JPH1151584A - Heat collector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the facility cost by installing an after cooler in the passage of a low temperature side gas being introduced to a heat exchanger and lowering the temperature of the low temperature side gas being introduced to the heat exchanger thereby increasing the logarithmic mean temperature difference of a gas passing through the heat exchanger and decreasing the heat transfer area of the heat exchanger. SOLUTION: A circulation passage circulates through a reaction furnace 1, a heat exchanger 2, a dehumidifier/dust separator 4, a compressor 5, the heat exchanger 2, a heating furnace 3 or a preheating furnace, and the reaction furnace 1. An after cooler is installed in the circulation passage 12 between the compressor 5 and the heat exchanger 2. When the temperature is lowered on the low temperature side by the after cooler 15 installed in the circulation passage 12 on the delivery side of the compressor 5, the logarithmic mean temperature difference of a gas passing through the heat exchanger 2 can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a heat recovery device,
In particular, the present invention relates to an apparatus capable of effectively recovering heat generated in a process related to a steelmaking plant using a large amount of thermal energy.

[0002]

BACKGROUND OF THE INVENTION Generally, the production of steel comprises the steps of converting iron ore to pig iron in a blast furnace and then converting pig iron to steel in a flat furnace or a converter. However, such a traditional manufacturing method requires large amounts of energy, equipment scale, and cost. Therefore, for small-scale steelmaking, iron ore is converted directly into steelmaking furnace raw material (solid) by direct ironmaking. A method of converting the raw material of the steelmaking furnace into molten steel by an electric furnace or the like is employed. Such direct iron production includes a direct reduction method for converting iron ore to reduced iron.

[0003] However, reduced iron produced by the direct reduction method has a strong reaction activity and generates heat by reacting with oxygen in the air. Therefore, transportation and storage require a treatment such as sealing with an inert gas. For this reason, the reaction activity is low, easy transportation,
Iron carbide, which is storable and contains a relatively high percentage of iron, has recently been used as a raw material for steelmaking in electric furnaces and the like.

[0004] A conventional process for producing such iron carbide is to powder iron ore and fill it into a fluidized bed reactor.
By reacting a mixed gas of a reducing gas (hydrogen gas) and a carbonized gas (for example, methane gas) at a predetermined temperature, iron oxide (hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ )) in iron ore , Wustite (FeO) and the like are reduced and carbonized in a single operation (referring to an operation performed by simultaneously introducing reduction and carbonization gas into one reactor). As this kind of prior art, Japanese Patent Publication No. Hei 6-50198 is disclosed.
There is one described in Japanese Patent Publication No.

FIG. 4 shows a schematic configuration of a conventional iron carbide manufacturing apparatus. In FIG. 4, 1 is a fluidized bed reactor, 2 is a multitubular heat exchanger, 3 is a heating furnace, 4 is a dehumidifier and a duster, and 5 is a compressor. In order to facilitate understanding of the present invention, an outline of a method for manufacturing iron carbide by a conventional iron carbide manufacturing apparatus will be briefly described. The fine-grained iron oxide charged into the fluidized bed reactor 1 through the pipe 6 is heated to about 680 ° C. in the heating furnace 3 and reacts with a reaction gas having a predetermined composition introduced into the reactor. Then, a gas at about 630 ° C. is discharged from the top of the fluidized bed reactor 1 through the passage 7. This exhaust gas is passed through the heat exchanger 2
After being dehumidified and removed by the dehumidifier 4, the compressor 5 exchanges heat with a gas of about 70 ° C., which is pressurized to a constant pressure. As a result, the exhaust gas at about 630 ° C. is cooled to about 160 ° C., and is introduced into the dehumidifier 4 via the route 8. In the dehumidifier 4, water vapor in the gas is condensed and removed by the water sprayed through the pipe 9 through the pipe 9, and the water vapor is discharged from the pipe 10 together with the dust in the gas. The gas thus dehumidified and dedusted is raised to a constant pressure of about 70 ° C. by the compressor 5 via the path 11 and then supplied to the heat exchanger 2 via the path 12 to about 70 ° C. After the heat exchange with the exhaust gas of 630 ° C, the temperature becomes about 580 ° C. This gas is
Is supplied to the heating furnace 3. The gas heated to about 680 ° C. in the heating furnace 3 is introduced into the fluidized bed reactor 1 and subjected to the above reaction. The iron oxide charged into the fluidized bed reactor 1 is converted into iron carbide having a predetermined carbonization rate after a certain reaction time by the above-described process to form a pipe 14.
And used as a raw material for electric furnaces.

By the way, the heat transfer area of the heat exchanger has a relation inversely proportional to a so-called log-average temperature difference when the heat exchange amount is the same. The logarithmic mean temperature difference (LMTD) means that the temperature of the high-temperature fluid introduced into the heat exchanger is T ₁ , the temperature of the low-temperature fluid is t ₂ , and the temperature of the high-temperature fluid is t ₁
It is lowered, if the temperature of the cold side of the fluid is to be heated to T _2, is defined by the following equation.

If t ₁ -t ₂ = △ α and T ₁ -T ₂ = △ β, LMTD = | △ α- △ β | / ln (△ α / △ β)... The required heat exchange amount cannot be obtained unless the heat transfer area of the heat exchanger is large.
In order to design the heat exchanger economically, it is preferable that the LMTD be as large as possible. Conversely, if the heat transfer area of the heat exchanger is constant, increasing the LMTD can increase the amount of heat transfer.

The conventional iron carbide production process in which the above-described reduction and carbonization are performed in a single operation has the advantage of being systemically simple. Due to the disadvantage that the reaction gas composition and the reaction temperature cannot be set individually and flexibly, the present applicant uses the gas used in the first reaction operation to have the optimum composition only for the reduction reaction, and uses the gas used in the second reaction operation. `` Perform the remaining reduction reaction and carbonization reaction after the first reaction operation that performs a part of the reduction reaction of the iron-containing raw material for iron making so that the gas can be made to have the optimum composition for the remaining reduction reaction and carbonization reaction. Patent Application for an invention relating to "a method and apparatus for producing iron carbide characterized by proceeding with a second reaction operation" (JP-A-9-486)
No. 04). According to the present invention, various operations can be taken for each operation, which is impossible with the conventional method of manufacturing iron carbide in a single operation, and the process becomes flexible. There is an advantage that the flow rate of the gas can be greatly reduced.

As described above, the method of performing the reaction in two steps when obtaining iron carbide from the iron-containing raw material has many advantages, but the reaction gas in the first reaction operation is steam generated as a result of the reduction reaction with hydrogen. And the gas dew point is high (about 100 ° C.), while the amount of water vapor contained in the reaction gas in the second reaction operation is small, so that the gas dew point is low (about 70 ° C.). As a result, dew condensation may occur on the pipe wall on the low temperature side of the heat exchanger. For example,
When the gas temperatures at the inlet and outlet of the heat exchanger 2 are as described above, the temperature of the inner surface of the pipe at the outlet side of the heat exchanger 2 through which the high-temperature gas passes is approximately equal to the gas temperature. 160 ° C
Is estimated. On the other hand, it is estimated that the minimum temperature of the pipe outer surface at the outlet side of the heat exchanger 2 is about 70 ° C. Therefore, it is estimated that the metal temperature of the tube wall at the outlet side of the heat exchanger 2 is approximately 115 ° C., which is an average value of the two temperatures. If the temperature of the gas at the outlet of the heat exchanger is lower than the metal temperature, dew condensation occurs on the tube wall at that portion, causing dust to adhere and corrode.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to reduce the equipment cost of a heat exchanger by reducing the heat transfer area of the heat exchanger. Another object of the present invention is to provide a heat recovery device capable of preventing dew condensation from occurring on the low temperature side of the heat exchanger. Alternatively, an object of the present invention is to provide a heat recovery device capable of increasing the amount of heat transfer by increasing the LMTD, even if the heat transfer area of the heat exchanger is the same.

[0011]

In order to achieve the above object, the gist of the present invention is to provide an aftercooler in a path of a low-temperature side gas introduced into a heat exchanger, and to provide a low-temperature side gas introduced into the heat exchanger. To reduce the equipment cost of the heat exchanger by increasing the logarithmic mean temperature difference (LMTD) of the gas passing through the heat exchanger by decreasing the temperature of the gas and decreasing the heat transfer area of the heat exchanger. Becomes possible. Alternatively, by increasing the LMTD, it is possible to increase the amount of heat exchange and increase the gas temperature at the outlet of the heat exchanger to reduce the load on the heating furnace, even for heat exchangers having the same heat transfer area. That is, it is possible to reduce the equipment cost of the heating furnace and reduce the fuel gas consumption. Further, by adjusting the cooling temperature of the low-temperature side gas by the aftercooler in accordance with the dew point of the high-temperature side gas passing through the heat exchanger, it is possible to prevent the occurrence of dew condensation on the heat exchanger outlet side.

[0012]

That is, the gist of the present invention is that after passing from a reactor through a heat exchanger, a dehumidifying and dedusting machine and a compressor, passing through the heat exchanger again and passing through a heating furnace or a preheating furnace, In a heat recovery apparatus having a circulation path reaching a reaction furnace, an aftercooler is installed in a circulation path between a compressor and a heat exchanger.

In the present invention configured as described above,
When the temperature of the low-temperature gas is reduced by an aftercooler installed in the circulation path on the outlet side of the compressor, the logarithmic average temperature difference of the gas passing through the heat exchanger can be increased. For example, the temperature of the gas on the outlet side of the compressor is 70 ° C. as described above, the temperature of the gas discharged from the reactor is 630 ° C., and the gas of 630 ° C. on the high temperature side passes through a heat exchanger. The temperature is lowered to 160 ° C., and the low-temperature gas at 70 ° C.
When the temperature is raised to 80 ° C., Δα = 160−70 = 90
° C and β = 630-580 = 50 ° C, the logarithmic mean temperature difference (LMTD) is as follows.

LMTD = (90-50) ° C./ln(90
/50)=68.0° C. In this case, the low temperature side gas is cooled to 50 ° C. by the aftercooler.
When cooled down, the logarithmic mean temperature difference (LMTD)
It looks like this:

That is, Δα = 160−50 = 110
° C, β = 630-560 = 70 ° C, LMTD
= (110-70) ° C / ln (110/70) = 88.
It will be 5 ° C. Thus, the logarithmic average temperature difference can be increased by installing the aftercooler in front of the heat exchanger and cooling the low-temperature side gas.

However, depending on the dew point of the high-temperature side gas passing through the heat exchanger, dew condensation may occur on the low-temperature side of the heat exchanger outlet. Therefore, cooling of the low-temperature side gas by the aftercooler prevents this dew condensation from occurring. It is preferable to set the temperature at a low level.

[0017]

Embodiments of the present invention will be described below. FIG. 1 is a layout diagram showing a schematic configuration of the heat recovery apparatus of the present invention. FIG.
The same components as those described above are denoted by the same reference numerals, and redundant description will be omitted. In FIG. 1, an aftercooler 15 is provided in a path 12 between the compressor 5 and the heat exchanger 2, and a refrigerant flow control valve 16 of the aftercooler 15 is provided.
Is controlled by the temperature of the gas passing through the path 8 detected by the temperature indicating controller 17.

Next, Tables 1 and 2 show examples of the gas temperature, the outlet tube wall temperature of the heat exchanger, and the LMTD of the heat exchanger at each design point in FIG.

[0019]

[Table 1]

Table 1 shows an example of each temperature when there is no aftercooler (case 1) and when the aftercooler is added to change the cooling temperature (cases 2 to 5). The heat exchanger is an example of a case where existing equipment is not downsized by increasing LMTD. FIG. 2 shows the cost merits of Cases 2 to 5 with respect to Case 1 when an aftercooler is added to the 2,000 ton / day iron carbide manufacturing apparatus. When converting the amount of heat recovered into money, 1
Billion B. T. U. $ 4 per unit. That is, FIG. 2 shows the amount obtained by subtracting the fuel cost in the heating furnace from “the increase in the aftercooler equipment cost + the reduction in the heat exchanger equipment cost + the increase in the heating furnace equipment cost”. If the bar graph is directed downward, it indicates that even if there is an increase in after-cooler capital investment, the amount of heat recovered by the heat exchanger is large, so that the effect of saving fuel for the recovered heat is great. However, in case 5, the aftercooler outlet gas temperature approaches the temperature of the cooling water, the aftercooler equipment becomes excessively large, and the investment amount increases. In case 4 (when the aftercooler outlet side temperature is 38 ° C.), it can be said that it is the most economical. Naturally, the most economical point depends on the unit price of the recovered heat (that is, the heating furnace fuel).

In order to prevent dew condensation on the heat exchanger, it is an absolute condition that the outlet temperature of the heat exchanger is determined in consideration of the dew point of the high-temperature side gas. For example, when a gas having a relatively high dew point is targeted, such as a reaction gas after the first reaction operation in the case where iron carbide is produced in two stages, the outlet temperature of the heat exchanger is also relatively high. By adjusting the outlet gas temperature of the aftercooler to make it higher,
Heat recovery up to the limit becomes possible. On the other hand, the reaction gas of the second reaction operation has a sufficiently low dew point, and the most economical heat recovery system up to the dew point can be selected.

In the case where iron carbide is produced in a single operation, the dew point of the reaction gas is substantially intermediate between the dew points of the reaction gas of the first reaction operation and the reaction gas of the second reaction operation in the two-stage method. In some cases, the system cannot be configured at the most economical point due to the restrictions of condensation. In this regard, the two-stage method is superior to the single operation method.

Next, similarly to Table 1, another embodiment of each temperature in the case where there is no after-cooler (case 1) and in the case where the after-cooler is added and the cooling temperature is changed (cases 2 to 5) are shown. It is shown in Table 2.

[0024]

[Table 2]

This embodiment is an embodiment in which the capacity of the heat exchanger can be increased or decreased according to the LMTD. Also, as in FIG. 2, FIG. 3 shows the cost merits of Cases 2 to 5 with respect to Case 1 when an aftercooler is added in the iron carbide manufacturing device of 2000 tons / day. In addition,
As a fuel, by-product gas can be used. T. U. It was $ 0.5 per. In this embodiment, the LMTD
The heat exchanger can be downsized by the increase of the cost, and a cost advantage is generated. That is, in case 3 (when the aftercooler outlet side temperature is 48 ° C.), it can be said that it is the most economical. Also in this case, in order to prevent dew condensation on the heat exchanger, it is an absolute condition that the outlet temperature of the heat exchanger is set to be higher than the dew point of the high-temperature side gas.

[0026]

According to the present invention, it is possible to provide a heat recovery apparatus capable of reducing the size of a heat exchanger and reducing equipment costs by adding an aftercooler. It also saves fuel by increasing heat recovery, and
A heat recovery device capable of suppressing dew condensation on the low temperature side of the heat exchanger can be provided.

[Brief description of the drawings]

FIG. 1 is a layout diagram showing a schematic configuration of a heat recovery device of the present invention.

FIG. 2 is a view showing a cost merit when an aftercooler is added to an iron carbide manufacturing apparatus.

FIG. 3 is another diagram showing a cost merit when an aftercooler is added to the iron carbide manufacturing apparatus.

FIG. 4 is a layout diagram showing a schematic configuration of a conventional heat recovery device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Fluidized-bed reactor 2 ... Heat exchanger 3 ... Heating furnace 4 ... Dehumidification / dust remover 5 ... Compressor 15 ... Aftercooler

Claims (1)

[Claims] 1. A heat recovery system having a circulation path from a reactor through a heat exchanger, a dehumidifier and a dust collector, and a compressor, passes through the heat exchanger again, passes through a heating furnace or a preheating furnace, and then reaches the reactor. A heat recovery apparatus, wherein an aftercooler is installed in a circulation path between the compressor and the heat exchanger.