FI61214C - AOTERVINNINGSFOERFARANDE FOER KOKKEMIKALIER - Google Patents

Description

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WA 1 J '} UTLÄGGNINGSSKRIFT ° 1 ^ 1 H

C Patent (pushed on 10 June 1982

Patent (') ^ (51) K ».ik? /Int.ci.3 D 21 C 11/0 ^

FINLAND I - FI N LAN D (21) P »t * nttlh« k «mu * —P» em »dknlnf 79011U

(22) H * k * mitpilvi - Ansttknlngsdag 19-01.79 ^ ^ (23) Alkupllvi — Glltlghstsdag 15-01.79 (41) Interpreters Public - Bllvit offmtllg l6.07 - 80 β (44) Date of issue and date of issue. -.

Patent and registration authorities Ansttksn utlagd och uti.ikriftan publicsrad 26.02. Ö2 (32) (33) (31) Privilege requested — Begird prloritet (71) Rauma-Repola Oy, Helsinki, FI; Rauma Tehtaat, 26100 Rauma 10, Finland-Finland (FI) (72) Ilpo Hassinen, Rauma, Reino Räsänen, Rauma, Ismo Reilama, Rauma,

Lauri Alava, Rauma, Finland-Finland (FI) (7 * 0 Ruska & Co (5k) Recovery method for cooking chemicals - Atervinningsförfarande för kokkemikalier

The invention relates to a process for carbonating green liquor with concentrated carbon dioxide gas of more than 50% by volume in a stirred reactor at a pressure higher than the total back pressure of the green liquor solution at the desired degree of carbonation and to separate sulfur from carbonated green liquor as hydrogen sulfide. In the process according to the invention, sulfur in sulphide or bisulphide form contained in green liquor obtained from the combustion of waste liquor from cellulose production is separated from said solution as hydrogen sulphide by first clarifying the green liquor to separate . The carbonated green liquor is then passed to a sulfur-hydrogen separator, where the alkali bicarbonate already shoulder libisulfide reacts, releasing hydrogen sulfide.

Several methods are known for separating sulfur from an alkali sulfide solution or an alkali carbonate-containing solution to regenerate cooking chemicals used in the production of cellulose. These include the Sivola process, the Stora process and the Tampella process.

61214 2 The process according to the invention is mainly characterized in 3 that in a carbonation reactor the solution is stirred at a power of 6-14 kW / m, preferably 9-11 kW / m to maintain gas dispersion and accelerate the formation of sodium bicarbonate so that 90% of the crystalline sodium bicarbonate less than 0.1 millimeter, and to prevent agglomeration of the resulting crystals and that the velocity of the liquid and the crystals projected on the reactor wall is more than 3 m / s in the vicinity of the walls.

The method according to the invention is as follows:

Green liquor, clarified and pre-carbonated or non-carbonated in a known manner, is carbonated with a concentrated carbon dioxide gas of more than 50% by volume, the impurity being mainly gases soluble in said solution under the conditions described below, such as hydrogen sulphide. The carbonation is carried out in a reactor apparatus in which, with vigorous stirring, the gas is dispersed into small bubbles in the green liquor so as to form a dispersion in which the volume fraction of the gas can also be maintained at 10-50% by stirring. During carbonation, a pressure is maintained that is greater than the total back pressure of the sum of the hydrogen sulfide and water vapor of the solution and the partial pressures of the other compounds in the solution. This prevents the release of hydrogen sulfide from the solution during the carbonation step. The separation of hydrogen sulfide from the solution takes place in the process step following carbonation, where the solution is allowed to discharge into a stripping column under reduced pressure. Steam is fed to the bottom of the column, which may be secondary steam, for example from a waste liquor evaporator.

With the increased pressure and efficient reactor, the advanced degree of carbonation causes the sulfur-hydrogen to be concentrated in a concentrated manner from the top of the hydrogen sulfide separation column. The hydrogen sulphide content of the non-condensable gas system is caused to rise by more than 95% by volume and the steam consumption remains low, as the proportion of hydrogen at the top of the column in the total amount of gas rises from I0 to 20% by volume. Table 1 below shows three examples of process implementation.

3 C 1214 TABLE 1

Composition of the solution Example case __ ~~ 1 | 2 3

After precarbo- Na 2 CO 3 mol / l 1.63 1.76 1.55 NaHCO 3 0.04 0.01 0.00

NaHS "1.55 1.32 1.80

Carbo-Na 2 CO 3 "0.66 0.85 0.36 after KaHCO 3" 1 1 * 83

NaHS 1.55 1.32 1.80

Hydrogen sulfide Na 2 CO 3 "2.21 2.18 2.17

Kr * MaHco3 ·. ° -36 ° · 50

NaHS "0.02 0.01 0.03

Carbonation temperature ° C 72 75 80

Carbonation pressure bar (abs.) 4.2 4.1 4.5

Pressure in hydrogen sulphide stripping bar (abs.) 0.21 0.20 0.22 Steam stripping

Hydrogen sulphide content after stripping before 14 12 16 condensing Jo

Hydrogen sulphide content <97 c5 97 of dry gases> "4 (: '7 1 4 The advantages of this method are highlighted in both operating and investment costs:

The advanced degree of carbonation and the efficient operation of the reactor, as already mentioned above, keep the steam consumption of the hydrogen sulfide separation stage low. The stripping used under vacuum also allows the utilization of waste heat. Concentrated hydrogen sulfide gas guarantees the economy of the vacuum process in the hydrogen sulfide separation stage, since after the vacuum pump or ejector, there is considerably no need to do unnecessary work to pump the hydrogen sulfide-diluting gas components.

Concentrated hydrogen sulfide gas is also preferred for the separation of hydrogen sulfide in the following process step: combustion of hydrogen sulfide and absorption of Si 2 in the preparation of cooking solutions. The Si 2 content can be as high as 10% by volume. In the combustion of hydrogen sulphide, concentrated hydrogen sulphide, the development of which is steady and the concentration variation is small, guarantees good efficiency in the heat recovery associated with combustion. When a waste boiler is used, evaporation is achieved which clearly exceeds the steam demand for hydrogen sulfide separation. Due to the efficient transfer of the substance, the equipment required is small and the investment costs remain low.

The actual inventiveness of the method is primarily directed to the green liquor carbonation step. As is known, the difficulty of the carbonation process is that the solubility limit of the sodium bicarbonate concentration in many of the processes used has to be exceeded * It follows that the carbonators are more or less intermittent in function due to their susceptibility to clogging. This requires oversizing of the coronation devices for resuscitation, i.e. washing to ensure capacity during cycles.

Carbonation equipment in industrial applications is large in unit size, such as liquid-filled parquet towers used in the soda industry and also in the side recovery process. Their large size makes them expensive to pressurize. When operating at normal pressure, hydrogen sulphide is released in a highly diluted gas used for carbonation. With these conventional absorbers suitable for carbonation, the degree of carbonation required for sufficient sulfur-hydrogen separation is not achieved by a single, successive carbonation and stripping treatment, but requires several treatments or even crystallization and crystallization to produce sufficiently pure cooking acid. In the process of the present invention, sufficient hydrogen sulfide separation is achieved by one carbonation step and a subsequent hydrogen sulfide separation step.

61214 5

It is also essential for the process according to the invention that the concentrated carbon dioxide gas used is obtained from the circuit of cooking chemicals. Examples of these carbon dioxide sources are: the preparation of sodium sulphite from sodium bisulphite and sodium carbonate solution as part of the preparation of the cooking solution ** and the neutralization of the cellulose cooking process or the digestion of the effluent from the digester using sodium carbonate.

One of the undeniable advantages of using concentrated carbon dioxide is that the formation of Na2S2O3, which is critical for the cellulose cooking process, does not occur to a measurable extent. This has avoided one of the disadvantages of using carbon dioxide-containing flue gases.

The following reactions take place in the carbonation of green liquor:

No 2 CO 3 + CO 2 + H 2 O φ 2 NaHCOg (1)

NaHS + NaHCO 3 * No 2 CO 3 + HgS (2)

As already mentioned above, the progress of the reaction (2) from left to right in the carbonation step is prevented by using increased pressure. The main reaction of the carbonation step remains reaction (1). In the hydrogen sulfide separation step, the main reaction is reaction (2).

The temperature of the green liquor in the solvent of the recovery boiler is approx. 90 ° C. In interim storage and clarification, the temperature drops so that the temperature of the green liquor in the carbonation step is 60-80 ° C. The carbonation temperature of the process of this invention is 20 to 100 ° C, but more preferably 60 to 80 ° C, in which case no heat exchangers are required to heat or cool the green liquor. The operation of heat exchangers with concentrated brine solutions, such as green leaves, is hampered by the crystallization of salt on the heat exchange surfaces, which makes their operation more or less intermittent. With a proper selection of the process temperature, this disadvantage can be avoided.

In carbonation, it is generally necessary to exceed the solubility of the sodium bicarbonate formed in the green liquor if, as in this method, the degree of carbonation required for the separation of hydrogen sulfide is achieved in one carbonation step. This brings with it the following disadvantages, the avoidance of which is based on the inventiveness of this method: The main reaction in the hydrogen sulfide separation step is the above reaction (2), in which sodium bicarbonate reacts with sodium bisulfide to release hydrogen sulfide. The rate of hydrogen sulphide removal from 6 (, 1 2 1 4 solutions is determined by the following equation: NH2S = Kga1 ^ PH2S ^ "PH2s) V 'iossa - g is the amount of hydrogen sulphide leaving the solution, mol / h

O

- K aj is the device-specific combined total transfer factor mol / (m ° h at) 9

B

- is the partial pressure of hydrogen sulphide in the green liquor at equilibrium, at - P | l j is the partial pressure of hydrogen sulphide in the gas phase, at 3 - V is the volume of the device, m

Thus, hydrogen sulfide mass transfer can be influenced by device dimensions and device solutions that achieve a high total mass transfer factor. The partial pressure of hydrogen sulfide in the gas phase can be influenced advantageously from the point of view of the desired mass transfer by calculating the total weight prevailing in the device and / or by using more stripping steam to dilute the gas phase.

According to reaction equation (2), the partial pressure of hydrogen sulphide in green liquor depends not only on the sodium bisulphide concentration but also on the sodium bicarbonate concentration, the sodium carbonate concentration and, of course, the temperature:. '[NoHS] [NoHCC ^], ......

PH S = K1 - tasapaxnovakxo Kj \ A) 2 rNa ^ CO »n is temperature dependent

However, it should now be noted that the NaHCO 3 concentration in Equation (4) refers to dissolved sodium bicarbonate, so it can be concluded that the solubility of sodium bicarbonate limits the hydrogen sulfide desorption.

Another disadvantage to be faced when exceeding the solubility of sodium bicarbonate is the susceptibility of the device to clogging.

More specifically, the method according to the invention is as follows:

The carbonation of the green liquor is carried out in a stirred reactor with four important mixing functions: 7 61 21 4 - With efficient stirring, the carbon dioxide-containing gas is dispersed into small bubbles in the green liquor to form a dispersion with a gas volume fraction of 10-50%, preferably 30%. in order to create. In this way, for example, at a temperature of 80 ° C, a specific mass transfer area of approx. 2000 m ”is achieved when a power of 10 kW / m ° is used for mixing.

- The rapid gas absorption obtained by efficient stirring creates a supersaturation state of sodium bicarbonate in the solution phase, which is discharged into a very abundant formation of crystal nuclei, therefore the crystal size distribution becomes advantageous for crystal dissolution, i.e. very abundant and small crystals.

An example is the cumulative number of crystals measured from a factory-scale prototype reactor as a function of length:

_ L

N = 2700 e "” ö'0162, where (5) N is the number of crystals, L is the length of the crystal, mm - Efficient mixing prevents agglomeration of individual crystals by more than 90%, which also accelerates the dissolution of sodium bicarbonate in the hydrogen sulphide separation step.

- Efficient mixing above 6 kVf / nr and design of the reaction vessel, which ensures sufficient turbulence throughout the reaction vessel, prevents crystallization on the walls and clogging of the apparatus. This keeps the operation of the carbonine reactor continuous and avoids the need for additional capacity required by the washing steps inherent in other processes.

In carrying out the carbonation temperature for example 80 ° C, reached carbonyl Λ nointivaiheessa connected to the gas side mass transfer coefficient of 30 kmol / (m ° h at) the mixing power of 10 kW / m. There is no clogging of the reactor apparatus, the crystal size distribution shown in equation (5) after carbonation and a sufficient resolution of 95-99.5% for the preparation of the hydrogen sulfide cooking solution are achieved in the spent vacuum stripping column (cf. Table 1).

The carbonation reactor used in the process according to the invention is small in size (cf. mass transfer coefficient) and does not require heat exchangers on the solution side,

Claims (14)

61214 8 as a result, its investment costs compared to the soda towers used in the Sivola recovery process are only about 10%. Furthermore, compared to said soda towers, the operation is continuous and not intermittent, the reaction heat is bound to the process stream and utilized in hydrogen sulfide separation and is not bound to cooling waters, which due to low thermal potential find it difficult to find useful use in pulp mill hot water management. The process according to the invention uses concentrated carbon dioxide for carbonation from different sources in the cooking chemical cycle and does not use flue gases for carbonation below pH 10.5. This avoids harmful Na2S2OQ-generating side reactions between flue gas oxygen, hydrogen sulfide, sulfur dioxide and green liquor. At the same time, the formation of said cooking chemical circulation (X 1) sources as odor gas emission sources is prevented. 1. A process for carbonating green liquor with concentrated carbon dioxide gas of more than 50% by volume in a stirred reactor at a pressure higher than the total back pressure of the green liquor solution at the desired degree of carbonation. , preferably to maintain a gas dispersion of 9-11 kW / m and to accelerate the formation of sodium bicarbonate so that the crystalline sodium bicarbonate crystallized in length is 90% less than 0.1 millimeter in length, to prevent agglomeration of the resulting crystals and to project liquid and crystal in the vicinity of more than 3 m / s. Process according to Claim 1, characterized in that the carbonation is carried out at a temperature of 50 to 100 ° C, preferably 60 to 80 ° C. Process according to Claim 1 or 2, characterized in that the concentrated carbon dioxide is leached as a product of the reaction between sodium bisulphite and sodium carbonate and / or sodium bicarbonate from the process for preparing the cooking solution. Process according to one of Claims 1 to 3, characterized in that the concentrated carbon dioxide is obtained as the product of the reaction of the acid cooking broth of the cellulose cooking process between sodium carbonate and / or sodium bicarbonate 9 61 21 4 from the digester or from a separate device outside it. Process according to one of Claims 1 to 4, characterized in that the carbonation reactor has a volume fraction of gas in the dispersion of 10 to 50%, preferably 25 to 35%, Process according to Claims 1 to 5, characterized in that the device used for the separation of hydrogen sulfide is of the bottom column type. Process according to one of Claims 1 to 5, characterized in that the device used for the separation of hydrogen sulfide is of the packing column type. Process according to one of Claims 1 to 7, characterized in that the hydrogen sulfide is separated off at an absolute pressure of 0.1 to 0.6 bar, preferably 0.15 to 0.3 bar. Process according to one of Claims 1 to 8, characterized in that the hydrogen sulfide separation is carried out by disturbance stripping. Method according to Claim 9, characterized in that secondary steam from the waste liquor evaporator is used as the stripping vapor. Process according to one of Claims 1 to 10, characterized in that the green liquor has been clarified and pre-carbonated in known ways before carbonation. Process according to one of Claims 1 to 11, characterized in that one green liquor carbonation treatment is sufficient, if necessary, for the separation of hydrogen sulfide. Process according to Claim 11, characterized in that flue gases are used for the precarbonation when the pH of the solution to be carbonated is at least 10.5. Process according to Claim 13, characterized in that the flue gas used for the precarbonation is washed with a solution containing sodium carbonate and / or sodium sulphite and / or sodium hydroxide in order to reduce the formation of sodium thiosulphate before it is fed to the precarbonator. 10 6121 4