Becht Experts Participate In Q&A In Recent Issue Of Digital Refining
Participants: Gerrit Buchheim, Mel Larson, Fred Lea, and Robert Ohmes
To review the Q&A article in Digital Refining, please click HERE
We didn’t expect severe corrosion issues in the carbon steel of our amine unit. What’s causing it? – Gerrit Buchheim
Severe corrosion in amine units is not rare and should be expected. Depending on where in the unit (absorber, hot rich exchangers/piping, regenerator bottom, regenerator top, or in the hot lean amine) corrosion is occurring will have different root causes. Typical culprits are excessive amine loading of H2S or CO2 or high velocities in the rich amine system, particularly in the hottest sections. Regenerator bottoms and hot lean amine piping and the reboiler return piping can be quite corrosive due to a lack of FeS protective scale, in hot lean amine systems, high heat stable salt loading and specific salts can be very corrosive. High velocity in the lean amine system aggravates corrosion rates. The regenerator overhead can be corrosive due to acid gases (H2S) and can be very corrosive if NH3 builds up if there is no purge on the regenerator acid gas outlet overhead drum reflux stream, to prevent ammonium bisulfide corrosion. Many units have upgraded austenitic stainless steel in the hottest rich and lean amine parts of the unit because of excessive corrosion. Beyond corrosion, PWHT is important to minimize amine SCC in lean amine and wet H2S SOHIC cracking in rich amine.
What’s the best route for maximum propylene from a resid feed to the FCC? – Mel Larson
The “best” route is not “generic”. Propylene yield from the FCC is 75% thermal reaction based with 25% catalytically defined. The typical petrochemical FCC using massive amounts of steam to lower the partial pressure in the reaction zone with riser outlet temperatures between 1035-1050°F (557-565°C). Secondly, maximizing propylene should not be viewed in isolation of the other yield components. Other considerations are in the use of ZSM-5 and or naphtha recycling to the riser with secondary cracking to light olefins. A whole unit analysis is necessary to determine constraints and limitations to achieve or accomplish the target or objective.
Our air compressor trips off immediately after it powers on. Is this a mechanical or process issue? – Fred Lea
This is a bit like saying my car is green, is it a Chevy or a Ford? Depending on the type of compressor (Centrifugal, Integrally Geared, Rotary Screw, Reciprocating), it could be any number of things. Contingent on how long it runs, it could be vibration, lube oil pressure, or simply a loading issue where the machine is not starting up unloaded.
Check your control logic, it should have a “First Out” indication on the panel. This would indicate the trip mechanism for the shutdown. If it does not and the motor trips on overload, I would lean toward the starting up loaded as the cause.
For best product quality from vegetable oils should we hydrotreat them with conventional petroleum feed or use them as straight feed? – Robert Ohmes
Addressing this question requires review of multiple factors:
- Type and Quality of Vegetable Oils
- Capabilities of existing hydroprocessing unit
- Product blending system
For item 1, each of the vegetable oils (canola, soybean, corn, etc) have their own distinct properties, both before and after hydroprocessing. After hydrotreating, these streams should meet or exceed typical specifications of ultra low sulfur diesel, such for specific gravity, distillation, cetane, and sulfur. The key quality that impacts this decision is cold properties. Given the highly paraffinic nature of the resultant products, the renewable diesel product will not meet cloud, pour, and/or CFPP without additional processing or blending. The primary options for meeting cold properties are 1) blending with conventional fossil fuel distillate streams, 2) cold property enhancement additives, or 3) use of dewaxing catalysts within the hydrotreater. Processing vegetable oils with conventional fossil fuel distillates in a hydrotreater is the most straight forward path to making renewable diesel, but the percentage of vegetable oil allowed may be dictated by the resultant cold properties. Blending of other hydrotreated fossil fuel distillate streams with the coprocessing unit’s product can allow for additional vegetable oil processing up to the same controlling limit. Maximizing vegetable oil processing likely requires use of a dewaxing catalyst to isomerize the paraffins, improve cold properties, and meet final product blending requirements.
Building off this element is the capability of the existing hydroprocessing unit (item 2). The high organically bound oxygen content of vegetables oils (at least 10%+) requires significant hydrogen addition to meet final product specifications, along with breaking the backbone of the triglycerides. Hence, the hydrotreating unit must have the assets to manage and complete hydrodeoxygenation. For instance, the makeup hydrogen compressors, as well as hydrogen generation assets, must have sufficient capacity to meet the vegetable oil processing target. If either of these elements are limited, then the percentage of vegetable oil processed will be limited. Similarly, this high level of hydrogen consumption requires sufficient recycle gas capacity not only for sufficient treat gas into the reactor to ensure proper catalyst distribution, hydrogen availability for the reactions at the catalyst site, and a heat sink for the resulting exotherm, but also quench capacity to maintain reactor heat balance and avoiding runaway reactions. In addition, the metallurgy and contacting systems must be able to handle the CO, CO2, and water that are generated by the oxygen removal reactions. If the unit is designed for a significant percentage of vegetable oil processing, then the unit should have capacity for higher processing levels. However, if an existing asset was repurposed and moderately modified to allow for a small percentage of co-processing, then increasing that processing percentage will be a challenge. For existing assets, if some of the reactor catalyst volume was used for dewaxing catalyst, then the conventional fossil fuel processing and vegetable oil processing will be impacted as one balances improving cold properties while hydrotreating the renewable and conventional feeds to meet oxygen, sulfur, and cetane requirements. In summary, the asset capacity and capabilities of the hydroprocessing unit will impact this decision (see diagram for some key considerations).
Finally, hydrotreated vegetable oils produces high quality diesel material – high API gravity / low SG, high cetane, low sulfur, and ideal distillation range. However, as stated above, the poor cold properties require processing or blending mitigations to meet specifications. The advantage of coprocessing and coblending of hydrotreated vegetable oils with conventional distillate streams is that the organization can use each blendstock to create a product that meets all the diesel qualities with minimal giveaway. Hence, a typical refiner with cold property giveaway and “spare” hydrotreater capacity can examine coprocessing of vegetable oils to not only take up that giveaway but generate renewable credits and reduce carbon intensity. Similarly, a facility that is cetane limited can coprocess or blend renewable diesel to create space within the cetane pool to allow for further optimization of upstream fractionation and/or additional processing of low cetane feedstocks. Producing a pure renewable diesel product does give the most flexibility from a blend perspective and, in some regions, provides clearer accounting and capturing of renewable credits, but making pure renewable diesel does require a robust processing scheme and hydroprocessing unit as well as the ability to manage cold properties of the standalone stream.
In summary, the decision on achieving best qualities is multifactorial and is a combination of sources for vegetable oils, capability of the treating assets and auxiliary systems, blending infrastructure, ability to capture renewable credits, and target market. A holistic analysis is required to review the processing capabilities, limitations, feedstock sources, market potential, incentives, and blending approaches.
