Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2000-06-23
2002-05-28
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S446000, C034S524000
Reexamination Certificate
active
06393729
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method, control paradigm and means for controlling and monitoring, during a drying process, process variables of a process gas flowing through a drying chamber used for the drying. Process gas flows continuously through the drying chamber. The conditions in the drying chamber are manipulated by monitoring and controlling a number of different process variables of the process gas according to the inventive control paradigm. The inventive device also includes a device for manipulating, in accordance with the inventive control paradigm, the conditions in the drying chamber by controlling and monitoring a number of process variables of the process gas flowing through the chamber.
PRIOR ART
For drying a water containing material, as well as for evaporation of a solvent or a binding agent during lacquering or manufacturing of plastics materials or ceramic green bodies, a variety of different methods are used, in which the material to be dried is heated for evaporation of a substance, such as water or a solvent. In order to achieve an efficient drying, the evaporated substance is in many processes removed from the zone adjacent to the dried material by means of a drying gas, flowing adjacent to said dried material, preferably air. In the following, prior art and the present invention are described from the point of view of drying a material containing moist or water, or a product containing moist or water, but the technique is of course useful also for other drying processes, in which an evaporated substance is taken up by a process gas, and transported in said gas, away from the dried product.
In order to control the process gas, and in particular its ability to take up and remove the evaporated substance, a process chamber is used for many processes. For stationary batch processes the chamber is often referred to as a drying oven, or a drying cabinet, and for continuous drying of web material or products it is often called a dryer hood. Other drying processes, primarily designed for treating grain size material use drying chambers in the form of rotating cylinders or fluidized beds, in which the grain size material meets and mixes with the drying gas.
Drying cabinets are used for wet water containing pulp material, such as pulp for manufacturing paper or boards, pulp for the mineral, ceramic, metallurgical and chemical industry, for drying solid products such as natural materials, e.g. chips, wood or dewatered products in for instance the ceramic industry, and for wood fibre based boards and some mineral and/or mineral fibre based construction and insulation products, in particular formed products, such as sheet rock, rock wool, glass wool and construction and insulation boards adapted to replace asbestos materials, and also for evaporation of solvents during lacquering, et c. The heat which is necessary for the drying process may be supplied to the material to be dried in any desired way, for instance by means of radiation from infra-red heaters, micro-waves or high-frequency radiation, thereby generating heat in the material to be dried, although it is preferred to use the heat content of the drying gas used. The result of the drying is however highly dependent on the efficiency and the uniformity of the evaporation of the water/solvent which is taken up by the drying gas and removed from the drying cabinet.
Drying cabinets and similar drying chambers are used in continuous web processes for paper, carton, sheet rock, rock wool boards et c. For these processes it is also possible to use a variety of different methods in order to achieve the desired evaporation of water and/or solvent, preferably by supplying/generating heat according to any of the above mentioned methods. Also in this regard, the ability of the drying gas to take up and remove the substance evaporated from the product is important for the result of the drying process. For web drying, for instance, it has since long been common to use dryer hoods. Said hoods are, for drying thin web material such as paper moving with high speed, arranged in combination with drying cylinders adjacent to the web. The paper moves with high speed along a path through the dryer hood, and the result of the drying is mainly influenced by the vapour pressure inside the cylinders, but it is also influenced by the process variables inside the dryer hood, and mainly, then, by the process variables of the process air flowing through the dryer hood.
For paper drying, as well as for a variety of other drying processes, there are, apart from requirements on the efficiency of the drying process, a number of additional boundary conditions, such as
a uniform drying of the entire surface of the product, which means that the product, after drying, has a uniform and reduced moisture content over its entire length and width;
good heating economics;
small leakage of moisture and heat to the surrounding environment;
low noise level;
high accessibility; and
easy access for maintenance, adjustments and repair.
In order to achieve the desired process conditions in a drying chamber, one or more process variables of the process air which flows through the drying chamber are monitored and controlled. A commonly monitored process variable is the water content of the process air, and usually it is the water content of the exhaust air of the drying chamber. Moreover, the water content of the exhaust air influences the process heating economics, and thus the overall process economics. The same goes for a drying process in which a solvent or the like is volatilised.
Another process variable which is significant for the drying process is the internal pressure or the pressure conditions in the drying chamber, which are usually expressed as the zero level of the drying chamber (mm), i.e. that level, at which the pressure difference &Dgr;P=P
surr
−P
hood
, between the pressure in the drying chamber and the surrounding pressure is zero. Normally said process variables are monitored by means of sensing and other means for measurement, arranged adjacent to the drying chamber. The measuring means are arranged in control loops which additionally, according to prior art comprise a conventional controller and actuators. The actuators influence a manipulated process variable which is known to influence the monitored process variable which is being measured. Examples of manipulated process variables used for controlling the process air and thereby influencing the drying conditions in a drying chamber are supply air flow and exhaust air flow (m
3
/min). The supply air flow and the exhaust air flow are controlled by means of an actuator, such as a damper or, as an alternative, a speed control for a variable speed fan. Known and conventional methods for process air control use control loops comprising one controlled and monitored process variable, associated with only one manipulated process variable, and controlled by said variable. In this type of process control, the fact that a change of a manipulated process variable can influence several monitored process variables is neglected, as is the fact that a monitored process variable may be dependent on changes of a plurality of manipulated process variables. Accordingly, this necessitates meticulous and extensive tuning methods for controlling the process air during tuning of a new drying equipment. It is further rather complicated to compensate the process air for changes in other process variables in the drying process, or for changes in other conditions for the drying process, for example changes in the previous process. Further, the process air is in some cases very sensitive to disturbances. What is stated above is also valid for process gases other than air.
It is known, within the field of process control, e.g. from other processes than the drying process in paper making, to use a control loop based on a control paradigm which apart from sensing means, a controller and actuators comprises a dynamic model of the controlled process, and in some cases also a corr
Forsman Krister
Sigvant Roger
ABB AB
Dykema Gossett PLLC
Lazarus Ira S.
Rinehart K. B
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