In an attempt to reduce water consumption, pulp and paper mills have increased the recirculation rate of white-water for process use [1,2]. In some thermomechanical pulp (TMP) newsprint mills, fresh water is used only for high-pressure showers on the paper machine, and this accounts only for a fraction of their total fresh water use . To further reduce fresh water demand, such mills must now cut non-process water usage for cooling, sealing of pumps and for liquid ring vacuum pumps. Re-use of biologically-treated effluent  will also reduce fresh water consumption but it would not decrease the hydraulic load to effluent treatment plant. Based on a survey of 75 US mills , the majority of the reporting mills had a vacuum pump seal water conservation program in place in 1982. Fresh water was still used extensively for seal water make-up, but clear white-water, as well as clarified primary effluent, were also used for this purpose. Closed-loop recycle of vacuum seal water, with a controlled blow-down to prevent excessive accumulation of dissolved solids, has also been practised .
Cascading the seal water from high vacuum pumps to low vacuum pumps, as well as its re-use in the paper machine white-water loop, are other examples of the fresh water conservation strategies applied. A close-loop vacuum seal water system, equipped with water cooling and filtration, was proposed by Jonsson . Although historically, the vacuum systems were designed for a maximum water inlet of about 40°, it now appears that this temperature can be increased considerably, without restricting the paper machine operation . It should be noted, however, that seal water temperature may affect the vacuum pump capacity. For high vacuum pumps, a temperature increase from 40 to 50° was reported to reduce the capacity by 20- 40% . Since the operating cost of vacuum systems can reach $1 million (US) per year , the vacuum pump capacity losses can represent a substantial manufacturing expenditure.
With several hundred pump sealing units operating at a typical mill (BCTMP, 800 air-dried tonnes per day (adt/d)) , saving in fresh water use can be also attained by reducing the water flow to pump sealing systems. Savings resulting from closed-loop cooling water system, including pump seal water, at an integrated newsprint/linerboard mill (1600 adt/d) were estimated at about 27 m3/adt . In most cases, closed-loop pump seal water systems will require oil separation and filtration of water before re-use. Even in open system, fresh water saving can be achieved by monitoring and control of water flow to each seal unit, with the provision of shutting down the flow when the equipment is not operating , and by replacing the compression packing staffing boxes by mechanical seals . A new self-lubricating sealing compound to reduce seal water flow was installed in at least two mills . The compound was believed to perform best at high pressures (3.4 MPa) and temperatures (1000°C).
PROJECT SCOPE, OBJECTIVE
The Donohue mill in Amos, QC, has investigated the possibility to achieve a full reduction of water consumption with the following specific objectives:
Decreased hydraulic charge to the effluent treatment system;
Reduced pumping and treatment costs of fresh water from the Harricana River; and
Reduced heating costs for fresh water used for showers and as make-up for broke dilution.
The following three measures have already been implemented by the mill since the beginning of 1995:
Partial recycle of non-contaminated cooling water;
Partial recirculation of the effluent from the secondary treatment system; and
Re-use of a part of the vacuum pumps discharge as sealing water for vacuum pumps.
An analysis of all water networks in the mill is also in progress. It is based on a dynamic simulation, using the CADSIM plus PAPDYN platform, which will be used to identify and evaluate alternative scenarios for water re-use. This work has been carried out in co-operation with a research team at École Polytechnique [10, 11, 12] and with the support of Paprican.
This paper summarizes the work already completed or in progress, as well the results obtained.
The Donohue mill in Amos, Québec, is an integrated mill producing newsprint from TMP pulp and a small amount of purchased deinked pulp. The paper is produced on a double-wire Papriformer with a capacity of 630 adt/d. The average daily production is 500 adt/d. The mill is equipped with primary and secondary (activated sludge) effluent treatment systems in which all effluents are treated with the exception of non-contaminated water (cooling water) segregated since the construction of the mill. Its consumption of fresh water for the process (paper machine showers) of about 11 m3/adt while its non-process fresh water usage 36 m3/adt . The main uses for fresh water are, Fig. 1:
PM showers: about 6 000 m3/d
Sealing of vacuum pumps: about 5000 m3/d
Sealing of other pumps: about 900 m3/d
Cooling: 8000 m3/d to 14 000 m3/d, depending of the season; the effluent is considered as non-contaminated water (NCW)
Make-up primarily to the surplus white-water chest (SWWC) for dilution of the broke.
INCREASED RECIRCULATION OF NON-CONTAMINATED WATER
Since the construction of the mill, the non-contaminated water (NCW) was segregated, as indicated in Fig. 1. But only a fraction, i.e. approximately 2900 m3/d, was recycled. At the beginning of 1995, a pump was installed to recycle more NCW to the fresh water tank. A benefit of this measure is that the water in this tank is at a higher temperature than when only fresh water is used, especially during winter (8 to 10°C instead of 2°C), which helps maintaining higher temperatures in the wastewater treatment system. The reduction in fresh water intake from the river during winter is substantial. For instance, during the winter of 1993-94, i.e. before increased recycling, the average intake was 23 000 m3/d, while it had dropped to 17 500 m3/d during the winter of 1994-95 when NCW was used. In summer, the non-contaminated water is not recirculated since the temperature of the fresh water is already quite high, and addition of recirculated water would raise it further.
The segregation of NCW reduces the hydraulic load to the wastewater treatment plant by 8000 to 14 000 m3/d, but recycling has no effect on the hydraulic load.
THE RECIRCULATION OF TREATED EFFLUENT
In the earlier study mentioned above , which was based on a steady-state simulation of the machine white-water networks, it was shown that the use of the biologically treated effluent to replace part of the fresh water intake could result in important reductions of heating costs for the water used for showers . During the 1995-96 winter, 4400 m3/d of treated water from the secondary clarifier was pumped to the sand filters normally used to remove suspended solids from the river water, while 12 800 m3/d of fresh water was sent directly to the fresh water tank, Fig. 2. The filter required abnormally frequent backwashes because of the accumulation of viscous bio-solids originating from the effluent treatment, and the experiment was interrupted after three months of operation. However, no paper quality problems traceable to effluent recycling were observed during the trial.
This recirculation scheme had no effect on the hydraulic load to the wastewater treatment system. If fresh water supply is limited, or if regulations impose constraints on fresh water use, this measure could be reconsidered, but an additional filtration step would be required to remove the viscous suspended solids. One of the separation technologies potentially applicable for removal of suspended solids is the sludge mat filtration process, developed recently at Paprican [13, 14].
RE-USE OF VACUUM PUMPS EFFLUENT
The two measures described above are based on a recirculation loop in which water leaving the mill site is reintroduced at the of fresh water entry point. A major concern of the mill is the large hydraulic load on the wastewater treatment plant, especially at the primary clarifier where flocculation aids are needed to enhance removal of suspended solids. The mill is therefore interested in recirculation schemes that would reduce this hydraulic load, while also reducing the overall water consumption. An analysis of the breakdown of fresh water use shows that one of the most interesting candidates for recycling is the water used to seal and cool the liquid ring type vacuum pumps (about 5000 m3/d).
The effluent from the pumps is only slightly contaminated as compared to mill white-water, but it must nevertheless be treated before re-use to remove pulp fibres. The simplest way to reduce the net water consumption at that point is to recycle part of the pumps effluent as a substitute for the fresh water used for sealing and cooling.
Following measurements taken by the mill during the fall of 1996, it was found that the main concern about closed circuit operation was the temperature of the input water to vacuum pumps. The measured temperature difference between the outlet and the inlet water was about 30° and this value was used to evaluate various recirculation scenarios. Furthermore, the upper limit recommended by the manufacturer for the outlet temperature is about 55°, especially for vacuum pumps operating at high level of vacuum. Using those temperature constraints, an evaluation made by computer simulation for the conditions given in Fig.3, indicated a poor prospect for recycling the vacuum pumps effluent during summer, unless a heat exchanger is used to cool the effluent. The fact that the hydraulic load is at its maximum during summer makes this scenario less attractive.
An experiment was carried out during the winter of 1996-97, in which 1800 m3/d (28%) of the total vacuum pumps effluent (6300 m3/d) was substituted for fresh water and used as seal water for all the vacuum pumps. The inlet water temperature increased from 10 to 27°C due to recycling. It was then observed that the temperature rise between inlet and outlet is not constant and depends upon the inlet temperature. The temperature rise decreases as the inlet temperature increases; the design *T of 30°C is only applicable to cool inlet water at approximately 10°C.
To better understand the thermal behaviour of the vacuum pumps, a model was developed by the Polytechnique partners. The thermodynamic model takes into account all transfer of heat between the air and vapor mix and the sealing water, as well as the mechanical energy gained by the water during the compression step. This shows that when cool sealing water (*10°C) is used in a high vacuum pump, approximately 70% of the total heat transfer occurs with the condensing vapor, and only 30% of total heat tranfer is due to compression. When the sealing water temperature is increased, less condensation occurs and the temperature rise of the sealing water is less pronounced. A comparison of the results given by the model to that of data measured at the mill showed that the model predicts liquid ring temperature, outlet temperature and volumetric flow rate with good accuracy. The computed volumetric flow rate was compared to results given by a manufacturer model. Table I summarizes results given by the model for a NASH CL9002 high vacuum pump. For a lower vacuum pump, the model is conservative and gives an outlet temperature about 4° above the measured temperature.
Table II shows the recirculation potential at different fresh water temperatures, corresponding to the span of temperatures measured at the mill during winter (10°C) and during the hottest period of the year (25°C).
This study shows that without using any cooling step, it is possible to reduce water consumption by about 47 %, even during the summer. Installation of a strainer is recommended to avoid plugging of the vacuum pump inlet orifices. To achieve a higher rate of recirculation, the installation of a heat exchanger would be required. It should be noted that a higher rate of recirculation would result in accumulation of contaminants that could accelerate vacuum pump corrosion. Some mills control the pH to avoid this problem.
Other applications for this warm water containing low concentrations of contaminants could be paper machine showers, other pump sealing water or other applications for which the fresh water now utilized must be treated before being sewered .
Questions have been raised concerning the effect on the efficiency of vacuum pumps when the inlet water temperature is increased. Experiments were carried out during the summer of 1997 on a high-vacuum pump (26 kPa abs.), in which the inlet water temperature was varied from 25 to 39°C. The vacuum variation resulting from this modulation was only about 2 kPa; such a change in vacuum is inferior to the observed fluctuations during the normal operation. A possible explanation is that the velocity of the incoming air through the small suction orifices is close to the speed of sound in air, which is the maximum possible velocity. This situation occurs when the pump is overdesigned. The condensing process is reduced when hotter sealing water is used. Normally, the condensation of vapor increases the volumetric flow of gases, but this has a little effect on the vacuum produced when the volumetric flow is near its maximum value.
A dynamic simulation of the entire mill has been developed to determine the impact of system closure on the accumulation of contaminants in the white-water networks. The simulation was developed using the CADSIM plus PAPDYN platform. It is also used to analyse process modifications in support of process management procedures and strategies.
The following example is typical of an analysis that can only be accomplished by dynamic simulation. In the current process configuration, clear water from the saveall is sent to a surplus white-water chest (SWWC). This reservoir normally overflows to the sewer, except during broke repulping when water is withdrawn from this reservoir for dilution. If frequent long breaks occur, make-up fresh water may be required and this water must be heated with live steam. This situation occurs quite often and contributes importantly to fresh water consumption. The simulation can be used to study scenarios in which white-water from other parts of the process would be used to repulp and to dilute the broke, or to determine an optimal sizing for the SWWC tank which might be underdesigned for its current use. The dynamics of white-water networks in response to sheet breaks on the machine have been characterized for a generic process model . The current work constitute an applied extension of this exploratory study.
This analysis tool will be also used to determine the possibility of using filtered white-water or water from the vacuum pumps effluent as make-up for the paper machine shower water.
Measures have been implemented in the Donohue mill in Amos, QC, to reduce the fresh water demand for process and non-process needs. The approach encompasses simulation and analysis of white-water networks, modelling of crucial operations (vacuum pumps) and carefully monitored experimentation at the mill. Three measures have been implemented: increased recycling of non-contaminated cooling water, partial recycling of the treated effluent and re-use of the vacuum pumps effluent as sealing water. The total amount of fresh water saved is about 7300 m3/d, or 32% on current combined process and non-process water consumption. These measures could be implemented without addition of cooling equipment. Performance of the vacuum pumps is not affected by the increase in temperature and no deleterious effects on paper quality have been observed. Other possibilities for increased system closure are presently being investigated using detailed dynamic simulation.
The development of enhanced analysis tools such as dynamic simulation greatly facilitates the study of more complex and integrated closure scenarios. But the process engineer is often in the situation of not having enough information on design criteria and equipment limitations under operating conditions that deviate strongly from current practice, as is often the case when implementing system closure. For example, what is the tolerable suspended solids or dissolved solids concentration for a given type of showers sprinklers? What is the maximum allowable temperature of inlet water to vacuum pumps to achieve a certain degree of vacuum? What is the maximum concentration of contaminants in water used as seal for pumps? These are questions which are still unanswered.
These design criteria are critical and essential to guide the design for system closure.
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Résumé: L'usine Donohue d'Amos (Québec) a implanté trois mesures permettant de réduire la consommation d'eau fraîche et la charge à l'unité de traitement des eaux usées. La recirculation des eaux non-contaminées au cuvier principal d'eau fraîche permet une diminution d'environ 5500 m3/d en hiver. Les besoins en refroidissement limitent le taux de recirculation pendant l'été. La recirculation d'une partie de l'effluent traité a été expérimentée; l'engorgement des filtres a forcé l'abandon de cette mesure. L'usine effectue actuellement des expériences sur la réutilisation partielle de l'effluent des pompes à vide comme eau de scellement des pompes à vide. La simulation à l'aide d'un modèle thermodynamique des pompes à vide permet de prévoir une réduction de la demande en eau fraîche pour ces pompes de 47 à 64% sans qu'il y ait besoin d'installer de nouveaux équipements; la charge hydraulique au système de traitement des eaux usées serait réduite du même montant. On développe une simulation dynamique de l'usine complète, ce qui permettra d'élaborer d'autres scénarios de fermeture des circuits plus complexes et intégrés.
Abstract: The Donohue mill in Amos, Quebec, has implemented three system closure measures to reduce the usage of fresh water and the flow to the effluent treatment system. The recirculation of non-contaminated water to the fresh water reservoir reduces by 5500 m3/d fresh water consumption during winter. However, cooling requirements limit the applicability of this scheme during summer. Partial recirculation of the treated effluent has been tried and abandoned because of excessive fouling of the sand filters. Mill trials have also been performed on the partial re-use of the effluent from vacuum pumps as sealing water to vacuum pumps. Thermodynamic modelling and simulation of the vacuum pumps indicate that a 47-64% reduction in fresh water requirement for the vacuum pumps could be implemented without addition of new equipment; the hydraulic charge to the effluent treatment system would be reduced by the same amount. The development of a dynamic simulation of the entire process will permit the evaluation of more complex and integrated system closure scenarios.
Reference: HOULE, J.E., BROUSSEAU, Y., DORICA, J., PARIS, J. Reduction of fresh water consumption for process and non-process uses in an integrated newsprint mill. Pulp Paper Can. 100(3): T76-79 (March 1999). Paper presented at the 84th Annual Meeting of the Technical Section, CPPA, in Montreal, QC, on January 26 to 30, 1998. Not to be reproduced without permission of Pulp and Paper technical Association of Canada. Manuscript received on December 22, 1998. Revised manuscript approved for publication by the Review Panel, January 11, 1999.
Keywords: PAPER MILLS, NEWSPRINT, QUEBEC, CLOSED SYSTEMS, FRESH WATER, WATER CONSUMPTION, VACUUM PUMPS, EFFLUENTS, SEALANTS, WATER, EFFLUENT TREATMENT.
|TABLE I. Predicted water temperatures (°C).Seal water||Predicted temp b (°C)||DT b (°C)|
|inlet a (°C)||liquid ring||Outlet||(outlet-inlet)|
|a Assumed value||b Computed values|
TABLE II. Maximum use of vacuum pump effluent as sealing water.
|Seal water||Recycle rate (%)a at FW|
|inlet (°C)||Temperatures (°C) of:|
a % of vacuum pump effluent