Gas separation: processes – With control responsive to sensed condition – Pressure sensed
Reexamination Certificate
2002-10-23
2004-06-15
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Pressure sensed
C095S280000, C055S283000, C055S302000
Reexamination Certificate
active
06749665
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to a method for cleaning a barrier filter, comprising a plurality of filter elements, of woven fabric or felt, arranged to separate particles out of a polluted gas. The filter elements are cleaned, separately or in groups, by pressurised air pulses, the frequency, the maximum pressure, and the duration of which can be varied in order to minimize, by an adjustment, the total emission of dust and maximise the life of the filter elements.
The method is in particular intended for optimising the cleaning of textile barrier filters, having filter elements in the form of tubes manufactured of woven fabric or felt.
BACKGROUND OF THE INVENTION
When separating particulate pollutions out of a streaming gas, one of the most common methods of cleaning is letting the gas stream through a medium, on the surface of which or in the interior of which the particles are deposited. A general term for these filters is barrier filters. Barrier filters can in principle be built-up of almost all thinkable solid materials, but the build-up principles are commonly either a rigid medium, such as a porous ceramic or a gravel bed, or a flexible medium, such as a woven fabric or felt.
During operation, particles are accumulated on the filter medium and a dust heap is built up. This provides an increased flow resistance and consequently leads to an increased pressure drop over the barrier filter. In the long run, it can completely clog the filter medium. A secure operation requires exchanging or cleaning of the filter elements/filter medium, either on site or by being taken out and, for instance, washed or brushed. To purify gases having a low particle content, disposable filters or filters which are taken out and cleaned were frequently used, and to purify gases having a high particle content, barrier filters which are cleaned on site were frequently used.
The cleaning on site can be done in several ways. In smaller plants it can, for instance, be carried out by movable suction nozzles, but in plants for purifying large gas flows the purification is in the most cases carried out by back-washing, shaking, or a combination of thereof, by way of a short pressurised air pulse providing a shock-like displacement of the filter medium, at the same time as the normal gas flow is replaced by a short-time, counter-directional gas flow.
The effectiveness/efficiency of a barrier filter increases by an increasing thickness of the heap of separated dust. A consequence thereof is that the degree of separation will decrease, when a filter element is cleaned. Thus, it is desirable that one does not clean too often and neither completely removes the formed dust heap. The frequency of the cleaning as well as the intensity of the cleaning should therefor be chosen so that an optimal function is achieved. By optimal function is generally meant either that the time mean value of the dust discharge becomes as small as possible or that the energy consumption for the dust separation is minimised under the sub-condition that a given upper limit for the time mean value of the dust discharge is not exceeded.
A common principle of controlling is to let the operation proceed under increasing flow resistance until the pressure drop over the barrier filter reaches a predetermined upper limit value and then to start a cleaning cycle implying that all filter elements, for instance filter tubes or filter cartridges of textile material, are consecutively cleaned, separately or in groups, whereby all receive a similar treatment. After a completed cleaning cycle, the pressure drop is smaller and, thereafter, one waits until the pressure drop, due to the increasing heap of dust, reaches the predetermined upper limit value, at which the next cleaning cycle is started. Since the pressure drop is not only dependent on the filter medium with dust heap, but also increases with the gas flow, one usually regards resistance as a common term for pressure drop or pressure drop corrected with regard to the volume flow of the gas. Henceforth, resistance referring to this extended meaning is used.
As an alternative, the cleaning cycle can be interrupted when the resistance has decreased with a predetermined difference or reaches a predetermined lower limit value. In such cases, the interrupted cleaning cycle is continued when the pressure drop again reaches the upper limit value, so that the cleaning frequency becomes the same for all filter elements.
THE OBJECT OF THE INVENTION
The main object of the present invention is to devise a method for determining the frequency and intensity for cleaning barrier filters in order to achieve an optimal function, which generally implies aiming at the lowest time mean value for the emission of dust.
A second object is to devise a method for determining the frequency and intensity for cleaning barrier filters, providing an increased life for the filter elements relative to known strategies of cleaning.
A third object is to devise a method for determining the frequency and intensity for cleaning barrier filters, allowing an individual adaption of the cleaning for separate filter elements or groups of filter elements in dependence of the dust load for the particular separate filter element or the actual group of filter elements, and thereby dynamically follow changed operational conditions.
The present invention refers to a method for cleaning a barrier filter, comprising a plurality of filter elements, of woven fabric or felt, arranged to separate particles out of a polluted gas. The filter elements are cleaned, separately or in groups, by pressurized air pulses, the frequency, the maximum pressure, and the duration of which can be varied by an adjustment in order to minimise the total emission of dust and maximise the life of the filter elements.
In the method according to the invention, the frequency and/or the maximum pressure and/or the duration of the pressurised air pulses are varied for a separate filter element, for a group of filter elements, or a plurality of groups of filter elements. After each pulse, the maximum value for the instantaneous emission, the emission peak, is determined, and the emission peak is used, after cleaning a certain group of filter elements, for selecting the frequency and/or the maximum pressure, and/or the duration of the pulses for this group of filter elements during continued operation.
In the ideal picture of a barrier filter, all dust is caught on the surface of the filter elements facing the dust-containing raw gas. However, in practice some dust penetrates into the filter material, usually a felt, and a small share thereof passes through.
The cleaning of barrier filter elements, in the form of tubes, bags, or cartridges, where the dust-containing gas streams from outside and into the element by pressurised air pulses, has to be done with regard to several side effects. With the object to achieve as low emission of dust as possible, one should allow a certain thickness of the dust heap on the filter element. This improves the separation, but as a negative consequence it provides an increased resistance and thus an increased energy consumption. In order to prevent the emission immediately after cleaning from being too large, one does not want to remove the entire dust heap in connection with the cleaning. This sets a limit for the size of the pressurised air pulses (cleaning pulses).
When the pressurised air pulse; during the cleaning, rushes as a pressure wave along the filter material, the latter is moved under large acceleration in a direction opposite to the normal gas flow. The movement is deccelerated abruptly, when the filter material is stretched out and thereafter a reversed movement occurs, which is interrupted when the filter material is stretched against the basket or the like holding the filter element stretched during the operation. At the second decceleration, forces of inertia lead to the fact that remaining dust penetrates deeper into the filter material and the amount which thereby passes through the element pr
Bjarnø Odd
Lindau Leif
Alstom Power N.V.
Burns Doane Swecker & Mathis L.L.P.
Spitzer Robert H.
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