SE541075C2

SE541075C2 - Ultrasound-assisted separation

Info

Publication number: SE541075C2
Application number: SE1700048A
Authority: SE
Inventors: Håkan Envall; Ulf Håkan Dahlberg
Original assignee: The Packaging Greenhouse Ab
Priority date: 2017-03-10
Filing date: 2017-03-10
Publication date: 2019-03-26
Also published as: SE1700048A1

Abstract

The present invention relates to a method and arrangement for separating a first constituent from a second constituent comprised in a liquid mixture contained in a separator, said first constituent having a lower density than said second constituent, said method comprisinga. generating at least one horizontal standing ultrasonic wave field in said mixture for a predetermined period of time whereby said first constituent is accumulated at antinodes and said second constituent is accumulated at nodes of said standing ultrasonic wave fieldb. withdrawing a separated first constituent at the surface from said liquidmixture.

Description

Ultrasound-assisted separation The present invention relates to a method of separating a first constituent, e.g. a liquid or solution, from a second constituent, e.g. a liquid or solution, having different densities. In particular, the invention relates to a method of separating tall oil soap from black liquor or from separating tall oil soap from tall oil. The invention also relates to an arrangement for separating said first constituent from said second constituent. The invention also relates to the use of ultrasonic radiation to accelerate floatation and settling processes for separating such constituents.

Background of the invention It is known in the art that liquids can be separated from each other by means of conventional operation units, e.g. separation of tall oil soap from black liquor, a by-product obtained from the sulphate pulp process. Since the tall oil soap has a lower density than the black liquor, the soap will rise to the surface while the heavier black liquor will settle. In a separation tank, a soap-black liquor interface will be formed above which purified soap is accumulated.

However, unaided separation of soap in a tank without any accelerating tools is slow and not cost-effective. Also, the purity of the soap withdrawn from such conventional separation process based on unaided floatation of soap/settlement of black liquor may result in too poor quality of the end product. Slow separation of soap also implies greater energy losses due to the fact that the black liquor to be purified from soap cools down resulting in a greater demand of steam for subsequent evaporation of the purified black liquor. A greater remainder of soap in the purified black liquor further results in worse mass balance of chemicals in the liquor cycle in the pulp mill.

The present invention intends to provide a cost-efficient separation method allowing shorter time of separation. In particular, the invention regards provision of a further purified black liquor and a higher yield of purified soap. The invention can also be used to provide a further purified tall oil and a higher yield of purified tall oil. The invention also intends to provide a process and an arrangement without utilization of expensive and energy-consuming separation machinery. More generally, the invention relates to a method of separating any constituents such as liquids having different densities in a more cost-efficient way resulting in purer end products.

The invention The present invention relates to a method for separating a first constituent, e.g. a first liquid or solution from a second constituent, e.g. a second liquid or solution, comprised in a liquid mixture contained in a separator, said first constituent having a lower density than said second constituent, said method comprising a. generating at least one horizontal standing ultrasonic wave field in said liquid mixture for a predetermined period of time whereby said first constituent is accumulated at antinodes and said second constituent is accumulated at nodes of said standing ultrasonic wave field b. withdrawing a separated first constituent at the surface from said liquid mixture.

It has been found that by generating a horizontal standing ultrasonic wave field in a liquid mixture comprising constituents having different densities, separation of the constituents can be obtained speeding up natural floatation and settling processes known in the art. The interaction between said first and said second constituents is reduced and rising of the first constituent having a lower density is faster than the natural rising time in the absence of a standing ultrasonic wave field. As a consequence of the separation, settling of the second constituent is also faster. An overall faster separation process is thus obtained by said wave field. The first constituent having risen to the surface of the liquid mixture can be withdrawn in a conventional way.

When generating a standing ultrasonic wave field, the wall of the separator acts as a reflector for the standing waves. A sonotrode, as further described herein, is sending out ultrasonic waves resulting in generation of a standing ultrasonic wave field.

The term “standing wave”, as used herein, is a wave in a medium, in this particular case in a liquid mixture, in which each point on the axis of the wave has an associated constant amplitude. The locations at which the amplitude is minimum are called nodes, and the locations where the amplitude is maximum are called antinodes.

As further taught in Phys. Chem. Chem. Phys., 2016, 18, 21-46, p.35-36, section 4.3.4, in a standing acoustic pressure field ultrasound generates an acoustophoretic force F which causes the migration of particles towards the pressure node or antinode as further described in figure 5 herein illustrating this migration. The value of the radiation acoustophoretic force depends on the physical properties of a particle with a characteristic acoustic contrast factor ?. If ? < 0, the particle moves to the pressure node and vice versa. Rigid particles and most cells have a negative ?-factor while gaseous bubbles and lipids are collected at the antinode. Particles, which are equally affected by acoustic radiation forces, can be separated by altering the density of the fluid. Acoustic standing waves are generated in a fluid-filled cell and satisfy the path length being half the ultrasound wavelength. The displacement nodes appear at a spacing of ?/2 and the pressure nodes are formed at the positions of the displacement antinodes.

Preferably, the first constituent is tall oil soap. The term “tall oil soap" refers to the soap present in the black liquor obtained from a Kraft or sulphate pulp mill as further described in e.g. US 8419897 where the tall oil soap is also called black liquor soap. Preferably, the second constituent is black liquor. Preferably, the liquid mixture fed to the separator comprises tall oil soap and black liquor, preferably with a tall oil soap content of 1 to 5 wt%. According to one embodiment, the liquid mixture is biodiesel containing glycerin to be separated from the biodiesel. According to one embodiment, the liquid mixture is tall oil containing tall oil soap which soap is to be separated from the tall oil.

In particular, the present invention relates to a method for separating tall oil soap from black liquor comprised in a mixture contained in a separator comprising a. generating at least one horizontal standing ultrasonic wave field in said mixture for a predetermined period of time whereby tall oil soap is accumulated at antinodes and black liquor is accumulated at nodes of said standing ultrasonic wave field b. withdrawing the tall oil soap separated at the surface from said mixture.

The present invention also relates to a method for separating tall oil soap from tall oil comprised in a mixture contained in a separator comprising a. generating at least one horizontal standing ultrasonic wave field in said mixture for a predetermined period of time whereby tall oil soap is accumulated at antinodes and tall oil is accumulated at nodes of said standing ultrasonic wave field b. withdrawing the tall oil soap separated at the surface from said mixture.

By the term “separator” is meant to include e.g. a settling tank, container or other separator unit in which separation of at least one constituent can be performed. The separator may be static or rotatable.

When exposing the mixture to ultrasonic radiation to generate a standing ultrasonic wave field, the density of the soap is lowered due to increased agglomeration of bubbles resulting in faster rising of soap and faster settling of black liquor. Yet a further advantage of the method is that the ultrasonic energy added is transferred to heat energy in the liquor thus contributing to faster rise of soap since the heat energy lowers the viscosity of the soap.

According to one embodiment, the liquid mixture, or solution, treated herein comprises plural constituents, e.g. more than two constituents which may be in a liquid and/or solid state.

Preferably, the separation method is continuous. For example, the cleaned tall oil soap is continuously withdrawn from the separator; black liquor with tall oil soap are continuously fed to the separator via an inlet stream; and cleaned black liquor is continuously withdrawn from the separator.

Preferably, said standing ultrasonic wave field is generated by a sonotrode system comprising at least one sonotrode.

According to one embodiment, at least one sonotrode sends out ultrasonic waves pulse-wise for periods ranging from 1 to 30 seconds, for example 1 to 10 seconds or 1 to 5 seconds. According to one embodiment, the sonotrode system is switched off between ultrasonic wave pulses for about 1 to 30 seconds, for example 1 to 10 seconds or 1 to 5 seconds.

According to one embodiment, said at least one sonotrode is arranged in the liquid mixture sending out standing ultrasonic waves in a frequency range from 20 to 50 kHz.

According to one embodiment, air is injected to the liquid mixture. It has been found air bubbles can accelerate the separation process by lowering the density of agglomerated tall oil soap resulting in more rapid rise of the soap to the surface of the liquid mixture.

According to one embodiment, a level measurement sensor for said first and second constituents is arranged to monitor the level of the respective constituents.

The invention also relates to an arrangement for separating a first constituent from a second constituent in a liquid mixture, said first constituent having a lower density than said second constituent, said arrangement comprising a. a separator for retaining the liquid mixture b. a sonotrode system comprising at least one ultrasonic sonotrode arranged to generate at least one horizontal standing ultrasonic wave field in said liquid mixture c. said separator being equipped with an inlet for said liquid mixture d. said separator being equipped with an outlet for withdrawal of a cleaned first constituent at the surface of the liquid mixture e. said separator being equipped with an outlet for withdrawal of a cleaned second constituent.

Preferably, said separator is equipped with an inlet for air injection. Preferably, the separator is cylindrical or has a square-shaped cross section. Preferably, the separator is equipped with means for controlling the level of said first and second constituents. Preferably, especially for speeding up separation of tall oil soap from black liquor, the diameter of the separator may be from about 3 to 20 meters, e.g. 10 to 20 meters. The height of the separator may be from about 5 to 20 meters, for example 12 to 18 meters.

According to one embodiment, the separator is a hydrodynamic separator.

According to one embodiment, the separator is a cylindrical tank comprising a centrally mounted driving shaft rotating an element, preferably a rake, mounted on the shaft and preferably extending toward the side wall of the tank. In this embodiment, the separation can be speeded up since the rotating element forms “channels” and turbulence in the liquid mixture to be separated, especially with regard to high viscosity liquids such as soap.

By the term “sonotrode system” is meant to include an ultrasonic transducer, which can include a rod, distributing ultrasonic wave energy to at least one sonotrode whereby the sonotrode can send out ultrasonic wave. An ultrasonic transducer usually consists of a stack of piezoelectric ceramics attached to a tapering metal rod. An alternating current oscillating at ultrasonic frequency is applied by a separate power supply unit (ultrasonic transducer) to the piezoelectric ceramics. The current variation causes them to expand and contract.

According to one embodiment, the frequencies used with said at least one ultrasonic sonotrode ranges from 20 kHz to 70 kHz, preferably from 20 kHz to 40 kHz. The amplitude of the vibration is preferably 10 to 100 micrometres.

According to one embodiment, at least one sonotrode is arranged centrally along the center line of a cylindrical separator with equal distance to the side walls of the separator.

According to one embodiment, the separator is a cylindrical tank comprising at least one ultrasonic sonotrode mounted on a rod centrally and coaxially positioned with respect to the side walls of the tank.

According to one embodiment, at least one sonotrode can be arranged at any suitable position in the separator provided a horizontal standing ultrasonic wave field can be generated. According to one embodiment, the inlets for liquid mixture and optional air injection and outlets for cleaned constituents are positioned at suitable points of the separator to allow for efficient floatation and settling of the constituents to be cleaned.

The invention also relates to the use of a sonotrode system for generating a horizontal standing ultransonic wave field in a liquid mixture for separating constituents having different densities such as tall oil soap and black liquor.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included in the scope of the following claims.

Brief description of the drawings Figure 1a shows a cylindrical separator in which ultrasound-assisted floatation of tall oil soap and sedimentation of black liquor is performed.

Figure 1b shows a magnified view of a sonotrode system comprising an ultrasonic transducer and two sonotrodes, connected to the transducer, generating standing wave fields.

Figure 2 shows a tank for ultrasound-assisted and optional air bubble injection-assisted separation comprising sonotrodes mounted at the side wall of the tank.

Figure 3 shows a tank with sonotrodes mounted on the side wall of the tank and a slowly rotating rake having a central driving shaft.

Figure 4 shows a hydrodynamic separator.

Figure 5 shows a schematic separation of particles (constituents) by exposing a particle mixture to a standing ultrasonic wave field.

Detailed description of the invention Different embodiments of the invention will be described more in detail below. While the embodiments below illustrate arrangements for separating soap from black liquor, any liquid mixtures or solutions composed of separable constituents, e.g. liquids, having different densities may be separated by the method according to the present invention.

In figure 1a, an embodiment of the invention is illustrated comprising a cylindrical tank to which black liquor comprising typically 1 to 5 wt% tall oil soap is fed via inlet 1. Along the center line of the tank, concentrically with the surrounding side wall 11 of the tank reflecting ultrasonic wave, sonotrodes 10 sending out ultrasonic waves are mounted along a metal rod 9 distributing ultrasonic wave energy to the sonotrodes 10. The metal rod 9 is in turn connected to an ultrasonic transducer 8 (piezoelectric or magnetostrictive) sending out ultrasonic wave energy. The sonotrode system is adapted to establish standing horizontal ultrasonic wave fields 12, 13 between the sonotrodes and the side wall 11 of the tank. The distance between sonotrodes 10 and side wall 11 has to be taken into account to satisfy the conditions to establish a standing wave field. Preferably, the ultrasonic radiation is performed intermittently for a predetermined period of time, for example 1 to 30 seconds, for example 1 to 10 seconds or 1 to 5 seconds before the ultrasonic transducer 8 is switched off. The ultrasonic transducer 8 is then preferably repeatedly turned on and off during regular time intervals whereby predetermined periods of standing wave fields 12, 13 are intermittently formed in the tank. By exposing tall oil soap and black liquor in the tank to standing ultrasonic wave fields 12, 13 as described, particles of tall oil soap and black liquor respectively will collect at nodes and antinodes as described herein as a function of their contrast factor ?. Tall oil soap droplets can thus start to agglomerate at the lower standing ultrasonic wave field 12 and agglomeration thereof will continue at the upper standing ultrasonic wave field 13 resulting in even bigger soap agglomerates. This embodiment may also work satisfyingly with establishment of only one wave field. Also, as the interaction of particles of tall oil soap and black liquor is reduced, the soap with lower density will more rapidly rise to the surface (arrows 5 indicate floating agglomerated soap droplet clusters rising upwardly). Also, cleaned black liquor having a higher density will more rapidly settle towards the bottom (arrows 4 indicate heavy settling black liquor). A cleaned tall oil soap layer 7 can thus more rapidly be withdrawn from the tank at outlet 3 positioned in the upper section of the tank and settling clean black liquor 6 can be withdrawn from the lower section of the tank at outlet 2. The separation process is not only faster, it also provides a higher and purer quality of soap and black liquor respectively with lower quantities of impurities. In order to safeguard black liquor by mistake is not withdrawn from the overflow outlet, a soap and liquid level measurement sensor is preferably arranged in the tank.

In figure 1b, a magnification of the sonotrode system of figure 1a is shown. The sonotrode system comprises an ultrasonic transducer 8, a metal rod 9 distributing ultrasonic wave energy (vertical arrows) to sonotrodes 10 sending out ultrasonic radiation horizontally (horizontal arrows). Dashed lines in the upper portion of the tank indicate a liquor-soap interface (lower dashed line) above which cleaned tall oil soap is accumulated. The upper dashed line indicates the surface of clean soap to be withdrawn. Black liquor is accumulated below the dashed interface line and withdrawn as described under figure 1a.

Figure 2 shows a tank for separation of tall oil soap from black liquor. Inlet stream 1 contains black liquor comprising 1-5 wt% tall oil soap. The separation process is ultrasound-assisted. Sonotrodes 10 are arranged at the side wall 11 of the tank establishing a standing ultrasonic wave field 12 between sonotrodes and also reflected by the tank wall 11 around the opposing sonotrodes in the tank resulting in agglomeration of soap droplets to clusters. An ultrasonic transducer (piezoelectric or magnetostrictive) is connected to sonotrodes 10. Sonotrodes 10 are preferably positioned in pairs 180 degrees relative to one another. The shape of the tank may vary, e.g. cylindrical or tanks having a square-shaped cross section can be used. Preferably, the arrangement is provided with an even number of sonotrodes sending out ultrasonic waves, e.g. 2 to 36 sonotrodes, especially in a cylindrical tank. In a square-shaped tank, but also in a cylindrical tank, the number of sonotrodes may be from 2 to 12, or 4 to 10 sonotrodes or any other possible even number in the range. Optionally, the process is also air injection-assisted by injection of air via inlet 15. Soap and optional air bubbles agglomerated with the soap will separate from black liquor. Air bubbles further lower the density of agglomerated soap generated by ultrasonic radiation. The rise of floating agglomerated soap droplet clusters (cf. upwardly pointing arrows 5) towards the surface and clean soap layer 7 will become even more rapid while heavy black liquor will settle in the tank (downwardly pointing arrows 4). Also in the absence of air bubbles, ultrasonic radiation increases the rise of soap to the surface by agglomerating the soap at antinodes of generated standing waves. Over time, the ultrasonic radiation will be transferred to heat energy which also contributes to faster rising of soap since the viscosity is lowered as the heat energy increases the temperature of the soap and lowers its viscosity. Outlet 3 is provided for withdrawal of clean soap from the surface and outlet 2 for withdrawing clean black liquor 6. Outlet 14 indicates a ventilation outlet for odor destruction.

Figure 3 shows an arrangement comprising a separating tank. Black liquor with a content of 1 to 5 wt% tall oil soap enters the tank at inlet 1. The arrangement further comprises a rotatable rake 36 mounted on a driving shaft driven by an engine. The rotation of rake 36 generates some turbulence in the black liquor increasing formation of soap bubble agglomeration. This reduces the resistance of high viscosity soap to rise towards the surface (cf. arrows 30 indicating floating agglomerated soap droplet clusters). Sonotrodes 33 connected to an ultrasonic transducer (piezo or magnetostrictive) send out ultrasonic waves. The sonotrodes 33 are preferably arranged as further described in figure 2. At least one sonotrode 33 sends out ultrasonic waves to generate at least one horizontal standing ultrasonic wave field between the sonotrodes and reflecting tank wall 35 around opposing sonotrodes 33. The arrangement in figure 3 is optionally air injection-assisted by injecting air via an inlet stream 38 to further increase the rise of soap. Cleaned soap from clean soap layer 32 is withdrawn via outlet 3. Arrows 29 indicate heavy settling black liquor. Cleaned black liquor 31 is withdrawn via outlet 2. Ventilation to odor destruction is indicated by outlet 37.

Figure 4 shows an ultrasound-assisted hydrodynamic separator. By means of sonotrodes 46, a horizontal standing ultrasonic wave field 47 is established between the sonotrodes and the wider inner pipe wall 48 acting as reflector for ultrasonic waves resulting in agglomeration of soap droplets to clusters. Inlet stream 39 represents black liquor containing 1-5 wt% of tall oil soap. Optionally, the process is also air injection-assisted by feeding a stream 50 of air bubbles to further lower the density of floating soap droplet cluster 43.

Heavy black liquor 42 is settling to eventually be withdrawn as clean black liquor at outlet 40. An internal recirculation loop 45 is provided to create more liquid rotation in the entire separator. A valve 44 controls the overflow circulation from the top and back to the uncleaned black liquor. Stream 49 indicates ventilation to odor destruction. The liquid is rotated to increase the rate of rise of soap for subsequent withdrawal of cleaned soap at outlet 41.

Figure 5a schematically shows intermixed particles in a mixture such as black liquor containing tall oil soap. The particles, low density particles 51 and high density particles 52, have not yet been exposed to any ultrasonic radiation generating a standing ultrasonic wave field. Figure 5b shows a schematic presentation of the principle of acoustophoresis in which the standing waves are formed by a transducer producing pressure nodes and antinodes resulting in particle separation. The radiation acoustic forces F transport particles to different locations of nodes or antinodes due to the difference in the particle density, conductivity and compressibility (acoustic contrast factor). An ultrasonic standing wave is depicted with the pressure node in the center and antinodes on the sides. The suspended particles collect either at the pressure node or antinodes. Black liquor of high density particles 52 collect at nodes and soap of low density particles 51 collect at antinodes. As evident from the disclosure herein, this effect is utilized to separate particles, constituents of different densities from each other.

Claims

1. . Method for separating a first constituent from a second constituent comprised in a liquid mixture contained in a separator having a diameter ranging from 3 to 20 meters and a height ranging from 5 to 20 meters, said first constituent having a lower density than said second constituent, wherein i) said first constituent is tall oil soap and said second constituent is black liquor; or ii) said first constituent is tall oil soap and said second constituent is tall oil; or iii) said first constituent is biodiesel and said second constituent is glycerine; said method comprising a. generating at least one horizontal standing ultrasonic wave field in said liquid mixture for a predetermined period of time, wherein at least one sonotrode is arranged in the liquid mixture sending out standing ultrasonic waves in a frequency range from 20 to 70 kHz whereby said first constituent is accumulated at antinodes and said second constituent is accumulated at nodes of said standing ultrasonic wave field b. withdrawing a separated first constituent at the surface from said liquid mixture.

2. Method according to claim 1, wherein the interaction between said first and said second constituents is reduced by said standing ultrasonic wave field.

3. Method according to claim 1 or 2, wherein said liquid mixture comprises more than two constituents which may be in a liquid and/or solid state.

4. Method according to any one of claims 1 to 3, wherein cleaned tall oil soap is continuously withdrawn from the separator.

5. Method according to any one of claims 1 to 4, wherein the separator is a cylindrical separator and said at least one sonotrode is arranged centrally along the center line of the cylindrical separator with equal distance to the side walls of the cylindrical separator.

6. Method according to any one of claims 1 to 5, wherein air is injected to the liquid mixture.

7. Method according to any one of claims 1 to 6, wherein at least one sonotrode sends out ultrasonic waves pulse-wise for periods ranging from 1 to 30 seconds.

8. Method according to any one of claims 1 to 7, wherein a level measurement sensor for said first and second constituents is arranged to monitor the level of the respective constituents.

9. Method according to any one of claims 1 to 8, wherein at least one sonotrode is arranged in the liquid mixture sending out standing ultrasonic waves in a frequency range from 20 to 50 kHz.

10. Method according to any one of claims 1 to 9, wherein the separator is a hydrodynamic separator.

11. Arrangement for separating a first constituent from a second constituent in a liquid mixture, said first constituent having a lower density than said second constituent, said arrangement comprising a. a separator for retaining the liquid mixture having a diameter ranging from 3 to 20 meters and a height ranging from 5 to 20 meters. b. a sonotrode system comprising at least one ultrasonic sonotrode arranged to generate at least one horizontal standing ultrasonic wave field and adapted to generate waves in a frequency range from 20 to 70 kHz in said liquid mixture c. said separator being equipped with an inlet for said liquid mixture d. said separator being equipped with an outlet for withdrawal of cleaned first constituent from the surface of the liquid mixture e. said separator being equipped with an outlet for withdrawal of cleaned second constituent.

12. Arrangement according to claim 11, wherein said separator is equipped with an inlet for air injection.