Zero-wait

If one unit is used per stage and the plant is operated with zero-wait transfer, the cycle time is ... 

For the ZW (zero wait) policy idle times (slacks) between consecutively produced batches may appear in all stages including the one that defines the bottleneck. It should be noted that the slacks are only a function of consecutive pairs of batches. Therefore, the slacks for each pair of batches can easily be calculated a priori with the binary variable for any two consecutive batches ... 

All of the schedules considered so far involved transferring material from one step to another, from a step to storage or from storage to a step without any time delay. This is known as zero-wait transfer. An alternative is to exploit the equipment in which a production step has taken place to provide hold-up. In this situation, material is held in the equipment until it is required by the production schedule. A schedule using equipment hold-up is shown in Figure 14.17c. This reduces the cycle time to 15 h. 

The following process is proposed for the production of Product C. For the scheduling of the production campaign, overlapping is allowed and a zero-wait transfer is applied for storage policy. The process is represented in Figure 14.28. 

The other operational philosophies that are generally encountered are the mixed intermediate storage (MIS), zero-wait (ZW), finite wait (FW) as well as the unlimited wait (UW) operational philosophies. The MIS philosophy is encountered in a situation where at least 2 of the aforementioned operating philosophies coexist in one process. It is indeed very seldom in most practical applications to have only one philosophy throughout the operation. A combination of different philosophies in often the case. 

Another important aspect of process flow requirements is reflected in inventory policies. These often involve finite and dedicated storage, although frequent cases include shared tanks as well as zero-wait, non-intermediate and unlimited storage policies. Material transfer is often assumed to be instantaneous, but in some cases such as in pipeless plants delay is significant and must be accounted for. 

Zero-Wait (ZW) the material is unstable and so it must be processed immediately after being produced. 

The so-called zero-wait transfer policy is used when a batch at any stage is immediately transferred to the next stage because there is no-intermediate storage vessel available or when it cannot be kept in the current vessel. This policy is extremely restrictive. The other extremum is the unlimited intermediate storage policy, where a batch can be stored without any capacity limit in a storage vessel. Finally, there is a transfer option called no-intermediate storage, which allows the batch to be kept inside the vessel. Normally, the zero-wait transfer reqnires the longest CT. In practice, plants normally have a mixtnre of the three transfer policies. 

The addition of parallel units can increase the efficiency of the plant considerably. Let us consider a simple biorefinery that is operated in zero-wait mode. Each batch is the same (1000 kg). In the first stage, the biomass is converted in a bioreactor to ethanol, which takes 12 h. In the second stage, a distillation is performed to pnrify the bioethanol. This separation step takes 3 h. The Gantt chart of the process is depicted in Figure 12.5. It is obvious that the CT for each batch is 12 h. 

Adding a parallel unit to stage 1, the plant can be operated as shown in Figure 12.6. The CT has been reduced to 6 h. The CT for a zero-wait transfer policy plant can be calculated from... 

To derive the proposed models, one production line with fixed assigned equipment units and zero wait transfer policy is assumed. The scheduling is addressed within a time horizon of one month. Finally, inventory costs and penalties for production shortfalls, proportional to the amount of underproduction, are adopted for each product,... 

The duration, Dj , and the finish time, 7j , of a task are calculated through constraints (5), and (6) and (7), respectively. The elimination of start times, 75j , is made through constraint (8) and the time matching between time points and finish times is achieved through constraints (9) and (10). Note that in the general case a task may finish at or before a time point n [constraint (9)], whereas a task must finish exactly at a time point [constraints (9) and (10)] if it produces a state for which zero-wait policy applies ... 

What is the difference between a zero-wait and a uis-process ... 

A batch process runs on a zero-wait-time schedule (see Chapter 3). It has been determined that a 20% increase in capacity is possible if three equally sized storage tanks are purchased, and the processing schedule is altered appropriately. The cost of manufacture is 1.5 million/yr with annual revenues of 2.75 million/yr. The internal hurdle rate for process improvements is 20% p.a. over six years. 

---