Feed pretreatment

There are many reasons why in some cases the feed stream needs to be pietreated prior to entering the membrane system. When the membrane has great fouling potential with respect to the feed components, or when the membrane pore size is close to or significantly larger than the size of the species to be retained, both the permselectivity and permeability may be compromised. 

Several prelreatmcnt techniques have been practiced in conjunction with the use of organic polymeric membranes. The choice of a particular technique to apply depends to a great extent on the chemical nature of the feed, membrane resistance toward the feed stream and the product requirements. The characteristics of the feed such as the panicle size or molecular weight distribution, effective fluid viscosity and pH (and therefore the 

In other applications such as oil-water separation and electrocoat paint recovery, it may be necessary to adjust the feed pH to prevent precipitation or adsorption of certain species on the membrane surface or into the pores. 

Wastewater reclamation was pioneered using advanced conventional treatment processes to upgrade the water quality of wastewater to reusable standards. When RO was first introduced to produce water closer to drinking water quality from wastewater, a conventional treatment process was employed as pretreatment to the RO. A typical conventional pretreatment configuration would include flocculation, lime or alum clarification, recarbonation, settling, filtration, and activated-carbon adsorption. Biological activity is controlled by chlorination. 

A simplified process used in smaller systems is in-line flocculation followed by pressure filtration. The simplified process produces water of lower quality than the lime clarification process but the equipment is smaller and simpler to operate (71). Experience with in-line filtration showed that optimal dosage of alum was rarely achieved due to fluctuating influent turbidity (72,73). 

Studies on in-line filtration showed that effluent turbidity of less than 2 NTU is achievable with alum dosage of 0-8 mg/L and cationic polymer dosage of 0-0.5 mg/L (73). However, the performance of this system is dependent on feed water quality. 

MF and UF membrane processes are increasingly being used in the water and wastewater treatment. The outcome of rapid developments in membrane industry is low-cost high-productivity membranes making membrane processes economically feasible. Numerous studies and site experience have lead to better understanding of process parameters, allowing process optimization making membrane processes more technically feasible. 

Pilot plant studies conducted at Canary Islands (Spain) showed that microfiltered secondary effluent from Tfas WWTP contained below 1.0 mg/L of suspended solids and turbidity below 1.0 NTU. Total and fecal coliforms were also absent from the micro-filtered water. The SDI of the microfiltered water was below 3.0. Average removal achieved for BOD, COD, and TOC were 81%, 40%, and 27% respectively. The MF achieved water recovery of about 85% (76). 

The crude vegetable oils must be carefully purified before they are used. Free fatty acids are neutrahzed with alkali, while pigments and poisons, such as alkyl soaps, phosphatides, thioglucosides, and amino acids are bleached with fuller s earth. Oils are carefully filtered and dried to remove water, which can produce fatty acids by hydrolysis during hydrogenation and thus damage the catalyst. 

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Hollow-fiber designs are being displaced by spiral-wound modules, which are inherently more fouling resistant, and require less feed pretreatment. Also, thin-film interfacial composite membranes, the best reverse osmosis membranes available, have not been fabricated in the form of hoUow-fine fibers. 

Measurable Process Parameters. The RO process is relatively simple ia design. It consists of a feed water source, feed pretreatment, high pressure pump, RO membrane modules, and ia some cases, post-treatment steps. A schematic of the RO process is shown ia Figure 2a. 

In the feed pretreatment section oil and water are removed from the recovered or converted CCI2F2. The reactor type will be a multi-tubular fixed bed reactor because of the exothermic reaction (standard heat of reaction -150 kJ/mol). After the reactor the acids are selectively removed and collected as products of the reaction. In the light removal section the CFCs are condensed and the excess hydrogen is separated and recycled. The product CH2F2 is separated from the waste such as other CFCs produced and unconverted CCI2F2. The waste will be catalytically converted or incinerated. A preliminary process design has shown that such a CFC-destruction process would be both technically and economically feasible. 

Process water applications include boiler water feed pretreatment before ion exchange or electrodialysis. RO is also used for ultrahigh-purity water production for use in laboratories, medical devices (kidney dialysis), microelectronic manufacturing (rinse fluids per ASTM D-19 D5127-90, 1990), and pharmaceutical manufacturing (purified water or water for injection as specified by USP). 

With all membrane processes, the condition of the feed has a significant influence on the performance of the unit. This often means that some feed pretreatment is necessary to minimize fouling and the potential to damage the membrane. 

As with reverse osmosis, feed pretreatment can be used to minimize membrane fouling and degradation, and regular cleaning is necessary. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Heavy fractions (e.g., vacuum gas oils) and residues HDP might involve both, hydrotreatment and hydrocracking operations. HDT, in this case, is a feed pretreatment, for preparation to another process unit, which might be a HCK unit. This process combination, HDT-HCK can be used on Cycle Oil (FCC, coker), VGO (SR and coker) and SR residues (atmospheric and vacuum). It can be carried out in a single reactor with more than one catalyst, or in more than one reactor. 

When HDT is a feed pretreatment for HCK, the unit could consist of one or two reactors, with or without intermediate scrubbing. The use of zeolitic-noble metal catalysts imposes careful scrubbing of ammonia and H2S prior to the corresponding HCK reactor. 

The recycled hydrogen is usually employed for quenching, as explained in Section 2.2. In the case of feed pretreatment (particularly HCK), the build up of ammonia or H2S... 

Kinetics studies of the hydrotreatment (and hydrocracking) of VR has led to the conclusion that most of the metals, sulfur and nitrogen removal takes place during the first 50% of the whole VR conversion [119-123], More than one reactor was needed for HDM and HDS of a Maya VR, when HDT is used as feed pretreatment [119,120], Although vanadium removal appears easier and faster than nickel removal, their kinetics results showed very similar values of the activation energy for the demetallization reactions [122],... 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

The effects of heavy oils on cracking can be met by process modifications, feed pretreatment or by the use of specially designed catalysts. 

This method is used for curing coatings and inks on plastic cups, tubs, tubes, or metal cans. The parts are placed on mandrels, which are attached to a rotating device. This device moves them through the individual stations feed, pretreat (most frequently corona or flame, for plastics), printing, curing, and take-off. The printing is done by dry offset (see Section 7.5.3). 

At slightly higher temperatures (100°-200°C) catalysts consisting of chlorided alumina in combination with a noble metal, such as platinum, are used. As a cocatalyst HC1 or an organic chloride is supplied with the feedstock. The high reactivity of these catalyst systems requires careful feed pretreatment for removal of deactivating materials. Several plants (1, 2) using this type of catalyst, and one version of this process especially developed to convert C5/C6 feed, have recently been built. 

At about 250° C a catalyst consisting of a low sodium zeolite and a noble metal is used in a recently developed process (3). It is claimed that no extensive feed pretreatment is required and that the stability of the catalyst is not impaired by common feed impurities. An older process using a catalyst consisting of platinum supported on amorphous silica-alumina (4) operates at 400° C. Naturally the higher the operation... 

Improvements in feed preparation and pretreatment have made important contributions to the advances in alkylation technology (12, 17). The ability to design better fractionators has made higher quality feedstocks available, and feed pretreatment facilities have been developed to remove water, mercaptans, sulfides, and diolefins effectively. The benefits of these advances have been realized as higher alkylate yields and octanes, lower acid consumption, and reduced corrosion. 

Colloids Si02, Fe(OH)3, Al(OH)3, etc. Negative Agglomerate on membrane surface MF or UF feed pretreatment higher feed flow rate lower concentration ratio... 

Organics Polysaccharides, proteins, polyelectrolytes, humate, surfactans, etc. Negative Attach to membrane surface AC, MF, or UF feed pretreatment alkali rinsing, EDR process... 

Feed Pretreatment. The type and complexity of the feed pretreatment system depends on the content of the water to be treated. As in reverse osmosis, most feed water is sterilized by chlorination to prevent bacterial growth on the membrane. 

From the foregoing it becomes obvious that the FCC processability of resid feedstock and the choice of a possible FCC feed pretreatment (for instance Resid Hydroprocessing) will depend on the feed quality and hence its origin. 

Naber et al (9) have demonstrated that FCC still has a considerable potential to remain the (resid) conversion "workhorse" of the oil industry. At present about 45% of the world s crude can be envisioned to be within the frontiers of Resid FCC (figure 1). Apart from the importance of FCC feed pretreatment and FCC unit design, also the impact of FCC catalyst performance is crucial to allow the processing of heavier feeds. 

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