EP0971784A1

EP0971784A1 - A system for preventing the emanation of vapors, odors, smoke and dusts in open spaces

Info

Publication number: EP0971784A1
Application number: EP98960064A
Authority: EP
Inventors: Ram Lavie; Yoav Rokman; Yehoshua Benjamin; Alexei Griniakov; Ela Harlev
Original assignee: Hme Separation Technologies Ltd
Current assignee: Hme Separation Technologies Ltd
Priority date: 1998-01-14
Filing date: 1998-12-15
Publication date: 2000-01-19
Also published as: AU1574499A; WO1999036157A1; IL126862A0

Abstract

The present invention relates to a system for preventing the emanation of entrained substances such as: vapors, smoke, odors, or dusts, from open spaces. In particular, the invention relates to a system for collecting and recovering organic vapors which emanate in open spaces and their recycling in a liquid form to be reused, or disposed of. The system consists of a directed nozzle (a) or set of nozzles, located close to the source of the emanating entrained substances and facing away from the source and a pumping device (b) with suction from the nozzle(s) and discharging its flow into an accumulation or recovery system (c). The entrained substances accumulation system is selected from an adsorbing material, an absorbing liquid, a washing liquid, or a dust bin.

Description

A SYSTEM FOR PREVENTING THE EMANATION OF VAPORS, ODORS, SMOKE AND DUSTS IN OPEN SPACES.

The present invention relates to a method and a system for preventing the emanation of entrained substances such as vapors, smoke, odors or dust from open spaces. More particularly, the invention relates to a method and a system for collecting and recovering organic vapors, which emanate in open spaces and their recycling in a liquid form to be reused or disposed of. An open space is defined as a region in space, at some distance from the source of the emission ("the Source"). This Source is freely connected to other regions in space ("the Surroundings") that are to be protected from the intrusion of the said entrained substance. For example: the immediate region surrounding an emission source in the open air is an "Open Space" according to our definition. Another example: the immediate region surrounding an emission source, when both are contained within a permanent total enclosure (a building, or a room, or an enclosure, that has been set specifically around a working region such as to contain a discharge of gas or vapor to other regions in space, etc.) is also defined as an "Open Space".

BACKGROUND OF THE INVENTION.

As known, there are many situations where harmful and/or valuable volatile liquids, emanate from an open vessel or existing vents and leaks. A typical example is the situation where degreasing baths, as widely used in the electronic field, or in the metal-working, or in the optical and the aerospace industries. In all these cases, there is a need for an access to the respective bath, thus causing the escape of the volatile liquid, generally organic solvents, to the surrounding atmosphere. In this manner, a health hazard is raised for people in the vicinity, besides the loss of valuable vapors of the respective solvent. The common solution, which is generally used, is to add freeboard heights to the bath and cooling the perriphery by refrigeration. Another problem is encountered with a "cooking" vessel which has to be opened for visual inspection, or for adding some ingredient, in which exists a suction hood at the upper section of the working space. However, this solution is not so effective due to the dilution of the vapors with large amounts of the surrounding air, which complicates the recovery of the valuable vapors. A method recently encouraged by the Environmental Protection Agencies is to contain a plant or a piece of equipment including a Source, in a Permanent Total Enclosure (PTE), such as to minimize emission to the environment. This approach, while quite effective from the point of view of protecting the environment, is often too restrictive for convenient operation and maintenance of the plant or equipment. Thus, it may be advantageous, to have within the PTE a local system for the collection and recovery of the emission.

The above serious problem is evidenced by a large number of quite recent patents. Thus, according to U.S. Patent Number 5,681 ,369, an apparatus is described for recovering volatile liquid vapors from an air-volatile liquid mixture, based on the adsorption of a volatile vapor, followed by regeneration of the adsorbent through pumping and absorption.

In another recent U.S. Patent Number 5,671,612, a process and apparatus are described for recovering volatile vapors from a mixture of air and a volatile compound. The process includes the cooling of the mixture to condense volatile liquid vapors and moisture and collecting thereof. The cooled and dehumidified mixture is circulated through a bed of adsorbent, thus desorbing and recovering the volatile liquids from the bed by absorption and condensation. According to the U.S. Patent Number 5,581 ,911 , a system is provided for recovering volatile-liquid vapor from an air-volatile liquid vapor mixture. The system includes a reaction vessel, which has a bed of adsorbent for adsorbing the volatile liquid vapor, which is then recovered by means of a vacuum pump.

Another approach was described in the U.S. Patent Number 5,426,945 where the volatile organic compounds are recovered from an air-volatile liquid vapor mixture. The process consists of condensation followed by adsorption and regeneration by means of an absorption tower.

According to the U.S. Patent Number 5,006,138, a vapor recovery system is described, wherein the condensable organic compounds, contained in a hydrated air mixture are removed, first by dehydrating the vapors using refrigeration, then followed by an indirect heat exchange with a refrigerant gas at cryogenic temperature in a chiller.

The desirability of preventing discharge of solvent vapors to the environment has also received attention. According to the US Patent Number 5,051 ,135, a cleaning method whereby care is taken to carry out the cleaning operation in a closed tank is described.

Attention has also been given in the recent patent literature to the means of collecting the vapors and ducting them toward the recovery apparatus.

According to US Patent Number 5,565,070, a solvent vapor sucking method is described where a suction pipe is located at a position within the container between the liquid surface of the solvent and an upper edge of the container, wherein the opening of the suction pipe is facing downward, and drawing the vapor from the upper surface of the solvent in the container. The above brief review clearly shows the importance attributed to the recovery of organic vapors, and preventing the emanation of unpleasant smoke, odors, or dust, which emanate in open spaces, all being referred to hereafter as "Entrained Substances".

It is an object of the present invention to provide a system for preventing the above "Entrained Substances" from substantially reaching the surrounding environment, by collecting them through a suitably located and directed suction nozzle(s) at a well defined rate. It is a further object of the present invention to recover the collected vapors in a liquid form to facilitate the recycling or disposal of the respective liquid. It is in particular an object of the present invention, to provide a system for the recovery of solvent vapors emanating from vapor degreasing baths. An additional aspect addressed by the present invention concerns the prevention of harmful or unpleasant Entrained Substances from polluting nearby surroundings and the environment. A common problem afflicting modern civilization is the release of vapors, smoke, odors and dust in various human activities that it would be beneficial to prevent or reduce. For example, cigarette, cigar and pipe smoking, vapors and odors that emanate from a cooking stove, smoke and odors from a barbecue stand, etc. It is a further object of the present invention to provide a mechanism that can be adapted to such undesirable emissions and reduce their expansion to the surrounding space.

BRIEF DESCRIPTION OF THE INVENTION.

The invention relates to a system for preventing Entrained Substances, as defined above, in open spaces from spreading the surrounding space and their accumulation or recovery. The following description pertains specifically to the collection and recovery of a vapor. The system consists of three main parts:

(a) a directed nozzle or system of nozzles , located close to the source of the Entrained Substances and facing away from the source, the rate of suction through the nozzle(s) being calculated to generate a forced flow field that is directed away from the open surrounding space and towards the nozzle(s), and having a velocity such as to counteract closely the spontaneous convection and diffusion currents of the Entrained Substances from the source to the surrounding. When winds or currents may disrupt the directed flow field, a shroud around the source and nozzle(s) region may be added to protect this region; (b) a pumping device with suction from the nozzle(s) and discharging its flow into a an accumulation or recovery system wherein the Entrained Substances are accumulated for future recovery and/or disposal, or wherein they are converted into a liquid or solid condensed form for immediate recovery or disposal.

Optionally, when a further separation is necessary, the remaining gas-vapor mixture is conveyed to a vapor separation device thus reducing the vapor content of the effluent stream to an acceptable level, including a second condenser and liquid separator. According to a preferred embodiment, a drier is provided, to remove water vapors from the separated gas-vapor mixture, prior to the first or the second condenser and liquid separator. The major inventive step in the present invention follows the following: By collecting the vapors as prescribed above, one prevents the emission at or close to the source without unnecessary dilution of the vapors with air that would force the handling of a much larger volume and also encumber the recovery of the vapors. At the same time, this collection is done such as not to promote unwarranted emanation of vapors from the source, as would be caused for example, if a nozzle is directed toward a liquid interface source, thereby promoting evaporation.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 illustrates in a schematic form the system of the present invention.

The system consists of three main parts: (a) a directed nozzle (or system of nozzles on a common manifold), (b) a pumping device (or compressor, as may be the case), and (c Jan Entrained Substance accumulation or separation and condensation unit that separates the collected stream into a purified stream of non-condensable (air) on the one hand, and a condensed phase on the other. The accumulation device as used in the present invention, may be solid adsorbent(s), liquid absorbent(s), liquid wash or a filter and dust bin. Figure 2 illustrates in a schematic form the system of the present invention, applied, as an example to vapor conservation and recovery from a Vapor Degreasing Bath, which comprises:

• The Vapor Degreaser.

• A collection system consisting of a suction nozzle (a) and a pump (b). • A primary separation system including a cooler/condenser/separator.

• A gas dryer element (d).

• A heat-mass-exchange unit (HME), comprising a heater (e), a desorption unit (g) and an adsorption unit (i) which are provided with a cooler and separator (h). Figure 3 illustrates in a schematic form the system of the present invention, applied, as an example, to vapor recovery from a manufacturing plant, which may or may not be itself contained in a Permanent Total Enclosure. Here the vapor separation unit is based on low temperature condensation, preceded by a membrane dryer to prevent ice formation and followed by a finishing step purifying the gaseous effluent to specification.

DETAILED DESCRIPTION OF THE INVENTION.

A typical example, which is encountered in many places, is the case when it is desired to collect vapors which emanate in an Open Space and recovering the condensed vapors in order that the respective liquid could be reused. An important element of the present invention is the collection method of the respective vapors, which can be characterized by two main requirements: (a) The location and direction of the suction orifices create a mass convection vector, which opposes the natural convection of the vapors towards the surrounding, thus preventing the diffusion of the vapors through the dilute layers of air or gas, which separate the bulk of the emanating vapors from the surrounding atmosphere, and

(b) The particular location of the suction orifices should be close to the vapor emission source, where prevails a relatively high concentration of vapors.

According to a preferred embodiment, the suction orifices will be located between the vapor source and the open air, at a distance in the range of about 0 to 50 cm from the source, the exact location depending on the geometry of the surroundings, the nature of the Entrained Substance and the rate of its emanation. The orifices position should, if possible, avoid obstructing the access to the working space. When necessary, the orifices may be made to swivel and retract upon touch.

A most important property, which is required for the method, is that the applied suction should balance as closely as possible the natural tendency of the escaping vapors to diffuse to the surrounding. To this effect, the opening of the suction pipe must be directed away from the source of the emanating Entrained Substance and towards the open space. This creates the necessary bulk convection field that will balance out the natural tendency of the Entrained Substance to flow outward. Directing the suction pipe otherwise would subvert the purpose of preventing the Entrained Substance from reaching the surrounding open space. For example, consider directing the suction pipe downward toward the upper surface of a liquid solvent in a container as is done in US Patent Number 5,565,070. This will obviously enhance evaporation from the liquid surface but will certainly not prevent a substantial part of the emanating vapors from diffusing to the surrounding open space. The main factors, which have to be considered for the necessary suction capacity, are as follows: • the concentration of the Entrained Substance at the suction point.

• the geometry of the space surrounding the emanation point.

• the diffusion coefficient of the Entrained Substance through the gas layer. The suction capacity may be expressed by the sum of the source Entrained Substance rate plus the natural convection and diffusion across the air layer, which separates the source from the ambient air. As an approximation, it could be illustrated by the following formula: Q > QV + D_AB^X Aavg^x C/ L (1 ) wherein:

Q : is the suction capacity, expressed in cm³/sec.

Q_v : is the rate of emanation of the Entrained Substance at the source, expressed in cm³/sec. D_AB : is the binary diffusion coefficient of the Entrained Substance , across the air layer B, expressed in cm² /sec.

A_avg : is the cross section for diffusion, as an average over the travelling distance of the Entrained Substance from the source to the surrounding air expressed in cm² (the cross section of the stack when it exists or the shell of a sphere when emanating in open air). C: is the concentration of the Entrained Substance at the source expressed in mole fraction.

L : is the distance in cm. from the source to the surrounding free air, where the concentration is very small as compared to C. When the Entrained Substance is a vapor, due to the relatively high concentration of the collected vapor, a quite large fraction of the collected vapor would be recovered by condensation. As a result, only a relatively small amount of a stream consisting of gas-vapor mixture will need to be purified prior to its release to the atmosphere. This purification being the most difficult and expensive operation of any vapor removal process, the system according to the present invention leads to considerable overall savings. The purification step following the primary/condensation/phase-separation step can be performed by means of a well-known gas purification method, such as HME, or membrane separation, or conventional adsorption, or absorption, etc. The HME method is the preferred method since it does not necessitate the expulsion of a relatively concentrated side stream, as do most other purification methods. Thus, when a method other than HME is used, a side stream will be produced, which it will be necessary to recycle, the consequence being an increase in the amount of gas-vapor mixture that must be processed. The invention will be hereafter illustrated by the following Examples, being understood that these examples are presented only for a better understanding of the invention without implying any limitation to the invention, which is covered by the appended Claims.

EXAMPLE 1 :

Solvent vapors recovery from a batch vapor-deαreasinq bath using HME (figure 2).

The recovery system in our example has been applied to a typical small vapor degreaser using Trifluorochloroethylene solvent to degrease electronic boards. It has a cross section of 50 x 35 cm and a neck depth of 60 cm from the open top of the bath to the highest liquid solvent level in it.

The parts to be washed are generally, inserted from the top of the equipment manually, or by means of a robot. This implies that the top lid must be open during operation causing a substantial loss of solvent vapors to the atmosphere. The parts are first dipped into a liquid wash section, and then they are hanged for a while within a vapor wash space before being removed. The bath is normally equipped with cooling coils around its neck to condense some of the emanating vapors thereby reducing losses. However, this is not very efficient because the cooling can not reach the interior vapor space and the coils temperature may not be reduced much below freezing temperature because atmospheric moisture would soon solidify on the coils surface. Furthermore, water condensation on the cooling coils, due to ambient air humidity, drips into the solvent bath, causing acidification of the solvent to the detriment of the parts being cleaned and of the equipment. In addition, the emanated vapors constitute a health hazard to the operators as well as a substantial cost in lost solvent. As depicted in Figure 2, the recovery system consists of:

(a) An upward directed suction nozzle(s), located just above the vapor wash level, leading to

(b) a compressor pumping 30 Nliter/min, generating by its suction through the nozzle a downward convection flow moving at an average speed of 0.57 cm/sec, opposing the otherwise natural upward motion of solvent vapors by natural convection and diffusion. The compression raises the pressure to 500 kPA, thus facilitating the condensation of the solvent in

(c) a primary cooler-condenser at +5°C. Following separation of approximately 50g/min of condensed liquid, the separated vapor-laden gas is then dried in (d) a desiccant bed designed to selectively remove water vapors and it is then heated to 150°C in

(e) a heater and then directed to -

(f) a HME unit consisting of a pair of periodically switched adsorbent beds, each containing one liter of active carbon. (g) The first bed desorbing the previously adsorbed solvent vapors, thereby enriching the stream and thus facilitating the condensation of the solvent vapors in (h) the secondary cooler-condenser at -20°C followed by liquid separation. Finally, the vapor-laden gas exiting the secondary separator is returned to (i) the second adsorbent bed of the HME unit, where the final purification takes place, thus releasing

( j) an essentially solvent-free stream of approximately 15 Nliter/min to the atmosphere, (k) The condensed liquid from both liquid separators is returned to the degreaser through a decanter, in order to separate whatever liquid water is present. The two adsorbent beds constituting the HME unit are periodically switched in roles.

EXAMPLE 2.

Emission Prevention and Solvent Vapor Recovery from a continuous manufacturing process using Low Temperature Condensation and Adsorption (Figure 3) The recovery system is applied to collect, recover and recycle a solvent (Chloroform) participating in the manufacturing of a polymer product. The polymer solution is spread into thin molds travelling on a belt conveyor where it is cured and degassed by moderate heat radiation on a last section of the belt. Therefrom, the emanating solvent vapors would otherwise (if not using the subject of our invention) be collected in an overhead hood with the help of a suction blower, and the diluted air-vapor mixture would thus all be released to the atmosphere. This because access to items on the belt may be necessary throughout the process. The solvent (chloroform) being both expensive and detrimental to the health of the workers and the environment, it is necessary to prevent the expulsion of the diluted air-vapor mixture to the surrounding working space. Moreover, recovering and recycling the valuable solvent is a substantial economic incentive.

A system essentially as described in figure 3 is installed to replace the original suction hood and stack. The system consists of the following items:

(a) a set of suction nozzles located at about 30 cm above the degassing section of the belt and directed upward (away from the belt). The nozzles are manifolded to the suction of (b) a compressor rated to compress the emanating vapors together with some air at a rate given by the formula (1 ), indicating Q = 500 Nliter/min, up to a pressure of P = 500 kPa. followed by (c) primary cooling and refrigeration to +5°C, thereby condensing a substantial fraction of the compressed chloroform vapor together with some atmospheric moisture, followed by gas-liquid separation in a separator; (d) directing the residual gas phase, Q1 = 384 Nliter/min which is made up of air saturated with Chloroform plus water vapors at 470 kPa and +5°C, to a membrane dryer, where it is dried to remove most of the water vapors from the stream to a Dew Point well below -20 °C . The permeated water vapor being flushed to the atmosphere, using the solvent-free effluent of the recovery plant.

(e) The dry air - Chloroform mixture is then refrigerated down to -20°C, condensing most of the Chloroform into a liquid that is then separated from the gas phase. The gas phase consisting essentially of air saturated with chloroform at P2 = 440 kPa and T2 = -20°C, contains

Chloroform amounting to 3% of the original amount of Chloroform brought into the recovery plant through the suction nozzle. Still, this may be more than the acceptable limit permitted by Environmental Protection Regulations. (f) If necessary, it may be further purified in a Pressure Swing pair of Carbon Adsorption beds. At each adsorbent regeneration cycle, pressure is released followed by a brief wash of the adsorbent by means of some of the purified stream, and a short duration concentrated stream of Chloroform vapors is recycled to the suction of the compressor.

(g) The purified gas, now containing less than 4 ppm Chloroform is expanded to 100 kPa and released to the atmosphere, flushing on its way the water vapors permeated in the membrane dryer, (h) The separated condensed liquid from both stages of gas-liquid separation are recycled to the recycled solvent storage, following decantation and disposal of the liquid water. EXAMPLE 3

Emission Prevention and Vapor Recovery from a leak at an extremity.

Consider the following scenario: A leak develops at an externally accessible point on a tank containing a liquefied Volatile Organic Compound, or at some non-ducted point on the piping servicing the tank, say a relief valve or a bursting disk. A transportable Vapor Recovery System essentially as depicted in Figure 1 , is brought to the site. It is equipped with a set of nozzles on a manifold that is connected by means of a flexible pipe to the pumping device and recovery unit. The suction nozzles system is applied to the leak and the Vapor Recovery Unit is operated for as long as needed until maintenance becomes possible.

EXAMPLE 4

Hazardous Spill Containment

Consider the following scenario:

A Road Tanker overturns in an accident on the road, spilling a valuable or hazardous liquefied Volatile Organic Compound over the road surface. A transportable Vapor Recovery System, essentially as depicted in Figure 1 , is brought to the site. It is equipped with a set of nozzles on a planar manifold that is connected by means of a flexible pipe to the pumping device and recovery unit. The planar manifold and nozzles system is spread over the spill and the Vapor Recovery Unit is operated for as long as needed until some other, more appropriate, containment method becomes preferable. EXAMPLE 5

Cooking Stove vapor/odor removal

It is common practice to install above a cooking stove a ventilated hood pumping the extracted polluted air out of the building or recycling it to the kitchen following its filtration through a carbon adsorbing pad. This hood is both expensive and cumbersome (unless desired as a decorative item). Including a central nozzle or a set of evenly distributed nozzles at the surface of the stove, directed upward and connected through a manifold to a small pumping device will perform the task normally assigned to the hood while pumping a much smaller amount of air. This will facilitate disposal and substantially reduce the equipment cost.

A similar device may be applied to a barbecue stove, thus providing a solution to a problem that has been largely ignored in the past.

Claims

C L A I M S :

1. A system for preventing Entrained Substances such as vapors or smoke or odors or dusts, emanating from a source in an open space from substantially reaching the surrounding space and for collecting or recovering them in a condensed form, the system consisting of:

(a) A directed nozzle or set of nozzles, located close to the source of the emanating Entrained Substances and facing away from the source, the rate of suction through this nozzle being calculated to generate a forced flow field that is directed away from the open surrounding space and towards the nozzle(s), and having a velocity such as to counteract closely the spontaneous convection and diffusion currents of the Entrained Substances from the source to the surrounding.

(b) - A pumping device with suction from the nozzle(s) and discharging its flow into an accumulation or recovery system wherein the Entrained Substance is accumulated for future recovery and/or disposal, or wherein they are converted into a liquid or solid condensed form for recovery or disposal.

2. The system as described in Claim 1 , where the pumping device is a compressor.

3. The system according to Claim 1 , where the Entrained Substance accumulation system is an adsorbing material.

4. The system according to Claim 1 , where the Entrained accumulation system uses an absorbing liquid, or a washing liquid.

5. The system according to Claim 1 , where the Entrained Substance accumulation system is a dust bin.

6. The system according to Claim 1 , where the Entrained Substances are a vapor and the accumulation/recovery system is a condenser/liquid separator in which the pump discharge flow is cooled, thus condensing the vapors and causing the separation of the liquid from the remaining gas-vapor mixture.

7. The system according to the preceding Claims, where the sources and the directed nozzles are protected from winds or currents by means of a shroud.

8. The system according to the preceding Claims, which includes a second purification step of the non-condensed gas phase consisting of:

-a vapor separating or concentrating device separating the vapor from the accompanying gas, yielding two gaseous effluents, one of which is substantially vapor free and the other is a vapor concentrate;

- a secondary condenser/liquid separator or adsorber in which the concentrated vapor laden gas stream exiting the vapor separation device is processed, yielding on the one hand the vapor in a concentrated or condensed liquid or solid form, and on the other hand a relatively dilute vapor- containing gas stream, and

-a liquid return line and decanter to separate water from the condensed vapor.

9. A system according to Claim 8, wherein a drier is provided to remove water vapors from the gas-vapor mixture, prior to the first accumulation/recovery step or the second purification step.

10. A system according to Claim 9, wherein gas exiting the second purification step is subjected to a further purification in an adsorption system to remove traces of the vapor or residual odors, prior to its release to the surrounding space.

11. A system according to Claim 9, where the first accumulation/recovery step used in Claim 1 is skipped, thereby the pumped vapor is directly introduced into the second purification step.

12. A system according to the preceding Claims, wherein the suction nozzle(s) are located between the source and the open air at a distance up to 100 cm from the source.

13. A system according to Claim 12, where a non-obstructing system of nozzles connected to a common manifold are used to collect the vapor.

14. A system according to the preceding Claims, wherein the suction nozzles are distributed on a surface and connected to a manifold, as necessary to generate a desirable suction flow distribution.

15. A system according to the preceding Claims , wherein the suction nozzles are radially directed out of a virtual sphere containing the source.

16. A system according to Claim 8, where the vapor separation/concentration device is a pair of adsorbent beds, such as an HME device.

17. A system as described in Claim 8, where the vapor separation device is a membrane based concentrator.

18. A system as described in Claim 8, where the vapor separation device is a pressure swing adsorption system.

19. A system as described in Claim 8, where at least a part of the relatively dilute vapor-containing gas of the second purification step is recycled to the suction of the pumping device.