2. 1 heat exchanger selection
In the process of ethanol distillation, the commonly used heat exchanger at the top of the tower is a tubular heat exchanger, so the tubular heat exchanger is selected in this design.
The type of tubular heat exchanger is mainly determined by the temperature difference between the tube side and the shell side of the heat exchanger. In the process of ethanol distillation, ethanol is condensed at atmospheric saturation temperature, with the inlet temperature of 76℃ and the outlet temperature of 45℃. The cooling medium is water, the inlet temperature is 24℃ and the outlet temperature is 36℃, and the temperature difference between the two fluids is not very large. According to the description of various types of heat exchangers in the overview, fixed tube-plate heat exchangers can be selected comprehensively.
2.2 Selection of fluid flow rate
The choice of fluid velocity involves heat transfer coefficient, flow resistance and heat exchanger structure. Increasing the flow velocity can improve the convective heat transfer coefficient, reduce the formation of fouling and improve the total heat transfer coefficient; But at the same time, the flow resistance and power consumption are increased; Choose high flow rate to reduce the number of pipes. For a certain heat exchange area, we must use longer tubes or increase the number of passes. If the tube is too long, it is not conducive to cleaning, and the average heat transfer temperature difference will decrease when the single pass becomes multi-pass. Therefore, it is generally necessary to choose the appropriate flow through various trade-offs. Table 1-3 lists the commonly used velocity ranges for design reference. When choosing the flow rate, laminar flow should be avoided as much as possible.
Table 1 Common speed range in shell-and-tube heat exchanger
Fluid type: Generally speaking, fluid is easy to scale, liquid gas.
Velocity, m/s, 0.5 ~ 3.0>1.05.0 ~ 30.
Shell side 0.2 ~1.5 >: 0.53.0 ~15
Table 2 Common flow rates of liquids with different viscosities in shell-and-tube heat exchangers
Liquid viscosity, mPa·s & gt；; 1500 1500 ~ 500 500 ~ 100 100 ~ 35 35 ~ 1 & lt； 1
Maximum speed, m/s 0.60.751.1.51.82.4.
Table 3 Safe allowable velocity of flammable and explosive liquid in shell-and-tube heat exchanger
Liquid name: ether, carbon disulfide, benzyl alcohol, ethanol, gasoline acetone.
Safe allowable speed, m/s
Because the cooling medium used is well water, it is easy to scale, while ethanol is not easy to scale. The viscosity of water and ethanol is very small. According to the data in the above three tables, it can be preliminarily selected that the tube side speed is 0.9m/s and the shell side speed is 7m/s.. ..
2.3 Determination of fluid outlet temperature
The inlet temperature of cooling medium water is 24℃ and the outlet temperature is 36℃, so the qualitative temperature of water can be obtained as Tm=30℃.
The hot fluid ethanol condenses at saturation temperature, and it can be determined that the inlet temperature and outlet temperature are the same, so the qualitative temperature TM of ethanol is 60.5℃.
2.4 Determine the number of tube side and shell side
When the heat exchange area of the heat exchanger is large and the tubes cannot be long, more tubes must be arranged. In order to improve the flow velocity of the fluid in the tube, it is necessary to divide the tube bundle. However, too many passes will lead to an increase in flow resistance and power consumption, and at the same time, multiple passes will reduce the average temperature difference, which should be considered in design. There are four tube passes in the series standards of shell-and-tube heat exchangers: 1, 2, 4 and 6. When multiple passes are used, the number of tubes in each pass should usually be equal.
The number of passes n is calculated as follows:
N=u/v
Where u refers to the appropriate velocity of the fluid on the tube side;
V—— the actual velocity of the fluid at the pipe side. Chapter II Process Design Calculation
1 determination of physical performance data
The qualitative temperature of water is Tm=(24+36)/2=30℃, and that of ethanol is Tm=(76+45)/2=60.5℃.
Physical property data of two fluids at qualitative temperature
Material ties
liquid
Ethanol 60.5 757 0.6942 2.83 0. 1774
Water 30 996 0.0.8 4.20 0.6438+07
2 Determination of heat load and heat transfer area
1, calculate heat load
Condensation = 3.5 1 kg/s
Heat load q1= r = 3.51× 2.83× 31= 307.93kw.
2. Calculate the cooling water consumption
Heat load loss of heat exchanger: 3% of total heat transfer;
Q2 = q/(1-0.03) = 317.46kw.
The flow of water can be calculated by heat balance, that is
= = 317460/4.2 (36-24) = 9.35kg/s.
3. Calculate the effective average temperature difference:
Countercurrent temperature difference℃.
4. Select the value of empirical heat transfer coefficient k..
According to the circulating water in the tube side and ethanol in the shell side, the total heat transfer coefficient k is temporarily taken as follows:
5. Estimate the heat exchange area
3 Determination of approximate size of heat exchanger
Diameter and velocity in pipeline
Select φ25×2.5mm advanced cold-drawn heat transfer tube (carbon steel), and take the flow velocity in the tube u1= 0.8m/s.
Number of tube passes and number of heat transfer tubes
The number of unidirectional heat transfer tubes can be determined according to the inner diameter and flow rate of the heat transfer tubes.
According to the calculation of the two-way pipe, the required heat transfer pipe length is
According to the design of double-pass tube, the heat transfer tube is moderate, and the double-pass structure can be adopted. According to the actual situation of this design, take the length of heat transfer tube l=4m, and the number of tube passes of this heat exchanger is
Total number of heat transfer tubes N=38×2=76 (root)
3, the average heat transfer temperature difference correction and shell side number
The average temperature difference correction coefficient is:
R=2.6 P=0.23
Double shell pass, double tube pass structure, found ε=0.923.
Average heat transfer temperature difference
Because the average heat transfer temperature difference correction coefficient is greater than 0.8, and the fluid flow in shell side is large, double shell side should be adopted.
4. Inner diameter of shell
Number of conduits passing through the center of the number of tubes
When calculating the inner diameter of the shell, the following formula can be used:
D=t
B. Measure the outer diameter of the heat transfer tube, and then:
D = 32 (10-1)+50 = 338mm.
D=350mm is preferred according to the high-grade gear of the rolled shell.
The specifications of horizontal fixed tube-plate heat exchanger are as follows:
Nominal diameter 350 mm
Nominal heat exchange area
Number of passes 2
Tube number n ................................ 76
Pipeline length l ..............................................................................................................................................................................
Pipe diameter ..........................
The pipeline layout is regular triangle.
5. Baffle
Use the arch baffle, the annular gap height of the arch baffle is 20% of the inner diameter of the shell, and the annular gap height h = 0.20 * 250 = 75 mm
Take baffle spacing B=0.3D, then
B=0.3*250= 105mm, preferably B= 150mm.
Number of baffles N= length of heat transfer tube/baffle spacing-1= 8000/150-1= 26 (block)
4 area and total heat transfer coefficient accounting
1, shell side surface heat transfer coefficient
2. Heat transfer coefficient of the inner surface of the tube
There is a formula:
Cross-sectional area of tube-side fluid flow
Tube-side fluid velocity
prandtl number
Pr=5.446
Ai=2.2。
3. Fouling thermal resistance and pipe wall thermal resistance
Fouling thermal resistance outside the tube
So the fouling thermal resistance in the pipe.
According to the thermal resistance calculation of the pipe wall, the thermal conductivity of carbon steel under this condition is 50.29 W/(m k). therefore
4, heat transfer coefficient k
According to the formula of heat transfer coefficient
5, heat transfer area margin
The calculated heat transfer area Ap is:
The actual heat transfer area of the heat exchanger is
The area margin of the heat exchanger is
5. Pressure drop check
1, calculate the pressure drop on the tube side.
(Proportional correction coefficient, number of tube passes, number of shell passes)
If the wall roughness of carbon steel is 0. 1mm, and Rei=9700, then
Yes, the pipeline is
& lt praseodymium
Therefore, the pressure drop on the tube side is within the allowable range.
2, calculate the shell side pressure drop
Calculate by formula
, ,
Resistance of fluid flowing through tube bundle
F=0.5
The shell-side fluid velocity and Reynolds number are respectively:
take
Resistance of fluid flowing through the gap of baffle
，B=0.2m，D=0.5m
total drag
Chapter III List of Calculation Results
The main structural dimensions and calculation results of the heat exchanger are as follows:
Project results unit
Nominal diameter of heat exchanger d350
Number of tube passes of heat exchanger 2-
The total number of heat exchanger tubes is n76.
Single tube length of heat exchanger is L 4m.
Heat exchanger tube size mm
The arrangement of heat exchange tubes is a regular triangle staggered arrangement.
The distance between the tube centers is 32 mm.
The distance from the center of partition to the nearest pipe center is s22mm.
The distance between adjacent pipes in each process is 2S 44mm.
Baffle spacing b150mm
The number of baffles is N 26
The outer diameter of the baffle is 365 mm.
The thickness of the baffle is 5mm.
Shell thickness10mm
Shell side fluid inlet nozzle specification mm
Shell side fluid outlet nozzle specification mm
Specification of inlet and outlet nozzles of tube side fluid mm
Head thickness10mm
Head inner diameter 350 mm
Head surface height 100 mm
Head diameter height 20 mm.
Heat transfer load Q 3 17.46 kw
The flow rate of ethanol is 3.5 1 kg/sec.
The circulating water flow rate is 9.35 kg/s.
The primary total heat transfer coefficient is 450W/m2.
The heat transfer area is preliminarily estimated to be 23.9 meters.
The tube side speed is 0.8m/s.
The shell-side heat transfer coefficient is 925.4 w/m2.k.
Heat transfer coefficient of tube side i2200w/m2.k.
Total heat transfer coefficient k575.4w/m2.k.
The required heat transfer area is a20.3m.
Actual heat transfer area A 2 1.34 m
Heat transfer area margin H5. 1%-
Tube side pressure drop Pt 3200Pa
Shell lamination pressure drop Ps 5400Pa
Chapter IV Heat Exchange Tube Diagram (see attached figure)
Chapter IV Flowchart (see attached figure)
Chapter IV Design Review
Based on the analysis of heat transfer and resistance performance characteristics of shell-and-tube heat exchanger, the energy coefficient K/N is proposed to evaluate it.
When valence enhances heat transfer, more attention should be paid to improving its heat transfer performance. In this design:
,
K/N=0.0669
It meets the requirements and has good performance.
By checking the area, pressure drop and other calculations, the design can meet the requirements, the heat transfer efficiency can meet the requirements, and the task can be completed well.
Evaluation of economic and environmental benefits: life cycle method is a process of evaluating the impact of products or production processes on the environment. By identifying and quantifying the consumption of energy and materials and the resulting waste discharge, it evaluates the impact of energy and material utilization on the environment in order to seek ways to improve products or processes. This evaluation runs through the whole life cycle of product production and technical activities, including the exploitation and processing of raw materials, product manufacturing, transportation, sales, product use and reuse, maintenance, recovery and final disposal. This design uses water as coolant, which is pollution-free, low-cost and free of harmful gases. The whole process is simple, easy to operate and has good environmental and economic benefits.
In this design, the area, heat transfer coefficient, pressure drop, etc. Are well guaranteed. Even if there is a big error in production and use, the equipment structure can ensure that there is no safety damage accident and has good and reliable safety guarantee.
Chapter V Personal Summary
This course design is a bridge between theory and practice, and it is our initial attempt to learn the basis of chemical engineering design. Through the course design, comprehensive use of the basic knowledge of this course and prerequisite courses, comprehensive and independent thinking, to complete the designated chemical design tasks within the specified time, so as to get the preliminary training of chemical programming. Through the course design, we have deepened our understanding of the basic contents of engineering design, mastered the procedures and methods of chemical engineering design, and cultivated our ability to analyze and solve practical engineering problems.
In addition, through this course design, we have improved our ability in the following aspects:
1 Be familiar with consulting literature and searching related materials. Choose the right formula.
2. Calculate the process design of the main equipment for process calculation accurately and quickly.
Use refined language, concise words and clear charts to express the calculation results of your design ideas.
I also found many shortcomings of myself, and I was unfamiliar with what I learned, wasting a lot of time.
Chapter VI References
1. Qian, editor-in-chief, heat exchanger design manual, chemical industry press, 2002.
2. Jia, Chai, etc. , Course Design of Chemical Engineering Principles, Tianjin University Press, 1994.
3. Kuang and Shi Qicai. Course Design of Chemical Unit Process and Equipment, Chemical Industry Press, 2002.
4. Wang Zhikui, Editor-in-Chief, Principles of Chemical Engineering, Chemical Industry Press, 2004.
5. Principles of Chemical Engineering (Volumes I and II) by Chen Minheng and Condez (Second Edition). Beijing Chemical Industry Press 2000.
6. How, Principles of Chemical Engineering, Science Press, 200 1.
Other related information and so on!