I DESIGN
by E. L Clark 6551 Dalzell Place, Pittsburgh 77, Pa
EQUIPMENT AND A
W O R K B O O K
F E A T U R E
E C
Design Considerations for Continuous Pilot Plants at Elevated Pressure Operation of pilot plant equipment at elevated pressures is a natural result of high pressure research in applied research laboratories ANY problems of equipment de M, sign, which only 10 or 15 years ago plagued the experimenter, have been greatly alleviated. Until recently each high pressure reactor had to be personally designed and guided through the various fabricating steps by the researcher. Today vessels, pumps, compressors, valves, and piping materials for pressures up to 25,000 p.s.i. are stock manufac turer's items. Data are readily avail able which permit the experimenter to select designs of components for much higher pressures. However, the assembly of these component parts into an operable and safe pilot plant is still a problem which must be faced. Pilot plant operation has some ad vantages over the research laboratory. By the time a process has reached the pilot plant stage, the chemistry of the major reactions is well defined. Usually, the range of process vari ables investigated in the continuous unit is considerably narrower than that of the research laboratory. While the possible build-up of minute quantities of hazardous materials in a continuously recycled process must be considered, there is gener ally less hazard from unexpected re actions than in exploratory research. The continuous system is inher ently safer than batch operation,be cause of the possibilities for trans fer of reactants and products, the ability to discontinue flow, and the smaller quantity of material in the
reactor for any specified daily pro duction than required for a single batch reaction. Figure 1 is a flow diagram of a hy pothetical plant. Indicated rates of flow provide a numerical basis for some of the design parameters. The plant is divided into several areas, each distinguished by separate prob lems of design and operation. These areas are denoted as supply, operat ing, high-pressure, and high tem perature-high pressure. Problems in Design of α Continuous Pilot Plant
Assumed in Design Gas and liquid feed; gas and liquid product Reactor system operates at 900° to 1000° F. and up to 10,000 p.s.i. Effect of Assumptions on Design Leakage intensified Larger weights of gas handled require added provision for safe release of this gas Equipment or operating fail ure will be more severe at high pressure High temperature results in assigning to equipment limi ted life of operation based on time of deformation Connections from high pres sure side to low pressure auxiliary systems (sampling, feeding reactants, etc.) offer unlimited opportunity for manipulative errors of great potential hazard I/EC
The first criterion is the space re quired for each area. Obviously, the physical space should be ample for containing the equipment and for normal maintenance require ments. Another consideration de pendent on space is the fluid con tent of the various pieces of equip ment within any area, as the con tained fluid might be released in a high pressure installation. The most serious problem involved in the re lease of flammable material is the danger of an explosion, and in the case of mixtures of air and flamma ble gases, the speed of such an ex plosion is sufficiently great to be characterized as "high explosive" or of great brisance and destructiveness. Flammable limits of most common gases have been determined and in most cases flammable mixtures can be considered explosive. Some ratio of quantity of gas contained in equip ment to that quantity needed to cause a flammable mixture within a room volume may be a suitable Criterion for design. Considering hydrogen, whose lower flammable limit in air is 4.0%, a room 20 X 40 X 10 feet would re quire only 480 cubic feet of hydro gen to reach this lower limit for the entire volume of the room. This amounts to the hydrogen in two to three standard gas cylinders at 2000 p.s.i.g. It is not a very large quan tity. Even large ventilation instal lations will not completely dissipate the potential hazard when consider-
W
ORKBOOK
F E A T U R E S 61 A
E/EC
EQUIPMENT A N D DESIGN
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A
Workbook
Feature
GAS FEED 3 0 0 CU.FTYHR. LIQUID FEED 30LB./HR. 4GAL./HR.
COMPRESSOR OIL TRAP 0.2 CU. FT.
PREHEATER
H.BOILING PRODUCT TRAP 0.4CU.FT.
REACTOR 0.4CU.FT.
L.BOILING PRODUCT TRAP 0.3CU.FT.
COOLER 0.1 CU.FT.
GAS PRODUCT
COOLER O.ICU.FT.
Figure 1 .
LOW BOILING HIGH BOILING PRODUCT PRODUCT High-pressure pilot p l a n t f l o w d i a g r a m
ing sudden release of this volume. Careful elimination of ignition possibilities is important, and the use of proper electrical systems is customary in such installations. However, the energy required for ignition of such gaseous mixtures is very low, particularly if the gases are at high temperatures, and sometimes the static charge caused by the turbulent discharge of gases through a small opening or impact of tiny particles picked up by such a gas stream may cause ignition. There is a potent reason for barricading or shielding portions of the plant to protect personnel against the consequences of an accidental discharge of gases or spillage of volatile liquids. This approach, based on carefully determined data, is only an empirical one, as any gas^release even in a very large room will provide some volume in which an explosive mixture exists. However, even this empirical approach has utility as a guide for limiting hazards and general design approaches. A consideration of a hypothetical plant will illustrate this point. The Supply A r e a
This area is actually a storage depot where feed and products are kept (Figure 2, A). Problems of liquid 62 A
storage are relatively simple. The most common type is the used 55gallon oil or chemical drum, although most inefficient. For most nonvolatile fluids the standard household oil tank of 275 gallons is an ideal container and probably the cheapest storage unit, costing about $20 in most areas. In the pilot plant of Figure 1, such a tank would hold about 3 days of feedstock. The operating schedule should determine whether a very large quantity of a single feedstock is required. Storage of gases is much more complicated and expensive. A 3-day supply of gas would require a 20,000cubic foot gas holder, which would cost $25,000 to $30,000. Feed rate of 300 standard cubic feet per hour is just within the possible use of gas cylinders. A commercial hydrogen cylinder holds about 200 SCF and 12 cylinders would operate the pilot plant for an 8-hour shift. However, as it requires several days for delivery of hydrogen in commercial cylinders, at least 100 cylinders would be needed as a working stock to keep the plant operating. A manifold for 12 to 14 gas cylinders split into two banks should be adequate for operation, and the design of such a manifold is relatively simple. Facilities for fastening each
INDUSTRIAL AND ENGINEERING CHEMISTRY
cylinder securely are needed; high pressure tubing for the manifold will permit the use of one reducing valve for each bank instead of for each cylinder; and the low pressure line from the manifold to the compressor should be adequately protected by a relief valve of sufficient size to vent the contents of a full manifold of full cylinders. The large concentration of flammable substances, both liquid and gaseous, dictates the necessity for having this area outdoors. Assuming that some 25 gas cylinders will be in this area with a total volume of 5000 cubic feet, a very large building volume is necessary to keep the potential gas release below the lower inflammable limit of almost any combustible gas. For hydrogen, a room volume of over 100,000 cubic feet would be needed. In addition, storage of several hundred gallons of flammable liquid -must be outdoors unless very special precautions are taken, such as fireproof doors, integral fire extinguisher systems, and control of air supply. Operating Areas
These areas (Figure 2, B) contain low pressure equipment or high pressure items of low fluid content, such as pumps or compressors. These high pressure components normally do not require shielding. T h e main problem in these areas, where most of the manipulations of the pilot plant take place, is the connection to either a high pressure system or a supply area, both of which contain large quantities of flammable materials. The instrument area is shown separately because of the special problems involved in the elimination of ignition hazards in , most control instruments. Connections to the supply area are usually lines at moderate pressures and the possibilities of failures in such lines are small. Major hazards in these connections are possibilities of leakage. Frequently in a high pressure installation the low pressure portion of the plant is neglected, and this is a hazard in and of itself. Periodic pressure-tests of these low pressure lines should be made, particularly those connected to large gas holders or storage tanks. The compressor inlet line and the auxiliary piping requires special
A Workbook GAS SUPPLY
Feature
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EQUIPMENT AND DESIGN
I/EC
HIGH PRESSURE HIGH T E M P E R A T U R E AREA
FEED a PRODUCT STORAGE
CYLINDER MANIFOLD
TO FEED COMPRESSOR
TO FEED PUMP
FROM PRODUCT PUMPS
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A. SUPPLY AREA
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