Publication No. 11
Nitroglycerine /Nitroglycol
Handling of Spent Acid and Effluents
Contents
Section
1.
INTRODUCTION
2.
MANUFACTURE OF NG
2.1.
2.2.
2.3.
2,4.
2.5.
2.6.
2.7.
2.8.
Preparation of mixed acid
Preparation of glycerine/glycol
Nitration.
Separation of ng spent acid
Handling of spent acid
Dilution of spent acid
Storage of spent acid
Stability of spent acid
3.
HANDLING OF EFFLUENTS
4.
REPAIR AND MAINTAINANCE WORK
5.
SUPERVISION
APPENDIX 1.
FREEZING POINTS
APPENDIX 2.
SOLUBILITIES
APPENDIX 3.
APPENDIX 4.
SOLUBILITIES
STABILITY
INTRODUCTION
During recent years FEEM has noticed an increased rate of accidents in the manufacture of NG,
particularly in the handling of Spent Acids. For a variety of different reasons, NG has often separated
from the Spent Acid and has accumulated in unexpected places where, over a period of time, it can
build up to dangerous proportions.
As acidic NG is a chemically unstable and sensitive mixture, all possibilities of unintentional
accumulation must be avoided.
The conditions for the safe nitration of Glycerine and/or Glycol are well known by all manufacturers
and are not dealt with in detail here. This Code gives advice on the handling of Spent Acid and
effluents from the stabilisation process, based on the joint expertise of the members of the
Federation.
Most installations for the manufacture of NG are of the continuous type. Although different
continuous systems are available, the principal unit operations are virtually the same in each case.
These may be listed as follows, the appropriate section within this Code being in brackets
Preparation of Mixed Acid
(2.1)
Preparation of Glycerine/Glycol
(2.2)
*Preparation of washing liquids
Nitration
(2.3)
Separation of NG/Spent Acid
Spent Acid
Handling
(2.4)
(2.5)
Dilution
(2.6)
Storage
(2.7)
*Denitration
*Stabilisation, washing of acid NG
Handling of effluents from the stabilisation process
*Storage of Stabilised NG
* Not within the scope of this Code
(3)
2.
MANUFACTURE OF NG
2.1
Preparation of Mixed Acid
Depending on availability of raw materials,
the Nitric Acid content of the Mixed Acid can vary
between 45% to 67%. Every batch of mixed Acid should be analysed and proper records should be
kept.
The Nitrous Acid content should not exceed 0.2%.
Approved stainless steel should be used in the construction of mixers, coolers, storage tanks and
tubing. Mild steel as construction material will cause formation of sludge which can Interfere with
the subsequent separation of an acidic NG from the Spent Acid
The freshly prepared mixed acid should be filtered or settled before use in nitration.
Gasket materials should be tested for acid resistance. Explosions have occurred which have been
attributed to the nitration of organic components in these materials.
Pumps should always be started up from a protected place and preferably remotely.
Flange couplings should be so protected that there is no danger of acid burns, should a gasket blow
out.
Coolers should be regularly checked for tightness.
Special care should be taken to avoid organic coolants (e.g. methanol) coming into contact with
mixed acid in enclosed systems.
2.2.
Preparation of Glycerine/Glycol
The following are typical specifications
Glycerine:
Glycerine
Water
Refractive index nD20
Ash
Chlorides as NaCl
Saponification equiv.
% Na20 in deficiency
Acidity or Alkalinity
equiv. to Na20
Density, + 20 C
Reducing Substances
% min 99.0
% max 1.0
1.4725-1.4750
% max 0.02
% max 0.01
% max 0.10
% max 0.01
1.2588-1.2622
traces only
Glycol:
Monoethylene Glycol
Diethylene Glycol
Water
Refractive index nD20
Ash
Chlorides as NaCl
Saponification equiv.
% Na20 in deficiency
Acidity as HAc
Density, + 20oC
Reducing substances
% min 99.0
% max 0.3
% max 0.5
1.4315-1.4325
%.max 0.001
% max 0.001
% max 0.01
max 0.001
1.1125-1.1140
traces only
Sampling should be carried out according to accepted statistical methods. Proper records of the
analyses should be kept.
Note: As bulk transport of these raw materials Is becoming more and more common, unexpected
impurities can occur due to inadequate cleaning of the transport tanks and precautions should be
taken to prevent this occurring.
Glycerine and/or Glycol should be maintained at a temperature which corresponds to that at which
the flow control devices were calibrated.
2.3
Nitration
Irrespective of the continuous method used, the nitration reaction can be said to take place in a
“buffer solution” of Spent Acid.
The safety of the nitration process is largely dependent on the composition of the Spent Acid
(provided of course that raw materials are within specification).
The composition and the temperature of the Spent Acid determines the content of dissolved NG in
the Spent Acid as well as the content of dissolved Nitric Acid in the NG and the content of
Dinitroglycerine and/or Mononitroglycol in the NG.
The generally accepted recommendation is that the Nitric Acid content of the Spent Acid should not
be lower than 8%-9% but preferably 9%-11%. The water content should not be higher than 16% and
the Nitrous Acid content of the Spent Acid should not exceed 0.2%.
The composition of the Spent Acid is entirely dependent on the nitration temperature and the ratio of
Mixed Acid or Nitrating Acid/Glycerine and/or Glycol fed to the nitrator. This ratio is controlled by
flow control devices, redox measurement and/or reaction temperature measurement.
However as instruments can fail, it is most strongly recommended that complete analysis of the
Spent Acid should be carried out on a regular, routine basis.
A calculation of the output shall also be carried out regularly and all other relevant process
parameters shall be recorded.
2.4
Separation of NG/Spent Acid
NG and Spent Acid leave the nitration process in the form of an emulsion in which NG is emulsified
in the Spent Acid. Separation is performed in a static or dynamic separator, from which the NG
comes out as a clear liquid containing 7%-9% dissolved Nitric Acid. Normally the Spent Acid has a
dissolved NG-content of 2%-4% and can also contain small amounts of emulsified NG. The optimal
separation conditions are given by the equipment suppliers.
Poor separation is mainly due to impurities In the Glycerine/Glycol and in the mixed Acid. If the
emulsion is too cold it will increase the viscosity of the Spent Acid and thus affect the separation.
Bad separation can also be caused by slipping V belts on dynamic separators.
Regular checks should be carried out to ensure that there is no blockage or interference with the flow
of NG and Spent Acid from the separator.
2.5
Handling of Spent Acid - General.
All equipment In contact with Spent Acid should be of an approved stainless steel or suitable acid
resistant material. Accidents have occurred when loosening mild steel flanges secured with rusty
bolts and nuts.
Gasketing and gland packing materials should be tested for acid resistance and NG absorption.
Explosions have occurred which have been attributed to the nitration of organic components in
materials used for these purposes.
Pumps should always be started up from a protected place and preferably remotely.
Spent Acid lines should be supported and follow a gentle inclined slope. Supports and incline should
be checked regularly. In cold climates heat tracing is recommended. Sun shields are recommended
in hot climates.
Flange couplings should be arranged to avoid any kind of obstruction to flow, or places where
pockets of NG might collect.
Flange couplings should be so protected that there is no danger of acid burns, should a gasket blow
out.
Flange couplings on vertical lines should have the nuts on the upper side.
It is now well known that welded constructions, especially in thin material, can develop microscopic
cracks and pores where NG can leak. Welded closed spaces should therefore be avoided. Tank
supports for instance should always be capable of easy and ready inspection.
2.6
Dilution of Spent Acid
The Spent Acid leaving the separation process is saturated with dissolved NG, the amount depends
on the separation temperature. It can also contain droplets of free NG.
The Spent Acid should be diluted to avoid separation of acidic NG due to a fall in temperature during
the subsequent handling. Diluents, for example, can be water, 70%-72% Sulphuric Acid or 60%-63%
Nitric Acid.
A central feature is the need to ensure adequate solubility of the NG whilst maintaining stability and
taking freezing point into consideration. In general terms if the water content in the Spent Acid is
less than 14%, dilution with water is not advised. If the water content is greater than 14%, water
dilution can be an acceptable procedure. Reference should be made to the diagrams in the
appendices Nos. 1, 2, 3 and 4.
The dilution of Spent Acid and also the preparation of Nitrating Acid should preferably be carried
out continuously and immediately after the Spent Acid leaves the separation process.
All unintentional dilution of Spent Acid or acid sludge must be avoided. The risk of dilution from
condensation of water vapour must be considered. Water lines connected to Spent Acid lines or tanks
should have double valves with a drain between.
Appendix 1 shows the freezing point of the system HN03-H2S04- H20
Appendix 2 gives example of the solubility of NG in Spent Acid.
Appendix 3 gives example of the solubility of NG in Spent Acid at varying temperatures.
The complexity of defining these parameters (freezing point, solubility) with precision is formidable.
The diagrammatic analyses given in the Appendices cover a representative set of parameters and can
be taken as indication of the general operating area.
It is recommended that, for wider coverage, reference should be made to the following publications:
Freezing point
*Holmes, WC, Hutchison, GF, Zieber, B, Freezing Points of Mixtures of Sulphuric and Nitric Acids,
Industrial and Engineering Chemistry, 1931, Vol.23, No.10, 1102-4.
*Carpenter, CD, Lehrman, A, The Solid-Liquid Temperature………Read and discussed before
Cincinnati meeting, Dec. 4, 1925, Am. Inst. Chem. Eng ., 1925 35-73.
Solubility
* Urbanski, T, Chemistry and Technology of Explosives, 1965, Vol. 2, 39-40.
*Klarsen, H J,Humphrey, J M, Manufacture of Nitroglycerine by the Biazzi Continuous Process.
Chemical Engineering Progress, 1953, Vol.49, No.12, 641-6.
*Wallerius, G, Beitrge zur Kenntnis des Nitroglycerin-prozesses, The Royal Swedish Institute for
Engineering Research, Proceeding No.113, 1931.
hman, V, Beitrge zur Kenntnis des Nitroglycerin-prozesses. The Royal Swedish Institute for
Engineering Research, Proceeding No.114, 1931.
*hman, V, die Phasen- und Reaktionsgleichgewichte bie der Daratellung von Nitroglykol. The
Royal Swedish Institute for Engineering Research, Proceeding No.139, 1936.
Stability
*hman, V, Camera, E, Cotti, L, The Stability of Acidic Nitroglycerine and Spent Acid '
Explosivestoffe, 1960, No.6, 120-7, No.7, 148-58.
*hman, V, Camera, E, Cotti, L., The Stability of the Acidic Nitroglycol system, Explosivestoffe,
Mai 1961, No.5, 95-113.
*Urbanski, T, Chemistry and Technology of Explosives, 1965, Vol.2, 84.
*hman, V, Redox Measurements Show Safe Nitration Conditions, Svensk Kemisk Tidskrift, 1966,
Vol.78, No.1, 20-34.
2.7
Storage of Spent Acid
Tanks for the storage of Spent Acid should always be equipped with agitation and easily accessible
inspection facilities.
Outlets from Spent Acid tanks to pumps, denitration etc., should be arranged so that the top layer of
the contents always remains in the tank.
Storage tanks should be equipped with proper temperature recording devices. 1n hot climates sun
shields and facilities for water spraying are recommended.
Storage quantities should be kept to a minimum consistent with production requirements and
precautions taken to prevent unintentional dilution during this period of storage.
There should be a daily regular check for possibly separated NG.
Particular care should be taken to ensure that there is no possibility of separated NG In the transfer
lines.
Accidents have been caused by unintentional separation of NG and cases have been reported where
hundreds of kilograms have been involved. Therefore, In the planning of tank(s) installations, means
should be provided for taking care of separated NG. Handling of NG on a non-routine or irregular
basis is always accompanied by an increased risk.
Small amounts of separated NG can be dissolved by adding Mixed Acid under agitation. If larger
amounts of NG should separate, proper installations for displacement, collection and stabilisation
should be available.
In some nitration installations an overhead tank is provided for Spent Acid which is to be used as
starting acid or displacement
acid. The ambient temperature of this tank should be such as to
prevent separation of NG.
It is good practice to empty the tank completely on a regular basis.
To avoid build-up of pressure in case of a “fume-of” Spent Acid tanks should be properly vented.
2.8 Stability of Spent Acid
It Is emphasised that particular care must be paid to the stability of the diluted Spent Acid.
Appendix 4 indicates recommended maximum storage time for spent Acid of various compositions.
3.
THE HANDLING OF EFFLUENTS
The stabilisation of NG includes washing with alkaline solutions and water, Intermediate
separation(s) and emulsion transport(s). As NG has a certain solubility in these solutions,
recirculation is often adopted as a common means of reducing NG-losses.
The solubility of NG in aqueous solutions is dependent on the temperature. Even a slight decrease of
the temperature will cause precipitation of NG. Droplets of free NG may also be present due to
incomplete separation.
All equipment should be of stainless steel or suitable plastic material.
Transfer lines should be supported and follow a gentle even incline. Any situation which could load
to gravity traps must be avoided.
Tanks and labyrinths containing spent water should have an outlet at the lowest point, preferably
with a sight-glass or reliable level probe where precipitated NG can be detected. NG should be taken
away regularly during production runs, preferably with an educator arrangement.
When spent waters are recirculated the correct installation and type of pump must be carefully
considered. It should never be placed at the lowest point of the system and should be self-draining to
avoid any accumulation of NG. Use of self-draining submersible pumps is recommended.
Effluents leaving the process should be diluted with a sufficient amount of water to avoid
precipitation of NG in drains and collecting areas.
Welded closed spaces in the equipment, according to Item 2..5, must be avoided.
4.
REPAIR AND MAINTENANCE WORK
Dismantling of equipment, tubing etc. should be carried out with the utmost care. It should always be
assumed that joints may have been leaking and that underlying bolts and nuts may be contaminated
with NG. Spray rust dissolvers will phlegmatize NG and are recommended for use before loosening
stainless steel nuts. Before use the rust dissolver should, of course, be tested for compatibility with
NG.
Dibutylphtalate or acetone with 10% of water can also be used to ensure that stainless steel bolts and
nuts are safe before loosening.
Before being taken to a repair shop any item which may have been in contact with NG should be
thoroughly cleaned.
If any particular item cannot withstand the heat of an open fire then superheated steam, boiling
water, solvents and/or NG destroyer can be used. However, the safest way to destroy traces of NG is
to subject the item to an open fire, provided that a temperature indicating colour is used to assure that
the decomposition temperature of NG has been reached.
Routine replacement of parts as recommended by the equipment supplier is an essential part of safety
maintenance.
The precise specification of plastic material used in the manufacturing equipment should be carefully
checked against the equipment supplier's recommendation. It must be assured that the plastic is
compatible with the actual media (acid, acidic NG, alkaline solution and mixtures).
Before starting up production after maintenance work, a function test of the equipment and
instrumentation should always be carried out to assure proper reassembly.
5.
SUPERVISION
It is the responsibility of the management to see that persons in charge of
NG-manufacture have a proper theoretical and practical knowledge of all details of the
manufacturing process and that they are given the authority and means to perform
their task. Reference should be made to FEEM Code of Good Practice on
"Manufacturing Control Systems, Monitoring and Updating ".
In normal NG-production all parameters such as analyses, temperatures, redox-values,
solubilities, output etc are constant within accepted limits over a period of many years.
Any deviation from approved limits or sustained trends against established and
accepted limits must be investigated thoroughly and cause(s) ascertained and
corrected.
With the increased automation of the NG-process there is a tendency among managers
and operators to rely heavily on instruments, TV-monitors and indicators. However,
indicators and instruments can and will fail and give wrong information.
THEREFORE THE IMPORTANCE OF REGULAR ROUTINE PHYSICAL
INSPECTION OF THE EQUIPMENT OF ALL UNIT OPERATIONS
INVOLVED IN NG-MANUFACTURE CANNOT BE OVER EMPHASIZED.
APPENDIX 1
APPENDIX 2
APPENDIX 3
APPENDIX 4