Drying and gas or vapor contact with solids – Process – With contacting of material treated with solid or liquid agent
Reexamination Certificate
2003-07-21
2004-05-04
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
With contacting of material treated with solid or liquid agent
C034S330000, C034S062000, C034S080000, C095S119000, C095S120000, C096S130000, C096S135000, C096S136000
Reexamination Certificate
active
06729039
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method and a system for controlling airflow in a multiple bed desiccant drying system, particularly in a twin tower desiccant dehumidifier, and particularly during a regeneration phase, and during transition periods between phases.
Multiple desiccant bed systems used for drying a synthetic plastic material are known, in which a moisture-laden gas stream is formed as the exit gas from a hopper in which plastic granules are dried by a stream of drying air. During an adsorption phase, the exit gas from the hopper is conducted through one or more drying vessels filled with an adsorption medium, whereby the adsorption medium extracts the moisture from the gas so that the resulting dry gas can be used again as a drying gas for drying plastic granules.
When the adsorption medium in a drying vessel is saturated with moisture, the drying vessel is transferred to a regeneration phase in which heated ambient air is conducted through the adsorption medium which takes up and carries away the moisture which was adsorbed therein. The ambient air used to dry the adsorption medium typically itself contains moisture, which increases the drying time required to regenerate the adsorption medium.
Since the adsorption medium is heated during the regeneration phase by the heated regeneration air, the adsorption medium typically must subsequently be cooled with a cooling air flow prior to a transition to the adsorption phase. If moisture-laden ambient air is used as the cooling air, the adsorption medium will adsorb the moisture therefrom, reducing the efficiency (i.e. dryness) of the regeneration process.
By using a multiple bed system, the drying process can be continued essentially without interruption, by utilizing one or a portion of the beds for adsorption, while simultaneously regenerating other of said beds, by appropriately channeling the process air flow.
Prior art twin tower dehumidifiers typically use two four-way valves to divert the respective air streams to the process and regeneration desiccant towers. This is done by porting the left and right sides of the four way valve to the desiccant towers, and by using the top port for the process air and the bottom port for the regeneration air. In this manner twin tower units are constructed for simple selection of the process airflow to one tower and the regeneration airflow to the other tower. The control of regeneration heating is limited to turning the regeneration fan and heater on.
An alternate method of construction for a twin tower adsorber system is to use a poppet type valve that will divert a central, process air inlet port to the left or right tower, and will then “bleed” a fixed airflow for the regeneration of the opposite tower. In this “bleed” method of regeneration with a continuous regeneration airflow, the control of the regeneration is limited to applying heat, with no control of the airflow.
Multiple desiccant tower dehumidifiers typically use a rotating “carousel” which holds the desiccant towers. In these systems the control of the regeneration is governed by the rotation of the desiccant system and is limited to the progressive rotation of the towers from one stage to the next. There is no individual control of the regeneration airflow with this type of unit. Heating of the desiccant is normally accomplished by the use of an external heating unit with the regeneration airflow conveying the necessary heat into the desiccant tower.
Conventional multi-tower desiccant bed dehumidifier controls utilize a single point proportional integral derivative (PID) anticipating temperature controller in which a thermocouple at the heater unit outlet measures the result of the control operation and feeds this result back to the controller. The controller uses this feedback to make adjustments to the heater operation by balancing the heater on-off cycles accordingly. Such control systems are sensitive to inlet air temperature variations and thus are subject to the drawback that they are limited in the amount of inlet air temperature disruption they can manage without unacceptable temperature variations appearing at the process air outlet. Experience has shown that an inlet air temperature variation of ±10° C. (±18 to 20° F.) over a few minutes will disrupt such a controller to such an extent that unacceptable temperature fluctuations will occur in the process air stream. Since the inlet air temperature variations of as much as 55° C. (100° F.) typically occur when the towers of a twin tower desiccant bed dehumidifier are switched between the adsorption and regeneration phases, exchange of the towers results in an interruption of processing due to a loss of process of control and requires a restabilization period before processing can resume.
In commonly owned U.S. Pat. No. 5,926,969 to Crawford et al., the entire disclosure of which is expressly incorporated by reference herein, a system and method of operation is disclosed in which two towers are connected by a 4-valve system. The 4-valve system is controlled such that the process air stream is progressively moved from the saturated bed to the regenerated bed. In that invention the terminal disruption of the dewpoint is minimized by any residual heat that remains in the fresh tower being brought online.
A further improvement is described in commonly owned, prior U.S. patent application No. 09/554,680 to Crawford, the entire disclosure of which is also expressly incorporated herein by reference. This improvement to the twin tower desiccant dehumidifier uses a split air stream from the inlet to the system as the cooling medium. In a typical closed loop process system, this inlet air stream has a much lower water content than the ambient air used for regeneration heating. The benefit of this improvement to the regeneration cooling of the desiccant was developed through use of the relative dry process return air as the desiccant cooling medium. While this is a successful method of operation, it has required the use of closed loop control for the management of the cooling air stream to avoid disruption of the process air quality. This is accomplished by the use of modulating control of the four-way dry air control valves of the dehumidifier with monitoring instruments to assure the proper control of the process. As in most industrial processes, the economics of the process system are under substantial discussion, and a premium is placed on reduction of costs for the equipment. Since the control costs are a major concern, there has remained a need for a different method of air control which would enable a less expensive control system to develop equal or greater process performance.
In previously known drying systems, a problematic issue is disruption of the process air temperature and dew point quality when changing from the saturated tower to the freshly regenerated tower. Furthermore, in previously known drying systems, bringing a heat exchanger online at the appropriate time without undue complexity of valves is a known problem. As an additional issue, previously known drying systems may be contaminated by room air during diverter valve changes. In addition, since the common instrumentation used to determine the dryness of the process air stream may require from 30 to 60 minutes to recover from the exposure to a high intermittent dewpoint, it is difficult to determine and monitor the humidity level of the process air in previously known drying systems.
SUMMARY OF THE INVENTION
In view of the above, there is a need for an improved method and a system for controlling airflow in a multiple bed desiccant drying system.
There is also a need for a method and a system which can is be implemented in existing multiple bed desiccant drying systems with a minimum of components, effort, and cost.
There is a particular need for a regeneration air control method and system which can maintain adequate process air temperature and dew point stability while cycling of desiccant towers between adsorption and regeneration phases of opera
Crowell & Moring LLP
Lazarus Ira S.
O'Malley Kathryn S.
Universal Dynamics, Inc.
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