VirtECS Scheduler Case Study: Scheduling, Rescheduling & Design The technology underlying v6.0 of allows automated scheduling like no other engine When considering a scheduling solution for your operation, there are many important factors to consider. With this case study, explore several key scheduling challenges. Fully Automated Scheduling Engine Rapid and Flexible Rescheduling This case study will take a virtual tour of the highlights of the unique scheduling capabilities provided by. As an example we ll look at scheduling a fermentation operation, starting with Fermentation Process Model a very simple model, then adding more realistic and complex conditions. Next we ll consider some important rescheduling considerations, and finish by addressing some challenging scenarios for process modifications or redesign. Design and Retrofit Analysis to Identify Key Improvements Scenarios Considered Baseline Scenarios 1) Basic fermentation process 2) Process vessel storage 3) Unexpected equipment shutdown 4) New equipment considered 5) Rework within the process 6) Equipment Maintenance Process Improvement Scenarios 7) Advanced scheduling 8) Process time: fermenting 9) Process time: rework 10) Adding a new Column 11 Process time: Columns 12) Time to fill feed tank
Even basic processes may possess features quite difficult for standard scheduling applications to handle properly 1) Basic Pharmaceutical Fermentation Process This is a typical piece of a pharmaceutical process. While this system is relatively small, it illustrates a level of complexity which is non-trivial to schedule. The process description is given in the sidebar to the left. The Gantt chart below shows a single batch of Fermentation feeding two batches of Refining on one Column. Process Description 1000 lb of material is processed in a fermenter with a batch time of 1 hr When fermentation is complete, 500 lb of broth may be transferred to a feed tank, requiring 20 mins When the transfer is completed, the Fermenter is empty and available to start another batch Broth in the Tank is fed into the Column (10 mins) and the broth is refined, requiring 83 mins to produce 410 lb of product One fermenter batch requires two batches of Refining on one Column Process vessels often must hold material until there is some other place to hold the intermediate Even this small process contains constraints which most other schedulers on the market cannot handle properly. The addition of a larger demand requiring more batches illustrates this point. Note the storage activities are shown as pink boxes. 2) Accounting for Process Vessel Storage Note that the second batch of Fermentation cannot begin until the entire first batch has been fed to the Tank. This requires accounting for the time to feed the Broth into the Tank, start the Refine task, complete the Refine task, and feed the second half the Broth into the Tank just in time to begin the second Refine task. Only after the second transfer into the Tank is completed can we begin the second Fermentation task. always tracks where every intermediate material is stored.
Critical equipment items are not available 100% of the time, and sometimes they don t ask when they should malfunction 3) Unanticipated Equipment Shutdown Of course, if something happens which requires shutting down the Column (C1) for an hour, the schedule must be recomputed. For this reason, the timing of tasks on one piece of equipment interacts strongly with those on other pieces of equipment. VirtECS can reschedule rapidly and automatically in response to equipment downtimes It is easy to implement a drag and drop interface allowing boxes on the chart to be moved about willy-nilly. But such moves will, in the absence of very strong model-based underpinning, produce infeasible schedules which are useless. Suffice it to say that any scheduling system which cannot automatically produce a feasible schedule for this simple process (without user intervention) will also lack the necessary modeling support to ensure feasibility under drag and drop manipulation. Drag-and-drop is easy to implement in the user interface, but far more difficult to implement in the scheduling engine 4) Bring New Pieces of Equipment Online Now we begin to see the full power of VirtECS - the ability to automatically produce fully detailed feasible schedules for a realistic process. We have added a second fermenter and two additional columns with all three columns sharing a single feed tank. We can see here that the columns running the refining task are the rate limiting piece of equipment. allows easy user movement of tasks without allowing an infeasible plan.
With many biotech and industrial processes, rework is often necessary, and quite difficult for some schedulers 5) Rework Back to the Drawing Board Now we add one further complicating factor which is common in pharmaceutical processes. The Fermentation task not only produces good product, but a small amount of material which must be fed to a rework tank. This material accumulates until at some point there is enough available to feed a special rework task on one of the fermenters. The Rework task requires three hours to complete, but converts all the off-spec material into broth suitable for refining. The Rework task must be run with some regularity because of the limited storage in Rework Tank. Note that VirtECS handles these complications automatically, requiring less then two seconds of CPU time to produce the schedule shown. Because VirtECS does not have to be "taught" or "configured" to generate these schedules, it can also easily and automatically respond to contingencies which arise in daily operation, as shown next. VirtECS Scheduling engine is not rulesbased and doesn t have to be taught to handle unusual conditions or events Scheduled maintenance is a normal activity for many plants. How easy is it to ask the question When is the best time? 6) Reschedule with Planned Maintenance Here we see how effortlessly VirtECS is able to automatically generate schedules which walk around four different equipment outages. The ability to automatically reschedule lets VirtECS answer such questions easily
Section Two Case Study Question to Improve Capacity Some processes never change due to regulations, but others undergo continuous improvement. Can your scheduling solution also consider changes to the equipment or process? Capacity Planning and Improvement Considering Addition (or Reduction) in Equipment Considering Process Improvements (Yield, Duration, ) 7) Advanced Scheduling for Capacity Planning Here we begin to use VirtECS to determine the value of various proposed improvements to the process. This is the base case where we set the demand to require six days to complete. handles a wide variety of process and equipment changes easily To consider significant process changes, it s critical that your scheduler treat constraints as real, not mere suggestions 8) Process Improvement Study: Process Time Here we evaluate the capacity increase derived from reducing the process time on the ferment process from 60 to 50 minutes. The makespan is reduced by 0.14%. Highly manual scheduling solutions cannot match the speed of VirtECS in considering process improvements
Manual studies on process improvement may easily miss nonlinear effects where small improvements can have large effects 9) Changing Process Time for Rework When we reduce the process time for the Rework task from 3 hours to 2 hours 50 minutes, the makespan is reduced by 0.27%, twice the capacity increase achieved by the previous change. This result is surprising and counter-intuitive. The capacity increase to be had by a 6% reduction (10 minutes) in process time for the Rework task, which is run relatively infrequently is worth more than a 17% reduction in process time for the common Ferment task. can help you identify and act upon the Efficient Frontier of your process These results highlight that you need not necessarily make large changes to a process time to see a significant improvement in results. This highlights the need for a rapid automated scheduling engine, to allow you to consider a wide range of improvements and specific changes in process parameters. Storage pitfalls, rework, and process changes are hard to handle in isolation. How much more difficult is it to schedule a process with all of these? 10) Adding a New Column to the Process Here we use VirtECS to predict the capacity of the plant if we add a fourth column (C4). The capacity increase from the fourth column is a full 25%. The complex pattern of processing, storage, accumulation and Rework-Broth and reworking would be very difficult to work out using conventional methods. With VirtECS it took less than two minutes to make the changes to the model input data and three seconds to resolve and get results for the new test case. VirtECS 6.0 allows greater automation of schedule creation which allows analysis of many scenarios
The more efficient your operation becomes, the more important that your scheduler is efficient 11) Reduce Process Time in the Columns Another way to improve refining capacity is to reduce the process time in the columns. This case shows the result of reducing the process time of the Refine task by 10 minutes. The capacity increase is 6.9%, which is rather significant. The solver technology underlying the has been improving continuously over the past decade in power, flexibility and speed 12) Change the Time to Fill the Feed Tanks Even though the feed tank is not busy all the time, it may be a significant bottleneck in this process. Again, it is not a simple problem to address without the ability to automatically generate quality schedules. In this case we investigate the performance of the process if the time required to feed the material from Fermenters to the feed tank is reduced from twenty minutes to ten minutes. This modest change in the process generates a 2% improvement in capacity. Keep in mind that a scheduling engine can t consider improvements which reduce process storage time when it cannot even track process vessel storage If your operation has scheduled downtime storage limitations, multiple stages, or changes often be sure to consider
This Case Study has shown that even processes that seem simple may have features which make them hard to schedule Process Network Diagram: Revised Process readily handles very complex situations due to its underlying optimization technology When considering the acquisition of a scheduling solution, be sure to ask for a demo of a process with the same hard-to-schedule features as your process or better yet, ask the vendor to demo their Scheduler on your data For more information, contact Advanced Process Combinatorics Case Study: Conclusions This case study has examined a basic fermentation process, simplified to be able to visualize what s going on, but with enough complexity and realistic process constraints to highlight some of the key features of Version 6 of. The case study began with a simple process and added complexity including process vessel storage, unexpected equipment downtime, scheduled maintenance, consideration of new equipment, and a number of process improvements. The conclusions to the scenarios considered in the case study are: Decreasing fermenting process time from 60 to 50 minutes had a relatively small impact, decreasing makespan by 0.14%. Decreasing the rework process time from 180 to 170 minutes had twice the effect on makespan, reducing it by 0.27%. Adding a fourth column increased total capacity by 25% (this may not have been the case if other process steps were rate limiting). Observing that the columns were an important determinant of capacity, a new scenario reducing refining process time by ten minutes gave a 6.9% increase. Decreasing the time required to fill the feed tank from the fermenter improved total capacity by 2%. Here are some potential landmines in a process or operation which may cause great difficulty for other scheduling engines Process vessel storage and limited or time-sensitive storage Frequent rescheduling to respond quickly to equipment downtime Rework of material and cycles in the process network Adding new pieces of equipment, especially shared equipment Automatic enforcement of firm constraints which may not be violated was seen to perform admirably under these conditions. Final Comments APC is committed to helping you succeed and providing outstanding business value. The point of having superior technology is not for the sake of technology, but to allow our product to handle the details that give you a competitive advantage. If you have questions on what aspects of your process might make scheduling difficult, or to see in action with a demo on your process, please contact us. Advanced Process Combinatorics Copyright 2005 Advanced Process Combinatorics, Inc. All rights reserved. 3000 Kent Ave. West Lafayette, IN 47906 Tel: (765) 497-9969 FAX: (765) 463-5126 E-mail: info@combination.com Website: http://www.combination.com