HCl separation column; it is generally no greater than 1000 ppm and preferably from 50 to 500 ppm.

The overhead mixture from the CH3CI column which is to be dried may also contain chlorine? the concentration of this varies according to the efficiency of the chlorination j reactor and of the optional finishing reaction which may be performed in the HCl column. This concentration may go up to, say, 10,000 ppm.

The CH3CI separation column may be coupled with the 5 CH2C12 separation column or there may be a single column with an overhead output of CH3CI and the higher chloromethanes at various side offtakes or any combination thereof as is wellknown in distillation practice. The drier can also be at the head of this column in the gaseous phase consisting essentially of methyl chloride.

The following Examples further illustrate the invention.

EXAMPLE 1 A drier is made up of a glass column 0.5 m in height 15 and 0.45m in diameter, which is filled with 30 kg of CaCl2 granules of 3 to 8 mm over a height of 37.5 cm. The water content of the caCl2 is 2.3 %. A flow of gaseous CH3CI containing HCl, vater and chlorine is passed through this bed, from the bottom upwards, this being done for 408 hours.

The results are given in Table I.

V.

TABLE I Pressure bar absolute Time h Residence time (H20) i ppm o (Cl2) • 1 ppm o (HCl) • X ppm o 1 170 4 min 70 to 105 < 25 85 85 300 300 1 180 4 900 < 30 500 490 300 290 1 190 4 300 < 30 550 500 350 380 1 211 4 300 < 30 1050 1050 350 350 1 265 4.5 310 25-30 950 980 300 290 1 297 3.5 300 < 25 950 980 500 460 1.1 376 4 150 < 25 1.5 383 4.5 190 < 25 950 980 500 460 1 408 5.3 300 < 25 2100 2000 350 350 i = drier intake o = drier output The residence time is calculated for the empty drier EXAMPLE 2 The operation is carried out as in Example 1, but with the drier charged (starting from the bottom towards the top of the bed) with: kg of CaCl2 containing 22 % of water kg 11 II n 12 % n kg II H II 21. 5 % n kg II II II 10 % II kg II II H 12 % II The operation is carried out in this way for 32 hours and then a break-through of the bed is observed. The results are given in Table II.

TABLE II Time h Residence time min (H20) i ppm o (Cl2) • 1 ppm o (HCl) • 1 ppm o 0 4.6 520 130 1100 1050 300 300 10h30 4.6 520 130 n II II II 13 4.6 350 120 n n It II 14 6.5 450 100 II n N II 15 4.6 450 130 II If II II 32 4.6 300 130 If II II 11 EXAMPLE 3 The operation is carried out as in Example 1, but with the drier charged (starting from the bottom towards the top of the bed) with: kg of CaCl2 containing 22 % of water kg It «1 12 % II 3 kg Π If 21. 5 % II The operation is carried out in this way for 12 hours and then 15 kg of CaCl2 containing 1 % of water are added on top of the bed. The total operating time was 50 hours.

The results are given in Table III.

TABLE III Time h Residence time min (H20) • 1 ppm o (Cl2) • X ppm o (HC1) i ppm o 0 4.6 212 160 1100 1050 300 300 10 10.5 255 160 II IV II » 12 4.6 270 150 II II n n 13 4.9 230 25 II 1200 150 150 35 4.9 265 25 II 1050 n II 40 4.9 360 25 II II II 50 4.9 300 25 1260 1300 130 140 EXAMPLE 4 The operation is carried out as in Example 1, but with the drier charged (starting from the bottom towards the top of the bed) with: 7 kg of CaCl2 containing 22 % of water 5 kg II II If 12 % It 4 kg II II If 5 % » 14 kg It II II 1 % II The results are given in Table IV.

TABLE IV Time h Residence time min (H20) i ppm o (Cl2) PP® (HCl) ppm • 1 o • 1 o 10 4.5 280 25 1570 1550 240 240 35 4.5 300 25 1550 1550 300 300 EXAMPLE 5 (comparative) Chloromethanes are dried over molecular sieves. g of molecular sieve, that is 71 ml, are arranged over a height of 540 mm in a glass tube of 13 mm internal diameter and 700 mm in height. The chloromethanes are in a flask and flow under gravity into the molecular sieve layer at a variable flow rate regulated by a needle valve * and are collected in a conical vented into a wash bottle containing concentrated sulphuric acid.

A steady flow rate of chloromethanes is obtained by placing the flask under a constant pressure of nitrogen.

A 3-angstrom potassium sieve is employed in the form of 2-mm beads.

The results are given in Table V, in which MS stands for Molecular sieve.

TABLE V Liquid tested Flow rate linear speed Space velocity flow rate h2o HCl (1/h) (cm/min) (1/h) MS vol (1) ppm ppm Mixture (by wt.) CH2C12:17 % 1.5 19 21 38 4 CHC13 :60 % 2.0 25 28 38 It CCI4 :23 % 2.5 31.5 35 33 II H20 = 85 ppm HCl = 22 ppm Chloro- methane mixture 2.5 31.5 35 30 4 as above 3.0 38 42 18 n H2O = 145 ppm 3.5 44 49 18 11 HCl = 18 ppm The space velocity is expressed in litres per hour per litre of sieve.

It is found that HCl is retained, like the water, on the molecular sieve.

EXAMPLE 6 A device like that of Figure 1 is employed, in which the column 40, 800 mm in diameter, comprises 35 valve trays and operates at 9 bars absolute. The head temperature is 40°C, the bottom temperature 110eC. The drying agent, CaCl2 with a content of more than 95 % is arranged as a stationary bed in a vessel (50), 2800 mm in diameter, 5800 mm in height.

The condenser is fed with the dry CH3CI drier output.

The stream (6) contains: to 20 % CH3CI 50-100 ppm H2O 100-500.ppm Cl2 50-500 ppm HCl to 90 % CH2CI2+CHCI3+CCI4 The stream (8) constitutes the reflux of the distillation column, containing CH3CI, HCl, Cl2» whose H2O content is lower than 20 ppm.

The stream (7) represents the CH3CI extracted from the column, whose composition is identical with the flow (8) .

The stream (9) constitutes the proportion of CH3CI which is recycled, of the same composition as the flows (7) and (8).

Claims (10)

1. Process for drying a mixture containing at least one hydrocarbon, hydrochloric acid and water, which comprises contacting it with,as drying agent, (i) an 5 anhydrous metal sulphate, chloride or perchlorate or (ii) phosphorus pentoxide.

2. Process according to Claim 1 in which the hydrocarbon is a chlorinated hydrocarbon containing one or two carbon atoms. 10

3. Process according to Claim 2 in which the hydrocarbon is a chloromethane.

4. Process according to any one of Claims 1 to 3 in which the drying agent is calcium chloride.

5. Process for the synthesis of a chloromethane, 15 in which a mixture containing at- least one chloromethane, hydrochloric acid and water is brought into contact at any point in the process with a drying agent which is (i) an anhydrous metal sulphate, chloride or perchlorate or (ii) phosphorus pentoxide. 20

6. Process according to Claim 5 in which the chloromethane is essentially methyl chloride.

7. Process according to Claim 6 in which the mixture is the gaseous phase originating from a column for separating methyl chloride from the higher chloromethanes. 25

8. Process according to any one of Claims 5 to 7 in which the drying agent is calcium chloride.

9. Process according to claim 1 or 5 substantially as described in any one of Examples 1 to 4 and 6.

10. A mixture containing at least one 5 hydrocarbon, hydrochloric acid and water whenever dried by a process as claimed in any one of the preceding claims. Dated this the 14th day of February, 1990