MXPA00006385A - Recovery of pfc using condensation - Google Patents

Abstract

This invention is directed to a method, and a system therefor, for recovering PFC using condensation by passing a PFC-containing feed stream into a condenser, preferably reflux condenser, to effect liquefaction into a PFC-containing condensate and a carrier gas stream, and passing the PFC-containing product into a mass transfer unit to fractionate the PFC-containing condensate into a high volatility PFC stream and a PFC product.

Description

RECOVERY OF PFC USING CONDENSATION FIELD OF THE INVENTION This invention relates generally to the recovery of perfluoro compounds (PFCs). Specifically, this invention relates to a method and system for the recovery of PFCs using condensation, preferably reflux condensation.

BACKGROUND OF THE INVENTION PFCs are used in many manufacturing processes. In particular, they are widely used in the manufacture of semiconductor components. The nature of many of these manufacturing processes results in the atmospheric emission of PFCs. Being of high value and harmful to the environment, it is advantageous to recover these issued PFCs, so that they can be reused. Examples of PFCs are nitrogen trifluoride (NF3), tetrafluoromethane (CF4), trifluoromethane (CHF3), hexafluoroethane (C2F6) and sulfur hexafluoride (SF6). In general, PFCs are completely fluorinated compounds of nitrogen, carbon and sulfur. CHF3 is an example that is not fully fluorinated, but due to its similar chemical nature and application with other fluorine saturated PFCs, a PFC is considered. The manufacture of semiconductor components produces leaks that normally include PFCs, non-PFC gases, particulate matter and a carrier gas. The flow of a process tool can be so condensing of the PFCs is achieved by cooling the gas stream to temperatures below the dew points of the constituent PFCs. In order to achieve a high recovery efficiency of PFCs, it is necessary to cool the gas stream below the melting points of some of the lower volatility PFCs. The freezing of PFCs in the capacitor is undesirable because this would reduce the efficiency of the capacitor and prevent continuous operation. The present invention contains several facets, which prevent the freezing of PFC. One, a reflux condenser is preferably used to effect the condensation of PFCs. The condensate in a reflux condenser flows countercurrent to the gas flow from where it comes, and therefore, does not need to be further cooled. However, in general, a conventional condenser is applicable in this invention. Two, this flow regime means that the high volatility PFC condensate flows over the regions where the low volatility PFCs are condensed. Low volatility PFCs with a tendency to freeze are soluble in these high volatility PFCs and freezing can be avoided. Three, in the preferred embodiment, the concentration of PFCs of high volatility in the gas stream rises when recycled in the system.

This is achieved by separating the high volatility PFCs from the recovered PFC product, as re-added upstream. Increasing the concentration of high volatility PFCs in the gas stream decreases the concentration of low volatility PFCs in the PFC condensate and prevents the PFCS from freezing. Several solutions have been suggested to recover the PFCs of a carrier gas current stream, mitigating some the problem of freezing of PFC when using cryogenic media for recovery. However, nothing in the art shows or suggests the present invention. An earlier method for recovering PFCs from the carrier gas is the condensation / dissolution as shown in U.S. Pat. No. 5,262,023. A solvent is added to the gas stream, which is then cooled to condense the PFCs and any solvent evaporated. Low volatility PFCS with a tendency to freeze are soluble in the additive solvent. The additive solvent and PFCs are then separated by distillation and the additive solvent is reused. The solvent must be completely removed from the PFC product to prevent loss. U.S. Patent No. 5, 540,057 provides the removal of • the volatile organic compounds (VOCs) of a carrier gas by condensation of the VOCs in a reflux condenser. The carrier gas loaded with VOCs passes the shell side of a heat exchanger of tubes and shell, and then it is cooled along a continuous temperature gradient. The VOCs are condensed to different degrees at different levels and collected in special deflectors on the shell side, which can direct a portion out of the condenser and allow a portion to drip back down the condenser as reflux. The cold cleaning carrier gas is then mixed with coolant at the outlet of the shell side and passes down the pipe side to effect cooling of the shell side. The freezing of VOCs, especially benzene, can be inhibited by the addition of a solvent, specifically toluene, to the gas stream. 5 US patents nos. 5, 533,338 and 5,799, 509 are examples of condensation freezing to condense PFCs against a cryogenic fluid. The freezing of low volatility PFCs occurs due to the low temperatures required for high efficiency condensation of high volatility PFCs. This method is disadvantageous because it is necessary to periodically thaw frozen PFCs by removal. This results in low cooling efficiencies and requires equipment in duplicate in order to maintain a continuous operation. The membrane permeation recovers the PFCs of the carrier gas to through differences in membrane permeability. The gas stream is contacted with the feed side of a specific membrane, which allows the carrier gas to preferentially infiltrate while the PFCs are retained. High separation efficiencies require the use of multiple membranes. The PFCs have different permeation characteristics and vary in recovery efficiencies. The adsorption recovers PFCs from the carrier gas. The gaseous current is brought into contact with an adsorbent, which removes the PFCs. The PFCs are desorbed and are removed from the leach or adsorbent with a sweeping gas. Sweeping gas results in a low PFC product concentration. Additionally, the adsorption processes do not have the flexibility to adjust to large changes in PFC concentration and the carrier gas flow velocities, which are characteristic of gaseous effluent streams. Yet another method of recycling PFC is the process of intensive energy incineration. The gas stream is heated to a high temperature, which prevents the emission of the PFCs. The decomposition gases, such as hydrogen fluoride and nitrogen oxides are then removed from the flue gas. It is desirable that PFC recovery systems treat the escape of small groups of semiconductor fabrication tools rather than complete manufacturing facilities. If a system fails, only a fraction of the manufacturing tools are affected. Therefore, the present invention is primarily intended to deal with the escape of a small number of tools. However, it can be scaled to handle the escape of a complete semiconductor fabrication facility. It is also an object of this invention to mitigate the problem associated with PFC freezing, while recovering from a stream of carrier gas by cryogenic condensation.

BRIEF DESCRIPTION OF THE INVENTION This invention is directed to a system for recovering PFCs using condensation, preferably reflux condensation. A condenser provides indirect heat exchange of a gaseous stream containing PFC to effect liquefaction in a condensate containing PFC and a stream of carrier gas. A mass transfer unit is used to fractionate the condensate containing PFC into a high volatility PFC stream and a PFC product. This invention is also directed to a method for recovering PFC using condensation, preferably reflux condensation. A feed stream containing PFC is passed to a condenser to effect liquefaction in a condensate containing PFC and a stream of carrier gas. In addition, the product containing PFC is passed to a mass transfer unit to fractionate the condensate containing PFC in a high volatility PFC stream and a PFC product.

DETAILED DESCRIPTION OF THE DRAWINGS Other objects, features and advantages will be apparent to those skilled in the art from the following description of the preferred embodiments and the accompanying drawings, in which: FIG. 1 is a schematic diagram of recovery of PFCs in this invention. Fig. 2 is a graph showing the condensation of condensate in several stages in a reflux condenser of this invention.

DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "high volatility PFCs" means one or more PFCs having an atmospheric boiling point below 150 K. Examples include tetrafluoromethane (CF4) and nitrogen trifluoride (NF3) ). As used herein, the term "low volatility PFCs" means one or more PFCs having an atmospheric boiling point above 1 50 K. Examples include trifluoromethane (CHF3), hexafluoroethane (C2F6) and sulfur hexafluoride ( SF6). As used herein, the term "indirect heat exchange" means the transport of two fluid streams in heat exchange relationship without physical contact or intermixing of the fluids of one and the other. As used herein, the term "capacitor" describes a container that provides indirect heat transfer from a gaseous flow, in order to effect the liquefaction of a portion of that flow.

As used herein, the term "condensate" describes a liquefied gas. As used herein, the term "reflux condenser" describes a condenser, wherein at least a portion of the condensate is forced to contact a hotter heat transfer surface than that which condenses it. This re-heating causes at least a portion to re-evaporate. This is easily effected by cooling a rising gas stream. Then the condensate descends and warms up. The reflux condenser is preferred and the general use of a condenser is contemplated in this invention. As used herein, the term "reflux condensation" describes a condensation performed in a reflux condenser.

As used herein, the term "rectification column" describes a distillation or fractionation zone, wherein the liquid and vapor phases are brought into countercurrent contact to effect separation of a fluid mixture. A rectification column is preferred, but the general use of a mass transfer unit, which can perform a similar function as the rectification column, is contemplated in this invention. Returning now to Fig. 1, which is a schematic flow diagram, of a preferred embodiment of the system in this invention. The hot gas feed stream 1 0 consists of a carrier gas, high volatility PFCs and low volatility PFCs, pressurized to approximately 6.67 kg / cm2. Particulate impurities and non-PFC gases, such as hydrogen fluoride and fluoride, will have been removed in a pre-purification step. Stream 10 enters at high pressure after being compressed during the pre-purification step. Pressure swing adsorption, for example, requires pressurization of the gas stream. Other adsorption processes are applicable, including adsorption of thermal oscillation at the highest level of pressure. It is also found that pressurization helps to separate the PFCs from the carrier gas and reduces the size of the process equipment. The freezing points of the individual PFCs will also be decreased at high pressure. The current 1 0 is cooled by indirect heat exchange with a refrigerant stream 34 in the heat exchanger 1 2. Current 1 0 is cooled to a temperature above which the PFCs begin to condense, since be in liquid or solid form. It is important that the PFCs do not freeze in the heat exchanger 12 because there is no means to remove the frozen PFCs. The cooled gaseous feed stream 14 then leaves the heat exchanger 12 and enters the condenser 1 8., preferably a reflux condenser, where it is combined with a high volatility PFC stream 40. In the preferred embodiment, the stream 40 is a liquid. It will also have a significantly lower temperature than current 14. This causes current 40 to evaporate instantaneously, so that the resulting mixed stream 16 achieves a lower temperature than that of stream 14. The temperature of stream 16 should be approximately that temperature. to which the PFCs begin to condense. The addition of stream 40 to stream 14 results in a mixed stream 16 having higher concentration of PFC of high volatility than stream 14. Stream 16 is countercurrently cooled in condenser 1 8 medium indirect heat exchange with the cold refrigerant stream 32. The cooling causes the PFCs to condense and flow out against the current to current 16, forming a condensate stream of PFC 22. Low volatility PFCs, such as hexafluoroethane and sulfur hexafluoride, are they condense to the hottest end of condenser 1 8, and have a tendency to condense as solids because they are cooled below their melting points. High volatility PFCs, such as carbon tetrafluoride and nitrogen trifluoride, condense to the cooler end of the condenser and will not condense as solids, since their melting points are not reached. A feature of the operation of the capacitor 18 is that the high volatility PFCs are washed over the hotter end of the capacitor and act as solvents towards the low volatility PFCs. As a result, the freezing of low volatility PFCs is inhibited. An additional feature of this mode is that current 40 is added to stream 14 to increase the amount of high volatility PFCs with respect to low volatility PFCs. The addition of the stream 40 also stabilizes the composition and concentration of PFC in the capacitor 1 8. This allows the condenser 18 to operate under temperature conditions, which more closely approximate steady state and aid in the control of the process. The cold carrier gas stream 20 leaves the condenser 18 having been treated to remove the PFCs. The liquid cryogen stream 24 is added to the stream 20 via the control valve 26 to produce the stream 28. The carrier gas will more usually be nitrogen gas and the liquid cryogen will more usually be liquid nitrogen. The rate of addition of the stream 24 is determined by the cooling requirements in the heat exchanger 1 2 and the condenser 1 8. The stream 20 will be closer, more usually, to the dew point of the carrier gas, in order to condense enough of the high volatility PFCs and as such, the addition of stream 24 will not usually cause the corrugator 24 to evaporate completely. Therefore, current 28 will usually be two phases. Current 28 passes through a butterfly valve 30 to form a refrigerant stream 32. Expansion causes a drop in temperature, the required degree of which is determined by the temperature difference of the cold end of the condenser 1 8 and is controlled by the pressure drop through the butterfly valve 30. The current 32 passes through the condenser 18, is heated against the mixed stream 14 and exits at the bottom of the condenser 18, as stream 34. The stream 34 then passes to the heat exchanger 12 to effect the cooling of the current 1 0. The hot coolant stream 36 leaves the heat exchanger 1 2. A portion of the stream 36 can be used. to regenerate the adsorption beds in the pre-purification stage. It is also advantageous to use a portion as the addition to the semiconductor tool exhaust to keep the volumetric flow rate constant in the PFC recovery system. The stream 22 passes to the mass transfer unit 38, preferably, a rectification column, where the PFCs of high volatility and low volatility are separated, preferably, by cryogenic rectification. At the top of the mass transfer unit 38, the stream 40 is formed and recycled upon addition to the stream 14 at the condenser inlet. The stream 40 will also contain carrier gas that was condensed in the capacitor 1 8 during the PFC removal. Therefore, the mass transfer unit 38 also raises the efficiency of the PFC concentration of the system. At the bottom of the mass transfer unit 38, the product of PFC l liquid 42 is formed. Under conditions of steady state, with 1 00% recovery of PFC, the mass and relative proportion of PFCs entering the system into the current 10 will equal the mass and relative proportion of the PFCs leaving the system in the stream 42. In another embodiment, the addition of the stream 40 can take place at different points at the condenser inlet, such as, in the stream in any point of the capacitor 1 8, in the stream 16, so that it flows again as a liquid in the condenser 1 8, in the stream 14, before the condenser 1 8, directly in the heat exchanger 12 and anywhere before the heat exchanger 12, including the pre-purification step. The stream 40 may also be two-phase or completely gaseous. Another embodiment does not require the use of current 40. Accordingly, stream 22 is collected as a product. The mass transfer unit 38 is not necessary. This modality is particularly applicable where the 1 0 current comprises enough PFCs of high volatility to ensure that the low volatility PFCs do not freeze in the capacitor 1 8. Other types of capacitors can be used to perform the condensation of the PFCs, where use current 40 to prevent freezing in the condenser. Certain types of PFCs can be used as solvents. For example, low volatility PFCs that do not have a high vapor pressure at their freezing point. These include tpfl uoromethane (CH F3) and octafluoropropane (C3F8).

The mass transfer unit 38 is used to separate the high volatility PFC from the PFC product. Various devices other than the rectification column may be used, such as a deflegmator. Different means of adding cooling to heat exchanger 12 and / or condenser 18 can also be used. This includes: one, indirect heat transfer with a cryogen such as liquid nitrogen. Two, mechanical refrigeration produced by a steam compression cycle using a working fluid, which is a mixture of atmospheric gases, hydrofluorocarbons and / or PFCs. Three, mechanical cooling produced by the turbo expansion of dry air, nitrogen, argon or mixtures thereof. Four, cooling obtained from a pulsation tube cooler, preferably providing the input work to the pulsation tube by a linear motor compressor. In addition, it is convenient to expand the cold carrier gas stream 20 through a butterfly valve 28, before adding the refrigeration. Where the heat exchanger 12 and the condenser 18 are one unit, it is appropriate to conduct a condensation in the conventional manner without backflow action. Multiple capacitors can be used, or a condenser with multiple liquid outputs to produce condensed products of multiple PFCs. It is also contemplated to operate the system at pressures above and below 6.67 kg / cm2. For pressure swing adsorption applications, a pressure range from about 5.62 kg / cm2 to about 14.06 kg / cm2, preferably from 6.32 kg / cm2 to about 8.78 kg / cm2, and most preferably to about 6.67 kg / cm2 is desirable. . For a thermal oscillation adsorption application, a substantially higher pressure range is used.

Example Stream 10 comprises nitrogen carrier gas with 1 000 ppm CH4, 2,000 ppm C2F6, and 500 ppm SF6, having been treated to remove non-PFC gases, such as, HF, F2, H2O and CO2. The current 1 0 has a pressure of 6.60 kg / cm2 and a temperature of 288 K. The current 1 0 is cooled to 165 K in the heat exchanger 12 to form the stream 14, and then passes to the condenser 18. The current 40, comprising CF4 and a portion of nitrogen carrier gas, is evaporated instantaneously in stream 14 at the inlet to the condenser, raising the concentration of CF4 in the resulting stream 16 to 18,200 ppm and decreasing the temperature to 57K. The stream 1 6 is cooled in the condenser 1 8 so that the PFCs are condensed to form the stream 22 and the carrier gas exits as the stream 20. To ensure the high removal efficiency of the PFCs, a portion is also condensed of the nitrogen carrier gas in the reflux condenser. At 6.53 kg / cm2, this occurs at 97.3 K. Current 20 comprises nitrogen with 5 ppm CF. Fig. 2 shows the composition of the liquid condensate in several stages in the reflux condenser. Stage 1 corresponds to the upper part of the condenser and stage 5 to the bottom, and this is represented by the X axis. The Y axis represents the mole fraction for each of the compounds. In this example, stream 22 is pumped to rectification column 38, where it is separated to form stream 40 and stream 42. At 6.67 kg / cm 2, stream 16 has a temperature of 125K and comprises 87.9 mol% of CF4 and 1 3. 1% mol of nitrogen. At 6.74 kg / cm2, stream 42 has a temperature of 209K and comprises 28.6 mol% of CF4, 57.1 mol% of C2F6 and 14.3 mol% of SF6. The recovery efficiencies of CF4, C2F6, and SF6 are 99.5%, 100%, and 100%, respectively. The PFC product contains 1 ppm of nitrogen carrier gas. A system of 56.64 m3 / h consumes approximately 22.68 kg / h of liquid nitrogen refrigerant. The specific features of the invention are shown in one or more of the drawings only for convenience, since each feature can be combined with other features according to the invention. The alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims.

Claims (9)

1. REIVI NDICATIONS 1 . A system for recovering PFCs using condensation, comprising a) a condenser for providing indirect heat transfer from a gas stream containing PFC to effect liquefaction to a condensate containing PFC and a stream of carrier gas; and b) a mass transfer unit for fractionating said condensate containing PFC in a stream of high volatility PFCs and a product of PFCs.
2. 2. The system of claim 1 further comprises a heat exchange unit for cooling said feed stream containing PFC prior to passing said feed stream to said condenser.
3. The system of claim 1, wherein the cooling is added to said carrier gas stream for recycling to said condenser.
4. The system of claim 1, wherein said high volatility PFC stream is recycled back to said capacitor to be combined with said carrier gas stream and to be combined with said feed stream containing PFC.
5. 5. The system of claim 1, wherein said condenser is a reflux condenser.
6. 6. A method for recovering PFCs using condensation, comprising a) passing a PFC-containing feed stream to a condenser to effect liquefaction to a condensate containing PFCs and a stream of carrier gas; and b) passing said condensate containing PFCs to a mass transfer unit to fractionate said condensate containing PFCs in a stream of high volatility PFCs and a product of PFCs.
7. 7. The method of claim 6 further comprises cooling said feed stream containing PFCs in a heat exchange unit before passing said feed stream to said condenser. The method of claim 6, which comprises adding refrigeration to said carrier gas stream to be recycled to said condenser. The method of claim 6, which comprises recycling said high volatility PFC stream back to said capacitor to be combined with said carrier gas stream and to be combined with said feed stream containing PFCs. The method of claim 6, which comprises recovering PFCs using a reflux condenser.