WO2002046680A1

WO2002046680A1 - A method or removing deposits that are difficult to dissolve

Info

Publication number: WO2002046680A1
Application number: PCT/SE2001/002664
Authority: WO
Inventors: Thomas Ericsson; Sten Gylfe; Olof Hallgren
Original assignee: Stora Kopparbergs Bergslags Ab
Priority date: 2000-12-07
Filing date: 2001-12-04
Publication date: 2002-06-13
Also published as: SE518075C2; SE0004510D0; AU2002218621A1; SE0004510L

Abstract

A method of removing deposits containing oxalate that are difficult to dissolve, from the heating surfaces of a heat exchanger in an evaporation apparatus, having a vessel in which the heat exchanger is arranged. In accordance with the invention the following steps are performed: (a) stopping the evaporation process for evaporating spent liquor containing oxalate and emptying the vessel of its spent liquor contents; (b) rinsing the heating surfaces clean from residue of spent liquor with water and emptying the vessel of its contents of this mixture; (c) supplying heat to the vessel to raise the temperature therein to a predetermined minimum value; (d) causing gas to flow through the vessel to remove moisture therefrom until a dry environment is obtained so that the deposits become dry and assume said minumum temperature, whereby the deposits fracture into brittle particle structures; (e) discontinuing step (c) and step (d) and causing liquid to circulate through the vessel to remove the particles from the heating surfaces; and (f) drawing off the mixture of liquid and particles from the vessel.

Description

A method of removing deposits that are difficult to dissolve

The present invention relates to a method of removing deposits containing oxalate that are difficult to dissolve, from the heating surfaces of a heat exchanger in an evaporation apparatus comprising a vertical vessel in which the heat exchanger is arranged, and in which evaporation apparatus spent liquor containing oxalate is evaporated.

In the manufacture of paper pulp large quantities of COD are obtained, and also higher carbohydrate compounds and lignin, which are difficult to decompose in a biological cleaning process. When an integrated production plant increases its production of paper, cardboard or the like, therefore, a higher production of pulp is required. Increased pulp production results in increased load on the cleaning equipment of the plant. In a heavily-loaded biological cleaning apparatus, normally in the form of a biological active sludge plant, the extension alternative is often extremely expensive and the cleaning effect is not satisfactory since the easily decomposed substances are still easily decomposed whereas as the substances that are already difficult to decompose accumulate. Thus, upon increased production of paper pulp, the load increases on the biological cleaning process comprising a biological active sludge plant. To reduce this load, and because said compounds are difficult to decompose, the biological active sludge plant is supplemented by an evaporation plant having falling film evaporators for processing a part of the waste water or spent liquor from the paper pulp process. Such an evaporation plant comprises a plurality, e.g. seven, series-connected evaporation apparatuses, also termed effects. One problem that arises in such an evaporation apparatus is that hard deposits, primarily in the form of calcium oxalate hydrate, CaC₂0₄H₂0, are formed on the heating surfaces of the heat exchanger. The wood contains both calcium and oxalate which combine to form calcium oxalate during the pulping process. Calcium oxalate and its hydrate CaC₂0₄H₂0 are insoluble in both cold and hot water. The water-insoluble, hard deposits containing oxalate, so-called scaling, deteriorate the heat transfer and must therefore be removed in some way in order to restore at least a large part of the full degree of efficiency of the heat exchanger.

The internal heating surfaces of an evaporation apparatus are cleaned from insoluble deposits containing oxalate by acid washing with hydrochloric acid or nitric acid. However, acid washing must be performed at relatively short intervals and therefore causes many undesired shutdowns during a specific period of time. Acid washing constitutes an investment and thus increases the capital investment in the plant. It entails relatively high costs for chemicals and the cleaning is relatively expensive to perform. It is associated with handling waste material dangerous to environment which in itself is never desirable and which results in extra running costs. It is also unfavourable from the working environment aspect and does not produce fully acceptable results within a reasonable period of time.

The object of the present invention is to provide an improved method of cleaning an evaporation apparatus from deposits containing oxalate that are difficult to dissolve, in order to essentially reduce the problems and drawbacks mentioned above in conjunction with acid washing. ω ω t to ¹ cπ o π o LΠ O LJ1

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necessary after the initial rinsing. The time required for rinsing and lowering the temperature is about 4-5 hours .

When rinsing is complete and the rinsing water has been drawn off, a lower manhole is opened at the bottom of the vessel and an upper manhole at the top of the vessel . The vessel can now be inspected internally if deemed necessary, and any residue of the viscous waste water can be removed manually from the heat exchanger, using suitable tools, so that the deposits on the heating surfaces containing oxalate are exposed, said deposits reducing the capacity of the heat exchanger.

In the next step steam is supplied to the heat exchanger so that the temperature in the vessel is increased to at least 70°C, preferably at least 100°C. The moist air in the vessel thus becomes hot and water on the outside of heating surfaces of the heat exchanger and other places in the vessel is vaporised. The hot, moist air rises through the vessel and flows out through the upper opened manhole at the same time as new, drier air is drawn in from the surroundings through the lower opened manhole. This heat treatment is continued long enough so as dry an environment as possible to be obtained in the vessel, and particularly around the deposits containing oxalate on the heating surfaces of the heat exchanger. This normally takes about 12 hours. The dry, hot environment in the vessel causes the deposits containing oxalate to fracture and become brittle so that their adhesion to the heat surfaces is reduced. The hard deposits containing oxalate thus gradually assume a brittle structure of fine particles, some of which fall of their own accord to the bottom of the vessel.

The next step is to remove the particles from the heating surfaces and out of the vessel . The brittle deposits are

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^— ' X CQ Φ rt rt if < (-^■ Pi ft⁾ μ- φ Ω 03 Φ 3 μ- μ- ft) LQ ø ^ rt 0 rt rt μ- ft) d CQ Ω 3 tr ft) "< μ- μ- μ^j μ- ø O tr Φ fi φ CQ i rt fi tr tr rt <! fi if tr 0 0 ft) rt P fi ø -<! rt Ω μ- 0 < μ- rt CO μ-

3 (-^■ 3 ft⁾ 0 rt μ- Φ fi P fi X fi fi Φ fi 0 μ- 0 ø li Φ μ- rt ft) if μ- -3 0 H3 0 rt ft⁾ Φ Φ rt d ø φ ø μ- ft) . Pi ft⁾ 0 rt d 0 if LQ ^{• 4} fi 03 tr 0 03 ft) if ø tr

^>< ft⁾ P. LQ Φ LQ 0 ft) LQ rt ø -3 rt 3 tr ø ø Φ μ- Pi CQ Ω φ Φ

CQ μ- tr Hi 03 φ rt ft) μ- Φ tr Φ Φ φ Pi fi 0 Φ ft) 03 fi tr 0 Φ tr rt μ- 03 ft⁾ ft⁾ 0 fi rt 03 01 μ- ø Φ μ- Φ X ø rt ft) μ- φ μ- 0 μ- 3

Φ ø rt ø ft) 0 Φ Φ CQ Ω =3 rt 0 fi ø 0 ft⁾ <! tr CQ φ 0 ø Pi ø CQ ø μ- φ

CQ Ω ⁴ P. ft rt 03 • tr tr ft) X 03 rt μ- Φ 0 ø LQ d LQ μ- rt 3 rt

CD 0 3 d Φ φ Pi tr 3 fi o rt ft) ft) rt rt •* ft) H rt Φ Ω Ω P. φ μ- if d 3 ft⁾ ϋ ft⁾ μ- fi 0 0 0 Hi Hi Φ rt O if rt 0 ø ft⁾ ft) fi 0 Φ φ fi ø o

TJ Φ 0 ϋ Φ TJ Φ ^ d -V 0 fi ft) 0 φ if Φ 0 Φ 0 μ- 0 3 P. μ. fi <! ft) Pi

TJ ^ Φ X TJ φ ft⁾ - Φ φ rt μ rt rt Pi ft) 3 d TJ tr ft⁾ ft⁾ rt l-^J 0 ø ft⁾ i ϋ CQ CQ P. Ω 0 ø⁴ 3 Φ tf Φ Hi H Ω φ μ- CQ 03 ft) Φ tr μ^J Φ μ- μ- Hi 0 rt 3 Φ ft⁾ ft ft d Φ 03 <! Φ tr 0 Pi tr ø 3 3 fi ft) --^■ CQ CD ø

Φ rt TJ CD rt Φ tr d ft⁾ μ- d fi •* ft) rt φ TJ rt μ- CO φ ^><

Pi ft if * . H CO d -J ft) ft⁾ Φ TJ ft TJ CQ rt 3 ø rt 0 ft⁾ 0 03 X tr rt if Φ Φ d fi o 3 <! CQ if 0 LQ μ- tr fi fi 0 -3 TJ " φ if φ fi 3 fi Φ 0 φ μ- Φ fi Pi 0 0 μ- ø o rt Φ 0 φ Φ o rt Φ

Hi 0 φ o μ- μ- ø ft) ii 0 Hi ø Pi ft⁾ μ^j ø ø -3

0 tr 3 03 0 - 03 ft⁾ CQ rt μ- Φ LQ ø ft) §, CQ LQ Φ fi Φ ft⁾ rt 0 Hi Φ Φ ft⁾ rt μ- Φ φ Φ ft⁾ Hi CQ P- CQ rt ft⁾ fi rt ø

CQ 0 tr CQ

instance, by means of the gas in step (d) , which gas is dry in_. order to remove moisture in the vessel while flowing through it, any residual heat being supplied to the heat exchanger. The hot gas supplied to the vessel in step (d) may thus be responsible for the whole increase in temperature.

In accordance with a modified method, the deposits containing oxalate are dried by means of freeze-drying, i.e. by means of decreasing the pressure instead of increasing the temperature.

Claims

C L A I M S

1. A method of removing deposits containing oxalate that are difficult to dissolve, from the heating surfaces of a heat exchanger in an evaporation apparatus, comprising a vertical vessel in which the heat exchanger is arranged and in which evaporation apparatus spent liquor containing oxalate is evaporated, characterized in that it comprises the following steps: (a) stopping the evaporation process and emptying the vessel of its spent liquor contents,

(b) rinsing the heating surfaces clean from residue of spent liquor with water and emptying the vessel of its contents of the mixture of water and spent liquor thus obtained,

(c) supplying heat to the vessel to raise the temperature therein to a predetermined minimum value,

(d) causing gas to flow through the vessel to remove moisture therefrom until a dry environment is obtained in the vessel so that the deposits containing oxalate become dry and assume said minimum temperature due to said supply of heat, whereby the deposits fracture into brittle particle structures,

(e) discontinuing step (c) and step (d) and causing liquid to circulate through the vessel to remove the particles from the heating surfaces, and

(f) drawing off the mixture of liquid and particles from the vessel.

2. A method as claimed in claim 1, characterized in that the temperature in step (c) is at least 70 °C.

3. A method as claimed in claim 1 or claim 2, characterized in that said gas in step (d) is ambient air which is drawn in through a lower inlet in the vessel, the moisture-absorbing air flowing out through an upper outlet by virtue of its increased temperature.

4. A method as claimed in any one of claims 1-3, characterized in that said heat in step (c) is supplied via said heat exchanger which is supplied with steam having a temperature of at least 70 °C.

5. A method as claimed in claim 1 or claim 2, characterized in that at least a part of the heat supplied to the vessel in step (c) is supplied by means of the gas in step (d) , which gas is dry in order to remove moisture in the vessel on its passage through the vessel .

6. A method as claimed in claim 5, characterized in that the hot gas supplied to the vessel in step (d) is also entirely responsible for the increase in temperature .

7. A method as claimed in claim 1 or claim 2, characterized in that steps (c) and (d) are performed for a period of time adapted to the current situation and where higher temperature and a higher through-flow rate for the air results in shorter period of time.

8. A method as claimed in claim 1 or claim 2, characterized in that steps (c) and (d) are performed for a period of at least about 5 hours, preferably at least about 10 hours .

9. A method as claimed in any one of claims 1-8, characterized in that the brittle deposit is loosened prior to step (e) with the aid of mechanical influence in the form of light blows against parts of the heating surfaces . P1496SE TP1 020318