Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
l. FIELD OF THE INVENTION: The invention relates
to a method and composition for effecting the cleaning of an
isolated section of a hydrocarbon transmission conduit, such as
a pipeline, to remove paraffin deposits therefrom by kinetically
and chemically effecting the dissolution of the paraffin deposits.
2. HISTORY OF THE PRIOR ART: As used herein, "hy-
drocarbon transmission conduits" refers to any cylindrical
means for transmitting hydrocarbons above the top surface of a
subterranean well, such as means extending from the Christmas
tree to and through the sales line, pipelines, conduits trans-
mitting hydrocarbons to and within refineries, and the like.
The accumulation of paraffin deposits in hydrocarbon transmis-
sion conduits, particularly subsea pipelines, is a problem that15 1
has been plaguing the oil-producing industry. By reference to
"paraffin", we mean a wax-like organic precipitate from hydro-
carbons of the alkane group. These paraffinic waxes are either
normal, branched, or cyclic alkanes and are described by the
following formula: Cn H(2n~2)~ where n is a positive integer.
These paraffins are generally inert to acids, bases, and oxi-
dizing ager.ts. Typically, when a crude oil containing paraf-
fins flows through pipelines at comparatively reduced tempera-
tures, such as would be encountered in a subsea pipeline, the
paraffins will start precipitating out of the crude in the form
of tiny crystals. As these paraffin crystals fall out of
solution, they will adhere to the walls of the conduit, and the
fluid passing through is exposed to an ever increasing number
of nucleation sites. Contemporaneously, the rate of paraffin
precipitation generally accelerates because of the abundance of
4ia4
1 1 these nucleation sites. Eventually, the effective flow area of
j the conduit is significantly reduced and, hence, cleaning of
the pipeline to eliminate the paraffin deposits is desired.
~ Prior art methods of removing paraffin deposits from
pipelines have been expensive and time-consuming. One method
has employed a mechanical action by using "pigs" which effect a
ii wiping action within the bore of the pipeline. Such pigs have
been pumped through the partially clogged section of the pipe to
mechanically remove the paraffins from the walls and then force
the paraffins out to an accesible portion of the pipeline where
¦ the paraffins can be removed from the pipeline and properly
¦ disposed. However, this method is somewhat limited and is par-
ll ticularly effective only for the removal of a relatively small
il thickness of deposits of paraffins. As the paraffin deposits
15 ~ increase in thickness, the pressure required to force the pigs
through the pipeline goes up substantially, sometimes approach-
ing the pressure limits of the pipeline, and certainly the
il operator cannot afford to bulge the pipeline and thus destroy
the external and/or internal coating of the pipeline.
A second prior art approach to removing deposited
paraffins from the bore of pipelines and similar conduits used
to transmit hydrocarbons is to employ a heated solvent capable
I of dissolving the paraffin deposit and to flow the heated
! solvent over the paraffin. This method is characterize`d by
extremely high costs, both for the solvent solution and for the
, energy costs involved in maintaining the solvent at a suffi-
ciently high temperature to effect the efficient dissolving of
the paraffin deposit.
In U.S. Patent No. 4,178,993 to RICHARDSON, et al,
there is disclosed a method of initiating production from a
~ZS4~
i
gas well by injecting into a subterranean well an aqueous
liquid that contains reactants which form nitrogen gas within
the well or reservoir, and displaces enough liquid out of the
Il well to lower the hydrostatic pressure and cause fluid to flow
5; from the reservoir to the well. The same reactants produce
exothermic heat.
A later patent to RICHARDSO~, et al, U.S. No. 4,399,868,
I employs the same reactive ingredients as were utilized in the
~ earlier patent to generate nitrogen gas, to not only generate
10' nitrogen gas but, by the exothermic reaction, produce heating of
a solvent solution which is introduced into the producti~n zone
, of a well to effect the dissolution of paraffin and similar
deposits from the per~orations of the production zone.
I These operations are, however, complicated by the
15 1I presence of a large amount of brine solution which is normally
found in a producing zone of a well. Additionally, the solvent
solution is heated to temperatures on the order of 440 F.
Temperatures of this order of magnitude can be tolerated in the
~ production zone of a well, but would generate excessive stresses
20 ¦ if employed in a hydrocarbon transmission conduit, such as a
pipeline. More importantly, in such patented processes, the
well's ambient temperature was stated to be on the order of
l 175 F., thus usually preventing any dissolved paraffins from
I precipitating out of solution in crystalline form.
25 ~ U.s. Patent No. 4,219,0~3 to RICHARDSON, et al,
describes a process for cleaning well casing perforations by
injecting an aqueous solution containing nitrogen-gas-generating
reactants, an alkaline bufer providing a reaction-retarding pH,
and an acid-yielding reactant for subsequently overriding the
buffer and lowering the pH in order to trigger a fast-driving
-4-
12S~ 4
1 ' pulse of heat and pressure which causes a perforation-cleaning
backsurge of fluid through the perforations. Again, this
, method, as disclosed, might produce undesirable stresses if
~1 applied to the cleaning of a pipeline and would not prevent the
reprecipitation of the paraffins as the temperature falls, par-
ticularly in a subsea pipeline.
There is an obvious need, therefore, for a cleaning
method and composition which will minimize the cost of any
¦ solvents employed and substantially reduce the cost associated
10 ¦ with heating the solvent and assuring paraffin removal after the
¦ solution is cooled.
SUMMARY OF THE INVENTION
I The present invention relates to a process and com-
¦l position for removing paraffin deposits from a hydrocarbon
! transmission conduit, and particularly a subsea pipeline, by
5 '!
bringing chemically activated heat and a hydrocarbon solvent
into contact with the paraffin deposits. The heat is provided
by an aaueous liquid solution of nitrogen-generating reactants
selected for generating heat and nitrogen gas at a significant
but moderate rate. The aqueous solution is thoroughly emulsi-
fied with a hydrocarbon solution containing a solvent
selected for its ability to dissolve the particular paraffins
i in the deposit. In a preferred form where comparatively low
Il temperatures are encountered, such as with a subsea pipeline,
¦¦ the hydrocarbon solution further contains a crystalline modifier
25 ~!
selected to produce a minimal cloud point for the paraffins
involved in the deposit, and thus prevent the reprecipitation
of the paraffins once they have been dissolved in the heated
solvent and then subjected to the inherent cooling action
involved in the conduit. Either the aqueous solution or the
S41~4
hydrocarbon solution preferably also contains an emulsifier to
maintain an emulsion of the hydrocarbon solvent in the aqueous
solution.
i Lastly, a buffered pH modifier, such as a buffered
solution of HCl, is added to the aqueous solution to
j provide a pH sufficient to retard the
heat and nitrogen-generating reaction so that no significant
¦ portion of the reaction occurs until the conduit containing the
¦ deposits is filled with the emulsified mixture of the aqueous
10 I and solvent solutions.
As the chemical reaction proceeds, the emulsified
solution in the vicinity of the paraffin deposits is heated to
a temperature in the range of 50 to 200 F. so as to effect
l a kinetically energized fluid to provide the melting of the
15 , paraffin deposits. The nitrogen fluid or gas concurrently
generated with the release of the exothermic heat provides an
àgitation of the heated emulsified solution to bring the solvent
portion of the solution into intimate contact with the paraffin
' and thus assure the dissolving of the paraffin in the solvent
201i component of the emulsified solution.
When the heat and nitrogen-producing reaction is com-
,1 pleted, the temperature of the solution contained in the pipe-
li line falls rapidly to the ambient temperature which, in the case ;
¦, of a subsea pipeline, is on the order of 40 F. Notwithstanding
25!1 this drop in temperature, the additional incorporation of a
crystalline modifier in the solvent solution effectively pre-
vents the reprecipitation of the dissolved paraffins and permits
the removal of the reacted solution and solvent emulsion con-
taining the dissolved paraffins from the pipeline for appropri-
ate disposition.
. 12S41C~4
1 1 ~urther advantages of the invention will be readily
apparent to those skilled in the art from the following detailed
description, taken in conjunction with the annexed sheet of
drawing, on which is schematically illustrated the process of
this invention.
BRIEF DESCRIPTION OF THE DRAWING
.
' Figure l is a schematic block diagram illustrating
¦ schematically the apparatus used in the pipeline-cleaning pro-
cess of this invention.
DESCRIPTION OF T~E PREFERRED EMBODIMENTS
Referring to the drawing, a section l of a pipeline
Il is shown within which an accumulation of paraffin precipitates P
!; has occurred to the extent that the flow area of the pipeline l
is significantly impaired. To remove the paraffin precipitates,
'I the portion l of the pipeline containing the precipitates is
i, isolated from the rest of the system at points on both sides of
the location of the precipitate deposit, but preferably as close
l as possible, through utilization of access connections to the
20,; pipeline originally installed with the line. Thus, for example,
l¦ if the pipeline extends from a producing subsea well 2 to a
¦ collecting station 3 located on shore, there could very well be
¦~ several miles of pipeline containing the paraffin precipitates.
¦¦ There is provided an access connec-
tion 5 intermediate the pipeline l and the well 2 at the well-
head or other location and, similarly, there is an access con-
nection 4 located at the collecting station 3. Appropriate
valves (not shown) in the access 4 or conduit 5 may be closed
and the cleaning solution embodying this invention introduced
through the other valve. Of course, the flow of crude oil from
1~41C~4
1ll the well 2 is interrupted during the paraffin-removing or
cleaning process.
'I An emulsified mixture of an aqueous solution of ni-
! trogen gas-generating exothermic reactants, and a hydrocarbon
5 i solution of a paraffin-dissolving amount of a solvent containing
an emulsifying agent, is introduced into the one end of the
pipeline section l and the entire pipeline section l is filled
with the intermixed solution. If desirable, a crystalline mod-
ifier may be included within the aqueous solution. By use of
the term "crystalline modifier", we intend to refer to any chem-
ical, such as a copolymer of ethylene vinyl acetate or an ester- ~
ified olefin/maleic ;jnhydride copolymer or Cl8 to C22 methacrylate,
which will modify the crystal growth of a paraf~inic crude. The
modification of the crystal growth is thought to occur because
either; l) the active chemical comes out of a solution at a tem-
perature slightly higher than the cloud point temperature of the
li crude oil causing nucleation, or 2) the active chemical comes out
¦¦ of the solution and co-crystallizes with the paraffinic crystals.
1 In eitl1er case by incorporation of the crystalline modifier, the
normal crystal habit of the paraffin wax is sufficiently deformed
to stop or inhibit further grow~h of the crystal.
Preferably, as illustrated in Figure l, separate com-
ponents of the intermixed solution are provided from two tanks
6 and 7 by pumps 8 and 9. The one tanX contains an emulsion of
25 ¦ hydrocarbon solvent, crystalline modifier (when required), and
an emulsifying agent with an aqueous solution of one of the
reactants producing the nitrogen and the reaction-generated
heat. The other tank contains an emulsion of the hydrocarbon
paraffin -~olvent, the crystalline modifier (when required) an~
emulsifying agent, and an aqueous solution of the other reaction
--8--
12541(~4
1 , component. Thus, the tanX 6 may contain an aqueous solution of
sodium nitrite while the other tank 7 contains an aqueous solu-
tion of ammonium nitrate. The ammonium nitrate aqueous solution
also contains an aqueous solution of a buffering agent which
5 ~ includes hydrogen chloride, or the like, in a q~antity selected
to produce 2 pH of the resulting emulsion at a value selected
to slow the reaction rate to a desired level, as will be dis~
cussed in more detail hereinafter.
Il In the present invention, nitrogen-forming reactants
10¦¦ for use in the invention can include water-soluble amino nitro-
gen-containing compounds having at least one nitrogen atom to
which at least one hydrogen atom is attached and which are
capable of reacting with an oxidizing agent to yield nitrogen
, gas within an aqueous medium. These compounds typically include
ammonium salts of certain organic or inorganic acids, amines,
nitrogen-linked hydrocarbon-radical substituted homologs of such
compounds, so long as they react with an oxidizing agent to pro-
duce nitrogen gas and other materials which are liquid and
~¦ fluid and dissolve in water to form fluids which are substanti-
20 1 ally inert relative to the transmission conduits. Typical ofsuch nitrogen-containing compounds are: ammonium chloride,
ammonium nitrate, ammonium nitrate,ammonium acetate, ammonium
formate, ethylene diamene, formamide, acetamide, urea, benzyl
urea, butyl urea, hydrazine, phenylhydrazine, phenylhydrazine
hydrochloride, and the like. Particularly suitable are the
ammonium salts, including ammonium chloride and ammonium formate.
Such nitrogen containing reactants are well known in the art,
and are as disclosed, for example, in U.S. Patent No. 4,399,868.
Buffering compounds are systems suitable for incorpor-
ation in the present invention and comprise substantially any water-
1~54:~4I!
1 soluble buffer which is compatible with the nitrogen-generating
reactants and their products and which tends to maintain the
pH of an aqueous solution at a value of at least 4.
Il Examples of suitable buffering materials include the alkali metal
and ammonium salts of acids such as carbonic, formic, acetic,
¦I citric, and the like, acids. When relatively high pH systems
are required, salts of amines or amino-substituted compounds
such as EDTA, and the like, can be utilized. Again, such buffering
1I compounds are well known to those skilled in the art, and may
101¦ be as disclosed in U. S. Patent No. 4,399,868.
j! The portion of the transmission conduit containing the
paraffin and which is required to be treated to remove such
paraffin therefrom, may be "isolated" by closing off the in-
terior of the conduit at a pre-determined point upstream of
the paraffin deposit and upstream of the injection point for
introduction of the emulsified mixture of the aqueous and hy-
ll drocarbon solutions. Such isolation may also include closing
¦l off a yalve, or the like, downstream of the paraffin deposit
I at the location in the conduit which i~ required to be treated.
20l However, downstream isolation may not be required in some in-
i! stances, particularly where a continuous treatment is provided
¦l over an extended period of time, with the treatment including
¦l at least a nominal fluid flow for ejection of the treatment
l~ solution with the removed paraffin deposit. Isolation may
25 11 also be effected by providing a balancing of the pressure
between the section of the conduit to be treated and a point
in the conduit upstream of the treatment area.
There are a number of ways of introducing the emulsi-
fied treating solution into the area of the conduit to be
treated, such as by spotting, batch, continuous, cyclical, or
--10--
1 12S41(~
1 other treatment means well known to those skilled in the art
of treating pipelines and other hydrocarbon conduits above the
top of a subterranean well for the removal of pariffin deposits.
' More specifically, the solution A contained in tank 6
may comprise, for each 50 barrels, aboyt 43 barrels of fresh
water and about 9,660 pounds of sodium nitrite, yielding a
mole ratio of 8. Solution B contained in tanX 7 constitutes,
for a 50-barrel tank capacity, about 33 barrels of water, about
Il 11,200 pounds of ammonium nitrate, yielding a mole ratio of 8,
10ll and about 11.7 gallons of 37 percent hydrochloric acid. On
¦ laboratory-equivalent preparations, these quantities of reac-
tants will produce maximum temperatures on the order of about
i 160 F. However, in actual practice, the temperature may ex-
I! ceed about 400F, depending on the ambient temperature sur-
rounding the pipeline and other variables. Additionally, the
solutions A and B are augmented by the addition of a hydrocar-
bon solvent ranging in volume from about 25 to about 50 percent
of the total tank volume of the aqueous solution. Since the
, hydrocarbon solvent typically will be the most expensive element
20 ' employed in the process, it is obviously desirable to use as
little solvent as required to effect the dissolution of all of
the paraffin precipitates. As previously mentioned, such
¦ hydrocarbon solvent may also include a cloud point depressing
Ij amount of crystalline modifier and an emulsifying agent in
25 ` order to produce a good emulsion of the hydrocarbon solu~ion
; with the aqueous solution.
The amount of, for example, buffered acid added to
the tank as a pH modifier is largely dependent upon the distance
of the major deposit of paraffin precipitate in the pipeline
from the access connection through which the cleaning solution
iZS41Q4
is introduced. Normally, the cleaning solution is lntroduced
through the access connection S immediate to the wellhead and
removed after the cleaning is accomplished through the access
connection 6 at the collecting station 3. A computerized
S calculation has been developed to indicate the pH value
required to delay the substantial initiation of the chemical -.
reaction between the sodium nitrite and the ammonium nitrate
until the inserted solutions have had time to fill up the
isolated pipeline section 1 and surround the precipitate
layer P. While this calculation can be performed by anyone
ordinarily skilled in this art, the details of the
calculation are set forth below in the interest of
completeness of disclosure. In such calculation and the
working examples below, the phrase "N-SITU" has been
employed to represent the aqueous solution of the reactants
which react exothermically to produce nitrogen gas and a
heat spike. N-SITU is a Trademark for products and services
associated therewith of the Assignee.
In order to locate the heat spike in the pipeline
generated by the injection of an N-SITU and paraffin solvent
mixture, it is necessary to determine the physical
properties of the mixture.
The physical properties, i.e., heat capacity,
vapor pressure, heat of vaporization, thermal conductivity,
and viscosity, of the immiscible liquid mixtures may be
determined as follows.
1. Heat capacity, Cp:
(CP)m = (Cp)l Xl + (Cp)2 X2 + --
where (Cp) = heat capacity of the mixture,
(Cp)mj = heat capacity of the component i
X. = weight or molar fraction of component i
(j = 1, 2, ...!
l;c~S4~
2. Heat of vapori~ation, Lv:
(LV)m = (Lv)l Xl + (LV)2 X2 t ,,.
where (Lv) = Heat of vaporization of the mixture
(Lv)im = heat of vapori~ation of the component i
(i = 1, 2, ...!
Xj = weight or molar fraction of component i
3. Thermal conductivity, K:
(2KC + Kd - 2~d (Kc- Kd))
(2KC Kd ~d (Kc Kd))
where K = thermal conductivity of the mixture
Km = thermal conductivity of the continuous phase
Kd = thermal conductivity discontinuous phase
= phase volume fraction of continuous phase
~d = phase volume fraction of discontinuous phase
4. Yiscosity,
for ~d < 0 03
(1 + 2.5 ~d) (~d + 0 4~c)
., = 11
-m -c (~d ~c)
for larger values of ~d:
( )~d( )~c
where subscript m, c, and d denote the mixture, continuous,
and discontinuous phases, respectively, and where ~ is phase
volume fraction.
CALCULATION OF THE TEMPERATURE PROFILE IN THE FLOW STREAM
The mixture of N-SITU and paraffin solvent plus
additives is immiscible; therefore, the heat and Nz gas
generation from the reaction of N-Situ solution is not
affected. The temperature profile in a flow pipe during a
treatment of N-SITU and paraffin solvent mixture can be
estimated by calculating the heat balance of a single small
volume of the mixture in the pipe as it transits from one
end to the other end of the pipe. The sing'le sma'll vo'lume
is referred as an "element". The heat balance of an element
can be expressed as:
HG = HL + HH + Hv
-!3-
12~:i41U4
where HG = heat generated by the reaction of N-SITU solution
in calories
HL = heat transferred across pipes to/from tile
surrounding in calories
H = Heat used to raise the temperature of the mixture
H in calories
Hv = heat lost/gained due to ~aporization in calories.
5uppose a mixture of N-SITU and paraffin solvent,
plus additives was pumped into a flow pipe at a constant
rate Q(BPM~. The mixture contains X~ barrels of N-SITU
solution and X2 barrels of paraffin solvent, plus
additives. The temperature rise in the element is
calculated as descrlbed below. At each position of the
element (starting at the inlet and ending at the outlet of
the pipe), the following factors are considered:
1. Heat generated by the N-SITU solution in the
time during which the element is located at
the position;
2. Heat transferred across pipe to/from
surrounding;
3. Heat lost/gained due to vaporization of the
mixture; and
4. The nitrogen gas generated by the N-SITU
solution in transit down the pipe is allowed
to change the volume of the element according
to the existing pressure and temperature of
the element.
2s When the temperature rise is calculated, the
element is moved to the next position of the pipe (as
indicated by the transit time, t, selected for the ele~ent
and the calculation repeated until the element is at the
outlet of the pipe. This will give the temperature profile
in the pipe f~r that element. While the volume Gf the
element changes in transit down the pipe, the ~olume of the
liquid in each element is considered fixed and is determined
by:
l~S41U4
V = 159 Q t
where V = volume of the liquid in each element, liters
Q = pump rate, barrels per minute
t = retention time for the element in each position,
minutes
159 = convers;on factor from barrel to liter
The volume on N-SITU solution Vl, and the volume
of the solvent, plus additives, V2, can be calculated as
follows:
Vl = V [Xl/(Xl + XZ)]
V2 = V lX2/(Xl + X2)]
The volume of the gas in each element is depended
on the pressure and temperature of the element. The
pressure, P, in the element depend on the position of the
element in the pipe and on the inlet pressure. Thus:
P = Pj ~ X(PO - Pj)/L + 14.696
where P = pressure in the element, psia
P. = pressure at the inlet of the pipe, psi
P0 = pressure at the outlet of the pipe, psi
X = position of the element, feet
L = Length of the pipe, feet
The maximum temperature of the element is limited
by the boiling point of the mixture. This can be determined
by the Antoine equation below:
log (Pv) = A - B/(T + C)
where Pv is vapor pressure in mm of Hg
T is the temperature in degree C
A, B, and C are constants for a particular substance
For water, A = 7.96681; B = 1,668.21; C = 228
For other substances, accuracy is best when all three
constants are evaluated from data
The boiling temperature of the mixture at pressure
P of the element is given by:
Tb = 273 - C - 8/tlog(P(760/14.696)) - Al; Deg. Kelvin
DETER~IIilATI0~l Of HEAT GEtlERATED, HG
The amount of heat generated is determined by the
mole of nitrogen generated as follows:
-15-
1~541()~
HG = Cr V I N2
where HG ~ V I are already defined above
G = 70,000 = heat of reaction, calories/mole of
r nitrogen
N2 = moles of Nz generated in the element per liter
of N-SITU solution during transit time t and is
determined by the following equation assuming the
evolution of nitrogen is a second order reaction
N2 = a - a~(l + akt)
where t = retention time in the element, minutes
a = starting concentration of N-SITU solution in the
element, M/L
k = rate constant of second order reaction and can be
determined as a function of temperature by the
following Arrhenius equation:
; - k = S exp (- Ea/RT)
where S is a frequency factor, E is the energy of
activation, R is the gas constan~ and T is absolute
temperature in Kelvin. Replacing E /R by H and T by
current element temperature T ~ ~T,athe abovae equation
becomes:
k = S exp (- Ha/(T + ~T))
DETER~lINATION OF HEAT USED TO RAISE THE TEMPERATURE OF THE
MIXTURE
The amount of heat used to raise the temperature
of the mixture is determined by the following equation:
HH = 1,000 Cp p V ~T
Where C is the heat capacity of the mixture in cal/g, p is
the densnity of the mixture in g/cc at current temperature of
the element, V is the volume of liquid in the element, and
~T is the temperature rise in the element in degree Kelvin.
Let Cl = 1,000 Cp p and substituting in the above
equation yields:
HH = Cl V ~T
DETER~llNATION OF HEAT LOST/GAINED DUE TO VAPORIZATION, HV
The amount of heat lost/gained due to vaporization
is depended on the amount of vapor formed in the element
during transit time t and can be expressed as follows:
1~4iU4
HV ~ Lv V Va
where Hv, and V are already defined above, Lv is the latent
heat of vaporlzation of the mixture in cal./g.-mole, Va is
moles of vapor in the element per liter of mixture during
transit time t. The amount of vapor is calculated as
described below.
S The volume ratio of vapor to nitrogen gas, VNR is given by:
or; VNR Pva/PN2
VNR = Pva/(P~Pva)
where PN is the partial pressure of nitrogen in the
element, and PVa is the vapor pressure of the mixture.
The vapor pressure of the mixture at current
element tempëratuë, PVa is determined by the Antoine
equation:
i t(A-B)/(T + ~T - 273 + C)] (14.696/760), psia
where T = initial temperature of the element then moved to
its current position, degree Kelvin
~T = temperature rise in the element, degree Kelvin
The amount of vapor, Va is given by:
Va = (YNR XN2 -VNRl XNl) (Vl/V), mole/liter of solution
where VNR, VNRl are current and previous vapor nitrogen
ratio, respectively, and XNz is cumulative moles of nitrogen
generated in the element per liter of N-SITU solution and is
given by:
XN2 = XNi+N2
where XNI is cumulative moles of nitrogen generated
previously in this element per liter of N-SITU solution, and
N2 is already described above.
The cumulative volume of nitrogen gas in the
element per liter of the N-SITU solution is calculated using
ideal gas equation:
VN2 = 1.206 Xrl2 (T+~T)/P, liter N2/liter N-SITU solution
The volume of mixture vapor in gas per liter of
solution~ YVa is:
Vva = 1.206 VNR XN2 (T+~T!/P, liter vapor/liter solution
1~;411~
The total gas volume per liter of solution, Vg is:
9 (\IN2+ Yva~ (Vl/V)
The total volume of the element, Ve ;s:
Ve = V + V Vg, liters
The length of the element, Le is then given by:
Le Ve/159.0 Pc , feet
where Pc is the pipe capacity in barrels per feet.
DETE~MINATION OF HEAT TRANSFERRED ACROSS PIPES TO/FROM THE
SURROUNDING
The heat transferred across pipes to/from the
surrounding is calculated as follows: -
HL = Bl Ao t (T+~T-Ta), calories
where A = area based on O.D. of the pipe, cm2
= 3.1416 POD L 77.42
t = transit timeein minutes
Ta = surrounding temperature in degree Kelvin
B1 = overall heat transfer coefficient in cal/cm2-
C-min. Bl is depended on the type of flow, pipe
- dimension, and properties of fluid and surrounding
material.
Substituting all of the heat terms in the energy
balance equation, the following expression is obtained:
Cr Y~ Nz = Cl Y ~T = Lv V Va + B1 Ao t (T+~T-Ta)
All three terms on the right contain the variable
~T (the temperature rise in the element). This variable can
be determined by rearranging the above equation as follows:
F(~T) = Cr Yl N2 - Cl Y ~T ~ Lv Y Ya + 81 Ao t ~T+~T-Ta) = O
then applying Biscetion method to solve for variable ~T.
This program is affected and is based on
"activation energy units", as opposed to units of pH. Of
course, there are other ways known to those in the art for
BO determining the proper buffered pH required to abate the
reactior, time.
-18-
1 ~Z541(~
1 ,j From these calculations, the pH value of the aqueous
reactant solution to produce the heat spike at the location of
the paraffin precipitate P can be determined.
~~ It should be noted that the buffered control of the
5 ~ pH value of the intermixed reactinq solutions does not effect
the total amount of heat generated nor the total amount of ni-
, trogen gas liberated. This is determined solely by the relativeproportions of the reacting COmponents contained within the
11 emulsified aqueous solutions. The pH value is selected at a
10 ¦ sufficiently high level to delay the speed of the reaction so
that no significant part of the reaction occurs until the fluid
has filled up the isolated section of pipeline and is in contact
¦ with the precipitated layer P of paraffin.
I The hydrocarbon solvent employed is preferably selected
15 , by a bench test in a manner that will be readily apparent to
those skilled in the art. There is a wide variation in the type
' of paraffins produced by various crude oils, but the specific
il type or types precipitated as a deposit in a pipeline can be
il determined by periodic sampling of the crude oil passing through
the pipeline. Such paraffins are then employed in bench tests
to select a solvent that is the most efficient in not only
effecting the dissolution of the paraffins in a liquid state,
Il but also in retaining the paraffins in solution after the tem-
i~ perature has dropped below the melting point of the particular
25 ~ paraffin. Thus, the invention has been successfully employed
utilizing an aromatic solvent sold under the Trademark "ParaSolv
504" by Amerigo International, Houston, Texas. A large variety
of suitable hydrocarbon solvents are available in the market-
place and routine bench testing wiil aiways determine the sol-
vent that can be most efficiently employed for the particular
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1 types of paraffin contained in the particular pipeline.
In similar fashion, the crystalline modifier employed
1ll may be determined by bench tests. The process of this invention
¦I has worked successfully with a crystalline modifier sold by
5ll MAGNA Corporation of Houston, Texas, under the name designation
¦1 "MAGNA D-Wax 9S0" and referred to herein as "A-950". Conven-
¦, tional bench tests will indicate which crytalline modifier and
¦ the required amount will be the most effective to depress thepour point of the paraffin and thus insure that the paraffin is
retained in solution in the aromatic solvent.
The emulsifier employed again may constitute any com-
mercially available material employed to maintain an emulsion
of oil in water between aqueous base fluids and hydrocarbon
base fluids that will not chemically react with other fluids.
; It is believed essential to the successful practice
of this invention that bench tests be employed to select the
¦l best aromatic solvent and (when required) the best crystalline
modifier for the particular paraffins to be removed. This is
clearly shown in the results from the following bench tests
201! wherein a fixed quantity of paraffin was immersed in a variety
¦ of different aromatic solvents and different crystalline modi-
¦ fiers were employed. The paraffin was heated
to convert it to a liquid state and the resulting
solution was stirred to attempt to dissolve the paraffin in the
aromatic solvent. If solution of the paraffin was produced,
the solution containing the dissolved paraffin was then lowered
in temperature to approximately 4~ F., representing a typical
subsea pipeline temperature, and the solution was closely
observed to detern1irle whether the paraffin again precipated out
of solution. Out of the following listed nine examples, only
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I
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1 ll the last three examples resulted in satisfactory dissolution
¦ and retention of the paraffin in solution in the aromatic
; solvent.
~I EXAMPLE I
TABLE OF BENCH TEST RESULTS
I Chemical System Observations
I
1, (1.)
20 ml N-SITU, Paraffin solidified after spending
10 ml Paraffin. and cooling of N-SITU to 75 F.
(2.)
10 20 ml N-SITU, i Paraffin solidified after spending j
2 ml (10%) ParaSolv 504, and cooling of N-SITV to 75 F.
10 ml Paraffin.
2G ml N-SITU, Paraffin solidified after spending
2 ml ParaSolv S04, and cooling of N-SITU to 75 F.
.1 ml (.5%) A-301 Crystal-
line Modifier,
10 ml Paraffin.
15 I (4 )
20 ml N-SITU, Paraffin did not solidify, but was
2 ml ParaSolv 504, viscous after spending and cooling
~ .1 ml x-44a Crystalline of N-SITU.
i Modifier,
I 10 ml Paraffin.
(5-)
l 20 ml N-SITU, Paraffin solidified after spending
20,2 ml ParaSolv 504, and cooling of N-SITU to 75 F.
.1 ml A-901 Crystalline
Modifier,
10 ml Paraffin.
(6.)
20 ml N-SITU, Paraffin did not solidify, but was I
2 ml ParaSolv 504, viscous after spending and cooling `
.1 ml A-950 Crystalline of N-SITU.
lModifier,
251 10 ml Paraffin.
.
i (7.)
I 20 ml N-SITU, Paraffin was softened and dispers-
2 ml ParaSolv 504, ed, but did not remain flowable
.4 ml A-901 Crystalline after cooling.
Modifier,
; .4 ml A-921 Dispersant,
lG ml Paraffin.
, .
i
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:.
:~2~4i(~
t8)
l 20 ml N-SITU, Paraffin emulsified with N-SITU;
I 10 ml (50~`~ ParaSolv 504, it remained dispersed, emulsified,
I .4 ml A-950 Crystalline and flowed after cooling to 45 F .
Modifier,
.4 ml A-921 Dispersant,
¦~ 10 ml Paraffin.
5 i (9)
I¦ 20 ml N-SITU, Paraffin/N-SITU emulsion was not
~ 10 ml ParaSolv 504, as flowable as above mixture with
!~ .4 ml X-448 Crystalline A-950.
~! Modifier,
.4 ml A-921 Dispersant,
10 ml Paraffin (different
l than Test 8).
10 2~ ml N-SITU, Paraffin partially melted and was
10 ml ParaSolv 504, dispçrsed in the ParaSolv 504;
.4 ml A-921 Dispersant, clumps of paraffin were visible but
10 ml Paraffin. tne system was partially emulsi-
fied and flowableOafter spending
and cooling to ~5 F.
Ii In the above table, N-SITU constitutes the previously
15 ll described aqueous solution of equal molar quantities of sodium
nitrate and ammonium nitrate; ParaSolv 504* constitutes the
aromatic solvent product of Amerigo International; A-301 is a
crystalline modifier of NALCO sold as "NALCO ASP-348*"; A-901
is a crystalline modifier sold by MAGNA Corporation as "D-Wax
901*"; and A-950 is a crystalline modifier of MAGNA sold under
the name "D-Wax 950*". A-921* constitutes a dispersant or emul-
sifier sold by NALCO C0.; and X-448* constitutes a crystalline
modifier sold by NALC0 Chemical Company of Sugarland, Texas,
under the name "NALCO ASP-448*".
25jl The foregoing test results clearly indicate the
desirability of selecting the crystalline modifier as a result
of bench tests wherein varying quantities and types of the
; crystalline modifier are tested with a particular aromatic
solvent to determine the solubility retell~ion of the particular
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* Trade Marks
lZS41(~4
.1
1 1l paraffins which are to be removed from the pipeline. The abcve
bench tests comprised adding the specified quantities of the
N-SITU to a selected quantity of an aromatic solvent and a
selected quantity of a crystalline modifier. These elements
were placed in a beaker with a fixed quantity of paraffin in
I solid form and, within a matter of minutes, the exothermic
¦! reaction of the N-SITU components elevates the temperature of
the beaker contents to approximately 175 F. and converts the
paraffin to a liquid state. The resulting nitrogen agitates
the solvent into intimate engagement with the paraffin and, if
the selected solvent is correct for the particular paraffins
~I being tested, the paraffins will go into solution. When the
¦I reaction is completed, the beaker is placed in an ice bath and
¦I the contents thereof are cooled to 45 F., which proximates the
ambient temperature of a subsea pipeline. During this cooling,
¦ if the crystalline modifier is required and is proper for the
il particular paraffins and the particular aromatic solvent, the
paraffin will remain in solution and not precipitate out as
!i solid crystals.
20 i With the foregoing description, those skilled in the
art will be readily able to maXe the proper selection of aro-
¦ matic solvents and crystalline modifiers to provide optimum
! dissolving of the liquid paraffin as a result of the heat gene-
l¦ rated by the exothermic reaction of the N-SITU reactants and to
25 l retain the paraffin in solution even though the temperature is
subsequently dropped to about 45 F. The selection of an emul-
sifier is not particularly critical, but is desirable to achieve
an emulsion of the N-SITU components and the solvent so as to
expose the solvent to both the hea~ ard the agitating effect
lZ54 ~
1 1 of the nitrogen generated by the exothermic reaction of the
N-SITU reactants.
Vpon completion of the selection of the best aromatic
~ solvent, the best crystalline modifier and the computation of
5 ¦ the pH value required to insure that the reaction will be
,¦ delayed until the isolated pipeline section is substantially
,I filled with the emulsified liquid containing the reacting
chemicals the resulting solutions are made up in tanXs 6 and 7
and are simultaneously introduced into the one end of the
pipeline 1 and forcibly mixed by kinetic energy imparted to the
respective fluids by pumps 8 and 9.
Most of the reaction of the reacting chemicals con-
i tained in the mixed solution occurs after the isolated section
~¦ of pipeline 1 is at least partially filled with the emulsified
5 l¦ solution of solvent and reacting chemicals. The total amount
!~ of heat generated by the reaction is, of course, a function of
¦I the amount of reacting chemicals contained in the aqueous
~,¦ 601ution. Typically a range of a~out 5 to about 10 molar for
Il each of the sodium nitrite and ammonium nitrate components
20,i would be employed. A preferred mole concentration for such
¦ components is about 8. The maximum heat generated by the
il exothermic reaction is preferably on the order of about 175 F.
Il Temperatures in excess of this figure would represent a threat
¦¦ to,the integrity of the pipeline covering, due to expansion of
25 1I the metal constituting the pipeline. Temperatures less than
, this value diminish the solubility of the liquified paraffinin the aromatic solvent.
The mixed solutions are permitted to remain in the
isolated section of pipeline 1 unti] sufficient time has
l~S~ 4
1 ,1 elapsed to insure that the chemical reaction has been com-
pleted. The solution is then pumped out of the isolated sec-
tion of the pipeline, carrying with it the paraffin precipi-
I tates dissolved in the aromatic solvent.
Although the invention has been described in terms of
specified embodiments which are set forth in detail, it should
¦ be understood that this is by illustration only and that the
invention is not necessarily limited thereto, since alternative
embodiments and operating techniques will become apparent to
those skilled in the art in view of the disclosure. Accordingly,'
modifications are contemplated which can be made without depart-
ing from the opirit of the described invention.
15 ~
i
I
25 j
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