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.

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