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How to select the best refinery catalyst for your process?

-van der Grift Catalyst Specialist

Catalysts are used in many refinery processes to enhance conversion towards desired products with lowest possible operating cost.Selection of the best catalyst for your process is thus important to maximize the value creation potential of your assets. To bring about this value creation, the catalysts need to exhibit different functionalities simultaneously.

Optimal Catalyst Performance
First of all the catalysts need to poses a high activity and selectivity towards the desired products. Next the catalysts need to show a good stability and maintain its intrinsic activity and selectivity over a prolonged period of time. Finally, the catalysts need to poses good physical properties to allow handling in the refinery with minimal losses.Sometimes combinations of catalysts are needed to achieve the refinery goals.

Here we describe criteria that can be used to select the best refinery catalyst for your process.Catalyst price can be a secondary factor in the selection. Other factors such as anticipated feedstocks, available reactor vessels and operating conditions, and market demand for products are much more important factors in optimizing the catalyst selection. These factors should be translated into quantitative catalyst performance requirements; activity, selectivity and stability, and then suitable catalysts need to be tested against these objective selection criteria.

In the end we are concerned with margin per day and how to create best capabilities for margin enhancement through intelligent catalyst selection.

First we look at what we know: unit monitoring

The recent past is known and may be used as a guide for the near future. In case we have an operating unit we can look at the feedstock quality we have processed, we know the operating conditions that we have applied, and we know the incumbent catalyst and how it has performed.This unit monitoring is a good starting point for any catalyst selection. The first question to ask is; Is the refinery satisfied with the performance of the unit?If yes, then the advice should be re-load with the same catalyst and expect a similar performance. If not, we want to find out where a catalyst performance improvement is required and how much additional margin would be generated if such improvement can be achieved.If feedstock quality is going to change or product specifications are going to change it is always a good idea to look for different catalysts and how they would perform under these new conditions.A test run with the incumbent catalyst can be used to get a reference point (or base case) on how the unit performs under the new conditions.If any constraints of the incumbent catalyst come to light these can be addressed in the new catalyst selection.


Catalyst performance improvement: activity, selectivity, and stability


With the hardware as installed and the future feedstock or different probable feedstocks known, it is possible to develop multiple scenarios where the catalyst is the key to producing the desired products at high rates and minimal costs. The installed unit hardware will always put some constraints on the selection of the catalyst. Without a unit revamp or de-bottlenecking, the selection of the catalyst(s) decides the capabilities of the refiner to take advantage of future market conditions and thus increase the operating margin over the entire complex within those hardware constraints.

Activity
A high catalyst activity is needed to achieve a high conversion under mild operating conditions (temperature, pressure) in a small reactor volume.In an existing unit the reactor volume is fixed, but in that case a higher activity can be used to achieve a higher production rate with shorter residence time, or it can be used to achieve a same production rate under milder conditions, e.g. lower temperature and pressure, thus reducing costs and potentially achieving a longer catalyst cycle by reducing the severity.When setting targets for activity it is necessary to specify the feedstocks and the operating conditions (hardware constraints) as well as the desired product specifications. In case multiple feedstocks should be considered be sure to request an activity forecast for each individual feedstock or blend. The feedstock quality will not only affect the catalyst activity, but also its selectivity and stability.

Selectivity
The selectivity is the catalyst’s ability to favour production of a certain product over alternative products and is usually expressed as a percentage of total product output.For instance in case of naphtha reforming we want to produce maximum C5+ yield and we want to keep production of the less valuable LPG to a minimum. When selecting a naphtha reforming catalyst the selectivity towards C5+ at a certain RON should be one of the selection criteria. Another example would be cracking dewaxing of diesel to improve cold flow properties. Here we want to selectively crack long chain paraffins to minimize wax formation at low temperatures, but we do not want to crack valuable diesel range molecules to naphtha and LPG. Thus, in cracking dewaxing the diesel yield (boiling range) at a certain cold flow improvement should be specified as a selection criterion.It is easy to calculate the additional value enabled by more selective catalysts as the value of additional C5+ barrels or additional arctic diesel barrels are known. Similar examples can be given for other hydroprocessing units and refinery optimization models may be used to set selectivity targets for new catalysts.Small improvements in selectivity can have a huge impact on overall profitability of the operation.

Stability
The catalyst stability is an indication of how long the activity and selectivity will be maintained under certain operating conditions.The stability can be seen as a characteristic that determines the catalyst cycle length.Stability is a function of operation severity (feedstock quality, and operating conditions), thus it may be that different feedstock scenarios will give rise to different cycle lengths. It is good to realize which parameters affect catalyst life, so that an informed decision can be made to treat those difficult feedstock molecules and accept a short cycle or to remove them from the feedstock and achieve a longer cycle length. It all depends on your alternative options and the cost / benefit of each solution. We are not always looking for longest cycle length, but we need a certain minimum cycle length to avoid too much downtime for catalyst change-outs.It is also important to realize that catalyst activity and selectivity will change during the time on stream, and catalyst selection needs to take into account average performance over the cycle.Sometimes the end of cycle will be dictated by heater output and bed inlet temperatures, or by selectivity not meeting minimum requirements.

It is a multi-parameter catalyst optimization

It is never an ”either/or” situation, we want optimal activity, and selectivity, and stability, but we need to be realistic and accept that with certain feedstock qualities and operating conditions even the best catalysts can only do so much. A refinery optimization program can be used to set certain minimum requirements for throughput, selectivity, and cycle length. Next you can set stretch targets and define the dollar value of each percent of additional throughput or higher selectivity, whilst still maintaining the other minimum requirements. A catalyst that is best for one set of conditions may not be the best for another set of conditions. In fact it may be that a certain feedstock requires a completely different catalyst loading diagram to achieve best results. Thus it is important to be as specific as possible with respect to operating conditions and develop several scenarios.

The catalyst vendor can provide performance forecasts for each scenario, and the better the input data the more accurate the output data or forecast. The catalyst vendor has his proprietary performance prediction model that collects data from many commercial operating units under a wide variety of conditions.These data are very valuable in terms of making predictions on how certain catalysts will perform under your proposed operating conditions. As you stretch your targets the catalyst vendor may stretch his predictions and in those cases it is a good idea to ask for references willing to confirm the catalyst performance under comparable conditions. If no references are available it may be necessary to do performance testing to compare different catalyst offers under your operating conditions.

Independent catalyst testing

To reduce risk associated with catalyst selection, many refiners are using side-by-side catalyst performance tests at independent test laboratories. These tests allow for a direct comparison of the catalyst performance under identical operating conditions using the refinery provided feedstock.Instead of using a performance prediction-model owned by the vendor, the refinery tests the performance of the catalyst under the conditions of interest to the refiner and bases its selection on the test results.Several test laboratories offer this service and they can provide expertise with respect to setting up the test to gain maximum information. These tests are costly and require significant planning on the side of the refiner to yield results in time for the selection. Sometimes vendors are willing to participate in a cost sharing to get their catalysts tested in a side-by-side comparison.

It is very important to test the catalyst under the conditions of interest, as a catalyst ranking under different conditions may give different results. Thus, it is important to agree up-front about the testing protocol. We want to collect data on the anticipated activity, selectivity, and stability of a catalyst under a variety of anticipated conditions to feed the refinery optimization model. A catalyst ranking in terms of activity, selectivity, and stability without knowing the economic impact is of little use.

Catalyst data evaluation and selection

In the end an economic evaluation will show which catalyst(s) enable(s) highest margin under anticipated operating conditions in a particular unit. It may be that the differences between the catalysts are small in which case other factors may be entered such as price, relationship with the supplier, technical service during the cycle, free hydrotreating courses or technical seminars.However, most of the time there will be a clear winner for the particular unit, and the differences in refinery margin will far exceed any difference in catalyst cost.

Thus, it is important to take catalyst selection seriously and spend adequate time and resources on the selection process. Without capital lay-out and additional downtime for hardware modifications, the selection of the optimal catalyst for your unit allows you to make the optimal margin under a variety of future operating scenarios.Individual refineries have little influence over the cost of the feedstock and the price of their products but they can increase their efficiency and thus operating margin by selecting the right catalyst and take good care of the catalyst when it is in use.

Our page Intelligent catalyst selectionmight be also interesting for you.

Catalyst-Intelligence is a consulting company providing independent & comprehensive advice on fixed bed catalysts to refineries and petrochemical plants. For more information on how we can help you we refer to our website: www.catalyst-intelligence.com

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