low temperature crystallization unit
TRANSCRIPT
-
L T t C t lli ti U it fLow Temperature Crystallization Unit for
Sulphate & Perchlorate RemovalFrom a Closed System Sodium Chlorate PlantFrom a Closed System Sodium Chlorate Plant
Kevin SundquistKevin Mamer
Allyse Kreiser
-
Low Temperature Crystallization Unit For Sulphate and Perchlorate Removal
From a Closed System Sodium Chlorate Plant
Report Prepared by: Allyse Kreiser Kevin Mamer
Kevin Sundquist
Department of Chemical Engineering University of Saskatchewan
2007-2008
-
i
EXECUTIVESUMMARY The objective of the project was to design a process to remove sulphate and perchlorate.
Experiments were performed to confirm that the low temperature crystallization unit
(LTCU) will effectively remove 70% of the sulphate from the slurry, although the LTCU
was only designed to remove 50%. Additional experiments showed that the lowest
concentration of sodium perchlorate that can be achieved through precipitation with
potassium chloride is 13 - 16 g/L, which is above ERCOs current perchlorate levels but
after the expansion the LTCU will effectively remove the perchlorate.
A small slip stream of the chlorate slurry will be fed to an agitated mix tank with
potassium chloride to facilitate perchlorate removal and hydrochloric acid to achieve a
pH of 4.5 to facilitate sulphate crystallization. This is then pumped to a crystallizer
where the slurry is cooled to a temperature of -5oC by calcium chloride brine flowing
through a cooling jacket. The crystals are then separated from the liquor by a centrifuge
and brine washed. The crystals are added to the existing chlorate dryer and the liquor
returns to the mother liquor tank at reduced sulphate levels of 10 g/L.
The expansion plans at ERCO make the LTCU a necessity, as the reduced sulphate levels
will improve the efficiency of ERCOs electrolysis process. As well, this process will
allow ERCO to become self-sufficient, and reduce its environmental footprint of the mud
disposal for the present purging mitigation. The production expansion increased revenue,
averted purge costs, power savings and the use of existing equipment make this project
economically successful, estimated to have a 42% discounted cash flow rate of return on
an initial investment of $1 949 000.
-
ii
ACKNOWLEDGEMENTS
We would like to thank the following individuals for their support throughout this
project:
Mr. Tim Barnstable, P.Eng ERCO Worldwide, Saskatoon - for all of the help
gathering resources and providing the experiment samples
Mr. Yuri Omelchenko ERCO Worldwide, Toronto - for additional support on
experiments and for testing the samples
Dr. Gordon Hill Chemical Engineering Professor - for all of his guidance
throughout the course and for the use of the glycol cooler
Mr. Chuck Edwards, P.Eng Engineer in Residence - for his guidance and
feedback obtained in the meetings
Dr. Richard Evitts Chemical Engineering Professor - for his help with the
understanding and implementing of control schemes
Mr. Dale Claude Chemical Engineering Laboratory Coordinator - for the use of
his lab and equipment
-
iii
TABLEOFCONTENTS
EXECUTIVE SUMMARY ................................................................................................. iACKNOWLEDGEMENTS ................................................................................................ iiLIST OF TABLES ............................................................................................................. viLIST OF FIGURES ......................................................................................................... viiiNOMENCLATURE .......................................................................................................... ix1. INTRODUCTION ...................................................................................................... 1
1.1. Background ......................................................................................................... 11.2. Expansion Plans .................................................................................................. 51.3. Current Mitigation .............................................................................................. 61.4. Project Deliverables ............................................................................................ 7
2. DESIGN ALTERNATIVES ....................................................................................... 92.1. Low Temperature Crystallization of Sulphate .................................................... 92.2. Low Temperature Crystallization of Sulphate and Perchlorate with the Addition of Potassium Chloride ................................................................................................... 102.3. Precipitation of Barium Sulphate by the Addition of Barium Chloride ........... 122.4. Sulphate Ion Exchange Resin ........................................................................... 13
3. EXPERIMENTS ....................................................................................................... 143.1. Bulk Density of Slurry Experiment .................................................................. 143.2. Preliminary Experiment .................................................................................... 163.3. Secondary Experiment ...................................................................................... 213.4. Additional Experimental Work Performed by ERCO ...................................... 26
4. DETAILED QUALITATIVE PROCESS DESCRIPTION ...................................... 284.1. Process Description ........................................................................................... 284.2. Control Scheme ................................................................................................. 324.3. Simulation ......................................................................................................... 334.4. Plot Space Plan ................................................................................................. 34
5. EQUIPMENT SIZING ............................................................................................. 375.1. Mix Tank ........................................................................................................... 375.2. Pump Material Selection ................................................................................... 375.3. Crystallizer Feed Pump ..................................................................................... 385.4. Crystallizer ........................................................................................................ 395.5. Centrifuge Feed Pump ...................................................................................... 485.6. Centrifuge ......................................................................................................... 495.7. Refrigeration Skid ............................................................................................. 515.8. Calcium Chloride Pump .................................................................................... 525.9. Brine Wash Heat Exchanger ............................................................................. 545.10. Pipe Sizing ........................................................................................................ 55
6. ECONOMIC ANALYSIS ........................................................................................ 586.1. Capital Costs ..................................................................................................... 586.1.1. Existing Equipment ....................................................................................... 586.1.2. Crystallizer Feed Pump ................................................................................. 596.1.3. Centrifuge Feed Pump .................................................................................. 59
-
iv
6.1.4. Refrigeration Skid ......................................................................................... 606.1.5. Calcium Chloride Pump ................................................................................ 606.1.6. Brine Wash Heat Exchanger ......................................................................... 616.1.7. Controllers and Valves .................................................................................. 616.1.8. Piping and Structural ..................................................................................... 626.1.9. Expansion Capital Costs ............................................................................... 636.1.10. Total Capital Costs and Implementation Time ............................................. 636.2. Operating Costs ................................................................................................. 646.2.1. Sulphate/Perchlorate Removal System ......................................................... 646.2.2. Expansion ...................................................................................................... 646.2.3. Total Operating Costs ................................................................................... 666.3. Revenues ........................................................................................................... 666.4. Cash Flow Analysis .......................................................................................... 676.4.1. Cash Flow with the Use of Existing Equipment ........................................... 676.4.2. Cash Flow without the Use of Existing Equipment ...................................... 706.4.2.1. Mix Tank ....................................................................................................... 706.4.2.2. Crystallizer .................................................................................................... 716.4.2.3. Pusher Centrifuge .......................................................................................... 716.4.3. Cash Flow with Sodium Chlorate Price Sensitivity ...................................... 72
7. PLANT SAFETY ...................................................................................................... 737.1. Material Safety Data Sheets .............................................................................. 737.2. Dow Fire & Explosion Index ............................................................................ 747.3. Hazard and Operability Study ........................................................................... 757.3.1. pH Conditioning of Slurry Node ................................................................... 777.3.2. Low Temperature Crystallization Node ........................................................ 817.3.3. Refrigeration Skid Node ............................................................................... 827.4. Alarms ............................................................................................................... 837.5. Interlocks ........................................................................................................... 847.6. Standard Operating Procedures ......................................................................... 85
8. CONCLUSIONS ....................................................................................................... 879. RECOMMENDATIONS .......................................................................................... 8910. REFERENCES ..................................................................................................... 90
Appendix A Project Description ................................................................................ 91Appendix B Literature Solubility Data ...................................................................... 93Appendix C Preliminary Experiment Sample Analysis Request ............................... 96Appendix D Preliminary Experiment Results for Processed Slurry Samples ........... 99Appendix E Secondary Experiment Procedure ........................................................ 101Appendix F Crystallizer Feed Pump Calculations ................................................... 104Appendix G CAPO Reactor Drawing ...................................................................... 106Appendix H Crystallizer .......................................................................................... 107Appendix I Centrifuge Feed Pump Calculations ..................................................... 115Appendix J CIMCO Refrigeration Skid Proposal ................................................... 117Appendix K Calcium Chloride Pump Calculations ................................................. 118Appendix L Goulds Pumps Calcium Chloride Pump Proposal ............................... 120Appendix M Brine Wash Heat Exchanger Sizing Calculations .............................. 121Appendix N Piping Calculations ............................................................................. 122
-
v
Appendix O Process Flow Diagram ........................................................................ 126Appendix P Low Temperature Crystallization Unit P&ID ...................................... 127Appendix Q ERCO P&ID 07-07100 ....................................................................... 128Appendix R ERCO P&ID 07-07130 ........................................................................ 129Appendix S Patents .................................................................................................. 130Appendix T Material Safety Data Sheets ................................................................. 131Appendix U Dow Fire and Explosion Index ............................................................ 132Appendix V Hazard and Operability Analysis ........................................................ 133Appendix W Sodium Chlorate Decomposition Reactions ....................................... 134
-
vi
LISTOFTABLES
TABLE 3. 1: DATA COLLECTED FOR THE DETERMINATION OF THE RELATIONSHIP BETWEEN BULK DENSITY OF SLURRY AND % SLURRY .............................................................. 14
TABLE 3. 2: PRELIMINARY EXPERIMENT SAMPLE PREPARATIONS ..................................... 19TABLE 3. 3: PRELIMINARY EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED
SAMPLES .................................................................................................................... 20TABLE 3. 4: SECOND EXPERIMENT SAMPLE PREPARATIONS .............................................. 22TABLE 3. 5: SECOND EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED
SAMPLES .................................................................................................................... 23TABLE 6. 1: EXISTING EQUIPMENT INSTALLATION COSTS ................................................. 58TABLE 6. 2: CONTROLLERS AND VALVES COSTS ............................................................... 62TABLE 6. 3: PIPING AND STRUCTURAL COSTS .................................................................... 62TABLE 6. 4: EXPANSION COSTS .......................................................................................... 63TABLE 6. 5: ERCO PURGE COST ........................................................................................ 66TABLE 7. 1: SAFETY PROPERTIES ....................................................................................... 73 TABLE 8. 1: MAJOR EQUIPMENT SIZING & COST ............................................................... 88 TABLE B- 1: SOLUBILITY DATA FOR POSSIBLE PRODUCTS ................................................ 93 TABLE D- 1: PRELIMINARY EXPERIMENT SAMPLE PREPARATIONS .................................... 99TABLE D- 2: PRELIMINARY EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED
SAMPLES .................................................................................................................... 99TABLE D- 3: PRELIMINARY EXPERIMENT RESULTS FOR WET CRYSTALS OF PROCESSED
SAMPLES .................................................................................................................... 99TABLE D- 4: PRELIMINARY EXPERIMENT RESULTS FOR DRIED CRYSTALS OF PROCESSED
SAMPLES .................................................................................................................. 100TABLE D- 5: PRELIMINARY EXPERIMENT RESULTS FOR OTHER DRIED CRYSTALS OF
PROCESSED SAMPLES ............................................................................................... 100 TABLE E- 1: SECOND EXPERIMENT SAMPLES ................................................................... 103 TABLE F- 1: CRYSTALLIZER FEED PUMP SYSTEM ............................................................ 104 TABLE H- 1: NACLO3 SLURRY PROPERTIES ..................................................................... 108TABLE H- 2: TUBE SIDE CALCULATION RESULTS ............................................................ 108TABLE H- 3: CACL2 BRINE PROPERTIES ........................................................................... 109TABLE H- 4: TUBE ARRANGEMENT CALCULATIONS ........................................................ 110 TABLE N- 1: CALCULATION SUMMARY ........................................................................... 122TABLE N- 2: MAIN SLURRY FLOW CALCULATIONS ......................................................... 123
-
vii
TABLE N- 3: POTASSIUM CHLORIDE FLOW CALCULATIONS ............................................. 123TABLE N- 4: HYDROCHLORIC ACID FLOW CALCULATIONS ............................................. 124TABLE N- 5: SODIUM CHLORIDE BRINE WASH CALCULATIONS ...................................... 124TABLE N- 6: CALCIUM CHLORIDE BRINE CALCULATIONS ............................................... 125
-
viii
LISTOFFIGURES
FIGURE 1. 1: SIMPLE CHLORATE CELL ................................................................................. 2 FIGURE 4. 1: DIAGRAM OF A PUSHER CENTRIFUGE ............................................................ 31FIGURE 4. 2: PLOT SPACE FOR F-100 CHLORATE MIX TANK ............................................. 35FIGURE 4. 3: PLOT SPACE FOR D-120 CRYSTALLIZER ........................................................ 36 FIGURE 5. 1: TOP OF CAPO REACTOR ............................................................................... 40FIGURE 5. 2: BOTTOM OF CAPO REACTOR ........................................................................ 41FIGURE 5. 3: INTERIOR OF CAPO REACTOR ....................................................................... 44FIGURE 5. 4: DIMENSIONS OF HEAT EXCHANGER JACKET OF CAPO REACTOR
(CRYSTALLIZER) VESSEL, (A) TOP VIEW: TUBES RUN THROUGH ANNULUS; (B) SIDE VIEW: 5 BAFFLES EQUALLY SPACED ......................................................................... 46
FIGURE 5. 5: EXISTING PUSHER CENTRIFUGE ..................................................................... 50 FIGURE B- 1: SOLUBILITY DATA FOR POSSIBLE PRODUCTS ............................................... 94FIGURE B- 2: SODIUM PERCHLORATE AS A FUNCTION OF THE POTASSIUM CHLORIDE IN THE
SYSTEM ...................................................................................................................... 95 FIGURE I- 1: CENTRIFUGE FEED PUMP SYSTEM ............................................................... 115 FIGURE K- 1: CALCIUM CHLORIDE PUMP SYSTEM ........................................................... 118 FIGURE O- 1: PROCESS FLOW DIAGRAM .......................................................................... 126
-
ix
NOMENCLATURE
Symbol Definition Units
A Surface Area m2
AD Annual Depreciation $
ADCF Annual Discounted Cash Flow $
ADCFT Cumulative Annual Discounted Cash Flow $
AI Annual Investment $
AIT Annual Income tax $
ALM Log-mean Surface Area m2
ANCI Annual Net Cash Income $
ANCIT Cumulative Annual Net Cash Income $
AS Annual Sales $
ATE Annual Operating Expenses $
BEP Break Even Point Years
C1 Loss Control Credit Factor- Process Control
C2 Loss Control Credit Factor- Material Isolation
C3 Loss Control Credit Factor- Fire Protection
CBM Bare Module Cost $
Conc Concentration (Mass) kg/m3
Cp Purchase Cost $
Cp Heat Capacity kJ/kg-K
CPI Chemical Engineering Plant Cost Index
Da Impeller Diameter m
-
x
DCFRR Discounted Cash Flow Rate of Return %
Di , ID Inner Diameter of Tubes m
Do, OD Outer Diameter of Tubes m
f Fanning Friction Factor dimensionless
F1 General Process Hazards Factor
F2 Special Process Hazards Factor
F3 Unit Hazards Factor
FBM Bare Module Factor
Fm Material Factor
Fp Pressure Factor
fd Future Interest Rate for Year i
g Gravitational Constant m/s2
h Convective Heat Transfer Coefficient W/m2K
H Enthalpy kJ/mol, kJ/kg
Hproducts Enthalpy of Product kJ/mol
Hreactants Enthalpy of Reactant kJ/mol
Hrxn Enthalpy of Reaction kJ/mol h Height m
i Interest Rate %
k Thermal Conductivity W/mK
K K-value for Valves and Fittings
Ksp Solubility Product
LD50 Lethal Dose that Kills 50% of Population mg/kg Administered to
-
xi
LTCU Low Temperature Crystallization Unit
Ltubes Length of Tubes m
MW Molecular Weight g/mol
m& Mass Flowrate kg/hr
NPV Net Present Value $
n Year number Years
N Impeller Rotational Speed rad/s, RPM
NL Number of Tubes in Bank of Tubes
NP Impeller Power Number dimensionless
NQ Impeller Pitch Flow Number dimensionless
NuD Nusselt Number dimensionless
P Pressure Pa
P Power Consumption W
Patm Atmospheric Pressure Pa
Pi Pressure at Position i Pa
Pi Pressure Change over Portion i Pa PBP Payback Period Years
Pr Prandtl Number dimensionless
q Heat Transfer Rate kW
q& Volumetric Flowrate m3/s
Q Volumetric Flowrate m3/hr
ReD Reynolds Number dimensionless
Rf Fouling Factor m2K/W
-
xii
SD Tube Spacing Diagonal To Flow m
SL Tube Spacing Parallel To Flow m
ST Tube Spacing Normal To Flow m
T Temperature oC, K
TLM Log-mean Temperature K
TLV Threshold Limit Value mg/m3
u Velocity m/s
U Overall Heat Transfer Coefficient W/m2K
um Mean Velocity m/s
Vmax Maximum Fluid Velocity Between Tubes m/s
Ws Shaft Power W
Correlation Factor dimensionless
Greek Symbol Definition Units
I Intrinsic Efficiency D Driver Efficiency Viscosity Pa-s Delta, Change in
Density kg/m3
-
1
1. INTRODUCTION
1.1. Background
The client that submitted the project was ERCO Worldwide, a division of Superior Plus.
ERCO operates a closed system sodium chlorate plant north of Saskatoon, Saskatchewan,
Canada. The technology used by ERCOs sodium chlorate plant is an electrolysis cell
reactor. A generalized schematic of a simple chlorate cell is shown in Figure 1.1.
-
2
Figure 1. 1: Simple Chlorate Cell
-
3
The principle overall chemical reaction in a chlorate cell is given by:
236
2 33 HNaClOOHNaCle ++ + (1.1)
If all sodium chlorate is made by the above equation only, the cell efficiency is 100%.
However, this is an extremely simplistic expression of a series of desirable chemical
reactions that occur in a chlorate cell and ignores a number of undesirable reactions. The
desirable reactions are as follows:
+ eClCl 22 2 (1.2) ++ OHHeOH 222 22 (1.3)
+ +++ ClHHOClOHCl 22 (1.4) + + ClOHHOCl (1.5)
+ +++ HClClOClOHOCl 222 3 (1.6)
In addition to the desirable reactions above, there are a number of undesirable reactions
which result in a loss of current efficiency in a chlorate cell. Undesirable reactions
occurring at the anode include:
+ eOSSO 222 28224 (1.7)
2212
42282 22 OSOHOHOS +++ + (1.8)
2242
282 242 COSOHCOHOS ++++ + (1.9)
+ ++ eOHOH 442 22 (1.10) + +++ eCOHCOH 442 22 (1.11)
+ +++ ClHCOCHOCl 222 2 (1.12) + +++++ eOClHClOOHClO 646236 22132 (1.13)
-
4
+ +++ eHClOOHClO 22423 (1.14)
The discharge of sulphate ions, Equation (1.7), occurs in a chlorate cell whenever the
sulphate content in the feed brine reaches relatively high levels. If metal anodes are
employed, such as in ERCOs chlorate process, sulphate ion discharge results in the
evolution of oxygen as in Equation (1.8). The accumulation of sulphate ions adversely
affects electrolytic power consumption. Equations (1.9), (1.11), and (1.12) are of concern
only when the anodes are composed of graphite.
Equation (1.14) illustrates the discharge of the chlorate ion to form perchlorate. This
reaction will occur whenever there is a low level of chloride in a cell together with a high
concentration of chlorate. All of these undesirable reactions at the anode can be
minimized or eliminated by adequate brine treatment and appropriate cell design and
operating conditions.
The cathode also has its share of undesirable reactions resulting in a loss of current
efficiency; undesirable reactions occurring at the cathode include:
+++ OHCleOHClO 222 (1.15) +++ OHCleOHO 663 23 (1.16)
Fortunately Equations (1.15) and (1.16) are almost completely inhibited by the addition
of sodium dichromate to the cell solution. The sodium dichromate forms a hydrated
chromium oxide layer on the cathode. Dichromate also acts as an excellent buffer
maintaining the pH of the cell at an optimum value via the reaction:
+ ++ HCrOOHOCr 22 242272 (1.17)
-
5
Other benefits of sodium dichromate are that it greatly reduces corrosion on steel surfaces
and it also inhibits the electrochemical discharge of hypochlorite or other ions which
evolve oxygen.
The importance of controlling the undesirable reactions without increasing capital cost is
obvious, when one considers that the cost of power accounts for half the cost of
producing sodium chlorate. Of the balance almost one third is the cost of capital. Thus,
the control of Equations (1.7), (1.10), (1.11), (1.13), (1.14), (1.15) and (1.16) is of
paramount importance to the chlorate cell designer and operator. Of equal importance to
the cell designer and operator is the cell voltage, since power cost is a function of voltage
as well as current efficiency. For this reason, the removal of sulphate and perchlorate are
of importance for reducing the power costs.
1.2. Expansion Plans
ERCO is developing plans to increase chlorate production at their Saskatoon sodium
chlorate plant to take advantage of the high market price of chlorate. The problem with
this being the brine evaporator, which limits the amount of sulphate entering the process,
is operating at full capacity. This means that further brine requirements would bypass the
brine evaporator and greatly increase levels of sulphate in the chlorate cells. This is
obviously undesirable given that sulphate, at higher concentrations, will cause decreased
cell current efficiencies and drive up operating costs. For this reason, ERCO submitted a
project to the Department of Chemical Engineering at the University of Saskatchewan to
achieve the low temperature removal of sodium sulphate (Na2SO4) and sodium
-
6
perchlorate (NaClO4) from their closed system sodium chlorate plant. The project
description submitted by ERCO can be found in Appendix A.
1.3. Current Mitigation
Currently, ERCO maintains acceptable levels of sulphate (and perchlorate) by purging
chlorate liquor product monthly. The minimum purge rate is governed by the amount of
sulphate entering the process with the feed brine. The higher the sulphate level, the
greater the chlorate liquor purge rate. When sulphate levels increase to ~29 g/L in the
reactors, the purge (at ~ 0.5 m3/hr) is started and ran until the sulphate levels drop to ~26
g/L. Overall ERCO ends up purging about 60 - 80 m3 of mother liquor per month. This
purge rate seems to maintain perchlorate levels at ~10-12 g/L. The current purge stream
is not operated continuously. The purge is drawn from the level control line that goes
from the Chlorate II 76-J-101 Crystallizer to the 74-T-104 Mother Liquor Tank. The
level control line is found on P&ID 07-07100 in Appendix Q. It is a 1" line that should
be indicated just before the level control valve LV-103.
The least expensive and technically preferred method to control the plants sulphate level
is by having a sufficient chlorate liquor product rate. This, of course, requires customers
willing to take the necessary quantities of chlorate liquor product. Fortunately, ERCO is
able to transport this chlorate liquor product to a sister plant where facilities exist to
process the sulphate by addition of calcium chloride to make a calcium sulphate mud.
This method of sulphate concentration maintenance is costing ERCO approximately $191
000 per year, and may not be feasible when production increases are implemented due to
-
7
the high levels of sulphate ingress which will result. Therefore, ERCO considers this a
potential threat to the long term operation of the sodium chlorate plant.
1.4. Project Deliverables
ERCO had indicated that patents show that low temperature crystallization of sodium
sulphate is feasible at temperatures below 0oC for concentrations experienced at the
Saskatoon location. (These patents can be found in Appendix S). ERCO was unsure of
the precipitation of sodium perchlorate, but internal evaluations showed that dual
precipitation could be economical. The project was to develop a conceptual process
design of a sulphate and perchlorate removal system. The project involved:
Literature search to confirm current data on sulphate and perchlorate solubility; Bench work and process simulations to confirm and optimize the design; Development of a conceptual process design based on the knowledge gained from
the above work;
Hazard and operability review of the design; Estimation of installed cost of the major equipment; Estimation of the operating cost and potential savings from the proposed design; Project justification.
The project deliverables included:
1. Conceptual process design that included a preliminary P&ID and a mass and
energy balance for the project;
2. Equipment sizing and estimated cost of major equipment;
-
8
3. Hazard Analysis of preliminary project design;
4. Project justification.
-
9
2. DESIGNALTERNATIVES
2.1. Low Temperature Crystallization of Sulphate
United States Patents exist for the low temperature removal of sulphate from a sodium
chlorate plant and can be found in Appendix S. At subzero temperatures, sodium
sulphate will crystallize from the slurry more readily than during normal operation. The
solubility of sulphate at various temperatures is located in Appendix B.
By drawing a small slip stream from the existing process, the chlorate slurry could be
cooled and placed in a crystallizer to remove the sulphate. If the entire stream was
cooled, the refrigeration requirements would be extremely high and unreasonable.
Removing the sulphate from the slip stream would adequately maintain the sulphate
levels in the process liquor and remove ERCOs present necessity to purge one railcar of
process liquor per month to keep sulphate levels in check. In fact, if the low temperature
crystallization of sulphate was chosen, this would allow sulphate levels to be steady
rather than purging when the levels reach their intolerable level. For this reason, this
system would allow for improved efficiency in the electrolysis process as the sulphate
ions inhibit the electrolysis process.
The crystallized sodium sulphate (Na2SO4) would be stable, thus it could be combined
with the sales product with minimal impact on quality as there is already sulphate crystals
-
10
present in the product. Thus, this alternative would remove the sister plants necessity of
using calcium chloride (CaCl2) to produce calcium sulphate (CaSO4) which then proceeds
to mud disposal. Thus this alternative would reduce ERCOs environmental footprint by
removing 4.1 - 5.6 metric tons of mud which requires disposal.
An additional benefit of this design is that it does not require handling of hazardous
materials handling other than the dichromate which is already present in the process
liquor. However, this alternative would require a number of key pieces of equipment
resulting in a high capital cost. Fortunately, ERCO has equipment from an old part of the
plant that was shut down that may be retrofitted for use in this process to reduce the
capital cost. (If ERCOs used equipment is suitable in the design, testing would be
required to ensure the equipment still meets safety standards under the new operation).
As well, this unit would be high operating costs from the refrigeration costs.
Since this design alternative was discussed in patents, experiments were performed in
order to ensure that the patent concepts would be applicable to ERCOs sodium chlorate
slurry. The experiments are discussed in Section 3.
2.2. Low Temperature Crystallization of Sulphate and Perchlorate
with the Addition of Potassium Chloride
The objective of this project was to design a system to remove sodium sulphate; the
removal of sodium perchlorate was a secondary objective. Sodium perchlorate and
sodium sulphate are undesirable components that build up in ERCOs sodium chlorate
-
11
process over time. Many patents have been developed for the removal of sulphate from a
sodium chlorate plant using low temperature crystallization, but do not contain discuss on
the removal of perchlorate. While the preliminary experiment focused on the primary
objective of the removal of sulphate, the second experiment also focused on the removal
of perchlorate. If the experiments indicate that the removal of perchlorate can be done in
conjunction with the removal of sulphate, ERCO will have the opportunity to develop its
own patent and avoid patent fees.
The addition of potassium chloride may be beneficial, as it would increase the removal of
perchlorate from the system. The literary search completed showed that the solubility of
potassium perchlorate is lower than that of sodium perchlorate; this solubility data is
located in Appendix S. However, the solubility data could only be found for
temperatures greater than 0oC, thus experimental data had to be collected to ensure that
the addition of potassium chloride to the slurry would indeed result in enhanced
perchlorate removal. As well, the solubility data indicated that there was a possibility
that potassium chlorate crystals could form, which would result in a loss of product
(sodium chlorate) rather than a removal of perchlorate.
The pros and cons of this alternative are very similar to that of the discussed in Section
2.1. Since the potassium chloride would be an additional cost, the experiments had to
show successful removal of perchlorate. If experiments did indicate that potassium
chloride addition increased the removal of perchlorate, this alternative would be chosen
over the Low Temperature Crystallization of Sulphate alternative. Similar to the
-
12
crystallized sulphate, the perchlorate crystal could be added to the chlorate product
without affecting the product quality.
2.3. Precipitation of Barium Sulphate by the Addition of Barium
Chloride
If barium chloride (BaCl2) was added to the slurry, it would result in the precipitation of
barium sulphate (BaSO4). This alternative would also remove the necessity to purge one
railcar of process liquor per month to keep the sulphate levels in check. However, unlike
the low temperature crystallization alternatives, this alternative has the advantage of
removing nearly 100% of the sulphate resulting in a higher efficiency in the electrolysis
process. Unfortunately, it would not remove any of the perchlorate. While the barium
chloride addition would be an expensive operating cost, this alternative would not require
a refrigeration unit and would therefore require a lower equipment cost.
The removal of the sulphate would not be 100% because the barium cannot be added in
excess as barium is a very toxic substance. If barium was added in excess, the barium
which would remain in the chlorate liquor would contaminate ERCOs entire process and
the consequences of barium in the electrolysis process are unknown. As well, it should
be noted that at ERCOs sister plant which handles the purged chlorate liquor containing
sulphate, barium is not used. The sister plant uses calcium chloride (CaCl2) to produce
calcium sulphate (CaSO4). The calcium sulphate then proceeds to mud disposal. Since
barium is a toxic substance, the disposal of the barium sulphate would be more delicate
-
13
than the present mud disposal of calcium sulphate. Therefore, this alternative was not
considered further.
2.4. Sulphate Ion Exchange Resin
Another alternative that was considered for the removal of sulphate was a sulphate ion
exchange resin.
Ion-exchange resins are light and porous solids, usually prepared in the form of granules, beads, or sheets; they contain any wide variety of organic compounds synthetically polymerized and containing positively or negatively charged sites that can attract an ion of opposite charge from a surrounding solution. In chemical analysis, ion-exchange resins are used for the separation or concentration of ionic substances (2008 Encyclopedia Britannica, Inc.).
However, a literature search did not turn up any information about this technology being
used for sulphate removal in a saturated solution. Because the ion exchange effectiveness
is unknown due to the other compounds in the system, this alternative would require
expensive bench scale and pilot plant testing. As well, a sulphate ion exchange resin
would be expensive and would require a sulphate-free stream to reactivate the resin.
Similarly to the Precipitation of Barium Sulphate by the Addition of Barium Chloride
alternative discussed in Section 2.3, while this alternative would not require refrigeration
and there would be no perchlorate removal. Due to the high cost and the unknown
effectiveness of exchange resins, this alternative was not considered further.
-
14
3. EXPERIMENTS
3.1. Bulk Density of Slurry Experiment
Sample mass and volumes were analyzed at ERCO to obtain a density trend based on
slurry volume percent. This analysis was conducted at 22.4oC in the lab. The data
collected can be found in Table 3.1. The data was used to determine the relationship
between the bulk density of the slurry and the slurry percent. Excel was used to
determine a line of best fit as shown in Figure 3.1.
( ) ( ) 3.1358%99.935%66.290 23 ++= SlurrySlurrymkgDensityBulkSlurry Table 3. 1: Data Collected for the Determination of the Relationship between Bulk Density of Slurry and % Slurry
Sample Description Tare
Mass of Cylinder
(g)
Total Mass of Sample & Cylinder
(g)
Sample Mass (g)
Total Sample Volume
(mL)
Slurry Volume
(mL) Slurry
%
Sample Bulk Density of
Slurry (kg/m3)
A Slurry 196.49 638.93 442.44 250.0 130 52% 1770 B Slurry 196.49 619.09 422.60 247.0 110 45% 1711 C Slurry 196.49 607.05 410.56 248.0 88 35% 1655 D Slurry 196.49 596.00 399.51 247.0 75 30% 1617 E Decant 152.93 289.40 136.47 100.5 0 0% 1358
-
15
Figure 3. 1 Relationship Between the Bulk Density Slurry and Percent Slurry
-
16
3.2. Preliminary Experiment
The first lab experiment was performed on Wednesday, October 17 at 1:30pm in the
Chemical Engineering undergraduate lab located in the Engineering building room 1D25.
The purpose of the preliminary experiment was to determine if low temperature
crystallization would effectively reduce the sulphate concentration in ERCOs chlorate
slurry.
Tim Barnstable, the design contact at ERCO, provided 12 - 1L samples of the chlorate
slurry from the 76-J-101 Primary Crystallizer for the Chlorate II system shown in P&ID
07-07100 in Appendix Q. He also provided the brine wash solution, hydrochloric acid
solution and potassium chloride salt.
Prior to the experiment, MSDS for hydrochloric acid, potassium chloride, sodium
dichromate anhydrous, potassium perchlorate, and sodium chlorate crystal were collected
and reviewed. These MSDS can be found in Appendix T.
The initial plan was to analyze the liquor samples for sulphate prior to being crystallized
and after a residence time of fifteen minutes in the cooler. Thus, to begin the experiment,
a blank sample supplied by ERCO (Sample 0) with no pH adjustment or cooling
completed was analyzed for sulphate using a Sulphate Test Kit Model SF-1. The
Sulphate Test Kit was provided by the Physical Chemistry Lab Coordinator, Bryan
Wilson, at the University of Saskatchewan. However, the kit did not give accurate results
-
17
for the feed sample which was expected to have sulphate levels of approximately 30 g/L.
Instead, the ERCO lab in Saskatoon completed sulphate analysis for the samples.
The first step of the procedure was to add 11% hydrochloric acid dropwise while stirring
continuously. The pH was analyzed using a pH meter. Initially, the pH of the sample
was at approximately 8 - 8.7, and was dropped down to approximately 4.5. If the pH
dropped below 4.0, there was a risk of releasing chlorine gas or chlorine dioxide gas due
to the composition of sodium chlorate. Thus, it was very important that each sample was
stirred continuously, and that each drop of hydrochloric acid was allowed to distribute
throughout the sample. The formation of chlorine dioxide gas is shown in equation (3.1)
and the formation of chlorine gas is shown in equation (3.2).
OHClClONaClHClNaClO 222123 2 ++++ (3.1)
OHNaClClHClNaClO 223 336 +++ (3.2)
In case the pH did drop below 4.0, despite the dropwise addition of hydrochloric acid,
this step was completed under the fume hood.
The pH adjusted sample was then placed in the glycol cooler which contained anti-freeze;
the anti-freeze temperature was set to a setpoint of 0oC, -5oC or -10oC depending on the
sample. A stirrer was placed in the beaker containing the slurry sample to maintain
fluidization of the sample. The slurry temperature was checked periodically; once the
sample reached the desired temperature, it remained in the glycol cooler for an additional
15 minutes. A residence time of 15 minutes was chosen based on time constraints, as
five samples were to be prepared that afternoon.
-
18
After the sample was removed from the glycol cooler, the volume increase in crystals was
noticeable to the naked eye indicating good crystallization. The crystals that formed had
distinct edges and good shape. As well, an additional thin crystal layer could be seen on
top of the initial chlorate crystals; the crystals in this layer were white in color and much
finer than the chlorate crystals. It was believed that this layer was the newly crystallized
sulphate crystals. A sample of these crystals was collected from the thin layer; once these
were placed in the crucible, it was noted that they were in a very high surface tension
fluid.
The sample was given time to settle and then the liquor was decanted. A portion of the
decant was collected in a plastic sample bottle and labeled by its sample number followed
by the letter A. A portion of the crystals (after mixing well) was placed in a funnel
containing a filter paper. An ejector was placed on the tap and used to vacuum filter the
sample to remove the remaining liquor from the crystals. These wet crystals were then
placed in a sample bottle and labeled with the letter B. Additional crystals were vacuum
filtered and placed in a small aluminum crucible; this crucible was then placed in the
oven at 100oC to dry overnight. The next day, the dry crystals were collected in sample
bags and labeled with the letter C.
For a couple of the samples, the vacuum filtered crystals were brine washed. However,
the brine wash was at room temperature and resulted in a substantial amount of
dissolution of the crystals. The brine wash did effectively remove the dichromate from
-
19
the crystals as after the wash the crystals were white. The brine washed crystals were
labeled with the letter D.
In addition to the sample labels, each sample was labeled as toxic and carcinogenic as a
precaution. All the samples were yellow from the dichromate in the solution and
dichromate is toxic and carcinogenic. Throughout the experiment, the proper personal
protection equipment was worn including safety glasses and gloves. All of the apparatus
and counter top area were cleaned and rinsed; all the waste (including the water used for
rinsing and all the soiled napkins and gloves) was collected in a hazardous waste
container and returned to ERCO for proper handling and disposal.
All five samples were prepared uniquely. A summary is shown in Table 3.2. Sample 0
was not cooled or pH adjusted, but used to compare the results of the samples which were
cooled and pH adjusted. Sample 1 was chilled to a temperature of 0oC but pH was not
adjusted. This was done to see if the pH adjustment was required.
Table 3. 2: Preliminary Experiment Sample Preparations
Sample ID Temperature pH Residence Time Desired KCl
Concentration Mass KCl
Added (oC) (min) (g/L) (g)
0A 35 8.7 1A 0 8.7 15 2A 0 4.6 15 3A -5 4.62 15 15 10 4A -5 4.65 15 5A -10 4.61 15
Sample 1 indicated that if the crystallization is done at a pH of 8.7, then sodium
dichromate also precipitates as expected. This is undesired as dichromate is carcinogenic
-
20
and toxic, and thus a stable precipitate cannot be washed away. The dichromate must
remain in solution so that it can be easily removed using a brine wash.
Potassium chloride was added to Sample 3 prior to it being chilled. When the sample
was removed from the chiller it was noted that it had poor settling qualities which made it
difficult to decant. This is a result of the potassium perchlorate forming fine solids that
settle very slowly.
All the samples were sent to the ERCO laboratory in Saskatoon. Perchlorate analysis was
not completed for this preliminary experiment as the laboratory in Saskatoon did not have
the necessary resources to analyze the samples for perchlorate. The request for the
analysis for the crystals can be found in Appendix C. The results for the decanted liquor
are shown in Table 3.3; the results of the wet and dry crystals can be found in Appendix
D.
Table 3. 3: Preliminary Experiment Results for Decanted Liquor of Processed Samples
Sample ID NaClO3 NaCl Na2SO4 Na2Cr2O7-2H2O
Na2SO4 Removal
Na2SO4 Removal
(g/L) (g/L) (g/L) (g/L) (%) (%) 0A 566.9 127.6 31.4 4.1 1A 405.1 138.4 27.1 4.5 13.7% 13.7% 2A 405.9 138.6 23.9 4.3 23.9% 23.9% 3A 378.8 146.6 20.9 4.2 33.4% 33.4% 4A 387.4 139.6 25.0 4.3 20.4% 20.4% 5A 371.9 139.9 33.9 4.4 -8.0% -8.0%
From these results it can be seen that sulphate does precipitate in the solution as a
significantly hydrated crystal product. As well, when sample 1 is compared to the other
samples it is obvious that the pH adjustment to approximately 4.5 does help to facilitate
-
21
the crystallization of sulphate. It should also be noted that as the samples are chilled, the
solubility of chlorate decreases from 567g/L at 35oC to approximately 382g/L at -5oC.
The maximum sulphate removal achieved was 33% and this sample had 15 g/L of
potassium chloride added. While it is unexplained, it will be reassessed during the
second experiment. In conclusion, the preliminary experiment confirmed that low
temperature crystallization is feasible.
3.3. Secondary Experiment
While the preliminary experiment focused on the primary objective of the removal of
sulphate, the second experiment also focused on the removal of perchlorate. Many
patents have been developed for the removal of sulphate from a sodium chlorate plant
using low temperature crystallization. If the removal of perchlorate in conjunction with
the sulphate is successful, ERCO will have the opportunity to develop its own patent and
avoid paying patent fees. By avoiding the existing sulphate removal patents,
implementation of the LTCU will be eased.
The second experiment was performed on Friday, January 4 at 9:30am in the Chemical
Engineering undergraduate lab located in the Engineering building room 1D25. The main
objectives of this experiment were as follows:
Confirm the composition of the liquor in the original slurry (Sample 0) of the preliminary experiment at 35oC;
Determine the liquor composition of the slurry chilled to -5oC and -10oC in order to compare the effective crystallization of sulphate and perchlorate;
-
22
Determine the effect of the addition of perchlorate would have on the liquor composition at -5oC;
Determine if the addition of potassium would increase the removal of perchlorate from the liquor at -5oC and -10oC.
Table 3.4 lists the samples that were prepared.
Table 3. 4: Second Experiment Sample Preparations
Sample ID Description Temp pH
Residence Time
Desired KCl Concentration
Mass KCl
AddedDesired ClO4- Concentration
Mass NaClO4-XH2O
Added (oC) (min) (g/L liquor) (g) (g/L liquor) (g)
0A Blank 35 9.8 1A -5 4.47 20 2A -5 4.47 30 3A -10 4.53 20 4A -5 4.65 20 5 2.9 5A -5 4.66 20 10 5.8 6A -5 4.25 20 15 8.6 7A -10 4.25 20 10 5.8
8/9A Blank 35 9.7 8A Spiked Blank 35 9.7 20 64 29 9A -5 4.54 20 64 29
The liquor samples were diluted 1:10 and the crystal samples were dissolved and
transported to Toronto. The analysis for the second experiment was completed in the
ERCO laboratory in Toronto as they had the analytical instrumentation needed for the
analyses of all the liquor components including sulphate, perchlorate and potassium.
The results for the decanted liquor are shown in Table 3.5.
-
23
Table 3. 5: Second Experiment Results for Decanted Liquor of Processed Samples
Sample ID NaClO3 NaCl Na2SO4
Na2Cr2O7-2H2O NaClO4 K
Na2SO4 Removal
NaClO4 Removal
(g/L) (g/L) (g/L) (g/L) (g/L) (g/L) (%) (%) 0A 548 125 26.7 3.3 10.71 1.6 1A 400 137 25.9 3.5 12.03 1.7 3.0% -12.3%2A 385 138 24.8 3.6 11.96 1.7 7.1% -11.6%3A 383 137 28.4 3.6 11.87 1.7 -6.4% -10.8%4A 387 140 22.3 3.6 11.89 3.4 16.5% -11.0%5A 384 143 18.3 3.6 12.07 3.3 31.5% -12.6%6A 386 141 22.3 3.6 12.03 3.1 16.5% -12.3%7A 361 142 19.7 3.7 11.98 2.8 26.2% -11.8%
8/9A 540 129 27.5 3.4 11.07 1.7 8A 497 128 27.5 3.4 46.90 1.7 9A 391 135 27.8 3.6 58.84 1.7 1.1% -25.5%
Sample 3A in this experiment as well as Sample 5A in the preliminary experiment had a
negative sulphate removal; both these samples were completed at -10oC suggesting that
dropping the temperature below -5oC will not increase the removal and may even inhibit
it.
At high doses of potassium (Sample 4 7) there is a strong possibility of precipitating
KClO3. Thus, the potassium concentration in the liquor is limited by solubility of KClO3.
This possibility of precipitating KClO3 is unfavorable because it would result in a loss of
sodium chlorate product. When the potassium chloride is added, a portion will
precipitate KClO4 and the remaining portion may precipitate KClO3.
From the results it appears that the highest KCl concentration in the liquor that can be
attained is around 2.8 - 3.4 g/L. The potassium chloride concentrations added were 5, 10
-
24
and 15 g/L; the potassium concentration results do not show an increasing trend. Thus, it
can be assumed that whatever additional KCl that is added and cannot remain in solution
will precipitate out as KClO3. The results indicate that 2.8 - 3.4 g/L is the maximum
achievable concentration of potassium in the liquor. For this reason, in order to
precipitate the potassium perchlorate, the concentration in the liquor must be greater than
16 g/L. This number was determined using the NaClO4 in the liquor as function of
steady state KCl concentration figure (Figure B-2 in Appendix B). Therefore, because
the current level of perchlorate in ERCOs liquor is so low, the removal of perchlorate is
not possible even at the maximum dosage of potassium.
However, the perchlorate levels are expected to increase as the expansion is brought
online, and therefore, a potassium chloride addition system was still put in place. For this
reason, sodium perchlorate was added to samples 8 and 9. An increase in perchlorate
concentration seemed to affect only the concentration of chlorate, the other components
did not tend to salt out. While these results do not look promising for perchlorate
removal, there appears to be errors as the level of perchlorate increased in the liquor after
chilling. In order to remove the required amount of perchlorate enough, potassium must
be added to first increase the background potassium concentration to the level that
corresponds to the expected level concentration of perchlorate in the treated stream and
secondly, form the necessary quantity of KClO4. Considering the samples that did not
have potassium chloride added had a concentration of 1.7 g/L potassium, only a small
amount of potassium chloride would have to be added to bring the background
concentration to 3.4 g/L.
-
25
After the decant was collected from Sample 1, the sample was returned to the cooler for
an additional 10 minutes resulting in a total residence time of 30 minutes. When the
decant results of Sample 1 and 2 are compared, it can be seen that the increased residence
time increased the amount of sulphate removal. The Low Temperature Crystallization
Unit (LTCU) will have a residence time of three hours in the crystallizer allowing for
longer precipitation times, so the sulphate removal is expected to increase. It is possible
that the 20 minute residence time in the glycol cooler was not sufficient time for reaching
complete equilibrium.
In addition to this, the results show that the amount of NaClO3 is decreasing, indicating
precipitation. As the chlorate drops out of the liquor, the solution volume decreases as it
contains fewer components in solution. As the volume decreases, the concentration of
the components not precipitating will increase. Therefore, if the sulphate concentration
in the liquor remains unchanged, this indicates that the sulphate has indeed precipitated as
well.
The maximum sulphate removal achieved in both experiments was 33% and these
samples which had the highest removal were those which had 10 - 15 g/L of potassium
chloride added. While it is unexplained, it seems to be the trend. A possible explanation
is the precipitation of K2SO4; however, the solubility data indicates that KClO4 and
KClO3 would precipitate first. For this reason, the potassium chloride will be added at 10
g/L despite the results indicating that only 2.8 3.4 g/L is required.
-
26
3.4. Additional Experimental Work Performed by ERCO
Yuri Omelchenko, an ERCO chemist, performed additional experimental work at the
ERCO laboratory in Toronto. The experiments he completed were done on a liquor
containing 379g/L NaClO3, 126 g/L NaCl, 62.8 g/L NaClO4 and 3.33 g/L Na2Cr2O7-
2H2O. This formulation is close to the liquor composition that was formed at subzero
temperatures during the preliminary and secondary experiments. Yuri Omelchenko drew
the following conclusions when he performed a perchlorate removal study on the liquor:
The experimentally solubility product, Ksp, value for KClO3 was found to be 0.0103 at -6oC which is somewhat higher than the calculated Ksp value of 0.0083.
The maximum achievable concentration of potassium in the liquor at 6oC was confirmed to be in the order of 3 g/L (5.7 g/L KCl).
The lowest concentration of NaClO4 that can be achieved through precipitation with KCl is 13 - 16 g/L. With the initial concentration of approximately 63 g/L
NaClO4, this shows that a 75% removal of perchlorate.
The precipitation performed using 22% KCl solution occurred quickly (within 10 minutes). If crystal KCl is added, the equilibration takes much longer (more than
an hour).
Experiments were also done at the laboratory in Toronto to confirm the removal of
sulphate. It was found that the concentration of Na2SO4 could be brought from 30 g/L
down to 9 g/L resulting in a 70% removal of sulphate at -6oC. It was noted that the
-
27
sulphate crystallization occurred after a notable delay; this explains why the results of the
preliminary and secondary experiment discussed in Section 3.2 and 3.3 only showed a
33% removal as the residence times were 15 and 20 minutes respectively.
-
28
4. DETAILEDQUALITATIVEPROCESSDESCRIPTION
4.1. Process Description
While this description states that the sulphate concentration entering the LTCU process
will be 20 g/L, initially the slurry will enter at approximately 30 g/L. Once this LTCU is
brought online, the target operating sulphate level will be 20 g/L. However, during the
first few months of operation, the LTCU will be required to lower the steady state
concentration of sulphate down to 20 g/L from the current level of 30 g/L.
A slipstream of the process slurry at the plant will be drawn at 2.5 m3/hr (4300 kg/hr).
The stream is drawn between the Chlorate II 76-J-101 Crystallizer and the 76-CF-101
Centrifuge as found on P&ID 07-07100 in Appendix Q. The Chlorate II plant was
chosen because it is the primary plant at ERCOs facility. The Chlorate I plant is the
secondary system and only runs part-time. Therefore, drawing from the Chlorate II
system would more effectively keep sulphate levels in check. The slurry entering the
system will have a sulphate level of 20 g/L and a perchlorate level of 36 g/L. The slurry
enters the F-100 Chlorate Mix Tank where it is combined with potassium chloride (KCl)
and hydrochloric acid (HCl) while an agitator keeps the slurry well-mixed. The 22 wt%
potassium chloride is pumped in at 0.096 m3/hr by a metering pump from a tote in order
to reach a mass concentration of 10 g/(L of liquor) of KCl in the mix tank. The KCl is
added to precipitate potassium perchlorate as it is less soluble than sodium perchlorate.
-
29
(Further solubility discussion can be found in Appendix B). The hydrochloric acid is
added dropwise into the mix tank until the pH reaches 4.5 from its original pH of 8.5.
The pH must be dropped to facilitate the precipitation of sodium sulphate and to prevent
the precipitation of sodium dichromate. The addition of HCl needed to be monitored and
controlled to prevent the pH from dropping to 4. If the pH drops to 4, chlorine and
chlorine dioxide gas may form; this must be prevented as both these gases are very
dangerous. In the event that gas forms in the mix tank there is a gooseneck vent to draw
air in from the surroundings, sweep the gases at the top of the tank and send them to the
off-gas fume scrubber. The line to the fume scrubber has a pressure indicator on it to
determine if there venting is drawing gas as designed. There is a recycle line on the mix
tank to ensure the slurry is well-mixed and to prevent blocking the K-110 Crystallizer
Feed Pump.
The slurry is then pumped by the K-110 Crystallizer Feed Pump to the D-120
Crystallizer. The slurry enters the top of the crystallizer and is forced down the draft tube
by the agitator. The slurry then comes back up through the tubes of the crystallizer where
it comes in contact with the cold calcium chloride brine (21 wt%). The calcium chloride
brine flows through the shell of the crystallizer and passes by five baffles to help with
heat transfer. The calcium chloride is cooled to -7oC by the CIMCO (50 ton)
Refrigeration unit and is pumped to the crystallizer by the K-123 Calcium Chloride Pump
at 130 m3/hr. The refrigerant used in the refrigeration skid is R-507. The calcium
chloride brine cools the process slurry to -5oC in the crystallizer. There is only a 2oC
temperature difference between the process slurry and the calcium chloride because of
-
30
scaling effects in the crystallizer. The temperature differential was provided by ERCO as
they have past experience with the crystallizer; however, it is recommended that when the
LTCU is brought online the temperature differential be increased gradually while scaling
does not occur. The process slurry then exits the bottom of the crystallizer where the pH
is analyzed again to verify previous readings in the process. The slurry passes by a
deionized (DI) water flush point to free any line blockages that may occur prior to or in
the K-130 Centrifuge Feed Pump.
The process slurry is then pumped to the H-140 Pusher Centrifuge by the K-130
Centrifuge Feed Pump. The slurry enters the center of the horizontal centrifuge where it
is spun and centrifugal forces force the liquid through the molecular sieves. The crystals
build up on the sieves and are pushed to the outlet of the centrifuge as shown in Figure
4.1.
-
31
Figure 4. 1: Diagram of a Pusher Centrifuge Sulphate free brine is added to the centrifuge at 0.15 m3/hr to wash the dichromate out of
the crystals and this is monitored by a flow indicator. Dichromate is added in another
portion of ERCOs plant to protect the plates in the electrolysis process and is
carcinogenic and toxic and must be removed from the crystal product. The brine is
cooled by the calcium chloride brine in the E-124 Brine Wash Heat Exchanger to prevent
the dissolution of crystals and the temperature of the brine is monitored and used to
control the flowrate of the calcium chloride brine.
The crystals are then transported to the existing Chlorate II 76DR101 Dryer, as found on
P&ID 07-07130 in Appendix R by the I-150 Dryer Feed Screw Conveyor. The mass
flowrate of crystals entering the dryer will be 2300 kg/hr. The solution from the
-
32
centrifuge will be gravity fed at 2000 kg/hr into the existing Chlorate Mother Liquor
Tank 74-T-104 with sulphate levels of 10 g/L and perchlorate levels of 16 g/L. This will
result in a removal of 50% of sulphate.
4.2. Control Scheme
All of the valves in the process are fail-closed valves in order to stop the flow in the event
that air pressure is lost. The flowrate of the HCl will be controlled with a control valve
and the pH of the recycle line on the mix tank is analyzed and controls a valve on the
inlet of the HCl line. There is also a sample point on the recycle line for operators to
manually check the pH. The level analyzer will monitor the level in the mix tank and
control a valve on the chlorate slurry feed into the mix tank to ensure there is no overflow
of the mix tank.
There is also a level controller on the crystallizer to control the inlet flowrate to the
crystallizer and to ensure that it is not overfilled. There is a flow indicator on the calcium
chloride line so the flowrate can be monitored. A cascade controller will be used for the
temperature controls between the refrigeration circuit and the crystallizer. With this
control, the inlet temperature of the CaCl2 coming from the refrigeration skid does not
have to be constant.
The cascade control system consists of a primary and secondary control loop. The
secondary control loop in this case is transmitting the temperature of the CaCl2 brine at
the outlet of the cooling jacket. The primary control loop is transmitting the temperature
-
33
of the chlorate slurry leaving the crystallizer. This control system is recommended
because the CaCl2 brine outlet temperature would indicate a disturbance more quickly
than the outlet slurry temperature. For example, an increase in the CaCl2 brine outlet
temperature would cause the VFD pump to speed up, whereas, it may take longer for the
slurry outlet temperature to show an increase in temperature and indicate that the slurry is
above -5oC. However, if the chlorate slurry outlet temperature transmitter did show a
disturbance from the desired -5oC temperature, then it would also cause the VFD pump
speed to change as it is the master controller and serves as the set point. The chlorate
outlet temperature will control the VFD pump. Considering the control system further, it
was decided to include a temperature transmitter on the CaCl2 brine line at the inlet of the
cooling jacket to ensure that there is only a 2 to 3oC difference between the CaCl2 brine
and the chlorate slurry outlet temperature.
There is a flow indicator on the inlet to the centrifuge to monitor the flowrate, as well as a
sample point to monitor product quality and calcium content to ensure the calcium
chloride brine is not leaking into the crystallizer.
4.3. Simulation
At the beginning of the project, a suitable simulation tool was sought after to aid with
calculations. HYSYS and ASPEN were determined to be not suitable due to the fact that
there are no crystallizer vessels in either software and the slurry is a saturated feed and
that would be difficult to model. After consultation with the Department of Chemistry
and the Department of Geology at the University of Saskatchewan, it was determined that
-
34
no suitable simulation software would be readily available to use in the design of this
project.
Microsoft Excel was used to aid in the calculation of mass and energy balances of the
process.
4.4. Plot Space Plan
The F-100 Chlorate Mix Tank will be placed on the third floor of ERCOs facility to
allow gravity to assist with the flow as can be seen in Figure 4.2. The D-120 will be
placed on the ground floor of ERCOs facility as there is room available as shown in
Figure 4.3. Because the plant is enclosed, it will be quite difficult to move these vessels
into position. This resulted in high installation costs for these pieces of equipment. The
refrigeration skid will be placed on the roof of one of the lower levels of the plant to ease
of installation and as a result of space limitations.
-
35
Figure 4. 2: Plot Space for F-100 Chlorate Mix Tank
-
36
Figure 4. 3: Plot Space for D-120 Crystallizer
-
37
5. EQUIPMENTSIZING
5.1. Mix Tank
The mix tank is an existing piece of equipment that ERCO has from previously shut
down plants. It is made with titanium to prevent erosion/corrosion from the chlorate
slurry and has an agitator. It has a height of approximately 4.3 m and a diameter of 1.1 m
with a total volume of 4 m3. This more than meets Ulrichs requirement for a feed tank
with a residence time of 1800 s. With the chlorate flowrate of 2.5 m3/hr, a feed tank with
a volume of 1.25 m3 would suffice but since ERCO already has this tank it will be
utilized. The tank will be equipped with a gooseneck vent to sweep any gas that may
form and send it to the off-gas fume scrubber. It will also have a recycle line to ensure
the slurry is well-mixed and to provide some flexibility in the flowrate to the crystallizer.
5.2. Pump Material Selection
Centrifugal pumps are used throughout the process because of the low slurry flows and
low pressures in the system. Fortunately, they are relatively inexpensive. The process
slurry pumps are stainless steel with wetted lined parts to prevent erosion/corrosion issues
because of the solid crystals in the slurry. Stainless steel with wetted lined parts was used
instead of titanium because it is significantly less expensive.
-
38
It was determined that a centrifugal stainless steel pump would also be sufficient for the
calcium chloride brine flow. A proposal for the K-123 Calcium Chloride pump was
provided by Goulds Pumps (shown in Appendix L) based on the conditions supplied by
SMK Integration Solutions.
5.3. Crystallizer Feed Pump
The K-110 Crystallizer Feed Pump was sized based on the pressure required to reach the
D-120 Crystallizer from the F-100 Chlorate Mix Tank. The top of the mix tank and the
top of the crystallizer were assumed to be at atmospheric pressure (101.3 kPa). The
suction pressure of the pump was based on the head of the mix tank and was calculated to
be 175 kPa. The pump will have to pump the slurry up 4.88 m for the recycle of the mix
tank, over the length of pipe from the pump to the crystallizer (6.1 m or 20 ft) and
through a globe valve with a K-value of 9.5 (1/2 open). Therefore, the discharge pressure
on the pump had to be 243 kPa. The pressure increase across the pump will be 68 kPa.
Chemical Engineering: Process Design and Economics; A Practical Guide (Ulrich,
2004) was used to determine the size of the pump needed from the pressure increase in
the pump using the following equation:
is
PqW = && (5.1)
The intrinsic efficiency of the pump, I, was determined to be 0.144 from the following equation:
)1)(12.01( 8.027.0 = qi & (5.2)
-
39
The viscosity of the slurry was supplied by ERCO to be 0.003 Pa*s. The shaft power,
sW& , was then determined to be 328 W.
The driver efficiency, D, was then determined from Figure 4.2 in Ulrich to be 0.82 and the power consumption, P, for the pump was found to be 400 W from the following
equation:
Di
PqP = & (5.3)
Further details of the calculations can be found in Appendix F.
5.4. Crystallizer
When operating a sub-zero temperature crystallizer, industry experience indicates that
heat transfer should have a temperature difference of no more than 2oC. This poses
restrictions on the operation of crystallizer and needs to be quantified. The crystallizer
vessel that will be used is a CAPO Reactor and is shown in Figures 5.1 and 5.2 below.
-
40
Figure 5. 1: Top of CAPO Reactor
-
41
Figure 5. 2: Bottom of CAPO Reactor
-
42
SMK Integrated Solutions performed a theoretical analysis of the CAPO Reactor vessel
based on a maximum temperature difference of 2oC. The analysis, based on first
principles and correlations presented in Fundamentals of Heat and Mass Transfer 5th
Ed., (Incropera & DeWitt, 2002) showed that the required rate of heat transfer could not
be achieved with the existing CAPO Reactor vessel and a 2oC temperature difference.
The analysis of the crystallizer as a heat exchanger was not a straight-forward calculation.
The difficulties are present in numerous aspects of the analysis; however, these
difficulties could be handled by making some assumptions, such as:
1. Exact measurements of the tube size and thickness were not known. Estimates of
tube sizes were made based on photographs of the end of the tube bundle. These
estimates were then verified against standard gauge tubing sizes. ERCO indicated
that the tubes were likely 18 gauge (referenced from a similar style titanium tube
sheet at their plant site), and were made of Grade 2 titanium.
2. Physical properties of sodium chlorate slurry relating to heat transfer were not
readily available:
a. The effective viscosity of the slurry was assumed to be 0.003 Pa-s. This
was based on a previous estimation made by ERCO when designing a
pump. This effective viscosity was estimated for slurry at process
temperature (18-35oC). The effect of cooling the slurry was not accounted
for in the use of this effective viscosity.
b. A Prandtl number could not be found in any literature for a saturated
solution of sodium chlorate, nor for the slurry. A Prandtl number was
-
43
found for the calcium chloride brine to be Pr(0oC, 30 wt% CaCl2) = 0.7.
This value of Prandtl number seemed reasonable and was assumed to be
the same for the sodium chlorate slurry for the purposes of the heat
transfer analysis.
3. The alignment of the tubes around the annulus of the agitator draft tube could not
be accurately described as either aligned or staggered arrangements, but a
combination of the two arrangements, as can be seen in Figure 5.3. Therefore,
best estimates of measurements of aligned and staggered arrangements were used
to calculate convective heat transfer coefficients and the average of the two results
was taken to be accurate.
-
44
Figure 5. 3: Interior of CAPO Reactor
-
45
4. The flow orientation is parallel flow as the slurry flows down the annulus and up
through the vertical tubes and the calcium chloride brine flows up the baffled shell
side of the heat exchanger as shown in Figure 5.4. Assuming that the exiting (hot)
calcium chloride brine can reach a temperature within 0.5oC of the exiting (cold)
slurry, the log-mean temperature difference (driving force for heat transfer) was
calculated to be 1.08oC. Due to the low low-mean temperature difference, a very
high flow of calcium chloride brine and chlorate slurry will be required to achieve
the required amount of heat transfer, and may lead to operability issues with
respect to pressure drops and pumping requirements for the refrigerant fluid.
5. When accounting for the heat transfer resistance of conduction through the
titanium tubes, the temperature difference of 2oC will not be sufficient to produce
the required heat flux.
-
46
Figure 5. 4: Dimensions of Heat Exchanger Jacket of CAPO Reactor (Crystallizer) Vessel, (a) Top View: Tubes Run Through Annulus; (b) Side View: 5 Baffles Equally Spaced
Slurry flow out (h,o)
Slurry flow in (h,i) 1360mm
300mm
1830
mm
267mm
(a) (b)
CaCl2 brine flow in (c,i)
CaCl2 brine flow out (c,o)
-
47
The calculated heat transfer rate when accounting for all resistances, with no fouling
resistance, under the conditions of 150 m3/hr CaCl2 brine flow in the shell side and 0.5-2
m/s flow velocity of slurry on the tube side, was only 2.2 kW. If the conduction
resistance is ignored from the calculation, there would be more than enough heat transfer,
but this would be technically incorrect.
After discussing this concern with ERCO and referencing design handbooks, it was
concluded that this result did not seem reasonable. Perrys Chemical Engineering
Handbook (1997) showed that a typical heat exchanger in chilling service would have a
typical U value (overall heat transfer coefficient) of 1135-1420 W/m2-K (200-250
Btu/oF.ft2.hr). Ulrichs Process Design and Economics: A Practical Guide (2004) showed
that a typical U value for a brine-brine service shell and tube heat exchanger was 800-
1000 W/m2-K. Using a U value of 1300 W/m2-K, the heat transfer capability was
determined to be 156 kW, slightly under the required rate of 190 kW. The client decided
this was accurate enough to assume the crystallizer will work for this application. SMK
Integrated Solutions recommends that ERCO verify the heat transfer capabilities of this
vessel before installing it for service in the proposed process.
Other parameters that were determined for successful operation of the existing CAPO
reactor vessel as the low temperature crystallizer include the impeller rotation speed and
the pressure drop across the shell side of the heat exchanger. To produce a slurry velocity
of 2 m/s in the tubes, the impeller would need to rotate at approximately 41 rotations per
minute (RPM). This parameter was calculated based on best estimates (from photographs
-
48
of equipment) and correlations from Perrys Chemical Engineering Handbook (1997).
The power requirement for the impeller operation was estimated to be 8.7 kW. The
pressure drop across the shell side of the heat exchanger jacket was estimated to be 500
kPa when pumping at a rate of 150 m3/hr. The pressure drop across the shell side of the
crystallizer was then calculated and it was determined to be only 64 kPa.
Detailed calculations of the heat transfer, pressure drop and impellar rate can be found in
Appendix H.
5.5. Centrifuge Feed Pump
The K-130 Centrifuge Feed Pump was sized based on the pressure required to reach the
H-140 Pusher Centrifuge from the D-120 Crystallizer. The top of the crystallizer and the
inlet of the centrifuge were assumed to be at atmospheric pressure (101.3 kPa). The
suction pressure of the pump was based on the head of the crystallizer and was calculated
to be 178 kPa. The pump had to be able to pump the slurry over 15.2 m (50 ft) and up
6.1 m (20 ft). Therefore, the discharge pressure on the pump needed to be 296 kPa. This
meant that there was a pressure increase in the pump of 118 kPa.
Ulrich (2002) was used to determine the size of the pump needed from the pressure
increase in the pump using the following equation:
is
PqW = && (5.4)
-
49
The intrinsic efficiency of the pump, I, was determined to be 0.144 from the following equation:
)1)(12.01( 8.027.0 = qi & (5.5) A slurry viscosity of 0.003 Pa-s was used for the calculations; this value was supplied by
ERCO. The shaft power, sW& , was then determined to be 569 W.
The driver efficiency, D, was then determined from Figure 4.2 in Ulrich (2007) to be 0.83 and the power consumption, P, for the pump was found to be 686 W from the
following equation:
Di
PqP = & (5.6)
Further calculations for the centrifuge feed pump can be found in Appendix I.
5.6. Centrifuge
The H-140 Pusher Centrifuge is an existing centrifuge currently at the ERCO facility but
in a shut down section of the plant and is shown in Figure 5.5. The centrifuge is titanium
to prevent erosion/corrosion issues and has a maximum flowrate of 3 m3/hr. Since the
flowrate in the process is 2.5 m3/hr, this centrifuge is acceptable. The solid crystals leave
the centrifuge and are transported to the Chlorate II Dryer by the I-150 Screw Conveyor
that is attached to the centrifuge. Using the existing 76-CF-101 Primary Centrifuge was
considered, however the 35oC chlorate slurry mixing with the chlorate slurry from the
LTCU would have resulted in dissolution of the sulphate crystals.
-
50
Figure 5. 5: Existing Pusher Centrifuge
-
51
5.7. Refrigeration Skid
The refrigeration skid was sized based on the amount of cooling needed to reduce the
crystal temperature from 35oC to -5oC in the crystallizer. The heat capacity of the slurry
was provided by ERCO and the flowrate was known; therefore, the amount of energy that
had to be removed from the slurry could be determined. There is also heat generated
from the crystallization of chlorate in the crystallizer and this was determined from the
amount of chlorate that will be crystallized and the heat of crystallization of chlorate.
Therefore, the total heat that will be necessary to remove is 190 kW. ERCO then
contacted CIMCO Refrigeration for a 50 ton of refrigerant (177 kW) refrigeration skid
proposal; the proposal is located in Appendix J. ERCO has also discussed using a 100
ton of refrigerant (354 kW) refrigeration skid that they have at their facility that is not
being used. After some discussion, ERCO was satisfied with the amount of cooling the
CIMCO refrigeration skid would provide, thus it was chosen to be implemented into the
process. The refrigeration skid includes the following major equipment:
(2) Copeland Semi Hermetic Screw Conveyors (1) Standard Refrigeration DX Chiller (1) Armstrong Pump (2) CIMCO Shell and Tube Condenser (2) CIMCO High Pressure Receiver (1) CIMCO Control Panel (standard relay-logic type c/w all motor
starters)
Associated valves, piping and fittings Armaflex insulation
-
52
Structural steel base Corrosive paint Compresser Oil initial charge Initial drier cores Initial refrigerant charge
Not included in the CIMCO refrigeration skid is any electrical wiring and/or electrical
components and any water connections.
5.8. Calcium Chloride Pump
The K-123 Calcium Chloride Pump was sized based on the pressure required to pump the
calcium chloride brine through the shell of the D-120 Crystallizer from CIMCO
Refrigeration unit. The suction pressure of the pump was assumed to be 203 kPa and the
pressure drop across the refrigeration unit was estimated to be 200 kPa. The pressure
drop through the crystallizer was estimated to be 500 kPa. The pump had to be able to
pump the slurry over the length of pipe from the pump to the crystallizer (46 m, 150 ft),
through the crystallizer, up 9.1 m (30 ft), over the length of pipe from crystallizer to the
refrigeration unit (46 m, 150 ft), and through the refrigeration unit back to the pump.
There is also a slipstream of calcium chloride going to the E-124 Brine Wash Heat
Exchanger but the flow in this line is so small that the pressure drop was assumed
negligible. Therefore, the discharge pressure on the pump needed to be 1153 kPa. This
meant that there was a pressure increase in the pump of 951 kPa. (The proposal can be
found in Appendix L).
-
53
Ulrich (2002) was used to determine the size of the pump needed from the pressure
increase in the pump using the following equation:
is
PqW = && (5.7)
The intrinsic efficiency of the pump, I, was determined to be 0.728 from the following equation:
)1)(12.01( 8.027.0 = qi & (5.8)
The viscosity of the calcium chloride brine used was 0.0045 Pa*s. The shaft power, sW& ,
was then determined to be 72.8 kW.
The driver efficiency, D, was then determined from Figure 4.2 in Ulrich to be 0.94 and the power consumption, P, for the pump was found to be 77.2 kW from the following
equation:
Di
PqP = & (5.9)
Details of the calculations discussed above can be found in Appendix K.
-
54
5.9. Brine Wash Heat Exchanger
The Brine Wash Heat Exchanger, E-124, was sized based on the brine wash flowrate
provided by ERCO. This brine wash flowrate was specified to be 2.5 L/min. Assuming
that the inlet temperature of the brine fluid was 20oC or 293 K, and the outlet temperature
was -5oC or 268 K, the heat transfer rate required was calculated to be 4.3 kW.
This heat exchanger was initially designed as a double-pipe (single-tube) heat exchanger
as these are typically the cheapest exchangers available. The heat transfer area required
for this exchanger was calculated to be 0.45 m2. The smallest commercially available
double-pipe heat exchanger is 0.6 m2, according to Ulrich (2002) Figure 5.36, so this size
was used for costing. After determining the dimensions of the double-pipe heat
exchanger (15 meters in length), it was decided that it would be too long for the space
that ERCO has available for this equipment. Therefore, a plate and frame type heat
exchanger would be used due to plot space restrictions.
The plate and frame heat exchanger was sized based on the same operating conditions
specified for the double-pipe exchanger. Assuming a coolant flow of 0.5m3/hr, a log-
mean temperature difference of 6.4oC was calculated. From Ulrich, a typical U value for