Conceptual design of a process site waste heat recovery system

G. Oluleye, M. Jobson, R. Smith

Centre for Process Integration, School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, UK; tel: 044-1613064381, e-mail: gbemi.oluleye@manchester.ac.uk
In spite of depleting reserves of fossil fuels and increasing prices, energy is still being wasted. In the process industries, at least 40 %1 of the energy content of fuel is wasted. Using energy more efficiently can reduce demand for fuel, thereby conserving resources, reducing operating costs and carbon dioxide emissions.
Industrial waste heat is defined as heat rejected from the processes in a site and the site utility system designed to satisfy the energy demand namely heating, power, cooling and refrigeration. With respect to the site processes, waste heat is the residual heat after heat recovery within a process or heat recovery between processes on a site using total site integration2. With respect to a utility system, waste heat is heat rejected to cooling water and air from a utility system designed to satisfy the energy demand of a site2. Diverse mature and commercialized waste heat recovery technologies3 exist to recover energy in the form of work, cooling and heat from waste heat. Examples include the organic Rankine cycle, Kalina cycle, absorption heat pumps, mechanical heat pumps, absorption chillers and economizers for boiler feed water preheating. Mathematical models of these technologies showing the relationship between key variables have been developed2. The operational limits of these technologies and physical limitations of a site dictate what waste heat it is feasible to recover.
Concepts that can be applied industrially to recover waste heat include heat recovery (by heat exchange), heat upgrading (using thermal or mechanical energy), conversion of thermal energy to mechanical or electrical energy, and use of waste heat to provide sub- ambient cooling. These concepts may also be combined to maximize production of useful energy from the available waste heat in a process site. The combination of these concepts and waste heat recovery technologies is defined as a waste heat recovery system.
Waste heat recovery systems are designed to provide useful energy from the available waste heat in a process site for use within the site (on-site) and over the fence (off-site). The major challenge is deciding how to combine these concepts taking into account the quality and quantity of the various sources of waste heat available in a process site and constraints on capital investment and space in an existing process site.
In this work a methodology is presented for the conceptual design of a site waste heat recovery system. The developed methodology is in five stages: (1) Identification of the available waste heat in a process site; (2) The waste heat source profile is then generated by plotting the heat source temperature (shifted by an appropriate minimum temperature difference, Î?Tmin, to allow for heat transfer) against the net duties of the waste heat sources. For design purposes, two waste heat source profiles can be generated: a profile for waste heat rejected to cooling water and waste heat rejected to air. The waste heat source profile allows evaluation of the potential for waste heat recovery in a site2; (3) Identification of opportunities to utilize the recovered energy (in form of work, chilling and heating) from the available waste heat for use both within the process site and over the fence (heat export for domestic
use and power export); (4) Building a superstructure for different combinations of waste heat utilization opportunities and heat sources; (5) Multi-period optimization of the superstructure to find the optimal combination of concepts taking into account seasonal variations in the available waste heat. The optimization framework is designed to allow for interactions between the waste heat recovery system and the process site utility system. The problem is formulated as a Mixed Integer Linear Program (MILP) solved using the branch-and-bound method.
An output of the methodology is a site waste heat recovery system showing how heat recovery opportunities can be combined to maximize energy recovery from waste heat available in a process site.
The methodology is applied to a medium scale petroleum refinery with seven processing units; crude distillation unit, three hydrotreaters (for naphtha, kerosene and diesel), a vacuum distillation unit, a platformer, a visbreaker and a fluidized catalytic cracking unit. The results show that increase in the site energy efficiency, reductions in the operating costs of a site and CO2 emissions can be achieved.
The methodology is sufficiently generic to be applied across the process industries.
References
1. Ammar Y., Joyce S., Norman R., Wang Y., Roskilly A. P., 2012, Low grade thermal energy sources and uses from the process industry in the UK, Applied Energy, 89, 3-
20.
2. Oluleye G., Jobson M., Smith R., Perry S. J., 2014, Evaluating the potential of a process site for waste heat recovery, Chemical Engineering Transactions, 39 (submitted).
3. Viklund S. B., Johansson M. T., 2014, Technologies for utilization of industrial excess heat: potentials for energy recovery and CO2 emission reduction, Energy Conversion and Management, 77, 369-379.