Production of Nitric Acid for Fertilizer
Applications
Prepared
by:
Mamdouh Al-anazi
Mohammed Al-Ajlan
Sultan Al-Sebeai
Hussni Mandeeli
Submitted in partial Fulfillment of the Requirement for the Degree of Bachelor ofScience in chemical Engineering in the college of Engineering
Riyadh
1434 – 1435 H 2013 – 2014 G
Supervised by:
Dr. Farag Abd El-SalamDr. Malik Al-Ahmad Prof. Tariq Al-Fariss
2
Table of Contents
Contents
Page
Acknowledgement
4
Summery
5
Problem Statement
6
Objectives
6
Scope of Work
6
Chapter 1: Introduction
7
1.1
Backgrounds and History
8
1.2
1.2.1
1.2.2
1.2.3
1.2.4
The Nitric Acid Production Processes
The single Pressure Process
The Dual Pressure Process
The Process Selection
The Process Selection Conclusion
9
9
11
13
13
1.3
Main Uses of Nitric Acid
13
1.4
Physical Properties
14
1.5
Chemical Properties
15
1.6
Safety Properties
15
1.7
1.7.1
Market Survey
Economic Outlook
16
16
1.8
1.8.1
1.8.2
1.8.3
Preliminary Hazard Analysis
Summary of Previous Plant Accident
Inherent Safety Aspects
Local Safety and Environmental Regulations
18
18
20
22
1.9
1.9.1
1.9.2
Site Feasibility Study
Selection Criteria
Potential Site location
22
22
24
Chapter 2: Material Balance
25
2.1
2.1.1
2.1.2
Process Description
Process Flow Sheet
Justification of Equipment Selection
27
28
29
2.2
Overall Material Balance
30
2.3
Reactor Material Balance
31
2.4
Oxidation Material Balance
32
2.5
Absorber Material Balance
33
Chapter 3: Energy Balance
34
3.1
Vaporizer Energy Balance
35
3.2
Compressor Energy Balance
36
3.3
Superheater Energy Balance
37
3.5
Reactor Energy Balance
39
3.6
1
stCooler Energy Balance
40
3.7
Oxidation Energy Balance
41
3.8
2
ndCooler Energy Balance
42
3.9
Absorber Energy Balance
43
Chapter 4: Design and Sizing Equipment
44
4.1
Sizing of Pump
45
4.2
Design of Vaporizer
46
4.3
Sizing of Compressor
56
4.4
Design of Superheater
57
4.5
Sizing of Mixer
67
4.6
Design of Reactor
68
4.7
Design of 1
stCooler
71
4.8
Design of Oxidation
82
4.9
Design of 2
ndCooler
83
4.10
Design of Absorber
93
4.11
Design of Nitric Acid Tank
101
Chapter 5: Control Loop
105
5.1
Introduction
106
5.2
Control of Vaporizer
109
5.3
Control of Reactor
110
5.4
Control of Absorber
111
Chapter 6: Economic Analysis
112
6.1
Estimation of Capital Investment Items Based on
Delivered Equipment
113
6.2
Equipment Cost (at 2011)
114
6.3
Total Production Cost (TPC)
115
Chapter 7: Process Integration
117
Chapter 8: Safety and Loss Prevention
120
8.1
Plant Layout
122
8.2
Safety of Materials
123
8.3
Hazard and Operability Studies
126
Chapter 9: Waste Treatment
133
Conclusion
140
Reference
142
Appendices A: Material Safety Data
143
Appendices B: Physical Properties Data
152
Appendices C: Detailed Material Balance Calculation
162
Appendices D: Detailed Energy Balance Calculation
167
Appendices E: Equipment Design References
181
4
Acknowledgement
First, thanks are to Allah who helped us throughout this work. Then, it is our pleasure to thank those people who helped us in the completion of this project. Also, we would like to express our deep gratitude and apprec ation to our dear supervisors: Dr.Farag Abd El-Salam, Prof. Tariq Al-Fariss and Dr. Malik Al-Ahmad for their support, advise, valuable scientific knowledge and patience throughout this project. We would like to express our sincere thanks to the department of chemical engineering at the College of Engineering, King Saud University, for giving us this opportunity to carry out this project and we are grateful to our colleagues at King Saud University for their inspiration, help and encouragement without which we would not have been able to complete this project too. Last but not least we thank our dear families and relatives for their assistance and encouragement.
Summary
The results of the design project for the commercial production of nitric acid are presented. The project has been performed in two stages. The first part covers a literature review on various processes for nitric acid production, selection of the suit table processes, materials and energy balance calculations as well as the feasibility of the project. The second part presents the detailed equipment designs. From the investigation into project feasibility, it is proposed to construct a plant that will deliver 100000 tons per year of 60%(wt.) Nitric acid. This capacity is based on 8000 hours of operation per year, i.e. 330 days. It is envisaged that this nitric acid production facility will be centered within a larger chemical complex to be located in the eastern region of Saudi Arabia. Other plants on this site will include an ammonia plant and an ammonium nitrate plant. Approximately 70% of the product acid will be consumed in situ for the production of ammonium nitrate Fertilizer. The remaining acid will be available to exploit the neighboring export market. The process chosen for the nitric acid plant is the "single-pressure process" based on the technology developed by C & I Girdler. The most important use is undoubtedly in the production of ammonium nitrate for the fertilizer and explosives industries, which accounts for approximately 65% of the world production of nitric acid. So we made all our production to be used in the fertilizer.
6
Problem Statement
It is required to design a suitable process for the production of 100,000-ton/year of nitric acid from ammonia (60% concentration).
Objectives
The objectives of this design project include the following:
To integrate chemical engineering knowledge in a detailed design of chemical plants.
To design a nitric acid plant which is economically attractive, safe to workers and society and reduce harm to the environment?
To develop oneself in the applications of all the elements of knowledge and skills that have been accumulated throughout the undergraduate program for solving design related problems for typical process industrial plant.
To develop the skills for working as a team and to nurture leadership qualities.
Scope of work
This project is subjected to the designing phase of the process plant. All researches and literatures used for this project falls under the scope of the chemical compositions, current productions of nitric acid, preliminary hazards analysis, process design configurations and selections, and configuration of plant equipment (i.e. reactor, separation system, heat integration, etc.). The designing phase is then executed using manual engineering calculations .The project is aimed at achieving the objective of the plant design process.
Chapter 1
8 The initial design problem is to determine whether: ‘it is both economically and technically feasible to establish a facility to produce nitric acid in the world’. This is a diverse and complex undertaking that necessitates a full investigation into the uses, properties, market, process technology, and production economics, associated with this particular chemical. Having considered these aspects and several others, an appropriate plant to fulfill the assessed market requirements is sized and specified accordingly. [1], [10]
1.1 Backgrounds and History
Nitric acid is a colorless, highly corrosive liquid and a very powerful oxidizing agent that in the highly pure state is not entirely stable and must be prepared from its azeotrope by distillation with concentrated sulfuric acid. Nitric acid gradually yellows because of decomposition to nitrogen dioxide. Solutions containing more than 80% nitric acid are called fuming nitric acids. [1], [4]
Reagent-grade nitric acid is a water solution containing about 68% by weight nitric acid. This strength corresponds to the constant-boiling mixture of the acid with water, which is 68.4% by weight nitric acid and boils at 121.9°C. Nitric acid is completely miscible with water and forms a monohydrate (HNO3.H2O, melting point: - 38°C) and a dehydrate (HNO3.2H2O, melting point: -18.5°C). [7]
Scholars have known nitric acid for many centuries. Probably the earliest description of its synthesis occurs in the writings of the Arabic alchemist Abu Musa Jabir Ibn Hayyan (c. 721–c. 815), better known by his Latinized name of Geber. The compound was widely used by the alchemists, although they knew nothing of its chemical composition. It was not until the middle of the seventeenth century that an improved method for making nitric acid was invented by German chemist Johann Rudolf Glauber (1604–1670). Glauber produced the acid by adding concentrated sulfuric acid (H2SO4) to saltpeter (potassium nitrate; KNO3). A similar method is still used for the laboratory preparation of nitric acid, although it has little or no commercial or industrial value. [7]
The chemical nature and composition of nitric acid were first determined in 1784 by the English chemist and physicist Henry Cavendish (1731–1810). Cavendish
applied an electric spark to moist air and found that a new compound - nitric acid - was formed. Cavendish was later able to determine the acid’s chemical and physical properties and its chemical composition. The method of preparation most commonly used for nitric acid today was one invented in 1901 by the Russian born German chemist Friedrich Wilhelm Ostwald (1853–1932). The Ostwald process involves the oxidation of ammonia over a catalyst of platinum or a platinum-rhodium mixture. [2]
Today, nitric acid is one of the most important chemical compounds used in industry. It ranks number thirteen among all chemicals produced in the United States each year. In 2005, about 6.7 million metric tons (7.4 million short tons) of the compound were produced in the United States. [1]
1.2 The Nitric Acid Production Processes:
All commercially produced nitric acid is now prepared by the oxidation of ammonia. The requirement for a nitric acid product of 60%(wt.) Immediately restricts the choice of a recommended production process. Only two processes are possible, both highly efficient, each offering distinct advantages under different market conditions. [1]
These two main Processes are:
1.2.1 The Single-Pressure Process
The Single-Pressure process was developed to take full advantage of operating pressure in enabling equipment sizes to be reduced throughout the process. A single compression step is used to raise the pressure through the entire process sufficiently to favor absorption. Operating pressures range from 800 kPa as used by the Sumitomo Chemical Company Ltd. to 1100 kPa as used in the C&l Girdler single-pressure process. Increased ammonia oxidation and complete ammonia/air mixing, and uniform flow distribution can minimize increased consumption of ammonia due to the higher-pressure operation across the gauze "inside the reactor". The higher oxidation
10 temperature results in an increased consumption of platinum and rhodium and the need to rework the gauze every five to seven weeks .The higher temperature and the favorable pressure effect make possible a greater recovery of energy from the process. [10]
The process begins with the vaporization of ammonia at 1240 kPa and 35°C using process heat "as shown in the given flow sheet, Fig. (1). Steam is then used to superheat the ammonia to 180°C, filtered air is compressed by an axial compressor to an interstage level and then, following cooling, by a centrifugal compressor to a discharge pressure of 1090 kPa. A portion of the air is diverted for acid bleaching; the remainder is circulated through a jacket surrounding the tail-gas preheater and then used for ammonia oxidation. [10]
In this process; the heated air and the ammonia vapor (10.3% ammonia by volume) are then mixed and passed through the platinum/ rhodium gauze reactor where the heat of reaction (producing nitric oxide) raises the temperature to be between 927°C and 937°C. The reaction gas flows through a series of heat exchangers in which energy is recovered either as high-pressure superheated steam or as shaft horsepower from the expansion of hot tail gas. [6]
Approximately 70% of the oxidation to nitrogen dioxide occurs as the gas passes through the energy recovery train and is cooled to 185°C. After further cooling to 63°C in the primary cooler/condenser, separation of approximately one third acid product as 42% strength nitric acid is achieved. The remaining gas reaches a 43% oxidation conversion to nitrogen dioxide, with approximately 20% dimerization. The gas is combined with bleached air containing additional nitrogen peroxide; it then passes through an empty oxidation vessel and the secondary cooler. [6]
In cooling to 66”C, the gas provides heat to a recirculating hot water system used for vaporizing the ammonia. The gas entering the absorber is 95% oxidized to nitrogen peroxide. In the absorber deionized water is added to the top tray, and weak acid from the low-pressure condenser is added to a tray corresponding to its strength. [6], [5]
Down-flowing acid and up-flowing acid alternately mix as the chemical steps of action formation and nitric oxide oxidation take place with the liberation of heat. There are three operational zones in the absorber, these are the lower zone cooled with plant cooling water, the middle zone cooled with chilled water, and the upper zone which is essentially adiabatic. High efficiency of heat removal in the middle and
lower zones is particularly important due to its effect on the oxidation and dimerization reactions. [6]
For this process, chilled water at 7°C is used and the tail-gas exit temperature is approximately 10°C. Acid from the bottom of the absorber is bleached at 1010 kPa with partially cooled compressed air. The bleach air, containing nitrogen peroxide stripped from the acid, is then added to the main gas stream before entering the oxidation vessel. The cold gas is warmed by heat exchange with the hot compressed bleached air, and then heated to the expander inlet temperature of 620°C by two exchangers in the recovery train. The expander recovers 80% of the required compressor power. Expanded tail gas at 300°C flows through an economizer, providing heat to high-pressure boiler feed water and to low-pressure de ionized deaerator make-up water. Subsequently tail gas is exhausted to the atmosphere at 66°C and less than 1000 ppm of nitrogen oxides. [9]
The chilled water (7°C) for absorption refrigeration unit, using heat, supplies the absorber recovered from the compressor and intercooler as the energy source. Heat for ammonia vaporization, as previously noted, is available at 35°C and is recovered from the secondary gas cooler. The system uses circulating condensate as the energy transfer medium. [7]
1.2.2 The Dual-Pressure Process
The dual-pressure process was developed to take advantage of two factors: a) Low-pressure ammonia oxidation;
b) High-pressure absorption for acid production.
In addition to the higher conversion, the lower catalyst gauze temperature (associated with the low-pressure ammonia oxidation) results in a much lower rate of platinum deterioration. Both advantages are maximized at the lowest pressure. In contrast, however, absorption is best performed at the highest pressure. [7]
The low-pressure ammonia oxidation is usually performed in the pressure range of 101.3 kPa to 344 kPa. High-pressure absorption is usually performed in the operating range of 800 kPa up to 1010 kPa. This process begins with the vaporization of ammonia at 550 kPa and 7”C "as shown in its flow sheet of Fig. (2)" followed by superheating to 76°C using heat from the compressed bleached air. Filtered air is
12 compressed in an axial compressor to 350 kPa and is mixed with the superheated ammonia vapor (1 O-l 1% ammonia by volume) prior to entering the converter/reactor. In the converter, the gases react over the platinum/rhodium gauze catalyst. [7]
The gases leaving the reactor at 330 kPa and 865°C flow through a series of heat exchangers for recovery of energy, either as high-pressure superheated steam or shaft horsepower from expansion of hot tail gas. Approximately 40% of the oxidation to nitrogen dioxide occurs in the gas as it passes through the energy recovery train and is cooled to 135°C (exit from the tail-gas preheater). After further cooling to 45°C in the medium-pressure condenser, and separation of 20% of the acid product as 30% strength nitric acid, the gas reaches 50% oxidation to nitrogen peroxide with approximately 20% dimerization. [7]
The gas is combined with bleach air containing additional nitrogen peroxide and is compressed in a centrifugal nitrous-gas compressor to 1025 kPa. The exit temperature of 224°C is achieved due to the combined heat effects of the compression, the further oxidation to 80% nitrogen peroxide, and the virtual disappearance of the dimer. The compressed gas flows through an empty oxidation chamber, a high-pressure boiler feed water economizer, and a low-pressure deionized water economizer, and thus is cooled to 95°C. Residence time in the system and the effect of increased pressure result in at least 95% oxidation to nitrogen peroxide, but the dimerization is low due to the temperature level. [10]
The gas is then cooled to the dew point (50°C) for entry into the absorber. The dimerization increases to 48%, adding significantly to the heat removed prior to the absorber. The system uses circulating condensate as the energy transfer medium. The absorber is essentially the same as that previously described for the single-pressure process. [4]
Chilled water at 15°C is used in the absorber and the outlet gas temperature is 18°C. Refrigeration for the chilled water is provided by the ammonia vaporizer which operates at 7°C. [6]
Weak acid from the bottom of the absorber is let down to 330 kPa for bleaching with air from the axial compressor. This air, with nitrogen peroxide stripped from the acid, flows to the suction of the nitrous-gas compressor together with the main nitrous gas stream from the condenser. [5]
1.2.3 Process Selection
(Factors Favoring the Single-Pressure Process)
The single-pressure process uses a higher ammonia conversion. This higher pressure provides advantages in terms of equipment design, e.g. smaller converter dimensions and a single heat-exchanger-train layout. The higher temperature and the favorable pressure both increase the energy recovery from the process. The single-pressure process provides an extra 10% high-level recoverable heat energy. Plant capital costs in the USA have been estimated at 8 million (USS5.1 million) for the single-pressure process and 9.1 million for the dual-pressure process.
The 1.1 million higher cost of the dual-pressure process is accounted for by the larger vessels required at lower operating pressures. Estimates made for the two plants in European locations show a differential of 0.8 million, also in favour of the single-pressure process A discounted cash flow (DCF) analysis based on these US figures was performed by matching the capital cost advantage of the single pressure process against the production cost advantage of the dual pressure process. They indicate that it would take 21 years for the lower operating cost of the dual-pressure process. In this project, pressure process to finally overcome its initial capital cost disadvantage due to its smaller capital cost compared to the dual pressure process. [1] , [9] , [10]
1.2.4 Process Selection Conclusions
The single-pressure process appears to be preferred for our project. The capital cost advantage of this process surpasses the benefits of the superior operating cost structure of the dual-pressure process.
1.3. Main Uses of Nitric Acid
Nitric acid is predominantly used for fertilizer manufacture. It also finds use in the manufacture of adipic acid, nitroglycerin, nitrocellulose, ammonium picrate, trinitrotoluene, nitrobenzene, silver nitrate, and various isocyanates.
Nitric acid has enormously diverse applications in the chemical industry. It has commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent, and
14 catalyst and hydrolyzing agent. The most important use is undoubtedly in the production of ammonium nitrate for the fertilizer, which accounts for approximately 65% of the world production of nitric acid. [5]
Nitric acid has a number of other industrial applications. It is used for pickling stainless steels, steel refining, and in the manufacture of dyes, plastics and synthetic fibres. Most of the methods used for the recovery of uranium. Such as ion exchange and solvent extraction, use nitric acid [7].
An important point is that for most uses concerned with chemical production, the acid must be concentrated above its azeotropic point to greater than 95%(wt). Conversely, the commercial manufacture of ammonium nitrate uses nitric acid below its azeotropic point in the range 50-65%(wt). If the stronger chemical grade is to be produced, additional process equipment appropriate to super-azeotropic distillation is required. [2] , [7] , [10]
1.4. Physical Properties
In its commonest form nitric acid is a pungent, colorless liquid and pure (anhydrous) that boils at 86°C and solidifies at -42°C. Those are the most common nitric acid properties. It is used in varying dilutions across many industries and chemical processes from munitions thru to agriculture, cleaning and woodworking. As a pure acid HNO3 often emits white vapor when exposed to air and as a dissolved solution can give off a vapor that is reddy-brown leading to its common name ‘red fuming acid’. When stored in a diluted form for some length of time the acid can take on a yellow tinge. Nitric acid is completely soluble in water.
This mineral based acid is highly corrosive, even in dilute forms, and if splashed on skin will cause yellow blisters to be formed this should be expunged immediately with copious amounts of water. It is highly toxic.
Pure anhydrous nitric acid (i.e. undiluted) is not, however, stable and even at ambient temperatures can decompose, as temperatures increase so too does the rate of the acid’s decomposition and when heated vigorously it will divide into its component form of water, oxygen and nitrogen dioxide. Care is required, therefore in its storage and handling. [3]
1.5. Chemical Properties
Nitric acid can be seen to have a number of properties that can be describes as acidic, oxidizing, reactive and as passivation.
Its acidic nature means that varying degrees of corrosion can be anticipated dependent on the levels of dilution – leading to its use as a cleansing, etching and ‘ageing’ chemical in many applications. If being used as a reagent or cleanser then care needs to be taken to use vessels made from corrosion resistant alloys or metals to enable processes to take place.
As a strong and powerful oxidizing agent it reacts, sometimes vigorously and violently, with numerous non-metallic substances and compounds and the resultant reaction can be an explosive one. This holds true to most metals with the exception of those classed as ‘precious’ and it is for that reason that nitric acid is used in the cleansing and assessing of precious metal purity. Depending on the level of dilution used during oxidization nitrous oxide may be formed.
When used in conjunction with many metals the end result is that nitric acid will dissolve most of them and in the process creates nitrogen oxides. If combined with hydrochloric acid then Nitric acid can be used to dissolve what are known as ‘noble’ metals such as gold, platinum, iridium and others. [3]
1.6. Safety Properties
Nitric acid is a strong acid and a powerful oxidizing agent. The major hazard posed by it is chemical burns as it carries out acid hydrolysis with proteins (amide) and fats (ester), which consequently decomposes living tissue (e.g. skin and flesh). Concentrated nitric acid stains human skin yellow due to its reaction with the keratin. These yellow stains turn orange when neutralized. Systemic effects are unlikely, however, and the substance is not considered a carcinogen or mutagen.
The standard first aid treatment for acid spills on the skin is, as for other corrosive agents, irrigation with large quantities of water. Washing is continued for at least ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent secondary damage. Contaminated clothing is removed immediately and the underlying skin washed thoroughly.
16 Being a strong oxidizing agent, reactions of nitric acid with compounds such as cyanides, carbides, and metallic powders can be explosive and those with many organic compounds, such as turpentine, are violent and hypergolic (i.e. self-igniting). Hence, it should be stored away from bases and organics. [1], [3]
1.7. Market Survey
1.7.1 Economic Outlook
Nitric acid is not produced in Saudi Arabia. However, worldwide annual production of nitric acid is at present approximately 34 million tons. The USA, UK, Poland and France are the largest producers. The trend in the last decade has been for growth by the larger producers, very much at the expense of the smaller ones. The global scene is a much more stable market. This can be attributed historically to consumption being more broadly based with a sizable consumption in chemical production processes.
World nitric acid consumption peaked in the late 1980s before declining significantly through 1994. That decline was related primarily to economic turmoil in the Eastern bloc countries. Since then, the market has exhibited an upward trend. The largest market for nitric acid consumption is the production of ammonium nitrate (AN) and calcium ammonium nitrate (CAN). In 2010, this accounted for 80% of total world consumption of nitric acid. The major end use of AN fertilizer is in decline as a result of concerns about nitrate groundwater contamination and increased usage of solid urea, which has a higher nitrogen content (46%) than AN (34%), is less costly, and is less dangerous. Consumption of AN in explosives and blasting agent applications continues to grow, but is much more regulated since September 11, 2001. [9]
Figure 1 World producers of nitric acid [2]
Most nitric acid is consumed captively and the merchant portion of the market accounts for less than 10% of the total. International trade is minor and has little impact on the nitric acid balance. Although there has been a relatively steady increase in capacity, the world's average operating rate increased to 76% in 2010, indicating a much stronger market balance than previously.
It is estimated that AN (and CAN) production accounted for 80% of the world nitric acid market in 2010. The AN market is nearly three-quarters fertilizer and one-quarter industrial applications. However, other nitric acid–based products such as nitrophosphates and potassium nitrates are also used in fertilizer applications, accounting for an additional 2.5% of total nitric acid consumption. The remaining 17.5% of nitric acid is consumed in industrial (nonfertilizer) applications. The combined production of organic compounds, such as nitrobenzene, toluene diisocyanate (TDI), adipic acid, and nitrochlorobenzenes accounted for nearly 10% of total world nitric acid consumption in 2010.
Western Europe, China, the former USSR, Central Europe and the United States dominate the market statistics. Together, these five regions accounted for 81.4% of capacity, 82.0% of production, and 81.9% of consumption in 2010. Since 1994, the largest increases in capacity, production and consumption have occurred in China.
30 % 7 % 4 % 18 % 10 % 10 % 21 %
World Producers of Nitric Acid
USA Poland france USSR UK Other Estern Europe
18 One environmental problem affecting the consumption of nitric acid concerns the use of ammonium nitrate fertilizers. The loss of nitrogen to groundwater because of nitrification and leaching has become a significant problem and has negatively impacted the use of nitrogen fertilizers, particularly in Western Europe. A European Community directive has set a target of reducing nitrate levels in groundwater to a maximum of 50 milligrams per liter. There is also concern about nitrate levels in groundwater in the United States.
Current world production is approximately 34 million tons per annum, and over 30% is produced in the United States. Of the remaining production, about 60% is based in Europe. The USSR (6 million tons), United Kingdom (3.3 million tons), Poland (2.4 million tons) and France (1.5 million tons) are the main producers.
The plant should operate on a standard 8000 hour/year basis, with approximately 330 days of production. Nitric acid market price is $400 per ton can be obtained for the product.
Ammonia market price is $500 per ton; we need to 18687.08 tons for ammonia to produce 100000 tons of nitric acid. [9]
Table 1: Prices of raw materials and product
Component Price per ton Total Price
Ammonia $500 $9343540
Water $0.04 $963
Nitric acid $400 $40000000
Total Profit = 40000000 - 9343540 - 963 = $30655497
1.8 Preliminary Hazard Analysis
1.8.1 Summary of Previous Plant Accident
At about 6 a.m. on December 13, 1994, two explosions rocked the ammonium nitrate (AN) facility at the Port Neal, U.S.A Iowa nitrogen products manufacturing complex operated by Terra Industries. Four people were killed and 18 injured.
Approximately 5,700 tons of anhydrous ammonia was released, and releases of ammonia continued for nearly six days following the explosion. Chemicals released as a result of the blast contaminated groundwater under the facility. [10]
Figure 2 Terra's Port Neal, Iowa complex before the explosion. The ammonium nitrate plant is indicated [2]
Figure 3 Post-explosion aerial photograph of the Port Neal plant [2]
The Port Neal plant produced an 83 percent AN solution by reacting ammonia and nitric acid in a vessel called a neutralizer. The original neutralizer was replaced in 1980 and a major modification and upgrade of the plant occurred in 1992. A scrubber and new control system were also added in September 1994. In the two days prior to the explosion, the nitric acid plant was shut down for maintenance. With the nitric acid plant not operating, the AN facility was also shut down.
The accident occurred due to unsafe plant operations including poor maintenance and inadequate employee training. Specifically, during the shutdown period, the pH
20 of the neutralizer vessel contents dropped to an unusually low level and leaks in other equipment led to the introduction of chloride ions that catalyzed the final reaction. Unaware that the 18,000 gallon-capacity neutralizer vessel was in a highly acidic and contaminated condition, Terra employees injected superheated steam to try to keep the vessel contents from freezing due to the winter cold. The energy from injected superheated steam led to the runaway chemical reaction of the sensitized ammonium nitrate solution and resulted in the subsequent explosive detonations. [5]
1.8.2 Inherent Safety Aspects
Taking into consideration the inherent safety aspects such as substitution of hazardous chemicals, safe location, plant layout, transportation and storage can reduce accidents Potential consequences:
a) Hazardous Chemicals:
The following hazards may arise during nitric acid production: Equipment/piping failure because of corrosion
Explosion hazard due to the air ammonia mixture Explosion of nitrite/nitrate salts
(i) Equipment/Piping Failure
Corrosion protection is achieved by the well-proven use of suitable austenitic stainless steel where condensation can occur and by regular monitoring of the conditions.
(ii) Explosion Hazard due to the Air Ammonia Mixture
The air ammonia ratio is continuously controlled and kept below the hazardous range. Safety is ensured by the automatic closure of the ammonia control valve and separate shutdown trip valve when too high an air ammonia ratio is measured, either from each individual flow meter or indirectly from the catalyst gauze temperature. (iii) Explosion of Nitrite/Nitrate Salts
Any free ammonia present in the nitrous gas will give a deposit of nitrite/nitrate in a cold spot. Local washing and well proven operating practices will prevent the hazard. [1], [6]
b) Safe Location:
The selection of suitable location is a very important decision to make when there is new plant to build. Some of the factors that should be consider when selecting the location for a plant is for example the availability of raw materials used, also good transportation network, and availability of market, weather condition suitability and the water supply. From safety aspects, few factors should be taken in consideration before deciding whether the location is suitable. Most important is the distance from the residential area, availability of water and power supply. [4]
c) Plant Layout:
Another inherent safety is plant layout. After deciding on the plant location the overall plant layout, for example the processing areas, the absorber column, control rooms, roads and storage areas and other utilities must be planned carefully taking in consideration the future problems that might arise. Firstly and elementary layout developed first, these shows the fundamental relationship between the operating equipment and the storages area. Then the second step is the primary layout base of the flow of the material, unit operational storage, and future expansion. An efficient arrangement and coordination is very important to reduce the risk and hazards in plant by putting the element of health and safety into the design. [9]
d) Transportation:
Transportation to and from the plant is very important. Usually if the plant is big, it requires inside transportation and these kinds of vehicles should be ensured that it is safe and does not bring any hazards to the workers or the plant itself. Vehicles should be using diesel instead of petrol, as a diesel engine does not produce sparks that might ignite fire. For nitric acid transferring process, container should be constructed from insulating material. [9]
22 e) Storage:
Most of the large accidents in chemical or petrochemicals plants happen in the storage area. Storages room or tanks is where most plants stores whether their raw material or their products. Chemical storage areas shall be inspected at least annually and any unwanted or expired chemicals shall be removed. Adjusting the storage capacity or installing safety system will definitely reduce accident occurrence. The duration of material stored should also be taken in consideration. For example, longer store might change the material properties, which might cause to undesired accidents. Raw materials and products should not be stored for long period. [4]
1.8.3 Local Safety and Environmental Regulations
(Nitrogen oxides defined as nitrogen dioxide NO2) 1) Purpose:
The purpose of these standards is to prevent development of nitrogen dioxide concentrations, which could produce adverse health effects or lead to the production of significant concentrations of photochemical oxidants.
2) Standards:
a) During any 30 days period, one-hour average NO2 concentration shall not exceed 660-microgram/cubic meter (0.35 ppm) more than twice at any location. b) During any 12 months period, the annual NO2 concentration shall not exceed 100-microgram/cubic meter at any location. [11]
1.9 Site Feasibility
1.9.1 Selection Criteria
Several factors influence the selection of a site for the location of a chemical plant. The following list contains a few of the important considerations, but should not be considered exhaustive.
(a) Designation as a heavy industrial development area (‘light’ industry usually means assembly of electronic components, small metal fabricators, etc., and a major chemical plant would not be acceptable).
(b) Prior existence of similar chemical plants and location of other industrial centres. (c) Existing roads and services, e.g. electricity, gas, water, etc.
(d) Appropriate terrain, sub-surface, drainage, etc.
(e) Suitable access for transportation of raw materials and chemicals, and for construction of a chemical plant.
(f) Proximity to major transportation networks, e.g. roads, railways, airports, waterways and ports. This is a major consideration in the location of a plant. In some cases direct pipelines for the transportation of chemicals or utilities (e.g. water, gas, oil) may be the most economical method. The cost of transportation by tanker (road, rail or sea) is reduced if a return load can also be carried.
(g) Availability of a local workforce and distance from local communities. (h) Availability of domestic water and plant cooling water.
(i) Environmental discharge regulations.
(j) Proximity to both the raw materials supply and the market for the product chemical.
(k) Existence of services equipped to deal with a major industrial accident.
(1) Climatic conditions, e.g. humidity, maximum wind velocity and its prominent direction, rainfall, etc.
(m) Proposed or possible government restrictions regarding industrial development or discharge emissions.
(n) Room for expansion. (0) Price of land.
(p) Public opinion.
24 (r) Availability of government regional development grants or tax incentives, subsidies, etc. [1], [9]
1.9.2 Potential Site Location
The manufacture of nitric acid is categorized as a petrochemical project. The plant must therefore be sited in a special zone provided by the government. After conducting the feasibility and site survey, three existing industrial areas have been evaluated to choose the most suitable area for the acetone plant, which are Jubail Industrial City (JIC), Yanbu Industrial City (YIC) and Ras Alkhair Seaport.
Table 2: Information of site location
Factors
Site Location
JIC YIC Ras Alkhair
Seaport
Type of Industrial Area 10 10 10
Raw Materials IPA 9 0 10
Utilities Power 10 10 10 Water 10 10 10 Steam 9 8 6 Natural Gas 8 8 8 Available Area 9 8 10 Land Price 10 10 10
Space for Expansion 0 0 0
Cost of Living 5 8 9 Transportation Seaport 10 10 10 Railway 5 0 10 Roadway 10 10 10 Airport 5 10 3 Price of Utilities Power 6 6 6 Water 10 10 10 Existing Infrastructure 10 10 10
Existing Services for Industrial Accidents 10 10 10 Training Centre 10 5 5 Government Incentives 10 10 10 TOTAL SCORE 166 162 167 PERCENTAGE (%) 83 81 83.5 RANKING 2 3 1
In these cities, there are a lot of petrochemical plants and because the raw material (ammonia) plant is available in Ras Alkhair, we have chosen the location of our plant to be there.
Chapter 2
26
2.1 Process Description
The process begins with the vaporization of ammonia at 1240 kPa and 35°C using process heat. Steam is then used to superheat the ammonia about 170°C, filtered air is compressed by a centrifugal compressor, discharge pressure of 1200 kPa.
In our process; the air and the ammonia vapor are mixed and passed through the platinum/ rhodium gauze reactor where the heat of reaction raises the temperature to be between 650°C and 630°C. The reaction gas flows of heat exchangers to cooled down to 70 oC.
Approximately 95% of the oxidation to nitrogen dioxide occurs as the gas passes in the Oxidation unit, after that cooled to 60°C, then sent to absorber to produce nitric acid (60%) purity.
28
Figure 4 Process flow sheet for nitric acid production by the single pressure process." our selected process"
2.1.1 The Flow Sheet of the Selected Process
E-101 E-102 C-101 M-101 R-201 E-201 R-202 E-202 T-201 Vaporizer Superheater Compressor Mixer Reactor Cooler Oxidation Cooler Absorber
2.1.2 Justification for the Equipment Selection
Process Units
1. Air Compressor “Two stage compressors to achieve the high pressure required".
2. Ammonia Vaporizer: a shell and tube-type heat exchanger. This unit should contain internal baffles. This exchanger is made from mild steel.
3. Ammonia Superheater: a shell and tube-type heat exchanger of similar mechanical construction to the ammonia vaporizer. Also constructed from mild steel.
4. Mixer (usual mixing vessel).
5. Reactor: the reactor is a pressure vessel operating in the range 1050 kPa to 1100 kPa. The vessel must be designed to ensure even passage of the feed gas mixture over the platinum/rhodium catalyst gauze The catalyst gauze and accompanying platinum filter gauze are fixed in position by lateral supports across the width of the reactor.
6. Cooler: this shell and tube-type heat exchanger uses deionized water as its cooling medium. It has a design pressure of about 1200 kPa.
7. Oxidation Unit: the oxidation unit is a normal pressure vessel that takes input reaction gases and air.
8. Secondary Cooler: the secondary cooler takes the exit gases from the oxidation unit at 140°C and cools them down to 60°C, a suitable temperature for entry into the absorption column. It is a shell and tube-type heat exchanger.
9. Absorber: the absorber is usually a sieve tray-type column. It has a design pressure of 1200 kPa, and operates at a temperature range of 60oC to 30oC.
30 The material balances for the all units of the plant were hand calculated. A material balance for each unit presented below in tabulated form. The main equations used in the calculations are shown in their relevant sections. The detailed calculations are included in Appendix (C).
The general equation for material balance is
Input – Output + Generation – Consumption = Accumulation For steady state without chemical reaction
Input – Output = 0
2.2 Overall Mass Balance
Table 3: Summery of overall mass balance
Component Input S1 (Kg/h) Input S2 (Kg/h) Output S14 (Kg/h) Output S12 (Kg/h) Output S13 (Kg/h) O2 9142.88 - - 799.19 - N2 34394.64 - - 34490.82 - NH3 - 2335.88 - - - H2O - - 3007.51 646.00 5000.00 HNO3 - - - - 7500.00 NO2 - - - 288.20 - NO - - - 156.70 - Total 48880.91 48880.91
S 1
S 12
S 2
S 13
S 14
2.3 Reactor Mass Balance
R-201
S 7
S 8
Table 4: Summery of reactor mass balance
Component Input S7 (Kg/h) Output S8 (Kg/h) NH3 2335.88 - O2 9142.88 3756.58 N2 34394.64 34490.82 NO - 3916.05 H2O - 3709.93 Total 45873.4 45873.4
32
2.4 Oxidation Mass Balance
R-202 S 9 S 10 Component Input S9 (Kg/h) Output S10 (Kg/h) NO 3916.05 156.70 NO2 - 5764.39 O2 3756.57 1751.55 N2 34490.82 34490.82 H2O 3709.93 3709.93 Total 45873.40 45873.40
2.5 Absorber Mass Balance
T-201
S 11
S 12
S 13
S 14
Table 6: Summery of absorber mass balance
Component Input S11 (Kg/h) Input S14 (Kg/h) Output S12 (Kg/h) Output S13 (Kg/h) H2O 3709.93 3007.51 646.00 5000.00 HNO3 - - - 7500.00 NO2 5764.40 - 288.20 - NO 156.70 - 156.70 - O2 1751.55 - 799.19 - N2 34490.82 - 34490.82 - Total 48880.91 48880.91
34
Chapter 3
3.1 Vaporizer Energy Balance
E-101
S 4
S 5
For S4: NH3 at -15 oC and S5: NH3 at 35 oC ∑ 𝐻𝑜𝑢𝑡 = 1.78 𝐾𝐽/𝑚𝑜𝑙 ∑ 𝐻𝑖𝑛 = 0 𝑄̇ = 2902575.37 𝐾𝐽/ℎ36
3.2 Compressor Energy Balance
C-101
S 1
S 3
For S1: Air at 101 kPa and S3: Air at 1090 kPa
𝑊 = 8879734.37 𝑘𝐽/ℎ
3.3 Superheater Energy Balance
E-102
S 5
S 6
For S5: NH3 at 35 oC and S6: NH3 at 177 oC 𝐻𝑖𝑛 = 0 𝐻𝑜𝑢𝑡 = 5.442 𝐾𝐽/ℎ 𝑄 = 747758.01 𝐾𝐽/ℎ38
3.4 Mixer Energy Balance
M-101
S 3
S 6
S 7
For S3: NH3 at 177 oC, S6: Air at 262 oC and S7: NH3+Air at ?
𝑄 = 0 (𝐴𝑑𝑖𝑎𝑏𝑎𝑡𝑖𝑐) 𝑇𝑜𝑢𝑡 = 250 °𝐶
3.5 Reactor Energy Balance
R-201
S 7
S 8
For S7: NH3+Air at 250 oC and S8: Air+NO+H2O at 645 oC
Table 7: Summary of enthalpy components in reactor
Component Input S7 (kJ/mol) Output S8 (kJ/mol) H (Air) 6.654 19 H (NH3) 8.84 - H (NO) - 19.744 H (H2O) - 23
𝑄̇ =
−51820729.07 𝑘𝐽/ℎ40
3.6 Heat Exchanger Energy Balance (1
stCooler)
E-201
S 8
S 9
For S8: Air+NO+H2O at 645 oC and S9: Air+NO+H2O at 70 oC 𝐻𝑜𝑢𝑡 = 0
For air: 𝐻𝑖𝑛 = 17.693 𝑘𝐽/𝑚𝑜𝑙 For NO: 𝐻𝑖𝑛 = 18.400 𝑘𝐽/𝑚𝑜𝑙 For H2O: 𝐻𝑖𝑛 = 21.173 𝑘𝐽/𝑚𝑜𝑙
3.7 Oxidation Energy Balance
R-202
S 9
S 10
For S9: Air+NO+H2O at 70 oC and S10: Air+NO+H2O+NO2 at 140 oC
Table 8: Summary of enthalpy components in oxidation
Component Input S9 (kJ/mol) Output S10 (kJ/mol) H (H2O) (g) 1.521 3.919 H (Air) 1.312 3.37 H (NO) 1.345 3.47 H (NO2) - 4.5 𝑄 = −3707006.24 𝑘𝐽/ℎ
42
3.8 Heat Exchanger Energy Balance (2
ndcooler)
E-202
S 10
S 11
For S10: Air+NO+H2O+NO2 at 140 oC and S11: Air+NO+H2O+NO2 at 60 oC 𝐻𝑜𝑢𝑡 = 0 For H2O: 𝐻𝑖𝑛 = 2.737 𝑘𝐽/𝑚𝑜𝑙 For air: 𝐻𝑖𝑛 = 2.350 𝑘𝐽/𝑚𝑜𝑙 For NO: 𝐻𝑖𝑛 = 2.4230 𝑘𝐽/𝑚𝑜𝑙 For NO2: 𝐻𝑖𝑛 = 3.1796 𝑘𝐽/𝑚𝑜𝑙 𝑄 = −3911927.94 𝐾𝐽/ℎ
3.9 Absorber Energy Balance
T-201
S 11
S 12
S 13
S 14
Basis: Tref = 25oCFor S11: Air+NO+H2O+NO2 at 60 oC, S14: H2O at 20 oC, S12: Air+NO+H2O at 30 oC and S13: HNO3+H2O at 30 oC
Table 9: Summary of enthalpy components in absorber
Component Input S11 (kJ/mol) Input S14 (kJ/mol) Output S12 (kJ/mol) Output S13 (kJ/mol) H (H2O) (g) 1.18 - 0.166 - H (H2O) (L) - -0.375 - 0.375 H (Air) 1.02 - 0.145 - H (NO) 1.044 - 0.149 - H (NO2) 1.32 - - - H (HNO3) - - - 0.55 Q= -29876241 KJ/h
44
Chapter 4
Design and Sizing
Equipment
4.1 Sizing of Pump [7]
Pump Specification Data Ammonia
Flow rate = 2335.88 kg/h Temperature = -15°C
Density at -15°C = 656.67 kg/m3
Estimating the pump diameter required
Mass flowrate (G) =2335.88 3600 = 0.6488 Kg/s G = 0.6488 kg/s 𝐹𝑙𝑜𝑤𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒(𝑄) =0.6488 656.67 Q = 0.001 m3/s Piping Specification
Atypical velocity for fluid flow is 2 m/s. Determination of the pipe area:
Area of pipe (A) =𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤𝑟𝑎𝑡𝑒 Velocity Area =0.001 2 Area= 0.0005 m2 Diameter of pipe (D) = [4 × 𝐴𝑟𝑒𝑎 𝜋 ] 0.5 𝐷 = [4 × 0.0005 𝜋 ] 0.5 D = 25 mm
This value is compared with the result achieved by applying the ‘Economic pipe diameter’ formula for stainless steel from
Optimum diameter = 226 G0.5 ρ-0.35
Optimum diameter = 226 (0.6488)0.5 × (656.67)-0.35 Optimum diameter = 19mm
46
4.2 Design of Heat Exchanger (Vaporizer)
The objective is to design a heat exchanger (vaporizer) (shell and tube) to heat Ammonia of flow rate 2335.88 kg/h from -15oC to 35 oC using saturated steam entering at 1 atm.
Table 10: Data of ammonia
Component Mwt N M Mole fraction
Y
Mass fraction X
Kg/Kmol Kmol/h Kg/h
NH3 17 137.404 2335.88 1 1
Table 11: Physical properties of ammonia at Tave = 10oC and Pave = 1240 kPa
Component Cp Μ K Ρ
KJ/kg.K Pa.s W/m.K Kg/m3
NH3 5.02 1.531 x 10-4 0.5135 623.6318
Table 12: Physical properties of standard steam at 1 atm and Tave = 100oC = 373 K
Cpl Cpv μl μv ρl ρv kl kv Λ KJ/Kg.K Pa.s Kg/m3 W/ m.K KJ/Kg 4.24 1.888 0.27 x 10-3 1.295 x 10-5 953 0.596 0.681 0.0251 2256.9
Heat duty (Q) needed to be added to gas mixture: Q = m Cpmix ΔT [7] Q = 163 kJ/s (kw) For steam: Qlost = Qgain = m λ [7] msteam = 163/2256.9 msteam = 0.0722 kg/s
Finding ΔTlm: ∆Tout= T1− t2 = (100 − 35) = 65 Co ∆𝑇𝑖𝑛= 𝑇2− 𝑡1 = (100 − (−15)) = 115 𝐶𝑜 ∆𝑇𝑙𝑚 =∆𝑇𝑜𝑢𝑡− ∆𝑇𝑖𝑛 𝑙𝑛∆𝑇∆𝑇𝑜𝑢𝑡 𝑖𝑛 [7] ∆𝑇𝑙𝑚 = 87.63 °𝐶
The heat exchanger will consist of one shell pass and one tube passes: ΔTm = 87.63 oC
According to Table A in Appendix E, Cold fluid is Ammonia and hot fluid is condensing steam. The following value of overall heat transfer coefficient U can be estimated:
U= 500 W/ m2.oC
Required Area for Heat transfer: 𝑄 = 𝑈𝐴 𝛥𝑇𝑚 𝐴 = 𝑄/𝑈 𝛥𝑇𝑚 [7] 𝐴 = 3.72 𝑚2
VAPORIZER
T2 = 100 oC T1 = 100 oC t1 = -15 oC t2 = 35 oC48 Choosing 25 mm “OD”, 21 mm “ID” stainless steel tubes to resist corrosion problems. Placing condensing steam in the shell side. Take Tube length L=2 m [7]
Calculation of Number of tubes:
𝑁𝑡= 𝐴 𝜋 × 𝑑 × 𝐿 = 3.72 𝜋 × 25 × 10−3× 2 𝑁𝑡= 24 𝑡𝑢𝑏𝑒𝑠
Since the steam in the shell side is always clean, we will use triangular pitch arrangement with Pt= 1.25do:
Calculation of bundle diameter Db:
𝐷𝑏 = 𝑑𝑜[ 𝑁𝑡 𝐾1] 1 𝑛1 [7]
K1, n1 are constants given in Table C: K1 = 0.319 n1 = 2.142 𝐷𝑏 = 25 [ 159 0.319] 1 2.142 [7] Db = 187.9 mm
Calculating the shell diameter Ds: 𝐷𝑠 = 𝐷𝑏+ 𝑐𝑙𝑒𝑎𝑟𝑎𝑛𝑐𝑒 [7]
From Figure A in Appendix E and for split-ring floated heat type of heat exchanger we get, clearance= 48 mm
Calculating the heat transfer coefficients for tube side (hi): 𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑜𝑛𝑒 𝑡𝑢𝑏𝑒 = 𝜋 4𝑑𝑖 2 =𝜋 4(21) 2 = 346.36 𝑚𝑚2 𝑇𝑜𝑡𝑎𝑙 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑓𝑜𝑟 𝑤𝑎𝑡𝑒𝑟 = ( 𝑁𝑡 𝑁𝑜. 𝑝𝑎𝑠𝑠) × 𝜋 4𝑑𝑖 2 = 0.008312 𝑚2 [7] 𝑀𝑎𝑠𝑠 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝐺𝑚𝑖𝑥) = 𝑚𝑤𝑎𝑡𝑒𝑟 𝐴 = 0.64885 0.008312 = 78.06 𝐾𝑔 𝑚2. 𝑠𝑒𝑐 ⁄ [7] 𝐿𝑖𝑛𝑒𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝑣𝑚𝑖𝑥) = 𝐺𝑤𝑎𝑡𝑒𝑟 𝜌𝑤𝑎𝑡𝑒𝑟 = 78.06 623.6318 = 0.125 𝑚 𝑠𝑒𝑐⁄ [7] To calculated hi use the equation
ℎ𝑖𝑑𝑖 𝐾𝑓 = 𝐽ℎ× 𝑅𝑒 × 𝑃𝑟0.33× ( 𝜇 𝜇𝑤 ) 0.14 [7] hi = inside heat transfer coefficient di = tube diameter = 21x10-3 m
Kf = thermal conductivity of water fluid = 0. 5135 W/m.oC
Jh = factor for heat transfer given form Figure B in Appendix E in Appendix E by (Re)
𝑅𝑒𝑦𝑛𝑜𝑙𝑑′𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 (𝑅𝑒) =𝜌𝑣𝑑 𝜇 = 623.6318 × 0.125 × 21 × 10−3 1.531 × 10−4 = 10692.57 [7] (Turbulent Flow) 𝑃𝑟𝑎𝑛𝑑𝑡𝑙 𝑁𝑢𝑚𝑏𝑒𝑟 (𝑃𝑟) =𝐶𝑝𝜇 𝐾𝑓 = 5.02 × 10 3× 1.531 × 10−4 0.5125 = 1.49 [7] From Figure B in Appendix E at Re= 10692.57
Jh = 4 x10-3
From (eq. hi) and neglecting ( )0.14 w
and assume it approximately 1 Then:
ℎ𝑖(21 × 10−3)
0.5135 = (4 × 10
−3)(10692.57)(1.49)0.33 hi = 1192.93 W/m2.oC
50 Calculate heat transfer coefficients for shell side (ho):
Area for cross flow of the shell side As, using Kern Method: 𝐴𝑠 = [
𝑃𝑡− 𝑑𝑜 𝑃𝑡
] 𝐷𝑠𝐿𝐵 [7]
Pt = tube pitch
do = tube outside diameter Ds = shell inside diameter
LB = Buffle spacing (usually one fifth of shell diameter) do = 25 mm Pt = 1.25 do mm = 31.25 mm 𝐿𝐵= 𝐷𝑠 5 = 235.9 5 = 47.18 𝑚𝑚 𝐴𝑠 = [𝑃𝑡− 𝑑𝑜 𝑃𝑡 ] 𝐷𝑠𝐿𝐵 = [ 31.25 − 25 31.25 ] (235.9)(47.18) = 2226 𝑚𝑚 2 As = 0.002226 m2
Mass velocity (Gs) and liner velocity (vs): mshell = mass flow rate of steam = 0.0722 kg/sec
𝐺𝑠 = [𝑚𝑠ℎ𝑒𝑙𝑙 𝐴𝑠 ] = [ 0.0722 0.002226] = 32.43 𝑘𝑔 𝑚2. 𝑠𝑒𝑐 ⁄ [7] 𝑣𝑠 = [ 𝐺𝑠 𝜌𝑠 ] = [32.43 0.596] = 54.42 𝑚 𝑠𝑒𝑐⁄ [7] The equivalent (Hydraulic) diameter de: 𝑑𝑒 = 1.10 𝑑𝑜 (𝑃𝑡2− 0.917𝑑𝑜2) [7] 𝑑𝑒 = 1.10 25 [(31.25) 2 − 0.917(25)2] = 17.75 𝑚𝑚 = 0.01775 𝑚
Re & Pr for shell side: 𝛤ℎ = 𝑚𝑠 𝑁𝑡𝑙 = 0.0722 24 × 2 = 1.5 × 10 −3𝐾𝑔 𝑚. 𝑠 ⁄ [7] Pt do
𝑅𝑒 =𝐺𝑠 𝑑𝑒 𝜇𝑣 = (32.43)(0.01775) 1.259 × 10−5 = 45721.4 [7] 𝑁𝑟 =𝐷𝑏 𝑃𝑡 = 187.9 31.25= 6.01 [7]
The shell side coefficient ( hs or ho ) ℎ𝑠 = (0.95)(0.681) [ (953)(953 − 0.596)(9.8) (0.27 × 10−3)(1.5 × 10−3)] 1 3 × 6.01−16 hs = 13441.08 W/m2.oC
52 Calculating the overall heat transfer coefficient (U):
1 𝑈𝑜 = 1 ℎ𝑜+ 1 ℎ𝑜𝑑+ 𝑑𝑜ln (𝑑𝑜 𝑑𝑖 ⁄ ) 2𝐾𝑤 + 1 ℎ𝑖( 𝑑𝑜 𝑑𝑖) + 1 ℎ𝑖𝑑( 𝑑𝑜 𝑑𝑖) [7] Uo = overall coefficient based on outside area of the tubes ho = outside fluid film coefficient
hi = inside fluid film coefficient do = tube outside diameter di = tube inside diameter
Kw = thermal conductivity of tube wall material = 16 (W/m.oC) for "stainless steel" hod = outside dirt " fouling " coefficient = 3000 (W/m2.oC) (From Table B)
hid = inside dirt " fouling " coefficient = 3000 (W/m2.oC) (From Table B) 1 𝑈𝑜 = 1 13441.08+ 1 3000+ 25 × 10−3ln(25 21⁄ ) 2 × 16 + 1 1192.93( 25 21) + 1 3000( 25 21) Uo = 516 W/𝒎𝟐.oC
The value of 516 W/m2.oC is well above the estimated value of 500 W/m2.oC. Hence, the present design satisfactory. (ok)
Calculating the pressure drop (ΔP) for side tube and shell tube: 1.The tube side ΔPt
∆Pt= NP[8jf(L di) ( μ μw) −m + 2.5] [ρvt 2 2 ] [7] ΔPt = tube side pressure drop (N/m2)
Np = Number of tube passes
jf = fraction factor ' depending on Re ' Found from Figure 12.30 in the text book assuming baffle cut of 0.25
L = length of one tube = 2 m
vt = flow velocity inside the tube =0.125 m/sec m = exponent value depending on type of flow For laminar (Re<2100) m = 0.25
For Turblent (R>10000) m = 0.14
Now, assuming viscosity ratio =1 and finding jf corresponding to Re= 5594.35, we get; ∆Pt= [8(5 × 10−3) ( 2
21 × 10−3) + 2.5] [
623.6318 × 0.1252
2 ]
54 2. The shell side ΔPs
∆Ps= 8jf( Ds De ) (L LB ) (ρvs 2 2 ) ( μ μw ) −0.14 [7] ΔPs = shell side pressure drop (N/m2)
jf = fraction factor ' depending on Re , Found from Figure 12.30 in the text book assuming baffle cut of 0.25
Ds = shell diameter = 235.9 mm
De = equivalent diameter for shell side = 17.75 mm L = length of one tube = 2 m
Re= 45721.4
vs = flow velocity in shell = 54.42 m/sec LB = Baffle spacing = 47.18 mm
ρ shell = 0.596 kg/m3
Neglecting viscosity correction, we get: ∆Ps= 8(3.9 × 10−2) ( 235.9 17.75) ( 2 × 103 47.18 ) ( 0.596 × 4.422 2 ) ΔPs = 155.12 kpa (acceptable)
Finally, we summarize our present design as follows
1. The selected heat exchanger has one shell pass and one tube passes, in which ammonia flow inside the tube and condensing steam flows inside the shell “.
2. The selected tubes are made of stainless steel with 25 mm outside diameter, 21 mm inside diameter, the total number of tube is 24, while the tube length is 2 m, and the triangular pitch applied for this tube distribution is 31.25 mm.
3. The shell diameter Ds is 235.9 mm. and the baffles used are 25% cut, while the baffle spacing is 48 mm.
4. The first estimated value for overall heat transfer coefficient Uo is 500 W/m2.oC, while the final calculated value is 516 (W/m2. oC).
5. The pressure drop (ΔPt) for the tube side is 0.0307 kPa , while the shell side (ΔPs) is 155.12 kPa .
56 𝜌 =𝑃𝑀𝑤 𝑅𝑇 = 101 × 1000 × 28.9 298 × 8314 = 1.15 𝑘𝑔 𝑚3 ⁄ 𝑉1 =1 𝜌= 0.8733 𝑚 3 𝑘𝑔 ⁄ V = 12.693/1.145 = 10 .562 m3/s = 38023.2 m3 / h
From Figure C in Appendix E and for volumetric flow 38023.2 and discharge pressure 11 bars the recommended compressor is centrifugal compressor
Calculation the polytropic coefficient (n)
From Figure D in Appendix E and suction flow 10.562 m3/s Ep = 0.74 Average heat capacity of mixture at T = 30 oC
𝐶𝑝𝑎𝑣𝑒 = ∑ 𝑦𝑖 𝐶𝑝𝑖 = 29.099 𝐽⁄𝑚𝑜𝑙. 𝐾 𝐶𝑣 = 𝐶𝑝 − 𝑅 = 29.099 − 8.314 = 20.785 𝛾 =𝐶𝑝⁄𝐶𝑣 = 1.4 𝑚 =(𝛾 − 1) 𝛾(𝐸𝑝) = 0.386 𝑛 = 1 1 − 𝑚= 1.62 The polytropic work: 𝑊 = 𝑃1𝑣1 𝑛 𝑛 − 1[( 𝑃2 𝑃1 ) (𝑛−1) 𝑛 ⁄ − 1] Wpoly =376366.4 J/kg
Actual work required: Wpoly/Ep = 508731.6 J/kg Power required
Pac = W × m = 12.693 × 508731.6 = 6068.531 KW Electric power:
From table the approximate electrical efficiency, Ee is 0.97 Pe = Pac/Ee = 6068.531 /0.97 = 6256.2 KW
The objective is to design a heat exchanger (superheater) (shell and tube) to heat Ammonia of flow rate 2335.88 kg/hr from 350C to 177 0C using saturated steam entering at 40 bars.
Table 13: Data of ammonia
Component Mwti Ni Mi Mole fraction Mass fraction
kg/kmol kmol/h Kg/h Yi Xi
NH3 17 137.404 2335.88 1 1
Table 14: Physical properties at Tave = 106oC and P = 1240 kPa
Component Cp μi K ρ
kJ/kg.K Pa.s W/m.K kg/m3
NH3 2.38 1.32 x 10-4 0.0364 7.04
Table 15: Physical properties of standard steam at 40 bars and Tave = 250.3oC = 523.45 K
Cpv μv ρv kv 𝝀
kJ/kg.K Pa.s kg/m3 W/ m.K kJ/kg
1.958 1.84 x 10-5 20.12 0.0388 1712.9
Heat duty (Q) needed to be added to gas mixture: 𝑄 = 𝑚𝐶𝑝𝑚𝑖𝑥 𝛥𝑇 [7] 𝑄 = 219.28 𝑘𝐽/𝑠 (𝑘𝑤) For steam: 𝑄𝑙𝑜𝑠𝑡= 𝑄𝑔𝑎𝑖𝑛 = 𝑚 𝜆 [7] 𝑚𝑠𝑡𝑒𝑎𝑚 = 219.28/1712.9 𝑚𝑠𝑡𝑒𝑎𝑚 = 0.128 𝑘𝑔/𝑠 Finding ΔTlm:
58 ∆𝑇𝑜𝑢𝑡 = 𝑇1− 𝑡2 = (250.3 − 177) = 73.3 °C ∆𝑇𝑖𝑛= 𝑇2− 𝑡1 = (250.3 − 35) = 215.3 °C ∆𝑇𝑙𝑚 =∆𝑇𝑜𝑢𝑡− ∆𝑇𝑖𝑛 𝑙𝑛∆𝑇∆𝑇𝑜𝑢𝑡 𝑖𝑛 [7] ∆𝑇𝑙𝑚 = 131.79 °C
The heat exchanger will consist of one shell pass and one tube passes.
According to Table A in Appendix E, Cold fluid is Ammonia and hot fluid is condensing steam. The following value of overall heat transfer coefficient U can be estimated:
U= 160 W/ m2.oC
Required Area for Heat transfer:
SUPERHEATER
T2 = 250.3 oC T1 = 250.3 oC
t1 = 25oC t2 = 177 oC
𝑄 = 𝑈𝐴 𝛥𝑇𝑚 𝐴 = 𝑄
𝑈𝛥𝑇𝑚 [7] 𝐴 = 10.39 𝑚2
Choosing 25 mm “O.D”, 21 mm “I.D” stainless steel tubes to resist corrosion problems. Placing condensing steam in the shell side. Take Tube length L = 5 m [7]
Calculation of Number of tubes:
𝑁𝑡= 𝐴
𝜋 × 𝑑 × 𝐿 =
10.39
𝜋 × 25 × 10−3× 5 [7] 𝑁𝑡= 27 𝑡𝑢𝑏𝑒𝑠
Since the gas mixture in the shell side is always clean, we will use triangular pitch arrangement with Pt= 1.25do [7]
Calculation of bundle diameter Db: 𝐷𝑏 = 𝑑𝑜[𝑁𝑡
𝐾1] 1 𝑛1
[7]
K1, n1 are constants given in Table C: K1 = 0.319 n1 = 2.142 𝐷𝑏 = 25 [ 27 0.319] 1 2.142 [7] Db = 198.5 mm
Calculating the shell diameter Ds: 𝐷𝑠 = 𝐷𝑏+ 𝑐𝑙𝑒𝑎𝑟𝑎𝑛𝑐𝑒 [7]
From Figure A in Appendix E and for split-ring floated heat type of heat exchanger we get, clearance= 49.5 mm
60 Calculating the heat transfer coefficients for tube side (hi):
𝐶𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑜𝑛𝑒 𝑡𝑢𝑏𝑒 = 𝜋 4𝑑𝑖 2 =𝜋 4(21) 2 = 346.36 𝑚𝑚2 [7] 𝑇𝑜𝑡𝑎𝑙 𝑐𝑟𝑜𝑠𝑠 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑓𝑜𝑟 𝑤𝑎𝑡𝑒𝑟 = ( 𝑁𝑡 𝑁𝑜. 𝑝𝑎𝑠𝑠) × 𝜋 4𝑑𝑖 2 = 0.009351 𝑚2 [7] 𝑀𝑎𝑠𝑠 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝐺𝑚𝑖𝑥) = 𝑚𝑤𝑎𝑡𝑒𝑟 𝐴 = 0.64885 0.009351 = 69.39 𝐾𝑔 𝑚2. 𝑠𝑒𝑐 ⁄ [7] 𝐿𝑖𝑛𝑒𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝑣𝑚𝑖𝑥) = 𝐺𝑤𝑎𝑡𝑒𝑟 𝜌𝑤𝑎𝑡𝑒𝑟 = 78.06 623.6318 = 9.85 𝑚 𝑠𝑒𝑐⁄ [7] To calculated hi use the equation
ℎ𝑖𝑑𝑖 𝐾𝑓 = 𝐽ℎ× 𝑅𝑒 × 𝑃𝑟0.33× ( 𝜇 𝜇𝑤 ) 0.14 [7] hi = inside heat transfer coefficient
di = tube diameter = 21x10-3 m
Kf = thermal conductivity of water fluid = 0.0364 W/m.oC
Jh = factor for heat transfer given form Figure B in Appendix E by (Re) 𝑅𝑒𝑦𝑛𝑜𝑙𝑑′𝑠 𝑁𝑢𝑚𝑏𝑒𝑟 (𝑅𝑒) =𝜌𝑣𝑑 𝜇 = 7.04 × 9.85 × 21 × 10−3 1.32 × 10−5 = 110320 [7] (Turbulent Flow) 𝑃𝑟𝑎𝑛𝑑𝑡𝑙 𝑁𝑢𝑚𝑏𝑒𝑟 (𝑃𝑟) =𝐶𝑝𝜇 𝐾𝑓 = 2.38 × 103× 1.32 × 10−5 0.0364 = 0.863 [7] From Figure B in Appendix E at Re = 110320
Jh = 2.8 x10-3 [7]
From (eq. hi) and neglecting (𝜇 𝜇𝑤)
0.14
and assume it approximately 1 Then:
ℎ𝑖(21 × 10−3)
0.0364 = (2.8 × 10
−3)(110320)(0.863)0.33 hi = 510 W/m2.oC