Review: Important contributions in development and improvement of the heat integration techniques

Abstract

The chemical processes and utility industries are central issues to modern living standards. The society evolution dictates that chemical processes will need continuous development and the advantages obtained of using process integration techniques consist in process improvement, increased productivity, energy conservation, pollution prevention, and capital and operating costs reductions of chemical plants.

Therefore, the aim of this paper is to present in a comprehensive review the development through the years (1975–2008) of the heat integration and heat exchanger network synthesis (HENS) as a technique of process integration. From an impressive amount of studies related to this topic, a selection with the studies representing the turning points and the emerging trends in developing and improving of heat integration and HENS methods was made.

The relationships between domains, authors, and journals, related with the field of research, are presented in an easy understanding visual format through the diagrams provided by CiteSpace II software.

Introduction

The classical process integration has been developed as a consequence of the continuous evolution of the technologies for improving process plants design. Continuously improvement of the energy consumption and plant operation represents the key for increasing the efficiency and the economical benefits of a plant.

Process integration design is based on hierarchical decisions. Process design starts with the reactor choice (batch vs. continuous). Once the reactor is established the separation system is design and analyzed. At this level the mass and energy balance are analyzed. Based on the energy balance the heat exchanger network is designed establishing also the need of utilities.

The continuous rising of energy costs has constrained the chemical and petrochemical companies to find ways to decrease the energy consumption improving especially the heat exchanger network. Therefore, the process integration through heat integration due to its economical and environmental benefits became a very important field in chemical engineering.

The concept of heat integration was introduced for the first time by Linnhoff and Flower (1978). In 1983, Linnhoff and Hindmarsh affirmed and demonstrated that it is possible to save an important part from the necessary energy required by a plant through specific actions and therefore resulting saving related to capital and operational costs up to 15–45%.

The heat integration represents a process integration technique that must follow several steps as: simulation of the process with the real industrial data, collection of the simulation results needed to apply one of the heat integration techniques. The use of the heat integration consists in reducing the quantities of the necessary utilities in the process.

Such analysis is performed using one of the two main heat integration techniques: the Pinch analysis and the mathematical programming.

The most used heat integration technique is the Pinch analysis. This technique is the simplest one, easy to use, with immediate results. The technique demonstrated its efficiency and applicability in many industrial saving energy problems. The steps that need to be followed in Pinch analysis are: identification of the energy targets trough composite curves and Grand composite curve, establishing the optimum ΔT_min for the process, obtaining the new composite curve and Grand composite curve with the optimum ΔT_min, obtaining the pinch point and the minimum need of hot and cold utilities, establishing the proper heat exchanger network based on this results.

The mathematical programming started to be used as a method to design the heat exchangers networks and to reduce the energy consumption in a plant beginning with the 1970s. The mathematical programming consists in elaboration of complex mathematical models solved using numerical methods. Generally, a super flowsheet is built containing all the possible design alternatives of a heat exchanger network. Each subsystem of the superstructure is solved and the result is generated separately. The global result is given after analyzing all the results obtained. In this way the mathematical programming is more precise and more objective than the pinch analysis but it is also more difficult to be applied in concrete cases and does not offer quickly results.

For a better understanding of the impressive research activity related to improvement of the mathematical programming method the LP, MILP, MILNP, NLP algorithms used for solving the mathematical models are explained related to the mathematic statement.

The major problems for continuous optimization include linear programming (LP) and nonlinear programming (NLP). As for discrete event problems, they are first classified into mixed-integer linear programming (MILP) and mixed-integer nonlinear programming (MINLP).

Therefore, linear programming (LP) is a technique for optimization of a linear objective function which determines the way to achieve the best outcome (such as maximum profit or lowest cost) in a given mathematical model and given some list of requirements represented as linear equations. A number of algorithms for other types of optimization problems work by solving LP problems as sub-problems. A mixed-integer programming (MILP) problem represents a LP technique but some of the unknown variables are required to be integers. Nonlinear programming (NLP) is the process of solving a system of equalities and inequalities, collectively termed constraints, over a set of unknown real variables, along with an objective function to be maximized or minimized, where some of the constraints or the objective function are nonlinear. MILNP combined the MILP and NLP techniques for a better problem solution.

In time, the two techniques discussed above were improved and successfully applied on various chemical processes in order to obtain the optimum economical benefits. Even so, the modeling and control of a heat integrated chemical process remains the most challenging research area.

Due to the high interest on energy conservation, in order to identify and to summaries the ways the field developed in time, this paper is thought to represent a comprehensive review of the literature subjected to studies related to heat integration and heat exchanger network synthesis (HENS). In order to improve the comprehension of the studied knowledge domain and its evolution in time the CiteSpace II software visualization diagrams have been used. The reason of using the CiteSpace II visualization diagrams is explained by the need to identify the most relevant aspects and changes that occurred in a specific knowledge domain in a fast and easy way. Such findings represent a challenge in case of each knowledge domain due to the several characteristics: complexity, scale, diversity or dynamic nature of a research field.

Visualizing and understanding a knowledge domain in terms of past, present and future enables the possibility to obtain information about the development of the research area, about the leading topics and about the future trends of the research area; and more than that to reveal the researchers and the institutions involved or dealing with the research area.