Searching for enhanced energy systems with process integration in pulp and paper industries

European Symposium on Computer Aided Process Engineering - 13 A. Kraslawski and I. Turunen (Editors) © 2003 Elsevier Science B.V. All rights reserved.

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Searching for Enhanced Energy Systems vdth Process Integration in Pulp and Paper Industries Jarmo Soderman and Frank Pettersson Abo Akademi University, Heat Engineering Laboratory Biskopsgatan 8, 20500 Abo, Finland e-mail: [email protected]

Abstract The scope of this paper is to discuss some process integration options for enhancing the energy systems in pulp and paper industries. The focus is on the recovery of heat of the exhaust air streams from the paper machine dryer section and on the utihsation of eventual excess heat from thermo-mechanical pulping (TMP). With optimal heat recovery systems substantial savings can be obtained. New papermaking technologies like improvements in the press section and impingement drying have opened up new possibilities for utilisation of secondary heat energy. Production of electrical power with organic Rankine cycle (ORC) can be an option in that respect.

1. Introduction Production of paper requires large amounts of both electrical and heat energy. For production of one ton paper approx. 600 to 900 kWh of electrical energy and 1400 to 2000 kWh of heat energy is needed, depending on the paper grade and mill. The electrical energy demand for production of one ton TMP-pulp is typically from 2000 to 3000 kWh. Approx. 70 % of that energy can be recovered as low-pressure steam that is used in a large part at the paper machine dryer section in an integrated mill. Because of the high energy demands the enhancement of the energy systems with process integration are studied intensively.

2. Development of process integration methods for heat exchanger network design Pinch technology has been the most utilised process integration tool in the heat exchanger network (HEN) synthesis. An early step in the method development was the introduction of heuristic rules for the HEN synthesis by Masso and Rudd (1969). Hohmann (1971) introduced the idea of estabHshing the minimum utility target ahead of design. Huang and Elshout (1976) defined a bottleneck in the temperature vs. energy diagram of hot and cold composite curves and Umeda et al. (1978) called the touching point of the curves as pinch point. Linnhoff and Flower (1978) presented a systematic procedure for generation of energy optimal HEN. The ideas were developed to the pinch technology, presented for instance in Linnhoff and Hindmarsh (1983).

1062 Mathematical programming for HEN synthesis was developed from the early attempts of e.g. by Kesler and Parker (1969) to solve the HEN synthesis problem as an assignment problem, through the sequential transportation and transhipment models of Cerda and Westerberg (1983) respectively Papoulias and Grossmann (1983) and the three-step model of Floudas et al. (1986), to the simultaneous HEN synthesis model of Yee and Grossmann (1990).

3. Paper machine dryer section heat recovery system In a paper machine heat recovery system (HRS) the exhaust air streams of the paper machine dryer section are utilised for heating different cold streams. Condensation of the air moisture causes strong non-linearities in the system, notably the heat flow rates per °C and the heat transfer coefficients in the exchangers. A mixed integer non-linear programming (MINLP) model has been developed that takes into account these nonlinearities (Soderman et al., 1999). Additionally the heat transfer area prices can be given as concave price curves in the model and the climate of the mill location can be taken into account with a multiple period formulation. The model is based on partitioning the overall temperature range into a number of temperature intervals. Heat of a hot stream temperature interval can be transferred to the cold stream temperature intervals with lower temperatures. The heat can also be cooled with a cold utility or it can be discharged. For a cold stream interval the options are heat transfer from the hot stream intervals with higher temperatures or from a hot utility. Maximum savings are obtained by minimising the sum of running and investment costs of the HRS. A solution comprises the heat exchange matches to be included as well as the process parameters and the heat transfer areas. For a paper machine with impingement drying an example of input data for optimisation is shown in table 1. Table 1. A set of hot and cold stream data for a paper machine with impingement drying. HOT STREAMS stream type air flow, kg d.a./s temperature, °C moist.cont., I^g H20/kg d.a. COLD STREAMS stream type air, kg d.a./s; water, kg/s temperature in, "C temperature out, "C moist.cont., kg H20/kg d.a.

HI exhaust air 30.5 85

H2 exhaust air 17.2 236

H3 exhaust air 30.5 82

0.17

0.26

0.15

C2 01 imp.supply air hood supply air 48.9 17.2 28 28 95 350 0.02

0.02

C3 wire pit water 190 51 60

04 process water 100 32 60

05 circ.water 150 28 45

-

-

-

1063 The obtained optimal solution is shown in fig. 1. The heat transfer coefficients are calculated prior to the optimisation for each optional interval pair, taking into account the influence of condensation. HI H2 H3 GAS Hood Exhaust Air Imp. Dryer Exhaust Air Hood Exhaust Air , 85 °C 236 °C . 82 °C CI Imp. Supply Air 28.0 °C C2 Hood Supply Air 28.0 °C C3 Wire Pit Water 51.0 °C C4 Process Water 34.0 °C' 4495 mM C5 10884 kW Circulation Water 34.1 °C

2752 kW

-m

2575m^I

350 °C

i 2936 kW 3408 m^

95 °C

I 3365 kW

1096m21

1068 mH

60 °C

2925 kW

4233 kW

60 °C 1042 m^

38.4 °C

2144 m 4

45 °C

2410 kW

5288 kW

44.0 °C

42.7 °C

Fig.l. Flow diagram of the optimal HRS in the given example,

4. Upgrade secondary heat streams with a heat pump Different types of heat pumps can be applied to upgrade the secondary heat streams. The conventional compressor heat pump cycle (CHP) and mechanical vapour recompression (MVR) are widely utilised. With an absorption heat pump (AHP) or an absorption heat transformer (AHT) the compressor can be omitted, but the process becomes more complex. In fig. 2 an absorption heat transformer is applied for a paper machine dryer section. LiBr-H20 mixture can be used as working fluid. Around 50 % of the sum of the heat input in the evaporator and the desorber is obtained from the absorber at an elevated temperature.

>er i

Iter I

^~~7

exhaust I Evaporator air(i;

I I i '^**°'**''

I

^Ur a y

1

5^2

cooling

•€K

DRYER SECTION

exhausti air (2)

^M?—\ Qr

On

^»

Fig.2. Flow diagram andp, T-diagram of an AHT, that heats the supply air, glycol water for ventilation air heaters and process water with dryer section exhaust air.

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5. Electrical power production from excess heat In an integrated mill there may be a problem to find good use for all the available secondary heat. The situation accentuates, when impingement drying is applied in the dryer section. A part of the steam is replaced by hot air, heated by gas burners. The steam consumption in the dryer section is decreasing also thanks to the improvements in the press section. The option to utilise an eventual excess heat for production of electrical power with an organic Rankine cycle (ORC) is discussed here. ORCtechnology is applied in several geothermal power plants. Evaporator, HR-steam

HR-steam

Condensate

Fig. 3. Flow diagram of the basic ORC.

Fig. 4. ORC with a recuperator.

Basic ORC, fig. 3, is a conventional power cycle with one-component organic working fluid. The fluid is evaporated at an elevated pressure by a heat source, expanded in a turbine, condensed with a suitable cold stream and pumped back to the higher pressure. The cycle efficiency can be improved with a recuperator, fig. 4. ORC with high-speed technology has been developed, in which the turbine, the generator and the circulation pump is built on the same shaft in a hermetic unit. The rotational speed of the turbine is much higher than in a conventional ORC and consequently the equipment sizes are drastically reduced (Larjola, 1995). If the exhaust air from cylinder drying is used as a heat source, the working fluid can be for instance ammonia. The condensation of the exhaust air moisture starts at around 60 °C. The cold inlet water can be used as a heat sink. Due to the relatively small temperature difference the process efficiency is low. At winter period the outdoor air can be used instead of process water. Somewhat better efficiency could be obtained, but more heat transfer area has to be buih. Exhaust air from impingement drying could also be used as heat source. The condensation starts at around 70 °C. With a larger temperature difference a slightly better efficiency can be obtained than with the cylinder drying exhaust air. A much improved ORC-process can be obtained with heat recovery steam (HR-steam) from the TMP-steam reboilers. An integrated mill with a TMP-plant combined with a paper machine is taken here as an example. Paper grade is newsprint with a basis weight of 40 g absolute dry web/m^. With a machine speed of 1700 m/min and a sheet width of 10 m the production of paper is 12.3 kg/s air-dry paper with 92 % dry solids. The paper

1065 is dried in the dryer section from 48 % to 92 % dry solids. The specific energy consumption of the TMP-refmers is 2000 kWh/t of 90 % dry solids pulp, the pulp production capacity 14 kg/s and the total installed refiner effect 100 MW. In the reboilers the HR-steam production is 1 ton steam/MWh and the pressure of the clean steam 3 bar(e). The paper machine dryer section is built with 30 % impingement drying and 70 % cylinder drying. It is assumed, that due to the impingement drying 6 kg/s of HR-steam becomes available for ORG. With HR-steam as the heat source and ammonia as the working fluid the pressure of the NHs-gas before the turbine could be approx. 90 bar(a) and the temperature 120 °C, that is quite near the critical point. The condensation pressure of the NH3 would be approx. 12 bar(a). The relatively high pressures lead to elevated plant costs. Other working fluids, such as isobutane or isopentane, can be used to get lower pressure levels. The critical point of isobutane is approx. 36 bar(a) and 135 °C and of isopentane approx. 34 bar(a) and 187 °C. With isopentane as working fluid the pressure in the evaporator could be approx. 13 bar(a) at 130 °C and in the condenser approx. 1.1 bar(a) at 30 °C. With an overall efficiency of 14 % for the electricity production the generated power would be 1.8 MW. With a specific plant cost of 2000 euros/kWh the investment would be 3.6 million euros and the cost of electricity 0.04 euros/kWh with an annuity factor of 0.16. A binary working fluid NH3/H2O with for instance 80 % NH3 and 20 % H2O is used in the Kalina cycles. A flow diagram of one type of Kalina cycle (from Leibowitz and Mlcak, 1999) is shown in fig. 5. The cycle is applied here with HR-steam as heat source and process water as cooling media. In a Kalina cycle the concentration of the NH3/H2O mixture is varying. After the evaporation the fluid is partitioned in the separator into an NH3-rich gas stream, to be led to the turbine, and an H20-rich stream, to be cooled in a preheater and mixed back to the NHs-rich stream after the turbine. The mixture is then cooled in a recuperator, condensed and pumped back to the evaporator.

Condensate

Fig. 5. A Kalina Cycle.

1066 Kalina cycles have been given credit of higher efficiencies than the basic ORC, (e.g. DiPippo, 1999). If the steam from the TMP-refiners is used directly in ORC, instead of the HR-steam, the overall investment cost could be reduced. The steam reboiler for that part of the TMP-steam is replaced with the ORC-evaporator. The construction of the ORCevaporator is close to the reboiler construction. The ORC-process should be placed in a separate building, where both the fire risk and the exposure limits can be taken properly into account. Process water heating in an ORC-plant condenser opens up new possibilities to utilise the heat from the paper machine dryer section. Heat can be used for example for reduced-pressure evaporation of wastewater to recover clean water. Water recovery evaporation plants are in operation in several mills in Finland and Sweden.

6. Conclusions Conventional heat recovery from paper machine exhaust air streams can be designed optimally with respect to the climate at the mill location and the type of drying process involved. Different heat pump types can be applied, for example absorption heat transformers, to elevate the stream temperatures above the temperature of the excess heat. In an integrated mill with impingement drying the eventual excess low-pressure steam from the TMP-heat recovery, before or after the reboilers, can be utilised in an organic Rankine cycle for production of electrical power. The next step is to study the design and economic aspects of different types of ORC-plants.

7. References Cerda, J. and Westerberg, A.W., 1983, Chem. Eng. Sci., vol 38, 1723-1740. DiPippo, R., 1999, Geo-Heat Center Quarterly Bulletin, June 1999, 20, 1-8. Floudas, C.A., Ciric, A.R. and Grossmann, I.E., 1986, AIChE Journal, vol 32, 276-290. Hohmann, E.C., 1971, Optimal Networks for Heat Exchange. Ph.D. Thesis, University of Southern Cahfomia, Los Angeles. Huang, F. and Elshout, R., 1976, Chem. Eng. Prog., vol 72, Nr. 7, 68-74. Kesler, M.G. and Parker, R.O., 1969, Chem. Eng. Progr. Symp. Series, vol 65, 111-120. Larjola, J., 1995, Int. J. Production Economics, vol 45, 227-235. Leibowitz, H.M. and Mlcak, H.A., 1999, GRC Trans., vol 23, 75-80. Linnhoff, B. and Flower, J.R., 1978, AIChE Journal, vol 24, 633-642. Linnhoff, B. and Hindmarsh, E., 1983, Chem. Eng. Sci., vol 38, 745-763. Masso, A.H. and Rudd, D.F., 1969, AIChE Journal, vol 15,10-17. Papoulias, S.A. and Grossmann, I.E., 1983, Comput. Chem. Eng., vol 7, 707-721. Umeda, T., Itoh, J. and Shiroko, K., 1978, Chem. Eng. Prog., vol 74, Nr. 7, 70-76. Soderman, J., Westerlund, T. and Pettersson, F., 1999, 2nd Conf. on Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction, Budapest, Hungary. Proceedings, 607-612. Yee, T.F. and Grossmann, I.E., 1990, Comput. Chem. Eng., vol 14, 1165-1184.