Basic characteristics of scheduling problem

To explain the scheduling problem, we first explain the lactose production process in more detail. This section clarifies the flow of the lactose product between the processing equipment of each production step. In the first part of this section, we illustrate the overall lactose production and in the second part we explain each processing step into more detail.

Overall lactose production

The K1 and Post-K1 both consist of production tanks, while the post-evaporation and dissolving street both have continuous production equipment units. Figure 4.3 depicts the flow of lactose product.

Figure 4.3: Basic overview of the production equipment of the lactose

Permeate and SW Yellow both skip the Post-K1 step, these lactose products go directly from K1 to the dissolving street. The next part of this section explains how the lactose products flow through the different process steps. This information is required for solving the scheduling problem.

Pasta storage

The pasta storage consists of large storage tanks. The storage tanks of whey are dedicated, while permeate and SW Yellow share a set of storage tanks.

Production Post-evaporator

Figure 4.4 depicts the production at a post- evaporator. The lactose product flows from the pasta storage, through the post-evaporator, into a K1 tank. So, while the post-evaporator is in production, also a K1-tank is in use and this tank

29 Filling Cooling Waiting Emptying Cleaning Waiting for new batch Production K1 to Post-K1

As said, only the whey goes from the K1 to the Post-K1. These tanks are significantly bigger than the tanks at K1; one Post-K1 tank can contain approximately 8 K1 tanks. From now on we call the material in the Post-K1 tank a pooled batch. The pooled batch has to be at least 48 hours in the tank to account for enough crystallization. The production time starts when the last K1-tank is emptied in the Post-K1 tank. Figure 4.5 depicts this processing step.

Production Dissolving streets

There are two dissolving streets, each consists of different production steps that together can be seen as a semi continuous process. So during production part of the batch is in the tank in K1 or Post K1 and part of the batch is processed at the dissolving street. Figure 4.6 depicts this process. A dissolving street can process 72 hours non-stop with one product, then it has to be cleaned. When switching to another product, the street has to be rinsed with water first.

Processing steps of a single unit

The continuous processing stages, the evaporators and the dissolving street, both have the following statuses in their production cycle: Processing, Cleaning, and Stop. The K1 follows a fixed production cycle. Figure 4.7 shows the production cycle of K1. Each cycle step is the same for each lactose product. The cooling goes according to a fixed cooling scheme: the number of degrees the product is cooled down per hour. This cooling scheme differs per lactose product, but due to the fixed scheme the cooling time can assumed to be fixed. The emptying and filling time is dependent of the batch size and the speed of the evaporator; these times are the same for each lactose product. Waiting occurs when the K1 tank cannot be processed at the dissolving street (or Post K1 in case of whey).

Figure 4.8 illustrates the production cycle of a Post-K1 tank. The Post-K1 tank is filled with a certain number of K1 tanks, which is determined by the operator. The whey has to be at least 48 hours in the Post-K1 tank to account for enough crystallization. As said, this production time starts when the last K1 tank is emptied in the Post-K1 tank. Again, waiting occurs when the Post-K1 tank cannot be processed at the dissolving street and has to wait in the tank. Figure 4.9 shows the calculation of the processing times at the post-evaporators, dissolving streets, and K1.

Figure 4.5: Emptying whey from K1 to Post-K1 tank

Figure 4.6: Processing at the dissolving streets

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Figure 4.8: Production cycle of Post-K1 tank

Figure 4.9: Explanation of the processing times of the stages of the lactose production

From the above information we already derive some basic constraints of the scheduling problem:  When scheduling lactose product at a post-evaporator, a K1-tank must be available.

 A K1-tank with permeate or SW Yellow can only be emptied if a dissolving street is available.  A Post-K1 tank can only be emptied if a dissolving street is available.

 The K1 or Post-K1 tank becomes available after the whole content is processed at the dissolving street.

Now that we globally explained the production flow through the different production steps, we explain the decisions involved in the lactose production in the Section 4.3.