Change processes

Minor process modifications can lead to hazardous conditions. For instance, a change of catalyst can lead to a dramatic increase in the reaction rate. As another example, changing from a stainless steel reactor to one of mild steel caused a violent decomposition, which was catalysed by traces of iron which had dissolved in the reaction mixture. It is impossible to define all modifications which are potentially hazardous. Some examples are  

No process or plant changes should be carried out until the hazards have been reassessed — often relatively simple tests or calculations will be sufficient. It is important that all process changes are recorded in writing. 

Incremental increases in the amount of combustibles can eventually render the existing fire protection systems inadequate. A more subtle change is the gradual replacement of metal parts in a warehouse with plastic ones of the same size and shape. 

Seemingly inconsequential changes to a process, such as a slight increase in temperature, pressure, or flow rate, can lead to a major increase in hazards. Materials of construction also need to be considered. Many fires have been caused by the replacement of a material or part with what was assumed to be an equivalent replacement.  

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Consider the composite curves in Fig. 12.1. Any process change which... 

Figura 12.1 The plus/minua principle guides process changes to reduce utility consumption. (From Smith and Linnhoff, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

The Tradeoffs Between Process Changes, Utility Selection, Energy Cost, and Capital Cost... 

Having to readjust the capital/energy tradeoff after every process change would be a real problem if it were not for the existence of the total cost targeting procedures discussed in Chap. 7. 

Process Changes for Improved Heat Integration—Summary... 

If the reactor proves to be inappropriately placed, then the process changes might be possible to correct this. One option would be to change the reactor conditions to bring this about. Most often,... 

The tendency of the strong, highly crystalline fibers to fibnUate, ie, to develop a hairy surface on wet-abrasion has, for the textile appUcations, been minimized by process changes both in fiber production and fabric manufacture. However, for nonwoven or speciaUty paper appUcations, this property can aUow potential users to develop ceUulosic microfibers during processing. 

Horizontal filter surfaces also allow a high degree of control over cake formation. Allowances can be made for changed feeds and/or different cake quality requirements. This is particularly tme of the horizontal belt vacuum filters. With these units the relative proportions of the belt allocated to filtration, washing, drying, etc, as well as the belt speed and vacuum quality, can be easily altered to suit process changes. 

Options. Traditional control options for overexposure are material substitution, process change, containment, enclosure, isolation, source reduction, ventilation, provide personal protection, change work practices, and improve housekeeping. A simple way of looking at selection of control options is to find the cheapest option that results in the desired amount of exposure reduction. It is not actually that simple, however, because the various options differ in ways other than cost and degree of control. Some of the other factors to consider in selection of control options are operabiUty, rehabiUty, and acceptabihty. 

Specifications and Standards, Shipping. Commercial iodine has a minimum purity of 99.8%. The Committee of Analytical reagents of the American Chemical Society (67) and the U.S. Pharmacopoeia XXII (68) specify an iodine content not less than 99.8%, a maximum nonvolatile residue of 0.01%, and chlorine—bromine (expressed as chlorine) of 0.005% (ACS) and 0.028% (USP), respectively. In the past these requirements were attained basicaHy only by sublimation, but with processing changes these specifications can be met by direct production of iodine. Previously the impurities of the Chilean product were chiefly water, sulfuric acid, and insoluble materials. Improvements in the production process, and especiaHy in the refining step, aHow the direct obtainment of ACS-type iodine. Also, because of its origin and production process, the Chilean iodine has a chlorine—bromine impurity level of no more than 0.002%. 

The pilot plant must also be carehiUy designed so that its control and safety systems are "fad-safe" and any unexpected equipment or utdity fadure brings the unit into a safe and de-energized condition. Unexpected or rapid process changes, if they can herald or lead to dangerous conditions (eg, mnaway exothermic reaction), should be continuously monitored by appropriate instmmentation and suitable automatic action provided (1,55—67). 

In the slurry process, the hydrolysis is accompHshed using two stirred-tank reactors in series (266). Solutions of poly(vinyl acetate) and catalyst are continuously added to the first reactor, where 90% of the conversion occur, and then transferred to the second reactor to reach hiU conversion. Alkyl acetate and alcohols are continuously distilled off in order to drive the equiUbrium of the reaction. The resulting poly(vinyl alcohol) particles tend to be very fine, resulting in a dusty product. The process has been modified to yield a less dusty product through process changes (267,268) and the use of additives (269). Partially hydroly2ed products having a narrow hydrolysis distribution cannot be prepared by this method. 

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