Integration of absorption heat pumps in a Kraft pulp process for enhanced energy efficiency
Introduction
The pulp and paper (P&P) industry is characterized by its very large energy requirements, in the form of electricity and, as heat for chemical pulping, paper and pulp drying, liquors evaporation and other operations. Energy can also be required for cooling to maintain critical streams below temperature limits, especially when intensive internal reuse of water is practiced [1]. The temperature of the treated effluent must also be controlled to ensure proper operation and not endanger sensitive species in the receptor streams [2]. Efforts have been directed at reducing energy costs and very significant gains have been achieved. Early measures have dealt with production vs. supply management, equipment modifications and a range of simple process improvements to increase energy efficiency, liberate steam capacity or reduce consumption of fossil fuel. Pinch analysis® [3] has been applied to the P&P industry to maximise internal heat recovery [4] and is now routinely used [5]. Seasonal cooling by steam ejectors or barometric condensers, heating directly with steam or heating process streams in the 60–90 °C range could be performed by absorption heat pumps (AHP) with significant advantages over other current technologies.
AHPs constitute an emerging approach to upgrade heat by exploiting the effect of pressure on an absorption–desorption cycle to accomplish the temperature lift needed for the heat pump effect. AHPs use environmentally benign working fluids which do not contribute to greenhouse gas accumulation. They are thermally driven and can be activated by combustion of low-cost biomass or even by waste heat. They have captured an appreciable share of the market for water chillers in large scale air conditioning units [6]. They are also used for deep refrigeration of food products [7]. To the knowledge of the authors there are only very few reports of actual implementation of AHPs in the P&P industry in the scientific literature. The first reported AHP unit installed in a P&P mill has been in operation since 1993 [8]. This 200 kW heat transformer delivers steam at 125 °C using secondary vapour at 85 °C from one of the liquor evaporation plants. Experimental developments and feasibility studies have also been reported [9]. A steam generating heat pump was developed and tested at the National Research Council of Canada with a view to upgrading waste heat for the P&P and other industries [10]. The application of AHP for energy conservation in paper drying has been evaluated but there are no reports of its implementation [11]. The possibility to combine an AHP and cogeneration to produce power, cooling and heat upgrading in a Kraft process has been investigated [12]; the potential benefits should increase as the cost of the technology and of energy continue to evolve. The appropriate insertion of AHPs as stand-alone devices or as components of tri-generation units is a key subject that will require particular attentions [13].
AHPs are amenable to a broad range of operating conditions (operational temperature range and temperature lifts) in single or multi-stage configurations. Although they require a significantly higher initial investment than conventional vapour recompression heat pumps (VRHP), they could be competitive with those devices when abundant sources of free heat are available.
The work presented here illustrates the potential use of AHPs for heat upgrading in the P&P industry. Three examples have been chosen that represent a broad spectrum of machine configurations, mode of operation and integration context. The first example is a double lift heat transformer used to produce a significant amount of low pressure (LP) steam; the second one is a double effect chiller used to cool unstable bleaching agents while also producing middle pressure (MP) steam; the third example concerns the coupling of an AHP with a cogeneration unit. The data utilized in the three case studies were taken from the same kraft pulp manufacturing mill currently in operation. The mill is located in the Canadian province of Quebec. Before the three examples are developed, the specific Kraft process on which they are based will be described and the principles of AHPs briefly reviewed.
Section snippets
The Kraft pulping process
Pulp is a key intermediate product in the sequence of operations transforming a source of ligno-cellulosic material (wood chips in this work) into a finished paper product. The mill on which this study is based produces high-grade bleached Kraft pulp sold to manufacturers of special paper products. It is a small-size mill producing about 750 t of dried pulp (12% moisture) per day.
The pulping process consists in reducing fragments of wood such as chips into individual cellulosic fibres. In the
Review of principles
The heat upgrading process performed by a heat pump essentially consists of a closed loop in which a heat carrying fluid, commonly called the refrigerant, is circulated between an evaporator (E) and a condenser (C), as shown in Fig. 2a. In the evaporator, a quantity of heat Qe, available at low potential and often of zero cost (waste or ambient heat) is transferred to the refrigerant which is at temperature Te and pressure Pe. In the condenser, a quantity of heat Qc, the upgraded or useful
Basis for design and economic evaluation
This preliminary feasibility follows the principles of the conventional pre-engineering process design and cost estimation method presented in a number of engineering design manuals such as [15], [16]. For the dimensioning and costing of AHPs which is not treated in those manuals, specific empirical correlations were used. The data that were necessary for each AHP considered are, (i) functional parameters such as temperatures, pressures and heat loads, (ii) estimates of the installed and
Case I: heat recovery from blow down tanks
The current process configuration of case I is shown in Fig. 6. The digesters consume 52 t/h of MP steam and additional LP steam is supplied to the pulping plant at a rate of 13 t/h for other uses. Contaminated steam (46 t/h) from the blow tanks is released slightly below atmospheric pressure (96 °C) and used in a direct condenser and recirculation loop to pre-heat fresh water to 60 °C. A double lift AHT working with the H2O/LiBr pair could be driven by this contaminated steam (Fig. 7) with a COP of
Conclusions
The implementation of absorption heat pumps in a Kraft pulping process has been illustrated by three exploratory case studies. The flexible technology can be adapted to different process environments, taking advantage either of high-efficiency cycles (double effect) or increased temperature lift (double lift). It has also been shown that a reversed AHP cycle, the absorption heat transformer, can be used to upgrade heat for the production of process steam. Simple pay back times in the range of
Acknowledgements
This work was supported by the R&D Cooperative Programme of the National Science and Engineering Research Council of Canada. Mr Sebastian Schuster as a research intern from the University of Dresden at Ecole Polytechnique de Montreal received a scholarship from the P&P association of Germany.
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