Gas separation: processes – Degasification of liquid – Plural successive degassing treatments
Patent
1996-06-19
1999-01-19
Silverman, Stanley S.
Gas separation: processes
Degasification of liquid
Plural successive degassing treatments
95241, 95261, 96216, 96196, 554591, 55409, 210188, 494900, 162190, 162 55, B01D 1900, D21D 526, F04D 704
Patent
active
058610529
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an apparatus and a process for separating a gas such as air from a liquid or a liquid suspension such as water or papermaking stock in connection with pumping said liquid. The invention also relates to the use of the apparatus for pumping backwater draining through a forming fabric of a paper machine, for pumping fluid material separated by flotation and for removing air from papermaking stock.
REVIEW OF THE RELATED TECHNOLOGY
In many cases gas enclosed or generated in a fluid to be pumped causes problems. For example, when pumping fluids containing gas with a conventional centrifugal pump, the gas disturbs the pumping. The gas separating from the fluid under the influence of the centrifugal forces in the pumping chamber forms a gradually growing gas bubble, which decreases the power of the pump until the bubble finally is discharged. In processes requiring a constant flow such irregular function causes great disturbances.
As an example backwater drained through a forming fabric in a papermaking process, normally, contains a large amount of enclosed air. Since the short circulation of a papermachine requires a particularly constant flow, disturbing air is normally removed by conducting the drained backwater, by means of special pipe or channel systems, from the dewatering box or boxes to an open backwater tank, from where it is pumped back to the fiber process of the short circulation preceding sheet forming.
Air contained in thin stock of a papermaking process also disturbs the stability of flow in the short circulation and disturbs the forming of a paper sheet by causing voids or holes in the sheet and by retarding the drainage of water.
For the above reasons it is particularly important for the paper forming process to have an efficient deaeration of the drained backwater. Consequently, backwater tanks and other parts of the system are traditionally designed for very low flow speeds in order to provide sufficient time for the enclosed air to emerge and be removed before the backwater is pumped into the closed part of the short circulation. It is also common to incorporate separate deaeration devices into the short circulation in order to remove air which has not been removed from the back water or brought into the short circulation together with the stock.
Due to slow flow speeds, the recycling of backwater into the fiber process takes a significantly extended time, which in turn retards the reaching of a new equilibrium state after process changes, such as a change of the paper grade. In addition thereto the slow flow speeds cause dirt buildup in tanks and pipings due to deposit therein of dispersed material and because of biological activity.
In cases where flotation is used for separating materials, such as in connection with de-inking of recycled wastepaper or the recovery of fibers from the long circulation in a paper machine, air separation, defoaming and pumping are particularly difficult and mostly require separate defoaming or settling tanks.
Pumps, which are able to separate gas from a fluid to be pumped are well known as such, but the objective of such pumps is normally just to remove a sufficient portion of the gas to enable regular pumping. The known pumps are normally not capable of removing enough gas for achieving the degree of freedom of air, which is required for using the fluid directly for example in a papermaking process without further deaeration. Deaerating pumps have also been designed for the treatment of fiber suspensions with a rather high solids content and high viscosity. In such pumps the separation of gas is generally caused by high shear forces required for fluidizing the highly viscous fluid which is to be pumped.
Examples of prior art pumps capable of uniformly pumping fluids which contain gas are disclosed in Patents such as FI 67591, U.S. Pat. No. 4,410,337, U.S. Pat. No. 5,039,320, FI 73023, FI 75912. Said pumps are so called MC-pumps developed for pumping high consistency (about 5% to 20%) pulp which must b
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Fortuna Jos'e A.
POM Technology Oy AB
Silverman Stanley S.
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