JPS6247401B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a method for removing scale adhering to a multi-effect evaporator.

<Prior Art> It is well known that multiple effect cans are used as devices for concentrating solutions.

Multi-effect cans are industrially useful as devices for effectively utilizing heat, but depending on the type of solution, solutes may adhere in the form of scale inside the evaporator body in the latter stage or inside the multiple tubes of the heater. , which has the disadvantage of lowering heat transfer efficiency and evaporation capacity.

Therefore, in the past, for example, a spare evaporator was installed, as in the method described in Japanese Patent Publication No. 54-62992,
A method has been proposed in which a low-concentration solution is passed through an evaporator with scale to remove the scale, and a high-concentration solution is passed through a spare evaporator.

Although this method is an excellent method since there is no interruption of the concentration operation, it has the disadvantage that a spare evaporator must be provided.

Therefore, it has been desired to establish a method for removing scale adhering to a multi-effect evaporator without interrupting the concentration operation and without requiring a spare evaporator.

On the other hand, as a method for preventing scale from adhering to a multi-effect can, a method described in Japanese Patent Application Laid-Open No. 18265/1983 is known. In this method, seawater is desalinated, and seawater heated to approximately the same temperature as the concentrated seawater is mixed with the concentrated seawater discharged from the canister in the previous stage than the final canister as dilution water. This makes it possible to operate under conditions close to the limit of calcium sulfate scale precipitation in utility cans.

This method converts seawater into freshwater and is an excellent method for collecting freshwater, but the concentration of concentrated seawater discharged is not constant, and the concentration of concentrated seawater in this case is on a scale. It does not reach the precipitation limit. Therefore, it is not possible to conversely use a multi-effect canister to concentrate a solution to obtain a uniformly concentrated solution that can be reused. In other words, when trying to obtain a solution with a constant high concentration that can be reused, it is impossible to avoid scale adhesion to the final pot, so we decided to use the method described in Japanese Patent Application Laid-Open No. 18265-1985 for scale precipitation. Even if applied to a method of obtaining a solution with a constant high concentration under harsh conditions that exceed the limits, the initial objective cannot be achieved.

The object of the present invention is to provide a plurality of (n) evaporators AO ₁ ,
A solution is sequentially passed through A ₂ ..., A _o-1 , A _o and concentrated to obtain a reusable solution with a constant high concentration. During this operation, scales attached to the final evaporator A _o and its heater B _o are The objective is to provide a method that can be efficiently removed and operated continuously.

<Means and effects for solving the problems> The configuration of the present invention includes a plurality of (n) evaporators A ₁ ,
The solution is sequentially passed through A ₂ ..., A _o-1 , A _o while controlling the liquid level, and when the solution in the final evaporator A _o reaches the specified concentration, the solution is drawn out from the final evaporator A _o . During the normal operation of continuous concentration, when scale occurs on the inner surface of the final evaporator A _o and its heater B _o , reducing its heating or evaporation capacity, (1) When the final evaporator A _o The supply of solution from the immediately preceding evaporator A _o-1 to the final evaporator A _o is substantially stopped, and (2) the solution L _o in the final evaporator A _o that has reached the specified concentration is derived. After lowering the liquid level, (3) other evaporators A ₁ , A 2 , except for the above-mentioned evaporator A _{o- 1} ...
(4) Supply low concentration solutions L ₁ , L ₂ or/and unconcentrated solution L ₀ to be concentrated to the evaporator A _o to reduce the concentration of the solution L _o in the evaporator A _o ; When the solutions L _o , L ₀ in the can A _o reach the specified amount, the supply of the low concentration solutions L ₁ , L ₂ ... or/and the unconcentrated solution L ₀ is stopped, and (5) the heating is performed by the heater B _o . By heating, the solution L _o in the evaporator A _o is circulated through the heater B _o and the evaporator A _o to dissolve scale adhering to the heater B _o and the evaporator A _o , and to remove the solution L _o . (6) After the solution L _o in the evaporator A o reaches the liquid level during the normal operation, the solution L _o in the evaporator A _{o-1 is} gradually concentrated.
restarting the supply of solution L _o-1 to the evaporator A _o from
While controlling the liquid level of multiple (n ₎ evaporators A ₁ , _{A 2 .} A method for removing scale attached to a multi-effect evaporator is characterized by removing scale attached to the final evaporator A _o and heater B _o .

Hereinafter, the method of the present invention will be specifically explained in detail with reference to the drawings. The drawing is a conceptual diagram of a multi-effect can (Figure 1)
A graph showing the concentration distribution of the solution in the evaporator and the evaporator (second
Figure).

In the present invention, a multi-effect evaporator means a plurality of n
evaporator, preferably 3 or 4 evaporators. It consists of a plurality of (n) evaporators A ₁ , A _{2 .} . . , A _o-1 , A _o , and a heater built in or external to each of these evaporators.
B ₁ , B ₂ ..., B _o-1 , B _o , supply pipe C ₀ for supplying the unconcentrated solution L ₀ to be concentrated to the evaporator, supplying the solution L ₁ from the first evaporator A ₁ to the evaporator A ₂ supply pipe that supplies to
C ₁ , supply pipe C ₂ that supplies solution L _{2 from} evaporator A 2 to evaporator A _{o-1 ,} supply pipe C o that supplies solution L _o -1 from evaporator A _o-1 to evaporator A _{o -1} , a discharge pipe C _o is provided for discharging the concentrated solution L _o from the evaporator A _o . Steam V ₀ to heater B ₁ , steam Y ₁ from evaporator A ₁ to heater B ₂ , steam Y ₂ from evaporator A ₂ to heater B _o-1 , and steam from evaporator A _o-1 Steam pipes D _{1 ,} D ₂ , and D _o-1 are provided, which respectively supply steam Y _o-1 to the heater _{B o .}

Also, a vacuum tube D _o for communicating the gas phase portion of the evaporator A _o with a vacuum source (not shown), and a circulation tube E ₁ for circulating the solution between each evaporator and each heater installed outside. , E ₂ , E _o-1 , E _o , drain discharge pipes F ₁ , F ₂ , F _o-1 , _F o from each heater, steam is auxiliary introduced into the heater B _o of the evaporator A _o A steam pipe D _o+1 is provided for this purpose.

A solution L _o that has reached a certain high concentration is discharged from the discharge pipe C _o
is derived. This is achieved by opening the flow control valve G when the concentration of the solution L _o detected by the concentration detector H installed in the evaporator A _o reaches a specified high concentration. When the concentration has not been reached, the flow control valve G is closed.

The on-off valve I _{o-1 provided in the supply pipe C o-1 opens} the discharge pipe C o when the solution L _o in the evaporation pipe A _o reaches a specified concentration _.
The liquid level of the solution L _{o in the evaporation tube A o is} controlled so that it opens when the liquid level of the solution L _o in the evaporation tube A _o becomes less than a specified amount, and closes when it exceeds the specified amount. Similarly, the on-off valve _I2 provided in the supply pipe C2 opens and closes to control the liquid level of the solution L _o-1 in the evaporator A o-1 within a specified range, _and the valve I2 is opened and closed to control the liquid level of the solution L _o- 1 in the evaporator A _{o -1} within a specified range. The on _- off valve _I1 provided in the supply pipe _C1 is opened and closed to control the liquid level of solution _L2 in the evaporator _A2 within a specified range, and the solution L2 is supplied to the evaporator _A2 . A ₁ to evaporator A ₂
Supply solution L ₁ to. The on-off valve I ₀ provided in the supply pipe C ₀ opens and closes to control the liquid level of the solution L ₁ in the evaporator A ₁ within a specified range, and supplies the unconcentrated solution L ₀ to the evaporator A ₁ . conduct.

The supply pipe C ₀ and the supply pipe C _o-1 are the evaporator
A supply pipe C _o+1 is provided which directly connects A ₁ , A ₂ ..., A _o-1 without diameter, and an on-off valve I o _{+ 1 is provided in the supply pipe C o+1.}
is provided. In this case, the branch between the supply pipe C ₀ and the supply pipe C _o+1 is located at a position where the unconcentrated solution L ₀ does not pass through the on-off valve I ₀ , that is, on the supply side of the unconcentrated solution L ₀ from the on-off valve I ₀ . The connection between the supply pipe C _o-1 and the supply pipe C _o+1 is made on the side of the evaporator A _o rather than the on-off valve I _o- , and the supply pipe C _o+1 and the evaporator A _o are directly connected. It's okay.

As mentioned above, the solution L _o that has reached the specified concentration is led out from the discharge pipe C _o , and during the normal operation of continuously concentrating the solution, scale is formed on the inner surface of the final evaporator A _o and the heater B _o . When this occurs, the heating or evaporation capacity of the solution L _o decreases, and the concentration efficiency decreases, the following operation is performed.

First, the on-off valve I _o-1 is forcibly closed, and the solution L _o (solution concentrated to a specified concentration) in the evaporator A _o is forcibly led out from the discharge pipe _Co. In this case, the on-off valve I _o-1 is completely closed when the solution L _o is discharged from the evaporator A _o in a short time, but by completely closing it, the solution L _o is not concentrated. A very small amount of solution L _o-1 that can be corrected if it exceeds the specified range
The supply of the solution L _o-1 is substantially stopped.

After lowering the liquid level of solution L _o in evaporator A _o ,
The on-off valve Io ₊₁ is opened and the unconcentrated solution _L0 is directly supplied to the evaporator _Ao . The concentration of the solution in the evaporator A _o decreases rapidly, and the flow rate control valve G closes.

Instead of the unconcentrated solution L ₀ supplied to the evaporator A _o , other solutions L 1 from the evaporator _{A 1 except the solution L o- 1} concentrated in the evaporator A o- ₁ , and the solution L ₁ from the evaporator A ₂ Solution L ₂ may be supplied. That is, these solutions L ₀ , L ₁ , L ₂
may be supplied alone or in combination.

By the above operation, the evaporator A _o and the heater B _o
The inside contacts with a solution whose concentration is significantly lower than that of the previous solution. Of course, this concentration is the supply pipe C _o+1
The concentration of the solution passing through it also depends on its feed rate and the proportion of solution remaining in the evaporator _Ao .

During the time when on-off valve I _o-1 is stopped, evaporator A
_o , substantially equal to the time for descaling heater B _o . Since the solution is not discharged from the evaporator A _o-1 , the solution in the evaporator A _o-1 is concentrated.
Since there is a risk that scale may adhere to the inside of the evaporator A _o-1 and the heater B _{o-1 while removing the scale inside the evaporator A o and the heater B o , in} the present invention, these The concentration in the evaporator A _o and the closing time of the on-off valve I _o-1 are determined within a range in which scale is not generated.

By adopting the method of the present invention, scale can be removed without stopping the operation of the multi-effect evaporator. Figure 2 shows the changes in the concentration of the solution in each evaporator before and during this removal operation. Indicated.

The hatched bar graph in FIG. 2 represents the concentration of the solution in each evaporator during normal operation of the four-effect evaporator before closing the on-off valve Io _-1 . Immediately before normal operation is resumed after scale elution in the evaporator Ao _{, etc.} , the concentration changes as shown in the bar graph without diagonal lines. That is, while the concentration of the solution in the evaporator A _o is slightly lower than the normal concentration, that in the other evaporators becomes significantly higher.

The scale can be removed because the concentration inside the evaporator A _o is low.

Next, when the operation is restored to its original state, a highly concentrated solution enters the evaporator _Ao , and the concentration of the solution in the evaporator _Ao increases again.

When this concentration reaches a predetermined concentration, the discharge pipe C _o
Deliver the solution through.

In addition, in the present invention, when scale is generated on the inner surface of the evaporator A _o and the heater B _o means when to remove the scale. If this is done, it is naturally possible to remove scale in a short time, but the number of scale removals per certain period increases. On the other hand, if you do it after a lot of scale builds up, the number of times it will be removed will be reduced, but it will take longer to remove the scale and restore the evaporation ability, so consider these factors and decide whether to carry out the scale removal work economically. Bye. However, in general, this should be done when the evaporation capacity begins to drop below about 90%.

<Example> Example: A dilute aqueous solution containing caprolactam and its oligomers such as dimers and trimers obtained during the scouring of polycapramide has the ability to concentrate a caprolactam concentration of 6% to 70% in _L0
A 150 t/time self-compression type quadruple effect evaporator shown in Figure 1 was operated as follows.

Transfer of liquid from the third evaporator A _o-1 to the fourth evaporator A _o Close the on-off valve I _o-1 to interrupt normal continuous operation, and then remove the liquid from the fourth evaporator A _o and the heater. An aqueous caprolactam solution L _o with a concentration of 70% held by B _o was led out from the discharge pipe C _o . The amount drawn out was set at 60% of the specified capacity of the evaporator _Ao . After that, on-off valve I
_o+1 is opened and unconcentrated solution is added using supply tube C _o+1.
L ₀ was fed directly to the evaporator A _o . The caprolactam concentration of the unconcentrated solution L ₀ (caprolactam aqueous solution) to the evaporator A _o is 6%, the same as that of a normal unconcentrated solution, and the supply amount was set to be higher than the liquid level of the solution L _o during normal operation. In order to speed up the dissolution of the liquid in the evaporator A _o , steam was added to the steam from the steam pipe D _o-1 and supplied from the steam pipe D _o+1 shown in the figure. After the on-off valve I _o-1 is closed and the scale in the steam canister A _o and the heater B _o is completely removed, three hours later, the on-off valve I _o-1 is opened and the supply of steam from the steam pipe D _o+ is stopped. The four-effect evaporator was returned to normal operation.

The above-mentioned evaporator A _o and heater B _o were descaled periodically once every 7 days and operated continuously for one year, but there was no decrease in the concentration ability. In addition, the natural circulation of the liquid by the thermosiphon between the external heater B _o and the evaporator A _o was not impaired.

After one year of continuous operation, we inspected the inside and found no scales such as oligomers on the inner walls of all evaporation tubes and heaters.

For comparison, when a four-effect evaporator was operated without performing the above removal operation, it became inoperable after three months.

<Effects of the Invention> According to the present invention, there is no need to substantially stop the operation of the multi-effect can. There is no need to install a spare evaporator.

In addition, a low concentration solution can be made into a constant extremely high concentration solution under harsh conditions that exceed the limit of scale precipitation in the evaporator and heater of the multi-effect can, and the scale can be smoothly removed. It can be operated continuously and the operating efficiency of the multi-effect can can be improved.

Furthermore, if scale is periodically removed by the method of the present invention during the continuous operation, although a highly concentrated solution cannot be drawn out continuously during the time required for scale removal, the solution in the preceding evaporator will be lower than during normal operation. After scale removal, the final stage evaporator reaches a specified high concentration much faster than during normal operation, resulting in a large amount of descaling, which effectively reduces the time required for scale removal. Most areas can be covered and productivity can be significantly improved.

Furthermore, especially when used for concentrating polycapramide scouring water, the resulting highly concentrated solution can be used as a raw material again as it is, resulting in significant effects such as effective use of resources, reduction of energy consumption, and prevention of environmental deterioration. have

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus to which the method of the present invention is applied, and FIG. 2 is a graph showing the concentration distribution of the solution in each evaporator. A ₁ , A ₂ , A _o-1 , A _o ...evaporator, B ₁ , B ₂ , B _o-
1 , B _o ...heater, C ₀ , C ₁ , C ₂ , C _o-1 ... solution supply pipe, C _o ... discharge pipe, D ₁ , D ₂ , D _o-1 , D _o+1 ... evaporation pipe , D _o ... Vacuum tube, E ₁ , E ₂ , E _o-1 , E _o ... Circulation pipe, F ₁ , F ₂ , F _o-1 , F _o ... Drain discharge pipe, G... Flow rate control valve, H... Concentration Detector, I ₀ , I ₁ , I ₂ , I _o-1 ,
_Io , Io ₊₁ ...on/off valve, _L0 ...unconcentrated solution, _L1 , _L2 ,
L _o-1 , L _o ...solution, _V0 , _V1 , _V2 , V _o-1 , V _o ...steam.

Claims (1)

[Claims] 1. A plurality of (n) evaporators A ₁ , A ₂ ..., A _o-1 , A _o
During a normal operation in which the solution is sequentially passed through the final evaporator A _{o while controlling the liquid level, and when the solution in the final evaporator A o reaches} a specified concentration, the solution is taken out from the final evaporator A o and continuously concentrated. When scale occurs on the inner surface of the evaporator A _o and its heater B _o , and its heating or evaporation capacity decreases, (1) When the final evaporator A _o-1 immediately before the final evaporator A _o The supply of solution to the evaporator A _o is substantially stopped, (2) the final solution L _o in the evaporator A _o that has reached the specified concentration is drawn out to lower the liquid level, and (3) the above Other evaporators A ₁ , A ₂ except evaporator A _o-1
Low concentration solutions L ₁ , L ₂ ... or/and unconcentrated solution L ₀ to be concentrated are supplied to the evaporator A _o to reduce the concentration of the solution _{L o in} the evaporator A o, (4) When the solutions L _o , L ₀ in the evaporator A _o reach the specified amounts, the supply of the low concentration solutions L ₁ , L ₂ ... or/and the unconcentrated solution L ₀ is stopped, and (5) heating is performed. The solution L _o in the evaporator A _o is circulated through the heater B _o and the evaporator _{A o by heating with the heater B o} , and the scale attached to the heater B _o and the evaporator A _o is dissolved. (6) After the solution L _o in the evaporator A _o reaches the liquid level during the normal operation, the solution L _{o is gradually concentrated.}
restarting the supply of solution L _o-1 to the evaporator A _o from
While controlling the liquid level of multiple (n ₎ evaporators A ₁ , _{A 2 .} A method for removing scale attached to a multi-effect evaporator, the method comprising removing scale attached to the final evaporator A _o and heater B _o .