The present invention relates to an apparatus for mixing both liquid and gaseous substances, such as chemicals, into a medium formed of solid and liquid substance, especially to fiber suspensions being generated during processing of wood or other vegetable-originating substance. To this kind of suspensions typically belong the fiber suspensions of the pulp and paper industry, such as chemical pulp and mechanical pulp suspensions, as well as pulp suspensions of paper production.
For mixing chemicals and gases into fiber suspensions, dynamic mixers are used, which typically are provided with a rotating rotor for effecting the mixing, and static mixers. In the latter, some sort of throttling has been arranged in the flow channel, where the flow rate is increased and the static pressure is decreased. Chemical is introduced into a lower static pressure zone or it can be introduced upstream of the point of throttling. In a static mixer, by throttling the flow, i.e. decreasing the cross-sectional area, an increase in the flow rate is achieved and due to the throttling and the shape of the flow channel a higher turbulence is generated, whereby the introduced chemical will mix into the actual flowing medium. Static mixers are typically provided with, in addition to or alternatively to the throttling, flow barriers arranged in the flow channel for generating turbulence.
Fiber suspension is a demanding material flow in view of mixing, because in order to obtain a good mixing result the fiber network (fiber flocks) are to be decomposed. In the mixing, the turbulence is to be at a level that decomposes the fiber flocks into microflocks and individual fibers, whereby the bleaching chemical is made to be distributed in the vicinity of the individual fibers. Traditionally, with medium consistency fiber suspensions high-capacity fluidizing chemical mixers have been used, wherein the rotor of the mixers generates the turbulence required for the mixing. Although modern fluidizing chemical mixers are reasonably small, intensifying energy consumption creates needs to decrease the amount of energy used for mixing chemicals.
The operation of static mixers is based on utilizing the pressure loss taking place in the apparatus and/or dividing the suspension flow into partial flows and combining them in the flow direction so that the concentration differences upstream of the mixer will be equalized.
European patent 1469937 (WO 03/064018) describes an apparatus for admixing a gas or a liquid into a material flow. In this apparatus a tube with a circular cross section is provided with a chamber for the material flow. The chamber has an inlet part, the cross section of which later changes from circular to oval while the area remains unchanged and an outlet part, the cross section of which later changes from oval to circular while the area remains unchanged. Gas or liquid is fed into the material flow at the narrowest point of the apparatus, which is provided with e.g. small circular holes around the chamber. The change of the material flow from laminar to turbulent state takes place when the minimum height of the oval cross section is defined in a proper way. The gas or the liquid is added in the turbulent zone.
For adding steam into a fiber suspension, direct heat injection heaters are used. In those the steam is admixed directly into a flowing fiber suspension to be heated, whereby the heating takes place quickly. Although direct steam injection heaters are efficient, fiber suspensions with flocking matter tend to clog the heater, if the suspension is to flow through bends and turns. U.S. Pat. No. 6,361,025 describes a direct steam injection heater that is designed for viscose material flows, such as fiber suspension, and in which the steam is introduced into the suspension flowing axially through a tube. In the construction according to U.S. Pat. No. 6,361,025 steam feed takes place in a cylindrical perforated part mounted through the device. The number of open holes can be regulated by means of a cover located inside the cylindrical tube, by means of which the steam feed can be totally closed if needed and also the passing of pulp to the interior of the cover or further into the steam feed piping can be prevented. The perforated cylindrical pipe, referred to as a Mach-diffuser, is mounted transversely with respect to the axial flow of the suspension, whereby it divides the flow space into two parts. Small jets of steam are easily scattered in a viscose suspension and distributed before the steam has a possibility to combine into large bubbles which may generate pressure shocks as steam is suddenly condensed. The smaller the bubbles of the condensing steam, the smaller is the pressure shock caused for the piping. Condensing of a large steam bubble on the inner surface of the pipe causes a strong pressure shock and noise and strong mechanical vibration in the piping. The cover is preferably rotatable with respect to the longitudinal axis of the Mach-diffuser mounted transversely with respect to the flow of the suspension. When the cover is rotated open, holes are freed both from the upper and the lower side, wherethrough the steam flows into the by-passing pulp.
The direct steam injection heat exchanger of the above-mentioned US-patent is advantageous as a steam feeding device, because the steam is fed via several small holes into the by-flowing pulp. As long as the pressure drop across the small open steam holes is adequate, the flow of the steam into the suspension remains even. When the velocity of the steam is adequately high, even condensing of the steam is obtained due to high turbulence caused by the steam feed. The steam condenses evenly, as the condensing takes place near the feed point.
When the pressure loss generated in a static mixer is utilized for effecting the mixing, the mixing result often varies depending on the pressure loss. If a static mixer is based on dividing and combining partial flows, the mixing result is not proportional to the generated pressure loss. A possible problem in this type of devices may be clogging of the partial flow channels and clear deterioration of the mixing efficiency, in addition to an increased pressure loss.
Based on the above, when treating fiber suspensions, the static mixer should break the fiber network, preferably fluidize the through-flowing suspension to an adequate extent, and the mixing result should not be dependent on the generated pressure loss, and partial clogging of the device should not affect the mixing result. In the designing of a device for mixing fiber suspension attention is to be paid to the possibility of ensuring the functioning condition of the device even if the suspension has been thickened e.g. due a disturbance situation at the mill. This means that when the mixer is taken into use it will reach an adequate operational level at the same time as the chemical feed is initiated. If a pressure loss generated in the device is utilized for generating turbulence in static mixers, but it is still desired to limit the extent of the pressure loss, the chemical feed is to be as even as possible with respect to the flow cross-sectional area.
In addition to steam or other gas, it is necessary to introduce into the fiber suspension flow also one or more liquid chemicals, such as bleaching chemical, which has to be distributed and mixed efficiently into the fiber suspension in order to ensure adequately quick and efficient reactions between the suspension and the chemical. For instance the above described apparatus presented in U.S. Pat. No. 6,361,025 and manufactured by Hydro-Thermal Co. has been found an advantageous steam feeding apparatus. However, the mixing capability of static mixers to e.g. mix bleaching chemicals into a fiber suspension should be improved.