Measuring and testing – Embrittlement or erosion
Reexamination Certificate
1999-06-22
2001-06-26
Williams, Hezron (Department: 2856)
Measuring and testing
Embrittlement or erosion
C436S006000, C073S061620, C073S579000, C073S116070
Reexamination Certificate
active
06250140
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method for rating the fouling propensity of industrial fluids, rating the effectiveness of anti-fouling treatments, and for the application of process controls to minimize fouling. More specifically, the present invention relates to a method of assessing the mass rate of deposition resulting from a chemical reaction in a process fluid caused by a heat flux through the immersed equipment surfaces also subjected to an electrochemical polarization.
BACKGROUND OF THE INVENTION
The operating efficiency of industrial and domestic systems largely depends on cleanliness of their surfaces exposed to process fluids and subjected to natural or induced heat transfer and electrochemical conditions. Untreated process fluids contain a number of constituents the solubility of which can substantially decrease in certain temperature and pH ranges resulting in scaling or precipitation on the surface of an apparatus or a vessel. These processes, otherwise known as fouling, impede the proper flow of heat through the equipment surfaces, which leads to an overall decrease of the system operating efficiency.
Further, in a system where the fluid or liquid is flowing or being pumped, the formation of scales and deposits decreases the diameter of passages, increases the flow resistance and mechanical stresses thereby increasing the risk of structural damage as well as energy costs. Also, the formation of scales and deposits on metal surface favors localized and under deposit corrosion, thereby reducing the operating lifetime of the equipment.
Fouling can be a function of many factors: liquid temperature and chemistry; physical characteristics of the flow such as Reynolds number, shear stress and viscosity; geometry of the equipment; materials of construction; and temperature of the heat transfer surface. The most important liquid characteristics are the level of dissolved solids, the presence of microbiological matter and the process chemistry. Liquid velocity, shear stress and viscosity are the determinant flow characteristics.
Induced fouling deposits can form on surfaces that are either colder or warmer than the temperature of the bulk liquid. For example, in industrial processes employing water-cooled heat exchangers, silicate scale deposits can form on surfaces that are colder than the bulk water while carbonate and sulfate deposits can form on surfaces warmer than the bulk water. Another example of fouling of a colder surface is the formation of ice from water or the solidification of wax laden hydrocarbons while transporting fluids containing these substances in pipelines exposed to low temperatures.
Further, an electrochemical polarization in the form of potential or current naturally or intentionally applied to a heat transfer surface may significantly affect fouling due to the electrochemical reactions induced at the equipment surface. For example, the surface pH increase induced electrochemically by corrosion results in the increase of the deposition rate of calcium carbonate scale on a mild steel heat exchange surface compared to that made of stainless steel. Corrosion results in the formation of anodic and cathodic sites on mild steel surface immersed in water at ambient conditions. Reduction at the cathodic sites of the oxygen dissolved in water leads to a near surface pH increase that favors precipitation of carbonate scales. In another example mild steel industrial heat exchangers are often protected against corrosion using cathodic polarization using sacrificial anodes or imposed current. A commonly accepted cathodic protection criterion for mild steel parts is the application of a negative potential which results in the increase of the near surface pH which favors carbonate scaling. The use of two or more different metals in constructing a heat exchanger can subject one of them to a positive potential sufficiently high to result in water oxidation which produces a near-surface excess of H
+
ions, and thus, a pH decrease will occur resulting in the scale dissolution.
The decrease of the heat exchange at a surface due to fouling is defined by the fouling thermal resistance, R
f
as:
R
f
=1
/U
fouled
−1
/U
clean
(1)
where 1/U
clean
and 1/U
fouled
are the heat transfer coefficients of the surface in clean and fouled conditions, respectively. The heat transfer coefficients are defined as:
T
wall clean
−T
bulk
=1
/U
clean
(Q/A) (2a)
T
wall fouled
−T
bulk
=1
/U
fouled
(Q/A) (2b)
where T
wall clean
and T
wall fouled
are the temperatures of the surface in clean and fouled conditions, respectively; T
bulk
is the bulk temperature of the process fluid; Q/A is the heat flux through the heat exchange surface having area A.
Thus, the fouling resistance may be determined by measuring the change of heat transfer through a given surface over time.
As shown in Equations 2a and 2b above, the measurement of fouling resistance requires the knowledge of the heat transfer resistance of the same surface at clean conditions. This brings unavoidable uncertainty because the heat transfer resistance of a clean surface depends on the thermal resistance of the clean surface itself, R
wall
, and the thermal resistance of the process fluid, R
fluid
:
1
/U
clean
=R
wall
+R
fluid
(3a)
combining equations (1) and (3a):
1
/U
fouled
=R
f
+R
wall
+R
fluid
(3b)
The thermal resistance of the fluid is highly dependent upon the fluid flow rate as shown in Equation 4 below:
1
/U
clean
=R
wall
+C/V
n
(4)
where C is the constant, V is the velocity of the fluid and R
wall
, C and n can be obtained through calibration of the heater.
Measurements as described above may result in artificially low or even negative values of thermal resistance during the initial operating period. This happens due to the initial deposits increasing the roughness of the heat exchange surface which consequently decreases surface-to-fluid thermal resistance. As a result, the actual increase in thermal fouling resistance may not be detected. As reported by J. Knudsen, “Apparatus and Technologies for Measurement of Fouling of Heat Transfer Surfaces, and Fouling of Heat Transfer Equipment”, Proceedings of an International Conference, Rensselaer Polytechnic Institute, pp. 57-82 (1979), as the fouling layer thickens, the effect of the lower thermal conductivity dominates the improved local heat transfer coefficient due to roughening and the fouling resistance again becomes positive. Therefore, the common heat transfer resistance measurements have a minimal fouling limit below which they cannot reliably detect or determine R
f
. This presents a substantial obstacle especially when effective anti-fouling treatments are being screened or tested.
Although important from the technical standpoint, the measurement of fouling thermal resistance does not provide a strict quantitative answer as to how a certain treatment affects the deposition mass rate of the fouling deposit. This deposition mass rate represents the velocity of mass accumulation of fouling deposit per square unit of area per unit of time and is expressed as follows:
m
f
=R
f
(&rgr;
f
k
f
) (5)
wherein m
f
is the deposit mass per unit area per unit of time; R
f
is the thermal fouling resistance; &rgr;
f
is the density of the fouling deposit; and k
f
is the thermal conductivity of the fouling deposit.
Traditionally, coupons or forensic investigation were used to determine m
f
, the mass of deposit on the scaled surface. However, the employment of a piezoelectric microbalance makes this task relatively easy to accomplish in real time and in situ. The principle of piezoelectric mass measurement is based upon the property of a quartz resonator to change its mechanical resonance frequency fo proportionally to the mass and viscoelastic properties of the deposited material. U.S. Pat. No. 5,201,215 discloses a method for the simultaneous measurement of the mass load
DiSanto Louis
Johnson Donald
Kouznetsov Dmitri
Kraus Paul R.
Moriarty Barbara E.
Breininger Thomas M.
Brumm Margaret M.
Nalco Chemical Company
Williams Hezron
Wilson Katina
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