Operational philosophies

Differences in each country s regulatory requirements may, in turn, cause differences in operating philosophies. It is important to clearly review expectations with regard to health, safety and environmental matters during initial assessments of foreign tollers. 

Process Flowsheet Batch vs. Continuous operation Detailed unit operations selection Control and operation philosophy Information above plus process engineering design principles and experience... 

An operating philosophy that trains and rewards personnel for shutting down when required by safety considerations is inherently safer than one that rewards personnel for taking intolerable risks. 

Many companies Bnd it useful to express corporate goals as vision statements, which articulate top management s operating philosophy concerning a given... 

The cat cracker s operational philosophy is dictated by refinery-economics. Economics of a refinery are divided into internal and external economics. 

The unit operating philosophy and its apparent operating limits often dictate unit constraints. For example, limitations on the main column bottoms temperature, the flue gas excess oxygen, and the slide valve delta P often constrain the unit feed rate and/or conversion. Unfortunately, some of these limits may no longer be applicable and should be reexamined. Some of them may have resulted from one bad experience and should not have become part of the operating procedure. 

In the analysis, synthesis and optimization of batch plants complexity arises from the various operational philosophies that are inherent in time dependent processes. In a situation where the intermediate is allowed to wait in the same unit from which it is produced until the next unit is available, the operational philosophy is commonly known as no intermediate storage (NIS) operational philosophy. This philosophy is depicted in Fig. 1.3. NIS operational philosophy is usually adopted if operational space is of essence, since intermediate storage tanks can occupy considerable area. 

The second operational philosophy that exploits intermediate storage is the unlimited intermediate storage (UIS) operational philosophy. This philosophy is similar to FIS philosophy, except that the availability of storage is always guaranteed. The implication thereof is that whenever the intermediate material is produced it can immediately be stored without limitations or constraints on storage capacity. In practical terms, this can be achieved if the capacity of storage is too large compared to the capacity of production units as shown in Fig. 1.5. 

The third philosophy that makes use of intermediate storage is common intermediate storage (CIS) operational philosophy, shown in Fig. 1.6. CIS philosophy involves the sharing of storage by various tasks within the batch plant. Needless... 

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. 

The ZW, FW, and UW are in most instances a consequence of product stability. In a situation where the intermediates are unstable, it is always advisable to proceed with the subsequent step(s) in the recipe as soon as the intermediates are formed, hence the ZW operational philosophy. Due to its nature, ZW does not require any dedicated storage for the intermediates and could be depicted by a flowsheet similar to that shown in Fig. 1.3. On the other hand, the intermediate could be partially stable and only commence decomposition after a certain period. In this case storage time has to be finite in order to prevent formation of unwanted material, hence the FW operational philosophy. The UW operational philosophy is applicable whenever the intermediates are stable over a significantly longer time than the time horizon of interest. In both FW and UW operational philosophies, storage of intermediates can either be within the processing equipment or dedicated storage unit. 

Process Intermediate Storage Operational Philosophy The New Operational Philosophy... 

The models developed to take the PIS operational philosophy into account are detailed in this chapter. The models are based on the SSN and continuous time model developed by Majozi and Zhu (2001), as such their model is presented in full. Following this the additional constraints required to take the PIS operational philosophy into account are presented, after which, the necessary changes to constraints developed by Majozi and Zhu (2001) are presented. In order to test the scheduling implications of the developed model, two solution algorithms are developed and applied to an illustrative example. The final subsection of the chapter details the use of the PIS operational philosophy as the basis of operation to design batch facilities. This model is then applied to an illustrative example. All models were solved on an Intel Core 2 CPU, T7200 2 GHz processor with 1 GB of RAM, unless specifically stated. 

The PIS operational philosophy is novel and thus requires further explanation. When a batch operation is scheduled a Gantt chart is usually generated, such as... 

When the PIS operational philosophy is not used, as in Fig. 3.3, 50 t of dedicated intermediate storage unit, ds,s, was required. The reason for this is that the... 

In order to illustrate the uses of this novel operational philosophy, i.e. PIS operational philosophy, this chapter has been divided into two parts. Firstly, the applicability of the operational philosophy will be proven and used to determine the minimum amount of intermediate storage required while maintaining the throughput achievable with infinite intermediate storage. Secondly, the PIS operational philosophy will be used to design storageless batch plants. 

The model in the above form does not take into account the possibility of using latent storage, i.e. PIS operational philosophy. There are a number of additional constraints needed to fully capture this operational philosophy. 

In order to ensure the completeness of the model that takes the PIS operational philosophy into account, the basic scheduling model developed by Majozi and Zhu (2001) has to be modified as follows. 

The balance over a dedicated intermediate storage unit has to be modified because of the possibility of latent storage. Constraint (3.34), provides the link for the inlet and outlet mass balance between units, as shown in constraints (3.17) and (3.18). Constraints (3.35), (3.36) and (3.37), are similar to constraints (3.4), (3.5) and (3.6), however they apply to the case where the PIS operational philosophy is taken into account. 

Fig. 3.7 Literature example without using the PIS operational philosophy...

The model was solved using GAMS and the CPLEX solver version 9.1.2. The computational results for case 1 are shown in are shown in Table 3.3. From these results it is clear to see the potential benefits for using the PIS operational philosophy, with a 50% increase in throughput. 

MILP and MINLP models have been developed to take into account the PIS operational philosophy for assessing the efficacy of this philosophy and design, respectively. The MILP model is used to determine the effectiveness of the PIS operational philosophy by, firstly, solving the model with zero intermediate storage with and without the use of latent storage. In the illustrative example a 50% increase in the throughput was achieved. 

Secondly, the minimum amount of intermediate storage is determined with and without the PIS operational philosophy. In both cases the production goal was set to that which was achieved when the model was solved with infinite intermediate storage. In the illustrative example a 20% reduction in the amount of intermediate storage is achieved. The design model is an MINLP model due to the non-linear capital cost objective function. This model is applied to an illustrative problem and results in the flowsheet as well as determining the capacities of the required units. 

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