GB2084034A - Recovery of a light organic compound from a mixture with a heavy organic compound - Google Patents

Abstract

A light organic compound is recovered from a liquid mixture with a heavy organic compound by passing the mixture through successive similar falling film evaporators E1, E2, E3, each comprising a vertical wall on a first surface of which the mixture flows as falling film, whereby the light organic compound progressively evaporates, the vapours produced being used as heat-supplying fluid in another evaporator E2 operated at lower temperature and pressure, the heat transfer between the liquid mixture and the vapours being effected through the said wall which is provided with cavities of a diameter ranging from 50 mu m to 2 mm, formed, for example, by projection of particles of a hard material against said wall. <IMAGE>

Description

SPECIFICATION Recovery of a light organic compound from a mixture with a heavy organic compound This invention is concerned with the recovery of light solvents. This operation is often necessary in chemical or physico-chemical operations involving the use of organic compounds of high molecular weight, particularly heavy hydrocarbons, which consequently have high normal boiling temperatures and/or viscosities. In practice, these heavy compounds are often admixed with much more volatile solvents.Among the reasons for making use of such mixtures are the following: (a) decrease in the viscosity and increase in the thermal capacity of the heavy product, thus facilitating thermal control of a rapid chemical reaction, this being the case, for example, with sulphonation or chlorination of heavy hydrocarbon charges; (b) decrease viscosity in separation by filtration or ultrafiltration;; (c) a specific physico-chemical action such as the precipitation of asphaltenes in the deasphalting of heavy hydrocarbons (straight-run residues or vacuum residues or even crude oils) by making use of a light hydrocarbon (e.g. an aliphatic or olefinic C3, C4, C5, C6 or C7 hydrocarbon), or paraffin removal from oil by the combined action of cold and selective solvents (e.g. methyl ethyl ketone or methyl isobutyl ketone, either alone or admixed with C6 or C7 aromatic hydrocarbons).

In all of these operations, the ratio of solvent volume to volume of heavy charge is high, and in most cases from 1:1 to 5:1. At the end of the chemical or physico-chemical treatments, it is accordingly necessary to recover the solvents.

In view of the large differences in the respective volatilities, such recovery is in most cases achieved by evaporation of the light solvent, but the large amounts of solvent involved make is desirable to improve the known techniques in order to decrease the energy consumption required by such evaporation. This evaporation is usually effected in one or more stages at variable pressures after reheating the mixture in a conventional heat-exchanger.

Evaporators with several stages operating at different temperature and pressure levels are already known. See, for example, German Patent Specification DE-871 742 and French Specifications FR-1, 178 135, FR-1 404264 and FR-1 431 037. It has also been proposed that the contact surfaces of the heat exchangers be modified by deposition of a layer of porous material (see U.S. Patent Specification No. 3 384154).

U.S. Patent Specification 4214975 also describes the principle of multiple installations. According to this principle, the operation is conducted by carrying out successive evaporation steps. Between consecutive steps, the liquid is pumped to attain a higher pressure so that its boiling temperature is increased from one stage to the next one, by 20 to 70C degrees.

The vapour produced in this way at higher pressure and temperature may release its condensation heat in an exchanger through which a liquid passes at a lower pressure: this is placed upstream in the evaporation system. In this way, it is to be expected that successive stages will make it possible to reuse some of the heat that has been introduced into the system under the highest temperature and pressure.

The same type of arrangement has already been used for a long time for producing distilled water or for concentrating aqueous solutions by evaporation.

It must be emphasized, however, that the procedure, which is effected with conventional apparatus, i.e. an exchanger and an evaporator, e.g. a "flash" drum, requires substantial temperature differences, and hence a relatively high ratio between the pressures at the different stages (about 7:1). The multiplication of these requirements concerning the temperature and pressure differences limits the number of effects that can in practice be achieved to three. Moreover, the increasing pressures in the direction of the overall liquid flow require the use of pumps that are capable of operating at high temperature.

By completely modifying the above-mentioned type of evaporation apparatus, it has been discovered that the principle of multiple effects may be applied under particularly economical conditions even when non-aqueous solutions, which are known as being operated with low-heat-transfer coefficients. As a matter of fact, the high temperature differences required to achieve the transfer under economical conditions are the consequences of the low film coefficients achieved in conventional evaporators operating with non-polar or slightly polar liquids, particularly heavy hydrocarbon oils.

As a general rule, total transfer coefficients of 250 to 800 watts/m2.C deg.) are obtained with these conventional evaporators, only to the extent that the temperature differences remain within the abovementioned range.

The present invention is based on the discovery that good results can be obtained with evaporators with multiple effect using falling-film apparatus in which the evaporation surface has been treated mechanically to obtain nucleate boiling.

In accordance with the invention there is provided a method of recovering a volatile organic compound from a liquid mixture containing the said compound with at least one liquid organic compound of lower volatility, comprising circulating the liquid mixture through two or more serially arranged evaporation stages each of which comprises a vertical wall, causing the liquid mixture in each stage to flow as a falling film on a first side of the wall of the said stage so as to progressively evaporate the volatile organic compound, the said first side having cavities of an average diameter from 50 #m to 2 mm, maintaining each stage at a temperature and pressure different from those of the other stages, causing the vapour of the volatile organic compound produced in at least one evaporation stage to flow in contact with the second side of the vertical wall of at least one other evaporation stage operated at a temperature and a pressure lower than those of the said one stage so as to condense the vapour and to transfer its condensation heat to the liquid mixturethroughthe corresponding wall, maintaining a difference of temperature from 2 to 30C degrees between the vapour and the liquid mixture flowing respectively in contact with the second side and the first side of the vertical wall.

The treatments of the surface to produce an evaporator for use in the method of the present invention leads to a relatively rough surface condition with craters having an average size that must be in the range 50 jrbm to 2 mm, preferably in the range 100 clam to 500,am. This may be achieved for example by projecting fine particles of a relatively hard material on the surface to be nucleated, for example by means of an air stream driving at high speed particles of sand or silicon carbide. The so-created cavities generally have a depth corresponding to about 20 to 60% of their average diameter.

Under such conditions, global heat transfer coefficients as high as 2000 to 3000 wattsj(m2.C deg.) have been obtainedfortemperature differences between the fluids circulating on both sides of the wall in the range to 2 to 30 and preferably 5 to 10 C degrees. It is consequently possible to make use of the recovery technique of this invention. The fundamental schemes are of two types but, of course, it is possible to combine these two types and to perfect a number of alternative embodiments.

Reference in the subsequent description will be made to the accompanying drawings, in which each of the two figures show diagrammatically embodiments of apparatus for use in accordance with the invention.

In the first typical scheme, a mixture consisting of a light solvent and a heavy product is introduced at the top of a first falling film evaporator exchanger E (figure 1). The mixture descendsthe wall of this exchanger (which may for example have the shape of a plate, a tube or any other convenient geometrical shape, for example, a bundle of tubes arranged in a shell) where it receives the heat from a heatcarrying fluid circulating on the other side of the wall. In most cases, this heat-carrying fluid is a condensing vapour.

This heat carrying fluid is at a temperature from 2 to 30 and preferably 5 to 10 C degrees higherthan the boiling temperature of the mixture to be evaporated under the pressure prevailing in the evaporator. The descending mixture on the evaporation side will be, in this type of scheme, at a pressure P1, which is the highest pressure in this installation.

Under nucleate boiling conditions, this mixture will thus produce vapour at this pressure P1 and this will be achieved by means of a moderate temperature difference from the heat-carrying fluid. The liquid mixture recovered at the bottom of the evaporator E1 is then expanded through a regulator valve V1 so that this liquid attains a boiling temperature between 2 and 30 and preferably from Sto 10 C degrees lower than the temperature prevailing in exchanger E1.

Thus, the vapour produced in E, may be used as heat-carrying fluid in a falling-film exchanger evaporator E2 of the same type as that of E1. In this second exchanger the evaporation recovery of the light solvent is performed at a pressure P2 and a temperature T2 lowerthan that prevailing in E1. This procedure may be repeated several times as long as the pressure and temperature levels remain compatible with an economical operation of the installation.

The number of stages involved may thus be defined from the point of view of an economical optimization. It must be noted that this scheme does not require any mechanical apparatus for transferring the fluid to be evaporated and the vapour produced from one stage to the next: it is the initial pressure that provides the required driving power. This scheme is particularly advantageous as it saves heating energy and accordingly, it is preferred.

In the second type of scheme the mixture from which the solvent must be recovered by evaporation is introduced into a first evaporator E1 at the lower pressure level of the installation (Fig. 2). To this pressure corresponds, as in the preceding scheme, a boiling temperature T1 in the range from 2 to 30 and preferably 5 to 10 C degrees lower than the temperature T2 at which the solvent vapour has been produced in a preceding stage E2, under a pressure P2 higher than P1.This time, the liquid, partially freed from its solvent, is fed to the top of exchanger evaporator E2 by means of a pump; the pressure difference between E1(P1) and E2 (P2) is so selected that the temperature in E2 is from 2 to 30 and prefer ablyfrom Sto 10 Cdegrees higherthanthatprevail- ing in E1, so that the vapour produced in E2 may be used as heat-carrying fluid for heating E1. This procedure is repeated as in the preceding scheme up to the attainment of the maximum temperature in the exchanger evaporator En (E2 in the case of Figure 2) where heat is introduced into the system by means of a heat carrying fluid (in most cases steam).As in the preceding case, the number of stages and the initial and final levels of the temperature and pressures will advantageously result from an economical optimization.

In the two schemes, the condensed vapours are collected and fed back to the same solvent storing means for optional re-use. The last traces of solvent may be removed from the heavy product in an auxiliary equipment. The vapour issued from stage n in scheme 1 (from stage 1 in scheme 2) is, of course, condensed in an exchanger before being fed to the storing means.

The two fundamental schemes presented here are illustrated by two examples, which concern recovery of pentane from a mixture of pentane and heavy oil from a deasphalting unit.

EXAMPLE 1 (Figure 1) In a falling-film apparatus, there is introduced a mixture of pentane and deasphalted vacuum gas oil at a rate of 2500 Kg/h. The mixture contains a proportion of 4 kg of pentane per kg of gas oil. This apparatus is formed of three tubes (P1, P2, P2) each of 5 cm diameter and 6 m in length. These tubes have been treated by shot blasting so that their surface is formed of a succession of craters having an average diameter from 0.2 to 0.5 mm. By means of a convenient distributor placed at the top of each tube, the liquid is substantially regularly distributed over the internal face of the tube. The introduction is effected through line 1 and the liquid is supplied at 3 MPa at 1900C. In exchanger E1,400 kg/h of pentane is evaporated. The required heat is supplied through steam introduced in the shell of exchanger E1 through line 10. This steam is at a temperature of about 195 C.35 kg/h of steam is condensed in this shell and discharged through line 11. The effluent from exchanger E1 consists of a stream of 2100 kg/h of liquid mixture and 400 kg/h of pentane vapour, which are easily separated at the bottom of the exchanger. The liquid discharged through line 3 is expanded through valve V1 to 2.5 MPa. As a result, pentane partially evaporates and the temperature decreases to 1800C. This mixture is introduced into an evaporator E2 identical to evaporator E1.The pentane vapour from evaporator E1 through line 2 is introduced, without expansion, into the shell of evaporator E2 where, as a result of the lower temperature, it condenses. The condensed vapour is removed through line 8. As a result of the expansion and the heat transfer from the condensed pentane vapour in the shell of appratus E2, 725 kglh of pentane is evaporated and separated at the bottom of exchanger E2 from the 1375 kg/h of an evaporated liquid.The liquid mixture is discharged through line 6 and, as at the output of exchanger E, it is expanded through valve V2 down to 1.75 MPa, thereby producing a new evaporation with a decrease of the temperature to 1700C. The 725 kg/h of pentane vapour is introduced without expansion into the shell of evaporator E2 through line 7 and discharged through line 5. The pentane vapour is also condensed by taking advantage of a temperature difference of 10 C degrees. The apparatus E2 is identical to the two preceding apparatuses E, and E2.

The above mentioned conditions for apparatus E2 provide for a new evaporation of 815 kg/h of pentane which is discharged through line 4.

The liquid consisting of 500 kg/h of oil and 60 kg/h of pentane is discharged through line 9 to a final treatment section where evaporation is completed up to the last traces of pentane, for example by stripping with steam. The only steam consumption amounted to 35 kg/h. In a conventional installation the steam consumption would have been close to 150 kg/h.

EXAMPLE2 (Figure 2) A mixture of pentane and vacuum-deasphalted gas oil is introduced into a falling film apparatus at a total rate of 2500 kg/h. The mixture contains 4 kg of pentane per each kg of gas oil. This apparatus comprises 3 tubes, each of 5 cm diameter and 6 m in length.

These tubes have been subjected to shot blasting in such a mannerthat their surface comprises a succession of craters having an average diameter from 0.2 to 0.5 mm.

The liquid streams are substantially regularly distributed, through a convenient distributor, onto the internal walls of the three tubes. The introduction is effected through line 21 at a temperature of 1 500C and a pressure of 1.9 MPa. By introducing 550 kg/h of pentane vapour at 1 600C in the shell of exchanger E1, 200 kg/h of pentane evaporates and the gas-liquid mixture is heated to 1 600C. This pentane vapour, introduced in the shell of apparatus E, through line 25 is obtained from an evaporation step taking place in evaporator E2, as explained hereinafter. This pentane vapour, condensed in the shell of appratus E" is discharged through line 24.

The pentane vapour (200 kg/h) produced in exchanger E, is separated from the liquid phase and discharged for condensation or direct recycling at another point of the installation, through line 22. The 2300 kg/h of liquid mixture is then fed through duct 23 to the falling film apparatus E2, identical to the preceding one. The apparatus E2 operates at 2.2 MPa and 1 700C so that a pump P, is necessary to increase the liquid pressure from that of E, (1.9 MPa) to that of E2 (2.2 MPa). The liquid is then reheated and partially evaporated in apparatus E2 by means of 1200 kg/h of pentane vapour produced in exchanger E3, as explained hereinafter. The condensed pentane is discharged through line 27.This evaporation generates 550 kg/h of pentane vapour which, after separation at the bottom of apparatus E2, is fed through line 25 to the shell of apparatus E,. The liquid mixture remaining at the bottom of exchanger E2 is discharged through line 26 and compressed by pump P2 up to the feeding pressure of evaporator E3. The stream passing through pump P2 amounts to 1750 kg/h. Evaporator E2 is identical to E, and E2. It is operated at a pressure of 2.4 MPa and at a temperature of 180 C. The 1750 kg/h of liquid fed at the top of this exchanger is reheated and evaporated by heat exchange with 120 kg/h of steam supplied to the shell of apparatus E2 through line 30. This results in the evaporation of 1200 kg/h of pentane, which is fed to the shell of apparatus E2 through line 28. The unevaporated liquid mixture, consisting of 50 kg/h of pentane and 500 kg of gas oil, is discharged through line 29 and the condensed pentane is discharged through line 28. This mixture may be subjected to a final treatment for recovering pentane traces, for example by stripping with steam. The vapour consumption was 120 kg/h. In a conventional installation 320 to 340 kg/h of steam would have been necessary.

The condensed steam is discharged through line 31.

Claims (13)

1. A method of recovering a volatile organic compound from a liquid mixture containing the said compound with at least one liquid organic compound of lower volatility, comprising the circulating liquid mixture through two or more serially arranged evaporation stages each of which comprises a vertical wall, causing the liquid mixture in each stage to flow as a falling film on a first side of the wall of the said stage so as to progressively evaporate the volatile organic compound, the said first side having cavities of an average diameter from 50 clam to 2 mm, maintaining each stage at a temperature and pres- sure different from those of the other stages, causing the vapour of the volatile organic compound produced in at least one evaporation stage to flow in contact with the second side of the vertical wall of at least one other evaporation stage operated at a temperature and a pressure lower than those of the said one stage so as to condense the vapour and to transfer its condensation heat to the liquid mixture through the corresponding wall, maintaining a difference of temperature from 2 to 30 C degrees between the vapour and the liquid mixture flowing respectively in contact with the second side and the first side of the vertical wall.

2. A method according to Claim 1, in which the stages are operated under pressures that decreases from the inlet for the mixture to be evaporated to the outlet for the compound of lower volatility, the vapours produced and the liquid mixture flowing as a whole co-currently through the evaporation stages, which are serially arranged, the vapour produced in a given stage being condensed in the next stage, the circulation on both sides of the wall at a given stage being effected co-currently or counter-currently, the first stage being itself heated by means of an externally supplied heat-carrying fluid.

3. A method according to Claim 2, in which in at least one stage the vapour formed in this stage and the liquid mixture from which this vapour has formed descend co-currently on the same side of the wall.

4. A method according to Claim 2, in which in at least one stage the vapour formed in this stage flows upwardly in counter-current with and on the same side of the wall as the liquid mixture from which it has formed.

5. A method according to Claim 1, in which the evaporation stages are operated under pressures that increase from the inlet for the liquid mixture to be evaporated to the outlet for the organic compound of low volatility, the vapour produced in one stage being condensed in the preceding stage, thereby creating a counter-current between the mixture to be evaporated and the vapour produced in the evaporation stages, the vapour being condensed and the liquid being vaporized circulating cocurrently or counter-currently at each stage, the last stage being heated with an externally supplied heat-carrying fluid.

6. A method according to Claim 5, in which in at least one stage the vapour formed in this stage and the liquid mixture descend co-currently on the same side of the wall.

7. A method according to Claim 5, in which in at least one stage the vapour formed in this stage flows upwardly in counter-current with and on the same side of the wall as the liquid mixture from which it has formed.

8. A method according to any one of Claims 1 to 7, in which the cavities have an average diameter in the range 100 ,um to 200 clam.

9. A method according to any one of Claims 1 to 8, in which the cavities have been produced by projecting particles of a hard material against the wall.

10. A method according to any one of Claim 1 to 9, in which the liquid mixture comprises at least one deasphalted heavy oil and at least one hydrocarbon containing from 3 to 7 carbon atoms in the molecule.

11. A method according to Claim 1 carried out in apparatus substantially as hereinbefore described with reference to Fig. 1 or 2 of the accompanying drawings.

12. A method according to Claim 1 carried out substantially as hereinbefore described in Example 1 or2.

13. Volatile organic compounds recovered by a method according to any one of Claims 1 to 12.