FRIEDEL-CRAFTS REACTIONS BASIC INFORMATION

Several chemicals are manufactured by application of the Friedel-Crafts condensation reaction. Efficient operation of any such process depends on:

1. The preparation and handling of reactants
2. The design and construction of the apparatus
3. The control of the reaction so as to lead practically exclusively to the formation of the specific products desired
4. The storage of the catalyst (aluminum chloride)

Several of the starting reactants, such as acid anhydrides, acid chlorides, and alkyl halides, are susceptible to hydrolysis. The absorption of moisture by these chemicals results in the production of compounds that are less active, require more aluminum chloride for condensation, and generally lead to lower yields of desired product.

Furthermore, the ingress of moisture into storage containers for these active components usually results in corrosion problems.

Anhydrous aluminum chloride needs to be stored in iron drums under conditions that ensure the absence of moisture. When, however, moisture contacts the aluminum chloride, hydrogen chloride is formed, the quantity of hydrogen chloride thus formed depends on the amount of water and the degree of agitation of the halide.

If sufficient moisture is present, particularly in the free space in the container or reaction vessel or at the point of contact with the outside atmosphere, then hydrochloric acid is formed and leads to corrosion of the storage container.

In certain reactions, such as the isomerization of butane and the alkylation of isoparaffins, problems of handling hydrogen chloride and acidic sludge are encountered. The corrosive action of the aluminum chloride–hydrocarbon complex, particularly at 70 to 100oC, has long been recognized and various reactor liners have been found satisfactory.

The rate of reaction is a function of the efficiency of the contact between the reactants, i.e., stirring mechanism and mixing of the reactants. In fact, mixing efficiency has a vital influence on the yield and purity of the product. Insufficient or inefficient mixing may lead to uncondensed reactants or to excessive reaction on heated surfaces.

PLANT CAPITAL COST ESTIMATION VIA SCALING FACTOR

Given that the total capital investment (TCI) of a 50,000-ton/year polypropylene unit is $60,000,000 (in 2002 dollars), find the TCI required for a 75,000-ton/year polypropylene unit via the scaling factor method.

Calculation Procedure
1. Apply the appropriate power-function formula. In the scaling factor method, the TCI is estimated via the following formula (a power function):
TCI2 = TCI1(C2/C1)E
where TCI1 and TCI2 = total capital investment of existing and planned unit, respectively, in dollars
TCI1 = $60,000,000
C1,C2 = capacity of existing and planned unit, respectively, in tons/year
C1 = 50,000 and C2 = 75,000
E = scaling exponent = 0.70

Thus
TCI2 = 60,000(75,000/50,000)0.70 = $80,000,000 (rounded)

Related Calculations. The scaling factor method is an appropriate procedure for estimating the TCI only under the following conditions:

1. The existing and planned units are identical (or nearly so), in terms of processing steps, end products, major equipment items used, and other respects.

2. The desired estimate falls within the category of “order-of-magnitude/screening/scoping” cost estimates (i.e., those estimates with a presumed accuracy less precise than ±30%).

3. The capacity of the planned unit falls within the capacity range for which the scaling exponent is valid. Rarely is the power function relationship between TCI and capacity a smooth curve over the entire capacity range.

Typically, the scaling exponent increases in value with increasing capacity. However, as the scaling exponent approaches unity, it becomes less costly to build two units, each with half the capacity of the large plant, than to construct a single, large-capacity plant.

4. The costs of both the existing and planned units are expressed in dollars of the same period. In this example, the TCIs are in 2002 dollars. If the costs are not of the same vintage, the cost of the existing plant (which is likely older) will have to be adjusted to the same year dollars as that of the planned unit.

However, unless the cost vintages are much different (e.g., five years or more), adjustments for escalation would be “fine tuning,” compared to the relative inaccuracy of these scaling factor estimates.