Writing above and over double arrow

Question

I am writing several chemical formulas and reactions and I want to write over a reversible reaction the reaction constants for the Forwards and reverse reaction.

So far I have the following:

But I would like to have this:

Thanks in advance.

My MWE is:

\immediate\write18{makeindex \jobname.nlo -s nomencl.ist -o \jobname.nls}
\documentclass[a4paper,11pt,oneside]{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{mathpazo}
\usepackage{geometry}
\usepackage[colorinlistoftodos]{todonotes}
\usepackage{tablefootnote}
\usepackage{graphicx}
\usepackage{adjustbox}
\usepackage{array}
\usepackage{mathtools}
\usepackage{float}
\usepackage{subfig}
%, nccmath}
\usepackage{siunitx} 
\usepackage{mhchem}
\usepackage{chemfig}    
\usepackage[colorinlistoftodos]{todonotes}
\usepackage[autostyle]{csquotes}
\usepackage{multirow}
\usepackage{xcolor}
\usepackage{booktabs}
\usepackage{soul}
\usepackage{acro}
\usepackage{pgfplots}
\usepackage{enumitem}
\usepackage{lscape}
\usepackage{tabularx}
\usepackage{nomencl}
\makenomenclature
\usepackage{amsmath}
\usepackage{lipsum}
%\usepackage[toc,page]{appendix}
%\usepackage{tikz}
\usetikzlibrary{intersections, calc}
\pgfplotsset{compat=1.14}
\usepackage{filecontents}
\acsetup{first-style=short}
\DeclareAcronym{PFD}{
short = PFD ,
long = Process Flow Diagram , 
class = abbrev
}
\DeclareAcronym{PIB}{
short = PIB,
long = Polyisobutene (OPPANOL\textsuperscript{\textregistered}),
class = abbrev
}

\DeclareAcronym{BDIS}{
short = BDIS,
long = Process Information Management System,
class = abbrev
}

\DeclareAcronym{BD}{
short = BD,
long = 1-3 Butadiene,
class = abbrev
}

\DeclareAcronym{IOC}{
short = IOC,
long = Octenes isomers,
class = abbrev
}

\DeclareAcronym{Raf}{
short = Raf,
long = Raffinate,
class= abbrev
}

\DeclareAcronym{VLE}{
short = VLE,
long = Vapor-liquid equilibrium,
class= abbrev
}   
\DeclareAcronym{CSTR}{
short = CSTR,
long = Continuous Stirred Tank Reactor,
class = abbrev
}

\DeclareAcronym{wrt}{
short = wrt,    
long = with respect to,
class = abbrev
}

\DeclareAcronym{SQP}{
short = SQP,    
long = Sequential quadratic programming,
class = abbrev
}

\DeclareAcronym{PFR}{
short = PFR,    
long = Plug flow reactor,
class = abbrev
}

\usepackage[backend=bibtex,style=authoryear,natbib=true]{biblatex} 

\addbibresource{example.bib}

\begin{document}
\frontmatter
\tableofcontents
\listoffigures
\listoftables 


\mainmatter

\begin{center}
\ce{B <=>[k1] A <=>[k2] C}
\end{center}

\end{document}