Critical attention must be paid when designing the crude distillation unit (CDU) preheat train. Many pitfalls can easily lead to unnecessary expenditures or unsatisfactory operations. Key design criteria include high energy efficiency, operational flexibility and reliability. Some designers may elect to replicate a previously proven design, without full consideration of the feedstocks characteristics. Others may focus on implementing designs with computer-aided tools to generate the heat exchanger network (HEN) without sufficient attention to the effects of heat integration on the whole plant operation. A practical and systematic design approach can integrate process design, control, simulation and pinch analysis, as outlined in Fig. 1.
| Fig. 1. Crude preheat train design flowchart.|
Basis of (developed) design
A major energy demand within refineries is the required heat input for the crude feedstock upstream of the CD column to obtain the desired flash and distillation yields. Conversely, heat removal is needed to provide the internal refluxes and cooling of CDU products. The main design objective of the crude preheat train is minimizing total energy consumption by maximizing heat recovery. However, there is more to designing a crude preheat train than the HEN alone.
When starting the design of the preheat train, the basic parameters, such as feedstock, product yields and product specifications are determined but other operational philosophies must be developed for the engineering process. Generally, the approach for a revamp design will be different from a new unit. The revamp project must use many of the existing constraints as set by the present configuration and plant equipment.
Operational considerations. The refinerys operations team may have strong views on the preheat train configuration, especially if it is a revamp. Redundancy for some heat exchanger services may be necessary to allow exchanger cleaning to maximize CDU run length. Operators may also have strong views on CDU controls and how the unit is started up and shut down.
A strategy should be developed to start up the CDU and vacuum distillation unit (VDU), and operational needs should include:
- Establishing hot and cold circulations to remove water and to achieve on-specification products in both the CDU and VDU.
- Heating the feeds and cooling products. This may require additional startup and shutdown lines and involve alternate uses for equipment, especially for atmospheric and vacuum residue.
- Alternate routings may require more severe operating conditions for some equipment and affect mechanical design conditions.
If inter-unit heat integration will be considered, then the shutdown schedule of all units will have to be compared and reconciled. The design strategy should include operating strategies so that processing operations can continue in the event other process units are shut down. This may result in additional equipment.
Basis (further development) of design
The overall philosophy for the heat integration must be established. This could involve heat integration within the unit or heat integration with other process units. Although most consideration is given to intra-unit heat integration, inter-unit heat integration is equally important for total energy efficiency. Besides providing heat to the liquefied petroleum gas (LPG) fractionation and gas-recovery plants, heat integration with other refinery units should also be considered. Excess heat from cokers and fluid catalytic crackers (FCCs) is used for steam generation. These process units are high-level heat sources and can be considered for preheating the crude to reduce the overall refinery fuel consumption. However, a careful analysis of the unit shutdown philosophy should be evaluated. This could result in additional investment for equipment to support refinery operation during unit shutdowns and process upsets.
The refiner will generally have a payback philosophy; it can be in terms of simple payback or internal rates of return on incremental investment. For revamps, the extent of heat recovery may be limited to a predetermined overall unit cost budget. The owner may have energy targets or expectations for the preheat train. This could be expressed in terms of total energy targets for the unit, temperature approaches or target heater inlet temperatures. Usually global (whole unit) targets are set, with local minimum temperatures preset for individual exchangers, which may be close to any temperature pinch.
Initial simulation analysis
For a new design, the target product cutpoints, feed/product rates and specifications are set during the initial simulation and generally are derived from a linear programming study (Fig. 2). The total heating and cooling curves are generated from the initial simulation. These curves are used as a basis for a high-level heat integration analysis.
| Fig. 2. A typical CDU schematic.|
At a high level, the total heating and cooling curves will show if there is an excess or a shortfall in available heat from the process to meet the preheating requirements of the crude feedstock. Also, it can give an indication of the scope for integration with other process units or facilities. The balance between heat availability and demand depends on the crude feedstock. For lighter crudes, more heat is recovered at lower temperatures, and, consequently, lower preheat temperatures are usually obtained, as shown in Fig. 3.
| Fig. 3. Composite heating and cooling curves.|
It is likely that a stand-alone CDU will have a shortfall in available heat from the hot streams within the unit. If this is the case, energy (heat) input from other units to the CDU may be feasible, and steam may be the best medium for reboiling the light-ends columns in the gas plant, as there will be no available heat from the CDU. However, where the CDU is integrated with a VDU, excess heat may be available, which can be used for reboiling light-ends columns, generating steam or supplying heat to other users.
The effects from the number of pumparounds (PAs) and draw temperatures on the heater inlet temperature should be investigated to achieve an optimum design. The target global approach temperature is set at the initial design stage. Fig. 4 shows the results of this analysis. This figure shows the optimum global temperature approach for a particular network.
| Fig. 4. Total cost targets for the CDU.|
Startup and shutdown scenarios should be considered at an early stage, especially when there is heat integration between units. For example, if the CDU and VDU are heat-integrated, then the VDU will receive hot feed directly from the CDU. While this is good for heat integration, it does make starting up and shutting down the unit more difficult, especially if the timing of these operations is not identical.
For a revamp, modeling the present operation and bench-marking the results against operating data are necessary before simulating the new operations. Such evaluations are helpful in rating the existing heat exchangers and determining if the required heat inputs and removals can be achieved by the present configuration. If a reconfiguration is required, a review of the heating and cooling curves can provide an overview of the heat-recovery possibilities and present cross-pinch heat exchange. However, the constraints within the existing configuration may dominate setting new targets.
The pressure profile must be developed and managed in parallel with the heat integration. The preheat train will often have multiple pumps and many heat exchangers in seriesthus, the operating pressure may approach the 600-lb flange rating limits in some areas. Addressing the pressure profile ensures that the design pressures remain below key flange rating limits. This will give substantial capital cost savings, compared with a design that exceeds the flange rating limits. The practical design approach establishes the pressure profile at an early stage; the pressure profile is continually reviewed to ensure a good design.
The pressure profile starts at the crude-feed tank. The first decision is whether the crude is blended and supplied from a crude-blending tank or blended inline downstream of the storage tanks. Inline blending is often accomplished by using low-pressure (LP) blending pumps within the tank farm (some may be on recycle), and a crude-charge pump boosts the pressure at the CDU unit. The crude-charge pump allows the tank to be drawn to a lower level, because the net positive suction head (NPSH)1 required for the LP pumps is less than that required for higher-pressure crude-charge pumps. The discharge pressure of the blending pumps is more than adequate to meet the NPSH requirements at the crude-charge pump suction. Using a crude-charge pump at the CDU battery limit has the further advantage that operating the unit on total recirculation becomes straightforward and simplifies startup. Fig. 5 is a typical CDU preheat train; it has three main sections:
Preheat before the desalter. The crude must be heated to remove bottom sediment and water by the desalter. Often, the temperature is allowed to float within an operating range so that the crudes viscosity permits good water removal, while ensuring the wet crude remains below its vapor pressure. For a revamp, it may be necessary to control the temperature more precisely to stay within operating constraints. In some designs, booster pumps are installed downstream of the desalters.
Flash drum/column. Following the desalter, further heat recovery takes place. A flash drum column may be installed, especially for lighter crudes in which a high-preheat temperature is achieved causing high vapor pressures.
Heater inlet. Finally, the crude is heated up to the heater inlet temperature.
| Fig. 5. A typical preheat train schematic.|
In evaluating design pressures, heat exchanger burst-tube scenarios may also need to be considered.
At the start of the design, a rough pressure profile should be developed for the preheat train. It is based on an estimate of the number of exchanger services in series. This will often give a first indication that the configuration may need to be changed to limit the pressure drop. The pressure profile should be updated as the heat exchanger design progresses. For revamps, the design pressure constraints can influence where new exchangers can be fitted into the design. For a new unit, the startup and shutdown requirements may influence the decision on the location of any crude booster pumps.
It is usually preferable to set the pressure upstream of the control valve at the inlet to any preflash/prefractionator; it should be above the bubble-point pressure. This will suppress vaporization, improve control of the operation, reduce the risk of fouling by salt deposition, and avoid mechanical problems due to slug flow. However, if the pressure balance is marginal and two-phase flow is expected due to variations in the crude feedstock, the unit must be designed accordingly.
Operating within a two-phase flow regime can provide advantages in terms of better heat recovery and lower operating pressure. Units have operated successfully in this manner without excessive fouling problems, provided that the upstream desalting operation (double- or triple-stage desalting may be required) is operated efficiently. If a unit is designed for two-phase operation, then it is vital to consider the whole range of crudes to be processed by the CDU. Operation with less vaporization than the design basis will lead to lower fluid velocities, poorer heat-transfer coefficients and lower performance.
The operating pressure of the preflash/prefractionator will affect the amount of light ends being flashed directly to the crude column and the vaporization of crude from the heater, as well as the CDU performance. The optimum pressure should be selected based on a parametric analysis of the effect of pressure on the crude column overflash/distillation and the efficiency of the heat integration.
To ensure that the flowrate to the furnace is controllable, the operating pressure should be high enough so that no vaporization occurs upstream of the control valves on the parallel furnace passes. For a new design, the operating pressure should be set to provide a margin over the bubble-point pressure and consider:
- Clean and fouled heat exchanger operation
- Potential use of heat exchanger bypasses
- Preflash vessel operating pressure flexibility
- Maximum Reid vapor pressure (Rvp) of the design crude
- Alternative crudes.
Also, low heater-flow trips only protect against low-pass flow and not the total loss of crude flow. With the loss of crude pressure, vaporization upstream of the flow orifices will occur, and any flow trips will become unreliable due to the higher vapor velocities.
Pinch analysis on the HEN design
Pinch analysis is a key design tool to achieve optimum energy efficiency. Modern software allows a HEN to be generated automatically for defined heating and cooling streams. However, engineering judgment is critical to ensure a practical and robust design. Designers should never rely solely on the automatically generated solutions. The HEN generated by the software may represent the best energy efficiency, but it can often lead to a design with high capital requirements and little flexibility. A practical design demands thorough analysis, and it requires that the designers take charge to configure the HEN, using the software only as a support tool. In addition, commercially available software may have limitations in the size/complexity of system it can accommodate. For example, while a preheat train for a CDU may be manageable, an integrated CDU and VDU may be too complex for the software to analyze. Key points to consider for a practical heat exchanger network design are:
- The total heat recovery system design should be optimized to achieve an economic target for energy usage when compared to total capital expenditure (CAPEX) and should account for both the preheat train and cooling equipment.
- For inter-unit heat integration, there is a trade-off between the total plant energy efficiency and operability. HEN design software often presents solutions with a large number of small heat exchangers and in an arrangement that could be awkward to build and control. These designs should be rationalized to obtain a good overall solution.
- Where the achievement of the targeted cooling temperatures is critical (for example to ensure a safe rundown of products to tankage), cooling utilitiessteam generation, cooling water or air coolingshould be used for final cooling. Often, the trim coolers are sized, based on the maximum rundown rates from different crude or cutpoint operations or, for clean exchanger operations, when some exchanger bypassing may be required to avoid temperature pinch-out.
- On larger-scale CDUs, it is common to split the feed crude line into several parallel streams from section to section. This ensures that heat exchangers remain within their mechanical size limits, while increasing the level of heat integration and enhancing heat recovery. However, an increased number of splits can lead to operational and control issues, especially during periods where the feed crude ratio is being adjusted. There is always a trade-off between energy efficiency and operability. However, sophisticated control systems can be configured to use ramping functions to help the operators optimize the splits and smooth out the transitions during crude composition or cutpoint changes.
- The crude preheat upstream of the desalter needs careful consideration to ensure that the temperature remains within the desirable range120°C to 150°C. For revamps, it may be necessary to control the desalter temperature more tightly to avoid constraints. In this situation, one solution is to split one of the duties into two, with one part duty upstream of the desalter and the second part duty downstream. This will allow adjustment of the desalter temperature without significant effects on the heat recovery.
- The HEN should be designed to allow the column fractionation performance to be controlled by varying the circulating reflux and/or quench duties. This is achieved by varying the flowrate of the circulating refluxes and/or providing a trim cooler in one or more PAs.
- Temperature control by bypassing exchangers on the crude side is not recommended due to increased fouling at low crude velocities.
- In the event of excess heat availability, recovery of low-level heat from product rundown streams is preferable to recovery from overhead streams, but this should be confirmed by a review of the composite curves. Heat recovery from overhead streams can lead to dewpoint corrosion problems and potential contamination problems in the event of tube leakage. If the exchangers are elevated, then significant extra costs are incurred due to the need to mount the exchangers in a structure. In some instances, running warm side products to the receiving units will provide better total heat integration. However, warm rundown limits the heat that can be recovered from the rundown streams, and heat recovery from overhead streams may need to be considered.
- For fouling services, if long run lengths are required, then the exchangers should be placed in parallel/series line-up to allow online isolation and cleaning. This is particularly an issue with crude and residue streams. Strategies include providing isolation block valves and bypasses on both sides to allow for cleaning and designing for increased flows through some exchangers while others are out of service. Pressure drops will also increase with fouling. While control valves will operate with a considerable pressure drop at start of run, there will be minimal pressure drop available at end of run.
As the HEN design progresses and the PAs are allocated against crude splits, it is necessary to re-evaluate the PA rates. Ideally for a particular heat exchanger service, the mass rate, M, multiplied by the specific heat, Cp, (M x Cp) for either side should be similar in value. Thus, the heating and cooling curves will be approximately parallel, and the heat exchanger area will be minimized. It is possible to optimize the PA rates and crude splits, where there are parallel exchangers, to minimize the heat exchanger area by adjusting these rates.
The single most important factor affecting the design of the preheat train is the volume of each product. This is dependent on the crudes characteristics. The optimized HEN can be significantly different for light and heavy crudes due to the differences in the heating and cooling curves. Some typical crude yield variations are shown in Fig. 6.
| Fig. 6. Some typical crude yield variations.|
As the crude-to-product-price differential dominates refinery margins, refiners try to process less-expensive (opportunity) crudes and/or to produce more higher-value products to improve profitability. Therefore, the CDU feedstock may be expected to vary significantly during the refinerys lifetime. Many refiners try and cover this variation by specifying multiple crudes and blends in the design basis. An alternate approach would be to allow for higher vapor pressures, design temperature and pressures, and higher heater duties in the design to provide more operational flexibility.
If the unit is designed for more than one crude, then different parts of the preheat train will be designed based on different cases. The quickest option is to specify each individual piece of equipment for the controlling case or design mode. This typically results in the colder end of the preheat train being designed for the lighter crudes and the hotter end of the preheat train for the heavier crudes. The unit controls should be capable of compensating for the resulting over-design, and the designer will gain an understanding of the performance by converting the simulation into rating mode. In rating mode, the key equipment parameters such as heat exchanger geometries or, more simply, heat transfer coefficient, U, times area, A,(U x A)values are input into the simulation, and the uncontrolled process conditionsincluding temperatures, pressures and heat exchanger dutiesare resultant from the specified equipment data.
Rating mode can be used to simulate the unit operation (including over-design) for the normal operating cases, with the exchangers in a clean, or a moderately or heavily fouled state, or at turndown. Such simulations would allow the controls to be tested for a range of scenarios, and may identify that over-performance in one area results in poorer performance in another, due to temperature pinch-out. This may result in a need for additional bypasses or an increase in some utility cooling duties. In some cases, the utility cooling design duties may be set by the rating cases.
The other major use of rating simulations is to reduce the net over-design resulting from the overlay of several design cases. This is done by trial and error, based on assessing the effects of the over-design from one case on the other cases. An ideal scenario would be for the design area for each case to be approximately the same, and with no additional area having to be added to the utility cooling design cases. This last option can be time-consuming and costly at the design stage, but could give significant CAPEX benefits. HP
The author acknowledges the assistance provided by her Foster Wheeler colleagues: Christopher M. Jones, Group Manager Process Technology; Bernard M. Hagger, Chief Engineer Refining; and Philip G. Marden, Principal Consultant Process Engineer in the preparation of this article.
1 The NPSH is the difference in meters (or feet) of fluid between the absolute pressure at the pump suction, taking into account the tank level minus the pressure drop in the suction pipe and the vapor pressure of the fluid.
Jenny Zhang is a principal process engineer working as a member of the Process Technology Group within Foster Wheelers Process Engineering Department in Reading, UK. Ms Zhang has over 20 years of experience, specializing in process design, simulation, process RAM studies and energy optimization covering refining, LNG, GTL and CCS projects. She holds an MPhil degree in chemical engineering from UMIST, Manchester, UK, and a BSc degree in chemical engineering from East China University of Science and Technology in Shanghai, China.