(488b) Mitigation of Refinery Pre Heat Train Fouling by Optimisation of Operating Conditions and Application of Heat Exchanger Technologies

Abstract

Fouling is
considered as one of the major unsolved problem in heat exchangers. It
generates additional resistances to heat transfer, a decrease of the thermal
effectiveness and an increase of pressure drop in industrial units. In order to
maintain stable crude inlet conditions to the distillation column, the energy
consumption is increased with detrimental environmental and economical
consequences.

Hydrocarbon
fouling is a complex phenomenon as often several fouling mechanisms act in
parallel and are also linked with each other:

-        
Fouling due to the fluid physico-chemical
properties (precipitation or sedimentation) that is predominant at lower
temperatures.

-        
Fouling due to a change in fluid chemical
composition (chemical reaction or corrosion) that is promoted by high
temperature.

In a crude
preheat train, fouling is principally due to chemical reaction fouling and
concentrated downstream of the desalter where temperatures are higher.

Chemical
reaction fouling can be modelled by two simultaneously acting but opposing
phenomena:

-        
a deposition term which depends on hydrocarbon
velocity (Reynolds number), bulk and wall temperatures and hydrocarbon
composition.

-        
a removal term which depends on hydrocarbon
velocity and heat transfer surface geometry (shear stress).

The work
presented here is based on an innovative test loop specifically designed and
commissioned to achieve the following objectives:

-        
Identify the main parameters involved in
hydrocarbon fouling (crude oil and atmospheric distillation residue);

-        
Assess fouling resistance in a conventional shell
and tube heat exchanger (reference);

-        
Study the fouling behaviour of a cross-flow
plate heat exchanger;

-        
Compare the experimental results to fouling
models prediction.

-        
Predict industrial exchangers fouling rates and
improve the efficiency and the design of the pre-heat train.

Experimental
setup

The fouling
rates encountered in the refinery are often relatively slow (of the order of
few months). Thus in order to hasten the process in a laboratory setup, in most
of the fouling rigs described in the open literature, crude is heated by an
external heating source.

Increasing the
wall temperature accelerates the kinetics of reaction leading to increased fouling
rates. Thus, often test sections are not representative of real exchanger
conditions. Most of the test rigs use single tube or annular tube as test
sections wherein fouling occurs only on one side. Currently, no fouling test rig
was thus identified through literature that approached both hydrodynamic and
thermal conditions of crude preheat train.

So, in order to
better understand hydrocarbon fouling in refinery pre-heat train, an innovative
test loop was designed and commissioned. This pilot allowed to investigate the fouling
behaviour of various heat exchanger technologies. Tests were conducted in
controlled conditions to mimic typical refinery operating conditions (nature of
fluids, velocity, temperatures and fouling rate).

The pilot plant
consists of five closed loops in cascade, with each loop connected to the subsequent
one with a heat exchanger. The first loop comprises of an electric furnace
which heats non fouling thermal oil until 360°C. The fluid further heats the atmospheric residue circulating in the second loop to the required temperature .The crude
oil which is circulating in the third circuit is heated by the residue
atmospheric, through the test section exchanger. The crude oil is then cooled
by two subsequent loops operating with thermal oil and water respectively.

Pilot plant and
especially the test sections were designed to simulate industrial heat exchangers
geometries, operating conditions and fouling at both side of an exchanger.

Since the fluids
are operated in a closed circuit, hence an inherent risk of depletion of
fouling species present in the fluid had been identified. In order to mitigate
the risk, a non ?invasive sampling system was designed and samples were
collected at regular intervals during the complete test campaign. Several
analyses were conducted to monitor any variations in fluids chemical
composition. In case of modification a fresh feed was to be used.

Tests sections

            -
Shell and tube heat exchanger

The first technology
examined was a shell and tube heat exchanger. It was designed to have the same heat
flux, overall heat transfer coefficient, shear stress and material of
construction as an industrial heat exchanger. Two crude oils were tested in
this heat exchanger. The operating conditions used for this test campaign were
chosen to be representative of a refinery. They are listed in table 1.

Table
1: Shell and tube overall dimensions and operating parameters

            -
Cross-flow plate heat exchanger

The second
technology tested was a cross-flow plate heat exchanger. It was designed so as to
transfer the same duty as the shell and tube heat exchanger under the same
operating conditions.

In order to
compare the two tests sections, tests were conducted with identical fluids and
same pressures and temperatures range. Further crude velocities were estimated
to achieve the shear stresses as in a pre-heat train of a refinery.

Results:

            -Influence
of operating conditions

Fouling rates
were calculated from temperature and flow rates measurements. Flow-meter as
well as temperature sensors, with high precision, were installed at the inlets
and outlets of each heat exchanger. Indeed, once fouling is initiated the
temperature profile changes and global heat transfer coefficient will decrease.
This coefficient is computed by:

U (W∙m^-2∙K^-1)
depends on the heat exchanger duty, Q (W), the heat transfer area, A (m²)
and the logarithmic mean temperature difference, LMTD (K).

The monitoring
of fouling resistances of the three heat exchangers in which hydrocarbons circulated
confirms that fouling mechanisms were detected on both sides of the test
section (atmospheric distillation residue and crude oil). Moreover, detection
of fouling deposits on the two sides of this exchanger while dismantling also
confirmed it.

The proof of
concept of the fouling test rig was validated by measuring fouling rates close
to literature and industrial values.

A fouling index
(I_f) is used to compare experimental results and is given by:

Where,

U_t=0 is
the overall heat transfer coefficient at the beginning of the test (clean test
section) and U_t is the overall heat transfer coefficient at time t.

Figure 1
represents the fouling index of one of the fouling tests.

Figure
1: Fouling Index curve

In the figure 1
above, a linear drop in I_f with time signifies a loss in thermal
efficiency of the exchanger due to fouling on the hot exchanger surface.

Tests were
carried at 3 different crude velocities keeping the temperature constant and
vice versa. Each operating point was tested for ten days.

Figure 2 represents
the fouling index for tests realised at three velocities (u1>u2>u3) and
at different temperatures

Figure
2: Effect of velocity and temperature on fouling index

For the three
temperatures, results show that fouling index increases with an increase in of crude
velocity indicating that fouling rate decreases with velocity. This can be
attributed to the fact that for higher velocity shear stress increases thus
enhancing deposit removal rates.

Further it was
demonstrated that fouling index decreases non-linearly with increasing bulk and
film temperatures. An increase of the temperature promotes chemical reactions and
thus fouling deposit rate.

            -
Influence of heat exchanger geometry

The impact of
heat exchanger technology was also examined. The same volumetric flow and shear
stress were applied into the two test sections. Figure 3 represents the fouling
index for tests conducted at the same crude oil volumetric flow.

Figure
3: Influence of the heat exchanger geometry on fouling index.

The fouling index
of the shell and tube heat exchanger decreases with time whereas the fouling
index of the plate heat exchanger remained almost constant. It seems that due to
higher turbulence, the plate heat exchanger technology could reduce crude oil
fouling at given operational conditions.

            -
Use of fouling models

Experimental
results were compared to several semi-empirical fouling models based on the
threshold fouling concept- Ebert & Panchal (1995; 1999), Polley (2002). This
threshold concept corresponds to theoretical zero fouling for a given operating
temperatures and velocities. From the model, typically for low temperatures and
high velocity, fouling is considered to be negligible. The fouling rate can be estimated
by the difference of two opposing factors: a term of deposit and a term of
removal:

The deposition
term is a function of Reynolds number, activation energy, temperature (film and
wall) and for certain models, also on the Prandtl number. The removal term on
the other hand depends upon the shear stress or Reynolds number.

Fouling rates measured
in the fouling rig were a reasonably good fit to those predicted by the Ebert
& Panchal fouling models and those observed at industrial scale.
Experimental results were also comparable with experimental data found in the
literature.