Feed purification

Multiple reactions also can occur with impurities that enter with the feed and undergo reaction. Again, such reactions should be minimized, but the most efiective means of dealing with byproduct reactions caused by feed impurities is not to alter reactor conditions but to introduce feed purification. 

Can the loss of useful material in the purge streams he avoided or reduced by feed purification If the purge is required to remove b5q)roducts formed in the reactor, then this is clearly not possible. 

Reducing waste from feed impurities which undergo reaction. If feed impurities undergo reaction, this causes waste of feed material, products, or both. Avoiding such waste is most readily achieved by purifying the feed. Thus increased feed purification costs are traded off against reduced raw materials, product separation, and waste disposal costs (Fig. 10.2). 

Feed purification. Impurities that enter with the feed inevitably cause waste. If feed impurities undergo reaction, then this causes waste from the reactor, as already discussed. If the feed impurity does not undergo reaction, then it can be separated out from the process in a number of ways, as discussed in Sec. 4.1. The greatest source of waste occurs when we choose to use a purge. Impurity builds up in the recycle, and we would like it to build up to a high concentration to minimize waste of feed materials and product in the purge. However, two factors limit the extent to which the feed impurity can be allowed to build up ... 

Additional separation and recycling. Once the possibilities for recycling streams directly, feed purification, and eliminating the use of extraneous materials for separation that cannot be recycled efiiciently have been exhausted, attention is turned to the fourth option, the degree of material recovery from the waste streams that are left. One very important point which should not be forgotten is that once the waste stream is rejected, any valuable material turns into a liability as an effluent material. The level of recovery in such situations needs careful consideration. It may be economical to carry out additional separation of the valuable material with a view to recycling that additional recovered material, particularly when the cost of downstream effluent treatment is taken into consideration. 

Perhaps the most extreme situation is encountered with purge streams. Purges are used to deal with both feed impurities and byproducts of reaction. In the preceding section we considered how the size of purges can be reduced in the case of feed impurities by purifying the feed. However, if it is impractical or uneconomical to reduce the purge by feed purification, or the purge is required to remove a byproduct of reaction, then the additional separation can be considered. 

Increasing process yields through feed purification to reduce losses in the reactor and separation and recycle system. 

Monomers for manufacture of butyl mbber are 2-methylpropene [115-11-7] (isobutylene) and 2-methyl-l.3-butadiene [78-79-5] (isoprene) (see Olefins). Polybutenes are copolymers of isobutylene and / -butenes from mixed-C olefin-containing streams. For the production of high mol wt butyl mbber, isobutylene must be of >99.5 wt % purity, and isoprene of >98 wt % purity is used. Water and oxygenated organic compounds iaterfere with the cationic polymerization mechanism, and are minimized by feed purification systems. 

Building Cycle Gas and Reaction Feed Purification and Vaporization ... 

Administration Building. To reduce the risk from the potential blast that could occur from either gas compression/reaction or feed purification/vaporization, safety film was installed over the windows, a catch system was provided to capture flying window fragments, and overhead fixtures were secured. This reduced the risk ranking for the administration building to IV. 

The qualitative assessment identified multiple opportunities for risk reduction in the feed purification and vaporization areas. These included providing gas detectors in the area, interlocked with automatic isolation valves. Upon detection of a gas release, the system would be shut down, significantly reducing the amount of hydrocarbon that could be released, thus reducing the likelihood of a damaging explosion. It was decided that, with the gas detectors and shutdown controls in place, the frequency of event Scenario 4 would be reduced from a "3" to a "2."... 

Even with the improvements in the frequency rating, the overall risk ranking for Scenario 4 (the most severe scenario for the feed purification area) was II (undesirable). 

Based on the results of the qualitative assessment, the decision was made to construct a new control building of blast-resistant design with more separation distance from both the cycle gas compression and feed purification areas. Construction of a new control building was deemed the most appropriate option since the control building spacing and design were considerably out of... 

In this first stage after the reactor the products and by-products are separated from any unreacted material. If in sufficient quantity, the unreacted material will be recycled to the reactor. They may be returned directly to the reactor, or to the feed purification and preparation stage. The by-products may also be separated from the products at this stage. 

It is important to highlight the interactions taking place among the three aforementioned subsystems which are also indicated in Figure 7.2. The feed streams can be directly fed to the reactor system, or they can be directed to the separation system first for feed purification and subsequently directed to the separation system where valuable components are recovered and are directed back to the reactor system via the recycle system. Note that if several reactors are employed in the reactor system (e.g., reactors 1 and 2) then alternatives of reactor 1-separator- recycle-reactor 2-separator-recycle may take place in addition to the other alternatives of reactor system-separation system-recycle system. 

From raw materials to reactor purification and conditioning of feeds (purification, heating, pressure)... 

Figure 5.10. Feed purification catalysts for different impurities. (Reproduced by permission of Siid-Chemie AG)...

Some comments are helpful. The first decision with strong implications regards the feed purification. If the impurity will affect neither the reaction nor the separations, this should enter the process. Otherwise, its removal is imperative. 

Another advantage is that a prereformer may be preferred in flowsheets for processing heavier feedstocks. The prereformer converts heavier hydrocarbons to methane before they are fed to the steam reformer. This allows the steam reformer to be designed for methane service. The prereforming catalysts are very sensitive to poisons. Therefore a good feed purification section is essential.70... 

Description The methanol plant concept consists of these process steps feed purification, steam reforming, syngas compression, synthesis and crude methanol distillation. 

The products are not necessarily single compounds. Polymers for example are a range of compounds of varying molecular weights but which are chemically identical. For commodity chemicals the expense of feed purification, a more exotic catalyst, the corrosion resistant metallurgy and the need for a product purification train, start to add cost. But like refinery processes these are dedicated to one product, run 24 hours a day and are part of an integrated complex benefiting from heat and waste recovery. As the cost to build these refinery and petrochemical complexes is many billions of dollars, time is needed to pay back the investment. Once built, both commodity chemical and refinery... 

Benedict, Pigford, and Levi have carried out mathematical analysis of the GS process. An exhaustive treatment of the process, including calculations for flow rates, dependence of composition on number of stages, effect of solubility and humidity on process analysis, temperature profile in cold towers, simultaneous heat and mass transfer in heat transfer section, concentration reversal in heat transfer section, corrosion, materials of construction, feed purification, and safety, etc. have been reviewed by Dave, Sadhukhan and Novaro. ... 

The main drawback of the GS process is the highly corrosive nature of its aqueous solutions. A 400 Mg/yr GS plant requires an inventory of 800 Mg of H2S, which is an extremely toxic, flammable, and corrosive gas with a distinct, disagreeable smell even at low concentrations. Hence, adequate measures must be taken for material selection, fabrication, feed purification, feed and waste discharges in water and the atmosphere, safety of staff, the surrounding population, and environment. ... 

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