Emissions in Remission - Albemarle Corporation

More documents

Recommendations

Info

Achieving greater profitability with opportunity feeds in the FCC unit Opportunity feeds can be characterized as having concarbon, nitrogen, aromaticity, contaminant metals levels, or boiling ranges much different from a typical vacuum gas oil (VGO). Examples include gasoline, diesel, distillates, light cycle oil (LCO), resids, slurry, coker stocks, visbreaker and deasphalted oils. With a wide range of crackability and coke-making tendency, opportunity feeds can present serious processing challenges. The penalty in yields for processing some of these feeds can be very high, and the best medicine to limit these problems is the use of the proper catalyst technology. The challenges involved in processing opportunity feeds can include high coke formation tendency, poor crackability, high levels of metals contamination, heaviness of the feed inhibiting feed/catalyst contact, and a challenge to upgrade the heavy molecules 1. Two key formulas Two key relationships in understanding the impact of cracking opportunity feeds are often simplified and summarized as follows: Formula 1: Coke yield ~ a*ConCarbon + UF coke [ (b*FF coke)*(c*PV coke)* (d*Activity coke)] Formula 2: Conversion K~UF conversion[(e*FF conversion)*(f*PV conversion)* (g*Activity conversion)] a, b, c, d, e, f and g are constants. FF coke is the coke feed factor, or the propensity of a given feed to generate catalytic coke. PV coke and Activity coke are terms. UF coke is the coke unit factor. Conversion K= conversion/(100-conversion), the kinetic conversion or second order conversion. From a bulk property perspective, the primary properties which influence FFcoke are the aromatic nature of the feed (often referred to as CsubA), nitrogen, the boiling range, and metals. Other properties such as sulfur and API are important, but are usually cross-correlated with the primary properties. PVcoke and Activitycoke are terms which relate the effect of process variables and catalyst activity on coke. UFcoke is the coke unit factor which contains the intrinsic catalytic coke making nature of the FCC catalyst. 12 Courier ■ Issue 66 For Conversion K, the definitions of the terms are analogous to those for coke in formula 1. FFconversion is often called the feed crackability factor, or the propensity of a given feed to generate “conversion” (conventionally defined as the disappearance of 430°F (221°C) and heavier material from the feedstock). PVconversion and Activityconversion relate the effect of process variables and catalyst activity to conversion. UFconversion contains the intrinsic conversion nature of the catalyst which is beyond what is explained by an activity measurement. For example, two catalysts could have the same catalyst activity as measured by conventional lab scale methods but, once in a commercial FCC and after detailed model analysis is performed to normalize all other variables, one catalyst may result in higher conversion. Some intrinsic property of the catalyst, for example its mass transfer nature (accessibility) or metals trapping ability, may not have been captured by the activity test, yet did manifest itself in the commercial FCC unit. Coke-selectivity is typically defined as the coke yield divided by second order conversion. For units which are against regenerator temperature limits or have room to move the circulation rate, the coke-selectivity may arguably be the most important value to optimize. Typical opportunity feeds Cracking gasoline in a FCC unit will usually reduce gasoline yield. However, gasoline does have a very low FFcoke, and the heat balance will call for increased catalyst circulation. Therefore, it is possible that increased conversion due to heat balance or C/O effects from both the low intrinsic cokemaking nature of gasoline and riser quench can at least partially offset any net loss in gasoline. Re-cracking gasoline is also a means to reduce gasoline sulfur. Refinery economics and unit constraints will dictate if cracking gasoline range material is profitable. Catalyst selection is also critical as the refiner should choose a catalyst which does not have mass transfer limitations, and which is designed so the LPG products resulting from recracking gasoline have the highest possible olefinicity.
“Nevertheless, resid stocks can be cheap and therefore very tempting to process in the FCC unit.” Distillate range material, generally 430- 650°F (221-343°C), can also be cracked in an FCC unit. Contrary to a common myth, most virgin distillate range oils are very crackable and make relatively little coke and gas per conversion. These virgin distillates usually have a low FFcoke and a high FFconversion. As a result of the low FFcoke, the regenerator cools and the catalyst circulation increases. Cracked oils in the 430-650°F (221-343°C) range, however, generally make poor opportunity feeds. LCO, for example, has a low FFconversion. The FFcoke is higher than virgin distillate material, but most LCOs do not have a significant effect on the regenerator temperature. In most cases there is little if any benefit to recycling LCO to the FCC unit. FCC unit slurry is almost always the least valuable product from the unit and minimizing its yield is always an objective. Slurry has a very high FFcoke and a very low FFconversion. Sending slurry to the FCC unit will heat the regenerator, slow catalyst circulation, and decrease overall conversion. In addition, given its low FFconversion (or crackability), the overall FCC unit conversion is further decreased. However, the slurry yield usually decreases and more LCO is generated. For refiners who are not against a limit (e.g., air, regenerator temperature), economics may favor processing additional fresh feed instead of recycling slurry. Catalyst selection should play a major role in the decision process. As with most incremental or opportunity feeds, coker products have high coke factors and low crackability factors. Unless added to a base feed slate dominated by resid, the result is lower overall conversion and higher regenerator temperatures. Historically, coker products were usually the highest priority feed to be hydrotreated before going to the FCC unit because they are very aromatic. However, with the advent of low sulfur product specs, some refiners are evaluating the use their cat feed hydrotreaters as product treaters. Lower pressure treaters may then be substituted for the cat feed hydrotreaters. The net result of rearranging hydrotreating options would be to send a relatively higher coke making, lower crackability, mildly hydrotreated coker feedstock to the FCC unit. As the quality of the feedstock deteriorates, the choice of cracking catalyst becomes even more critical. Even small errors in a catalyst decision with a poor quality feedstock can result in large yield losses and non-optimum operation of the FCC unit. The most popular and contentious opportunity feedstock is resid, generally high in concarbon, metals, asphaltenes and boiling point, qualities that drive up the coke. Although portions of some resid stocks can be reasonably crackable, the heat balance effects due to the high coke making propensity generally results in very low incremental conversion. Nevertheless, resid stocks can be cheap and therefore very tempting to process in the FCC unit. Another challenge with resids are their typically high metals content, which can result in a dramatic increase in catalyst deactivation, a problem made worse by higher regenerator temperatures. The high coke from resid results in dual penalties – lost circulation and lower activity. To combat the activity losses, an aggressive catalyst plan with the proper catalyst design is necessary. The use of a poor catalyst when processing resid will simply act as a very expensive metals flush and not provide the selective cracking necessary for optimum operation. Sulfur and nitrogen are two other contaminants to take into account when processing any opportunity feed, especially resids. These two compounds cause an increase in SOx and NOx emissions, respectively. Nitrogen (prevalent with coker stocks and resids) is also a temporary poison as under reactor conditions it forms basic compounds, which neutralize the acid sites of the catalyst. Given all these issues, it is obvious that not all FCC units nor catalysts are capable of cracking resid with the same efficacy. Generally speaking the more ‘room available in the heat balance’ (i.e., air and regenerator temperature), the more efficiently resids can be processed. Resid cat crackers are often classified as either delta coke limited or activity limited. Delta coke limited operations are limited by regenerator temperature and therefore require very low delta coke catalysts and a high catalyst turnover to Issue 66 ■ Courier 13
Page 1 and 2: Catalysts Courier Emissions in Remi
Page 3 and 4: John Nicols assumes leadership role
Page 5 and 6: “The refinery will be able to pro
Page 7 and 8: SOx (ppm) “Using SOxMASTER we hav
Page 9 and 10: “The growing use of light vehicle
Page 11: “In addition to lower product sul
Page 15 and 16: Americas Hydroprocessing Courses -

Emissions in Remission - Albemarle Corporation

Create successful ePaper yourself

Delete template?

Save as template?