CS253061B1 - A method of heat exchange between two separate, countercurrent fluid streams - Google Patents

Abstract

The method of heat exchange between separate countercurrently arranged liquid streams consists in reducing the pressure in the cooled stream in one or more stages, and the resulting steam transfers its condensation heat back to the heated stream in one or more stages.

Description

The invention relates to a method of heat exchange between two separate countercurrently arranged liquid streams.

Heat exchange is carried out in a well-known indirect way in heat exchangers (recuperators). The exchange takes place by heat transfer through a dividing wall. From a thermodynamic point of view, the most advantageous method is exchange in counter-current devices. To achieve advantageous permeability coefficients, this exchange is carried out in single- and multi-pass devices. This simple arrangement is practically impossible if solid deposits form in one or both streams.

The solution according to the invention concerns the case where one of the streams has this unpleasant property. As will be shown below, the method according to our proposal is advantageous even in cases where deposits do not occur, because in the proposed arrangement it is possible to achieve up to double the values of the heat transfer coefficients.

The essence of the method of heat exchange between two separate countercurrently arranged liquid streams according to the invention is that the pressure of the cooled stream is reduced in one or more stages, the resulting steam transfers its condensation heat again in several stages of the heated stream. The pressure reduction can be achieved by a throttle valve, a vacuum pump or a steam ejector.

The method according to the invention achieves significant energy savings and consequently also improves the environment.

The essence of the method according to the invention is clear from Fig. 1 to Fig. 4.

Fig. 1 shows the case where heat exchange takes place at a pressure higher than atmospheric pressure, where any gas (inert) present is discharged through a throttle valve 17 into the atmosphere. Fig. 2 shows the case where heat exchange takes place at a pressure lower than atmospheric pressure and any gas (inert) present is discharged by vacuum pumps 18. In our case, steam ejectors are considered as vacuum pumps 18. The steam required for the operation of vacuum pump 18 is further consumed for heating a colder stream in a separate condenser 16.

Heat exchange takes place in several stages (four are shown in the figures). The cooled stream 3 ^is fed to the lower part of the exchangers 21, 22, 23, 24, where it evaporates at pressures corresponding to the temperatures Tg, Tg, Tg, T^. The resulting steam condenses on the tubes of the exchangers 21, 22, 23, 24 located in the upper part of the individual parts. The condensing steam heats the stream of heated liquid Pg with its condensation heat.

Example 1

Utilization of waste heat from withdrawn liquors in the production of calcium chloride.

The withdrawn liquors (referred to as DS liquors), which have a temperature of 90 to 95 °C, contain dissolved calcium chloride and sodium and insoluble substances, calcium hydroxide, calcium carbonate, calcium sulfate and others. After the insoluble components settle, an equilibrium amount of sulfate and calcium hydroxide is present in the solution, which are the source of the following problems. Cooling the solution shifts the equilibrium and precipitates solid sulfate and calcium hydroxide, which settle on the tubes of conventional heat exchangers and prevent heat transfer. A description of the proposed method for producing calcium chloride by utilizing the waste heat of DS liquors according to the invention is shown in Fig. 3.

DS liquor from soda plant J flows into decanter A, where the solid phase settles, which is continuously pumped to sludge ponds 2. The clarified DS liquor 2 ^is divided into two streams. Part 4 flows into tank B and part 5 flows into heat exchanger C, where it transfers its heat to the thickened solution. After cooling in the heat exchanger, stream 6 is divided into part 2< going to the Elbe and part 2, used in an amount of 11 t/h in the siloxide production plant. The actual thickening of the clarified liquor takes place in tower D, where the air flow evaporates the water from the solution. By admitting (continuously) liquor £ and withdrawing the thickened solution 12, the concentration of the circulating solution is kept constant. From the reservoir B, the solution 9_ is led to the heat exchanger C. The heated stream 10 is divided into a recycle 11 to the cooling tower D and a part 12, taken for further processing in tubular evaporators to the final product of this device, in the cooling tower D, part of the water evaporates and the evaporated water mixed with air is discharged into the atmosphere 13.

The proposed method eliminates the above-mentioned problems. In the schematic diagram (Fig. 4) of the i-th part, the lower part shows the flow of the warmer medium (withdrawn liquor) with a mass flow rate M ₂ and the corresponding temperatures T ₂ £ ₊₁ and T ₂ £, and in the upper part the flow of the heated medium with a mass flow rate M| and temperatures and. Heat exchange is carried out using water vapor, which is released from the warmer medium by expansion and its flow rate is denoted by M and temperature T ₂ ^. This eliminates direct contact of the solution with the cooler walls of the tubes and the transfer of heat from the warmer liquid to the cooler one is ensured by water vapor. Since the warmer medium is fed into the heat exchanger section at a certain temperature and the temperature inside the exchanger is lower (corresponding to the water vapor pressure at this outlet temperature), expansion and release of water vapor occur in the openings of the supply pipe of the warmer medium. The necessary pressure inside the exchanger part (more precisely, negative pressure, because the temperatures are below the boiling point of water) is provided by a vacuum pump, e.g. an ejector.

In the individual parts of the exchanger, pressures corresponding to the saturated water vapor pressure at the temperatures of the outgoing warmer medium, i.e. T ₂ ^ (see the table of temperature distribution in the individual parts), are maintained.

The total heat balance equation:

^M 1 ^C 1< ^T 1Í - T ₁ ._ ₁ ) = m ₂ c

2 ^(T 2i-l ^T 2i ⁾

If we denote the ratio of the heat capacities of both streams B, then:

^T li = ^T li-1 ^{+ B(T} 2i ₊ l - ^T 2i>

For the outlet temperature from the pipe system (system with distributed parameters), the following can be derived:

^T li = *2i - ^(T 2i - ^T li-l' ^eX P ^{T 2i} decrease in the temperature of water vapor above the solution.

Assuming that the individual parts of the heat exchanger have the same heat transfer area and the same heat transfer coefficient, the exponential term in the equation is constant. If we denote this term A, then:

A = exp / K.F. \ (--Τη· = τ, . , + (T-. - T,. - T, . , \ IX n-1 2x 2i lr-1 ) (1 - A)

The temperature distribution in both streams in the individual parts of the exchanger can therefore be expressed as:

where B = ratio of heat capacities of the currents.

For any number of heat exchanger sections, the following relationship can be derived (we assume that T2Í is the same for all sections):

^(T 21 ^T 2

(1 - A) (1 - B) where T _ln ^and T _lo are the outlet and inlet temperatures of the heated medium ^T 21 ^inlet P ⁿ 4 temperature of the heat-supplying stream n is the number of sections of the split exchanger.

If we denote the heat exchange, we can calculate the size of the heat exchange area of the ith part from:

F. = ^{1 1} ln 1 + —-— B

K 1 - B

F total = n.F.

Entered calculation parameters:

1. Warmer medium T ₂₃ = 40 °C (desired outlet temperature)

T _2n+3 = 85 °C (input value)

2. Heated medium = 70 °C (desired output value)

T^q = 30.3 °C (input value)

3. Ratio of heat capacities of streams B = 0.88227586

-2 -1 -1

4. The heat transfer coefficient K was chosen as K = 8,373.6 kJ.m.h.deg.

5. The temperature of the released steam T ₂ ^ is different from the temperature of the outgoing warmer stream T ₂ ^ because the dissolved substance causes the temperature of the steam and the boiling solution to differ. The appropriate value of 1 °C was read from the tables, so T ₂ = 1 °C.

Calculation part: C = 1.608881

Number of parts n F.(m ² ) ^F total ^{= n} 4 17,555 70.22 5 8,279 41.40 6 5,787 34.72 7 4,498 31.48

Significant reduction in overall transfer rate A five-piece exchanger was therefore chosen.

area occurs when changing from a four- to a five-section exchanger. Calculation of the temperature distribution of both streams on a five-section exchanger:

Heated stream Warmer current _T1Q = 30.30 °C ^T21 = 40°C Τ _1χ = 36.80 °C _T22 = 47.37 °C _Τχ2 = 43.96 °C _T23 = 55.48 °C _T13 = 51.82 °C _T24 = 64.40 °C _Τχ4 = 60.48 °C _T25 = 74.21 °C _T15 = 70°C ^{T26 = 85} ° ^C

For n = 5, F. = 8.279 m.

Table of balance values of individual streams:

Current No. Mass flow rate (th“ ¹ ) Specific heat of the stream (kJ.kg l.deg Stream temperature (°C) 1 168.30 3,672 93 2 34.41 3,672 84 3 - 133.89 3,672 85 4 12.96 3,672 85 5 120.94 3,672 85 6 120.94 3,672 40 7 109.94 3,672 40 8 11.00 3,672 40 9 151.79 3,316 30 10 151.79 3,316 70 11 144.17 3,316 70 12 7.62 3,316 70 13 5.33 - - 14 138.84 3,316 24.5

The balances carried out show that in loss-free operation, 7.62 t/h of 17% calcium chloride solution can be produced by using the waste heat of the DS liquor, i.e. approximately 1.3 t/h of 100% chloride g

The described method uses about 20.10 kJ/h of heat from waste DS liquor.

By using the waste heat of DS liquor to thicken from 9% to 17% calcium chloride, savings of 6,000 kg of steam per hour or 800 kg of oil per hour can be achieved.

Claims (1)

Method of heat exchange between two separate, countercurrently arranged fluid streams, characterized in that in the cooled stream in one or more stages the pressure is reduced, the resulting steam transfers its condensation heat again in one or more stages of the heated stream.