T H E IMPORTANCE 0F CHEMISTRY IN PROCESS DESIGN R. L. B U C H A N A N
n process design, the implications of apparently simple
I straightforward
choices among several alternate chemical routes may have far-reaching effects. Too frequently, the importance of such choices is overlooked. This article is intended as a reminder that engineers and chemists must work together to design an optimum plant. Numerous examples can be quoted where importance of the choice in chemical route is so great and obvious that no discussion is required. For example, the facilities required for manufacturing carbon tetrachloride from carbon disulfide bear little resemblance to those for making carbon disulfide by methane chlorination. Similarly, production of phenol via monochlorobenzene requires a plant entirely different from that for the cumene process. However, where the chemical route chosen appears on the surface to have little significance, selection is often made with little or no formal consideration. To illustrate this a hypothetical example is used, even though this approach may lead to some minor inconsistencies, because citing actual examples is rather difficult without disclosing confidential information. Consider for example the hydrolysis of an organic nitrile to produce the corresponding carboxy acid : RCN
+ 2 HzO
-P
RCOOH
+ NH3
or more commonly, RCN
+ 2H20 + mineral acid -+
RCOOH
+ NH4salt
In actual practice, this reaction is carried out in the presence of a mineral acid which reacts with ammonia to form the corresponding ammonium salt. The hydrolysis step can be illustrated by a simplified flow diagram shown at right (Figure 1). 50
INDUSTRIAL A N D ENGINEERING CHEMISTRY
At this point, the question to be answered is a simple one: Which mineral acid should we use? In practice, several acids may be available, but for the purpose of this discussion, only sulfuric and hydrochloric acids are considered. I n Terms of Raw Material Cost, and Product and ByProduct Value, Choice of Acid Makes L i t t l e Difference
(MINERAL ACIDCOSTAND BY-PRODUCT VALUE) RCN f 2Hz0
+ HCI
-L
RCOOH
36.5 RCN
+ 2Hz0 f '/zHzS04
+ NHqCI 53.5
+ RCOOH
+ '/z
49
(NH1)zSOI 66
Acid Cost for 10,000 moles of product per year, using 15% excess acid HC1 36.5 X 0.05 X 10,000 X 1.15 HzS04 49 X 0.01 X 10,000 X 1.15
=
421,000 $ 5600
Value of ammonium salt product
HCI 53.5 X 10,000 X 0.06 = $32,100 HzSOi 66 X 10,000 X 0.02 = $13,200
If we assume that a 15yoexcess of either acid will be required and pick a production rate on the order of 10,000 moles of product acid per year (this might range from 0.5 to 2.0 million pounds, depending on molecular weight), we see that a commitment to use HC1 might involve a raw material cost penalty on the order of $15,000. On the other hand, the value of the byproduct ammonium salt would be greater than with sulfuric acid so that the raw material cost picture corrected for by-product credit might well be a stand-off. The prices, taken from the Oil,Paint, and Dru,g Reporter, may or may not be representative for a particular circumstance where location, quantity, and integration with other processes might be involved. Similarly, the
!
completed process where all chemical decisions have been made is no more logical than handing
chemists a process design completed ly engineers
value of the ammonium salt does not necessarily indicate that we could enter a new market and compete at that figure. On the basis of mineral acid cost and product value alone, profits using either acid could be about 'the same. However, other factors are involved--e.g., materials of construction and size of equipment. For example, hydrolysis rates for hydrochloric acid are higher, and therefore a small reactor can be used.
R. L. BuchaMn is in the Engincning Snuicc Division, Engiruning Department, E. I . du Pont de Nemours @ Go., Im.,Wilminglon, Del..AUTHOR
NITRILE
Velocity Constants for Hydrolysis ef Nitribs
I
4M& HdO.
CHGN CHICH.CN COOHCHICN CHnCHOHCN CHIOHCHICN
I
' w
Figure 1. . Which ~ ' ' Iod sdccl? Sdlfunc or hydrochloric?
1
I
6Mold 4 M & HdO. HCI
2.6
6
4.6 1.2 6.1 2.7
9.1 2.6 17.5 6.3
2.1 3.6 1.2
6 MoId
HCI
I
8Mold
7.7 14.4 0.2 259 6.6
28 1.9
HCI 39 78 62 1022
35
J. Am. Clun. Soc.61,560 (1939).
For the Hydrolysis step Sulfuric Acid Requimabout 10% Less Investment Cost
(PAQUTIES&QUIREO)
I
E l Acid storage
30,000-gd., rubber-
Pump
lined 50-g.p.m., Chlorimet
Hydrolysis maCtDr Agitator Heat exch. Rccire. pump Installed cost
v
1
4,WOgal., glass-lined 5 hp. 500 q.h.; Hartdloy 50-g.p.m., Chlorimet $220,000
HdO,
__
I 12,OOO-gal. 316 SS 25 hp.
600 4.ft.: 316 SS 50-g.p.m. Durinn 20 s200,lmo
Investment in facilities is a function of both size and materials of construction. Selection of HC1 requires a considerably larger storage tank than is required for sulfuric (because of differences in concentration) and requires that the storage tank be rubber-lined instead of simple carbon steel construction. This is partly compensated for by the fact that the hydrolysis reactor with HCI is only 4000 gallons compared to 12,000 gallons capacity -required with the lower hydrolysis rate obtained with sulfuric acid. However, the HCI process has the additional disadvantage of requiring a Hastelloy VOL. 5 5
NO. 1 2
DECEMBER 1963
51
heat exchanger for removal of heat of reaction, w4erm Type 316 stainless steel can be used with sulfuric. These estimates which show investment costs, including items such as auxiliaries, lines, instrumentation,&nd buildings, amount to $220,000 for the HCI system compared with $200,000 for the sulfuric system. In the separation step, the assumption is made that organic acid product cannot be readily volatilized because perhaps of degradation at the temperature required for volatilization or polymerization which might be the case with an unsaturated acid. When sulfuric acid is used, the organic acid ie extracted, the extractant is steam stripped, and the waterextractant mixture is decanted to permit return of the extractant to the process. Water introduced in steam stripping is discarded. The aqueous phase from the extractor is neutralized with ammonia, and exceas water is evaporated to crystallize by-product ammonium sulfate which is then filtered and dried. Mother liquor from the filter is returned to the evaporator. The facilities including items such as allocated steam capacity and lines involve an investment of $280,000 (Figure 2). On the other hand because HCI is volatile, it may be steam stripped from the hydrolysis products, neutralized, and discarded. Addition of a suitable organic liquid may then be employed to reduce solubility of the byproduct ammonium chloride permitting it to be removed by filtration and dried. Because of the presence of the organic component in the dryer, an inert gas system is now required for drying. In this case, mother liquor from the filter is steam stripped to remove the salt-out SEPAUBONS SYITIM HdOi ROUTE
reagent leaving crude organic acid. The salt-out reagent must then be rectified to permit disearding of excess water and return of the organic to the salt-out reactor. Investment cost is $430,000, considerably more than that required by the sulfuric acid route. Thus, the mineral acid used affects product quality, yield, hydrolysis rate, hydrolysis equilibrium, materials of construction, separation techniques, and value of by-product salt. In practice there are frequently one or more overriding considerations which dictate the choice-e.g., product quality may be affected by undesirable and sometimes unknown by-ptoducts produced in the' presence of one acid but not the other. Yields may differ considerably. One by-product may be salable, fit an existing company product line for easy marketing, or be usable as a raw material in other company processes. Perhaps data available for one route will permit plant construction without expenditure of time and money which the other route may require. The optimum chemical plant is a product of the best decisions in chemistry and engineering. A sequential procedure in which all chemical decisions are made and a completed process is delivered to the engineers for process design is no more logical than one in which a process design is completed by the engineers and handed to the chemists to develop appropriate chemistry. Tomorrow's optimum plants will be the result of engineers and chemists working together to determine the best balance among engineering, and chemical, and economic requirements.
Figrrrr 2. Wi?hhydrocrJoric m'd, mort cnpilul iwcslrnnt is wc&d for the repmalion slcp S E P A R A W IymY HU MUIE
rIIRIPPER
STY
Nd
HI0
1 IEUlRAllZER 1
1
CRUDE ORGANIC ACID SEPARITIONI INVESTMENT,
nmm
52
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
I DRYER
i
