CA2132165A1 - Method for rapid ozone bleaching of wood pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for rapid bleaching of high consistency wood pulp using ozone gas provides for generating ozone in a carrier gas at a concentration of five percent to fifteen percent by weight, introducing the ozone in its carrier gas together with the wood pulp into a bleaching reactor at a pressure of more than one atmosphere, retaining the pulp and ozone gas in the bleaching reactor for a required bleaching time, and discharging the bleached pulp to a receiving vessel for further processing.

Description

~ Docket No. 1958-IR-PA

METHOD FOR RAPID OZONE BLEACHING OF WOOD PULP

This invention relates generally to bleaching of wood pulp and more particularly to rapid rate bleaching of pulp at more than twenty percent consistency, using gaseous ozone as the bleaching reagent.

Wood pulp bleaching with ozone and other relatively high reaction rate gases promises a significant reduction of objectionable pulp mill effluents to streams and to the environment. It is to be expected that great economic and ecological benefits can be achieved by elimination of chlorine compounds from the bleaching sequence. However, incorporation of these noncholorine bleaching reagents can impose significant capital costs on the mills due to the requirement for large complex mechanical contactors and reaction vessels. Because of these high capital equipment costs, virtually no production scale experiments have been done, despite the availability of such large reaction vessels.

Aside from its high cost, another deterrent to application of bleaching~with ozone gas has been the difficulty in obtaining uniform bleaching results. This is primarily attributable to the rapid reaction rate of ~ J Docket No. 1958-IR-PA

ozone with wood pulp, which results in overbleaching of -some fibers which are first contacted by the ozone bearing carrier gas, and underbleaching of other fibers which are contacted after the ozone concentration in the carrier gas has been severely reduced. It is obvious that, if each pulp fiber were individually presented to the bleaching atmosphere, all such fibers would be uniformly bleached. To this end, the pulp industry has directed intense efforts toward ever more vigorous comminution to achieve finer fiber bundle particle sizes in high consistency pulp in order to achieve greater uniformity of contact between pulp fibers and ozone bleaching gas. In addition, numerous machinès of varying degrees of complexity have been employed in attempts to improve the uniformity of bleaching by agitating the pulp in the presence of the bleaching gas. These efforts have -been directed toward elimination of channelling of the ~-~
bleaching gas through the pulp bed and increasing the contact of unbleached fiber with the ozone gas.

Mixing reactors, however, frequently aggravate the nonuniformity of bleaching by causing compaction of some of the pulp into relatively impervious masses which are not amenable to penetration by the bleaching gas. This ~-compaction also causes increased power consumption, thereby further reducing its economic viability. Of ~ Docket No. 1958-IR-PA

course, the longer such intense agitation is required for completing the bleaching reaction, the greater will be the size of the agitating reactor required for a given pulp production rate, and the greater will be the required operating power. It is clear that currently available agitating reactors are not capable of avoiding the problems described above.

The foregoing illustrates limitations known to exist in present devices and methods. Thus it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

SUMMARY OF THE INVENTION

In one aspect o~ the present invention, this is accomplished by providing a method for a rapid bleaching of wood pulp at a consistency of more than 25% using ozone gas, including the steps of introducing ozone in a carrier gas, at a concentration of five percent to fifteen percent, by weight, into a bleaching reactor at a pressure of at least one and one-half atmospheres;

retaining the pulp and ozone gas in the bleaching reactor ~ A ~ Docket No. 1958~IR-PA

for a required bleaching time; and discharging bleached pulp to a receiving vessel for further processing.

The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
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BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic block diagram illustrating a preferred embodiment of the present invention;

~, Fig. 2 is a graphical representation showing the relative ozone consumption rates for two different ozone feed gas concentrations; and -~

Fig. 3 is a graphical illustration of the effect of pressure on ozone consumption rate in a cocurrent flow pulp bleaching reactor at a twelve weight percent ozone feed gas concentration.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 illustrates~a portion of a pulp processing line in which the present invention is embodied. Pulp from conduit 5 enters pulp thickener 10 and is thickened )J
Docket No. 1958-IR-PA

to a consistency of twenty percent or greater before being discharged through conduit 15 to a fluffer 20.
After being fluffed, the pulp is transported through conduit 25 to contacting reactor 300 in which the fluffed pulp particles are mechanically tossed in the presence of ozone in a carrier gas which is introduced through conduit 67. After the required bleaching time in the contacting reactor, the pulp, together with the reaction gases, is discharged through conduit 35 to receiving vessel 40. Here the gas is separated from the pulp and discharged through conduit 45 to a gas purifier 60, in which reaction products and other impurities are removed from the carrier gas. The pulp, having been separated from the gas in receiving vessel 40, is discharged through conduit 55 to further processing. The carrier gas from purifier 60 is discharged into conduit 62 where it mixes with fresh ozone from ozone generator 100, which is introduced through conduit 105. This gas mixture enters gas compressor 65 in which the gas mixture may be compressed to as much as fifteen atmospheres pressure.
The pressurized gas is metered through conduit 67 to contacting reactor 300.

' The process just described is continuous and is made possible by formation of a pulp plug at the juncture of conduit 15 and pulp fluffer 20 and another plug at the ., ~

Docket No. 1958-IR-PA

juncture of conduit 55 and receiving vessel 40. These pulp plugs are being continuously added to at their rearward faces and continuously subtracted from at their forward faces, thereby providing continuous throughput of pulp while retaining higher than atmospheric pressure in the system between pulp fluffer 20 and receiving vessel 40. This permits operation of contacting reactor 300 at higher than atmospheric pressure which increases the rate of the pulp bleaching reaction. The rate of reaction of lo ozone with wood pulp is directly proportional to the ozone concentration and the pressure at which the reaction is carried out. Thus, it is clear that by maintaining the plugs previously described, it is feasible to conduct a continuous reaction at elevated pressure in which pulp is introduced at one end and discharged at the other end without a loss in pressure which would upset the reaction. In the past, the maximum ozone concentration which was economically available from standard ozone yenerators was approximately six percent.
Recently, however, it has become possible to generate ozone at concentrations approaching fifteen percent.

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Figs. 2 and 3 graphically illustrate consumption of ozone in wood pulp bleaching as a function of time of reaction. In ~~ig. 2 the effect of ozone concentration is .

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Docket No. 1958-IR-PA

shown, and as predicted, the consumption rate of ozone at twelve percent is considerably higher than at three percent. Here, also, it should be noticed that consumption rates for cocurrent and countercurrent flows are significantly different at a yiven ozone concentration. In the case of a high intensity mixed reactor, the time for 100% consumption of the ozone applied to the pulp can be reduced by as much as 50%.

The effect of pressure on the ozone pulp reaction rate is indicated in Fig. 3. Again, as expected, the ozone consumption rate in the pulp bleaching reaction is significantly increased by increased pressure. It follows that combining high concentration ozone as in Fig. 2 with high pressure ozone as in Fig. 3 will produce a pulp bleaching reaction which is significantly faster than was previously possible. This makes it possible to accomplish the same pulp production in a significantly smaller reactor.

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Claims (3)

1. A method for rapid bleaching of wood pulp at a consistency of more than 25% using ozone gas, comprising the steps of:
introducing ozone in a carrier gas, at a concentration of five percent to fifteen percent by weight, together with wood pulp, into a bleaching reactor at a pressure of at least one and one-half atmospheres;
retaining the pulp and ozone gas in said bleaching reactor for a required bleaching time; and discharging bleached pulp to a receiving vessel for further processing.

2. The method of claim 1, wherein ozone is introduced in said carrier gas at a concentration of approximately eight to thirteen percent by weight.

3. The method of claim 1, wherein the ozone is introduced into the bleaching reactor at a pressure of approximately eight to twelve atmospheres.