GB2126496A - Removal of condensable vapour - Google Patents

Abstract

Gas containing condensable vapour (e.g. siloxane) is passed over a cooling coil 16 and simultaneously subjected to a spray 18 of cooling liquid. Sprayed liquid and solid condensate collect in a reservoir 34 and are separated. The liquid may be cooled by refrigeration plant 52 and recycled. Heat rejected by the plant 52 may be used to reheat the treated gas, which is then recycled through an evaporation chamber 10 containing articles from which the vapourisable material is to be extracted. <IMAGE>

Description

SPECIFICATION Removal of condensable vapour The present invention relates to a method and apparatus for the removal of a condensable vapour. It particularly relates to the removal by solidification of a vapour from air. In another aspect, it relates to the removal and recovery of a volatile solid from a material containing it.

The present invention is particularly applicable to the removal and recovery of a siloxane from a stream of gas such as air. Siloxanes are used in the fabrication of certain moulded products, e.g. to render a waxy material more easily pourable. It may be desired to remove the siloxane from the moulded product, and this may be achieved by heating the product slightly above room temperature, by passing warm air over it. The siloxane vapourises, and is carried away by the airstream.

It is becoming increasingly unacceptable simply to discharge the siloxane-bearing air into the environment. Attempts have been made to treat the air before it is discharged, by passing it through a burner. This is not an ideal solution, since it merely changes the nature of the pollutant. Furthermore, such use of a burner is wasteful of energy, and of course the siloxane is wasted too.

According to a first aspect of the present invention, there is provided a method of extracting from a gas a material in varpour form, comprising passing the gas over a cooling coil and simultaneously subjecting it to a cooling liquid spray so that at least a part of said material is solidified and removed from the gas.

It will generally be desirable that the liquid should not react with the material to any significant extent. To facilitate the recovery of the material, it may be preferable for it to be substantially insoluble in the liquid, and of a substantially different density. For example, a siloxane is conveniently extracted using a spray of water, in which it is insoluble, and on which if floats. Thus the sprayed liquid and the solidified material can be collected in a reservoir, from which the material can be easily recovered.

In another aspect, the invention provides a method of recovering a vapourisable material from a product, comprising passing air over the product at a temperature sufficient to entrain vapour of the material, and then extracting the material from the gas as described above. The heat required for the vapourising step can be supplied, at least in part, by the heat rejected by a refrigeration plant used to cool the coil and the liquid for the spray.

In further aspects the invention provides apparatus for performing the methods described above.

A preferred embodiment of the invention will now be detached in greater detail with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a plant intended for the removal and recovery of a siloxane; Figure 2 is a more detailed view on a larger scale of a spray chamber tank in vertical longitudinal section; and Figure 3 is a schematic diagram of the water circuitry.

An evaporation chamber 10 (which will generally contain racks of products from which siloxane is to be evaporated) has at one end an inlet for heated air. At the opposite end, the chamber narrows to form a take-off head 12 for extraction of air. The air, containing siloxane, passes thence to a spray chamber 14. Here it is cooled by passing over a cooling coil 16 while cooling water is sprayed through it from a spray header 18. This precipitates the siloxane, as will be described later. The air then passes upwardly through a conduit 20 to an eliminator section 22 where liquid particles entrained in it are removed. Thence it passes to a heater 24.

This comprises two batteries (or extended fin coils): a heat recycling battery 26, and an electrically heated battery 28. The rate of transfer of heat to the gas by the heater 24 may be under the control of a thermostat 30 in the evaporation chamber 10. From the heater 24, the air passes back into the evaporation chamber 10 through a conduit 32 containing a fan.

The temperature required in the drying chamber to obtain a satisfactory rate of evaporation of siloxane may depend on such factors as the nature of the products and the air velocity through the chamber. Of course, use of a high temperature means that much heat has to be removed in the spray chamber. Furthermore, the products may be heat-sensitive. Good results have been obtained with a drying chamber temperature in the range 1 8-35'C, preferably 25-28 C, with an air velocity of about 1 m/s, and an air throughput of about 4 m3/s. Good removal of the siloxane can then be achieved in the spray chamber by using temperature for the cooling coil and cooling spray water in the range 5-1 5,C, preferably about 7 C.

The combined effect of the chilled water spray and the cooling coil is that solidifed siloxane collects on the cooling coil 16, from where it falls into the collection tank 34, which also collects the spray water.

It has been found that the combination of the chilled spray and the cooling coil is remarkably more effective than either on its own.

One form of collection tank 34 and associated apparatus is shown in Fig. 2. The collection tank 34 is partitioned by a number of weirs and barriers. Thus, a central reservoir 36 is bounded by left- and right-hand weirs which extend across the tank 34. The left-hand weir 38 is slightly higher than the right-hand weir 40. Within the central reservoir 36, near the right-hand weir 40, a barrier wall 42 extends downwardly some way below the level of the top of the righthand weir 40. The spray from the chamber 14 and the solidified material are arranged to fall into the central reservoir 36 between the left-hand weir 38 and the barrier wall 42.

In the central reservoir just to the left of the barrier wall 42 there may be a spray header 46.

This extends across the reservoir 36, and is arranged to emit a spray of chilled water substantially horizontally towards the left-hand weir 38.

Thus, in use, spray water trickling down from the spray chamber 14 passes into the central reservoir 36, and overflows across the right-hand weir 40. Lumps of siloxane float on the water in the reservoir 36, and are driven by the spray from the spray header 46 to the left, so that they fall over the left-hand weir 38. Of course, some water will accompany them. The barrier wall 42 prevents any significant amount of solid from passing to the right. But most of the water does pass to the right, into the end chamber 48, through which it can be drawn off through conduit 50. The wall 42 also provides an air trap.

Reverting to Fig. 1, a refrigeration plant 52 is represented schematically by an evaporator 54 and a condenser 56. The evaporator 54 receives water circulating through the collection tank 34 and chilling assembly (16, 18, 46), and returns it thereto in a chilled state. From the tank 34, water is taken off (from the compartment beyond the left-hand weir 38} by means of a spray pump 58. All or part is circulated through the cooling coil 16, and it is then sprayed through the spray header 18 (and header 46 is present). Its passage through the coil 16 and, as spray from the headers 18, 46, causes it to be warmed before it returns to the tank 34. It is then recycled by pump 55 through the evaporator 54 to be cooled.

The heat taken from the cooling water in the evaporator 54 is transferred to a heating fluid in the condenser 56. This heating fluid (e.g. water) is circulated through the heat recycling battery 26, where it gives up heat to the air which has passed through the spray chamber 14. Thus, in the spray chamber 14, the air gives up heat to the cooling water, and this heat is returned to it by the recycling battery 26 via the evaporator 54 and the condenser 56. However, since the refrigeration plant 52 cannot work with perfect efficiency, the amount of heat it produces is geater than that which it extracts in the evaporator 54. Thus, the amount of heat potentially available from the condenser 56 is greater than the amount extracted from the air in the spray chamber 14.To prevent the air from getting ever hotter as it recycles, a closed circuit water cooler 58' is provided, for rejecting heat to the exterior. This cooler 58' comprises a coil in parallel with the battery 26. A three-way valve 59 controls the routing of water through the battery 26 and/or the cooler 58', and serves to control the pressure in the condenser. A bypass conduit 60 bridges the battery 26. Passage through it is controlled by a three-way control valve 62. This enables the battery 26 and the condenser 56 to be decoupled, or coupled to a desired extent. (Figs. 1 and 3 show alternative configurations of the bypass conduit 60 and valve 62, but it will be appreciated that they can achieve similar results.) The valve 62 serves to control the heat output of the battery 26. This valve 62 (and/or valve 59) may be controlled by the chamber thermostat 30.Possible electrical control circuitry is indicated in Figs. 1 and 3 by broken lines.

For situations when there is insufficient heat available from the condenser 56 for heating the air (e.g. during maintenance or breakdown of the refrigeration plant) there is also the electrically heated battery 28 which can be actuated.

Under normal running conditions, an actual embodiment of the apparatus employed in the evaporation chamber 10 air at 25 C. The water in the spray chamber 14 was at 7 C, and this chilled the air to 1 8'C. The heat recycling battery 26 was sufficient to return this to 25'C. (This was with a flow-rate of about 4 m3/s as described above.) Of course, other circumstances and other volatile materials may lead to other conditions being preferred. To give more detailed actual figures of an example, referring to Figs. 2 and 3, the airflow through the condensor 56 was 10.9m3/h. This was partitiond between the battery 26 (7.3m3/h) and the cooler 58' (3.6m3/h). Water passed through the evaporator 54 at 1.8m/h and was returned to the reservoir at 7 C.

For recovery of siloxane, water is a convenient liquid to spray. Siloxane does not react with it, and the floating lumps are easy to collect. The material can simply be dried, and then, if necessary, purified by redistillation.

The material with which the illustrated embodiment is concerned is polydimethysiloxane, whose repeating unit is -(CH3)2Sio-. Some physical data follow.

Boiling point 1 76 C Flash point 52 C Freezing point 18'C Vapour pressure 74 mm Hg at 20 C Vapour density (air = 1) 10 (by volume) Specific gravity of solid 0.95 Evaporation rate (ether = 1) 0.03 The skilled reader will appreciated how the process conditions should be adapted for other materials.

Claims (17)

1. A method of extracting from a gas a material in vapour form, comprising passing the gas over a cooling coil and simultaneously subjecting it to a cooling liquid spray so that at least a part of said material is solidified and removed from the gas.

2. A method according to claim 1 wherein the material is substantially insoluble in the liquid, and of a substantially different density.

3. A method according to claim 1 or claim 2 wherein the solidified material floats on the sprayed liquid, and wherein the liquid and the solidified material are collected in a reservoir whence the material is collected.

4. A method of extracting material from a gas substantially as described herein with reference to and as illustrated in the accompanying drawings.

5. A method of recovering a vapourisable material from a product, comprising passing air over the product at a temperature sufficient to entrain vapour of the material, and then extracting the material from the gas by a method according to any preceding claim.

6. A method according to claim 5 wherein the coil and liquid spray are cooled using a refrigeration plant, and heat rejected thence is used in the vapourisation step.

7. A method of recovering a vapourisable material from a product substantially as described herein with reference to and as illustrated in the accompanying drawings.

8. Apparatus for recovering a volatile material comprising a cooling chamber containing a cooling element and means for spraying a cooled liquid, the arrangement being such that gas containing vapour of the material can be passed through the chamber and subjected simultaneously to the element and the spray to effect condensation of the material.

9. Apparatus according to claim 8 having a reservoir for collecting sprayed liquid and condensed material, the reservoir having means for separating the sprayed liquid from the condensed material.

10. Apparatus according to claim 9 wherein liquid collecting in the reservoir is cooled and recycled through the spray and/or through the cooling element to provide its cooling effect.

11. Apparatus according to claim 9 or claim 10 for use with material which condenses as a solid which floats on the sprayed liquid, the reservoir having a weir over which the liquid tends to pass, and a barrier extending adjacent the level of the top of the weir for restraining floating solid from passing over the weir.

12. Apparatus according to claim 11 further including a spray header arranged to spray liquid across the surface of the liquid in the reservoir to drive floating solid away from the weir.

13. Apparatus according to claim 12 wherein the reservoir has a second weir, higher than the first, and the barrier and spray header are intermediate the two weirs so that floating solid tends to pass over the second weir with not more than a minor proportion of the liquid.

14. Apparatus according to any of claims 8 to 13 further including an evaporation chamber for treating material comprising the volatile material in a condensed state with gas to volatile at least some of the volatile material; and means for conducting gas containing volatilised material thence to the cooling chamber.

15. Apparatus according to claim 14 including refrigeration plant arranged to draw heat from the cooling element and spray and transfer it to gas for use in the evaporation chamber.

16. Apparatus according to claim 15 including means for recycling gas through the evaporation and cooling chambers, and means for dumping excessive heat in the gas from the system.

17. Apparatus for recovering a volatile material substantially as described herein with reference to and as illustrated in the accompanying drawings.