Technical background:

The present invention relates to a process and an arrangement for separation of melt and preferably also solid particles from a gas flow, wherein the said flow arises from the gasification of spent liquor from the production of pulp, especially black liquor, and contains both gaseous substances and melt and possibly also solid particles.

State of the art: Nowadays, ever stronger demands are being made, in connection with a variety of different processes, for recovering substances involved in the process, which can be reusable chemicals as well as energy that has been added or liberated during the process. These demands have mainly arisen with regard to the environment and for economic reasons. A general problem is that, in certain processes, gas mixtures are obtained containing both melt and vapour phase, which gas mixtures are difficult to handle in regard to the recovery of their content of chemicals and energy. In particular it is the melt particles which cause problems, since, in connection with an associated heat exchange process, these particles often settle on the convection surfaces of the heat exchanger, something which is undesirable. This problem arises during the use of the known process according to SE-B-328 179.

An industrial process area in which problems of this nature arise is in connection with the gasification of spent liquors from paper pulp manufacture. The gasification of spent liquor takes place in a reactor in which the organic substances contained in

the spent liquor are pyrolysed and partially burnt so that the heat energy can be obtained and the chemical content can be separated off for recovery. In the following the invention will be discussed in relation to the gasification of black liquor in order to be able to describe the invention in a more concrete manner, which, however, does not thereby limit the invention to this specific area.

Black liquor is obtained as a residual product, spent liquor, in the manufacture of pulp according to the sulphate cellulose method. The black liquor contains alkaline chemicals in the form of a variety of sodium compounds, which it is desired to recover for reusing in the process. The recovery process includes a heating stage, a reactor, in which the organic molecular chains in the black liquor are broken down to energy-rich gas and the inorganic content is mainly present as melt particles which can be recovered. This occurs by pyrolysis and partial incineration of the organic components in the black liquor. The pyrolysis and incineration thus release heat energy, usually the temperature is about 1000°C in the reactor, and produce a flammable gas. It is desirable to recover the liberated heat and utilise the energy-rich gas as a fuel. SE-B-448 173 describes a method for recovering chemicals and energy from black liquor. According to SE-B-448 173 it has been possible to recover the sensible heat in the gas and simultaneously avoid deposition problems by allowing the gases to pass directly through a liquid bath. As well as the melt particles being taken up by the said liquid, which is green liquor, the gas is simultaneously cooled. The gas which leaves the fluid bath, having been freed of melt particles, thereby has a temperature which in an unpressurised system amounts to something in the order of 80°-90°C, which in turn also corresponds to the approximate temperature of the green liquor bath. Thus,

in this instance a melt-free gas flow at about 90°C and a fluid bath at about 90°C are obtained.

From the point of view of energy recovery these temperatures are too low to be optimal. The reason is that it is desired in the preferred case to transfer the heat at a higher temperature level so that it can, for example, be obtained in the form of superheated steam, especially for operating turbines in order to generate electricity. The greatest efficiency in operating steam turbines is obtained when the superheated steam is delivered at high pressure and high temperature, for example 150 bar and 600°C. Thus it is desirable to achieve heat exchange at as high a temperature as possible. However, in the abovementioned method the gas is cooled down to a temperature of around 100°C, which renders efficient heat recovery impossible. On the basis of the above reasoning it is evident that it will be desirable to extract the heat at a higher temperature so that it can be recovered in the form of superheated steam. A process for recovery from spent liquor is also previously known through SE-B-363 651, wherein the gases from the reactor are cooled to a level which is too low to be optimal from the point of view of recovery.

Solution and advantages:

An initial object of the invention is thus to offer a process and an arrangement with which it is possible to separate and utilise the melt particles from a gas stream, without drastically lowering the temperature of the gas.

The abovementioned object is achieved with the aid of a process according to Patent Claim 1 and an arrangement according to Patent Claim 7, i.e. by means of the said flow of gaseous substances containing said particles being forced to move along a curved path and at least the outer periphery of the said curved path being

limited by a mobile layer of liquid, wherein the said particles are separated from the gas stream by being amalgamated with the said liquid layer.

It is indeed in principle previously known that "liquid film cyclones" can be used, but not, however, in the present context. Thus, for example, cyclones of this type for separating off solid particles are previously known through US-A-793 110 (Uehling) , US-A-2 259 034, etc. Such known cyclones are, however, not intended for the separation of melt particles which produce accretion, particularly not such as have been produced in the gasification of cellulose spent liquors, and therefore have inner components which after a period of accretion of hardening melt particles would cause malfunction. Such malfunction is consequently avoided with an arrangement according to the invention.

A further object of the invention is to offer a process whereby carbonate formation in the melt particles is retarded, in favour of the amount of sodium hydroxide, by carrying out the separation of the melt particles at an elevated temperature, so that the high content of CO-, is separated off before binding to the sodium has time to take place.

An additional object of the invention is to facilitate the production of green liquor with different Na/S proportions (sulphidity) by making it possible to separate extracted organic chemicals selectively in at least one sulphur-free and at least one sulphur-containing flow. A further object of the invention is to offer a process whereby it is possible to obtain direct conversion of the melt particles into active digestion chemicals, i.e. NaOH and Na ₂ S.

An additional object of the invention is to present a process whereby it is possible to use alternative methods for recovering the sensible heat content of the gas, inter alia in the form of pre-heated

gas, for example air, for the gasification reactor alternatively in the form of superheated steam, preferably with a pressure of up to about 150 bar and a temperature of up to about 600°C, this being independent of the operating pressure of the reactor.

Description of the figures:

The invention will be described in more detail below in relation to the attached figures, in which:

Fig. 1 shows a preferred embodiment of an arrangement according to the invention, seen partly cut away in perspective from the side, Fig. 2 shows a horizontal cross-section through an arrangement according to

Fig. l, Fig. 3 shows a somewhat modified embodiment of an arrangement according to Fig. 1, and

Fig. 4 shows a basic process plant with two arrangements according to the invention.

In Fig. 1 is shown a reactor 1 which is directly connected to an arrangement 2 according to the invention, which hereinafter will be termed liquid film cyclone. In the example shown the arrangement is used in connection with the extraction of chemicals and energy during black liquor gasification. Black liquor is a residual product which is obtained during digestion in the production of pulp according to the sulphate cellulose process. To reactor 1 are connected a first inlet conduit 3A intended for black liquor and a second inlet conduit 3B intended for air. In the reactor, pyrolysis and partial incineration of the black liquor take place. During this process a gas mixture is formed containing partly a gas portion and partly a melt

portion. The temperature in the reactor is normally between 800°C and l,500 ^o C. The chemical melt is now present in the form of drops 5 in the gas 4. The gas and melt mixture 4, 5 is allowed subsequently to pass out of the reactor 1 into the liquid cyclone 2. The inflow into the liquid cyclone 2 takes place tangentially (see Fig. 2) in relation to the cylindrically-shaped wall of the liquid cyclone 2.

The liquid cyclone 2 consists of a casing 6, 7, 8 with an outflow 9 at the bottom. A liquid 12, for example water (H ₂ 0) or weak liquor at a temperature of about 100°C, which temperature can be increased if the pressure in the cyclone 2 increases, is conveyed to the liquid cyclone 2 via an inlet conduit 10 and distribution conduits 11 connected thereto in the top 8 of the liquid cyclone 2. Guide bars 15 are arranged on the inside of the liquid film cyclone 2 for distribution and guidance of the liquid layer 12. The liquid film cyclone 2 is placed with its longitudinal axis 13B running in the vertical plane. In the top gable 8 is an outlet opening to which is attached a pipe 14 for removal of gas.

The basic function of the arrangement according to the invention is as follows. The hot gas , 5 containing both gas 4 and melt particles 5 streams out from the reactor 1 at a relatively high velocity. The stream is brought to the liquid film cyclone 2 in a tangential and essentially horizontal direction. In this connection the centripetal acceleration (F = mv/2r) will ensure that separation of the melt particles from the gas stream will take place, since the melt particles are heavier than the gas. The melt particles will thus be thrown out towards the periphery and thereby be caught up in the liquid stream 12. By contrast the gas 4 will move spirally upwards towards the opening in the top gable 8 and will be transported onwards via the pipe 14. During this process the gas will be in contact with the liquid

film for a very short time. Furthermore only a relatively small portion of the gas will be in direct contact with the liquid film. As a consequence of this a relatively small transfer of heat can take place from the gas to the liquid film. Thus the gas 4 in the pipe 14 will have a relatively high temperature, in the preferred case a temperature decrease is obtained of only about 100-500°C. Since the gasification takes place in the presence of a sub-stoichiometric supply of oxygen, the gas that is formed will have a significant proportion of combustible components, such as hydrogen gas (H ₂ ) , carbon monoxide (CO) and methane (CH ₄ ) .

The melt particles 5, which have been picked up by the liquid 12 in the liquid film, will, in the preferred case, be first cooled by the liquid 12 and concomitantly also dissolved in it, and then, as a result of the movement of the liquid film, be transported with it downwards in the liquid film cyclone 2. Insoluble solid or melt components will also be picked up by the liquid. The particles picked up in this way will be led out from the cyclone 2, either in dissolved form or in undissolved form, via an outflow 9 in the opening at the bottom of the cyclone 2.

From Fig. 1 it is evident that the cyclone 2 in the preferred case has a central section 7 which is slightly conical. The reason for the slight conicity is to bring about a compensation for liquid evaporation. When the hot gas 4, with its content of melt chemical particles 5, meets the liquid film 12, which preferably consists of a liquid with a boiling point below the temperature of the gas (e.g. water) a certain evaporation will take place. In order, despite this, to preserve a certain given thickness in the liquid film 12, a part 7 of the cyclone 2 is made slightly conical, so that, despite the decrease in volume of the liquid 12 during its downward movement, a virtually constant thickness of the liquid film is obtained.

It is further evident from Figures 1 and 2 that the liquid ^" film cyclone 2 is fitted with longitudinal guide bars 15. The purpose of these guide bars 15 is, as has already been mentioned, the distribution and guidance of the liquid layer 12, in particular in order to ensure the existence of the liquid film and to resist the powerful flow of gas and melt A , 5.

.In a possible operational example the reactor temperature is about 950°C and the pressure on the system about 30 bar. Weak liquor or water which because of the pressure on the system has a boiling point of about 230°C, is used as the film liquid 12. In this example, the temperature of the gases leaving the liquid film cyclone 2 is about 600°-700°C, which temperature favours the production of superheated steam in a subsequent heat exchange.

In Figure 3 is shown an alternative embodiment of an arrangement according to the invention. The reactor 1, like the liquid film cyclone 2, is here so placed that its longitudinal axis is orientated in the vertical plane. Here too, gasification of the black liquor takes place preferably by means of pyrolysis and substoichiometric supply of oxygen. The gas 4 and the melt particles 5 are conducted from the reactor 1 to the liquid film cyclone 2 via a connecting conduit 16 which diverts the flow so that a tangential essentially horizontal inflow into the liquid cyclone 2 is obtained. Otherwise, the construction of this arrangement is in principle the same as for that described previously. Thus, it is evident, for example, that here too distributors 11 and guide bars 15 are used to ensure that the liquid film of the desired thickness is obtained within the relevant areas. In Fig. 4 is shown an exemplifying basic part of a process plant for the recovery of the content of chemicals and energy in black liquor with the

aid of arrangements according to the invention. The plant consists of a reactor 1 to which black liquor is conveyed via a conduit 3A. The black liquor has a temperature of about 130°-190°C and a dry matter content of about 65-85%. Pre-warmed secondary air, which has a temperature of about 500°-700°C, is conveyed via a second conduit 3B. A two-stage process for the separation of melt particles 5 from the gas stream 4 is used in the plant, wherein a first liquid film cyclone 2A has been arranged for a first separation at a higher temperature and a second liquid film cyclone 2B for a second separation at a lower temperature. This can be of advantage, since the content and the concentration in the liquid which is removed from the respective cyclone can, at least to some extent, be varied/determined by different separation temperatures.

The gas 4 with the melt particles 5 streams out from the reactor 1 into the first liquid filin cyclone 2A at which juncture a first separation takes place in accordance with what has previously been described. The gas 4 which leaves this first liquid film cyclone 2A contains a certain quantity of gasified sodium compounds and is conducted via a conduit 14A to the second liquid film cyclone 2B. Only after this second separation stage is the gas freed of melt particles conducted to a first heat exchanger 18B. Here, the hot gases 4 give off a portion of their heat content, which for example is extracted as superheated steam, appropriately at a temperature of about 600°C and a pressure of nearly 150 bar. Prior to entering the heat exchanger 18 the hot gases have a temperature of about 600-800°C.

After the passage through the first heat exchanger 18 the gas 4 is first conducted into a venturi scrubber 19 and subsequently into a nozzle scrubber 21 after which the gas 4 is led off via a conduit 22. As a consequence of the high degree of purification of the gas that is achieved in this way

(< 1 ppm alkali) it can be used as fuel for a gas turbine, for example for efficient electricity production. A relatively- weak solution of alkali is preferably used as washing liquid in the nozzle scrubber 21 and liquid which has been obtained from the nozzle scrubber 21 via a conduit 35 is preferably used in the venturi scrubber.

The first liquid film cyclone 2A has been arranged with two separate outflows 23A, 26A at the bottom. The first of these outflows 23A is designed to collect green liquor for subsequent conversion to digester house chemicals. The green liquor that has been recovered in this way is pumped with the aid of a pump 24 via a conduit 25 to a collecting receptacle (not shown) . The second outflow 26A that is obtained from the first liquid film cyclone 2A is recirculated with the aid of a pump 27 through a conduit 28 which leads the liquid, i.e. the green liquor, to the top of this first liquid film cyclone 2A for continuous replenishment of liquid 12 in the liquid film. With the aid of a heat exchanger 18A in this circuit 28 a part of the heat in the green liquor is recovered.

The liquid which is taken out via the conduit 23B from the second liquid film cyclone 2B is circulated in a circuit 28B with the aid of a pump 32. In this circuit 28B, as well, there is a heat exchanger 18C for recovering heat energy from the green liquor before it is recirculated. Additionally, a conduit 28C for liquid supply is connected to this circuit 28B. A conduit 29 leads off from this circuit 28B and is connected to the circulation circuit 28A of the first cyclone 2A. This conduit 29 is used for supplying liquid in the first cyclone 2A. Green liquor is also collected (pump 36) from the nozzle scrubber 21 for removal to a collecting receptacle (not shown) .

The invention is not limited by the above description but can be varied within the limits of

the subsequent Patent Claims. Thus, for example, the exemplified operating pressure indicated above is not limiting, and the system pressure can be varied to be from atmospheric pressure up to about 150 bar, depending, inter alia, on what is the required evaporation tempera¬ ture of the film liquid.

The expression "that the casing encloses a free space" is intended to imply that there are no components placed inside the casing at a distance from the periphery that are not bathed in water. For this reason, the melt particles which produce accretion cannot accumulate and build up a function-impairing coating, which is an important feature of the invention.

The gasification temperature, i.e. the temperature in the reactor at which thermal decomposition occurs, can be within a wide temperature range, at a temperature above 500°C. The most preferred range is however 800°-1600°C and even more preferred 800°-1400°C. When gasification occurs within the range 500°-800°C crystals rather than melt particles are obtained in the gas stream, for which reason a process within these temperature limits is also called the "dry" method. If the temperature in the reactor is between 800°C and 1000°C the inorganic substances will be present in the form of melt drops, mainly in the form of sodium carbonate (Na ₂ C0 ₃ ) and sodium sulphide (Na ₂ S) . At higher temperatures starting from about 1000°C carbonate formation will be retarded and for this reason a process at higher temperatures than about 1000°C offers the possibility of recovering alkali without causticisation.

A basic prerequisite for this to be possible is that the temperature in the reactor should be high and most preferably be within the range 1200°- 1500°C. This s because at high temperatures green liquor is obtained with greatly reduced carbonate content. This is because the reaction equilibria Na ₂ S + C0 ₂ + H ₂ 0 H ₂ S + Na,C0 ₃ and

2NaOH + C0 ₂ Na ₂ C0 ₃ + H ₂ 0 are greatly displaced to the left at high temperatures and the separation takes place at this high temperature level.

In the preferred case, the gasification takes place sub-stoichiometrically, by which is meant that an insufficient quantity of oxygen is supplied to commit complete combustion of the organic substances in the black liquor. Because of this, production is obtained of combustible gases such as hydrogen gas (H ₂ ) and carbon monoxide (CO), and others.

In the preferred case about 50% of oxygen is supplied in relation to the theoretical requirement for stoichiometric combustion. It is however obvious to the expert that this can be varied within wide limits. Additionally, it is obvious to the expert that the conical shape of the liquid film cyclone is a preferred embodiment and that the method also works with a cylindrical casing surface if sufficient thickness is imparted to the liquid film from the beginning, so that it can tolerate a certain diminution in thickness as a consequence of evaporation. The guide bars also represent a preferred embodiment. The effect of the gas flow on the liquid film can be compensated for by alternative means, e.g. by guiding the liquid film current at the outlet in such a way that the guide bars can largely be dispensed with. Additionally, the possible arrangement, size and form of the guide bars depend on the size and speed of the gas flow as well as the dimensions of the cyclone. Furthermore, it is naturally not necessary that the whole of the inner surface of the casing should be supplied with a liquid film, but only important parts thereof.

The temperature of the liquid in the liquid film cyclone can naturally also be varied within wide limits. Factors which affect which temperature is chosen are: possible requirement for rapid cooling of the particles in the gas stream (if the particles are to be

dissolved in a liquid, the speed of dissolution can depend on the temperature and thereby to a certain extent be the determining factor) , the operating pressure, the size of the casing surface, the gas flow and its temperature, and many others.

Instead of recovering the sensible heat content of the gas as superheated steam it can naturally be extracted in other forms, for example as preheated combustion air for the reactor 1. The treatment of the gas in nozzle scrubbers or a scrubber system for removing remaining chemicals is naturally also a preferred process. The washing is carried out partially for environmental reasons, i.e. to free subsequent discharges from impurities, and partially for operating reasons of a technical nature.

It will also be evident to the expert that it is possible to allow the gasification of the black liquor to take place entirely without any supply of secondary air, i.e. entirely without external supply of oxygen or oxygen-containing gas, in which case pyrolysis alone occurs.

Finally it should once again be pointed out that the use of an arrangement according to the invention for gasification of black liquor is simply exemplifying. An arrangement according to the invention, the liquid film cyclone 2, can be used within all the areas where a separation of melt particles from a, preferably hot, gas stream is required, and where there should be the least possible effect on the temperature of the gas stream during the separation. Additionally, it is appreciated that the constructions shown are also not limiting for the invention.

As the means for distributing the liquid 12 at the inlet 11 is meant in the first place simply the opening of the inlet 12, i.e. its shape and its alignment. It will, however, be evident to the expert

that distributors of the fixed blade type are one of many obvious means that are available.

Instead of taking the gas stream directly from a reactor it can be taken from a hot gas separator (no liquid layer) + > 900°C and release the heat to a heat exchanger before it is allowed to enter a liquid cyclone.