Quantifying the Benefits of Batch Scheduling

Transcription

1 Quantifying the Benefits of Batch Scheduling Steve Morrison, Ph.D. Chem. Eng Abstract: Would you buy a materials-only management package to schedule equipment? Unfortunately people have actually proposed this. In fact, it is all too easy for engineers to buy a batch scheduling solution, even a well-written one, without realizing that the package has only marginal overlap with the requirements that can really help their site. It can be helpful to see a grid of 32 potential benefits in the five main categories of scheduling benefits. Finally, some real world examples showing the differences in benefits are presented. Key words: benefits of batch scheduling, batch scheduling Introduction Scheduling of batch pharmaceutical, chemical, and food plants is paradoxically so difficult that many times it is done manually. Part of the difficulty is that process scheduling needs are so varied that there is no chance of having a shrink-wrapped scheduler that will work without modification for even most sites. Even within the same industry, a small batch plant, a large batch facility and a semi-batch or continuous site would expect very different kinds of benefits from scheduling. This paper details 32 potential benefits that might apply to any process plant. Then it gives guidelines for eliminating from the justification benefits that are insignificant for your specific situation, and quantifying the significant benefits that would justify a scheduling solution. The only reasons to have computerized scheduling are to meet orders better, improved profitability and cost, and fewer headaches. If a scheduling solution will not do these things for you, then to be candid, why should you even consider it? On the other hand, if there are substantial benefits, every day that your site waits for a solution that gives these benefits is significant amount of money lost each day in opportunity cost. MRP II, finite capacity scheduling, and dynamic scheduling using S88, SP95, or other standards each give different kinds of benefits; but they can be used together too. How do you choose which system to buy, or if you have an existing system, how do you choose whether or not to upgrade or replace it? Usually it is not just one person making a decision. One person s unsubstantiated gut feeling might not mean much to someone else. Quantifying the benefits is not only important to making the best decision, but to assure others in your company that your proposed solution is the best one to implement. 1

2 Figure 1 shows the five categories of benefits from scheduling systems: cost, time, inventory, agility, and predictability. Cost Time Inventory Agility Predictability Figure 1. Categories of potential scheduling and planning benefits. Following is a listing of the benefits in each category, adapted in part from Morrison (1996). One should estimate the magnitude of savings from each potential benefit before even considering the question of what type of scheduling software is really needed. Cost Savings of Equipment, Energy, Labor, and Environmental Global planning for the most profitable plant for efficiencies, costs, taxes, transportation, etc. given the realities of the loading on all plants Selecting the best equipment examining expense, unneeded components, excess capacity, availability, and keeping longer gaps for better contingency capability. Reducing setup and cleaning costs both by campaign ordering and equipment selections Piping, flexhose, pipe header, and manifold room scheduling Reducing energy costs, peak energy usage, and steam spikes Environmental scheduling for both microbe ponds and air pollution with weather changes Scheduling to reduce labor overtime, material, and other costs 2

3 Material Inventory Savings Determining and eliminating excess long-term inventory, leaving only what is needed for production targets, plus sufficient margin for changes Shortening the time feed, product, and work-in-progress (intermediate material) must be in inventory Better for mixed feeds and product blending Less frequent buying from competitors to meet customer commitments Stochastic modeling for uncertainty in demand and unforeseen events Time Savings for Orders Cutting order lead times through more intelligent and dynamic allocation of resources. Improving due date performance improved prediction of what is predictable, to adapt better to what is not Improved throughput of high demand products Agility in Changing Conditions Unexpected equipment outages, or even equipment available sooner than expected Responsive to situations of atypical feeds and off-grade material Rework or disposal of bad/contaminated material Immediate response to rush orders, quotations, order changes, feed availability, etc. Improved maintenance planning - Coordination between maintenance and operations of the tradeoffs in changing the time equipment or an entire plant is shutdown and started up Quick decisions due to real-time data Predictability Fewer scheduling mistakes due to both carelessness and being unaware of situations in other parts of the plant Shift view - a Log sheet of the future for Operators and Shift Production Supervisors Actual versus planned at any point in time Prediction of bottlenecks of energy, solvents, headers, and other resources Process Analysis for both plant engineers and process designers Reduce variability of both batch quality and predicted completion times Compare different equipment and sites Wiser spending of capital money. Plant retrofitting, upgrade, and training Investigating changes in capacity, different feeds, inventory levels, and peak energy usage Training tool for operators, engineers, and planning people Pre-planned changes under unpredictable events to reduce regulatory problems Prioritizing Benefits Quantifying benefits is difficult because some are intangible in the short term, yet very real in the long term. One suggestion for estimating the size of benefits combines subjective and objective 3

4 approaches. For your specific situation, go back over the previous list of 32 benefits and rank each one in order of priority, 1 being the most important. For example, in one batch pulp mill, the main savings were seen as: Better equipment selection Fewer scheduling mistakes Quick decisions on unexpected equipment outages or availability Shift view for a log sheet of the future Reducing variability Reducing steam spikes Improved maintenance planning On the other hand, at a different pulp mill, energy scheduling was the most significant benefit. In a specialty chemical plant that was studied with both batch and continuous parts for each batch, the main savings were somewhat similar: Better equipment selection (especially pipe headers and reactors) Reducing setup and cleaning costs Improved throughput of high demand products Shift view for a log sheet of the future Improved throughput Fewer scheduling mistakes On the other hand, at three other chemical plants that were visited, pipe headers were so plentiful as basically to be irrelevant to scheduling. In all of these cases, global planning, energy usage, and environmental were irrelevant to scheduling. By contrast, for a pharmaceutical company the main savings long-term were seen as: Better equipment selection based utilization, cost, availability, etc. Global planning for the most profitable plant The planner s time in coming up with a new schedule Understanding resource utilization Shift view for a log sheet of the future Fewer scheduling mistakes Reduce variability Reducing the costs of startup, cleaning, labor, pipe headers, etc. were not considered important for the first few releases of the software. Quantifying Benefits Starting with the seven or so most important benefits, estimate the annual cost of the yearly situation. Then estimate the annual cost if a the world s best scheduler for you could do a perfect job of eliminating all excess cost. Now estimate if a realistic scheduling solution could achieve most of that savings, half of it, a quarter of it, etc. As an example, according to three discrete manufacturing customers of i2 Technologies, the main benefits they report are in Table I. Benefit Customer 1 Customer 2 Customer 3 4

5 Improved due date 25% 60-90% 65% performance Cutting lead times 4 weeks to 1 week 10 weeks to 1 week 5-80 days to 2 days Days of inventory 85 days to days Faultless complete shipments 93-95% to 100% Table I. Main scheduling benefits. The research required to come up with numbers that are accurate +/- 20% can take some time. It requires not only looking at accounting numbers, but discussion with the engineers, operators, and supervisors too. Now quantify the benefits of more items, until the savings are less than 5% of what the sum of what you have so far. (After implementation, it is OK to report additional savings). Bear in mind that quantifying benefits can be tricky. For example, at one pulp mill there was tremendous potential savings in simulation simply to predict the outcome of mixing. (One could take 40% of a product in one mixture, 50% in another mixture, blend them together, and have 70% of the product in the blend, due to wrong numbers for the composition.) However, one the root cause of the problem was uncovered, correcting it (which did not need any computer program) made this particular benefit now insignificant. Now compare the initial ordering of benefits with the actual cost savings. Where a lower order item appears to have higher cost savings, go back and double check the savings of both of those items. Choosing the Initial Savings to Pursue The probability of finding a shrink-wrapped package that meets your needs is remote. For even some many standard packages, the cost for the vendor to customize the software is equal to the cost of the software. For implementing a solution you have three choices: 1) Implement a short-term solution to get a few of the most important benefits now, even though the solution will need to be thrown away and replaced to get the additional significant benefits. 2) Wait until the software is complete and all features are implemented, before receiving any benefits whatsoever. 3) Implement a growable solution that quickly gives you the most significant benefits, but can be extended to the complete solution with minimal data re-entry, re-training, and thrown away software and experience using the software. One cannot jump to the conclusion that one of these three is the best choice in every circumstance. For example, if the users have little experience in scheduling, are unsure of the benefits, and not able to articulate all of the requirements the first option is best, both to receive some of the benefits form scheduling and serve as a prototype for a more complete scheduler. One might think the first option only adds time and cost to the path of the ultimate solution, but scheduling can be complex enough that users might not be visualize the requirements for the 5

6 ultimate solution until they have actually used the simple solution for a year or so. In addition, adding time and cost is not always negative, if profits are being added at the same time. The second solution can be best if there are only a few features and they are all significant. If there is an existing framework, then the time to solution might not be long (only four or five months). The third solution is ideal for many complex scheduling problems where there are many potential benefits to justify a lengthy project, but the top two or three benefits are great enough to possibly fund the entire rest of the project. It might be that the quantification of some benefits if very inaccurate, and some benefits can be realized, while others can be explored. Conclusion Of thirty or so possible benefits of scheduling for your facility, you should be able to justify a scheduling solution with just the top seven or so. Work to quantify the yearly benefits as rigorously as possible. Then decide on the release strategy: there is no benefit until the first production release is delivered, but how complete should that release be is a key question. Often for the initial release it is best to focus on the primary purpose to get the top benefits of scheduling for your situation. How to determine the primary purpose is the topic of the next paper in this series. References 1. ANSI/ISA Batch Control Part 1: Models and Terminology. 2. ANSI/ISA Batch Control Part 2: Data Structures and Guidelines for Languages. 3. i2 Technologies Sales Literature. 4. Morrison, S.M., Edgar, T.F., and Burns, H.A. Dynamic Scheduling. American Institute of Chemical Engineers. 11/1991 Meeting. 5. Morrison, S.M. Scheduling Methods for Batch Digesters, Batch Process Plants, and Shops with the Conveyor Algorithm. Dissertation University of Texas at Austin, May