Patent Publication Number: US-2010108107-A1

Title: System and apparatus for fluoride ion cleaning

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to systems and apparatuses for fluoride ion cleaning, and more specifically to systems and apparatuses operable for in situ generation and capture of hydrogen fluoride gas used to clean components, including components comprising superalloy material. 
     Fluoride Ion Cleaning (FIC) is used to remove oxides from field-run hot gas path components in preparation for subsequent braze repair operations. Current FIC techniques either suffer from reduced effectiveness due to the limited availability of HF gas in the process or are burdened by high equipment and maintenance costs stemming from the use of bottled HF gas as a reactant. 
     Commercially available dynamic FIC equipment currently uses bottled HF gas as the source material. Because HF is an extreme toxin, its use requires expensive, relatively complicated equipment in order to safely handle the HF. In addition, since HF must be used in excess in the cleaning process, the effluent stream also contains HF, requiring a scrubber system and it&#39;s necessary ancillaries. This combination tends to drive users to employ larger equipment which is segregated from the normal process cells to obtain some economy of scale with the gas handling and treatment systems. 
     Accordingly, it would be desirable to have an effective cleaning method without the associated downfalls of the use of bottled HF gas a source material. Further, it would be desirable to provide an effective cleaning method that reduces or eliminates the need for a separate scrubber system to remove excess HF from the effluent stream. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned need or needs may be met by exemplary embodiments which provide systems and apparatuses for in-situ generation of HF within a cleaning retort and removes excessive HF before releasing an effluent stream from the cleaning retort. 
     Exemplary embodiments disclosed herein include a system including a cleaning retort operable at a temperature sufficient to promote an in-situ reaction between a liquid or gaseous halogenated feedstock and hydrogen gas to form hydrogen fluoride (HF). The system also includes a feedstock source for supplying at least one of a liquid or gaseous halogenated feedstock to the cleaning retort, and a hydrogen gas source for supplying hydrogen gas to the cleaning retort, wherein both feedstock source and the hydrogen gas source are disposed outside the cleaning retort. The system further includes a HF scrubber operable to substantially remove residual HF gas formed by the in-situ reaction, wherein the HF scrubber is disposed within the cleaning retort. 
     Exemplary embodiments disclosed herein include an apparatus comprising a cleaning retort operable at a temperature sufficient to promote an in-situ reaction between a liquid or gaseous halogenated feedstock and hydrogen gas to form hydrogen fluoride (HF), wherein the cleaning retort comprises a first region sized and dimensioned to hold parts in need of cleaning and a second region including a HF scrubber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic representation of a high temperature furnace containing a cleaning retort having at least a cleaning region and a scrubbing region. 
         FIG. 2  is a schematic representation of a high temperature furnace similar in certain respects to the furnace shown in  FIG. 1  and including a vacuum pump. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  shows an exemplary system  10  including a high temperature furnace  12  including a cleaning retort  14  into which parts or components  16  in need of cleaning are placed. Cleaning retort  14  is capable of containing the appropriate cleaning gasses introduced by gas stream  18 . The cleaning retort  14  includes at least two regions. A first region  20  is sized and dimensioned to hold parts and components  16  in need of cleaning. A second region  22  is operable as an HF scrubbing unit (fluorine getter). In an exemplary process, the retort  14  is preheated and purged with, for example, argon. Thereafter, a feedstock of a non-hazardous fluorine-containing compound  24  and hydrogen gas  26  are introduced into the retort  12 . The fluorine-containing compound reacts at temperature with hydrogen to form HF gas (gas stream  18 ) in the retort  14 . 
     The HF gas then acts to clean the parts via the conversion of oxides to semi-volatile fluorides which are carried away from the parts or components  16  by the flowing gas stream in a fluoride ion cleaning process. In an exemplary embodiment, prior to exiting the retort, the initial effluent stream  28  is scrubbed of fluorine in the second region  22  (fluorine getter) such that the scrubbed effluent stream  30  exiting the retort is substantially free of fluorine, and therefore less hazardous then in traditional fluoride ion cleaning processes. Optionally, the cleaning retort  14  may include a third region  32  (metal getter) operable to remove a majority of the metals found in the initial effluent stream  28  such as Al and Cr as discussed below. 
     In an exemplary embodiment, illustrated in  FIG. 2 , a similar system  100  is utilized. System  100  includes a high temperature furnace  112  including a cleaning retort  114  into which parts or components  116  in need of cleaning are placed. The cleaning retort  114  includes at least regions  120  and  122  operationally similar to regions  20 ,  22  as described above. Optionally, retort  114  may include region  132  as a metal getter. In an exemplary embodiment, the scrubbed effluent stream  130  is connected to a vacuum pump  140  operable to pulse or modulate pressure in the retort  114  to help evaporate semi-volatile fluorides from the surface of the parts  116  and to provide fresh HF gas  118  into cracks on the parts. A dessicant  142  may be placed in the effluent stream  130  to prevent moisture contamination of the vacuum pump. Alternately, a liquid ring type vacuum pump could be used to provide vacuum without the dryer. 
     In an exemplary embodiment, hydrogen fluoride gas (HF) is generated and subsequently destructed in-situ, thereby eliminating some of the hazards associated with prior fluoride ion cleaning (FIC) processes. A non-toxic, fluorine-containing compound such as Freon 134a (tetrafluoroethane) is used as a feedstock and thermally decomposed after mixing with hydrogen to form HF: 
       C 2 H 2 F 4 +5H 2 →4HF+2CH 4  (in situ HF generation) 
     The HF thus generated is utilized in a Fluoride Ion Cleaning process: 
       6HF+Al 2 O 3 →2AlF 3 +3H 2 O 
       6HF+Cr 2 O 3 →2CrF 3 +3H 2 O (cleaning) 
       H 2 O+CH 4 →CO+H 2    
     The effluent stream is treated to remove HF prior to exhausting from the retort. The process has an optional step which removes the majority of the metals found in the initial effluent stream such as Al and Cr. The metal fluoride compounds may be substantially stripped of their metal content so that reconstituted HF may be recycled to the cleaning process. Alternately the reconstituted HF may be more readily removed in a subsequent scrubbing unit. 
     These elements exist in the effluent stream as fluorides, and can be removed by reducing them to a metal alloy by combining them with a pure sacrificial metal such as iron: 
       2AlF 3 +2Fe+3H 2 →2AlFe+6HF (and) 
       CrF 2 +Fe+H 2 →CrFe+2HF (metal getting) 
     In the fluorine removal step, a packed bed is used to contact fluorine-containing species with a sacrificial high melting temperature material. The reactions result in formation of stable, high melting point fluoride compounds which may be subsequently disposed of after the retort has been returned to room temperature. In the preferred embodiment, the fluorine scrubber contains a fluorine getter such as CaO: 
       2HF+CaO→CaF 2 +H 2 O (fluorine getting) 
     Other salts or combinations of salts may replace CaO. For example a combination of CaO and NaCl may be mixed with Si. This fluorine-getter mixture may allow substantially all of the HF to be removed from the gas streams at elevated temperatures. The process generates non-hazardous, readily disposable solid wastes. The gaseous by-products may be combusted in the furnace hot zone resulting in CO 2  and water vapor emissions. 
     The result of this combination of in-situ generation and destruction of HF allows for tailoring the cleaning processes to the components requiring cleaning, rather than running a single common cycle for all parts regardless of the difficulty of cleaning certain components. 
     Thus, the exemplary embodiments disclosed herein provide an effective cleaning method without the associated downfalls of the use of bottled HF gas as a source material in situ generation of HF. Further, in situ removal of excess HF from the effluent stream reduces or eliminates the need for a separate scrubber system. 
     Embodiments disclosed herein present systems and methods of fluoride ion cleaning in which HF gas is generated in-situ in the cleaning retort using a liquid or gaseous halogenated feedstock combined with hydrogen at high temperatures such that no HF precursor material is required to be placed in the cleaning retort prior to initiation of the cleaning cycle and no HF gas is employed as a feedstock 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.