The Use Organic Solvents Evaporation
Losses System Calculation Control in Tanks of
Storage, Productive Systems and Transportation in
Brazil for Prevention of Environmental Impact;
Potential Fire and Incidents Risks
.
Chem Ing. Gilberto Saboia,BsC
Gilberto_saboia@hotmail.com
This paper has the intention of supplying subsidies to be a starting point for the Civilian
Defense, Fire Brigade and for the personal involved in environmental control; so that
they can evaluate an impact caused by the evaporation of solvents, and its inherent
potential risks; that may cause fires and environmental accidents.
This evaluation can be made even before the project or inspection phase of a new
industry, highway or on a new installation of storage tanks in storage parks project or
still as a forecast of the evaporation and risk in transport in a route or highway.
Besides the theoretical study we will be compare the real measurements data
of
physical inventory from 38 industries that answered to the questionnaire with the
studied theoretical data.
Comparing these inventory data with, for instance, the numbers obtained by the
mathematical experiment to analyze that an excessive value of losses should be verified,
it can indicate need to adapt or to install a system of recovery of the lost steams for the
atmosphere as form of control of the environmental emission during storage, transport
or process, but it can also indicate the need of larger logistic storage control ( for
example it can be having deviation for illicit ends and/or product can being stolen).
.
We interviewed 70 industries, among them the largest industries of paints and
stickers,05 Petroleum Refineries and 02 Petrochemical Plants installed in Brazil whose
results will be presented in this summary.
The Situation of Tanks of Storage and Transport - The Mathematical Study
Model Choice
The bulk tanks used for this kind of storage and rake-rail transport of solvents and
hidrocarbons ( included ethanol alcohol) in Brazil are steel cylinders manufactured
with commercial carbon steel plates or inox steel solded;with many varied capacities,
being more common 15, 30 and 50.000 liters.
Of the technical point of view these tanks are all atmospheric ones, that is, tanks that
are opened for the atmosphere through a breath through where the atmospheric air can
enter and steams of the solvent stored internally can leave.
In Petroleum Refineries and PetroChemical Plants we found storage systems with
flotation roof, inert gas or condensation systems for control and reduction of losses for
evaporation.
Then, we will study the most common situation found in the industries, storage site
parks of solvents and fuels, rake-rail transport that are the atmospheric tanks.
When the tank is empty after the exit of the solvent, it remains in its interior only mixed
atmospheric air with steams of the organic solvent.
Being the tank completely full, the whole kept steam will be expelled, for being
atmospheric tank this happens with a smaller pressure than 0,36 kPa.
Under atmospheric pressure, the mixture gaseous air / solvent steams behave as an ideal
gas. Bird et al, 1960. (hypothesis 1)
While the tank is being emptied, there will be a fast thermodynamic balance among the
phases liquid / steam, exercised by the pressure of steam of each solvent, that it happens
for the diffusion in stagnated gas. Bird et al, 1960.
When expelling the air of the interior of the tank, in the stuffing operation, actually all
the volume of the tanks (of storage, of transport or of production) are equal volume of
the tank, in liters, of air with solvent steams will be being expelled, lost for the
atmosphere.
The thermodynamic balance in the interface liquid-gas grows quickly.
The results below are them obtained as example, for a characteristic campaign
found in real situation in most of the brazilian medium or small industries and load
carriers ; that it is the movement of 15.000 liters every 2 days, but higher volumes can
be easily estimated.
They express volumes evaporated in liters for a movement of 15.000 liters in
stockpiling and production operation in conventional equipments of storage in
atmospheric tanks only closed with cover it doesn't stanch.
Randomic we had selected a period of the year with medium temperatures in the South
of Brazil : nor very high nor very cold
The medium temperatures were collected in the daytime (25 centigrade degrees) and
night (13 centigrade degrees) during the first semester of the year for the metropolitan
area of Curitiba City, Parana State ( 400 km south from Sao Paulo)
We also selected the main products found in the industries of paints and stickers, the
largest consumers of organic solvents.
In terms of fuels, we analyzed the alcohol and we can infer values for gasoline and
diesel, that possess many of these solvents in its composition (the gasoline for instance
is a hidrocarbons mixture with a distillation strip that is going from 39 to 240 centigrade
degrees; this means that it contains in its composition several solvents, as for instance
hexane, SPB (Special Boiling Point), toluene, xilens ,white spirits, C9 aromátics.
Even gasoline or Diesel fuel oil, however, are not so pure in chemical composition to
be a defined product in their compositions, their components percentage varies to
much, depending on the petroleum origin and process and for this reason we don't
have analytical data at this time that tecnically
mathematical study case.
Results of the Mathematical Study
allow us to include them in the
We will analyze with more details the first two products, ethyl acetate and ethanol, in
the sequence to present other results so that you can do the calculations and reach your
conclusions.
Product Ethyl Acetate:
Mês
Jan
Fev
mar/abr
mai/jun
jul/ago
Dia
9,13
9,28
8,71
7,67
5,88
Noite
4,11
4,22
3,99
3,47
2,96
Total
13,24
13,50
12,70
11,14
8,84
%
0,088
0,0900
0,0850
0,0740
0,0590
Analysis :
As expected, the volatile solvent evaporates more during the day where the temperature
is larger but you can confirm that it also evaporates during the night even with lower
temperatures.
The average January to August of this evaporation is 0,079%, it means that will lose for
the atmosphere 0,079% of the whole busy product in this period.
For instance, we will analyze the operation of a company or carrier of loads for a month
of 22 days.
It is going in to fill and to empty the tank 11 times; moving 15 000 liters every time, if
the tank is a 15.000 liters tank.
That means on that month it moved 165.000 liters.
165.000 x 0,079% = 130,35 liters that evaporated for the atmosphere, in only 22 days.
And if it was a 30.000 liters tank?
The calculation would be the same: 22 days are 11 operations x 30.000 liters = 330.000
busy liters in the month x 0,079% = 260,70 liters of loss in the month.
And so on.
Besides the financial loss for the company, because that is money that is getting lost has
some questions that the specialists can answer:
Which is the environment impact?
Which is the risk that these steams come to catch fire when/if the fire triangle happens(
ignition font, fuel, oxygen)?
Ethanol
Mês
Jan
Fev
mar/abr
mai/jun
Jul/ago
Dia
4,29
4,38
4,04
3,42
2,96
Noite
1,96
2,02
1,90
1,62
1,35
Total
6,25
6,40
5,94
5,04
4,31
0,0417
0,0427
0,0396
0,0336
0,0287
%
Evaporation range average % =0,037
An operation of an industry, carrier or gas station , working 22 useful days for a tank of
15.000 liters means that they have 165.000 busy liters in this period.
165.000 x 0,037% =61,47 liters evaporated in 22 days.
If the tank is of 30.000 liters it will be
330.000 x 0,037%= 122.95 liters evaporated in only 22 days.
How many trucks of alcohol, how many gas stations do have operating, which the
volume of busy ethanol in 2 days in our industry, city, State, Country?
Acetone(butanone)
Mês
Jan
Fev
Dia
21,69
Noite
Total
%
Mar/abr
Mai/jun
Jul/ago
22,08
19,57
16,88
14,85
10,46
10,75
10,17
8,91
7,66
32,15
32,83
29,74
25,79
22,51
0,0210
0,0220
0,1983
0,1719
0,1501
Jan
Fev
mar/abr
mai/jun
ul/ago
15,88
16,15
15,09
13,16
11,68
8,36
8,59
8,14
7,16
6,18
Hexane( Aliphatic C6)
Mês
Dia
Noite
Total
%
24,24
24,74
23,23
20,32
17,86
0,1616
0,1649
0,1549
0,1355
0,1191
Jan
Fev
mar/abr
mai/jun
jul/ago
Toluen
Mês
Dia
2,16
2,20
2,05
1,77
1,56
Noite
1,08
1,11
1,05
0,91
0,77
Total
3,24
3,31
3,10
2,68
2,33
0,0216
0,0221
0,0207
0,0179
0,0155
Jan
Fev
mar/abr
mai/jun
Jul/ago
%
Xilens
Mês
Dia
0,54
0,55
0,50
0,43
0,37
Noite
0,25
0,25
0,24
0,20
0,17
Total
0,79
0,80
0,74
0,63
0,54
0,0053
0,0053
0,0049
0,0042
0,0036
Fev
mar/abr
mai/jun
jull/ago
%
Aromatic C9 solvent
Mês
Jan
Dia
0,74
0,76
0,70
0,60
0,52
Noite
0,35
0,36
0,34
0,29
0,24
Total
1,09
1,12
1,04
0,99
0,66
0,0073
0,0075
0,0069
0,0066
0,0044
%
Field Research Results
From 70 consulted companies
adhesives industries with
answered this research 38
big, medium and small
paint industries and
sizes, 3 refineries and 2
Petrochemical Plants.
We verified that in their stock risings the real losses of mass are of the order of 0,5% in
Refineries, Central and companies with efficient system of evaporation storage control
systems like flotation roof, inert gas or condensation systems for control and reduction
of losses for evaporation.
In the larger industries or those that possess emission reduction systems such an as
valves of pressure relief and vacuous this loss only eats evaporation is among 1,8%
even to 2,7% in the other facilities where these systems are inexistent.
This percentage is on the total amount volume monthly solvents used ; it is different
from the the pictures thet we got in the mathematical formula because the mathematical
model doesn't take into account the turbulence generated in the production by the
mixers' rotation (usually mixture Cowles equipments- with motor-axis with turbine
stirrers) and it doesn't also consider kinetic and thermal energy generated inside these
equipments; some heat (in the dispersions mainly) up to 60 degrees centigrade or
more.
The mathematical formula doesn't also take into account some physical inventory that
were used
in the production but not written in the production orders such an as
solvents used in small cleanings and no thrown as consumption, leaks in the
productive equipment inter tranfers, etc.
How can we Reduce the Evaporation Losses With Low Cost
We believe that even in the storage systems and transport that commonly we found in
the industries of paints, adhesives and similar and also in the solvent and fuels dealers
in which the solvents-fuel reservoirs are cylindrical steel tanks atmospherically open,
with very low cost it is possible to reduce the losses for evaporation.
This could be obtained by the thermal isolation of the storage or transport tank using
double walls, for instance; or in quite economical way for the use of simple systems of
coils of cooling (some turns in coil in the breath tube with copper serpentine tubes
connected in a cheap cold water refrigeration/recirculation system, installed before the
relief and vacuous pressure valve.
In most used loading systems for the brazilian Petrochemical units and Petroleum
Refineries, it is already demanded underneath the shipment from the transporters with
equipment of recovery of volatile steams of the interior of the load tank, usually known
as Bottom Load System.
Another technical environmental recommendation is to install in the exit of the breath
for the atmosphere, after the relief and vacuous pressure valves, filters of adsorption of
steams, with stuffing of active coal, so that the little of steam of solvents that evaporates
from the tanks is absorbed, not being liberated for the atmosphere, as we can see in the
picture 1 below, got in the storage of solvents of the Carbono Química Enterprise (a
strong Brazilian chemical dealer installed in Bernardo Bernardo do Campo City- Sao
Paulo) where this system is being used for more than 8 years with good results.
Top of the Tank
Internal open filter view
For the mixture system and dispersion in industrial production elaborate with Cowles
equipment type, for instance, in which we registered the largest index of evaporation of
solvents in the industry of paints, a suggestion is the installation of systems of
condensers of steams of solvents, practice already applied in the USA and in Europe.
(as the system that we can see in the picture below, of a
Cowles
manufactured by Niemann GmbH; of 2001)
In this illustration we can see 4 copper condensers on the cap cover
dispersor
MATHEMATICAL CALCULATION - The BIRD EQUATION
Consideration 1 - open Tank
Consideration 2 - Diffusion in stagnated gas
:
Hypotheses:
1.
Solubility of the air in the organic liquids in subject is very low to the
atmospheric pressure.
2.
Thermodynamic balance in the interface liquid-gas grows quickly.
Equation:
Mass balance:
dN
Az
______________
dz
=
(1)
0
NAz – absolute molar flow of the organic in the gaseous phase (mol cm2 s-1)
Constituent equation (Fick´s Law):
dxA
NAz = c. DAB_______
+
.
XA .( NAz + NBZ )
dz
c–
molar concentration of the mixture (mol cm-3)
DAB - coefficient of diffusion of the organic in the air (cm2 s-1)
xA -fraction molar of the organic in the gaseous phase
NBZ – absolute molar flow of the air (mol cm-2 s-1)
NBZ = 0 – hypothesis 1 e 2
(2)
Integrating the equation (2) for constant NAz (equation (1)) along an arbitrary
diffuse course z:
NA
=
________
c. DAB .
________
(I-xA)
________
________
In
(3)
(I-xAO)
z
Under atmospheric pressure the gaseous mixture behaves as an ideal gas, soon
C =_____
_
,
RT
Where P is the pressure, T is the absolute temperature and R is the universal constant of
the gases
Admitting that far away from the interface the fraction of organic is despicable
(XAz = 0).
-P
NAZ = _______
DAB
_______
RT
(4)
In (I-xAO)
z
Integrating the flow along the free surface from the tank during a complete period of
emptying, it is obtained the evaporated total mass:
P
MT = 2L _____ DAB In
(
1
_______
) In (2 ) . t . M
A
RT
MT – evaporated total mass (kg)
L - length of the tank (5,4m)
P - atmospheric pressure (Pa)
R – universal gas constant (8,314,0 Pa m3 Kmol-1 k-1)
T - absolute temperature (k)
DAB – diffusion coefficientof the organic´s in the air (m2 s-1)
xAO – molar fraction of organic in the interface (psat/p)
(5)
psat – pressure of saturation pressure of the organic in T (Pa)
t – time (s)
MA – molecular mass of the organic´s in (kg Kmol-1)
To convert the mass evaporated in liters of organic liquid that were originally in the
tank is enough to divide her/it for the specific mass of the organic liquid:
VT
MT
= _____
Pa
VT - liquid total volume organic lost for evaporation in liters
Pa – specific mass of the organic liquid (kg litro-1)
The critical properties, density, parameters for Wagner's equation for calculation of the
pressures of saturation of the pure organic liquids were solitary of Reid, et al, 1986.
These data are shown in the table 1.
Table 1.
MA
TC
PC
K
x 10-5
VPa
VPb
VPc
VPd
PA
Kg/litro
Pa
Toluol
92,14
591,8
41,0
-7,28607
1,38091
-2,83433
-2,79168
0,863
Haxane
86,18
507,5
30,1
-7,46765
1,44211
-3,28222
-2,50947
0,659
Acetone
58,08
508,1
47,0
-7,45514
1,20200
-2,43926
-3,35590
0,790
Ethano
46,07
513,9
61,4
-8,51838
0,34163
-5,73683
8,32581
0,789
Xilen
106,17
630,3
37,3
-7,53357
1,40968
-3,10985
-2,85992
0,880
Ethyl
88,11
523,2
38,3
-7,68521
1,36511
-4,08980
-1,75342
0,901
128,26
568,0
23,3
-7,80573
1,68023
-4,50859
-0,78808
0,717
Acetate
C9
Wagner's equation for calculation of the steam pressures is:
1,5
___sat
( Pc ) ( ) [ (
p
In
=
1
_____
__T
VPA
1-
) (
T
+VPB
Tc
T
1- Tc
) (
+VPC
3
1-
1-
T
Tc
)
6
+ VPD
(
1-
T
Tc
Tc
Where:
psat- saturation pressure (Pa)
Pc- critical pressure (Pa)
Tc- critical temperature (K)
T – temperature (K)
VPA, VPB, VPC E VPD - they are parameters of Wagner's equation.
All the necessary data are presented in the table 1.
The coefficients of diffusion of the organic steams in the air were calculated using the
methodology proposed by Fuller, et al, 1966.
10-7 T1,75
DAB
=
(
)
1+ 1
MA
___________________________
MB
P
[(
VA
1
3
1
2
1
3
2
) (V ) ]
+
B
Where:
DAB – coefficient of diffusion of the organic in the air (m2s-1)
MA – molecular mass of the organic product (g mol-1)
MB – molecular mass of the air – 28,9 g mol-1
VA – volume of diffusion of the organic product (cm3mol-1)
VB – volume of diffusion of the air = 20,1cm3mol-1
P – pressure (atm)
)
]
T – temperature (K)
The data of VA are shown in the table 2.
Table 2. DifusionVolumes for the air difisivity components calculation .
Componente
VA cm3mol-1
Toluen (C7H8)
131,34
Hexane (C6H6)
126,72
Acetone(C34H60)
66,86
Ethanol (C2H6)
50,36
Xilens (C8H10)
151,80
Ethyl Acetate (C4H10O2)
Solvent C9 (C9H20)
96,76
188,10
Bibliography:
Avaliação Teórica Das Perdas por Evaporação em Tanques de Armazenamento de Solventes; Anais do
8o. Congresso Internacional de Tintas da ABRAFATI, 2003; eng. Gilberto Sabóia e eng. Paulo Auriquio
(Petrobras S.A)
Controle das perdas por evaporação de solventes orgânicos em sistemas produtivos de unidades
industriais de tintas no Brasil,Anais do 9º. Congresso Internacional de Tintas da Abrafati 2005.
eng.Gilberto Sabóia e Dr Roberto Giannini.
BIRD, R.B., Stewart, W.E e Lightfoot, E.N., “Transport Phenomena”, 1 ª ed., John Wiley
&
Sons, Singapura, 1960.
PERRY, R.H., Green, D.W. e Maloney, J.O., “Perry’s Chemical Engineers’Handbook”, 6 ª ed.,McGrawHill, USA, 1984.
REID, R.C., Praunitz, J..M. e Poling, B.E., “The Properties of Gases and Liquids”, 4ª ed., Mc Graw-Hill,
USA, 1987.
Niemann GmbH & Co brochures; www. niemann.com.de