JP2753392B2 - Method for cooling intermediate gas in multi-stage compressor for carbon dioxide and multi-stage compressor for carbon dioxide provided with intermediate gas cooling device - Google Patents

Description

The present invention relates to a method for cooling an intermediate gas in a multistage compressor in which a discharge gas is cooled from a first-stage compressor and is guided to a second-stage compressor. The present invention relates to a multi-stage compressor provided with a cooling device, and is particularly suitable for being applied to a carbon dioxide gas compressor used in a corrosive environment such as a factor causing chloride stress corrosion cracking.

[Conventional technology]

As this kind of conventional technology, Hitachi Review Vol.60 No.3 (1
March 978). Those described in this document, a multistage compressor for compressing carbon dioxide gas (hereinafter referred to as CO ₂ gas), front side (low pressure side) CO ₂ gas compressed by the compressor is an intermediate heat exchanger ( Intercooler)
And is sent to the next stage (later stage or high pressure side) compressor.

[Problems to be solved by the invention]

In general, when compressing a gas with a compressor, the relationship between the work per unit mass (head) given to the gas from the driver and the change in the state of the gas is expressed by equation (1).

Where H _th = theoretical head m = polytropic index R = gas constant Ts = suction temperature Pd = discharge pressure Ps = suction pressure ηpol = polytropic efficiency As is clear from equation (1), the same pressure is obtained by reducing the suction temperature Ts. The work per unit mass (H _th ) required to obtain a rise (ie, pressure ratio Pd / Ps) can be small. Therefore, in general, it is desirable that the gas temperature at the outlet of the heat exchanger be lower because the shaft power (energy) of the driving machine can be reduced. For this reason,
A heat exchanger is provided between the groups grouped by one or several stages to cool the CO ₂ gas.

On the other hand, the CO ₂ gas has the property of liquefying at a high pressure even at a temperature higher than ordinary temperature, so the gas temperature at the heat exchanger outlet must be higher than the temperature at which this liquefaction does not occur.
Further, after the CO ₂ gas is sucked into the compressor, a part of the CO ₂ gas leaks from the shaft seal portion to the outside of the machine. The pressure outside the machine is always lower than the pressure inside the compressor. Therefore, it leaks while expanding to the outside of the machine
If the CO ₂ gas is liquefied on the way (for example, in the labyrinth seal of the shaft seal portion), it will cause an erosion attack and cause harmful damage to the compressor. If the temperature of the sucked CO ₂ gas is low, it expands and liquefies. In consideration of these points, the temperature of the outlet of the CO ₂ gas heat exchanger is controlled.

Furthermore, when manufacturing the heat exchanger, the cooling water contains corrosive factors such as chlorine ions, so the design must be made with due consideration to corrosion, such as the heat exchanger tube material, tube temperature, and cooling water speed. It has been done. In addition, the tube heat transfer capacity is manufactured with a margin for heat exchange capacity in advance in consideration of contamination by cooling water during operation.

That is, it is manufactured so that the heat exchange capacity does not become insufficient even during long-term continuous operation.

Generally, the heat transfer rate K of the heat exchanger is represented by the following equation (2).

Here, K = heat transfer rate α ₁ = heat transfer coefficient on the gas side (tube side) α ₂ = heat transfer coefficient on the water side (shell side) f _W = dirt coefficient In the equation (2), the dirt coefficient f _W is the initial value. 0 when driving.
Although it is 0001 or less, in production, it is generally set to about 0.0004 in consideration of contamination on the cooling water side due to long-term operation.

On the other hand, the heat transfer amount Q is represented by the equations (3), (4), and (5).

Q = K × A × ΔTm (3) Q = G _w × C _pw × ΔT _w (4) Q = G _g × C _pg × ΔT _g (5) where Q = heat transfer amount A = heat exchange area? Tm = logarithmic mean temperature difference G = flow rate C _p = specific heat [Delta] T = heat exchanger inlet and outlet temperature difference index: w = coolant is g = gas.

Now, it is assumed that the exchanged heat quantity Q shown by the equation (5) is required in a certain operation state. The amount of heat passing through the tubes of the heat exchanger is Q represented by the equation (3).
(3) the heat transfer coefficient K of the heat exchanger is shown in equation (2), when the contamination of the tubes are different, adjusting the thermal heat transfer rate alpha ₂ in the water side in order to obtain a constant K There is a need to. That is, in the initial operation of the heat exchanger it is necessary to reduce the alpha ₂ because f _W is small.

For example, when f _W = 0.0004m ² h ° C / kcal, α ₂ = 5000kc
heat exchanger designed at al / m ² h ° C, f _W = 0.001 m ² h ° C / kca
When operating at l, alpha ₂ is not a comparable K value unless about 2000kcal / m ² h ℃. Since α ₂ is approximately proportional to the power of 0.6 of the cooling water amount, conventionally, the cooling water amount was reduced to reduce α ₂ . Reducing the amount of cooling water increases the temperature difference ΔT _{W as} is apparent from equation (4). That is, the temperature of the cooling water outlet rises, and as a result, the temperature of the heat exchanger tube rises. That is, in the conventional system for adjusting the amount of cooling water such that the temperature of the gas to be cooled at the heat exchanger outlet becomes constant, the amount of cooling water becomes insufficient in the initial stage of operation, causing the temperature of the heat exchanger tube to rise, and the scale by the cooling water flow velocity range. Therefore, there is a risk that chloride stress corrosion cracking may occur due to the adhesion of water and an increase in tube wall temperature.

An object of the present invention is to provide a method for cooling an intermediate gas in a multi-stage compressor and a multi-stage compressor equipped with an intermediate gas cooling device that does not cause an increase in tube wall temperature and does not cause liquefaction of a handled gas. is there.

[Means for solving the problem]

A first feature of the present invention to achieve the above object is that an intermediate gas compressed by a low-pressure side compressor is cooled by exchanging heat with cooling water in an intermediate heat exchanger, and then the cooled gas is cooled. In the method of cooling the intermediate gas in the multi-stage compressor that supplies the intermediate gas to the downstream compressor, part of the intermediate gas discharged from the upstream compressor bypasses the intermediate cooler, and the rest of the intermediate gas passes through the intermediate cooling. In the heat exchanger, the amount of water flowing in based on the gas temperature at the outlet of the intercooler and located upstream of the mixed point of the cooled gas and the bypassed gas is exchanged with the controlled cooling water to exchange heat with the gas. The temperature at which the gas cooled to near the gas-liquid critical point and a part of the gas obtained by mixing the cooled gas and the bypassed gas flows into the labyrinth seal portion of the subsequent compressor and does not liquefy even when expanded. Until after the mixing Controlling the flow rate of the intermediate gas obtained by the bypass based on the temperature of the gas is to increase the amount of water of the cooling water when the bypass gas amount is increased.

A second feature of the present invention is that a front-stage compressor, a rear-stage compressor, an intermediate gas line that guides an intermediate gas discharged from the front-stage compressor to a rear-stage compressor, and an intermediate gas line provided in the intermediate gas line. A multi-stage compressor having an intermediate cooler for cooling the intermediate gas by exchanging heat with the cooling water to cool the intermediate gas. Control valve, first temperature detecting means provided at the outlet side of the intercooler for detecting the temperature of the gas discharged from the intercooler, and the intermediate gas is detected based on the temperature detected by the temperature detecting means. A first control valve for controlling the first control valve so as to cool the gas to near the gas-liquid critical point;
A bypass gas line provided so as to branch off from the intermediate gas line, bypass the intermediate cooler, and merge downstream of the first temperature detecting means; and a bypass provided in the bypass gas line. A second control valve for controlling a gas amount, a second temperature detecting means for detecting a temperature of the gas after the merge of the intermediate gas line and the bypass gas line, and a gas for the post-stage compressor, A second control valve for controlling the second control valve based on the temperature detected by the second temperature detecting means so that the temperature does not liquefy even when the second control valve flows into the labyrinth seal portion and expands.
And the second control means controls to open the second control valve when the first control means opens the first control valve.

Preferably, the gas to be compressed is carbon dioxide (CO ₂ gas).

[Action]

With the above configuration, it is not necessary to reduce the flow rate of the cooling fluid even in the initial operation of the heat exchanger, and it is sufficient that the flow rate is within a range that is not very close to the gas-liquid critical point. The amount of supercooling is
By mixing with the heating means or the bypassed hot gas, it is possible to raise the temperature of the gas to such an extent that the gas sucked into the downstream compressor does not liquefy when expanded in the labyrinth seal portion of the compressor.

Therefore, it is possible to always supply sufficient cooling fluid to keep the temperature of the heat exchanger tube of the intercooler such that stress corrosion cracking does not occur.

〔Example〕

FIG. 2 is a diagram showing an overall configuration of a carbon dioxide gas compressor. In general, this type of compressor is used for pressure
The pressure is increased to 0 atm. The CO ₂ gas sucked at approximately atmospheric pressure is, for example, about 5 atm in the first group and about 5 atm in the second group.
The pressure is increased to 25 atm, about 90 atm in the third group, and about 260 atm which is the discharge pressure in the last fourth group.

FIG. 3 shows a state diagram of the CO ₂ gas. In FIG. 3, for example, when the outlet pressure of the second group of the CO ₂ compressor is 25 atm, the intersection with the liquidus line is about −11 ° C. If the temperature does not fall below this temperature, the CO ₂ gas does not liquefy. In general, water coolers are not cooled below 0 ° C,
No matter how much the first and second coolers are cooled, they do not liquefy. However, when the outlet pressure of the third group is 90 atm, the intersection with the gas-liquid critical line is about 40 ° C. (point X in the upper left in the figure).
℃ or more).

Furthermore, even if the temperature is 45 ° C. or higher, liquefaction occurs due to expansion when leaking out of the compressor. For example, in the case of 45 ° C., it intersects with the liquidus line due to expansion and liquefies (point X at the lower right in the figure). For this reason
Even when the CO ₂ gas expands, it is necessary to control the inlet temperature of the fourth group of the compressor to a temperature (for example, 68 ° C.) that does not cross the liquidus line. As described above, the temperature control at the outlet of the CO ₂ gas heat exchanger is performed in consideration of energy reduction, liquefaction prevention due to supercooling and expansion.

In addition, when a gas other than the CO ₂ gas is handled in the vicinity of the liquidus line or the gas-liquid critical line, the same can be said for the above-described example of the CO ₂ gas.

FIG. 1 shows a specific example of the present invention applied to a carbon dioxide (CO ₂ ) compressor. The CO ₂ gas compressed by the former-stage (low-pressure side) compressor 1 passes through the intermediate gas line 10 and is cooled by the intermediate heat exchanger or the intercooler 3, and at the same time, some of the gas is intermediate gas line.
The mixture is mixed with the cooled gas by bypassing the intermediate heat exchanger 3 by 11, adjusted to a certain constant temperature, and sent to the high pressure side (secondary stage) compressor 2 of the next stage. In the heat exchanger 3, the intermediate gas is cooled by the cooling water
Cooled through 3a.

The temperature of the cooling gas is detected by a first temperature detector 5a provided downstream of the heat exchanger 3, and the temperature of the control valve 4 is controlled by a temperature controller (first control means) 5 to a certain temperature. And the amount of cooling water is adjusted.

Next, the bypass gas amount is determined by detecting the gas temperature after mixing by the second detector 7a, and by the temperature controller (second control means) 7 so that the mixed gas becomes a certain temperature. The control valve 9 is controlled. The set temperature of the temperature controller 5 is set lower than the set temperature of the temperature controller 7. The set temperatures of the temperature controllers 5 and 7 will be described with reference to FIG. The temperature controller 5 sets the temperature near the minimum temperature at which the CO ₂ gas does not liquefy in FIG. 3 (for example, about 45 ° C. when the CO ₂ gas is 90 atm).
The temperature set by the temperature controller 7 is set to a temperature near the lowest temperature at which liquefaction does not occur even when the CO ₂ gas expands (for example, about 68 ° C. when the CO ₂ gas is 90 atm).

According to the present embodiment, the provision of the hot gas bypass line 11 allows the amount of cooling water to be sufficiently larger than when there is no bypass line. This will be described below.

Since the gas temperature at the outlet of the heat exchanger 3 is controlled to be lower than the gas temperature after mixing, it is necessary to always perform hot gas bypass. Therefore, the amount of gas passing through the heat exchanger 3 is reduced. Then, the heat transfer coefficient alpha ₁ of the gas side will be reduced in proportion to the approximately 0.8 square of flow rate. On the other hand, even when the hot gas bypass is performed, the required heat exchange amounts on the gas side and the water side are not different from those without the bypass. That is, the heat transfer coefficient K of the heat exchanger hardly changes. Even smaller gas side heat transfer coefficient alpha _1, in order to equalize the heat transfer coefficient K, increase the heat transfer coefficient alpha ₂ of the water side, it is necessary to remove dirt of the heat exchanger tubes . In other words, long-term continuous operation in consideration, when the heat exchanger is set at a certain dirt factor is operated without an initial stains, that may not reduce the alpha ₂ by using the hot gas bypass become. Since the heat transfer coefficient alpha ₂ of the water side varies in proportion to the approximately 0.6 squares of the cooling water, reducing the alpha ₂ would reduce the flow rate. For this reason, the outlet temperature of the cooling water increases, the temperature of the outlet side of the heat exchanger tube 3a increases, and when the cooling water contains a corrosive factor (when ordinary water is used as the cooling water), corrosion occurs. Promotes stress corrosion cracking. In the present invention, it is not necessary to reduce the heat transfer coefficient alpha ₂ of the water side, a cause of stress corrosion cracking can be prevented.

The operation of the present invention will be described in more detail with reference to FIGS.

Even if the hot gas bypass line 11 is provided as in the present invention, the required heat exchange amount of the heat exchanger 3 is not different from the case where the bypass line 11 is not provided. FIG. 4 is a schematic configuration diagram when there is no bypass line, and FIG. 5 is a schematic configuration diagram of the present invention. When the heat transfer part is surrounded by the inspection spaces 8 and 9, the amount of heat transfer between gas and water is generally the same in the conventional and the present invention. Further, in the case of the present invention, a hot gas bypass is required to reach the target gas temperature, but the amount of gas passing through the heat exchanger is reduced by the amount of the hot gas bypass. Then, alpha ₁ the gas-side heat transfer coefficient alpha ₁ of the heat exchanger tubes is proportional to approximately 0.8 square of the flow rate is reduced.

In order to make the heat exchange heat amounts the same, it is necessary to make the heat transmittance K shown in the above equation (3) substantially the same. That is, the (2) As is apparent from equation smaller the gas-side heat transfer coefficient alpha _1, it will increase the water side heat transfer coefficient alpha ₂ commensurate therewith. In order to increase the alpha _2, it is necessary to increase the amount of cooling water G _W as described above. The fact that the amount of cooling water can be increased while the amount of heat exchanged is the same means that the temperature difference of the cooling water can be reduced as is apparent from the equation (4), and that the temperature of the outlet of the cooling water is reduced.
Since the temperature of the cooling water in the tube 3a of the heat exchanger 3 can be reduced in this way, the temperature of the tube 3a itself can be reduced.

In the above embodiment, the bypass gas line 11 is provided, the gas cooled by the intermediate heat exchanger 3 and the hot gas are merged, and the cooling gas flows into the labyrinth seal portion of the downstream-side compressor and expands. Although the cooling gas is heated to a predetermined temperature so as not to be liquefied even though it is not liquefied, instead of providing a bypass gas line, another intermediate gas heating means is provided and the cooling gas is discharged from the intermediate heat exchanger 3. The incoming gas may be heated to the predetermined temperature.

〔The invention's effect〕

According to the present invention, since the tube wall temperature of the heat exchanger can be kept low and the liquefaction of the handled gas does not occur, the occurrence of stress corrosion cracking of the heat exchanger tube can be prevented and the life of the tube can be reduced. There is an effect that a method of cooling an intermediate gas in a multistage compressor that can be extended and a multistage compressor equipped with an intermediate gas cooling device are obtained.

[Brief description of the drawings]

Figure 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 CO ₂
Configuration diagram showing the overall system of the gas compressor, and FIG. 3 shows CO ₂
FIG. 4 is a diagram showing the relationship between temperature and entropy. FIG. 4 and FIG. 5 are configuration diagrams of a main part for explaining the operation of the present invention. FIG. The figure is a diagram showing a device provided with a bypass line. DESCRIPTION OF SYMBOLS 1 ... Low pressure side (front stage) compressor, 2 ... High pressure side (back stage) compressor, 3 ... Intermediate heat exchanger (intercooler), 4 ... First control valve, 5 ... Temperature controller (First control) Means), 5a: first temperature detector, 6: second control valve, 7: temperature controller (second control means), 7a: second temperature detector, 10: intermediate gas line, 11: bypass gas line.

Claims (2)

(57) [Claims] An intermediate gas compressed by a low-pressure side compressor is cooled by exchanging heat with cooling water in an intermediate heat exchanger, and thereafter, the cooled gas is supplied to a subsequent-stage compressor. In the method for cooling an intermediate gas in a multi-stage compressor for use, a part of the intermediate gas discharged from the pre-stage compressor bypasses the intermediate cooler, and the rest of the intermediate gas passes through an intermediate cooler outlet in the intermediate cooler. The amount of water flowing in based on the gas temperature at a position upstream of the mixed point of the cooled gas and the bypassed gas exchanges heat with the controlled cooling water to cool the gas to near the gas-liquid critical point. The mixed gas is cooled to a temperature at which a part of the gas obtained by mixing the cooled gas and the bypassed gas flows into the labyrinth seal portion of the subsequent compressor and does not liquefy even when expanded. Based on the temperature of Controlling the flow rate of the bypassed intermediate gas, and increasing the amount of the cooling water when the bypassed gas amount increases, in the multistage compressor for carbon dioxide gas. 2. A pre-stage compressor, a post-stage compressor, an intermediate gas line for guiding an intermediate gas discharged from the pre-stage compressor to a post-stage compressor, and the intermediate gas line provided in the intermediate gas line. A multi-stage compressor for carbon dioxide gas, comprising: an intercooler for cooling by exchanging heat with cooling water, wherein the first compressor is provided in a pipe for sending cooling water to the intercooler, and controls a flow rate of the cooling water. Control valve, first temperature detecting means provided at the outlet side of the intercooler for detecting the temperature of the gas discharged from the intercooler, and the intermediate gas is detected based on the temperature detected by the temperature detecting means. A first control means for controlling the first control valve so as to cool the gas to a temperature near the gas-liquid critical point; and a first temperature detection means for branching off from the intermediate gas line and bypassing the intermediate cooler. To join downstream A bypass gas line provided, a second control valve provided in the bypass gas line for controlling the amount of bypass gas, and a second control valve for detecting a temperature of the gas after the merge of the intermediate gas line and the bypass gas line. Temperature detecting means, based on the temperature detected by the second temperature detecting means such that the combined gas flows into the labyrinth seal portion of the subsequent-stage compressor and does not liquefy even when expanded. And second control means for controlling the second control valve, wherein the second control means opens the second control valve when the first control means opens the first control valve. A multi-stage compressor for carbon dioxide equipped with an intermediate gas cooling device, characterized by being controlled.