Abstract:
A system and method for drying particulate material in a dual flow hopper employs multiple flow paths by which heated gas is inserted into the hopper at different levels. Gas is extracted from the hopper and is divided for flow along two flow paths, one of the flow paths receiving gas that has been dried and the other flow path receiving gas that has not been dried.

Description:
BACKGROUND OF THE INVENTION 
   This invention is concerned with drying solid bulk materials, such as plastic pellets, powders or other particulates. 
   Commonly owned U.S. Pat. No. 6,289,606 issued Sep. 18, 2001 and U.S. Pat. No. 6,584,701 issued Jul. 1, 2003 disclose systems and methods for drying particulate material in a hopper using heated gas, both patents being incorporated herein by reference. 
   In U.S. Pat. No. 6,289,606, dehumidified air from a dessicant bed is supplied to a hopper and is returned from the hopper to the dessicant bed. When the dew point of the return air drops to a pre-set level, a diverter causes the return air flow to bypass the dessicant bed and to be re-supplied to a hopper without dehumidification. 
   In U.S. Pat. No. 6,584,701, air from a compressed air inlet flows through a membrane dryer and a heater and is supplied to a lower portion of a dual flow hopper. Another portion of the air from the inlet is mixed with air extracted from the hopper. The mixed air flows through a heater and is supplied to a higher portion of the hopper. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention takes advantage of a dual flow hopper having inlet ports at two different levels, without requiring a source of compressed air, a membrane dryer, or an air mixer. Gas extracted from the hopper is divided into two parts, a first part being returned to the hopper along a first flow path after being passed through a dryer, a second part being returned to the hopper along a second flow path without being passed through a dryer. By virtue of the fact that only part of the gas extracted from the hopper passes through the dryer, a smaller dryer can be used, which lowers the initial equipment cost, saves floor space, and reduces the energy required compared to a larger dryer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described in conjunction with the accompanying drawings, which illustrate preferred (best mode) embodiments, and wherein: 
       FIG. 1  is a diagrammatic side view of a system in accordance with the invention; 
       FIG. 2  is a diagrammatic top view of the system of  FIG. 1 ; and 
       FIG. 3  is a diagrammatic end view of a portion of the system. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIG. 1 , a drying system in accordance with the invention includes a chamber  10  containing material to be dried and a dryer  12 . The chamber  10  is preferably a dual flow hopper of the type described in the aforesaid U.S. Pat. No. 6,584,701, in which material to be dried flows downwardly from the top of the hopper and exits the hopper at the bottom, while drying gas (e.g., air, nitrogen or other gas) supplied to the hopper through ports at different levels flows upwardly and is extracted from the hopper for recirculation to the hopper. The dryer is preferably a dual tower dessicant dryer, such as that described in the aforesaid U.S. Pat. No. 6,289,606, with valves that adjust process gas flow so that one of the towers can be in use while the other is being regenerated. 
   The system of the invention is a closed loop system that includes three process flow paths  14 ,  16 ,  18 . Gas extracted from the hopper flows along the third flow path  18  and through a process filter  19  to a splitter  20  that divides the extracted gas into a first part and a second part for flow along the first and second flow paths  14  and  16 , respectively. Gas flow along the first flow path  14  is passed through a heater  22  and is inserted into a lower portion of the hopper via a diffuser  24 . Gas flow along the second flow path  16  is passed through a heater  26  and is inserted into a higher portion of the hopper via a diffuser  28 . From the splitter  20 , the gas to flow along the first flow path  14  passes through the dryer  12 . Gas from the splitter to the second flow path  16  does not pass through the dryer, and is returned to the hopper. A process blower  30  provides the gas flow along all three flow paths. 
   In operation of the system of the invention, gas laden with moisture from the material in the hopper  10  returns to the dessicant dryer  12  along the third flow path  18 , passes through the process filter  19 , and is circulated through the system by the process blower  30 . Part of the moisture-laden gas from the process blower recirculates directly back to the hopper along flow path  16  where it is heated by the heater  26  to a selected drying temperature before entering the hopper through an upper port. This gas stream heats material in the upper portion of the hopper while surrounding the material with gas that is generally drier than ambient, but not extremely dry. 
   Another part of the gas extracted from the hopper passes through the dessicant dryer  12 . Moisture is removed from this gas stream, which returns to the hopper along the first flow path  14 , where it is heated by the heater  22  to a required drying temperature before entering the hopper through a lower port. This very dry gas heats material in the lower portion of the hopper while it surrounds the material with a gas that is sufficiently dry for the material to reach its final moisture equilibrium content. 
   The system of the invention can be enhanced with optional gas flow controllers  32 , using dampers, for example, which may be either manually or automatically controlled, or using orifices, to limit the amount of gas returned to the hopper along the first and second flow paths. 
   In a fully automatic system, dew point sensors  34  can measure the dew point of the gas flow along each of the flow paths, and air flow sensors  36  can measure the air flow along flow paths  14  and  16 . The flow controllers  32  can then be set in response to the measured dew points and air flows to maximize drying while minimizing energy requirements. The controls can be used to determine the portion of dry gas required in flow path  14  compared to the recirculated gas in flow path  16  to achieve the desired dryness without overdrying, while minimizing the amount of energy used. Certain features of the dryer  12  are designated in the drawings by appropriate legends and are further described in the aforesaid U.S. Pat. No. 6,289,606. 
   While preferred embodiments of the invention have been shown and described, it will be apparent that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the following claims.