Chemical Engineering Plant Design

Chemical Engineering Plant Design LekWantha Lecture 04 Information and Batch Versus Continuous

Decide whether the process will be batch or continuous • Identify the input-output structure of the process • Identify and define the recycle structure of the process • Identify and design the general structure of the separation system • Identify and design the heat exchanger network or process energy recovery system A Hierarchical Approach to Conceptual Process Design

Input Information Reactions and reaction conditions Desired production rate Desired product purity or some information about price versus purity Raw materials and/or some information about price versus purity Information of the rate of reaction Information of the rate of catalyst deactivation

Input Information Any process constrains Other plant and site data Physical properties of all component Information concerning the safety, toxicity, and environmental impact of the materials involved in the process Cost data for by-products, equipment and utilities

Reaction Information Stoichiometry of the participating reactions - main reaction - side reaction Thermal and other physical properties Heats of reaction and equilibrium data Rate of reaction relating it to composition, temperature, pressures, catalysts, and so on Activity of the catalyst as a function of time Phase(s) of the reaction system

Reaction Information Catalyst deactivation and regeneration Some information on the product distribution versus conversion Stability and controllability of the process Special considerations of heat and mass transfer Corrosion and safety hazards

Specific Design Data Required products: their compositions, amount, purities, toxicities, temperatures, pressures, monetary values. Maximum yield: numerous processes have been designed to operate at the conditions of maximum yield, but this operation often does not correspond to the optimum economic conversion

Specific Design Data B Reaction: ABC where B is desired product A C

Specific Design Data Selectivity: and we refer to the conversion of A to C as a selectivity loss

Specific Design Data Raw materials costs and selectivity losses are the dominant factors in the design of a process The optimum economic conversion is less than the conversion corresponding to the maximum yield

Specific Design Data Production rate - is fixed by the maximum size of one or more pieces of equipment and marketing considerations Product purity - is normally is also fixed by marketing considerations

Specific Design Data Raw materials: their compositions, amounts, impurities, toxicities, temperatures, pressures, monetary values, and all physical properties. Constraints: safety, material decomposition, corrosive materials, etc. Any available existing plant data of similar process. Local restrictions on means of disposal of wastes.

Physical Property Data molecular weights, boiling points, vapor pressures, heats of vaporization, heats of reactions, liquid densities, and fugacity coefficients (or equation of state).

Basic Engineering Data • Characteristics and values of gaseous and liquid fuels that to be used. • Characteristics of raw makeup and cooling tower waters, temperatures, maximum allowable temperature, flow rates available, and unit costs.

Basic Engineering Data • Steam and condensate: mean pressures and temperatures and their fluctuations at each level, amount available, extent of recovery of condensate, and unit costs.

Basic Engineering Data • Electrical power: Voltages allowed for instruments, lighting and various driver sizes, transformer capacities, need for emergency generator, unit costs. • Compressed air: capacities and pressures of plant and instrument air, instrument air dryer.

Basic Engineering Data • Plant site elevation. • Soil bearing value, frost depth, ground water depth, piling requirements, available soil test data.

Basic Engineering Data • Climate data: Winter and summer temperature extreme, cooling tower drybulb temperature, air cooler design temperature, strength and direction of prevailing winds rain and snowfall maxima in 1 hr and in 2 hr, earthquake provision.

Basic Engineering Data • Blowdown and flare: What may or may not be vented to the atmosphere or to ponds or to nature waters, nature of required liquid, and vapor relief systems. • Drainage and sewers: rainwater, oil, sanitary.

Basic Engineering Data • Building: process, pump, control instruments, special equipment. • Paving types required in different areas. • Pipe racks: elevations, grouping, coding. • Battery limit pressures and temperatures of individual feed stocks and products.

Basic Engineering Data • Codes: those governing pressure vessels, other equipment, buildings, electrical, safety, sanitation, and others. • Miscellaneous: includes heater stacks, winterizing, insulation, steam or electrical tracing of lines, heat exchanger tubing size standardization, instrument locations.

Survey Literature • SRI Design Results • Encyclopedias • Handbooks and Reference books • Indexes • Patents (http://www.delphion.com)

Encyclopedias • Krik-Othmer Encyclopedia of Chemical Technology • the Encyclopedia of Chemical Processing and Design (McKetta and Cunningham) • Ullman’s Encyclopedia of Industrial Chemistry

Encyclopedias • McGraw-Hill Encyclopedia of Science and Technology • Van Nostrand’s Scientific Encyclopedia • the Encyclopedia of Fluid Mechanics • the Encyclopedia of Materials Science and Technology

Handbooks and Reference books • Perry’s Chemical Enginer’s Handbook • the CRD Handbook of Chemistry and Physics • JANAF Thermochemical Tables • Riegel’s Handbook of Industrial Chemistry • the Chemical processing Handbook • the Unit operations Handbook

Handbooks and Reference books • Process Design and Engineering Practice • Data for process Design and Engineering Practice • the Handbook of Reactive Chemical Hazards • Shreve’s Chemical Process Industries • Petrochemical Processes Handbook

Handbooks and Reference books • Refining Processes Handbook • Gas Processes Handbook

Handbooks and Reference books For reactions and kinetics • Chemical Reactor Design for Process Plants • Industrial and Engineering Chemistry Research • Journal of Catalysis - Academic Press • Applied Catalysis - Elseviser • Catalysis Today - Elseviser

Indexes • Applied Science and Technology Index • the Engineering Index • Chemical Abstracts • the Science Citation Index

Example: Input information for the hydrodealkylation of tuluene to produce benzene • Reaction information a. Reactions: Toluene + H2 Benzene + CH4 2Benzene Diphenyl + H2 b. Reaction inlet temperature > 1150 C (to get a reasonable reaction rate), reactor pressure = 5 psia.

Example c. d. Gas phase e. No catalyst • Production rate of benzene: 265 mol/hr • Production purity of benzene: xD = 0.9997

Example • Raw materials: Pure toluene at ambient conditions, H2 stream containing 95% H2, 5% CH4, at 550 psia. 100 F • Constrains: H2/aromatic  5 at the reactor inlet (to prevent coking), reactor outlet temperature < 1300 F (to prevent hydrocracking), rapidly quench reactor effluent to 1150 F (to prevent coking), x < 0.97 for the product distribution correlation • Other plant and site data to be given later

First Decision: Batch versus Continuous • Batch operation A batch process is one in which a finite quantity (batch) of product is made during a period of a few hours or days. The batch process most often consists of metering feed(s) into a vessel followed by a series of unit operations (mixing, heating, reaction, distillation, etc.) taking place at discrete scheduled intervals. This is then followed by the removal and storage of the products, by-product, and waste streams. The equipment is then cleaned and made ready for the next process. Production of up to 100 different products from the same facility can be done.

First Decision: Batch versus Continuous • Continuous process This process the feed is sent continuously to a series of equipment, with each piece usually performing a single unit operation. Products, by-products, and waste streams leave the process continuously and are sent to storage or for further processing.

First Decision: Batch versus Continuous • Production rate • Sometimes batch if less than 107 lb/yr • Usually batch if less than 106 lb/yr • Multiproduct plants • Market forces • Seasonal production • Short product lifetime

First Decision: Batch versus Continuous • Scale-up problems • Very long reaction times • Handling slurries at low flow rates • Rapidly fouling materials

Chemical Engineering Plant Design