Oct-2013

Heat integration projects for refining processes

Heat integration projects can deliver substantial savings, but they require detailed simulation and case-specific heat integration analysis

ALis¸AN DOg˘AN
Turkish Petroleum Refineries Corporation

Article Summary

Refining is a complex operation involving many kinds of processes. All these processes have different principles; some involve fractionation, some involve different reactions and some have both. All these processes have one thing in common: they need energy. It may be a need to heat “cold streams”: energy to make the required separation between cuts, energy to strip off unwanted gases, energy to perform a reaction and so on. The processes also have energy-giving streams (hot streams): column pumparounds, overhead streams, reactor effluents and so on, which are available to supply a portion of the necessary heat; furnaces burning fuel take care of the rest. Some processes are integrated; the product or residue of one process may be the feed to another. The better the heat integration in or between process units, the less fuel is burned in furnaces, which leads to more profit. In this article, the basics of heat integration studies performed in various heat integration projects for different refinery process units are considered. The methods, equipment and approaches used for heat integration (pinch) studies of various refining processes, their similarities and differences are discussed.

In order to make a pinch study of an existing unit, one should first define the overall picture, which is the energy balance and the temperature profile of all the related cold and hot streams. The heating cooling curves and potentials saving should be determined. A test run performed in the unit will give all the necessary information, such as flow rates, temperatures and lab results. Based on this test run, a simulation model of the unit should be made. For a start, only the heat exchanger network consisting of simple heat exchanger models may be enough to define the overall heat balance. However, when it comes to adding equipment, making accurate cost estimations and defining design data for retrofits and new equipment, a unit model with rigorous heat exchangers, columns and other equipment will be necessary. Furthermore, a complete unit model will let you find additional and more accurate saving opportunities through case studies and trial-and-error studies. At the end of the day, it is all about making the necessary investment in an existing unit to gain air- or water-cooled waste heat to decrease furnace loads or generate steam. Case studies will be necessary to be able to select the best investment option. When deciding on the design data of new equipment and retrofits, rating them with a second set of simulation data representing the unit (or units) will be wise in order to select the equipment based on a range of operations.

Main steps to making a detailed heat integration engineering study are given below. The procedure may change from study to study, but the principles remain the same:
•    Rigorous simulation modelling of the existing unit or units within the boundary
•    Formation of base case heating – cooling composite curves
•    Determination of base case minimum approach temperature and potential savings
•    Determination of possible retrofit paths to achieve potential savings
•    Making the necessary equipment additions to the unit model and simulating the new retrofit paths
•    Repeating the first five steps for another base case simulation model, preferably a case at the opposite end of the operation envelope
•    Rating the equipment to supply the needs of both operation cases
•    Determination of investment costs and benefits of all the different saving opportunities (options)
•    Selecting the most appropriate case
•    Extracting the process data necessary for new equipment design.

Savings may be further increased during the latter simulation stages by changing/shifting reflux duties and operation variables.

Heat integration studies performed on different refining units will now be discussed, taking into account similarities and differences in the approaches and their effects.

Crude and vacuum distillation processes
The first important step for a heat integration study of a crude distillation unit is drawing the boundary: is the unit integrated with the vacuum unit and, if not, should it be? Integrating a crude unit with the downstream vacuum unit is, most of the time, more profitable. Even if they are not integrated, the overall boundary should be drawn to include the vacuum side — the atmospheric residue (vacuum charge) preheating train.

Drawing the boundary
In an integrated CDU/VDU configuration, vacuum unit hot streams are used to heat the crude charge, and hot atmospheric residue is sent directly to the vacuum furnace to be heated. In a non-integrated configuration, atmospheric residue is sent colder to the vacuum unit after heating the crude oil. Therefore, in a non-integrated layout, atmospheric residue is first a hot stream giving energy to the crude side, then a cold stream, which is heated by vacuum unit hot streams such as the HVGO pumparound and vacuum residue run-down. This is an inefficient design from a heat integration point of view. Heat exchanger area is needed to first cool down the hot atmospheric residue in the crude side, and additional area is needed to heat it in the vacuum side. Furthermore, the heat that could be recovered would be higher in the integrated case, the HVGO pumparound (at high flow) and vacuum residue (at high temperature) being able to give more duty to the crude side and the atmospheric residue going much hotter to the vacuum furnace directly from the atmospheric column.

Even if integration of the CDU and VDU is not desired for a specific reason, atmospheric residue preheating should be included in the heat integration study when drawing boundaries. Including only the crude unit will prevent one from seeing the potential modifications, retrofits and benefits, which result in hotter atmospheric residue going to the vacuum unit.

Figure 2 is the composite curve for the CDU unit in Figure 1, which is not integrated with the VDU. The hot-side pinch point is 129°C and the atmospheric residue outlet from crude preheating is at 127°C. Therefore, the atmospheric residue temperature is at the closest point to the heating curve of the cold streams. The minimum temperature difference (DTmin) between the curves is 30°C. Although there is 44 Gcal/h of waste heat, the saving potential is limited to 7 Gcal/h even when you target a minimum temperature difference of 10°C, which is very hard to achieve. The reason for this is that if you bring only the CDU into the picture, it is not possible to gain substantially from the waste heat and keep the atmospheric residue temperature close to 129°C at the same time.