Process considerations

The methanol carbonylation process (Equation (1)) involves formal insertion of carbon monoxide into the C-O bond of methanol. However, the transition metal catalyst does not activate methanol directly, so a more reactive methyl substrate (reactant) must be generated in situ. In this respect, a key role of the iodide promoter (HI) is to convert methanol into methyl iodide (Equation (3)). 

Although most depictions of the catalytic cycle include Equation (3) as one of the reaction steps, this is, in fact, an oversimplification. For the continuous process, under typical industrial conditions, the principal component of the reaction medium is acetic acid, and so in a working catalytic system, esterification leads to conversion of most of the alcohol substrate into methyl acetate (Equation (4)). It is then methyl acetate (rather than methanol) that is activated by reaction with the iodide cocatalyst (Equation (5)), the net result of Equations (4) and (5) being Equation (3). 

The methyl iodide undergoes carbonylation by the transition metal catalyst to give (formally) acetyl iodide (Equation (6)), which is rapidly hydrolyzed to the product acetic acid (Equation (7)). The intermediacy of acetyl iodide is difficult to establish under aqueous conditions, and acetic acid may arise directly from hydrolysis of a metal-acetyl species. 

180-190 °C) on a continuous basis, and product solution flows from the reactor into a flash-tank where the initial separation of product from catalyst is achieved. Reduction of pressure in the flash-tank causes vaporization of most of the volatile components while the catalyst remains dissolved in the liquid phase and is recycled back to the reactor. The product stream is directed into a distillation train to remove methyl iodide, water, and heavier by-products from the acetic acid product. The heavies include propionic acid and higher-molecular-weight organics arising from condensation reactions of acetaldehyde. Higher alkyl iodides can also form, especially if iodide salts are added to the rhodium catalyst. 

The relatively low partial pressure of carbon monoxide in the flash-tank has implications for catalyst stability. Since the rhodium catalyst exists principally as iodocarbonyl complexes (e.g., [Rh(CO)2l2] and [RtKCO LJ-), loss of CO ligands and precipitation of insoluble species (e.g., Rhl3) can be problematic. The conventional Monsanto process operates with a relatively high water concentration (10-15%, w/w) that helps to maintain catalyst stability and solubility (as discussed later). However, this operation results in a costly separation process to dry the product, typically requiring three distillation columns. The presence of water also results in the occurrence of the WGS reaction (Equation (2)), in competition with the desired carbonylation process, resulting in a lower utilization of CO. 

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Sterile Filtration of Gases. Primary appHcations for sterile gas filtration are the sterilization of fermentor inlet air, fermentor vent gas, vents on water for injection tanks, and vacuum break filters during lyophilization. Operational and process considerations apply. Typically, the membrane in gas... 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Process considerations must not only take into account characteristics of the particular alcohol or phenol to be esterified, but also the self-propagating by-product reaction, which results in polymer formation. 

Conditions of high pressure and low temperature favor the formation of the complex, whereas low pressure and high temperature tend to release the complexed carbon monoxide from solution. These conditions typify the operation of the absorber-stripper shown in Figure 2. Specific design conditions for the process are given in references 86—88, and an excellent summary of processing considerations is presented in reference 85. 

The majority of the cyanuric acid produced commercially is made via pyrolysis of urea [57-13-6] (mp 135°C) primarily employing either directiy or indirectly fired stainless steel rotary kilns. Small amounts of CA are produced by pyrolysis of urea in stirred batch or continuous reactors, over molten tin, or in sulfolane. The feed to the kilns can be either urea soHd, melt, or aqueous solution. Since conversion of urea to CA is endothermic and goes through a plastic stage, heat and mass transport are important process considerations. The kiln operates under slight vacuum. Air is drawn into the kiln to avoid explosive concentrations of ammonia (15—27 mol %). 

Economic and process considerations usually dictate that agitated thin-film evaporators be operated in single-effect mode. Veiy high temperature differences can then be used many are heated with Dowtherm or other high-temperature media. This permits achieving reasonable capacities in spite of the relatively low heat-transfer coefficients and the small surface that can be provided in a single tube [to about 20 m" (200 ft")]. The structural need for wall thicknesses of 6 to 13 mm (V4 to V2. in) is a major reason for the relatively low heat-transfer coefficients when evaporating water-like materials. 

If there is water to be settled and withdrawn from hydrocarbon, the water s settling time requirement needs to be checked. The water settling requirement, rather than other process considerations, might set the liquid surge capacity. Therefore, the liquid surge capacity we have previously estimated from tables might have to be increased. 

Compressors are normally one part of an overall process train. When a compressor is selected for use in a process, consideration must be given to the required operating range. It will become quickly apparent when a compressor with a fixed capacity is installed in a process with a variable capacity requirement. Modifying capacity of the individual compressors was discussed in the earlier chapters. 

Th chapter presents an overview of the types of compresL . ,. eratic i for selecting a type of compressor, a procedure for estimating hors iwer and number of stages, and some process considerations for both ciprocating and centrifugal compressors. Chapter 11 discusses... 

It is essential that the ASME code requirements be understood by the designer and individual rating and specifying the installation details of the safety device. It is not sufficient to merely establish an orifice diameter, since process considerations which might cause overpressure must be thoroughly explored in order to establish the maximum relieving conditions. 

Spenlehauer (59) reported that in the case of cisplatin-loaded microspheres, irradiation only changes the processing considerations and does not influence drug release. This observation is in conflict with other literature reports showing increases in drug release rates (60,61). 

Though the total degrees of freedom is seen to be (C + 4) some of the variables will normally be fixed by general process considerations, and will not be free for the designer to select as design variables . The flash distillation unit will normally be one unit in a process system and the feed composition and feed conditions will be fixed by the upstream processes the feed will arise as an outlet stream from some other unit. Defining the feed fixes (C + 2) variables, so the designer is left with ... 

Waste-heat boilers are often used to recover heat from furnace flue gases and the process gas streams from high-temperature reactors. The pressure, and superheat temperature, of the stream generated will depend on the temperature of the hot stream and the approach temperature permissible at the boiler exit (see Chapter 12). As with any heat-transfer equipment, the area required will increase as the mean temperature driving force (log mean AT) is reduced. The permissible exit temperature may also be limited by process considerations. If the gas stream contains water vapour and soluble corrosive gases, such as HC1 or S02, the exit gases temperature must be kept above the dew point. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

The single-stage, horizontal, overhung, centrifugal pump is by far the most commonly used type in the chemical process industry. Other types are used where a high head or other special process considerations are specified. 

Pump selection is made on the flow rate and head required, together with other process considerations, such as corrosion or the presence of solids in the fluid. 

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