February 2023

Heat Transfer

Key considerations for the optimum design of a hot oil system

In the hydrocarbon processing industry, where direct heating is not possible, a heat transfer medium is used. Steam has traditionally been the preferred heating medium.

In the hydrocarbon processing industry, where direct heating is not possible, a heat transfer medium is used. Steam has traditionally been the preferred heating medium. Heat transfer by steam uses latent heat. In several applications, steam is not used in certain situations, such as a high required temperature, the need to avoid water and condensate treatment, plot plan limitations and product contamination. In such cases, hot oil (heat transfer fluids) is used as an alternative heat transfer medium. This article details the major components in hot oil systems, suggests alternate configurations and outlines options available in selecting equipment design parameters. The selection of a fit-for-purpose design involves considering the impact on capital and operating costs. Case studies have been included to demonstrate the selection methodology. This discussion is intended for complex processes involving multiple hot oil users and two or more temperature levels.

System components

Components of a typical hot oil system are depicted in FIG. 1. This system comprises multiple process users and two temperature levels of hot oil supply. Hot oil as a heat transfer medium can be used to recover waste heat by utilizing a waste heat recovery unit (WHRU), or it can be heated using fuel in a dedicated fired heater. The following provides a brief description of some of the major system components. Some of the components described here may not be required for simpler systems. 

The WHRU/heater configuration will depend on the required heat duty and other plant-specific considerations. From a hot oil system design perspective, the WHRU can be with/without a bypass duct for the exhaust gas. The bypass on the exhaust duct provides control on the amount of heat absorbed by the hot oil when using a WHRU. In fired heaters, this is controlled through fuel firing control.

A hot oil expansion drum provides a spot in a closed circulating system for fluid expansion and contraction, along with a reservoir for fluid and a venting point in the system for moisture and low boiling components. The expansion drum also provides a location where inert gas blanketing can be installed and makeup fluid can be added.

A hot oil filter is provided to keep the system free of particulates and degradation products. A slipstream of hot oil from the discharge of pumps is continuously recycled to the expansion drum via the filter.

Typically, a hot oil trim cooler is provided in parallel to the process users. This is a dump cooler that dissipates any excess heat that is absorbed but not utilized by the process users, which helps to maintain a required inlet temperature at the WHRU/heater.

Hot oil recirculation pumps circulate the hot oil between the source of heat (WHRU/heater) and the process users. A pressure differential controller (PDC) is provided to ensure that a minimum flow is always maintained through the WHRU/heater, ensuring that the system is protected.

A hot oil sump drum (not shown in FIG. 1) is typically provided to collect hot oil drained from the hot oil pumps, hot oil filter and any other equipment/piping inventory during maintenance. Inside the sump drum is the hot oil sump pump, which recycles hot oil from the hot oil sump drum to the hot oil expansion drum.

Some designs provide a dedicated hot oil storage tank and pump for inventory/de-inventory of system volume.

Process objectives

The availability of hot oil is essential for continuous and steady operation of plant processes. The system is designed to maintain:

1. Hot oil flow through WHRU/heater coils as per design requirements—If the hot oil flowrate reduces, whereas the heat input remains constant, then both the film and bulk temperatures of the hot oil will increase, leading to increased coking/degradation of the hot oil fluid.
2. The temperature of hot oil at the WHRU/heater inlet as per design requirements—An increase in inlet temperature, while the heat input and hot oil circulation in the WHRU/heater remains constant, will cause an increase in the temperature of the hot oil at the WHRU/heater outlet, making coking/degradation more likely.
3. The supply temperature of hot oil at each process user—Since hot oil transfers heat to process users, any variation in the inlet temperature leads to instability in control.

The following is a typical control scheme to achieve these objectives:

• Distribute hot oil from the pump(s) on flow control to the WHRU/heater. A temperature control bypass is provided on the hot oil stream to target the required temperature at the outlet.
• Use waste gas bypass duct damper control for the WHRU, or burner firing control for the heater, to modulate heat input.
• Bifurcate the hot oil stream into the upper-temperature and lower-temperature streams. The lower temperature is achieved by mixing cool hot oil by using a separate, dedicated temperature control bypass.
• Ensure that the hot oil to each process user is on flow control reset by the process outlet temperature.
• Keep the hot oil to the trim cooler on flow control reset by temperature of the hot oil return header to the hot oil expansion drum.
• Provide a differential pressure control bypass around the process users. This bypass is used indirectly as flow control for the hot oil pumps to maintain a required recirculation rate.

Selecting system configuration

Depending on specific user needs, the system can be configured in different ways. This section describes a few commonly used configurations, along with a case study to compare alternatives.

Scheme 1. Referring to FIG. 1, the primary features of Scheme 1 include:

• Hot oil circulation to all temperature levels and users through common pumps
• All process users arranged in parallel
• Hot oil to each user is on flow control reset by the process side temperature
• A trim cooler in parallel to process users
• A differential pressure controller between the supply and return headers.

Scheme 2: A separate circulation pump for lower temperature levels. This alternate scheme has a separate pump recirculating low-temperature hot oil. The hot oil from the WHRU/heater joins at this pump suction (FIG. 2). This pump is in addition to the main high-temperature hot oil circulation pump that is providing hot oil to users at higher temperatures. The second pump is located at the low-temperature user inlet, with the temperature control valve located downstream of the user. When the temperature control valve is opened, additional high-temperature oil is mixed. The main pump is sized for the heat load through the heaters, and the second pump recirculates larger volumes across low-temperature users.

Scheme 3: Hot oil exchangers in a series. This scheme involves configuring the lower-temperature hot oil user in a series to the high-temperature hot oil users (FIG. 3). The high-temperature hot oil user is supplied first with hot oil. The hot oil outlet from this exchanger is then connected to the other lower-temperature hot oil users.

Comparison of three alternate configurations

Schemes 1, 2 and 3 are compared in TABLE 1 regarding pump capacity, power requirements, line sizes and hot oil expansion drum sizes. The type of hot oil and heater/user duties are the same for all schemes. To summarize:

• Scheme 1 has a simpler control scheme and a lower equipment count.
• The pump power required is lowest for Scheme 2, even though it is a two-pump configuration, while the highest pump power is required for exchangers in a series (due to additional pressure drop in the circuit because of exchangers in a series).
• The hot oil expansion drum size is largest for Scheme 1, due to a higher circulation rate of high-temperature hot oil.
• Hot oil supply and return header line size reduces by two sizes for Scheme 2 and by one size for Scheme 3 vis-à-vis Scheme 1.

Optimizing exchanger design

Hot oil as a heating fluid in reboilers/exchangers transfers sensible heat to the process. The heat input is regulated by changing the heating medium flow through the exchanger, which, in turn, varies both the heat transfer coefficient and the ∆T across the exchanger. The exchanger response depends on whether the heat transfer coefficient variation or the ∆T variation is the dominant factor affecting heat input. Generally, at high ∆T across the exchanger, the heat transfer coefficient effect dominates, while at low ∆T across the exchanger, the ∆T effect dominates. Controls are improved with exchangers operating with the heat transfer coefficient effect dominating. With the ∆T effect dominating, the controls are sluggish.

Selecting the right temperature approach is key to achieving an optimized exchanger design and hot oil circulation rate. The approach temperature is defined as the difference in the hot oil outlet and process outlet temperatures. Generally, a minimum approach of 20°F–30°F is preferred. A case study is presented to demonstrate the impact of these parameters on exchanger design. The hot oil flowrate and exchanger temperature approach are varied, and the impact on the tube-side heat transfer coefficient and required exchanger area is tabulated. The study was performed for two exchangers, one each for high- and low-temperature hot oil service. The exchangers are part of a facility configured as per Scheme 2 described above. TABLES 2 and 3 detail the impact of temperature on exchanger design.

An increase in hot oil flow leads to a higher heat transfer coefficient and a higher approach temperature, which lowers the required heat transfer area. However, increasing the hot oil flowrate requires a higher-capacity hot oil circulation pump. To arrive at an optimum design requires evaluating the impact of temperature on the entire hot oil system. To analyze this impact, the hot oil system for this case study was configured for multiple users and two temperature levels (as per Scheme 2), and circulation flowrates were adjusted to match Base Case flowrates in the two exchangers. The flowrates were then changed to match Case 3 in TABLE 2 and Case 5 in TABLE 3. This is presented as a modified case, and the results are presented in TABLE 4.

The results indicated a marginal increase in the capacity of the high-temperature hot oil pump (~2%) for the modified case. The capacity increase required for the low-temperature hot oil pump was significant (~25%). The overall increase in pump power was 50 kW. While a significant reduction was achieved in the exchanger area (saving the capital cost of the exchangers), there was also an increase in capital and operating costs due to the higher-capacity pumps and a higher pump power requirement. To attain an optimum design requires annualizing all costs. The capital cost of exchangers and pumps must be annualized by applying the cost of capital (interest rate) and plant life considerations. Much of the operating cost will be for power. The annualized capital and operating costs of each case can then be tabulated to find the optimum design, considering overall lifecycle cost. 

Hot oil trim cooler duty

The purpose of the trim cooler is to maintain the temperature at the WHRU/furnace inlet by absorbing any heat imbalance between the WHRU/furnace and process users. The heat imbalance can be addressed in the following two ways:

• Bypassing waste gas to the WHRU or reducing fuel firing in the furnace, thus reducing the heat absorbed by hot oil
• Using constant heat input in the WHRU/furnace, while varying trim cooler duty to compensate for process user variations.

A system with a dedicated fired heater controlling heat input to the heater through fuel firing control provides good control, and, in such a system, trim cooler duty can be minimized. WHRU designs can be with/without a flue gas bypass. In WHRU designs without a waste gas bypass, the trim cooler duty is the difference between the operating scenario requiring maximum duty (equal to WHRU design duty) and the operating scenario requiring minimum duty. In designs with a waste gas bypass option, it is theoretically possible to keep the trim cooler duty significantly lower than the design duty of the WHRU. However, in practice, the extent to which this can be achieved depends on how the controls are configured. Important considerations include the following:

• The response of the damper control is not expected to be as fast as the fuel firing control in the heater.
• Stable operation requires that the hot oil temperature be maintained within a narrow range.

Within these limitations, it is possible to optimize trim cooler duty by segregating the step/cyclical changes in heat duty demand from the gradual and planned changes in heat duty. Step changes and frequent changes that operate cyclically are better managed by a trim cooler duty adjustment—gradual changes and planned operations (where an operator can monitor damper position and avoid any sluggishness in control) can be managed through a flue gas bypass. However, it should be noted that this scheme requires a bypass duct, dampers and additional controls, which adds costs against the savings achieved in a trim cooler.

Hot oil expansion drum elevation

It is common industry practice to place a hot oil surge vessel at the highest location in the network. One benefit of this scheme is that this reduces the vessel volume requirements since the vessel can be sized for expansion requirements and no drain-back volume needs to be considered. An alternative is to lower the elevation of this vessel, thus reducing the cost of any associated structure. However, the vessel at a lower elevation will be required to be sized for expansion volume plus system inventory, which can drain down, resulting in a larger vessel size. Vessel volume can increase significantly, thus reducing the benefits of the lower elevation. Another factor to cinsider is that a large elevation difference between the trim cooler and the expansion drum may lead to negative pressure at high points downstream of the trim cooler/temperature control valve. This must be mitigated either by moving the control valve assembly to a lower elevation, or, if that is not possible, by increasing the operating pressure of the expansion drum. For blanketing gas failure cases, the trim cooler and the connected piping must be designed for full vacuum.

Hot oil filter

Hot oil degrades over time due to thermal cracking, oxidation and contamination. Contaminates can also include dirt, sand, dust, mill scale and slag from piping that accumulates during maintenance downtime or from installation. Often, a cartridge-type strainer is installed on the slip stream taken from the pump downstream and routed to the hot oil expansion drum. Installing a filter in the loop has benefits, including that it:

• Removes particulates
• Maintains viscosity by reducing sludge buildup
• Extends the life of hot oil.

Hot oil fluid suppliers generally recommend between 3% and 10% of total circulated fluid to pass through the side-stream filter.

Draining hot oil

Hot oil is flammable, and draining it requires a closed-drain system. Closed drains can be classified as maintenance drains and operational drains. Closed drains for maintenance purposes drain a facility after it has been isolated and depressurized. The capacity of a closed-drain drum depends on the drainage/operating philosophy. General industry practice is to design the drum to handle maximum liquid inventory from the single largest equipment at a low operating liquid level. There is scope to optimize the design of closed-drain hot oil systems while meeting system objectives. This can be done by discussing the operating philosophy with the owner and capturing the requirements. Some considerations include:

• Some facilities may opt to not provide any closed drain. Oil is drained to barrels at low-point drains. Hot oil collected in barrels must be disposed of offsite and cannot be recycled. Oil must be allowed to cool before drainage to manage this process safely. The drawback to this approach is a loss of hot oil in every maintenance event, along with associated operational inconvenience. Routine maintenance is normally required for pumps and filters.
• Most large facilities provide a closed drain with a blanketed drum and pump to recycle drained hot oil back to the process. The philosophy for fixing drum capacity varies, depending on the owner’s maintenance philosophy. Typically, sites determine size for draining the inventory from pumps and filters. Some sites consider the largest inventory of hot oil in any equipment.
• Two approaches are taken in selecting the design temperature for a closed-drain system. One approach is to consider the design temperature of the hot oil circuit. This implies that the draining is considered “operational,” and the credit for positive isolation, depressurization and cooling before draining is not taken. An alternate approach typical to hydrocarbon drains is to design a closed drain, considering that liquid will be drained after cooling, thus limiting design temperature between 150°F and 170°F and simplifying piping requirements. Designing underground piping for high temperatures leads to piping stress issues, along with the provision of expansion loops, which may be challenging and expensive.
• Gravity draining is preferred with lines running underground. However, depending on the selected design temperature and plant layout, a mix of gravity and nitrogen-pressured drains may be needed.

Takeaway

Hot oil applications in large facilities with multiple heat sources and process heat users can be complex, entailing high capital and operating costs. These systems offer opportunities to seek design options that can meet owner requirements with reduced lifecycle costs. The awareness of alternatives in design, and the adoption of a rigorous economic evaluation framework, can help in optimizing capital and energy costs for the system. HP

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