Drying and gas or vapor contact with solids – Process – Gas or vapor contact with treated material
Reexamination Certificate
2001-08-31
2002-09-10
Lazarus, Ira S. (Department: 3749)
Drying and gas or vapor contact with solids
Process
Gas or vapor contact with treated material
C034S114000, C034S445000, C034S446000, C034S448000, C034S454000, C034S546000, C034S568000
Reexamination Certificate
active
06446356
ABSTRACT:
The object of the present invention is a method relating to the preamble of the independent claim presented below.
The invention relates in this case especially to a method for controlling the drying process taking place in the dryer section of a paper machine in such a way that an advantageous drying process is obtained from the viewpoint of quality and/or energy costs.
Paper webs have for long been dried mainly by means of drying cylinders, a large number of which are fitted in succession in the dryer section in one or two, or even more, rows situated on top of one another.
When drying with drying cylinders, the drying energy is obtained from the hot steam by means of which the cylinders are heated. In the dryer section the cylinders are combined into cylinder groups, typically into groups of 3 to 8 cylinders. Pressurised, saturated steam is passed through the cylinders in each cylinder group at a pressure calculated in advance for the said cylinder group. The exhaust steam that has flowed through each cylinder group and the condensate are passed to the condensate tank, from which the steam—now at a lower pressure—is passed to the next cylinder group. In this way, for example, live steam fed to the dry end of the dryer section at a pressure of 3 bars can be passed on through all cylinder groups in the dryer section, towards the wet end of the dryer section. In the first cylinder group of the dryer section the pressure is typically below atmospheric pressure. If necessary, that is, in order to obtain the desired pressure, live steam can be supplied to the various cylinder groups in addition to the exhaust steam.
Drying is controlled by regulating the pressure of the live steam supplied to the dryer section. Control may take place manually or automatically. The efficiency of the dryer groups is typically controlled on the basis of a recipe, such as cascade control, which is guided by the dry matter content of the web coming out of the dryer section. The recipes used are recorded set value recommendations which have been found to be advantageous from the point of view of the quality of each paper grade respectively. However, the operational point of the process varies during production, which means that the set values would have to be adjusted continuously. Generally, adjustments to set values required by changes in the drying process are not made until the measured quality values change to the extent that they go beyond the limits set for them.
Drying with drying cylinders in its present form functions relatively well—it has been possible to increase the speeds of paper machines and runnability has improved thanks to closed draws. The great length of the dryer section has, however, still presented a problem, as it incurs considerable construction costs. Neither has cylinder drying always been considered sufficiently effective. The aim has, therefore, been to find new and more efficient solutions for web drying.
For some time now, infrared heaters have been incorporated in the dryer section, the said heaters being, however, used mainly for controlling the cross direction profile of the web.
Air impingement drying, that is, evaporation drying carried out by blowing hot air or other suitable hot gas, such as superheated steam, towards the web, has proved to be an efficient drying method. Air impingement can, for example, be directed at the web as it travels, supported by the dryer wire, across the surface of a large vacuum roll, cylinder or other likewise linear surface, as disclosed for example in the Applicant's earlier Finnish patent applications FI 971713, FI 971714 and FI 971715. In air impingement, high-speed hot air jets or, for example, jets of superheated steam, are blown from a hood covering the said surface towards the web being dried, which travels on the said surface. Air impingement thus brings about a powerful evaporation drying effect. An efficient ventilation effect for blowing off the humidity that has evaporated from the web is also achieved by means of air impingement. The drying energy required for air impingement is obtained, for example, from natural gas or another suitable fuel which can be used for heating the impingement air. Air impingement also requires blowing energy, electricity, for circulating the hot air in the drying device, that is, for blowing the hot air towards the web and for removing humid air from the area surrounding the web.
By means of air impingement devices, it has been possible to improve the efficiency of drying taking place in the dryer section considerably. Due to more efficient drying, it has been possible to shorten the dryer section correspondingly. Moreover, by means of air impingement, the drying conditions can be changed much more rapidly than by means of drying cylinders.
However, the control of drying in the dryer section is not under the control of the process controller in a manner that would be desirable. The recipe-based set-value control of adjustments is not a sufficiently effective tool for controlling more efficient drying and for taking into account the drying requirements at different points of the dryer section. Neither do recipe-based adjustments take the cost factors relating to different forms of energy into account.
The aim of the present invention is, therefore, to achieve an improved method for controlling the drying process in the dryer section of a paper machine.
The aim is more particularly to achieve an improved method which allows better than before for the different drying requirements at different points of the dryer section and the cost factors relating to different forms of drying energy.
A further aim is to achieve a method, which allows both for quality requirements and for cost factors in drying control in the different parts of the dryer section.
In order to achieve the above aims, the method relating to the invention is characterised by what is specified in the characterising part of the independent claim presented below.
Controlling the drying process taking place in the dryer section of the paper machine so as to be optimal from the point of view of quality and costs can, according to a typical method relating to the invention, be carried out as follows:
a method known as such is first used to calculate the total power requirement of the dryer section, which in this application refers to the amount of energy transferred to the web in order to effect the evaporation desired. The total power requirement can be determined on the basis of the total evaporation requirement
Typically, total evaporation, that is, the amount of water to be evaporated, is calculated from the difference between initial moisture content and desired final moisture content, when the production rate is known.
Total evaporation can also be calculated on the basis of the amount of water discharged with the exhaust air from the dryer section, that is, by measuring the flow and humidity of the exhaust air, when the flow and humidity of the supply air are known.
On the other hand, the value of total evaporation can also be calculated by using physical and mathematical models known as such, when the process parameters are known.
The dryer section is then divided according to the paper grade, into imaginary drying segments, which behave differently as to drying or evaporation, or some other quality criteria. In this specification, the division of the dryer section or the division of the drying process has been described by the term “segment”. Alternatively, the terms “stage” or “phase” could also have been used.
The first segment typically covers that part of the dryer section in which the web is heated to a temperature advantageous for evaporation. In this first segment, little evaporation takes place, nor is a high evaporation efficiency required in this segment. The next, that is, the second segment typically covers that part of the dryer section in which the free water, that is, the readily evaporable water in the web, is evaporated from it. A high level of evaporation takes place in the second segment and thus also t
Hamström Kristian
Kari Jorma
Sundqvist Hans
Lathrop & Clark LLP
Lazarus Ira S.
Metso Paper Inc.
O'Malley Kathryn S.
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