January 2021

Heat Transfer

Selection and comparison of thermic fluids in a fired heater

Hot oil, also called thermic fluid, is a heat transfer fluid (HTF) used in oil refineries and chemical plants.

Karre, A. V., Worley Group Inc.; Valsaraj, K. T., Cain Department of Chemical Engineering, Louisiana State University

Hot oil, also called thermic fluid, is a heat transfer fluid (HTF) used in oil refineries and chemical plants. HTF transfers heat to the process fluid using sensible heat. Many limitations exist to using steam as a heat source. The steam pressure must be high enough to get the high temperatures required for the process; a steam system also requires water treatment and condensate collection systems; and steam systems are also a common leak point within plants. Conversely, steam is non-toxic and non-flammable, so there is zero negative impact to the environment.

Hot oil can also leak, but it does not require a high-pressure system. By selecting an appropriate HTF, high temperatures can easily be reached. The hot oil process is a closed-loop process. HTFs undergo thermal degradation over time due to the high-temperature process and will often need to be refilled by an external make-up. As shown in FIG. 1, a hot oil system consists of a fired heater, circulation pumps, expansion tank, filters, a hot oil cooler, process users and a slop oil system. The respective process equipment is located close to its process user to minimize both the piping and the required hot oil volume. Efficient design of hot oil equipment and the proper selection of the HTF is crucial to the performance of the hot oil system.1

FIG. 1. A hot oil system consists of a fired heater, circulation pumps, expansion tank, filters, a hot oil cooler, process users, and a slop oil system.

Circulating hot oil systems can limit CAPEX, depending on the chosen HTF. These hot oil systems are most advantageous to areas of the plant in which multiple users require process temperatures above what can be achieved with high-pressure steam. In some select cases, a circulating hot oil system can be utilized in which only one fired heater is required vs. installing independent fired heaters for each of the process users.

Many HTFs are available from manufacturers based on various properties, such as temperature range, density, thermal conductivity, heat capacity, vapor pressure, viscosity, corrosivity, flammability, flash point, fire point, thermal expansion, thermal stability, freezing point, flash point and autoignition temperature. The main parameters, such as heat transfer coefficient and line pressure drop, are used to determine the suitability of the hot oil.

A refinery client of the author’s company uses a proprietary base oila as the hot oil medium. The base oil degrades rapidly at an operating temperature of 575°F–600°F (302°C–316°C), causing fouling issues in the pumps, pipelines and exchangers. As an example, fouling caused by rapid degradation of hot oil in a heat exchanger is shown in FIG. 2. A client refinery condensate fractionation project required a reboiler duty of 48.1 MMBtu/hr. The additional duty for the project required a comparative study of different HTF systems for the service in the naphtha splitter reboiler, as the plant experienced degradation problems with the hot oil. The project team evaluated different hot oils that remain stable and are expected to have a 20-yr life span for operating temperatures ≥ 680°F (≥ 360°C). This study evaluates the hot oils by comparing their heat transfer characteristics, hydraulic benefits, cost benefits and other (vapor pressure, thermal stability, etc.) vital process parameters. The aims of the study are:

FIG. 2. Examples of fouling caused by rapid degradation of hot oil in a heat exchanger.
FIG. 2. Examples of fouling caused by rapid degradation of hot oil in a heat exchanger.
  1. Investigate thermal and hydraulic performance and suitability of the proprietary oil and ultra-low sulfur diesel (ULSD) hot oils at an operating temperature above 575°F–600°F (302°C–316°C).
  2. Compare different HTFs and recommend an optimal hot oil for operating temperatures above 680°F (360°C).

It is important to note that several operating refineries and chemical plants are still using the same hot oil that was selected 40 yr–50 yr ago. These old plants could be running their hot oil systems inefficiently and experiencing degradation problems, as mentioned here. Any plant or refinery can use this approach to solve a degradation problem and select an optimal HTF suitable and appropriate for their system.

METHOD PARAMETERS USED FOR COMPARISON

ULSD and proprietary heat transfer fluidsb,c,d were selected for comparison in this study because they exhibit the desired properties for the temperature range of 550°F–680°F (288°C–360°C). Note that ULSD has also been observed to break down fairly rapidly above 600°F (316°C) similar to the base oila. The following parameters are estimated for each hot oil fluid, and the description and significance of each parameter are detailed.

Vapor pressure

The vapor pressure is the pressure exerted by a pure component at equilibrium, at any temperature, when both liquid and vapor phases exist.2 Vapor pressure increases with an increase in temperature. Low vapor pressures are preferred in hot services to avoid any vaporization within the system. A two-phase hot oil system has higher frictional losses and requires more pumping energy, which equates to a higher operating cost. Also, a two-phase system requires additional piping supports, adding to project capital costs. Vapor pressure values for the proprietary heat transfer fluidsb,c,d are available.3,4,5 Typically, refinery diesel stream data and a proprietary softwaree simulation are used for estimating ULSD properties.

Fluid properties

Basic fluid properties—such as heat capacity (Cp, Btu/lb-°F), thermal conductivity (k, Btu/hr-ft-°F), viscosity (µ, cP) and density (ρ, lb/ft3)—are used to estimate the thermal and hydraulic process parameters. Fluid properties for the proprietary heat transfer fluidsb,c,d are available.3,4,5 Typically, refinery diesel stream data and a proprietary software simulationa are used for estimating ULSD properties. These fluid properties play an important role in determining the thermal and hydraulic performance of a hot oil.

Absorbed heat duty and mass flowrate

The heat duty (Q) is the heat absorbed by the hot oil heater, which is 48.1 MMBtu/hr. Based on the process requirements, the supply temperature from the hot oil heater is 680°F (360°C) and the return temperature is 551°F (288°C). The differential temperature is the difference between the supply and return temperatures (i.e., ∆T = 680°F–550°F = 129°F). The duty and differential temperatures are used to estimate the mass flowrate of a hot oil through the hot oil system. The mass flowrate can then be used to estimate volumetric flowrate of a hot oil. The heat equation (Eq. 1) is:

      Q = m × Cp × ∆T                          (1)

where m is the mass flowrate of the hot oil in lb/hr, Q is the heat absorbed, Cp is the heat capacity of hot oil in Btu/lb-°F, and ∆T is the differential temperature in °F.

Volumetric flowrates

The hydraulic equation (Eq. 2) is used to convert the mass flowrate into the volumetric flowrate (VF), which can then be used to estimate the velocity and the Reynolds (Re)number:

                                         (2), (3)

Velocity, Reynolds number and Prandtl number

The velocity through an 8-in. nominal diameter pipe (7.981-in. inner diameter) is estimated for each hot oil. This diameter is selected based on an approximate 10 ft/sec velocity for liquids. The corresponding Re and Prandtl (Pr) numbers are also calculated.6 Re and Pr numbers are then used to estimate the heat transfer coefficients, as shown in Eq. 4:

                                             (4)

where d is the diameter of the hot oil tube in inches (Eqs. 5 and 6):

                                                (5), (6)

Nusselt number and heat transfer coefficient

The Nusselt number (Nu) and the heat transfer coefficients (h) are based on the Dittus and Boelter equation.6 Since d and 0.023 are constant for all of the hot oils, Eq. 7 is rearranged to Eq. 8 and is defined as the modified heat transfer coefficient, H. The net change in modified heat transfer coefficient (ΔH) is also defined to compare the heat transfer performance of the new hot oils with the proprietary oil.a The required heat exchange area decreases as H and ΔH increase, because the heat transfer coefficient is inversely proportional to the surface area (Eq. 9):

                                      (7), (8), (9)

Frictional factor and pressure drop

The friction factor (f) and line pressure drop (ΔP) are estimated per approximations of the Colebrook equation.7 These parameters are used for estimating the hydraulic horsepower requirement and associated cost, as shown in Eqs. 10 and 11:

                                                      (10)

where ε is the pipe roughness factor of 0.0005 ft:

                                                                   (11)

where g is acceleration due to gravity (32.17 ft/sec2). ∆P per 100 ft of pipe (ΔP/100 ft) is estimated by substituting 100 ft for pipe length (L) in Eq. 11. ΔP per 2,000 ft of pipe is also estimated by using 2,000 ft for L.

Horsepower

Equivalent hydraulic horsepower (HH) and the actual horsepower (AH) for the calculated line pressure loss are estimated using pump Eqs. 12 and 13:

                                                                   (12)

Assuming a pump efficiency of 60%:

                                                                   (13)

Cost of horsepower

The cost of horsepower is estimated based on the cost of industrial electricity in the client’s location, and with the assumption of 8,000 hr of operation. The estimated costs are compared for the different hot oils: 1 hp = 0.7457 kw; cost of industrial electricity in client’s location, U.S. = $0.617/kwh;8 industrial hours in a year = 8,000 hr (Eq. 14):

      Cost = 0.7457 × 6.17 × AH × 8,000 × 0.01 = USD                                                   (14)

Cost of thermic fluid

The cost of proprietary heat transfer fluidsb,c,d products are very high ($1,600/bbl). The wax-free oila and ULSD are normally less than $100/bbl (more like $70 /bbl on average).

RESULTS AND DISCUSSIONS

As seen in TABLE 1, the proprietary base oila has the lowest vapor pressure at 680°F, but is not suitable at the operating temperature. The base oil degrades rapidly above 600°F, which causes fouling issues within the pumps and exchangers. ULSD has the highest vapor pressure, which means the system pressures need to be higher to avoid two-phase conditions within the system. The proprietary heat transfer fluidc has the lowest vapor pressure at both 680°F and 551°F, second only to the base oil. As shown in TABLE 1, the viscosities for all of the hot oils are relatively low when compared to water (1 cP). A lower viscosity leads to lower energy requirements for circulation within the system.

TABLES 2 and 3 show the hydraulic performance of different hot oils. The frictional factors for all of the hot oils are close to each other. ULSD requires the lowest mass flowrate. The base oila requires the lowest volumetric flowrate, but is not selected for use due to known fouling issues. One of the proprietary heat transfer fluidsc requires the lowest volumetric rate. That fluid also is lower in ΔP/100 ft, line pressure losses and hydraulic horsepower requirement compared to the other hot oils. The lowest cost found for that fluid shows that approximately $13,435/yr is needed, which is about 16% lower than ULSD and about 39% lower than both other heat transfer fluidsb,d. The lowest pumping cost for the heat transfer fluidc is due to the lowest volumetric flowrate compared with other hot oils.

As shown in TABLE 4, one thermic fluidc has a higher heat transfer coefficient than both ULSD and the base oila, but is still slightly lower than the second thermic fluidb. The third thermic fluid has a slightly higher heat transfer coefficient than both the first and second fluidsb,c. The higher the heat transfer coefficient, the lower the required area. The capital cost related to equipment could be lower due to the reduced area requirement, but the operating cost (pumping) would still be much higher for the third thermic fluidd, as shown in TABLE 3. The first and third thermic fluids discussed hereb,d have higher ΔH values than the second fluidc, but that fluid has a lower operating cost and lower volumetric rates.

Every hot oil system requires a certain amount of hot oil volume in the system, and frequent refill is required to account for losses and thermal degradation. The aforementioned thermic fluidsb,c,d are about 23 times more expensive compared with ULSD and the base oila, but they provide higher operating temperature required for the process.

Takeaway

Fluid properties and cost data of hot oils can be used to determine their suitability. Properties—temperature range, density, thermal conductivity, heat capacity, vapor pressure, viscosity, etc.—are used to estimate the hydraulic and thermal performance of a hot oil. The pressure drop through a line and the heat transfer coefficient are essential for the selection of a hot oil. The original hot oila was not selected due to its poor fluid properties and degradation at a temperature above 600°F. The low vapor pressure of the termic fluidc is needed at a high operating temperature of 680°F to keep the hot oil in the liquid phase. Other hot oils with a higher vapor pressure like the first and third fluidsb,d and ULSD require a higher system operating pressure, which increases the operating costs of pumping. The second fluid mentioned herec has the lowest line pressure drop and lowest cost per year, which minimizes the operating costs of pumping. Even though the third fluidd has the highest heat transfer coefficient, it is not preferred due to the higher pumping related operating cost. The cost of these fluids per barrel is very high compared with the base oila and ULSD. The second thermic fluidc was chosen for the project due to its stability at very high temperature and lowest pumping cost compared with the other two fluids discussed hereb,d. The base oila and ULSD are unsuitable for an operating temperature above 600°F.

Over a 10-yr–20-yr cycle, a hot oil system has exchanger leaks, flange leaks, solids building up in the system, etc., which can amount to substantial volume loss. Also, de-inventorying the system for 10-yr vessel inspections typically results in some fairly high system losses as some will ultimately get pushed to the flare, regardless of how well the plant minimize the losses. Large systems are harder to manage in a refinery system. Due to the sheer volume of the hot oil system, and the fact that the refinery frequently contaminates and purges high volumes of hot oil material (especially during an outage), the price-rendered specialized heat transfer fluidsb,c,d are infeasible. For this reason, the refinery went with a superheated 450# steam on another new project. If a hot oil system loses appreciable volumes of material, or simply needs a fresh load, it is going to take at least a month for the thermic fluidsb,c,d to generate and supply the necessary volumes. The refinery still uses hot oil systems such as the base oila and ULSD for operating temperatures below 600°F. Through trial and error in the refinery, the plant noticed that if the existing system is operated near 575°F, virtually no thermal degradation occurs, making the base oila and ULSD thermally stable below 575°F, ideally. Although, the base oila and ULSD are economically very attractive, these can only be used when the system operating temperatures are below 575°F.

This research can be utilized in any revamp, new grassroot refinery or a chemical project where hot oil fluids require comparison. It is vital to look for temperature range of processes, and vapor pressure is a critical parameter in selecting an appropriate hot oil fluid for a system. With the presented calculation approach, defining heat transfer and hydraulics process parameters can be estimated to properly select a hot oil type. Additionally, it is important to note that a single parameter is insufficient; analysis of many parameters provides a bigger picture of the system. Refinery experience with operation of a particular hot oil fluid and the cost of hot oil play critical roles in choosing a hot oil for new projects. HP

NOTES

      a Ergon 100-N oil by Ergon North & South America
        b Dowtherm A Heat Transfer Fluid by Dow
        c Dowtherm G Heat Transfer Fluid by Dow
        d Therminol 72c Heat Transfer Fluid by Eastman
        e ASPEN HYSYS

LITERATURE CITED

  1. Mitra, S., “A technical report on design of hot oil system,” University of Newcastle, June 2015.
  2. James, D. R. and G. Speight, Environmental Inorganic Chemistry for Engineers, 1st Ed., Butterworth-Heinemann, May 2017.
  3. Online: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098b/0901b8038098b395.pdf?filepath=/heattrans/pdfs/noreg/176-01466.pdf&fromPage=GetDoc
  4. Online: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098b/0901b8038098b391.pdf?filepath=heattrans/pdfs/noreg/176-01463.pdf&fromPage=GetDoc
  5. Online: https://www.therminol.com/sites/therminol/files/documents/TF-13A_Therminol_72.pdf
  6. Holman, J. P., Heat Transfer, Tata McGraw-Hill, October 2001.
  7. Crane Co., “Flow of fluids,” Technical Paper Number 410, 2010.
  8. Cost of electricity in Kentucky, U.S., Online: https://www.electricitylocal.com/states/kentucky/louisville/

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