(521f) Integration of Production Processes, Waste Management and Energy Utility Generation In Chemical Batch Plants

The production of chemicals on industrial scale heavily relies on the availability of utilities such as energy in different forms e.g., steam, electricity, cooling water, brine and others. These utilities represent a significant share of the production cost of chemicals and define to a major degree the environmental impact resulting from chemical production. The efficiency of utility usage in chemical industry is primarily defined by two major factors, the efficiency of the utility generation (e.g. steam production) and the efficiency in using the generated utilities in achieving a productivity of chemicals as high as possible.This description evokes the idea of two separated problems and unfortunately nowadays the two fields are often treated like this and consequently substantial integration potentials are overlooked resulting in increased cost and environmental impact. However, the generation and the usage of utilities in chemical production are highly interlinked and integration potentials must be identified during early design stages of both utility generation systems and chemical production processes. Major aspects in this context are waste treatment in general and solvent management in particular. Increased process integration, e.g., recycling of material flows, reduces the amount of waste to be treated and therefore the additional utility input required by many waste treatment technologies (e.g., stripping, sewage treatment). However, waste can also be a source of utility generation, e.g., by generating steam through waste incineration. This is particularly an issue for the huge amount of waste solvents that are generated in pharmaceutical and fine chemical production typically operating in batch mode.

Chemical batch plants are characterized by frequent changes in production portfolio which implies dynamic patterns in energy demand and waste generation. Since waste treatment plays an important role in the energy system of a chemical plant, these dynamic patterns can increase energy demand and simultaneously offer opportunities for primary energy savings. In constrast, utility generation systems cannot be changed frequently because high investments are associated and high efficiency can only be achieved for a well-defined utility mix (e.g. the proportion of electricity to steam in Combined Heat and Power devices).

In order to study the potential of an integrated design in this three-component system (i.e., production processes, waste management and energy utility generation) suitable models are needed. In the present study the energy demand of batch chemical production processes was based on bottom-up unit operation models adjusted to real process measurements using calibrated control valve openings. The difference between theoretical and real utility consumption was used to develop empirical parametric equations describing thermal losses due to inefficient heating/cooling system operations. Validation of these bottom-up models was performed at different aggregation levels, including equipment specific unit operations, production lines and overall batch plant energy consumption. The results indicated that for all types of energy utility, equipment and unit operation the consumption could be estimated with a relative error between 5% and 35% depending on the aggregation level.

Typical batch plant waste treatment facilities were considered like incineration, wastewater treatment and wet air oxidation, while solvent recovery was treated with continuous and batch distillation units. This waste management system can make full use of the dual properties of waste solvents which can be an energy source as combustible for incineration or energy sink as material for recycling in distillation units. In order to include life cycle analysis aspects in the investigation, multi-input allocation models were used for the waste treatment units to include energy requirements and pollutant emissions of waste handling. These models assess emissions and utility consumption with few waste stream data using linear transfer coefficients derived by extensive industrial studies of the respective operations.

The energy utility generation system includes steam boilers and the associated steam network, gas turbines, a refrigeration cycle producing brine and of course the waste incineration plant. The heat load provided by utilities corresponds to the heat demand of the process taking into account an average efficiency for heat exchangers. Concerning the process streams temperatures, it is possible to assume that they are ΔTmin lower (or higher, if the utility is a cold stream) than the utility lower (higher) temperature. The size of the corresponding heat exchanger network and the amount of necessary primary energy sources can be optimized using Pinch analysis.

For the optimization of the integrated system in a modular form, three nested optimization problems were solved: the production planning of the batch processes (MILP), the mixing problem of the generated waste streams (MINLP), and the heat integration problem for energy utilities geneartion (LP). The operational costs and diverse life-cycle impact oriented environmental indicators were used as objective functions. A synthetic case study plant consisting of a mono-product and a multi-product building was used for investigating the efficiency of the integrated design. The case study plant reflects the complexity of a real batch plant that provided the data for the development of the bottom-up energy consumption models, the desired production capacities, the waste management constraints and the utility generation system. In this way, it was possible to define a base case corresponding to the current plant operation, to which the optimization results were compared. It is shown that an optimal waste mixing together with a suitable production planning (i.e., more homogeneously distributed production over the time horizon) can result in a significant reduction of ancillaries in waste treatment operations and primary sources for energy utility generation (approx. up to 50% less energy consumption related operational costs and 20% less environmental impacts expressed in Eco-Indicator-99 points).