RU2342610C1 - Method of reducing heat consumption in rectification processes - Google Patents

Abstract

FIELD: heating systems.

SUBSTANCE: method of reducing the heat consumption required for rectification is realised by compressing the steam of the rectifying column upper part up to the pressure whereat steam condensation temperature is greater than liquid boiling point in evaporator, and by using heat of its condensation for heating the evaporator. Steam of the rectifying column upper part is compressed in a liquid ring-type compressor, and condensate formed during steam condensation of the rectifying column upper part in evaporator is used as a sealing liquid. The method allows utilising steam condensation heat of the rectifying column upper part for heating the evaporator.

EFFECT: reducing the power required for compression and eliminating the necessity of steam reheat stage before it is supplied to compressor.

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Description

FIELD OF THE INVENTION

The invention relates to methods for reducing heat consumption in rectification processes by using the heat of condensation of the vapor of the top of the column to evaporate bottoms liquid. The rectification process is the process of separation of mixtures by repeated partial evaporation and condensation. A feature of the process is the large heat consumption for the evaporation of bottoms liquid in a distillation column [1].

State of the art

The closest analogues of the invention are methods of reducing heat consumption due to recompression of the vapor of the top of the distillation column and the use of a heat pump [2, 3, 4].

The scheme for reducing heat consumption in a distillation column using a heat pump is shown in Fig.1. The liquid from the bottom of the column (K-1) is throttled through the throttle valve (Dr) to a pressure at which the boiling point of the bottom liquid falls below the condensation temperature of the vapor of the top of the column (K-1). When throttling bottoms liquid, a vapor-liquid mixture is formed, which enters the separation tank (E-1). The liquid from the tank (E-1) enters the heat exchanger (X-1), where it evaporates due to the heat of condensation of the vapor of the top of the column. The steam formed, together with the steam from the separation tank (E-1), overheats in the superheater (T-1) and is compressed in the compressor (Cr) to the bottom pressure of the column. The resulting compressed steam is returned to the cube distillation column (K-1) [2, 3, 4].

The closest prototype for the invention is a method of reducing heat consumption due to recompression of steam. The scheme of this method is shown in figure 2. Steam from the top of the column (K-1) enters the separation tank (E-1). Steam flows from the tank through the superheater (T-1) to the compressor (Cr). In most cases, piston and centrifugal compressors are used to compress steam [4].

In the compressor (Cp), the vapor is compressed to a pressure at which the vapor condensation temperature becomes higher than the boiling point of the bottoms liquid. Due to compression in the compressor, the steam becomes overheated. Such steam enters the evaporation tank (E-2), where it sparges through condensate. When bubbling superheated steam, an additional amount of saturated steam is formed, which, together with the bubbled steam, is sent to heat the cube of the column to the evaporator (P-1). The resulting condensate from the evaporator enters the tank (E-2). Liquid from the evaporation tank (E-2) is throttled in the throttle valve (Dr). As a result of throttling, a vapor-liquid mixture is formed, which enters the separation tank (E-1). The liquid from the tank (E-1) is taken in the form of reflux and distillate, and the steam together with the vapor from the top of the column enters the superheater (T-1) and is supplied to the compressor (Cr) for compression [2, 4, 5].

These methods allow for low energy consumption for compression to use the heat of condensation of the vapor of the top of the column for heating the evaporator [4].

The disadvantage of these methods is that only dry steam is allowed to be compressed in compressors. If droplets of liquid with steam get into the compressor, damage to different parts of the compressor may result in damage to it. To prevent undesired condensation, the steam is superheated before compression. This leads to the presence of a superheater (T-1) and additional energy costs. In addition, the presence of overheating negatively affects the thermodynamic efficiency of the cycle [4].

Another disadvantage of these methods is that the vapor is not cooled during compression. This leads to a significant overheating of steam at the outlet of the compressor and an increase in the required compression power [4, 6]. In addition, the high temperature of steam in many cases significantly complicates the operation of the compressor and increases the requirements for lubricating and sealing materials [4, 7].

Disclosure of invention

The objective of the invention is to reduce the required power to compress the steam and eliminate the stage of overheating of steam before being fed to the compressor.

The scheme of the proposed method is shown in figure 3. Steam from the top of the column is compressed in the compressor (Cp) to a pressure at which the vapor condensation temperature rises above the boiling point of the bottoms liquid. After that, the steam is sent to heat the evaporator (R-1) of the distillation column (K-1). The resulting condensate enters the collection tank (E-2). From the tank, the liquid is throttled to the pressure of the top of the column in the throttle valve (Dr). In this case, a vapor-liquid mixture is formed, which enters the separation tank (E-1). From the tank, the liquid is sent to irrigate the top of the column and is taken as a distillate, and the steam, together with the steam of the top of the column, is sent to the compressor for compression (Cr).

The proposed method differs from the prototype in that the vapor compression is carried out in a liquid-ring compressor. The liquid-ring compressor refers to volumetric single-rotor compressors, which has a blade type wheel. The rotating wheel is eccentric to the axis of the cylindrical body. The gap between the casing and the rotating blades of the rotor is filled with a sealing fluid discarded during rotation. Compression in this compressor occurs due to a change in volume between the blades and the liquid ring in the direction of rotation [7].

It is proposed to use condensate from a collection tank (E-2) as a sealing liquid. As a result, saturated vapor is in contact with the liquid during compression. This avoids overheating of the steam. The absence of overheating positively affects the thermodynamic efficiency of the cycle and leads to a decrease in the consumed power for vapor compression [4].

Liquid ring compressors allow fluid to exist in a compressible pair. This allows you to exclude from the circuit superheater (T-1), which leads to lower capital and operating costs.

When carrying out the invention, the following technical results can be obtained:

1. Decrease in required power for compression of steam.

2. Elimination of the stage of steam overheating before feeding it to the compressor.

Brief Description of the Drawings

The drawings show schemes for using the condensation heat of vapor from the top of the column. Figure 1 shows a diagram of the use of condensation heat using a heat pump. Figure 2 shows a diagram of the use of heat of condensation due to recompression of the vapor of the top of the column. Figure 3 shows a diagram of the use of heat of condensation due to recompression of the vapor of the top of the column in a liquid-ring compressor. Figure 4 shows the ideal reverse Rankine cycle and the ideal cycle of the liquid ring compressor using the example of the recompression of isobutanol vapor from 1.5 at. To 4.5 at.

Figure 1 - column (K-1), an evaporator for starting and controlling the column (P-2), a heat exchanger for condensing the vapor of the top of the column (X-1), superheater (T-1), compressor (Kr), throttle valve ( Dr), separation tank (E-1).

Figure 2 - column (K-1), an evaporator for starting and controlling the column (P-2), an evaporator heated by steam at the top of the column (P-1), a condenser for controlling the column (X-1), superheater (T-1 ), compressor (Cr), throttle valve (Dr), separation tank (E-1), evaporation tank (E-2).

Figure 3 - column (K-1), an evaporator for starting and controlling the column (P-2), an evaporator heated by steam at the top of the column (P-1), a condenser for controlling the column (X-1), a liquid-ring compressor ( Kr), throttle valve (Др), valve (В-1), separation tank (Е-1), collecting tank (Е-2).

Figure 4 - saturated dry steam of low pressure (1), superheated steam at the outlet of the compressor at the final pressure (2), conditional state of the steam when it is compressed in a liquid ring compressor (2 '), saturated dry steam of high pressure (3) , liquid at boiling point (4), vapor-liquid mixture after throttling to the initial pressure (5).

The implementation of the invention

The scheme of the proposed method is shown in figure 3. This method is as follows. The evaporator (P-2) and condenser (X-1) are used to bring the distillation column (K-1) to the desired mode. The condensate formed from the condenser (X-1) fills the separation tank (E-1) and the collection tank (E-2). After filling the collection tank (E-2) to the required level, close the valve (B-1) connecting the collection tank (E-2) with the separation tank (E-1). Condensate from the collection tank (E-2) is supplied to the liquid ring compressor (Cp) as a sealing liquid. The liquid ring compressor is a volumetric single-rotor compressor with a rotary vane wheel. The wheel is eccentric relative to the axis of the cylindrical body. As a result of the rotation of the blades, the sealing liquid is discarded from the center and completely fills the gap between the blades and the body [7].

After starting the compressor drive (Cp), the supply of cold and hot coolants to the condenser (X-1) and the evaporator (P-2) are reduced, respectively, and they are used only to control the column mode (K-1).

Steam from the top of the column through the separation tank (E-1) enters the suction window of the liquid-ring compressor (Cp). Due to the fact that the axis of the rotating compressor wheel (Kp) is eccentric relative to the axis of the housing, there is a change in the volume of space between the blades in the direction of rotation. Due to the changing volume, steam is compressed.

Due to the simple design, the absence of valves, moving cylinders and low rotor speeds, liquid-ring compressors allow the supply and compression of wet steam. In this case, there is no need for preliminary steam overheating, which allows to reduce capital and operating costs relative to the prototype [4, 7].

The increase in steam temperature during compression creates a temperature difference between it and the sealing liquid at the boiling point. This leads to heat exchange between the vapor and the liquid. As a result of this, the steam is cooled to a saturation state, and the heat released during cooling of the steam goes to the evaporation of part of the liquid. Since heat removal occurs due to condensation of the vapor in contact with the liquid, the efficiency of the heat transfer process is high [1]. Due to the high efficiency of heat removal, the resulting temperature difference between the contacted liquid and steam is negligible. In this case, during the compression in the liquid ring compressor, the steam remains saturated, and the heat transfer process is close to equilibrium.

Carrying out the compression process near the saturation line eliminates steam overheating, reduces the work required in comparison with compression in a conventional compressor and, as a result, reduces the required power for steam compression [4, 6].

In the compressor, the vapor is compressed to a pressure at which its condensation temperature becomes higher than the boiling point of the bottoms liquid, taking into account the required temperature difference in the evaporator (P-1). From the compressor (Cr), saturated steam enters the evaporator (P-1). During its condensation, heat is released, which goes to the evaporation of the still liquid in the column (K-1). The resulting condensate flows into a collection tank (E-2). Part of the condensate is used as sealing liquid in the compressor (Cr). The rest, passing through the throttle valve (Dr), is throttled to the pressure of the top of the distillation column. Since the condensate is on the saturation line, during throttling a vapor-liquid mixture is formed, which enters the separation tank (E-1). Part of the liquid from the tank (E-1) is fed to the irrigation of the top of the column (K-1). The rest is discharged as distillate. The steam generated during throttling, together with the vapor at the top of the column, is compressed into a liquid ring compressor (Cr).

Figure 4 shows the ideal cycles of the prototype and the proposed method on the example of the recompression of isobutanol vapor from a pressure of 1.5 atm to 4.5 atm. The prototype of the invention works on the reverse Rankine cycle. Saturated dry steam from state (1) is compressed isentropically in the compressor to state (2). In this state, the steam is superheated. When bubbling superheated steam in the evaporation tank, it isobarically cooled to state (3) and forms an additional amount of saturated steam. Then, saturated dry steam (3) completely condenses in the evaporator of the distillation column to a liquid state at the boiling point (4). The condensate that forms isentally throttled to the initial pressure (5) in the throttle valve.

The power consumption for vapor compression (W) in an ideal cycle is determined by the difference in the enthalpies of states (2) and (1). Actual power consumption is higher due to preliminary steam overheating and deviation of the real process from the ideal one [4, 5].

In the proposed method, the processes of compression and heat removal occur simultaneously. Due to the efficient heat exchange between the liquid and the vapor, heat removal in the compressor is close to equilibrium. Simultaneously, the ongoing processes of compression and equilibrium heat removal can conditionally be presented on the diagram in the form of two successive ideal processes: isentropic compression and isothermal heat transfer (Fig. 4). In this case, the vapor is isentropically compressed to the conditional state (2 '), at which its temperature is equal to the temperature of the saturated vapor at 4.5 ata (3). Heat is removed from superheated steam (2 ') through the isotherm to state (3). During the heat transfer process, part of the sealing fluid evaporates and an additional amount of saturated steam is formed. The compressor drive power consumption for vapor compression (W ') in this case will be determined by the difference between the enthalpies of the conditional state (2') and state (3). Due to the fact that heat removal flows through the isotherm, the power consumption in the proposed method (W ') is significantly lower than the cost of power for compressing the steam in the prototype (W).

Bibliography

1. Kasatkin A.G. Basic processes and apparatuses of chemical technology. - M.: Chemistry, 1971, 784 p.

2. Zakharov M.K. Energy-saving schemes of rectification processes. // Science & technology of hydrocarbons, 2002, No. 6, p. 3-8.

3. Zelvensky Y.D. Ways of energy conservation in the separation of mixtures by distillation. // Chemical industry, 2001, No. 5, p.21-27.

4. Ray D., McMichael D. Heat pumps: TRANS. from English - M .: Energoizdat, 1982.- 224 p., Ill.

5. Einstein V.G., Zakharov M.K., Nosov G.A. Optimization of a full heat pump in chemical technology processes. // Chemical industry, 2001, No. 1, p. 18-27.

6. Larikov N.N. Heat engineering: Textbook. for universities. - 3rd ed., Revised. and add. - M .: Stroyizdat, 1985 .-- 432 p., Ill.

7. Mikhailov A.K., Voroshilov V.P. Compressor machines. - M .: Energoatomizdat, 1989 .-- 288 p., Ill.

Claims (1)

A method of reducing the heat consumption of distillation by compressing the steam of the top of the distillation column to a pressure at which the vapor condensation temperature is higher than the boiling point of the liquid in the evaporator, and using the heat of condensation to heat the evaporator, characterized in that the vapor of the distillation column top is compressed in a liquid ring the compressor, and as the sealing liquid in the compressor, condensate is used, which is formed during condensation of the vapor of the top of the distillation column in the evaporator.

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