T
U
Delft
FVO Nr.
Fabrieksvoorontwerp
Vakgroep Chemische Procestechnologie
Subject
The processing of an aqueous HF containing wastestream from the Du Pont de Nemours Freon-22® process in economie valuable products.
Authors
R.A. Kautz
S.D. Raghoe
D. van Soest
M. van Wijngaarden
Keywords
Telephone
078 - 136314
070 - 3901713
010 - 4155920
078 - 157647
Aluminum fluoride, Azeotropic distillation, Evaporative crystallisation, Extractive distillation, Hydrogen fluoride
Date assignment
Date report
november
april
1994
1995
FVO Nr.
Fabrieksvoorontwerp
Vakgroep Chemische Procestechnologie
Subject
The processing of an aqueous HF containing wastestream from the Du Pont de Nemours Freon-22® process in economie valuable products.
Authors
R.A. Kautz
S.D. Raghoe
D. van Soest
M. van Wijngaarden
Keywords
Telephone
078 - 136314
070 - 3901713
010 - 4155920
078 - 157647
Aluminum fluoride, Azeotropic distillation, Evaporative crystallisation, Extractive distillation, Hydrogen fluoride
Date assignment
Date report
november
april
1994
1995
Summary
The production of Freon-22® by Du Pont de Nemours at Dordrecht produces a wastestream containing 15 wt% HF in water at 500 kg/h. Nowadays this process stream is coUected and transported to Germany where the hydrogen fluoride is separated from the water. Near the existing plant exists a site where a small plant can be build for the processing of this stream.
This report embodies two options for processing the wastestream. In part I a plant for the concentration to pure anhydrous hydrogen fluoride is designed, and in part
n
a plant for manufacturing anhydrous aluminum fluoride is designed.Part I
The direct application of pure hydrogen fluoride in the Freon® manufacturing plant is the major advantage of the concentrating of the wastestream. The proposed design yields a productstream of 580 ton annually and the economie evaluation shows commer-cial potential. The pay out time is 1.5 years, the return on investment is 27% and the internal rate of return is 14%.
Part 11
An entire different design is a plant for the production of anhydrous aluminum fluoride. Aluminum fluoride is a make-up ingredient in the molten electrolyte of the aluminum reduction ceU and in the electrolytic process for refining aluminium. The proposed design yields a productstream of 920 ton per year and the economie evaluation shows also commercial potential, but it is for Du Pont de Nemours an entirely new product, even productgroup. The pay out time is 1.13 years, the return on investment is 36% and the intern al rate of return is 27%.
Table of contents
PART I: HF CONCENTRATION 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -1-2. Base of design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -2-2.1 Feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -2-2.2 Product specifications . . . . . . . . . . . . . . . . . . . . . . . . .. -2-2.3 Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -2-2.4 Literature . . . -3-3. Process description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..-4-3.1 General process flow diagram . . .
-4-3.2 T3: the azeotropic column. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-4-3.3 T9: the extraction column . . . . . . . . . . . . . . . . . . . . ..
-4-3.4 T25: the separation column . . . . . . . . . . . . . . . . . . ..
3.5 Overall mass balance . . .
-5-3.6 Simulating columns in Aspen Plus . . . . . . . . . . . . . . . . ..
-5-3.7 Literature . . .
-6-4. Separation columns .... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-7-4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-7-4.2 Process description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-7-4.3 Design of T3: the azeotropic column . . .
-7-4.3.1 Base of design ... .. . .
-7-4.3.2 The number of plates at total reflux. Frenske's method . . .
-9-4.3.3 Approximate column sizing . . . . . . . . . . . . . . . . . . . . ..
-9-4.3.4 Plate contactors . . .
-10-4.3.5 Plate hydraulic design. . . . . . . . . . . . . . . . . . . . . . . ..
-10-4.3.6 Plate design procedure . . . .. . . .. . .
-10-4.3.7 Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-11-4.3.8 Entrainment . . . . . . . . . . . . . . . . . . . . . . . ..
-11-4.3.9 Weep point . . . .. . . .. . .
-12-4.3.10 Plate details . . . ... . . .. . .
-12-4.3.11 Plate pressure drop .... . . . . . . . . . . . ..
-13-4.3.12 Downcorner design . . . . . . . . . . . . . . . . . . . . . . . ..
-13-4.4 Results of the design of the separation columns . . .
-14-4.5 Literature . . . ... . . .. . .
-15-4.6 Nomenclature . . . .. . .
-17-5. Co st engineering . . . .. . .
-18-5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-18-5.2 Investment costs . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-18-5.2.1 General aspects investments . . . . . . . . . . . . . . . . . . . . ..
-19-5.5 Wages dependent operating costs . . . ,
-21-5.6 Total costs . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-21-5.7 Depreciation and capital charge . . .
-21-5.8 Benefits . . .
-22-5.9 Profits, taxes and cash flow . . . . . . . . . . . . . . . . . ..
-22-5.10 Profitability analysis . . . . . . . . . . . . . . . . . . . . ..
5.10.1 Pay out time . . .
-23-5.10.2 Return on investment . . .
-23-5.10.3 Internal rate of return . . .
-24-5.11 Literature . . . . . . . . . . . . . . . . . . . . ..
-24-6. Safety considerations . . .
-25-6.1 Cryogenic coolants . . . . . . . . . . . . . . . . . . . ..
6.2 High pressure . . .
-25-6.3 Steam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
-25-6.4 Corrosive process streams . . . . . . . . . . . . . . . . ..
-25-6.5 Hazardous substances . . . . . . . . . . . . . . . . . . . . . . . ..
6.6 Literature . . .
-26-7. Conclusions . . . . . . . . . . . . . . . . . . . . . . ..
-27-PART 11: AIF3 PRODUCTION 1 Introduction ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -28-2 Base of design . . . -30-2.1 Feed . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2.1.1 Aluminum hydroxide . . . -30-2.1.2 Hydrofluoric acid .... . . . . . . . . . . . . . . . . . . . . . . . .. -31-2.2 Product specifications . . . . . . . . . . . . . . . . . . . . . . . . . .. -32-2.3 Utilities . . . . . . . . . . . . . . . . . . . . . . . . .. -32-3 Process description . . . -34-3.1 Introduction . . . ". . . . ..
3.2 General process flow diagram . . .
-34-3.2.1 Make-up section . . . . . . . . . . . . . . . . . . . . .. -34-3.2.2 Distillation section . . . -34-3.2.3 Reactor section . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -35-3.2.4 Crystallization section . . . . . . . . . . . . . . . . . . . . . . . .. -35-3.2.5 Filtration section . . . 3.2.6 Dehydration section . . .
3.3 Overall mass balance . . .
3.4 Literature . . .
-36-4 Mixer design . . .
-37-4.1 Introduction . . . . . . . . . . . . . . . . . . ..
-37-4.3 Nomenclature
...
....
.
.
..
..
.
.
...
.
.
.
...
...
.
...
.
-42-4.4 Literature..
..
...
...
...
.
...
.
...
.
.
-42-5. Reactor design. . . . . . . . . . . . . . . . . . . . . . . . . .. -43-5.1 Introduction . . . . . . . . . . . . . . . . . . . . .. -43-5.2 Base of design . . . . . . . . . . . . . . . . . .. -43-5.3 Process discription .... . . . . . . . . . . .. -43-5.4 Equipment design. . . . . . . . . . . . . . . . . . . . .. -44-5.5 Nomenclature .. .... ... ... .. . . .. ... -45-~:? Literature ... .. ... .. ... .. . . " , ;. . . . ... -,15-6 Crystallizer design . . . -46-6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. -46-6.2 Basis of design . . . -46-6.2.1 Crystallizer feed . . . . . . . . . . . . . . . . . . ..-46-6.2.2 Material and thermal balances . . . . . . . . . . . . . . . . . . . . ..
-46-6.2.3 Crystallization kinetics . . . .. .... -50-6.3 Process description . . . -50-6.4 Crystallizer selection . . . . . . . . . . . . . . . . . . . . . . . .. 6.5 Equipment design . . . -51-6.6 Literature . . . . . . . . . . . . .. -52-6.7 Nomenclature .. ... . . -53-7 Filter design. . . . . . . . . . . . . . . . . . . .. -54-7.1 Introduction . . . . . . . . . . . . . . . . . . . . .. -54-7.2 Base of design . . . . . . . . . . . . . . . . . . . . . .. -55-7.2.1 Theory of filtration . . . . . . . . . . . . . . . . . . .. -55-7.3 Nomenclature . . . -59-7.4 Literature .. .. . . -60-8 Dryer design . . . . . . . . . . . . . . . . . . . . . . . . ., -61-8.1 Introduction . . . . . . . . . . . . . . . . . . .. -61-8.2 Base of design . . . . . . . . . . . . . . . . . . . . . . .. 8.3 Equipment design . . . -62-8.4 Nomenclature . . . -66-8.5 Literature ... .. . .
-67-9 Process con trol & operation . . . . . . . . . . . . . . . . . . . . . . . . . ..
-68-9.1 Plant start-up . . .
-68-9.2 Plant shut-down . . . . . . . . . . . . . . . . . . . . . ..
-68-10 Safety, Health and Environment . . .
10.1 Chemicals . . .
10.1.1 Hydrofluoric acid . . .
-69-10.1.2 Aluminum hydroxide & Aluminum fluoride . . . . . . . . . . . . ..
-69-10.2 HAZOP analysis . . . . . . . . . . . . . . . . . . . . . . . ..
-69-10.2.1 HAZOP analysis of the mixer . . . ...
-69-10.2.2 HAZOP analysis of the reactor . . . . . . . . . . . . . ..
-70-10.3 Conc1usions HAZOP analysis . . . . . . . . . . ..
-72-11 Economie evaluation . . .
11.1 Economie results . . .
-73-11.2 Literature . . . . . . . . . . . . . . . . . ..
-74-12 Conc1usions & Reeommondations . . . -75-APPENDICES PART I Column Caleulation: A Investment Caleulations: B Stream Tables C Equipment Lists D Specifieation Forms E Mass & Heat Balanee F
'. APPENDICES PART 11
Stream Tables G Equipment Lists H Specifieation Forms I Mass & Heat Balanee J
1. Introduction
As the starting point for most fluorine products and in volume of production, hydrogen fluoride is the most important manufactured compound of fluorine. Both anhydrous hydrogen fluorine and aqueous hydrogen fluoride are sold for use directly and as intermediates in making other fluorine-containing materiaIs, such as Freons®.
A process wastestream from the Du Pont de Nemours Freon-22® process containts 15 wt% hydrogen fluoride in water. Nowadays this process stream is collected and transported to Germany where the hydrogen fluoride is separated from the water. Du Pont de Nemours Dordrecht has a area near the existing plant to build a plant for processing this waste stream. A design for processing the waste steam to anhydrous HF is discussed in this report.
Anhydrous hydrogen fluoride is a colorless liquid or gas (b.p. 19.54 °C), completely water soluble and fuming strongly in contact with the atmosphere. The formula weight is 20.006, but hydrogen bonding between molecules produces extensive polymerization and the liquid and gas show large departures from ideal behavior. The compound can cause human injury in any form of contact and informed control of hazards in handling and use must be observed.
Hydrogen fluoride may have been made as early as 1670 when Schwankhard of Nürnberg etched glass with fluorspar and acid. Marggraf in 1764 showed that the etchant was a gas, Scheele showed that a new acid had been discovered, Ampère in 1810 and Davy in 1813 and 1814 proved that the now known as hydrogen fluoride contained a new element.
The earlier quite limited uses for HF, such as glass etching and polishing, foundry sc ale removal and minor production of metal fluorides, predominates until the 1930s. In the period 1935 to 1940, the large increase in aluminum production brought an equivalent requirement for HF, employed captively for aluminum refining. Anhydrous hydrogen was first made by Frémy in 1856, but there was no significant commercial manufacture until 1931 when the first bulk shipmant was made by Sterling Products company for use in making chlorofluorohydrocarbons as refrigerating fluids. Growth in the two mentioned industries brought the volume of HF production to a high level. Decrease in demand for HF is due to a greater efficiency in the aluminum industry and restrictions in using chlorofluorohydrocarbons as refrigerants.
This report starts with the base of design, where the boundary conditions are set that have to be satisfied. The next chapter gives a general process description and subsequently the processs, economie evaluation and safety considerations are discussed.
2. Base of design
In this chapter we discuss the base of design of the total process for the concentration of a 15 wt % HF wastestreám to its pure state. This is the basis from which the process is designed. The outline of the feed will be discussed as weU as the product specifications and the assumed availability of several utilities will be mentioned.
2.1 Feed
The feed is a hydrofluoric acid stream. A dilute aqueous hydrofluoric acid waste-stream containing 15 weight percent of the acid is concentrated to form pure hydrofluoric acid. The conditions and the composition of the aqueous hydrofluoric acid stream passed into the azeotrope distillation column T3 are:
Component Hydrogen fluoride 15 wt % Water 85 wt % Impurities None Temperature 25
oe
Pressure 1 bara Phase LiquidThe annual feed of hydrofluoric acid, having this concentration is 4380 tons.
2.2 Product specifications
The desired product of the concentration of the aqueous wastestream is pure HF.
2.3 Utilities
minimum purity
maximum water content
99.9 wt %
50 ppm by weight
Except for feeds some auxiliary equipment and flows will always be required. When necessary, it has been assumed that certain utilities are available on site. For the design these utilities are assumed to be present Grievink [1]:
a. Stearn
High Medium Low
pressure: pressure: pressure: Process 40 bar 410 °C 10 bar 220°C 3 bar 190 °C conditions:
Condensation 250°C 180 °C 133.5°C
temp.:
b. Electricity
All required voltages: 220 VAC; 380 V 'driefasendraaistroom'; 10.000 V 'driefasendraai-stroom voor grotere vermogens.'
"Process"-air excluded, available at 20°C, 7 bar and having a dewpoint of 40°C. d. Water
a. Drinking-water, pressure 7 bar. b. Demineralized-water, pressure 7 bar.
c. Cooling-water, pressure at mowing-field 3 bar (design inlet-temperature 20°C, maximum permitted outlet-temperature 40°C).
e. Air temperature
Design-temperature 25°C; Relative humidity 70 %; Maximum cooling with air has to be applied, if necessary aftercooling with water.
2.4 Literature
[1] J. Grievink, e.P. Luteijn and M.E.A.M. Thijs-Krijnen, 'Handleiding fabrieksvoor-ontwerp', TU Delft, (1994)
3. Process description
3.1 General process flow diagram
The concentration of an aqueous HF stream, as schematically given in Figure 3.1, consists of several sections. Each one of these sections will he discussed briefly.
Feed Water Azeotropic Distillation 15 wt%HF HCl recycle Extraction Column HF-HCl Separation Column Water Product: HF
Figure 3.1: Process flowchart
3.2 T3: the azeotropic column
The feed consists of hydrogen fluoride in a relative large amount of water. The desired pure hydrogen fluoride is 15 % by weight of this feed. The feed of 500 kg/h contains thus 75 kglh hydrogen fluoride and 425 kglh water.
The fITst step in the process is to remove a large amount of water from the proces and form a hydrogen fluoride-water mixture near its azeotropic composition. The azeotropic composition lies about 37.5 wt % HF. So in this column can approximately 300 kg/h water be removed, resulting in a remaining process stream of 200 kglh at near azeotropic composition.
The column is designed to contain a minimum loss of hydrogen fluoride. A design specification of 25 ppm HF in the waterstream from the top of the column means a total loss of hydrogen fluoride of 7.5 g/h.
3.3 T9: the extraction column
The feed to this column is a mixture of hydrogen and water near its azeotropic compositi-on. In this column is hydrogen chloride used to bring the mixture over its azeotropic composition by means of extraction. The amount of hydrogen chloride pumped through the system is 150 kglh; according to Hanson [5].
The column is designed for a mixture of HFIHCI with little to no water coming from the top and water with minor impurities coming from the bottom of the column.
3.4 T25: the separation column
In this column the extractive compount Hel is separated from the product HF. The main difference in operating this column in respect to the other two column is that this column is simulated under a pressure of 20 bar in stead of atmospheric circumstances. The main reason for operating at a pressure of 20 bar are the boiling points of both Hel and HF. The column is designed so that the bottomproduct HF at a temperature of 200
e does not contain more than 50 ppm impurities and Hel corning over the top at a temperature of
O°c.
3.5 Overall mass balance
The overall mass balance is schematically given in Figure 3.2 and the feed and product streams are quantatively given in Table 3.1.
Feed 15 wt %HF e-up Mak
He
1-.
.
Water fr om TlPROCESS
omT2 Water fr Product: HFFigure 3.2: Process streams
Table 3.1: Overall mass balance
I
IN & OUTI
HF in kg/hI
HeL in kg/h1 Feed 1 75 1 Make-up Hel 33 Product 67 Water from Tl Water from T2 8 33
3.6 Simulating columns in Aspen Plus
I
WATER in kg/h1
425
300 125
The binairy system hydrogen fluoride/water has an azeotrope at approximately 37.5 wt%
I
1HF. An azeotrope can cause serious problems when simulating towers in Aspen Plus. Aspen Plus separates purelyon differences in boiling points and therefQre systems with an azeotrope will not be properly simulated unless you supply Aspen Plus with experimental data. There are Data Packages available for some electrolytes. These inserts are stored in the Aspen Plus System insert library and they provide the complete list of electrolyte systems. Public library data were used in developing these data packages [6].
The data package MHP provides the solution chernistry and physical property model
parameters for a system containing water and hydrogen fluoride. The key physical property model is the Electrolyte NRTL model. The data sources are obtained from Munter et al. [1], Wagman et al. [2J, Brosheer et al. [3].
The data package MHCI provides the solution chemistry and the physical property model parameters for a system containing water and hydrogen chloride. The key physical property model is the Electrolyte NRTL model. The data sources are obtained from Wagman et al. [2J, Perry et al. [4J.
In literature no information could be found concerning the interactions between HF and Hel.
3.7 Literature
[1] Munter, P.A. et al., 'Industrial and Engineering Chemistry', 39(3), p. 427, 1947 [2] Wagman, D.D. et al., 'J. Phys. Chem. Re! Data', 11, suppl. 2, 1982
[3] Brosheer,
J.c.
et al., 'lndustrial and Chemical Engineering', 39(3), p. 427, 1947 [4] Perry, R.H. et al., 'Chemical Engineer's Handbook', 6th ed., McGraw-Hill, 1984 [5] Hanson, O.M. et al., 'Method for concentrating hydrogen floride', Unites StatesPatent Office, 2568889 (1951)
[6] ASPEN PLUS Electrolytes Manual, Edition August 1988, Aspen Technology Inc.,
4. Separation columns
4.1Introduction
This chapter covers the design of separation columns. Distallation is probably the most widely used separation process in the chemical and allied industries. The design of a separation column can be divided into the following steps:
1. Specify the degree of separation required: set product specifications. 2. Select the operation conditions: batch or continuous; operating pressure. 3. Select the type of contacting devices: plates or packing.
4. Determine the stage and reflux requirements: the number of equilibrium stages. 5. Size the column: diameter, number of real stages.
6. Design the column internals: plates, distributors, packing supports. 7. Mechanical design: vessel and intemal fittings.
4.2 Process description
The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. Vapour flows up the column and liquid counter-currently down the column. The vapour and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is retumed to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporised in the reboiler and retumed to provide the vapour flow. In the section below the feed, the more volatile components are stripped from the liquid and this is known as the stripping section. Above the feed, the concentration of the more volatile components is increased and this is called the rectifying section.
4.3 Design of T3: the azeotropic column
4.3.1 Base of design
The design of T3 will be taken as example for the design of separation columns and calculations are elaborated in Appendix A. The general concept for designing a column in section 4.1 is used to design the azeotropic column.
Water is to be removed from the aqueous HF-waste stream by continuous distillation. The feed contains 15 per cent w/w hydrogen fluoride. Hydrogen floride at its azeotropic composition is wanted, and the aqueous effluent must not contain more than approximately 25 ppm hydrogen fluoride. The feed to the column will first be heated up to its boiling point. There is no point in operating this column at other than atmospheric pressure.
The composition and the conditions of the HF feed are: Purity Temperature Pressure 15.00 wt % 25°C 1 bara
The desired product of this azeotropic distillation process is constant boiling HF and has the following composition and conditions:
Purity Temperature Pressure 37-38 wt % 112.4 °C 1 bara
The data package MHF is used, which provides the solution chernistry and physical property model parameters for a system containing water and hydrogen fluoride. The key physical property model is the Electrolyte NRTL model. The data sources are obtained from Munter et al. [2], Wagman et al. [3J, Brosheer et al. [4].
Munter et al. [2,3] reported the vapour-liquid equilibrium of the binary system HF-H20 at atmospheric pressure over a considerable range of composition. The boiling points and
vapour~liquid compositions of the binary system HF-H20 deterrnined by them are now utilized as the standard for hydrofluoric acid. From these boiling points can be conc1uded that the top temperature is restricted by the maximum 10ss of HF in the vapour stream. The bottom temperature is that of a constant boiling liquid 112.4
0c.
In Figure 3.3 is T3 schematically given with the temperatures and contents of the one inlet and two outlet streams.
1 + 2 0< xf < xaz T=277K 2 T=373 K (1) HF (2) H20 azeotrope T= 385 K
4.3.2 The number of plates at total reflux. Frenske's method
A shortcut distillation is normally used for retrieving the number of plates, however a shortcut distillation in Aspen Plus cao't separate ao azeotrope. There are several methods for estimating the number of plates. Frenske's method is used in this paragraph.
For conditions in which the relative volatility is constant, Frenske derived ao equation for calculating the required number of plates for a desired separation.
In most cases total condensation occurs in the condensor, so that
(:; U
::
1.
n + 1 = > "
-log
(1)
aod n is the required number of theoretical plates in the column.
An average value of a. is found by taking a temperature based mediao value.
4.3.3 Approximate column sizing
An approximate estimate of the overall column size cao be made once the number of real stages required for the separation is known.
The overall height of the column will depend on the plate spacing. Plate spacings from 0.15 to 1 m are normally used. Close spacing is used with small diameter columns, and where head room is restricted. For columns above 1 m diameter, plate spacings of 0.3 to 0.6 m will normally be used. An initial estimate of 0.4 m is chosen for this azeotropic column.
The principal factor that determines the column diameter is the vapour flow-rate. The vapour velocity must be below that which would cause excessive liquid entrainment or a high-pressure drop. The equation given below cao be used to estimate the maximum allowable superficial vapour velocity , and hence the column area aod diameter.
P2
- pUv = (-OJ. 71{ + 027{ - 0.047) 1 v
Pv
(2)
The column diameter cao be calculated using eq. 4:
~
vW'
D a
=
-TT Pv Uv
4.3.4 Plate contactors
Cross-flow plates are the most common type of plate contacters used in distillation and absorption columns. In a cross-flow plate the liquid flows across the plate and the vapour up through the plate. The flowing liquid is transferred from plate to plate through vertical channels called downcorners. A pool of liquid is retained on the plate by an outlet weir. Sieve plates are the cheapest and are satisfactory for most applications and therefore choosen.
4.3.5 Plate hydraulic design
The base requirement of a plate contacting stage are that it should: Provide good vapour-liquid contact
Provide sufficient liquid hold-up for good mass transfer (high efficiency)
Have sufficient area and spacing to keep the entrainment and pressure drop acceptable Have sufficient downcorner area for the liquid to flow freely from plate to plate
Satisfactory operation will only be achieved over a limited range of vapour and liquid flow rates. The upper limit to vapour flow is set by the condition of flooding. At flooding there is a sharp drop in plate efficiency and increase in pressure drop. Flooding is caused by either to excessive carry over of liquid to the next plate by entrainment, or by liquid backing up in the downcorners. The lower limit is set by the condition of weeping. Weeping occurs when the vapour flow is insufficient to maintain a level of liquid on the plate. Coning occurs at low liquid rates, and is the term given to the condition where the vapour pushes out the liquid back from the holes and jets upward, with poor liquid contact.
4.3.6 Plate design procedure
A trail-and-error approach is necessary in plate design. A typical procedure is set out below.
1. Calculate the maximum and minimum vapour and liquid flow-rates. 2. Collect, or estimate, the system physical properties.
3. Select a trail plate spacing.
4. Estimate the column diameter, based on flooding considerations.
5. Decide the liquid flow arrangement.
6. Make a plate layout: downcorner area, active area, hole area, hole size, weir hieght.
7. Check the weeping rate, if unsatisfactory return to step 6.
8. Check the plate pressure drop, if too high return to step 6.
9. Check the downcorner back-up, if too high return to step 6 or 3.
unsatisfactory return to step 6.
11. Recalculate the percentage flooding based on chosen column diameter. 12. Check entrainment, if too high return to step 4.
4.3.7 I>ianmeter
The flooding condition fixes the upper limit of vapour velocity . A high vapour velocity is needed for high plate efficiencies, and the velocity will normally be between 70 to 90 per cent of that which would cause flooding. For design, a value of 80 to 85 per cent of the flooding velocity should be used. The flooding velocity can be estimated from the correlation given by Fair (1961):
(4)
'. Kl can he obtained from Figure 11.27 Couldson & Richardson [1]. The liquid-vapour flow
factor FIv in Figure 11.27 is given by:
F = Lw
~v
Iv V P
w I
(S)
For liquids with an other surface tension than 0.02 Nim, multiply Kl with a factor:
K =
[ al°.2
- - *Klp>< 0.02 0.0 2 1 (6)
To calculate the column diameter an estimate of the net area An is required. As a first trail take the downcomer area as 12 per cent of the total, and assume that the hole: active area is lOper cent.
4.3.8 Entrainment
Entrainment can be estimated from the correlation given in Figure 11.29 Couldson &
Richardson [1], which gives the fractional entrainrnent '" as a function of the vapour-liquid
factor FIv, with the percentage approach to flooding as a parameter. The percentage flooding is given by:
Un
percentflooding--Ut
4.3.9 Weep point
The lower limit of the operating range occurs when liquid leakage through the plate holes becomes excessive. This is known as the weep point. The vapour velocity at the weep point is the minimum value for operation. The hole area must be chosen so that the lowest operating rate the vapour flow velocity is still above the weep point.
The minimum design vapour velocity is given by:
u h
=
[~ - 0.90 (25.4 - dh )].fPv
(l.)The constant K2 is dependent on the depth of c1ear liquid on the plate, obtained from Figure 11.30 Couldson & Richardson [1]. The clear liquid depth is equal to the height of the weir hw plus the depth of the crest of liquid over the weir how. The height of the liquid crest over the weir can be estimated using:
h =
750[~11
ow Pl{..
(2)
The height of the weir determines the volume of liquid on the plate and is an important factor in determining the plate efficiency. For columns operating at atmospheric pressure the weir heights will be between 40 to 90 mmo With segmental downcorners the length of the weir fixes the area of the downcomer. The chord length will normally be between 0.6 to 0.85 of the column diameter. The relationship between weir length and downcomer area is given in Figure 11.31 Couldson & Richardson [1].
4.3.10 Plate details
The area available for perforation will be reduced by the obstruction caused by structural members, and by the use of calming zones. The width of the calming zone is below 1.5 m diameter 75 mm, and above 100 mmo The width of the support ring is 50 to 75 mmo The unperforated area can be calculated from the plate geometry. The lationship between the weir chord length, chord leng th and the angle subtended by the chord is given in Fig. 11.32 Couldson & Richardson [1].
The hole sizes used vary from 2.5 to 12 mmo The hole pitch lp should not be less than 2 times the hole diameter, and the normal range will be 2.5 to 4 diameters. Square and equilateral triangular patterns are used. The total hole area as a fraction of the perforated area Ap is given by the following expres sion, for an equilateral triangular pitch:
The equation is plotted in Figure 11.33 Couldson & Richardson [1].
4.3.11 Plate pressure drop
The pressure drop over the plates is an important design consideration. There are two main sourees of pressure loss: that due to vapour flow through the holes and that due to the statie head of liquid on the plate.
A simple adaptive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hd); the head of c1ear liquid on the plate (hw + how); and a term to account for other residual losses hr. The total pressure drop becomes:
APt = 9.81xlO-3 ht PI (4)
The pressure drop through a dry plate can be estimated by:
(S)
Residual head can be estimated using:
125xl()3 hr ==
-PL (6)
The total pressure drop is given by:
h t == hd + (h w + h ) ow + h r (7)
4.3.12 Downcomer design
The downcomer area and plate spacing must be such that the level of the liquid and froth in the downcomer is well below the top of the outlet we ir on the plate above. In terms of c1ear liquid the downcorner back-up is given by:
(8)
The head loss in the downcomer can be estimated using:
h ==
166[
LWd ]2
de P A
L m
The clearance area under the downcomer is given by:
Aap = hap Zw (10)
where ~p is the height of the bottom edge of the apron above the plate. The height is normally set at 5 to 10 mm below the outlet weir height.
To predict the height of 'aerated' liquid on the plate, and the height of froth in the downco-mer, some means of estimating the froth density is required. This will be between 0.4 to 0.7 times that of the clear liquid. A criterium is set for safe design:
(11)
Sufficient residence time must he allowed in the downcomer for the entrained vapour to disengage from the liquid stream. A time of at least 3 seconds is recommended. The downcomer residence time is given by:
4.4 Results of the design of the separation columns
Simulation and calculation leads to:Tl: the azeotropic column Number of stages Pressure Top Temperature Bottom Temperature 24 1 bar 372.8 K 384.9 K
T2: the extraction column Numher of stages Pressure Top Temperature Bottom Temperature 12 1 bar 293.1 K 381.1 K Column Diameter Column Height Condenser Duty Reboiler Duty Column Diameter Column Height Condenser Duty Reboiler Duty 0.5 m 9.6 m -69 kW 259 kW 0.4 m 4.8 m -85 kW 100 kW (12)
T3: the distillation column Number of stages Pressure Top Temperature Bottom Temperature 12 20 bar ·273.2 K 402.9 K Column Diameter Column Height Condenser Duty Reboiler Duty 0.1 m 4.8 m -5 kW 3.5 kW Elaborate calculations for the three columns can be read in Appendix A.
4.5 Literature
[1] lM. Couldson and J.F. Richardson, 'Chemical engineering', Vol. 6, 6th edition, Pergamon Press, New York, (1983)
[2] Munter, P.A. et al., 'Industrial and Engineering Chemistry', 39(3), p. 427, 1947 [3] Wagman, D.D. et al., 'J. Phys. Chem. Re! Data', 11, suppl. 2, 1982
4.6 Nomenclature
Aa
Active area of plate m2Aap Clearance area under apron m2
~ Total column cross-sectional area m2
Ad Downcomer cross-sectional area m2
Ah Total hole area m2
~ Net area m2
~ Perforated area m2
Co Orifice coefficient
De Column diameter m
dh Hole diameter m
FLV Column liquid-vapour factor
hap Apron clearance m
hb Height of liquid back-up in downcomer m
hbc Downcomer back-up in terms of clear liquid head m
hd Dry plate pressure drop, head of liquid m
hde Head loss in downcomer m
how Height of liquid crest over downcomer weir m
hr Plate residual pressure drop, head of liquid m
ht Total plate pressure drop, head of liquid m
hw Weir height m
KI Constant in eq. 6
K 2 Constant in eq. 8
Lw Liquid mass flow rate kgls
LWd Liquid mass flow rate through downcomer kgls
lh Weir cord height m
lt Plate spacing m
lw Weir length m
n Number of stages
P Total pressure Pa
~ Residence time in downcomer s
Ua Vapour velocity based on active area mis
uf Vapour velocity at flooding point mis
Uh Vapour velocity through holes mis
un Vapour velocity based on net cross-sectional area mis
u min
v Minimum vapour velocity mis
Vw Vapour mass flow rate kgls
x Mol fraction of component in liquid fase
XI Mol fraction of HF in liquid fase
x2 Mol fraction of water in liquid fase
y Mol fraction of component in vapour fase
YI Mol fraction of HF in vapour fase
Y2 Mol fraction of water in vapour fase
Clav Average relative volatility
Density vapour fase Density liquid fase Surface tension
5. Cost engineering
5.1 Introduction
An essential part of designing a process is the economic evaluation. The total operating cost is subdivided in general operating costs and manufactoring costs. The manufacturing cost of a plant can be estimated using:
C =C +C t g m =C g +C +C 0 IJ +c.+C I S
with Ct = total costs Cg
=
general costsCm = manufacturing costs Co = plant overhead
Cv
=
volume dependent costs Cs = semi-variable costs Ci = investment costs(20)
In the early stages of a design, all operating costs, exept the volume dependent costs, are related to the investment and wages costs. The model can then be simplified to:
C t = a C IJ + t I + d W
with W
=
wages costsI
=
investment a = 1.13f = 0.13 d = 2.6
(21)
The values of a, f and d are based on the so-called 'best model' and have been compiled from various sources collected by Montfort [1]. This model does not take into account the capital charge and depreciation. Therefore, capital charge and depreciation are estimated sepeartely.
5.2 Investment costs
For calculation of the manufacturing costs the total investments are needed; for this Taylor's method and Zevnik-Buchanan's method (also called: Du Pont method) are used. Both methods calculate the manufacturing investments which are 64 % of the total investments.
5.2.1 General aspects investments
Taylor's method calculates the investments as a function of the capacity, the costliness index and the Cl index for process blocks. The costliness index is proportional to the complexity of the process block, e.g. temperature, pressure, materials of construction, reaction time, storage time, explosion risk and toxicity .
Process blocks are the equipment as well as inlet, recycle and outlet streams. In Taylor's method small equipment like heat exchangers and flash vessels are combined to one large unit.
Unlike Taylor, Zevnik-Buchanan uses process steps. The investments are calculated from throughput, applied materials and process conditions.
5.2.2 Formulas used for the investments
Taylor's method
'. The manufacturing investments Ci ($) are:
c
C. = 93f
* P
0.39*
_ II 300
with f the costliness index
P the production capacity in kT/y Cl PE index (UK)
Si total process step score Zevnik-Buchanan's method
The following procedure is used to estimate the investment costs:
(22)
1. Estimate the maximum temperature and read the temperature factor from Fig. 111-16 in Montfoort [1].
2. Estimate the maximum pressure and read the pressure factor from Fig. ill-I7 in Montfoort [1].
3. Estimate the material factor from Fig. Iil-I8 in Montfoort [1]. 4. Calculate the complexity factor:
C = 2
*
10 (Ft + Fp + Fm)f (23)
5. For productioncapacities below 4500 tonly is the degressionfactor: m
=
0.5 taken. 6. Calculate the investments:N
C; = 0.1
*
L
(CJi • p;m )*
Cl;=1
with Cl Chemical engineering plant cost index N Number of functional units
P Capacity RESULTS
(24)
The intermediary results are given in Appendix B. The investment costs by Taylor are 1.61 Mil The investment costs by Zevnik-Buchanan are 1.65 Mil
5.3 Production volume dependent operating costs
Table S.l: Volume dependent costsMaterial Consumption Price (fl) Costs (kfl)
Utility per year per year
Feed: 15 wt% HF 4.38 kton 0 0
HCI 0.29 kton 0 0
Electricity 210000 kWh 0.13 kWh-I
27.3 LP steam 5.3 kton 30 ton-1
159 Total costs of materials and utilities per year 186.3
The prices are taken from Grievink [2]. There are no costs for coolants and Hel, because there are copious quantities of coolants and HCI at Du Pont Dordrecht available. The production volume dependent cost is obtained by multiplying with a factor 1.13 resulting in 210.5 kfl annually.
5.4 Fixed capital dependent operating costs
The fixed capital dependent operating costs are calculated by multiplying the investments, as discussed in section 5.1, by a factor f
=
0.13. The costs inc1ude insurance and advanced profit.5.5 Wages dependent operating costs
These costs can he calculated using the Wessel-relation Montfoort [1]. The following
equation can be derived from this relation:
where
L = 32
*
N*
C 0.24L
=
total sum of wages in kflIyear N=
number of stepsC = capacity in ktonlyear
(25)
Eq. 25 is valid for a continuous proces in 1986 for 350 kfl per employee. We assume that an employee costs fl 400000,- per year, based on a 24 hours per day job. In 1986, the factor k was 1.7 for continuous processes. Due to a productivity increase of 6 percent per year, the factor k is 0.97 in 1995.
These two changes are taken into account by estimating the total sumo
Deciding on the number of process steps is a matter of experience. We assume that this process consists of 3 steps. Here, the assumption is made that every major piece of process equipment represents a process step. With a capacity of 0.584 kton HF/year, this results in 55 kfllyear. The total wages dependent costs are calculated by multiplying the wages by the factor d = 2.6, which results in 143 kflIyear.
5.6 Total costs
As calculated in section 5.4 the fixed capital cost are 214.5 kfl per year, the wages dependent costs in section 5.5 are 143 kfl per year and according to section 5.3 the production dependent operating costs are 186.3 kfl per year. These costs add up to the total operating cost per year, according to eq. 21. The total operating costs are 543.8 kfl per year.
5.7 Depreciation and capital charge
A linear model is used for calculation of the depreciation. This means that the depreciation rate is constant. This is a simplification of reality but is considered to be sufficient.
where
I-I
D = ___ W D=
depreciation I=
investment nIw
=
work capital (10% of the investment) n = project life in years (usually 10 year)The depreciation then comes to 148.5 kfl annually.
The linear depreciation model does not take in account that the capital charge is relative high in the first years and low in the last years with respect to the average over the whole depreciation period. This can be corrected by assuming a charge rate of 8% over a depreciation period of ten years. The charge costs are based on 60% of the total fixed capital:
Charge costs = 0.6 . charge rate . I (27)
The capital charge then is 79.2 kfl per year.
5.8 Benefits
The selling prices, arnount of product and the benefits are given in Table 5.2: Table 5.2: Benefits
Product Production Price Benefits
ton per year kfl per ton kfl per year
I
HFI
584I
2.34I
1367I
I
Total benefitsI
1367I
The Price of hydrogen fluoride are retrieved from the Chemical market reporter [3].
5.9 Profits, taxes and cash flow
Benefits
- total operation costs - depreciation - capital charge Gross pront - taxes (50%) Net pront + depreciation Cash flow
5.10 Profitability analysis
5.10.1 Pay out time
1367 - 543.8 - 148.5 - 79.2 595.5 - 297.8 297.7 + 148.5 kfl 446.2 kf1
The pay out time (POT) is the minimum number of years required to earn the initial investment minus the work capital back. The calculations are based on the exploration carry-over, Eo, which can be found by reducing the benefits with the production volume dependent operating cost. General costs like insurance, interest and taxes are not taken into account. where I - I POT = _ _ w Eo = Benefits - (a . Cv + d . W) I
=
total investment Iw=
work capitalFor this process the pay out time is estimated at 1.5 years.
5.10.2 Return on investment
(28)
total investment, multiplied by 100%.
P
ROl = - ·100%
I (29)
where P
=
net profit I = total investmentThe major disadvantage of this method is the fact that inflation and the variance of the benefits and the costs are neglected.
For this process the return on investment is 27 percent.
5.10.3 Internal rate of return
Discounted cash-flow analysis, used to calculate the present worth of future eamings, is sensitive to the interest rate assumed. By calculating the NPW, Net Present Worth, for various interest rates, it is possible to find an interest rate at which the cumulative net present worth at the end of the project is zero. This particular rate is called the internal rate of return and is a measure of the maximum rate that the project could pay and still break even by the end of the project life.
L
n = 0
NFW
- - - = 0
(1 + IRR)n
where IRR = internal rate of return
NFW = the future worth of the net cash flow in year n t
=
the life of the project in years(30)
The project has a lifetime of 10 years. For this process a internal rate of return is found at 14 %.
5.11 Literature
[1] A.G. Montfoort, 'De chemische fabriek, Deel Il.·Cost engineering en economische
aspecten', TU Delft (1991)
[2] J. Grievink, C.P. Luteijn and M.E.A.M. Thijs-Krijnen, 'Handleiding
fabrieksvoor-ontwerp', TU Delft, (1994)
6. Safety considerations
In thls chapter some safetyaspects of the proces will he discussed. Safety is an important point of attention, especially when there are some dangerous components in the system. When discussing safety, both considerations with respect to process equipment and considerations with respect to the hazard potentials of components have to be included.
6.1 Cryogenic coolants
The condensors of the towers T9 and T25 can't be cooled with water, so refrigerants are used, which are being made just around the corner. Working with refrigerants embodies the danger of 'cold burns'. Severe cold burns may be inflicted if the human body comes in contact with cryogenic fluids or with surfaces cooled with cryogenic fluids. Damage to the skin or tissue is similar to that of ordinary burns. Because the hu man tissue mainly consists of water, low temperatures cause to freeze the tissue thereby damaging or destroying it. To prevent burning by cryogenic eoolants, process equipment should be isolated very good, which is of course also desired for economie reasons.
6.2 High pressure
The high pressured gases are hazardous because of the stored energy. When confined gas is suddenly released through a rupture or break in a line a significant burst is to be expected.
6.3 Steam
The use of steam is often more safe than the use of other heat transfer fluids, because steam is not flarnmable and not toxic. However, there are dangers with the use of steam. Static electicity may build up where steam leaks occur and sufficient isolation has to be carried out to prevent burning of skin and human tissue.
6.4 Corrosive process streams
In thls process there are very corrosive substances present. Hydrogen chloride and hydrogen fluoride are already severely corrosive with even a small amount of water. The hole process is therefore very corrosive and over the hole plant the acids in the several units can cause severe burnings. One of the safety measures to be taken is water rinsing facilities around the units. In order to prevent hazards downstream in case of a break-through of hydrogen fluoride or hydrogen chloride, an additional tower with caustic solution will be required in order to neutralise these acids.
6.5 Hazardous substances
Hydrogen fluoride and hydrogen chloride are both substances to be handled with care. In Table 6.1 are the hazards and other data concerning safety presented Chemiekaarten [1].
Table 6.1: Data on toxity, flammability etc. of components
I
DataI
Hydrogen FluorideI
Hydrogen ChlorideI
MAC value 3 ppm 2.5 mglm3 5 ppm 7 mg/m3Flammability inflammabIe inflammabie
Hazards with air Exposure to air gives corrosive clouds of acids, which spread over the ground because they are heavier than air. Hazards with metals Reacts with many metals forming a highly flammabie gas:
hydrogen.
Hazards with 'other' Etching of glass and other Reacts vigirously with silicium containing materi- oxidants, forrning a toxic
als gas: chlorine
Hazards with water highly corrosive highly corrosive Toxity Corrosive to the eyes, skin Corrosive to the eyes, skin
and the resperatory system. and resperatory system. Breathing in HF cause Breathing in HCI cause suffication, which can be suffication.
lethal in severe cases.
6.6 Literature
[1] 'Chemiekaarten: Gegevens over veilig werken', 9th edition, Samsonffjeenk
7. Conclusions
The proposed process in Part I shows commercial potential. The total investment costs are 1.65 Mfl. The Pay Out Time is 1.5 years, the Return On Investment is 27% and the Internal Rate of Return is 14%.
The process could even be more valuable if the bottomstream of T9 could be processed in such a way that the last 8 kglh HF could also be recovered, however this is the major problem of the simulation. The loss of HF simulated in Aspen Plus couldn't be made smaller; more extractant caused excessive water over the top; Sulfuric acid as extractant caused the same problems; the stream further processed in another column didn't establish any separation whatsoever. The bottomstream of T9 is apparently a constant boiling mixture at its azeotropic composition. No literature could be found verifying this assumpti-on.
The loss of Hel is of minor importance, because Du Pont de Nemours dumps Hel anyway.
The Hel, used as extractant, has some impurities which are not taken into account, because the simulation crashed using them. Presurredrop over the column is also not taken into account, because Aspen Plus had difficulties enough.
1 Introduction
Chemical and medicinal usage of aluminous materials goes as far back as the Greek and Roman civilizations. The earliest reference to an aluminum compound dates back to the fifth century B.C. when Herodotus mentioned "alum". Materials with an astringent taste were named "alumen" by the Romans; their use as a mordant was described by Pliny around A.D. 80. The discovery of the aluminumrich ore bauxite, A1203·n H20, in 1821 provided a convenient source of aluminum for the preparation of alum, which was the principal aluminum compound of commercial value at that time [1].
Nowadays aluminum metal has a commercial value because it's not only used for airplanes but also for carparts and entirely whole cars. So more aluminum is needed for all these applications and probably in the future more aluminum will be used in other common things such as chairs, tables, etc.
Anhydrous aluminum fluoride, AlF3, is used in considerable amounts in the production of aluminum by the Bayer process, in which a bauxite-cryolite melt is electrolyzed. The fluoride is applied for adjusting the physical and electrolytic properties of the melt. Minor amounts are used in ceramics, in glasses, glazes, welding rod coatings, and as catalyst.
Cryolite, Na3AIF6 (sodium hexafluoroaluminate), is a relatively rare mineral and
deposits of commercial importance have so far found only in the south of Greenland at Ivigtut. As cryolite deposits were practically exhausted by 1962, and are today only a very minor contribution to the world's need for cryolite, nearly all the cryolite consumed is synthetic, which is often deficient in sodium fluoride. The published price range for synthetic cryolite in April 1979 was $562-606/t, approximately twice the 1965 price of $287/t.
As synthetic cryolite is a bulk chemical for the aluminum process, it isn't profitable to synthesize cryolite from the small aqueous wastestream of hydrogen fluoride leaving the Du Pont de Nemours Freon-22® process.
Synthesizing aluminum fluoride is more profitabIe because it's a make-up ingredient in the molten electrolyte of the aluminum reduction cell and in the electrolytic process for refining aluminum. The 1979 price for aluminum fluoride, anhydrous technical grade, in bulk quantities, was 38.6 i/kg.
In a typical wet process, aluminum hydroxide, AI(OH)3' is added to an aqueous hydrofluoric acid solution containing 15 weight percent of the acid. Solution of aluminum hydroxide and formation of crystalline AIF3.3 H20 takes place in continuous-flow crystallizers with the slurry held for about 3 hours at about 90°C with continuous agitation [2]. In a
similar process, employing continuous reaction of aluminum hydrate and acid, hydrofluoric acid of higher concentration from 40 to 60 weight percent of acid is used [3]. In the process, an average crystallization time of 15 to 30 minutes is employed. Generally, a temperature in the range of 90°C to 100°C, is preferred. The precipitated aluminum fluoride is separated by filtration, washed, and dried is calcined in rotating horizontal kilns at high temperatures
to remove the water of hydration. For economie operation loss of aluminum fluoride to the filter cake must be low.
Production of aluminum hydroxide is an intermediate stage of alumina production. Thus, the most important souree of aluminum hydroxides is the bauxite refining plant. More than 94 % of world alumina production is accounted for by the Bayer process for bauxite refining. The Bayer process, because of its economics, became a convenient and inexpensive supplier of pure aluminum hydroxide for the chemical industry in addition to its main function of producing metallurgical alumina for the aluminum industry. Today, this situation remains unchanged.
The objective is to report on the results obtained from a study we have done on a process for the conversion of a dilute aqueous hydrogen fluoride containing wastestream to aluminum fluoride. Based on a technical design and an economie evaluation, conc1usions will be drawn on the feasibility of such a process.
This report starts with the base of design, where the boundary conditions are set that have to be satisfied (Chapter 2). Chapter 3 gives a general process description of the process configuration as we recommended it. In the following chapters each specific section the process is comprised of, will be dealt with in more detail, discussing operating conditions and equipment design (Chapter 4 to 8). In Chapter 9 the overall process control is dealt with. A Safety, Health, and Environment (SHE) analysis is presented in Chapter 10. The economie evaluation of the proposed design is presented in Chapter 11. The report ends with conc1usions and recommendations (Chapter 12).
C. Misra, Industrial Alumina ChemicaIs, Washington, DC: ACS Monograph (1986), p. 184 2 1.R. Call aham , Chem. Met. Eng. 53 (3), 94 (1945)
2 Base of design
In this chapter we'll discuss the base of design of the total alurninum fluoride production process. This is the basis from which the process is designed. The outlines of the feed wiU be discussed as weU as the product specifications. The assumed availability of several utilities will be mentioned.
There are two process options for the production of aluminum fluoride:
• A wet process. Concentrated hydrofluoric acid, containing from 40 to 60 weight percent of hydrogen fluoride, and a slurry of aluminum hydroxide are continuously charged into a reactor, where the alurninum hydroxide and the hydrofluoric acid are reacted to form aluminum fluoride. The formation of crystalline AIF3·3 H20 takes place in a continuous flow crystallizer. An average crystallization time of 15 to 30 minutes is employed. Generally , a temperature in the range 90°C to 100°C, is preferred.
• A similar process. Aluminum hydroxide is added to an aqueous hydrofluoric acid solution containing 15 weight percent of the acid into a slurry tank. In the crystallizer, the mixture is maintained at a temperature of approximately 100°C for 2Y2 hours. The aluminum fluoride thus recovered represents about 32 percent yield. In using a 3Y2 hour crystallization time, approximately 55 percent is recovered.
Both options are feasible but only the first one will he discussed.
2.1 Feed
By continuously charging an aluminum hydroxide slurry and hydrofluoric acid, containing about 38 weight percent of hydrogen fluoride into a reactor a continuous process for the production of anhydrous aluminum fluoride is provided.
The supply-rate of hydrofluoric acid is fixed at 4.8 tons/day. The proposed process is continuous and runs 365 days/year.
2.1.1 Aluminum hydroxide
As mentioned in Chapter 1, the bauxite refining plant is the main producer of aluminum hydroxides. The mined bauxite, which consists mainly of A1203·n H20 (n=I-3), is the feed for the refining plant. Aluminum hydroxide, iron oxide and hydroxide, titanium oxide, silicon oxide, and alurninosilicate minerals are the major components of all bauxites. Two principal types of bauxite ores, gibbsitic and boehmitic, are of primary interest on today's commercial processes because they contain 30-60 % A1203•
In the bauxite refining plant all the impurities, which are various metal oxides as mentioned above, are removed and pure aluminum hydroxides are produced. One of the most weU-defined crystalline form is the aluminum hydroxide, gibbsite.
conditions of the aluminum hydroxide, gibbsite, feed are: Purity Temperature Pressure Storage Phase Particle size 100.00 % 298.15 K 1.00 bara Cone-bottom storage bins Solid 0.5 - 200 ~
The annual feed of aluminum hydroxide is 854 tons.
We assume that on the plant site no bauxite refining plant is present. The required aluminum hydroxide has to be delivered by commercial suppliers.
2.1.2 Hydrofluoric acid
The second feed is a hydrofluoric acid stream. Referring to Part 1, a dilute aqueous hydrofluoric acid waste-stream containing 15 weight percent of the acid is concentrated to form aqueous hydrofluoric acid azeotrope containing about 38 weight percent hydrogen fluoride and this may then be contacted with an aluminum hydroxide slurry to form the aluminum fluoride solution. The conditions and the composition of the aqueous hydrofluoric acid stream passed into the azeotrope distillation column are:
Component Hydrogen fluoride 15 wt % Water 85 wt % Impurities None Temperature 25°C Pressure 1 bara Phase Liquid
2.2 Product specifications
The desired product of the developed process is anhydrous aluminum fluoride. The specifications of the produced aluminum fluoride have to be the same as for the commercial available aluminum fluoride. Aluminum fluoride has the following sales specifications:
Maximum purity, typical Free alurnina as Al203, typical Maximum silica as Si02 Maximum iron as Fe203 Maximum sulfur as S02 Storage Phase Partic1e size
2.3 Utilities
90.00 wt % 9.5 wt % 0.1 wt % 0.1 wt % 0.3 wt % paper bags c10sed by end-sewmg Solid 1-2 ~Except for feeds some auxiliary equipment and flows will always be required. When necessary, it has been assumed that certain utilities are available on site. For the design these utilities are assumed to be present:
a. Steam
High Medium Low
pressure: pressure: pressure:
Process 40 bar 410 °C 10 bar 220°C 3 bar 190 °C
conditions:
Condensation 250.33°C 179.88°C 133.54
oe
temp.:
b. Electricity
"Process"-air exc1uded, available at 20°C, 7 bar and having a dewpoint of 40°C. d. Water
a. Drinking-water, pressure 7 bar. b. Dernineralized-water, pressure 7 bar.
c. Cooling-water, pressure at mowing-field 3 bar (design inlet-temperature 20°C, maximum perrnitted outlet-temperature 40°C).
e. Air temperature
Design-temperature 25°C; Relative hurnidity 70 %; Maximum cooling with air has to be applied, if necessary aftercooling with water.
