GB2145343A - Improved solvent recovery - Google Patents

Abstract

Solvent recovery from a gas mixture is effected with the mixture flowing upwardly past an array of horizontal tubes (5) which carry a heat exchange fluid (e.g. a primary refrigerant). Solvent condensed on the upper tubes in the array (5) drips down counter current to the gas mixture. Thermal recuperator coils (3, 7) may be linked in a run-around system by pipes (10) with a pump (11). Solvent in a sump (9) can be warmed by the incoming gas mixture via a spray pipe (6). <IMAGE>

Description

SPECIFICATION improved solvent recovery In a number of different industrial processes expensive and noxious organic solvents are used and need to be removed from an air based gas mixture, before the air is discharged to the atmosphere.

The extent to which solvent is removed from the gas mixture can be dictated by the cost of the solvent or by regulations relating to environmental pollution, but in general there is an increasing demand for higher proportions of solvent to be recovered from the gas mixture and for smaller amounts to be discharged to the atmosphere.

Removing organic solvents from a gas mixture by condensing the solvent as the gas mixture is cooled has proved an acceptable solution but only some 90% of the entrained solvent has usually been recovered in this way, a substantial part of the remaining 10% being absorbed in a bed of carbon.

Taking acetone as a typical example, the gas mixture leaving a processing plant might be at a temperature upwards of 50 C. contain a ton of acetone per hour and provide a gas mixture of 50:50 by weight air and solvent.

Passing the gas mixture through a heat exchange coil of a refrigeration plant causes it to be cooled and condensation of the acetone from the gas mixture would start at around +30 C. The higher the proportion of acetone which has to be extracted from the gas mixture the greater the cooling of the gas mixture has to be. To reduce the solvent content to 10% w/w an exit temperature of around -10 C would be required whereas to reduce the solvent content to 2% w/w would require a temperature of -35 C. Temperatures as low as -50 C might be required in some cases.The increasing difficulty of extracting the last traces of solvent by cooling the gas has been previously recognised and heretofore it has been accepted that the last few percent by weight of solvent will have to be taken out by a carbon absorber (and thus not directly recovered without a further extraction procedure).

As the outlet temperature of the tail gas from the heat exchange coil is reduced, not only do the operating costs rise sharply but the colder are the gas and liquid leaving the recovery plant. The colder are the gas and liquid, the greater are the problems relating to the thermal insulation of the ducts provided to convey them.

With the trend towards higher and higher levels of direct solvent recovery, there is a need for more effective and cost efficient solvent recovery plants and this invention relates to such an improved plant and to an improved method of recovering an organic solvent from a process off-gas mixture.

According to one aspect of this invention a method of recovering an organic solvent, present in its vapour phase in a gas mixture, from a carrier gas of the mixture, which method comprises cooling the gas mixture by feeding it past an array of horizontal tubes extending transversely of the stream of gas mixture, through which tubes a cooling fluid is passing and collecting solvent condensed on the surfaces of the tubes, is characterised in that the gas mixture is fed upwardly around the array of tubes so that the condensed solvent flows counter current to the upwardly flowing gas mixture.

In a known type of solvent recovery plant, the gas mixture passes horizontally around the array of horizontal transverse tubes. With this arrangement the tubes at the gas inlet end of the array tend to run with their exposed surfaces to a large extent dry of solvent and the solvent dripping off the tubes at the gas outlet end is at the lowest temperature.

Dry operation at the inlet end is now seen to be undesirable not only from considerations of optimising the heat exchange performance of the tubes, but also because it allows a build-up of particles carried over with the gas mixture, on the tubes at the inlet end. By the simple expedient of ensuring an upward flow of the gas mixture through the array of horizontal tubes, we have provided more efficient operation with no risk of fouling of gills or fins on the tubes by particle carry-over.

Desirably the array of tubes is preceded and succeeded by a linked pair of coils serving as a thermal recuperator, the lower of such a pair of coils then being flooded with solvent dripping from the array of tubes. The pair of coils can form a conventional closed circuit run-around system or recovered solvent can be pumped in a single pass through the upper coil to warm the tail gas leaving the tube array and the resulting cooled solvent discharged through the flooding header to precool the incoming gas mixture.

A sump for recovered solvent can be located below the lower of the pair of coils used for a runaround system and means provided for spraying solvent drawn from the sump on to the upper surface of the lower of the pair of coils. This arrangement ensures that the pre-cooling coil is always flooded with liquid.

The array of tubes can serve as the direct expansion evaporator of a compression refrigeration plant. Alternatively, for larger capacity plants, the array of tubes can form part of a flooded or pumped circulation primary refrigerant system, or it is possible to circulate a secondary refrigerant through the array of tubes.

Suitably, the speed of flow of the gas mixture upwardly around the array of tubes is not more than 1.27 metres/second (250 ft/minute), since this ensures negligible entrainment of droplets.

According to a further aspect of the invention, a solvent recovery plant comprises an upwardly extending tower having a tail gas discharge vent at or near the top and a solvent/air mixture inlet at or near the bottom, a solvent condenser coil intermediate the inlet and outlet with the heat exchange tubes thereof extending horizontally, means to circulate a cold fluid through the heat exchange tubes of the solvent condenser coil, precooling means located between the solvent/air mixture inlet and the underside of the solvent condenser coil, a heat re claim coil located between the top of the solvent condenser coil and the tail gas discharge vent, and duct means linking the precooling means and the heat reclaim coil to allow a heat exchange liquid to flow from one to the other.

The precooling means can be a spray means spraying cooled solvent into the upwardly flowing gas/solvent mixture, but desirably a spray means is located between the solvent condenser coil and a precooling coil in circuit with the reclaim coil and suitably this spray means draws liquid from a sol vent collection sump below the precooling coil.

A liquid-droplet eliminator can be provided above the solvent condenser coil.

A preferred plant according to the invention can show significant energy savings (e.g. between 30% and 60%) over comparable prior art solvent recov ery plants.

The accompanying drawing schmatically illustrates one example of a solvent recovery plant ac cording to this invention.

A vertical tower 1, contains in descending order a tail gas discharge vent 2, a heat reclaim coil 3, a droplet eliminator 4, a solvent condenser 5, a flooding header 6, a precooling coil 7, a solvent/air mixture inlet 8 and a solvent sump 9.

The coils 3 and 7 are linked via run-around pip ing 10 containing a pump 11 and linked to a header tank 12.

The eliminator 4 can be a mesh pad or other conventional design of liquid droplet eliminator but -is not essential since with the low gas flow rates employed in the tower 1 droplet entrainment oc curs to a minor extent.

The solvent condenser 5 comprises a plurality of vertically disposed side-by-side sinuous tubes each extending in parallel from a lower header 5a to an upper header 5b and providing horizontal runs of finned or gilled tubing. The condenser 5 serves as the evaporator of a compression refrigeration plant including a condenser 13, a compressor 14 and an expansion valve 15. The expansion valve is controlled (via line 16) from the outlet from the upper header 5b of the coil of the condenser 5 in the conventional manner. A solenoid valve 17 and oil separator 18 are also shown. Compressed liquid refrigerant is fed to the lower header 5a and this evaporates in each sinuous tube leaving the upper header 5b substantially entirely vaporised.

The flooding header 6 is fed from a pipe 19 which includes a pump 20, drawing solvent from the sump 9. A float valve 21 can operate a by-pass valve to direct solvent to a line 22 to a storage receiving vessel (not shown).

The apparatus shown operates as follows: A solvent vapour/air mixture is admitted through the inlet 8 from whence it rises through the precooling coil 7.

The pre-cooling coil utilises heat exchange with the heat reclaim coil 3 serving the dual purpose of heating the waste gas leaving the tower 1 at 2 and at the same time economising in refrigeration by pre-cooling the gas mixture at 7. The pre-cooled gas then passes the flooding header 6 wherein the collected solvent from the sump 9 is pumped at a controlled rate by the pump 20. The object of this is to again ensure that any cooling value in the solvent collected in the sump 9 is utilised for further pre-cooling and at the same time warms the solvent before this is delivered to the storage receiving vessel. The header 6 ensures that the precooling coil 7 is at all times flooded with liquid to ensure both maximum heat transfer and a reduction in the danger of its being fouled by particles entering with the gas mixture.

The pre-cooled air/vapour mixture is further cooled in the solvent condenser 5 from its entry saturated condition to the reduced saturated condition required to maintain the reduced percentage of solvent in the air leaving in the tail gas. The solvent condenser 5 shown in the drawing is a refrigerant evaporator and this could be either direct expansion (as described), a flooded refrigerant system, a pumped refrigerant circulation or even a low temperature secondary refrigerant coil. It is essential that the finned or gilled tubes in the tube array in the coil 5 are horizontal in order that the condensing solvent will drain off the coils falling naturally on to the flooding headers 6 and so through the pre-cooling coil 7. This ensures that this very cold solvent will effect a sensible heat exchange with the rising pre-cooled gas.

Although, as mentioned, the solvent condenser 5 is designed to operate at low velocity where there is little possibility of liquid droplet carryover, the droplet eliminator 4 also serves as a barrrier to radiant heat transmission between the low temperature coil 5 and the high temperature heat reclaim coil 3.

There are three main variants to the arrangement shown in the drawing, the first being the possibility of splitting the coil 5 into two sections each served by a separate refrigeration plant. This variant is of particular importance where there is a need to discharge tail gas directly to atmosphere and thus a requirement for a very low dew point temperature or a minimum concentration of solvent in the tail gas discharge.

Under these circumstances, it is normally advantageous to use two separate refrigeration plants which can operate at different evaporating temperatures rather than to supply one large unit capable of taking the complete load whilst operating at the lowest evaporating temperature and by necessity being very much less efficient in running costs.

The second variant utilises the collected solvent for pumping directly through the heat reclaim coil 3 and discharging the solvent through the flooding header 6 back to the sump 9. The coil 7 will be replaced by either a series of baffles to ensure intimate contact between the incoming solvent gas/air mixture and the liquid solvent or alternatively some form of mesh pad to achieve the same result.

The third variant would be the omission of the flooding header 6, which under certain circumstances is of limited value particularly when the solvent air mixture is rich in solvent.

The main advantages of the invention are: (1) A vertical gas flow. This is important in that droplets of liquid from the solvent condenser 5 and pre-cooling coil 7 drain vertically downwards which is a great advantage. This is particularly important in the case of the solvent condenser coil 5 where if the entry condidtion of the coil is e.g.

+20"C and the leaving condition of the coil at is -30 C, then solvent will be condensing at varying temperatures from the top to the bottom of the coil and the very low temperature condensate at -30 C will drain down over the face of the coil to increase the cooling effect at the base of the coil where the gas/air mixture is at very much higher temperature.

(2) The introduction of the heat recovery system coils 3 and 7. This reduces the capital cost of refrigeration and also the running cost in terms of compressor power. It further serves to heat the tail gas from the very low temperature at which it leaves the coil 5 to a very reasonable temperature of in the order of +10"C thus eliminating the necessity for very expensive insulation on the tail gas pipework.

(3) By using an interchange of heat between the incoming solvent/air mixture and the solvent returned to the sump this solvent is delivered at a moderate temperature, again avoiding the necessity for expensive insulation.

Claims (18)

1. A method of recovering an organic solvent, present in its vapour phase in a gas mixture, from a carrier gas of the mixture, which method comprises cooling the gas mixture by feeding it past an array of horizontal tubes extending transversely of the stream of gas mixture, through which tubes a cooling fluid is passing and collecting solvent condensed on the surfaces of the tubes, characterised in that the gas mixture is fed upwardly around the array of tubes so that the condensed solvent flows counter current to the upwardly flowing gas mixture.

2. A method as claimed in claim 1, in which the array of tubes is preceded and succeeded by a linked pair of coils serving as a thermal recuperator, the lower of such a pair of coils being positioned to be flooded with solvent dripping from the array of tubes.

3. A method as claimed in claim 2, in which the pair of coils are connected together in a closed circuit run-around system.

4. A method as claimed in claim 3, in which a sump for recovered solvent is located below the lower of the pair of coils used for the run-around system and means is provided for spraying solvent drawn from the sump on to the lower of the pair of coils.

5. A method as claimed in claim 1, in which an upper heat exchange coil is located above the array of tubes and recovered solvent is pumped in a single pass through the upper coil to warm the gas leaving the tube array and the resulting cooled solvent from the upper coil is discharged through a flooding header to precool the incoming gas mixture.

6. A method as claimed in any preceding claim, in which the array of tubes serves as the direct expansion evaporator of a compression refrigeration plant.

7. A method as claimed in any of claims 1 to 5, in which the array of tubes form part of a flooded primary refrigerant system.

8. A method as claimed in any of claims 1 to 5, in which the array of tubes is cooled by the pumped circulation therethrough of a primary refrigerant.

9. A method as claimed in any of claims 1 to 5, in which a secondary refrigerant is circulated through the array of tubes.

10. A method according to any preceding claim, in which the speed of flow of the gas mixture upwardly around the array of tubes is not more than 1.27 metres/second (250 ft/minute).

11. A method according to any preceding claim, in which there are two separate arrays of horizontal tubes around which, in sequence, the gas mixture is upwardly fed, each said separate array being fed with heat exchange medium from a respective refrigeration plant.

12. A method of recovering an organic solvent from a carrier gas substantially as hereinbefore described with reference to the accompanying drawing.

13. A solvent recovery plant comprising an upwardly extending tower having a tail gas discharge vent at or near the top and a solvent/air mixture inlet at or near the bottom, a solvent condenser coil intermediate the inlet and outlet with the heat exchange tubes thereof extending horizontally, means to circulate a cold fluid through the heat exchange tubes of the solvent condenser coil, precooling means located between the solvent/air mixture inlet and the underside of the solvent condenser coil, a heat reclaim coil located between the top of the solvent condenser coil and the tail gas discharge vent, and duct means linking the precooling means and the heat reclaim coil to allow a heat exchange liquid to flow from one to the other.

14. Plant as claimed in claim 13, in which the precooling means is a spray means adapted to spray cooled solvent into the upwardly flowing gas/solvent mixture.

15. Plant as claimed in claim 13, in which a spray means is located between the solvent condenser coil and a precooling coil in circuit with the reclaim coil.

16. Plant as claimed in claim 15, in which the spray means draws liquid from a solvent collection sump below the precooling coil.

17. Plant as claimed in any of claims 13 to 16, in which a liquid-droplet eliminator is provided above the solvent condenser coil.

18. A solvent recovery plant substantially as hereinbefore described with reference to the accompanying drawing.