**Introduction**

BALAS® is a steady state simulation package for chemical processes with emphasis on pulp and paper. The software has been developed at Technical Research Centre of Finland over the last 20 years and several Finnish paper mills, engineering companies and equipment manufacturers currently use it. The list of current users can be found here.

Figure 1. |
Screen shot of BALAS®. |

BALAS® operates under Windows operating systems and it is equipped with a graphical user interface (*Figure 1.*). Typical applications are

- Calculation of mass and energy balances
- Analysis of heat integration and heat recovery
- "What if" analysis
- Process optimisation
- Development of unit operation modules

**Unit operations and model libraries**

BALAS® has an extensive selection of unit operation modules. These unit operation modules enable the user to model the whole paper mill including mechanical pulping, heat recovery, utilities and wastewater treatment. New unit operations, including unit operations for kraft pulping process, are under development. A selection of ready-made model processes, which are listed below, are supplied with the software:

- TMP
- PGWS (News)
- PGW-70 (SC, LWC)
- DIP
- Paper machine (uncoated)
- Debarking plant
- Effluent treatment plant
- Water preparation plant
- CHP power plant
- Multi-effect evaporator

These model processes have been parameterised using typical industrial values. These processes can be pasted to any process model. Using these processes as a basis for modelling one’s own processes can speed up the modelling task considerably.

**Calculation modes***Steady-state simulation*

Process behaviour with pre-defined model parameters can be calculated using simulation mode. Two optional solvers are available for simulation calculations.

*Design*

Design (rating) calculation mode is used when one needs to define unit model parameters based on known output values based on e.g. mill measurements. In design mode any number of output values (e.g. temperatures) are fixed to their known value and an equivalent number of parameters (e.g. heat transfer area) are freed. During design calculation values for the parameters are sought to meet the given output requirements. Any number of design constraints can be introduced, and they are solved simultaneously with the rest of the process.

*Dynamic simulation*

Elementary dynamic simulations can be performed in this calculation mode. BALAS® is currently equipped with two dynamic process elements, which enable modelling dynamic behaviour of tanks and time delays caused by piping. Tank dimensioning and examination of process variability are typical applications of this feature.

*Optimisation*

BALAS® has a solver for non-linear optimisation problems. For these problems an objective function (the function to be minimised or maximised), equality (design) and inequality constraints, and free variables (design parameters) can be defined. The solver manipulates the values of free variables to obtain the optimal value for the objective function. Typical applications of this feature are water allocation problems and parameter optimisation problems.

*Parameter estimation*

A solver has been added to handle exclusively parameter estimation problems. In this mode sets of measured data (e.g. contaminant concentrations) in various parts of the process can be supplied. The free variables and constraints are given in similar fashion as in optimisation mode. The solver manipulates the variables to obtain the best possible fit between the measured data and simulated values using a least-square type of algorithm. A typical application of this feature is to estimate sources and sinks for dissolved and colloidal substances in processes.

**Excel-interface**

BALAS® comes with a ready-made link to Microsoft Excel. This link allows the user to access and manipulate all BALAS® parameter and stream data and run any process within Excel. Excel tools can then be used for visualisation and analysis of calculated data (*Figure 2.*).

Figure 2. |
Visualisation of process data using Excel. |

**Selected applications***Mass and energy balances*

Several mills have been modelled using BALAS®. Typically the models have been set up to investigate mass and energy balances (*Figure 3.*). The model in the Figure is a so-called Total Site model, which includes all the major processes and their inter-connections.

Figure 3. |
Simplified diagram of a Total Site model. |

The model includes external cooling and heating needs, process water requirements and effluent production of every major process. It also includes water preparation, effluent treatment and CHP power plant. With this model following aspects can be studied for any season or operational scenario:

- Water balance
- External heating and cooling needs
- Need for external fuel

*Energy studies*

Numerous energy studies have been conducted with the help of BALAS® software. These studies have ranged from typical energy saving studies to integration of new, efficient technology into an existing process (*Figure 4.*).

Figure 4. |
Comparison of different drying technology options on a mill. |

*Modelling dissolved and colloidal substances (DCS) in paper making processes*

BALAS® can handle modelling of DCS and non-process elements. Generation and adsorption/desorption of these elements has been modelled using user-defined reactions. These reactions can be readily defined in some unit operation models, e.g. refiner and bleaching tower, and there also exists a generic reactor module that can be used elsewhere in the process.

Measured concentrations of non-process elements can also be transformed into source and sink terms using the parameter estimation mode in BALAS®. Measured data from various parts of the process are given as data set(s) for the solver and reaction parameters are manipulated to achieve the best possible agreement between the measured and simulated concentrations (*Figure 5.*).

Figure 5. |
Conversion of measured concentrations to sorption terms and process parameters. |

*Water use reduction studies*

These types of problems arise when we are considering re-using slightly contaminated process water in other parts of the process in order to reduce the fresh water demand. The main objective is to allocate process water in such a way that the intake of fresh water is reduced as much as possible without exceeding contaminant concentrations in critical parts of the process, like bleaching.

The optimising solver in BALAS® can be utilised to solve these types of problems. One needs to build a structure containing all the possible connections between water sources and sinks (a so-called superstructure). The objective function is to minimise the fresh water intake and constraints can be set to ensure that contaminant concentrations do not exceed their maximum values in given parts of the process. The flowrates between the water sources and sinks are the free variables, which the optimisation routine manipulates to achieve the optimal solution. In Figure 6 is shown typical results of water use reduction studies performed by Technical Research Centre of Finland.

Figure 6. |
Influence of increased recycling on build-up of dissolved substances. |