Synthesis of zero effluent multipurpose batch processes using effective scheduling

Abstract

Wastewater minimization in batch processes has gained much attention in the very recent past. Mainly 2 reasons lie behind this heightened interest. Firstly, batch operations are inherently flexible, which renders them ideal for volatile conditions that characterize today's markets. Secondly, batch processes tend to produce highly toxic effluent streams, albeit in relatively small quantities in comparison to their continuous counterparts. The stringent environmental conditions militate against the latter characteristic of batch plants, hence the need to eliminate or minimize effluent. In most published literature, wastewater minimization is achieved through water reuse, water recycle, water regeneration reuse and water regeneration recycle. These concepts generally exclude the possibility of water being used as a key ingredient in the water using operations. However, some pharmaceutical and agrochemical operations, which traditionally operate in a batch mode, offer a unique opportunity to use recycle or reuse water as the main constituent of the final product. In some of these operations water constitutes more than 90% of the final product makeup. The problem addressed in this paper bears this feature, which consequently allows the batch processes to operate in a near-zero effluent mode. Moreover, the question of the number and size of the vessels used in a batch processing facility has always posed a problem. The incorrect approach to the synthesis of a batch plant can lead to the situation where processing vessels are over sized and even the possibility of idle processing vessels. This could pose a possible over capitalisation of an operation. In essence, an optimal design of a batch processing plant is determined by considering the scheduling of operations in the synthesis phase. The mathematically based method presented in this paper deals with the synthesis of a batch palnt operating in the fashion mentioned above. The method determines the optimal size and number of processing vessels and wastewater storage vessels, while scheduling the operation in such a manner as to operate in a near-zero effluent fashion