Abstract:
A device for the pneumatic conveyance of particulate substance having a specific gravity of 0.1 to 15.0 g/cm 2  and a particle size of between 0.1 to 300 μm includes a flat plate like filter element between a pump chamber and sources of positive and negative pressure. The flat plate filter element provides a desired pressure drop and is cleaned of particulate material during operation of the device upon application of positive pressure.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns an apparatus and a process for pneumatically conveying powdery substances having a container using at least one filter in pipe systems. The invention also concerns the use of said apparatus and/or said process. 
     EP-A-0 574 596 describes an installation for pneumatically transshipping cement from ships into silos by means of a so-called lock container comprising a plurality of container segments; disposed in the uppermost container segment is an exhaust air filter while the lowermost container segment tapers in a funnel-like configuration. 
     The chemical. pharmaceutical and foodstuff industries also involve conveying powdery substances and transporting them in a controlled atmosphere. The known installations for conveying powdery substances of that kind are generally matched in regard to their structure to the product which is to be subsequently conveyed; those installations involve individual fabrications which give rise to high levels of installation costs. A further disadvantage with the known installations is inter alia that the required filters already clog up after a short period of operation. As a consequence of that problem, production of the powdery substances is frequently subjected to disturbances which result in production failures that have an adverse effect on costs. It has not been possible at the present time to overcome those deficiencies. 
     The operation of introducing powder into reaction vessels or reactors within explosive areas is generally effected manually by way of a lock arrangement or a protective valve as most reactors do not have the container comprising a plurality of container segments; disposed in the uppermost container segment is an exhaust air filter while the lowermost container segment tapers in a funnel-like configuration. 
     The chemical pharmaceutical and foodstuff industries also involve conveying powdery substances and transporting them in a controlled atmosphere. The known installations for conveying powdery substances of that kind are generally matched in regard to their structure to the product which is to be subsequently conveyed; those installations involve individual fabrications which give rise to high levels of installation costs. A further disadvantage with the known installations is inter alia that the required filters already clog up after a short period of operation. As a consequence of that problem. production of the powdery substances is frequently subjected to disturbances which result in production failures that have an adverse effect on costs. It has not been possible at the present time to overcome those deficiencies. 
     The operation of introducing powder into reaction vessels or reactors within explosive areas is generally effected manually by way of a lock arrangement or a protective valve as most reactors do not have the necessary space for an adequate loading installation. Such a mode of operation however does not comply with the applicable safety rules for obviating the risk of explosion: if the reactor is inerted, the step of manually introducing powders from the manhole results in atmospheric pressures and eliminates the protective effect of the inert gas. Upon manual introduction of solid material, the inerting effect is eliminated within a short period of time (O2 concentration &gt;8%) and is not restored even after prolonged operational N2-flushing. 
     Furthermore the dust can result in contamination of the environment; the gas vapours which are developed give rise to the risk of asphyxiation for the operating personnel. The risks of explosion during the conveying procedure are possible in particular when the following factors come together at the same time: 
     oxidisable powder; 
     powder/oxygen ratio is within an explosion limit (varies according to the respect product involved); and 
     formation of an ignition source (electrostatic discharge, flame, hot spot, sparks). 
     In consideration of those aspects the inventor set himself the aim of eliminating the acknowledged disadvantages and permitting inexpensive conveying of powdery substances, including sticky substances. In particular the invention seeks to provide that powdery solids can be introduced into reactors or the like units with an enhanced level of safety. 2 Amended Page 
     That object is attained by the teaching set forth hereinbelow. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, in the above-mentioned apparatus for pneumatically conveying powdery substances of a specific weight of 0.1 to 15 g/cm 3  and with a grain size range of between 0.1 and 300 μm as the conveyed material, the ratio of the length of the container forming a pump chamber for temporarily receiving the conveyed material, to its internal diameter, is between 0.5 and 10.0 and the plate-like filter element is provided between a vacuum conduit of a vacuum pump for sucking in the conveyed material and the pump chamber, wherein a conveyor gas conduit of a conveyor gas source is connected to the space—in a dome cover—which is connected to the vacuum pump, and a respective automatic closure member is arranged both in the vacuum conduit and also in the conveyor gas conduit. 
     In addition, for handling of the filter, it has been found desirable for it to be in the form of a plate-like filter membrane which is preferably interchangeably disposed in a frame of a filter insert. 
     Advantageously, associated with the container according to the invention are four automatic blocking or closure elements which are controllable with each other, more specifically a respective one at each of the feed conduit and the discharge conduit and at the conduits for vacuum and conveying medium. 
     During a suction intake phase the closure element of the feed conduit opens whereas the discharge conduit remains closed. By virtue of the vacuum connection which is open in that situation, material to be conveyed is sucked into the pump chamber; after a predetermined period of time the feed conduit closes and the discharge is opened. The conveyed material is ejected by pressure—compressed air or nitrogen for filter cleaning purposes. The filter in the upper part of the container retains the finest particles and is cleaned in each emptying cycle. 
     Before the powder is introduced into the downstream-disposed reactor—for example a mixer, a crusher or the like unit in which a reaction takes place—air and powder are separated from each other by closure of the vacuum shut-off valve being delayed with respect to opening of the conveyed material intake. So that no gases of the reactor are sucked in when the discharge conduit is opened, the reactor is firstly put under pressure and only then is the emptying valve opened. Moreover the vacuum conduit can only be opened when the discharge conduit is closed. 
     A ratio of container length to container diameter in the range of between 0.5 and 10.0. preferably between 2.0 and 8.0. has been found desirable. The container width or the container diameter itself is advantageously between 10 and 500 mm, in particular between 50 and 400 mm, while the container length is between 200 and 1000 mm, in particular between 400 and 900 mm. This therefore involves a comparatively narrow container. wherein preferably the diameter of the container determines the size of the filter. 
     It is in accordance with the invention for the apparatus to be operated with a reduced pressure for sucking in the conveyed material of between 1 and 25 mbar, in particular between 5.0 and 20 mbar. The increased pressure for discharge of the conveyed material should for that purpose be between 0.5 and 5.0 bar. in particular between 1.0 and 3.0 bar. 
     In accordance with a further feature of the invention the filter is to be designed in such a way that a differential pressure of between 100 and 300 mbar occurs at its side remote from the vacuum pump. 
     It is also advantageous to provide a flat grid or mesh which at the vacuum side is associated with the filter as a support means. The preferred mesh size thereof is to be between 5 and 50 mm, preferably between 10 and 40 mm. A grid or mesh can also be provided at the other filter surface. 
     In addition., each grid or mesh can be connected to a vibratory drive and can thus be in the form of a vibration source for the filter. 
     In accordance with the invention, for the purposes of cleaning, associated with the filter during the emptying procedure is a counter-flow flushing operation which is controllable at periods of time, such an air jet can be provided at both filter surfaces. 
     Unlike the previous apparatuses and installations, smaller dimensions are possible while complying with the prerequisites in accordance with the invention, so that there are no space problems which have an adverse effect on the cost aspect. 
     A factor of particular significance is the possibility of using a plurality of these apparatuses jointly, for example in the form of a tandem installation, without any difficulties, for the purposes of increasing the through-put rate. Thus for example a plurality of the apparatuses are operated in juxtaposed relationship at the same rhythm or in an alternate rhythm. 
     It is also in accordance with the invention however, for the purposes of modifying the mixing ratio of the powdery substances, for at least two apparatuses to be operated side-by-side at different rhythms. 
     Preferably, cleaned compressed air, a reactive gas or an inert gas, in particular nitrogen, is used for discharge of the powdery substances by a pneumatic conveying action. 
     The described system permits the conveying of powdery products by way of a flat filter membrane which is installed in the upper part of a pump chamber; the diameter thereof substantially corresponds to that of the filter membrane. 
     Powdery products are conveyed by a vacuum and a pressure source being applied alternately to the pump chamber. The vacuum generated by a vacuum pump sucks the powder conveyed material into the pump chamber, and the filter separates particles sucked in by the vacuum pump from the air. The pressure of the conveying gas makes it possible to empty the pump chamber and at the same time to clean the filter by a counter-flow. 
     By virtue of those measures most problems in connection with transportation and metering of fine, sticky, contaminated powders can be resolved. 
     The following properties are to be viewed as particular advantages: 
     a mobile and compact system; 
     a simple structure with a cylindrical chamber. for many materials (SS, Haselloy, plastic material. glass); 
     a very simple cleaning operation; 
     an economical installation; 
     no damage to the powder during the conveying operation; 
     no clinging or adhesion of sticky powders with poor flow properties; 
     a completely closed-off system; no dust formation; 
     no oxygen feed possible in closed containers. 
     In addition the system considerably reduces the risk of explosion during the introduction of powders into reactors or similar vessels which contain combustible gases/vapors. As the powder conveying effect is achieved by suction, the risk of explosion in the conveyor conduit is considerably reduced. The powder/oxygen ratio is outside the explosion limit in most cases. As there are no rotatable parts, any kind of ignition or risk of explosion due to friction is also out of the question. 
     This procedure makes it possible for powders from sacks, big bags or silos to be introduced into a container which is under pressure, and it thus fully complies with the expectations in regard to the safety precautions in the chemical and pharmaceutical industries. There is the possibility of using various gases for emptying the pump chamber, for example nitrogen or argon. The use of a neutral gas for the emptying operation makes it possible for example to fill inerted reactors with powder, without introducing oxygen into the reactor. As a result the consumption of inert gas is low as it is not used during the suction intake phase for the powder conveying operation, but only for the step of emptying the pump chamber. In the pump chamber, the oxygen is separated from the powder and replaced by inert gas. 
     In systems which are available on the market, large filtering sleeves are required in order to prevent the filters from becoming excessively quickly gummed up; cleaning of a sleeve-type filter is difficult and not very efficient. In contrast, cleaning a flat filter is a simpler operation. Cyclic cleaning of the filter at frequent time intervals, in accordance with the invention, makes it possible to guarantee a constant level of filtration efficiency. 
     The volume of the chamber of hitherto conventional systems is large because of the large filter volume. Emptying of such installations is implemented by a gravimetric procedure. A reducing portion is normally necessary to permit the installation to be connected to a flange of standard size. The reduction frequently triggers off operational problems and requires the use of a vibrator or the like ancillary device for discharging the powder from the separator. 
     The use of the described apparatus and process is preferably implemented for acting on a reaction vessel in the chemical industry or the foodstuffs industry. in the pharmaceutical industry or in the industry producing inks and paints or lacquers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages, features and details of the invention will be apparent from the following description of preferred embodiments and with reference to diagrammatic views in the drawing in which: 
     FIG. 1 is a side view of an apparatus for pneumatically conveying powdery substances, 
     FIG. 2 is a detail from FIG. 1 on an enlarged scale, 
     FIG. 3 is a side view of a clamping closure of the apparatus, 
     FIG. 4 is a plan view of the apparatus, 
     FIG. 5 shows the apparatus in an indicated installation, 
     FIG. 6 shows a side view of a twin assembly, 
     FIG. 7 shows a plan view of the apparatus of FIG. 4, 
     FIG. 8 is a perspective view of part of a filter insert, and 
     FIG. 9 is a partial sectional view of an air jet for clamping the filter. 
    
    
     DETAILED DESCRIPTION 
     An apparatus  10  for pneumatically conveying powdery substances of a specific weight of 0.1 to 15.0 g/cm 3 , in a grain size range of between 0.1 and 300 μm from a silo  9  indicated in FIG. 5 to a reaction vessel or reactor  11  has a cylindrical container  12 —of electrolytically polished high-quality steel—of a length a of here 600 or 500 mm, the internal space of which, of an internal diameter d of 200 or 300 mm respectively, serves as a pump chamber  13 , and a coupling adapter connection  14   a  for a feed conduit  14  for suction intake of the conveyed material. The feed conduit  14  includes a so-called butterfly valve  16  as a locking or closure member in a connecting flange  15 . 
     As shown in FIG. 8, in an annular frame  42  of the filter insert  26 , a filter or a filter membrane  44  is associated with a flat grid mesh  46  associated at the vacuum side, as a support element, the grid mesh 46 being of small mesh size. It can be connected to a vibratory drive  50  and transmit the vibrations thereof to the filter membrane  44 . The latter is cleaned, see FIG. 9, by an air jet  52 —controlled at intervals of time—; it is also possible to involve a plurality of such air jets  54  which are directed on to both surfaces of the filter membrane  44 . A wide-mesh bar grid or lattice  48  can additionally support the filter membrane at the surface  45  which is remote from the grid mesh  46 . 
     In an upward direction. the container 12 terminates at a filter hinsert  26  which is spanned over by a dome cover  30  which is provided axially with a T-shaped connecting pipe  28 . The cover  30  is fixed by a further locking device  24   a  to tie hooks  32  of the container  12 . The upper part thereof is surrounded in FIG.  1 —together with the described container cover arrangement  26 .  30 —by a hood structure  34 . 
     Extending away from the connecting pipe  28  in FIG. 2 are on the one hand a vacuum conduit  27   a  with a vacuum valve  27  for a vacuum pump  27   b  which is arranged upstream of the latter, and on the other hand, a conveyor gas conduit  29   a  for a conveyor gas source  29 . the conduit  29   a  including a closure valve  17 . 
     During a suction intake phase the butterfly valve  16  of the feed conduit  14  is opened and the discharge conduit  22  remains closed. The pump chamber  13  now fills by virtue of a vacuum being built up by way of the vacuum conduit  27   a  up to a desired filling level, possibly also entirely. 
     After a predetermined period of time the feed conduit  14  is closed and the discharge conduit  22  is opened. The powder is ejected by means of pressure—for example of nitrogen for filter cleaning purposes—after opening of the closure valve  17 , into the conveying gas conduit  29   a . At the end of the suction intake phase the vacuum conduit  27   a  remains open for a given period of time before the butterfly valve  16   a  of the discharge conduit  22  is opened in order to remove the oxygen from the pump chamber  13 . 
     A point of particular significance in this procedure is the filter in the filter insert  26 , which retains the powder and at the same time maintains the suction capacity of the system. By virtue of its position between the pump chamber  13  and the conveying gas source  29  the filter is cleaned in each cycle and therefore retains its full filtration capacity. 
     The four closure elements  16 .  16   a ,  17 ,  27  are connected together in terms of control procedure to a control box  35 . During a suction intake phase the butterfly valve  16  of the feed conduit  14  opens whereas the discharge conduit  22  remains closed. By virtue of the vacuum valve  16  which is open in that situation. the pump chamber  13  is sucked full; after a predetermined period of time the feed conduit  14  closes and the discharge conduit  22  is opened. The conveyed material is ejected by pressure—compressed air or nitrogen for filter cleaning purposes. The filter in the upper part of the container  12  retains the finest particles and is cleaned in each emptying cycle. 
     Before the powder is introduced into the downstream-disposed reactor  11  air and powder are separated from each other by closure of the vacuum shut-off valve  16  being delayed with respect to opening of the conveyed material intake  14 . So that no gases of the reactor  11  are sucked in when the discharge conduit  22  is opened, the container  12  is firstly put under pressure and it is only then that the emptying valve  16   a  is opened. Moreover the vacuum conduit  27   a  can only be opened when the discharge conduit  22  is closed. 
     A preferred suction intake phase is one of between 10 and 12 seconds and the emptying time will be of the order of magnitude of between 3 and 5 seconds; in order to avoid an over-pressure in the change in cycle, there is provided a pneumatically regulated throttle means. A second is normally sufficient for that delay procedure. 
     In the same manner, by virtue of the time delay effects in the control procedure, closure of the vacuum for evacuation of the air and opening of the butterfly valve  16   a  for powder emptying can be mutually adapted. A delay of a maximum of one second should also suffice here. 
     The emptying pressure—compressed air or nitrogen—is so regulated that the total amount of powder sucked in is ejected, without unnecessary dust formation. upon opening of the pump chamber  13  (ideal pressure =1.5 to 2 bars). 
     When dealing with very sticky products the pressure can be increased to between 2.5 and 3 bars in order to provide for complete emptying and thorough filter cleaning. 
     By way of example, the following operating states can occur for opened valves: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Pressure 
                   
               
               
                   
                 Feed 
                 Discharge 
                 gas 
                 Vacuum 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Valve 
                   
                 16 
                 16a 
                 17 
                 27 
               
               
                 Suction-intake 
                 1 
                 — 
                 — 
                 — 
                 1-2 sec * 
               
               
                 phase 
                 2 
                 3-20 sec * 
                 — 
                 — 
                 3-20 sec *  
               
               
                 Intermediate 
                 3 
                 — 
                 — 
                 — 
                 1-2 sec * 
               
               
                 phase 
                 4 
                 — 
                 — 
                  1-2 sec * 
                 — 
               
               
                 Discharge 
                 5 
                 — 
                 5-10 sec * 
                 5-10 sec * 
                 — 
               
               
                 phase 
                 6 
                 — 
                 — 
                 — 
                 — 
               
               
                   
               
               
                 * Opening duration in sec.  
               
             
          
         
       
     
     In FIG. 6, two of the apparatuses  10  are mounted in parallel side-by-side relationship on carriers  36 ; their feed conduits  14  open into a common manifold or port pipe  38  with connecting flange  40  for a continuing conveyor conduit which is not shown in the drawing. If the two apparatuses  10  are caused to operate alternately in the described manner, it is possible to make a transition from a sequential system to a continuous system. 
     As shown in FIG. 8, in an annular frame  42  of the filter insert  26 , a filter or a filter membrane  44  is associated with a flat grid mesh  46  associated at the vacuum side, as a support element, the grid mesh  46  being of small mesh size. It can be connected to a vibratory drive (not shown) and transmit the vibrations thereof to the filter membrane  44 . The latter is cleaned by an air jet—controlled at intervals of time—; it is also possible to involve a plurality of such air jets which are directed on to both surfaces of the filter membrane  44 . A wide-mesh bar grid or lattice  48  can additionally support the filter membrane at the surface  45  which is remote from the grid mesh  46 . 
     The ratio of the length a to the diameter d of the container  12  is between 0.5 and 10, preferably between 2 and 8; with those structural factors, with a pressure of between 1 and 25 mbar—preferably between 5 and 20 mbar—on the suction intake side and a pressure of between 0.5 and 5 bars—preferably between 1 and 3 bars—for ejection of the powdery substance, it is possible readily to convey large amounts up to several tonnes per hour. 
     In order to prevent discharge sparks, all system parts such as hoses, valves and the like are conducting and must be grounded. 
     As investigations have shown, it is also possible to provide a metering action with a good degree of accuracy of &lt;10%, with the described pump system or conveyor. 
     Preferred dimensions of the container  12 , with predetermined operating parameters, are to be found in the Table hereinafter: 
     
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Throughput 
                   
               
             
          
           
               
                 Diameter d 
                 Length a 
                   
                 Double 
                   
                   
               
               
                 container 
                 container 
                 Pump 
                 pump 
                 Vacuum 
                 Suction pressure 
               
               
                 (mm) 
                 (mm) 
                 (t/h) 
                 (t/h) 
                 (Nm3/h) 
                 (mbar) 
               
               
                   
               
             
          
           
               
                 300 
                 850 
                 5 
                 8 
                 300 
                 5-20 
               
               
                 200 
                 800 
                 3 
                 5 
                 200 
                 5-20 
               
               
                 150 
                 750 
                 2 
                 3 
                 160 
                 5-20 
               
               
                 100 
                 650 
                 1 
                 1.7 
                 100 
                 5-20 
               
               
                 50 
                 400 
                 0.3 
                 0.5 
                 40 
                 5-20 
               
               
                   
               
             
          
         
       
     
     As the described principle is implemented for loading powders into the reactor  11  under inert gas protection, it is sufficient to replace the air for filter cleaning by inert gas. The internal oxygen content thus remains constant—or even decreases during the conveying period—and the level of N2-consumption remains low. 
     The powders can be very rapidly changed over, even while observing the strictest relevant standards. The suction intake bodies can comprise various materials such as high-quality steel, plastic material, Hastelloy or the like in order to comply with the most important limitations in the chemical sector. 
     The installation can also be connected to weighing systems so that the powders can be accurately metered directly into the reactors  11 .