Within the processing industry, such as the chemical industry and the cellulose industry, it is common prac¬ tice to deliver the chemicals to the factories in solid form or in the form of highly concentrated aqueous solu- tions. Prior to use, the concentration of the chemicals usually is lowered by diluting them with water. In for instance pulp mills, large quantities of concentrated sodium hydrate are used and in the industrial plants in such mills large quantities of low-energy vapour are produced, i.e. vapour having a poor heat content. The low-energy vapour usually is utilised to produce hot water. In modern pulp mills, increasingly higher tempe¬ ratures are requested, particularly in the bleaching plants. No doubt it would be an advantage to be able to utilise low-energy vapour in the preparation of compara¬ tively diluted chemical solutions and at the same time increase their temperature to a level allowing them to be used directly in the production process.

WO 91/18863 discloses a process for the recovery of lower aliphatic acids, such as formic acid, and sugars from the spent cooking liquor from a pulping process car¬ ried out with these acids. Spent cooking liquor is eva¬ porated in several steps in a multiple effect evaporator wherein the vapour is returned to previous stages and heats and dilutes the cooking liquor. This publication is silent with regard to the possibility of elevating the boiling points in order to recover maximum thermal energy from the system. Instead, the purpose of the invention described in that publication is to recover acids and sugars.

US-A- 4 755 258 describes multiple effect evapo¬ ration of spent liquor, wherein the formed vapour is restored to previous stages and heats the liquor. An

advantage is said to be that the dry solids contents of the spent liquor remain nearly unchanged. The plant in accordance with this prior-art publication does not either make use of the boiling-point elevation to recover high-quality energy.

The primary purpose of the subject invention is to provide a process and a plant for efficient recovery of energy upon dilution of liquids having a considerable elevation of boiling points. This purpose is achieved by means of the process defined in the preamble of claim 1, possessing the char¬ acteristic features specified in the characterising clause thereof. The plant is further defined in claim 5. The invention is based on the idea that by condensing vapour in a liquid containing chemicals while simulta¬ neously making use of the condensation heat and the heat of solution generated during the dilution it becomes pos¬ sible to produce hot water having a higher temperature than the vapour. Before giving a detail description of the inven¬ tion, some definitions of the terms used herein will be explained.

For instance, the expression "elevation of boiling points" designates the difference in boiling temperatures obtained when boiling an aqueous solution of a chemical and when boiling pure water at identical pressures.

The expression "chemical" is to be given a wide interpretation and relates to water-soluble organic or inorganic compounds and mixtures thereof. As non-limit- ing preferred examples of such chemicals may be mention¬ ed sodium hydroxide and sulphuric acid.

The invention will be described in closer detail in the following with reference to the accompanying draw¬ ings, wherein: Fig. 1 is an overall flow chart illustrating one example of application of the invention within the pulp industry, and

Fig. 2 is a schematic view of a preferred plant for performing the process in accordance with the invention, using an absorber of multi-compartment type.

Referring to Fig. 1, vapour is introduced into an absorber 1 comprising a cooler (not illustrated). The vapour has a temperature of approximately 53°C and is formed by the outflow vapour from an evaporator which is supplied with polluted liquor vapour at a temperature of approximately 58°C. This cleaning step is not necessary if the liquor vapour purity is regarded as acceptable. At the same time, a 50% sodium hydrate solution is introduc¬ ed into the absorber. The condensate that forms in the absorber upon condensation of the vapour therein is used to dilute the 50% concentration liquor and liquor at 20% is withdrawn from the absorber. Hot water having a tempe¬ rature of about 53°C is used as the medium for absorption of the heat that generates as a consequence of the con¬ densation and the heat of solution from the dilution, and this hot water is introduced into an heat exchanger (not shown) positioned in or adjacent to the absorber, through a separate inlet and is withdrawn through a separate out¬ let in the form of hot water having a temperature of about 70°C. The heat exchanger may for instance be a tube heat exchanger, wherein the hot water circulates inside the tubes and vapour and liquor externally thereof. The above values of concentration, temperatures, etc. , are to be regarded as examples only. The Table below gives exam¬ ples of various boiling point elevation values and para¬ meters related thereto.

Table of Boiling Point Elevation

Parts Parts % NaOH Boiling point Liquor tem¬ NaOH water elevation °C perature °C

1 1 50 43 96

1 1.5 40 25 78

1 2 33.3 15 68

1 3 25 9 62

The values given in the Table above indicate that the higher the elevation of boiling points, i.e. the higher the liquor concentration, the higher the air tem¬ perature and the amount of energy that may be recovered. The chemical solution that is introduced into the absor¬ ber should have a concentration of between 20% and the concentration of saturation, and preferably between 30 and 50%.

Fig. 2 illustrates schematically one embodiment of a plant, chosen by way of example and designed to perform the process in accordance with the invention. The absorber 1 is divided into three compartments, la, lb, lc, and one heat exchanger 3a, 3b, 3c is connected to each compartment. The various compartments are interconnected.

Vapour having a normal condensation temperature of 53°C is introduced into all compartments, and is condens¬ ed in the liquid contained inside the compartments. The condensation heat is withdrawn via the heat exchanger associated with the respective compartment. A 50% sodium hydroxide solution is added to compartment la. Upon con- densation of the vapour thus supplied the liquor is diluted to a concentration of 40%. The elevation of the boiling point increases the temperature of the compart¬ ment to a level above that of the condensation tempera¬ ture, however to a maximum of 78°C. Practically all con- densation heat is withdrawn by way of the associated heat

exchanger. From compartment la 40% sodium hydroxide is supplied to compartment lb. The vapour condenses and heat is drawn off in the same manner as described above. In this compartment, the liquor is diluted to a concentra- tion of 33.3%. As a result of the dilution, the tempera¬ ture drops to a maximum of 68°C. Surplus liquor is pass¬ ed to compartment lc. In this compartment, the sodium hydroxide is diluted to a concentration of 25%. The temperature drops to a maximum of 62°C. In this example, the condensation heat is used to heat the hot water.

Water having a temperature of 45°C is added to the heat exchanger in compartment lc, and is sequentially heated further, first in compartment lb and finally in compart¬ ment lc, to a temperature of 70°C. See Table below.

TABLE

Basis of calculation : 1 ton. : 100% liquor

Total Compart¬ ^■ Compart¬ Compart¬ ment 1 ment lb ment lc

Supplied sodium hydroxide solution

Part by weight of sodium hydroxide, % 50 50 40 33.3

Sodium hydroxide, ton 1.00 1.00 1.00 1.00

Water in sodium hydroxide 1.00 1.00 1.50 2.00

Temperature, °C 20 78 68 62

Condensing vapour, in ton 2.00 0.50 0.50 1.00

Withdrawn sodium hydroxide solution

Part by weight of sodium hydroxide, % 25 40 33.3 25

Elevation of boiling points, °C 25 15 9

Sodium hydroxide, ton 1.00 1.00 1.00 1.00

Water in sodium hydroxide 3.00 1.50 2.00 3.00

Temperature, °C 62

Hot water

Input temp. , °C 45 65 58 45

Output temp. , °C 70 70 65 58

Hot water quant. 40 40 40 40

In summary, essential advantages are obtained in accordance with the invention in that all energy in the process, with the exception of inevitable losses, is transformed into high-quality energy, and this is achiev¬ ed while using a smaller number of units than in accor¬ dance with prior-art technology. This in turn gives con- siderable savings in primary heat, evaluated to be in the order of 2 MSEK yearly in a normal-size pulp mill alone.

The primary advantage of the invention is that in one and the same process it becomes possible to make use of the secondary heat more efficiency while at the same time the desired dilution of chemicals is obtained. In accordance with the invention the elevation of boiling points thus has been taken advantage of in a most efficient manner.