Dynamic simulation aids recycling plant redesign

A poor design and high consumption costs shut this Pennsylvania paper recycling plant down, but an in-house implementation of full, dynamic process simulation got it all tuned up for a successful restart.

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 By Georges-Edouard Alexis

In the 90s there was great interest in North America in increasing paper recycling capacity. The enthusiasm to produce, combined with lack of experience with an emerging industry, led some to rush plant designs before they were fully vetted. At the time, there was a lot of speculation as to what it would take to produce high-quality recycled pulp.

Several of the mills built during the period have since closed. The reasons for their failure include poor design, poor water management, high energy consumption and high raw material cost due to the increased demand for waste paper. Our recycle mill in Northampton Pa. was one of those ill-fated plants. In 2001, the mill was purchased by Belkorp Industries Inc. and renamed Newstech PA, LP.

Newstech PA, is a deinking pulp mill with a design capacity of 420 air-dry metric tons/day. Our engineering group has been tasked with studying the redesign and feasibility of reopening and operating the plant.

At an early stage of the process study, it was determined that some form of process simulation software should be employed. When conducting a process study, it is important not to overlook any aspect of the operation. It is, therefore, very important to go through every control loop. Several different types of software were initially used to obtain a mill-wide mass balance, but always with the assumption of steady-state conditions.

       

VIRTUALLY THE REAL THING 

              
       

Ful, dynamic process simulation means never having to say you're sorry for poor design decisions after the plant starts up.
 

Full Dynamic Simulation
To replicate what really goes on in the mill, a full dynamic process simulation iwas called for, something that would show tank levels raising or lowering, valves reacting to meet certain setpoints and so on. Of the simulation tools available, we found Aurel Systems’ CADSIM Plus (CP) dynamic process simulator fit the bill.

The software has been used extensively during the study but in actual application the platform has proved to be more than a just simulator—it has been the tool used for the redesign of the entire plant. The simulator has allowed Newstech to build the process flow, input all the specifications and then use them to determine the projected quality, yield and energy consumption of the process on a mill-wide basis.

At the early stage of the study, we worked with a consultant who employed dated steady-state process simulation software. When it came time to compare numbers, the consultant argued that certain water loops were short on water when the CP balance indicated otherwise.

This was puzzling until we realized that their balance model the simulation was based on had no valves, pumps, or tanks. All the consultant’s software possessed was a series of blocks which they had designated as separators and other key system elements.

In a CP simulation, when a water or pulp loop requires more or less water, valves open or close to satisfy that need, similar to what actually happens in an operating mill. This prompts tank levels to rise or fall as needed during the simulation. In this case, it’s easy to observe if the tanks are sized too small. Not having this capability produces more work and may possibly cause an error on the part of the process engineer.

Moreover, looking at a CP balance is much like looking at a process flow diagram, because the software utilizes a CAD drawing front end that allows units and streams to be drawn in any form–from block diagrams to P&C quality drawings.

A Process Snapshot
After getting the balance to converge, the controllers tuned, and everything else running smoothly, a snapshot of the process can easily be captured. Once that is done, converting to AutoCAD is one button click away. We found that with the old steady-state software, we would still have to do a process control study, chest tanks size agreement, and finally, spend hours redrawing the flowsheet in AutoCAD.

Our process begins with two Valmet batch pulpers that discharge into a dump chest. As part of the redesign, we needed to know whether or not the existing dilution chest for the pulpers was too small for the given tonnage that we wanted to push through the mill.

With CP we were able to simulate the batch sequence by inputting all of the parameters, such as batch times, batch consistency, discharge consistency, and discharge flow rate. After determining the parameters, we were able to see how quickly the pulper dilution chest would be depleted of water.

In the original design, the pulper dilution chest was on level control. The simulation model allowed us to trend the surge caused by the pulper on the white water system. The simulation helped us realize that a constant flow with the overflow routed back to the dilution chest was the optimum solution. This eliminated the surge in the first loop white water chest that occurred every time the pulpers were being diluted for discharge.

Engineering was also able to increase the size of the simulated dilution chest until the group felt comfortable with the level during discharge. However, since the pulper dilution chest was already present in the process, the group concluded that a second unit would have to be installed.

An Easier Calculation
Most of the water in the front loop comes from pulper dilution, because at that point, the stock is diluted from 8-95.5% moisture. It then became apparent that the most efficient way to cool down the first loop would be to lower the pulper dilution chest temperature enough to account for the heat added in that loop from motors and from the excess in the previous loop. The mass and energy balance made that calculation very easy since the total BTU value is calculated by the software.

While pulp dewaters extremely well at higher temperatures, high temperature can have a reverse effect on certain temperature-sensitive chemicals. In the mill’s deinking cells, for example, a soap with the right amount of foam and ink/dirt collector is used. While too much foam can be a serious problem, not enough can create quality issues. Temperature plays an important role in the soap’s ability to foam. It is therefore necessary to keep the temperature high enough to help its drain-ability while not too high to affect quality.

During the redesign we wanted to reuse as many of the existing pumps, motors and equipment to keep the capital budget as low as possible. Dynamic process simulation allowed us to dynamically change the feed consistencies of equipment while keeping a close eye on the hydraulic capacities of the different equipment in the plant.

The forward flow cleaners for example, operate more efficiently at lower feed consistencies. But the mill’s fluidized drum washer thickeners are notorious for high fiber loss. The drum washers were placed by the mill’s original designers right behind the forward flow cleaners, a configuration that can significantly affect the mill’s total system yield due to the unit’s high filtrate consistency when fed at a consistency that is too low.

A CP simulation allowed us to monitor total fiber loss at the drum washers, while observing how the lowering of the feed consistency of the cleaner feed would dramatically increase the unit’s feed volumetric flow at constant feed tonnage. Cleaner efficiencies are directly proportional to their feed and accept pressure differential. Since feed volumetric flow and feed pressure are also proportional, one can clearly see the balancing act needed to optimize the operation in the front loop. The CP software helped us monitor all these variables. It also allowed us to quickly identify bottlenecks, which is important to be able to increase production.

Separate Water Loop
In order to produce a top-quality pulp, while efficiently using the mill’s limited fresh water supply, the engineering group had to design a totally counter-current system. The model helped us to separate the mill’s water loops, manage the temperatures in those loops, and most importantly, keep the fresh water system separate. Separating the water loops is critical to preventing the reintroduction of contaminants once they have been removed.

With temperature being an important variable for the removal of stickies and other temperature-sensitive contaminants, it was important not to have the bleach loop warm up the screening loop, which would significantly lower our screening efficiency. The simulation allowed us to input the entire equipment energy load and accurately size a heat exchanger to remove the required heat.

As part of the redesign, we were also able to simulate disaster situations to make sure that the system was not under-designed. The group was able to model pulp drier sheet breaks and monitor their effect on the fresh water system. Since dryer sheet breaks generate a large fresh water demand in the back of the process, we were able to see how quickly a sheet break might deplete our fresh water reserves. Seeing the high density storage chest filling with pulp and water during a sheet break showed us that if the process is not stopped quickly enough, we would not only end up with more water than we could handle, but also run out of fresh water. The resulting data showed us how long we could continue pulping with a sheet break.

Don’t Forget Training
Operator training is another benefit we’ve derived from the simulation software. Having been in operating mills and knowing what kind of problems occur during a run, experienced staff can safely simulate these problems and help operators-in-training come up with ways to solve or prevent them. Because every operator has his or her own style of running the mill, we are now able to instruct others on which techniques work and which do not. Process simulation is helping to eliminate the guesswork.

From a quality standpoint, with the software we are able to simulate the addition of contaminants into the balance and follow them through the whole white water system. Most mills have figured out how to remove some of these contaminants from the pulp, but keeping them out of the pulp can be difficult. As noted previously, contaminants can inadvertently be reintroduced later in the process via reuse of water.

Simulation modeling assisted us in monitoring the contaminants’ concentration in the different rejects streams and allowed us to reuse some of those streams into earlier parts of the process. In so doing, we could lower the concentration of contaminants in that stream, increase the overall system yield and keep a close eye on the contaminants within the whole mill-wide water inventory.

That In Control Feeling
Process simulation allowed us to feel more confident and in control of the redesign effort. Making a decision now has become a routine matter of simulating the idea to check its effect on the mill’s system.

Also, while tracking of all the solids in our system we knew exactly what solid loading we were sending to our effluent treatment. Knowing that allows us to accurately size the mill’s flotation clarifiers to handle the load. Applying for an effluent discharge permit became simpler since we knew the resulting solid loading, the volumetric flow and temperatures. As a whole, process simulation software, and specifically CADSIM Plus, has helped us in more ways than we would have imagined when we began this project. As a company, we feel that no mill should be designed without the use of dynamic process simulation software. We are confident that when Newstech PA is recommissioned, that it will start and operate with minimal problems.

Purchasing the software and developing in-house simulation expertise has turned out to be one of the best investments we could have made. Consultants can cost tens of thousands of dollars or more to provide mills with dynamic balances. Significant savings were realized by doing most of this work in-house.

With the ever-increasing cost of raw materials, demand for a more cost-effective product, and high operating costs, every mill is forced to keep optimizing and cut cost. Simulation modeling with a true dynamic simulator takes the guesswork out of making changes in the mill when one can simulate a change and see its effect on the whole process.

 

 


Georges Edouard Alexis is a Project/Plant Engineer for Newstech PA LP, based in Northampton Pa. Since his graduation from West Virginia University in 2001 with a BS in Chemical Engineering, he has been managing and implementing projects in the Pulp & Paper industry. He’s been involved in the design and construction of a roofing paper mill in Tuscaloosa, Ala. and has been directly involved in several mill acquisition due diligence studies. He is now working on the redesign of a newly acquired Deinking Pulp Mill.
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